PALEOZOIC STRUCTURE, METAMORPHISM, AND TECTONICS … 1997.pdfpaleozoic structure, metamorphism, and...

PALEOZOIC STRUCTURE,

METAMORPHISM, AND TECTONICS

OF THE BLUE RIDGE OF WESTERN

NORTH CAROLINA

EDITORS:

KEVIN G. STEWARTMARK G. ADAMS

CHARLES H. TRUPE

CAROLINA GEOLOGICAL SOCIETY1997 FIELD TRIP AND ANNUAL MEETING

Trip Leaders: Kevin Stewart, Mark Adams,Chuck Trupe, Rick Abbott, and Loren Raymond

Banner Elk, North CarolinaSeptember 26-28, 1997

CAROLINA GEOLOGICAL SOCIETY1997 FIELD TRIP GUIDEBOOK

September 26-28, 1997

PALEOZOIC STRUCTURE,

METAMORPHISM, AND TECTONICS OF

THE BLUE RIDGE OF WESTERN

NORTH CAROLINA

Edited by:

Kevin G. StewartDepartment of GeologyUniversity of North CarolinaChapel Hill, NC 27599-3315

Mark G. AdamsDepartment of GeologyAppalachian State UniversityBoone, NC 28608

Charles H. TrupeDepartment of GeologyUniversity of North CarolinaChapel Hill, NC 27599-3315

iii

TABLE OF CONTENTS

Foreword And Acknowledgements .....................................................................................iv

Dedication ..............................................................................................................................iv

Road Log And Stop Descriptions ........................................................................................1

Paleozoic Structural Evolution Of The Blue Ridge Thrust Complex,Western North Carolina .......................................................................................................21

Kevin G. Stewart, Mark G. Adams, Charles H. Trupe

Conditions And Timing Of Metamorphism In The Blue Ridge ThrustComplex, Northwestern North Carolina And Eastern Tennessee ....................................33

Mark G. Adams And Charles H. Trupe

Structural Relationships In The Linville Falls Shear Zone, Blue RidgeThrust Complex, Northwestern North Carolina ...............................................................49

Charles H. Trupe

Petrology And Tectonic Significance Of Ultramafic Rocks Near The Grandfather Mountain Window In The Blue Ridge Belt, ToeTerrane, Western Piedmont Zone, North Carolina ...........................................................67

Loren A. Raymond And Richard N. Abbott, Jr.

Petrology Of Pelitic And Mafic Rocks In The Ashe And AlligatorBack Metamorphic Suites, Northeast Of The Grandfather Mountain Window ............87

Richard N. Abbott, Jr. And Loren A. Raymond

iv

FOREWORD ANDACKNOWLEDGEMENTS

The Blue Ridge of western North Caro-lina has served as a classic field area for thestudy of the tectonics of crystalline rocks.Beginning with Arthur Keith at the turn of thecentury, the work of innumerable Blue Ridgefield geologists has provided the sound baseupon which we now build our research. Cer-tainly one of the greatest contributors was J.Robert Butler from the University of NorthCarolina at Chapel Hill.

The papers in the guidebook covervarious aspects of the structural and metamor-phic history of the Blue Ridge thrust complex.In addition we have provided a detailed roadlogto accompany the field trip stop descriptions.We hope this additional information will pro-vide participants with an understanding of thegeology between field stops.

We thank the authors of the papersincluded in the guidebook. We should note thatthe first two papers (Stewart et al. and Adamsand Trupe) are syntheses of previously pub-lished data and did not undergo peer review.The remaining papers were reviewed and wethank the reviewers for their careful work.

We are grateful to the sponsors of the1997 CGS meeting. As of the time of thiswriting, we have received generous donationsfrom Vulcan Materials, Olson Enterprises,Turner Environmental Consultants, and Appala-chian Resources. The generosity of our spon-sors allowed us to keep the registration costsdown.

We also thank the student assistants whoprovided invaluable help in many phases of themeeting: Lauren Hewitt, Shelly Kitchens, CalebPollock, and Cheryl Waters. We are especiallyindebted to Jane Gue who handled all of theregistration.

Dedication to the memory of

J. Robert Butler

We dedicate the 1997 Carolina Geologi-cal Society Guidebook to the memory of J.Robert Butler. Bob Butler was a geologyprofessor at the University of North Carolina atChapel Hill for over thirty years and was wellknown for his work on the geology of theCarolinas. Bob’s passion for field geology wasunmatched and he was a regular participant andtrip leader at past Carolina Geological Societymeetings. Our own interest in the geology ofthe Blue Ridge can be almost entirely attributedto Bob. His wealth of ideas, encyclopedicknowledge of Carolina geology, and readinessto head into the field made Bob a perfect col-league and mentor. Many of the ideas in thisguidebook originated during conversations andfield trips with Bob Butler. Bob’s influence isespecially evident when one considers howmany of his former students have continued towork in the Blue Ridge and on geologic prob-lems throughout the Carolinas. We believe it isonly fitting that we dedicate this book to hismemory.

Kevin Stewart, Mark Adams, and Chuck Trupe

v

Bob Butler and his Blue Ridge “Bullys”. Pictured from left to right: MarkAdams, Rod W illard, Chuck T rupe, Bob Butler . Not pictur ed: Kevin Stewart.Photo taken during SEGSA Field Trip, Banner Elk, NC, Mar ch 21, 1992.

1

INTRODUCTION

This field trip will focus on the structuraland metamorphic history of the Blue Ridgethrust complex in the vicinity of the GrandfatherMountain window in northwestern North Caro-lina. The Blue Ridge thrust complex is definedas the sequence of thrust sheets overlying theGrandfather Mountain window west of theBrevard zone. At least four thrust sheets havebeen recognized in the complex. These thrust

sheets include, from structurally lowest tohighest, the Pardee Point, the Beech Mountain,the Pumpkin Patch, and the Spruce Pine thrustsheets.

The rocks of the Blue Ridge record atectonometamorphic history that ranges fromthe Grenville orogeny (ca. 1 Ga) to the LatePaleozoic Alleghanian orogeny. The Grenvilleorogeny was a Mesoproterozoic collisionalevent associated with the assembly of a largePrecambrian supercontinent, Rodinia. Intru-

CAROLINA GEOLOGICAL SOCIETY1997 FIELD TRIP AND ANNUAL MEETING

ROAD LOG AND STOP DESCRIPTIONS

Grandfather

Mountain

window

Linville

Gossan-Lead fa

ult

Burnsville fault

Stone Mountain fault

Unaka M

ountain fault

Iron

Mou

ntain

fault

Holston M

ountain fault

Long

Brevard Fault z

one

Mountain City window

Spruce Pine thrust sheet

Pumpkin Patch thrust sheet

Beech Mountain thrust sheet

Pardee Point thrust sheet

0 5 10

Kilometers

NCTN

N

Boone

Banner Elk

Linville Falls

Burnsville

Bakersville

Falls fault

Ridge

faultStop 2 - 2

Stop 2 - 1

Stop 1 - 1

Stop 1 - 2

Stop 1 - 3

Stop 1 - 4

Stop 1 - 5

Stop 1 - 6

Stop 2 - 3

Linville

Pineola

Spruce Pine

Area oflarge map

Generalized tectonic map of the Blue Ridge thrust complex showing locations and numbers of field trip stops.

2

sions of mafic rocks and the presence of largerift basins (such as the Grandfather Mountainwindow) provide evidence for theNeoproterozoic break-up of Rodinia whichresulted in the formation of the North Americanpaleocontinent, Laurentia. Following the break-up of Rodinia was the accumulation of sedimen-tary strata along the passive margin ofLaurentia, and the formation of oceanic crustand marine sediments in the evolving basin ofthe Iapetus Ocean. Paleozoic orogenic eventscommenced with the destruction of theLaurentian margin during early stages of theOrdovician Taconic orogeny. Closure of theIapetus Ocean basin was associated with variouscompressional or transpressional events result-ing from arc-continent or continent-continentcollision. Paleozoic orogenic events in theAppalachian belt culminated with theAlleghanian orogeny. During this time, theBlue Ridge thrust complex was transported fromsoutheast to northwest (present geographiccoordinates) over rocks now exposed in theGrandfather Mountain window. Duplex faultingat structurally lower levels created doming ofthe area and subsequent erosion exposed rocksbelow the thrust sheets, thus creating the Grand-father Mountain window.

Formation of the Grandfather Mountainwindow has provided a unique opportunity toexamine the overriding thrust sheets and theirexposed boundaries. Within a relatively smallarea, there are exposures of rocks that recordevidence for the Grenville and subsequentPaleozoic orogenies. The objective of the fieldtrip is to present the participants with a geologiccross section through the Blue Ridge thrustcomplex. The field stops will range from theGrandfather Mountain window and its boundingfault, the Linville Falls fault, to the uppermostthrust sheet of the complex, the Spruce Pinethrust sheet. The trip will emphasize results ofrecent research by the authors. Day One willbegin with an examination of the shear zone andfault bounding the Grandfather Mountain

window at the base of the complex, followed bystops in the structurally highest thrust sheet toexamine ultramafic rocks, eclogite and otherhigh grade metamorphic rocks. Day two willconcentrate on the nature of deformation in theBeech Mountain thrust sheet, followed by a stopto recently discovered retrogressed eclogitenortheast of the Grandfather Mountain window.

Mi. DESCRIPTION

DAY ONE SATURDAY,SEPTEMBER 27, 1997

0.0 Leave parking lot at Holiday Inn ofBanner Elk. Turn left (south) on NC 194.The town of Banner Elk and the HolidayInn are at the northwestern edge of theGrandfather Mountain window. Theprominent mountain to the northwest isBeech Mountain, which lies outside thewindow. The prominent mountain to thesouthwest is Sugar Mountain.

1.1 Outcrop of Grandfather MountainFormation sandstone on left side of road.

2.0 Escarpment at shopping center showscontact between Montezuma Member(metabasalt) with sandstone of theGrandfather Mountain Formation.Continuing to the south along 194 theprominent mountain due south is Grand-father Mountain.

3.0 Intersection of NC 194 and NC 105 atLinville Gap. Turn right on 105 South.

3.8 Outcrops of Linville Metadiabase onright side of road.

7.0 Intersection of NC 105 and US 221.Proceed straight through intersection.

Carolina Geological Society

3

8.7 Intersection with NC 181 and US 221.Turn left on US 221/ NC 181 South.

10.5 Bear to the right on US 221 South.

13.5 Cross the western boundary of theGrandfather Mountain window into theLinville Falls shear zone in the hangingwall of the Linville Falls fault. For about2 miles, US 221 runs along the westernboundary of the window.

18.2 Entrance to Blue Ridge Parkway. Turnleft on Blue Ridge Parkway North.

19.3 Turn right on Blue Ridge Parkway spurto Linville Falls Visitor Center.

19.6 Cross Linville Falls fault back into theGrandfather Mountain window. Out-crops along road are Chilhowee Groupsandstone within the Table Rock thrustsheet.

20.8 National Park Service Linville FallsVisitor Center parking area.

STOP 1-1 LINVILLE FALLS FAULT ATLINVILLE FALLS, LINVILLE FALLS, NCQUADRANGLE

Location: Blue Ridge Parkway National Park,Linville Falls Overlook, approximately 0.3 mi Sof the National Park Service Linville FallsVisitors Center, Linville Falls, NC quadrangle.UTM coordinates: 416510mE, 3978520mN

Note: This stop is within the Blue Ridge Park-way National Park, and taking samples orbreaking rock is prohibited except by specialpermit. Cross the wall on the upstream side ofthe overlook to exposure of mylonite at the base

of the outcrop. Space at the outcrop is limited;we will lead groups of 25 people at a time toview this exposure.

Stop Leader: Charles H. Trupe

The Linville Falls fault frames theGrandfather Mountain window, and separatescrystalline rocks of the Blue Ridge thrust com-plex from underlying rocks of the window. AtLinville Falls, Neoproterozoic andMesoproterozoic rocks of the Beech Mountainthrust sheet overlie Cambrian Chilhowee Groupmetasedimentary rocks of the Tablerock thrustsheet. The fault was named for this exposure byBryant and Reed (1970), which they interpretedas a top-to-northwest thrust fault. Van Campand Fullagar (1982) and Schedl et al. (1992)obtained a Rb-Sr whole-rock date of 302 Ma formylonite from this location.

The Linville Falls fault dips gently westat this location. The overlook is on ChilhoweeGroup metaquartzite, which exhibits northwest-trending mineral stretching lineation, isoclinalfolds with hinges parallel to this lineation, andopen folds with northeast-trending hinges(Bryant and Reed, 1970; Hatcher and Butler,

Stop 1-1

Road Log and Stop Descriptions

4

1986). Crenulation cleavage is parallel to axialplanes of the folds. Hatcher and Butler (1986)stated that this cleavage (F3 cleavage of Butler,1973) is a penetrative feature that affects themylonite zone, and rocks in both the hangingwall and footwall.

Bryant and Reed (1970, p. 163) de-scribed the fault as “marked by 6 to 18 inches ofwhite to green finely laminated blastomylonitewhich is parallel to bedding in the quartzite andto foliation in the gneiss”. The “blastomylonite”separates “Cranberry Gneiss” in the hangingwall from Chilhowee Group quartzite of theTablerock thrust sheet in the footwall. Bryantand Reed (1970) noted extensive shearing inbasement rocks above the fault, and describedslices of quartzite, amphibolite, and mica schistintercalated with mylonitic gneisses along thefault at several locations around the GrandfatherMountain window.

New mapping in the vicinity of LinvilleFalls shows that the hanging wall of the faultlocally consists of tectonic blocks of alkalifeldspar granite, informally referred to as theLinville Falls granite (Trupe, this volume). The“blastomylonite” of Bryant and Reed (1970)occurs between the granite and ChilhoweeGroup quartzites in the footwall. The LinvilleFalls granite is similar petrographically andchemically to the Beech Granite, and mayrepresent tectonic slices of that unit. The gran-ite is typically strongly sheared, and myloniticfabric is locally overprinted by cataclasticfabric. Cataclastic fabric in the granite is well-exposed in outcrops on the north side of theLinville River across from the National ParkService Linville Falls Campground. ChilhoweeGroup metasedimentary rocks in the footwallare strongly sheared to quartz-sericite (± feld-spar) mylonite.

The Linville Falls granite is structurallyoverlain by a thick ductile shear zone consistingof several hundred meters of basement-derivedmylonite and ultramylonite, termed the LinvilleFalls shear zone by Trupe et al. (1990). The

mylonitic rocks record top-to-northwest shearsense, and were deformed at greenschist faciesconditions (Trupe, 1997, this volume). Theshear zone contains tectonic blocks of variouslithologies, including slices of magnetite-hornblendite, metagranite, and amphibolite.The Linville Falls shear zone extends to thecontact with the Ashe Metamorphic Suite, ~ 3kilometers southwest of the Linville Falls fault.

Some previous studies have assumedthat the Linville Falls fault is a meter-scalefeature at Linville Falls, and changes to a muchthicker zone in exposures along the northernmargin of the Grandfather Mountain window(e.g., Wojtal and Mitra, 1988; Newman andMitra, 1993). These previous workers failed torecognize the significant thickness of myloniticrocks present in the Linville Falls shear zone,the occurrence of cataclasites near LinvilleFalls, and the presence of slices of granite alongthe base of the shear zone. Models that attemptto explain thickening of the fault zone are notnecessary, as there is no large variation in shearzone thickness between Linville Falls andBanner Elk. The mylonite exposed at theLinville Falls overlook did not accommodate allof the strain associated with emplacement of theBlue Ridge thrust complex. This strain isrepresented by hundreds of meters of myloniteof the Linville falls shear zone.

ReferencesBryant, B., and Reed, J. C., Jr., 1970, Geology of the

Grandfather Mountain window and vicinity, NorthCarolina and Tennessee: United States GeologicalSurvey Professional Paper 615, 190 p.

Butler, J.R., 1973, Paleozoic deformation and metamor-phism in part of the Blue Ridge thrust sheet, NorthCarolina: American Journal of Science, v. 273-A, p.72-88.

Hatcher, R. D., Jr. and Butler, J. R., 1986, Linville Fallsfault at Linville Falls, North Carolina: GeologicalSociety of America Centennial Field Guide, South-eastern Section, p. 229-230.


5

Newman, J., and Mitra, G., 1993, Lateral variations inmylonite zone thickness as influenced by fluid-rockinteractions, Linville Falls fault, North Carolina:Journal of Structural Geology, v. 15, p. 849-863.

Schedl, A., McCabe, C., Montanez, I., Fullagar, P. D., andValley, J., 1992, Alleghanian regional diagenesis: Aresponse to the migration of modified metamorphicfluids derived from beneath the Blue Ridge-Piedmontthrust sheet: Journal of Geology, v. 100, p. 339-352.

Trupe, C. H., 1997, Deformation and metamorphism inpart of the Blue Ridge thrust complex, northwesternNorth Carolina: Ph.D. Dissertation, University ofNorth Carolina at Chapel Hill, 176 p.

Trupe, C. H., Butler, J. R., Mies, J. W., Adams, M. G., andGoldberg, S. A., 1990, The Linville Falls fault andrelated shear zone: Geological Society of AmericaAbstracts with Programs, v. 22, p. 66.

Van Camp, S. G., and Fullagar, P. D., 1982, Rb-Sr whole-rock ages of mylonites from the Blue Ridge andBrevard zone of North Carolina: Geological Societyof America Abstracts with Programs, v. 14, p. 92-93.

Wojtal, S., and Mitra, G., 1988, Nature of deformation insome fault rocks from Appalachian thrusts, in Mitra,G., and Wojtal, S., editors, Geometries and Mecha-nisms of Thrusting, With Special Reference to theAppalachians: Geological Society of AmericaSpecial Paper 222, p. 17-33.

COFFEE BREAK

Return to Blue Ridge Parkway.

22.3 Turn left (south) on Blue Ridge Park-way. Parkway continues within theLinville Falls shear zone (hanging wallof Linville Falls fault). The shear zone iscomposed of blocks of sheared andunsheared rocks incorporated intomylonites predominantly derived fromcrystalline basement rocks.

26.0 Outcrops on the right (mile marker 320).Buses will unload here, then continuesouth to Gillespie Gap and turn aroundat the Museum of North Carolina Miner-als.

STOP 1-2 CONTACT BETWEEN THELINVILLE FALLS SHEAR ZONE ANDTHE ASHE METAMORPHIC SUITE,BLUE RIDGE PARKWAY, LINVILLEFALLS, NC QUADRANGLE

Location: Blue Ridge Parkway National Park,Blue Ridge Parkway, mile marker 320, LinvilleFalls, NC quadrangle. UTM coordinates:413650 mE, 3977350 mN.

Note: This stop is within the Blue Ridge Park-way National Park, and taking samples orbreaking rock is prohibited except by specialpermit. There is limited space along the shoul-der of the Parkway. Participants must be ex-tremely careful of traffic on the Parkway.


The purpose of this stop is to examinethe contact between Ashe Metamorphic Suite(AMS) rocks in the Spruce Pine thrust sheet andmylonitic rocks of the Linville Falls shear zone.Bryant and Reed (1970) and Abbott andRaymond (1984) postulated that the contactbetween the AMS and underlying basementrocks is a fault. The basal contact of the Spruce

Stop 1-2

Blue Ridge Parkway


6

Pine thrust sheet was defined as a northwest-directed, post-metamorphic thrust fault (Butleret al., 1987), and subsequent work has verifiedthese relationships adjacent to and northeast ofthe Grandfather Mountain window (Mies,1990). Along strike to the southwest, the con-tact between the AMS and basement is a dextralstrike-slip fault (Burnsville fault, Stop 1-4) thatpredates the thrust fault adjacent to the window(Adams et al., 1995a). Field relations indicatethat the strike slip faulting is overprinted in thevicinity of the Grandfather Mountain windowby Alleghanian shearing and thrust faulting(Adams et al., 1995a). Farther to the southwest,other workers have stated that the presumablyequivalent contact is a pre-metamorphic thrustfault, correlative with the Hayesville fault(Hatcher, 1987; Merschat and Wiener, 1988).

At this stop, garnet mica schist (± kyan-ite) and amphibolite of the AMS overlie LinvilleFalls shear zone mylonite derived from biotitegneiss and granitic gneiss of the Beech Moun-tain thrust sheet. The mylonites contain alkalifeldspar and plagioclase porphyroclasts in amatrix of recrystallized quartz and fine-grainedbiotite, with minor epidote, sphene, and opaqueminerals. Foliation and compositional layeringin the hanging wall and footwall rocks dipsouthwest. Northwest-trending mineral stretch-ing lineation is well-developed in the myloniticfootwall rocks. Pegmatites in the AMS aresheared and boundinaged; asymmetric boudinsdemonstrate top-to-northwest shear sense.

Amphibolite facies assemblages in theAMS rocks have been sheared and retrogradednear the contact. Hornblende in the amphibo-lites is partially replaced by green biotite, andepidote is abundant. Garnet in the mica schistshas been partially to totally replaced green bybiotite (± chlorite). Plagioclase is sausseritic,and fine-grained white mica is abundant. Theearlier regional metamorphic foliation is over-printed by shearing. Muscovite fish and shearbands show top-to-northwest shear sense. The

retrogressive metamorphism and shearing areinterpreted to represent late Paleozoic deforma-tion during Alleghanian thrusting.

ReferencesAbbott, R.N., Jr., and Raymond, L.A., 1984, The Ashe

Metamorphic Suite, northwest North Carolina:Metamorphism and observations on geologic history.American Journal of Science, 284, 350-375.

Adams, M.G., Stewart, K.G., Trupe, C.H., and Willard,R.A., 1995a, Tectonic significance of high-pressuremetamorphic rocks and dextral strike-slip faulting inthe southern Appalachians: in Hibbard, J., van Staal,C.R., Cawood, P. and Colman-Sadd, S., editors, NewPerspectives in the Appalachian-Caledonian orogen,Geological Association of Canada Special Paper 41,p. 21-42.

Bryant, B., and Reed, J. C., Jr., 1970, Geology of theGrandfather Mountain window and vicinity, NorthCarolina and Tennessee: United States GeologicalSurvey Professional Paper 615, 190 p.

Butler, J.R., Goldberg, S.A., and Mies, J.W., 1987,Tectonics of the Blue Ridge west of the GrandfatherMountain window, North Carolina and Tennessee:Geological Society of America Abstracts withPrograms, v. 19, p. 77.

Hatcher, R.D., Jr., 1987, Tectonics of the southern andcentral Appalachian internides: Annual Review ofEarth and Planetary Sciences, v. 15, p. 337-362.

Merschat, C.E., and Wiener, L.S., 1988, Geology of theSandymush and Canton quadrangles, North Carolina:North Carolina Geological Survey Bulletin 90, 66 p.

Mies, J.W., 1990, Structural and petrologic studies ofmylonite at the Grenville basement - Ashe Formationboundary, Grayson County, Virginia to MitchellCounty, North Carolina: Ph. D. Dissertation,University of North Carolina at Chapel Hill, 330 p.

Turn around. Head north on Blue RidgeParkway.

29.7 Turn left into Linville Falls Picnic Area.

LUNCH

30.8 Exit for US 221. Turn right (north) onUS 221.


7

32.0 Intersection with NC 194. Turn left(south) on NC 194.

33.0 In this vicinity, cross from the LinvilleFalls shear zone into the Ashe Metamor-phic Suite of the Spruce Pine thrustsheet.

36.0 Turn left (south) on US 19E.

43.2 Intersection with NC 226. Continuesouth on US 19E through the Town ofSpruce Pine, “The Mineral City”, thecenter of the Spruce Pine mineral dis-trict. This area is the leading producer ofdomestic feldspar and is also a majorproducer of scrap and sheet mica, quartz,and kaolin. The area is also known formuseum-quality specimens of beryl,tourmaline, garnet, and other preciousand semiprecious minerals. Abandonedand active mica and feldspar mines arevisible on many of the mountainsides inthis area.

49.8 Turn right on NC 80 North. Park inparking area adjacent to buildings onright, or continue ~100 yards across ToeRiver Bridge and park on gravel pull-over on right.

STOP 1-3 NEWDALE DUNITE NEARNEWDALE, NC, MICAVILLE, NC QUAD-RANGLE

Location: NC 80 ~400 meters north of SouthToe River bridge, Micaville, NC quadrangle.UTM coordinates: 392830mE, 3974520mN.

Stop leader: Loren A. Raymond

The Newdale Dunite is one of the nu-merous ultramafic rock bodies present withinthe Ashe Metamorphic Suite of the Spruce Pine

Thrust Sheet (Toe Terrane). Immediately east ofthe Highway, a quarry was operated in thedunite during the 1970s, but termination ofmining operations allowed the quarry to fill withwater by the late 1980s. Dunite is exposed bothalong the highway and along an east trending,local road that intersects the highway near thesouth edge of the quarry.

The Newdale Dunite body, mapped aspart of the Spruce Pine District mapping projectof the USGS (Brobst, 1962), is an east-northeasttrending, elliptical body. The body is about440m long and 210m wide (Vrona, 1977).Brobst (1962) shows the body to be in contactprimarily with hornblende schist and gneiss(amphibolite). Vrona (1977) made a moredetailed map of the Newdale Dunite and foundthat the dunite is surrounded by hornblendeschist and gneiss, but it seemingly cuts a bodyof anthophyllite-plagioclase gneiss enclosedwithin the hornblende-rich rocks.

In spite of the exposures of ultramaficrocks in mine workings and road cuts, Vrona(1977) was unable to locate any exposures ofthe contacts. He noted, however, no evidence ofcontact metamorphism, he observed apparentcross-cutting relations between structures in theultramafic rocks and those in surroundinggneisses, and he recognized serpentine- and

Stop 1-3NC Hwy 80


8

talc-bearing rocks near the contacts. From thesedata, Vrona (1977) concluded that the contactwas tectonic and he figured it as a thrust fault.

The dominant rock of the Newdaleultramafic body is metadunite, butmetaharzburgite and metachromitite are presentlocally (Vrona, 1977). The metadunite is typi-cally somewhat serpentinized, but is character-ized by LPO (Lattice preferred orientation)fabrics (Vrona, 1977). Metaharzburgite occurswhere orthopyroxene forms layers, and chromiteoccurs both in layers and in one podiform mass(Vrona, 1977, p. 30). More commonly, however,sparse chromite and orthopyroxene are scatteredamong the dominant olivine grains within themetadunite. Tremolite is similarly distributed.Veins containing talc, anthophyllite, and magne-site, as well as locally extensive veins of serpen-tine minerals reflect local fluid-enhanced,retrograde metamorphism of the metadunite.Thin-section petrography reveals textures andmineral associations that suggest at least threesuccessive metamorphic “events.” An earlyolivine + chromite + pyroxene assemblage isoverprinted by an Amphibolite Facies olivine +chromite + tremolite + chlorite + pyroxeneassemblage, which in turn is overprinted by oneor more Greenschist Facies assemblages com-posed of serpentine + magnetite + chlorite + talc+ tremolite. The vein assemblage composed oftremolite + anthophyllite + chlorite + talc +magnesite may represent an Amphibolite Facies,fluid-dominated, localized metasomatic event.Textural evidence suggests that the GreenschistFacies metamorphism followed earlier Am-phibolite Facies metamorphism.

References:Brobst, D.A., 1962, Geology of the Spruce Pine district,

Avery, Mitchell, and Yancey Counties, NorthCarolina: United States Geological Survey Bulletin1122-A, 26 p.

Vrona, J.P., 1997, Structure of the Newdale ultramafite,Yancey County, North Carolina: M.S. Thesis,Southern Illinois University, 115p.

Return to intersection of NC 80 and US19E. Turn right (south) on US 19E.

55.0 Turn right (north) on NC 197.

59.0 Turn left on Clearmont School Road(NCSR 1416). Day Book ultramaficbody is on the right. Clearmont SchoolRoad traverses the Burnsville faultwhich separates the Spruce Pine thrustsheet from the Pumpkin Patch thrustsheet.

60.0 Park at gravel area adjacent toClearmont School Road. Walk back eastalong road for 0.2 miles to Stop 1-4.

STOP 1-4 SHEARED BAKERSVILLEGABBRO ALONG THE BURNSVILLEFAULT ZONE, CLEARMONT SCHOOLROAD, BURNSVILLE, NC QUADRANGLE

Location: Clearmont School Road (NCSR1416) approximately 0.3 miles east of intersec-tion with Jacks Creek Road (1336), Burnsville,NC quadrangle. UTM coordinates: 383390mE,3981720mN.

Stop Leaders: Kevin G. Stewart, Mark Adams

This outcrop contains strongly shearedBakersville Gabbro associated with theBurnsville fault zone. The rock is stronglyfoliated and lineated. Foliation strikes northeastand dips moderately to the southeast. The well-developed mineral-stretching lineation trendsnortheast and is subhorizontal. Northeast trend-ing, mineral-stretching lineations have beenrecognized along the Burnsville fault from theCarvers Gap quadrangle to at least the Mars Hillquadrangle to the southwest for a distance atleast 50 kilometers. Kinematic indicators fromrocks along the Burnsville fault are consistent


9

with dextral strike-slip movement (Mallard etal., 1994; Adams et al., 1995; Burton, 1996).

Petrographic observations and P-Testimates based on mineral chemistry indicatethat deformation along the Burnsville fault zoneoccurred under amphibolite facies conditions.Ribbons of dynamically recrystallized horn-blende indicate crystal-plastic deformation ofamphibole, which is thought to occur at 600-650° C (cf., Burton, this volume; Adams et al.,1995). Additionally, equilibrium grain boundarytextures in dynamically recrystallized quartz andplagioclase are consistent with deformationunder high temperature conditions. Adams andStewart (1993) reported P-T estimates of 6 - 8kbar and 580 - 640° C for sheared pelitic schistassociated with the Burnsville fault. Theseauthors interpreted these conditions to representfinal equilibration during shearing. Burton(1996) estimated temperatures of 640 - 730° Cat assumed pressures of 6 - 8 kbar for dynami-cally recrystallized hornblende and plagioclasefrom sheared amphibolite along the Burnsvillefault zone.

Amphibolites facies conditions ofdeformation along the Burnsville fault contrastwith greenschist facies conditions reported fordeformation along other major shear zones in

the Blue Ridge thrust complex (e.g., LinvilleFalls fault, Long Ridge fault, Fries-Gossan Leadfault, Stone Mountain fault).

The Burnsville fault juxtaposes rocks ofthe Ashe Metamorphic Suite with crystallinerocks of the Laurentian margin. This contacthas been interpreted by many workers to be theTaconic suture in this part of the Blue Ridge.Most tectonic syntheses of this area correlate theBurnsville fault with the Hayesville fault to thesouth (for details of fault correlations see Adamset al., 1995). If the Burnsville fault correspondsto the Taconic suture then it was originally anOrdovician thrust. Recent geochronology ofmylonites within the Burnsville shear zone,however, suggest strike-slip faulting occurredduring the Siluro-Devonian (Goldberg andDallmeyer, 1997). The Bakersville gabbro inthis outcrop is intruded by a Spruce Pine-typepegmatite (Siluro-Devonian; Kish, 1983).Spruce Pine pegmatites are generally restrictedto the Spruce Pine thrust sheet while Bakersvilleintrusive rocks are found only in the Laurentianbasement rocks. The presence of the pegmatitein the gabbro indicates that the Spruce Pine andPumpkin Patch thrust sheets were initiallyjuxtaposed prior to the intrusion of the pegma-tite, possibly during the Ordovician. Shearingof the pegmatite indicates that strike-slip fault-ing followed intrusion. These field relationscombined with the geochronology are consistentwith the interpretation that the strike-slip motionon the Burnsville fault is a Siluro-Devonianreactivation of an original Taconic thrust.

To the southwest, in the Barnardsvillequadrangle, the Burnsville fault zone widens toseveral miles and consists of large blocks ofrelatively unsheared basement and Ashe Meta-morphic Suite bounded by thick mylonite zoneswith consistent dextral strike-slip kinematicindicators.

To the northeast, approaching the Grand-father Mountain window, the nature of the faultchanges. The lineations become more north-westerly and the conditions of shearing are


Stop 1-4

10

upper greenschist. We believe this is an over-printing of later Alleghanian thrusting along theLinville Falls shear zone, as seen at Stop 1-2.North of the window, the base of the SprucePine thrust sheet (the Fries/Gossan-Lead fault)is a greenschist facies, northwest-directed thrust.This fault has also been interpreted as anAlleghanian feature and in our model wouldrepresent late transport of rocks which wereoriginally east of the Taconic suture.

ReferencesAdams, M.G., and Stewart, K.G., 1993, Tectonic implica-

tions of the occurrence of eclogite in the Blue Ridge,northwestern North Carolina, Geological Society ofAmerica Abstracts with Programs, v.25, p. 424.

Adams, M.G., Stewart, K.G., Trupe, C.H., and Willard,R.A., 1995, Tectonic significance of high-pressuremetamorphic rocks and dextral strike-slip faulting inthe southern Appalachians: in Hibbard, J., van Staal,C.R., Cawood, P. and Colman-Sadd, S., editors,Current Perspectives in the Appalachian-Caledonianorogen, Geological Association of Canada SpecialPaper 41, p. 21-42.

Burton, F.H., 1996, Kinematic study of the Taconicsuture, west-central North Carolina: M.S. thesis,University of North Carolina at Chapel Hill, 114 p.

Goldberg, S.A., and Dallmeyer, R.D., 1997, Chronologyof Paleozoic metamorphism and deformation in theBlue Ridge thrust complex, North Carolina andTennessee: American Journal of Science, v. 297, p.488-526.

Kish, S.A., 1983, A geochronological study of deforma-tion and metamorphism in the Blue Ridge andPiedmont of the Carolinas: Ph. D dissertation,University of North Carolina at Chapel Hill, 220 p.

Mallard, L.D., Adams, M.G., and Stewart, K.G., 1994,Kinematic analysis of a possible suture in thesouthern Appalachians, northwestern North Carolina:Geological Society of America Abstracts withPrograms, v. 26, n. 4, p. 25-26.

Return east along Clearmont SchoolRoad to NC 197.

61.0 Turn left (north) on NC 197.

62.6 Cross the Burnsville fault into thePumpkin Patch thrust sheet.

67.7 On north side of Toe River Bridge,beside railroad tracks. Outcrop ofmylonitic Bakersville gabbro exhibitingwell-developed, sub-horizontal, north-east-trending mineral stretching linea-tions.

68.3 Turn right (south) on NC 226.

69.6 Outcrops of Bakersville Gabbro alongleft side of road.

72.5 Outcrops of Mesoproterozoic biotite-hornblende gneiss typical of basementrocks of the Pumpkin Patch thrust sheet.

73.1 Cross the Burnsville fault back into theSpruce Pine thrust sheet.

73.8 Enter Bakersville, NC. Turn right fol-lowing NC 226. Then turn left at stoplight onto North Mitchell Avenue(NCSR 1211).

74.1 Turn left on Redwood Road (NCSR1217).

74.6 Unload from buses for Stop 1-5.

STOP 1-5 ECLOGITE AND RETRO-GRESSED ECLOGITE ALONGHONEYCUTT BRANCH, BAKERSVILLE,NC-TN QUADRANGLE

Location: NCSR 1217 ~ 3000 feet NE ofintersection with NCSR 1211, northeast ofBakersville, NC, Bakersville, NC-TN quad-rangle. UTM coordinates: 396550mE,3986740mN

Stop Leader: Mark G. Adams

Eclogite and retrogressed eclogite areexposed in the road cut along the northwest sideof the road. The exposure is continuous for


11

approximately 300 feet and discontinuous forapproximately 1000 feet. This occurrence is theoriginal outcrop of eclogite discovered by RodWillard and described by Willard and Adams(1994). This exposure is one of several largeblocks of eclogite that occur in the area. Themajority and the largest of the blocks occuralong Lick Ridge (8000 feet, N24E from here)and are described in Adams et al. (1995).

The outcrop shows a complete progres-sion from relatively pristine eclogite to thor-oughly retrogressed eclogite (amphibolite). Theprimary assemblage in the eclogite is garnet +omphacite + quartz + rutile. Variably the rockscontain retrograde minerals that include one ormore of the following: diopside, plagioclase,hornblende, sphene, ilmenite, and epidote. In themost thoroughly retrogressed samples, theresulting mineralogy is dominated by horn-blende + plagioclase + quartz ± sphene ± il-menite ± epidote. The most thoroughly retro-gress eclogite is petrographically indistinguish-able from other amphibolite in the Ashe, whichled Adams et al. (1995) and Abbott andRaymond (this guidebook) to speculate thatmuch of the Ashe Metamorphic Suite wasmetamorphosed to eclogite facies conditions,

but retrogressive metamorphism has obscuredmost evidence for earlier, high pressure condi-tions.

The eclogite shows a characteristiccompositional layering defined by alternatingconcentrations of garnet-rich and pyroxene-richmaterial. This layering is generally oblique tothe regional metamorphic foliation. Locally, asretrogression becomes more pervasive, thelayering is transposed into concordance with theregional foliation (a feature more prevalent inexposures on Lick Ridge; see photograph oncover of guidebook).

The blocks of eclogite are incorporatedinto sheared pelitic schist, gneiss, and amphibo-lite of the Ashe Metamorphic Suite. An expo-sure of the contact between this block and thepelitic schist is located in the bed of HoneycuttBranch off of the road bank to the east. Theexposure in the creek bed also shows severalsmall (~ 30 cm in diameter) pods of eclogiteincorporated into the pelitic schist matrix. Inaddition to these blocks of eclogite, small bodiesof ultramafic rock are also incorporated into thepelitic schist and amphibolite. One small bodyof altered ultramafic rock (predominantlyactinolite schist) occurs approximately 2000 feetto the southwest, but is not easily accessible.

Adams et al. (1995) reported P-T esti-mates from geothermobarometry based onmineral chemistry for eclogite and adjacentrocks in the area. Garnet-omphacite pairs fromeclogite yield temperatures ranging from 626°to 790° C. Omphacite-plagioclase pairs yieldpressure estimates ranging from 13 to 17 kbar.As plagioclase is a retrograde product, thesepressure estimates are considered to be mini-mum pressures.

The structural base of the Ashe Meta-morphic Suite is represented by the Burnsvillefault. This fault occurs approximately 2000 feetto the northwest. Locally, sheared rocks associ-ated with the Burnsville fault show mesoscopicand microscopic kinematic indicators (such as,shear bands, porphyroclasts with asymmetric

Stop 1-5

NCSR 1217


12

tails, and composite planar fabric) indicating amajor component of dextral shear sense alongthe fault.

References:Adams, M.G., Stewart, K.G., Trupe, C.H., and Willard,

R.A., 1995, Tectonic significance of high-pressuremetamorphic rocks and dextral strike-slip faulting inthe southern Appalachians: in Hibbard, J., van Staal,C.R., Cawood, P. and Colman-Sadd, S., editors, NewPerspectives in the Appalachian-Caledonian orogen,Geological Association of Canada Special Paper 41,p. 21-42.

Willard, R.A., and Adams, M.G., 1994 Newly discoveredeclogite in the southern Appalachian orogen, north-western North Carolina: Earth and Planetary ScienceLetters, v. 123, p. 61-70.

Walk 0.6 miles northeast to McKinneyCove Church. Very large, well-exposedoutcrops of eclogite occur on LickRidge, which is the prominent ridgeobservable to the northeast during thiswalk.

COFFEE BREAK

Return to North Mitchell Avenue, turnright. At stop light turn right onto NC226. Go ~ 100 feet to flashing light andproceed straight (north) on NC 261.

76.9 Cross Burnsville fault from Spruce Pinethrust sheet into Pumpkin Patch thrustsheet.

79.5 Outcrop of two-pyroxene granulite onleft side of road. Granulite facies rockshave been reported from a few scatteredlocations in basement rocks of thePumpkin Patch thrust sheet. PumpkinPatch Mountain is the prominent moun-tain on the left. Meadlock Mountain isthe prominent mountain on the right.

81.5 In this vicinity, enter back into theBurnsville fault zone. NC 261 continueswithin the fault zone for approximately3.5 miles, then turns north out of thefault zone into the Pumpkin Patch thrustsheet.

85.2 Outcrop of Meadlock Mountain gneisson left side of road. Continue 0.2 milesand park on wide shoulder on left side ofroad. Walk back to outcrop of MeadlockMountain gneiss.

STOP 1-6 MEADLOCK MOUNTAINGNEISS ALONG NC HWY 226 NEARROAN VALLEY, CARVERS GAP, NC-TNQUADRANGLE

Location: Along NC - 261 ~ 1.2 mi NE of GlenAyre, Carvers Gap, NC-TN quadrangle. UTMcoordinates: 402150mE, 3993830mN


This stop highlights the MeadlockMountain gneiss, a mafic basement gneiss in thePumpkin Patch thrust sheet. The Meadlock

Stop 1-6

NC Hwy 261


13

Mountain gneiss is juxtaposed with the AsheMetamorphic Suite along the Burnsville fault inthe eastern part of the Bakersville quadrangleand the western part of the Carvers Gap quad-rangle. The fault is located approximately 1000feet to the south. East of this area, the nature ofthe Ashe-basement contact changes from arelatively high grade (amphibolite facies),dextral strike-slip shear zone to a lower grade(greenschist facies), northwest directed thrustfault (Adams et al., 1995a). Adams et al.(1995a) suggested that the original juxtapositionof the Pumpkin Patch and Spruce Pine thrustsheets was a result of collision during theTaconic orogeny, the dextral movement resultedfrom reactivation of the fault during the Acadianorogeny, and the thrust fault overprinted theearlier strike-slip fault during the Alleghanianorogeny.

The stable metamorphic assemblage inthis rock includes garnet + hornblende + diop-side + biotite + plagioclase + quartz + rutile +ilmenite. Adams et al. (1995b) estimated “peak”P-T conditions of ~13 kbar at 725° C and finalequilibration conditions of ~9 kbar at 660° C.Thin sections from this locality show deforma-tion of primary metamorphic phases, but do notshow significant retrograde phases. Deformationis manifested by grain-size reduction by dy-namic recrystallization and the formation ofribbons of quartz, hornblende, and pyroxene.Equilibrium textures of dynamically recrystal-lized grains in pyroxene ribbons indicate defor-mation occurred under relatively high gradeconditions.

Gulley (1985) and Monrad and Gulley(1983) documented granulite facies conditions(6.5 - 8 kbar at 750° - 847° C) for the Cloudlandand Carvers Gap gneisses on Roan Mountain.These authors interpreted these granulite faciesconditions to represent metamorphism duringthe Precambrian. They also estimated upperamphibole facies conditions (10 - 12 kbar at 680- 760° C) interpreted to record Paleozoic(Taconic orogeny) metamorphism. The latter P-

T estimates are consistent with those from theMeadlock Mountain gneiss reported by Adamset al. (1995).

The Meadlock Mountain gneiss may becorrelative with parts of the Carvers Gap gneissas described by Gulley (1985) and Monrad andGulley (1983). However, these authors statedthat mafic components of the Carvers Gapgneiss constitute a minor volume of the gneiss.Geologic mapping (Adams, 1995) indicates thatthe Meadlock Mountain gneiss covers a signifi-cant area in map view. Although felsic zoneslocally occur (exposed ~ 500 feet east of thisstop) within the Meadlock Mountain gneiss, themafic component is the dominant lithologyalong the Burnsville fault from Bakersville, NCto the community of Valley in the Carvers Gapquadrangle (Adams, 1995).

References:Adams, M.G., 1995, The tectonothermal evolution of part

of the Blue Ridge thrust complex, northwesternNorth Carolina: Ph.D. dissertation, University ofNorth Carolina, 193 p.


Adams, M.G., Trupe, C.H., Goldberg, S.A., Stewart,K.G., and Butler, J.R., 1995b, Pressure-temperaturehistory of high-grade metamorphic rock along theeastern-western Blue Ridge boundary, northwesternNorth Carolina: Geological Society of AmericaAbstracts with Programs, v. 27, p. 33.

Gulley, G. L., Jr., 1985, A Proterozoic granulite-faciesterrane on Roan Mountain, western Blue Ridge belt,North Carolina - Tennessee: Geological Society ofAmerica Bulletin, v. 96, p. 1428-1439.

Monrad, J.R., and Gulley, G.R., Jr., 1983, Age and P-Tconditions during metamorphism of granulite-faciesgneisses, Roan Mountain, North Carolina-Tennessee,in Lewis, S.E., editor, Geologic investigations in theBlue Ridge of northwestern North Carolina: CarolinaGeological Society Guidebook, North CarolinaGeological Survey, Article IV, 29 p.


14

Walk back to buses.

Continue north on NC 261.

89.3 NC-TN state line at Carvers Gap. Op-tional stop: turn left at Carvers Gap toRoan Mountain overlook. The overlookprovides a panoramic view of the BlueRidge and Valley and Ridge Provinces.For detailed description of the geology atthis stop, refer to 1983 Carolina Geo-logical Society Field Trip Guidebook.

At the TN-NC state line, NC 261 be-comes TN 143. Continue north on TN143 to US 19E. Turn right (south) on US19E. Continue for approximately 7 milesto NC 194 in the town of Elk Park. Turnleft (north) on NC 194 toward BannerElk. Continue to NC 184 in Banner Elk.Turn right at stop light and continue 1.3miles to Holiday Inn.

DAY TWO SUNDAY,SEPTEMBER 28, 1997

0.0 Leave Holiday Inn parking lot. Turnright (north) on NC 184.

1.0 Low outcrop on left is stretched pebblemetaconglomerate of the GrandfatherMountain Formation.

1.3 Turn left at traffic light onto NC 184/NC 194.

1.6 Park in Sunrise Shopping Center lot onright.

STOP 2-1. LINVILLE FALLS FAULT ATBANNER ELK

Location: Intersection of NC highways 184 and194, Elk Park 7.5' quadrangle. UTM coordi-nates: 420990mE, 4002050mN.


The Linville Falls fault (Bryant andReed, 1970) is exposed on the west side ofBanner Elk, North Carolina, approximately 30meters northwest of the intersection of BeechMountain Parkway (NC 184) and NC 194(Figure 4). The fault juxtaposes Precambriancrystalline rocks of the Beech Mountain thrustsheet and underlying rocks of the GrandfatherMountain window. Approximately 50 meters ofhanging wall and footwall rocks are exposed ina recent excavation. Metaquartzite and sericiticquartz mylonite and ultramylonite in the hang-ing wall dip moderately northwest, concordantwith the orientation of the fault surface. Thefootwall consists of low-grade metasedimentaryrocks of the Grandfather Mountain Formation inwhich sedimentary layering dips moderatelynortheast and cleavage dips moderately east.Although the major Blue Ridge thrusts are


Stop 2-1

NC Hwy 194

NC Hwy 184

15

generally recognized as hinterland dippingstructures, the northwest dip of the fault atBanner Elk is due to large amplitude folding ofthe thrust surface during formation of theGrandfather Mountain window (Bryant andReed, 1970), possibly due to the presence of anunderlying thrust duplex (Boyer and Elliott,1982).

Recent work in the area has shown thatthe Linville Falls fault lies at the base of a thickshear zone, appropriately termed the LinvilleFalls shear zone (Trupe et al., 1990; Trupe, thisvolume). Adams and Su (1996) showed theshear zone to be approximately 1 kilometerthick in the Banner Elk area. Kinematic indica-tors in the mylonites of the shear zone yield top-to-northwest sense of shear (Trupe, 1989;Adams, 1990; Adams and Su, 1996). The shearzone contains tectonic slices of metaquartziteand exotic crystalline rocks (granoblastic lay-ered gneisses and metagranite) surrounded bygreenschist-grade mylonite and ultramylonite.Contrary to assumptions by Newman and Mitra(1993), the relatively small, exotic slice ofquartz diorite gneiss (Potts Cemetery gneiss ofAdams and Su, 1996) that crops out 500 metersabove the fault zone is not a likely protolith ofthe hanging wall mylonites.

Cataclastic deformation overprintsmylonitic fabric several meters into the hangingwall rocks at Banner Elk. A progression fromultracataclasite (2-3 cm) to cataclasite (1-2 m)to protocataclasite may be observed immedi-ately above the fault. Cataclastic rocks alsooccur at an exposure of the fault at Bowers Gap,4 km east of the Banner Elk exposure, and at thetype locality of the fault at Linville Falls, NC(Trupe, this volume). The Long Ridge fault(Adams and Su, 1996) to the northeast, and theStone Mountain fault farther to the northwestalso exhibit cataclastic deformation overprintingmylonitic fabrics, and are related to the LinvilleFalls shear zone .

ReferencesAdams, M.G., 1990, The geology of the Valle Crucis area,

northwestern North Carolina: Masters Thesis,University of North Carolina at Chapel Hill, 95 p.

Adams, M. G., and Su, Q., 1996, The nature and timing ofdeformation in the Beech Mountain thrust sheetbetween the Grandfather Mountain and MountainCity windows in the Blue Ridge of northwesternNorth Carolina: Journal of Geology, p. 197-213.

Boyer, S.E., and Elliott, D., 1982, Thrust systems:American Association of Petroleum GeologistsBulletin, v. 66, p. 1196-1230.

Bryant, B., and Reed, J.C., Jr., 1970, Geology of theGrandfather Mountain window and vicinity, NorthCarolina and Tennessee: United States GeologicalSurvey Professional Paper 615, 190 p.


Trupe, C.H., 1989, Kinematic analysis and deformationmechanisms in mylonites from the Blue Ridge thrustcomplex, northwestern North Carolina: GeologicalSociety of America Abstracts with Programs, v. 21, p.62.

Trupe, C.H., Butler, J.R., Mies, J.W., Adams, M.G., andGoldberg, S.A., 1990, The Linville Falls fault andrelated shear zone: Geological Society of AmericaAbstracts with Programs, v. 22, p. 66.

Leave Sunrise Shopping Center. Turnleft on NC 184 South/ NC 194 North.

1.9 Turn right at traffic light on NC 184South.

6.2 Turn left on NC 105 North.

14.4 Turn left on NC 105 North Truck Route.Felsic metavolcanic rocks of the Grand-father Mountain Formation are exposeon the left side of road.

15.4 Outcrops of Montezuma Member(metabasalt) and metaconglomerate ofthe Grandfather Mountain Formation.


16

15.7 Cross the Linville Falls fault along thenorthern border of the GrandfatherMountain window into the Beech Moun-tain thrust sheet. The first well-exposedrock on the left, north of the fault, is partof a slice of Broadstone Camp graniteincorporated into the Linville Falls shearzone.

17.3 Intersection of NC194 Truck Route withNC 194 in the town of Valle Crucis.Continue north on NC 194.

18.0 Cross Watauga River. In this area theflood plain of the Watauga River coin-cides with the location of the LongRidge fault zone.

18.7 Turn left on Mast Gap Road (NCSR1117). Outcrop on right is Valle Crucisgneiss (Adams, 1990) Cranberry Mine-layered gneiss of Bartholomew andLewis (1984); Cranberry Gneiss ofBryant and Reed (1970).

20.9 Turn left on US 321 North.

23.4 Cross Fork Ridge fault of Bartholomewand Lewis (1984). The Fork Ridge faultis a thrust within the Beech Mountainthrust sheet and separates Valle Crucisgneiss in the hanging wall from WataugaRiver Gneiss in the foot wall.

25.0 Park in store parking lot on right. Walk0.1 miles and turn right on Bull HarmonRoad. Walk about 700 feet to Stop 2-2.

STOP 2-2 LONG RIDGE FAULT

Location: Along Bull Harmon Road ~700 feetNW of intersection with US 321, Sherwood,NC-TN quadrangle. UTM coordinates:423500mE, 4012390mN


This exposure was designated as the“type locality” of the Long Ridge fault byAdams and Su (1996). Part of the fault wasmapped as the “upper branch of the StoneMountain fault” by Bryant and Reed (1970).Bryant and Reed (1970) mapped this fault fromthe Stone Mountain fault along the southeasternboundary of the Mountain City window to thesoutheast where it “dies out in the vicinity ofLong Ridge.” Adams (1990) traced the faultfarther to the southeast where it merges with theLinville Falls fault in the vicinity of ValleCrucis. Adams and Su (1996) interpreted theLong Ridge fault to be a low angle tear faultwithin the Beech Mountain thrust sheet, result-ing from differential movement of the thrustsheet during the Alleghanian orogeny. Mineral-stretching lineations and kinematic indicators

Stop 2-2US Hwy 321


17

are consistent with top-to northwest, dextralstrike-slip movement along the Long Ridge fault(Adams and Su, 1996)

The Long Ridge fault is similar to theborder faults (Linville Falls and Stone Mountainfaults) of the Beech Mountain thrust sheet inseveral respects. Associated with the LongRidge fault is a thick (up to 400 meters), mixed-rock mylonite zone termed the Long Ridgeshear zone. This shear zone contains blocks ofsheared and unsheared rock and discontinuousslices of metasedimentary rocks (predominantlymetaquartzite, meta-arkose, andmetaconglomerate of the Chilhowee Group and/or the Grandfather Mountain Formation) incor-porated into mylonite, ultramylonite, andphyllonite derived predominantly from base-ment rocks. In addition to ductile deformation,manifested by mylonite and ultramylonite; theLong Ridge shear zone shows evidence ofbrittle deformation evidenced by cataclasite,ultracataclasite, and pseudotachylyte (?)(O’Hara, 1992; Adams, 1994; Adams and Su,1996). The deformed rocks show evidence ofalternating episodes of brittle and ductile defor-mation. Adams and Su (1996) documentedevidence for overprinting episodes including asequence of ductile-brittle-ductile-brittle defor-mation. Adams and Su (1996) stated that bothstyles of deformation (brittle and ductile) oc-curred essentially synchronous under the sameregional P-T conditions.

At this stop, the outcrop shows a slice ofmetasedimentary rock incorporated into theLong Ridge shear zone. Also exposed areveinlets of pseudotachylyte (or ultracataclasite)cross cutting the basement rocks. The outcropalso locally shows ductilely deformedpseudotachylyte (?), indicating alternatingepisodes of brittle-ductile deformation. Isotopicevidence indicates that both the ultramyloniteand pseudotachylyte from this locality wereformed around 300 Ma. (Adams and Su, 1996).

References:Adams, M.G., 1994, Major- and trace-element constraints

on the petrogenesis of a fault-relatedpseudotachylyte, western Blue Ridge province, NorthCarolina - Comment: Tectonophysics, v. 233, p. 145-147.

Adams, M.G., and Su, Q., 1996, The nature and timing ofdeformation in the Beech Mountain thrust sheetbetween the Grandfather Mountain and MountainCity windows in the Blue Ridge of northwesternNorth Carolina: Journal of Geology, p. 197-213.


O’Hara, K., 1992, Major- and trace-element constraintson the petrogenesis of a fault-relatedpseudotachylyte, western Blue Ridge province, NorthCarolina. Tectonophysics, 204: 279-288.

Trupe, C.H., and Adams, M.G., 1991, Cataclastic defor-mation in the Linville Falls shear zone, western NorthCarolina Blue Ridge: Geological Society of AmericaAbstracts with Programs, v. 23, p.141.

COFFEE BREAK

Return to Buses.

Turn around and head south on US 321.

30.0 Intersection with US 421. The top of theprominent ridge directly in front (to theeast) is Ashe Metamorphic Suite of theSpruce Pine thrust sheet. The thrust fault(Fries/Gossan Lead fault) separating theSpruce Pine thrust sheet from the under-lying Beech Mountain thrust sheet isabout half way up the mountain. Turnright and head south on US 321/421. Thehighway parallels a relatively narrowstrip of the Beech Mountain thrust sheetbounded above by Fries/Gossan Leadfault and below by the Linville Fallsfault.


18

33.5 Intersection with US 321/421 TruckRoute. Continue straight on US 321/421South.

34.9 Enter Boone, North Carolina city limits.

37.1 Turn left (north) on NC 194. In this areaNC 194 crosses the Fries/Gossan Leadfault into the Spruce Pine thrust sheet.

38.7 Turn left on Howards Creek Road(NCSR 1306).

44.8 Unload from buses at open field on left(southeast). Walk 0.1 miles further toStop 2-3.

STOP 2-3 RETROGRESSED ECLOGITEON TATER HILL, ZIONVILLE, NC-TNQUADRANGLE

Location: NCSR 1306 ~ 300 feet E of intersec-tion with NCSR 1102, Southeast of Silverstone,NC, Zionville, NC-TN quadrangle. UTMcoordinates: 434620mE, 4014890mN.

Stop Leader: Richard N. Abbott

The purpose of this stop is to examinerecently discovered retrograded eclogite in theAshe Metamorphic Suite (AMS) north of theGrandfather Mountain window. The only otherarea of bona fide eclogite occurs in a similarstructural setting in the AMS, but southwest ofthe Grandfather Mountain window (Willard andAdams, 1994; Adams et al., 1995). The bestexposures of retrograded eclogite north of theGrandfather Mountain window (this stop) arealong the east side Howard Creek Rd, 50 to 100meters north of the junction with Tater Hill Rd.and the north end of Junaluska Rd (lat. N 36°16' 44.2", long. W 81° 43' 41"). While theoutcrop is clearly on a state right-of-way, pleaserespect the owners of adjacent properties.Permission to examine the outcrop should beobtained from Mr. Donald Price of Zionville.

The local distribution of retrogradedeclogite has not been mapped in detail. Bouldersof retrograded eclogite, on the south slope of thehill, immediately north of the roadside expo-sures, suggest that retrograded eclogite extendsat least 200 meters to the northeast. One otherlocation of retrograded eclogite has been discov-ered in a small outcrop approximately 2 km tothe south (lat. N 36° 15' 34", long. W 81° 43'22", along Junaluska Rd.).

The retrograded eclogites are close to thebase of the AMS, near the western edge of theSpruce Pine thrust sheet. The actual base of thethrust sheet, which is down hill to the west, isconcealed by colluvium. The location of theretrograded eclogites is consistent with the areaof highest metamorphic pressures in the AMS,as inferred from the general direction of increas-ing P (Abbott and Raymond, this volume). Theretrograded eclogites occur as thin (cm-scale),gray-green granoblastic layers in otherwisetypical garnet-hornblende schist. The essentialmineralogy consists of symplectic intergrowthsof diopside and plagioclase (representing formeromphacite), generally euhedral to subhedralgarnet (< 1mm), polygonal epidote, and quartz.Boundaries between layers of retrograded

Stop 2-3 NCSR 1306

NCSR 1102


19

eclogite and hornblende schist are gradational,showing a progressive replacement of theeclogite by hornblende schist. Plagioclaseoccurs only in the symplectite and with quartz incoronas around garnet. Adams et al. (1995) havesuggested that such coronas formed in responseto a retrograde reaction between garnet andsurrounding symplectite. Hence, at it’s highest-grade the rock was bona fide eclogite. Thedistinctive coronas are common in thesymplectite-free hornblende schist, especiallynear the base of the AMS. Interpreted as relictfeatures inherited from an earlier, high-gradecondition of the rock, the coronas suggest that asignificant volume of the hornblende schist inthe AMS is retrograded (amphibolitized)eclogite (Adams et al., 1995). The presence ofepidote distinguishes these eclogites from thosedescribed by Willard and Adams (1994), andprobably reflects a higher fugacity of O2. Addi-tional details of the petrography are given inAbbott and Raymond (this volume).

The presence of eclogite, or retrogradedequivalents, in the AMS is significant for atleast three reasons. (1) The eclogites record theearliest recognizable and highest grade meta-morphism affecting mafic rocks during theTaconic Orogeny. (Evidence for even earlier,and higher grades of metamorphism may bepreserved in ultramafic rocks of the AMS.) (2)The eclogites strongly support an ensimaticorigin for the AMS, as part of a subduction-related accretionary melange. (3) Much of theAmphibolite Facies metamorphism in thewestern part of the AMS is retrograde.

References:Adams, M.G., Stewart, K.G., Trupe, C.H., and Willard,

R.A., 1995, Tectonic significance of high-pressuremetamorphic rocks and dextral strike-slip faulting inthe southern Appalachians: in Hibbard, J., van Staal,C.R., Cawood, P. and Colman-Sadd, S., editors,


Current Perspectives in the Appalachian-Caledonianorogen, Geological Association of Canada SpecialPaper 41, p 21-42.

Willard, R.A., and Adams, M.G., 1994, Newly discoveredeclogite in the southern Appalachian orogen, north-western North Carolina: Earth and Planetary ScienceLetters, v. 123, p. 61-70.

Return to buses.

Turn around and follow Howards CreekRoad back to NC 194. Return to BannerElk via US 421 - NC 105 - NC 184.

21

ABSTRACT

The Blue Ridge thrust complex in theregion surrounding the Grandfather Mountainwindow consists of four major thrust sheets.Structurally lowest to highest these are thePardee Point, the Beech Mountain, the Pump-kin Patch, and the Spruce Pine thrust sheets.The Spruce Pine thrust sheet is especiallynoteworthy because it contains large bodies ofeclogite near its base.

All the thrust sheets were transported tothe northwest during the Alleghanian orogenyat the end of the Paleozoic. The Pumpkin Patchand the Spruce Pine thrust sheets, however,were juxtaposed prior to the Alleghanian andwere affected by the Taconic and possiblyAcadian orogenies. Published metamorphicmineral ages from amphibolite facies rocks inthe Pumpkin Patch and Spruce Pine thrustsheets and eclogite from the Spruce Pine thrustsheet are Ordovician, indicating that they wereinitially juxtaposed during the Taconic orogeny.The eclogite bodies within the Spruce Pinethrust sheet were probably generated in aTaconic subduction zone and thrust onto theLaurentian margin as relatively coherentbodies. The contact between the PumpkinPatch and Spruce Pine thrust sheets is now an

amphibolite facies dextral strike-slip shearzone. Preliminary mineral ages from rocks inthis shear zone are Devonian, suggesting thatthe original Taconic thrust in this area wasreactivated during the Acadian orogeny as astrike-slip fault.

The faults bounding the Beech Moun-tain and Pardee Point thrust sheets areAlleghanian northwest-directed thrusts. Thefault separating the Pumpkin Patch and SprucePine thrust sheets is cut by one of theseAlleghanian faults in the vicinity of the Grand-father Mountain window, indicating that thisfault was transported to the northwest alongwith the composite Blue Ridge thrust complexat the end of the Paleozoic.

INTRODUCTION

The Blue Ridge thrust complex refers tothe stack of crystalline thrust sheets west of theBrevard fault zone which structurally overliethe Linville Falls fault (Figure 1; Goldberg etal., 1989). Individual thrust sheets are distin-guished by changes in rock type, metamorphichistory, and the presence of thick mylonitic orcataclastic fault zones. The thrust sheets, fromstructurally lowest to highest, are the PardeePoint thrust sheet, the Beech Mountain thrustsheet, the Pumpkin Patch thrust sheet, and theSpruce Pine thrust sheet.

Other authors have subdivided therocks of the North Carolina Blue Ridge into

PALEOZOIC STRUCTURAL EVOLUTION OF THE BLUE RIDGE THRUST COMPLEX,WESTERN NORTH CAROLINA

Kevin G. Stewart, Mark G. Adams1 , and Charles H.Trupe2

Department of GeologyUniversity of North CarolinaChapel Hill, NC 27599-3315

________________________________________________Present addresses:1. Dept of Geology, Appalachian State Univ., Boone, NC28608 and Appalachian Resources, 205 Providence Rd.,Chapel Hill, NC 27515.2. 2619 Mt. Gilead Church Rd., Pittsboro, NC 27312.

22

different terranes or thrust sheets, using similarcriteria. Figure 1 in Raymond and Abbott (thisvolume) summarizes the classification schemesused by other authors. We have used the classi-fication of Goldberg et al. (1989) in our pastwork and will continue this practice in thispaper.

The structural history of the area out-lined in this paper is primarily based on kine-matic studies of the fault zones bounding thethrust sheets, structural geology of the rockswithin the thrust sheets, and the grade and ageof metamorphism. Although some of the thrustsheets record Precambrian deformation, meta-

morphism, and igneous activity, we are restrict-ing our analysis to Paleozoic events. Notsurprisingly, the record of early Paleozoicevents has been obscured by later Paleozoicevents and therefore our model becomes moregeneralized for early events. Our currentmodel should be considered one in a series ofsuccessive approximations which are refinedwhen new data become available. Neverthe-less, we feel that we can provide a reasonableinterpretation of the structural evolution of theBlue Ridge of western North Carolina from theOrdovician through the Pennsylvanian.

Grandfather

Mountain

window

Linville

Gossan-Lead fa

ult

Burnsville fault


Unaka M

ountain fault

Iron

Mou

ntain

fault

Holston M

ountain fault

Long

Brevard Fault z

one






0 5 10

Kilometers

NCTN

N

Boone

Banner Elk

Linville Falls

Burnsville

Bakersville

Falls fault

Ridge

fault

Area oflarge map

Figure 1. Generalized geologic map of the Blue Ridge of western North Carolina. Four thrust sheets of the Blue Ridgethrust complex are shown. All faults are Alleghanian thrusts and strike-slip faults except for the Burnsville fault, whichis Ordovician - Devonian. Modified from Adams et al. (1995).

Stewart, Adams, and Trupe

23

PALEOZOIC METAMORPHISM WITHINTHE BLUE RIDGE THRUST COMPLEX

Detailed descriptions of the metamor-phic grade of the four thrust sheets in the BlueRidge thrust complex can be found in Butler(1991) and Adams and Trupe (this volume).Ages of metamorphism of the thrust sheets canbe found in Goldberg and Dallmeyer (1997)and are summarized by Adams and Trupe (thisvolume). The following summary is based onthese studies.

ORDOVICIAN (TACONIC) AMPHIBOLITEAND ECLOGITE FACIES METAMOR-PHISM

Evidence for Ordovician amphibolitefacies metamorphism is restricted to the SprucePine and Pumpkin Patch thrust sheets. Al-though Goldberg and Dallmeyer (1997) reporta metamorphic mineral age of 451 Ma for asample within the Beech Mountain thrust sheet,this sample actually belongs to the PumpkinPatch thrust sheet (Adams and Trupe, thisvolume).

Eclogite facies metamorphism has beenrecognized in two places within the SprucePine thrust sheet: near Bakersville, NorthCarolina (Willard and Adams, 1994; Adams etal., 1995), and northwest of Boone, NorthCarolina (Abbott and Raymond, this volume).Sm-Nd mineral-whole rock isochrons fromeclogite near Bakersville give a range of agesfrom about 410-450 Ma (Adams et al., 1995).

SILURO-DEVONIAN (ACADIAN) AM-PHIBOLITE FACIES METAMORPHISMAND STRIKE-SLIP FAULTING

Goldberg and Dallmeyer (1997) reportSilurian and Devonian amphibolite faciesmetamorphism in the Pumpkin Patch andSpruce Pine thrust sheets (see also Trupe andAdams, this volume). There is no evidence of

Siluro-Devonian metamorphism in the BeechMountain or Pardee Point thrust sheet.

The Burnsville fault (Figure 1), whichseparates the Spruce Pine thrust sheet from thePumpkin Patch thrust sheet, is an amphibolitefacies dextral strike-slip shear zone (Adams etal., 1995; Burton, 1996). Goldberg andDallmeyer report a Devonian cooling age forrocks within the Burnsville fault zone.

MISSISSIPPIAN-PENNSYLVANIAN(ALLEGHANIAN) GREENSCHIST FACIESMETAMORPHISM AND THRUSTING

Evidence of significant Alleghanianmetamorphism is restricted to the BeechMountain and Pardee Point thrust sheets(Butler 1991, Adams and Trupe, this volume).The Beech Mountain thrust sheet experiencedpervasive greenschist facies metamorphism andthe Pardee Point experienced greenschist tosub-greenschist facies metamorphism.

The Alleghanian thrusts throughout theBlue Ridge thrust complex were active undergreenschist facies, or lower, conditions (Abbottand Raymond, 1984; Mies, 1990; Trupe, 1989;Adams, 1990; Schedl et al., 1992).

STRUCTURAL MODELS

ORDOVICIAN CONVERGENCE ANDCOLLISION

According to Hatcher (1989), theTaconic orogeny in the southern Appalachianswas the result of the collision between thePiedmont terrane and Laurentia. Convergencebegan in the Early Ordovician as the Piedmontadvanced over an east-dipping subduction zone(Figure 2). Raymond et al. (1989) and Adamset al. (1995) interpret the Ashe MetamorphicSuite (AMS) to be the metamorphosed accre-tionary wedge formed at the leading edge ofthe Piedmont terrane. The AMS containspelitic schist, amphibolite, and ultramafic rocks

Structural Evolution of the Blue Ridge

24

and commonly has a block-in-matrix texture,which is typical of accretionary melanges(Raymond et al., 1989).

Deeply subducted basaltic crust wasmetamorphosed to eclogite, which was eventu-ally detached and incorporated into the accre-tionary wedge. The uplift mechanism of high-pressure metamorphic rocks is an enduringproblem in structural geology. Cloos (1982)proposed that eclogite blocks in the FranciscanComplex of California were entrained in anupward-flowing mud melange. In his model,only blocks which are less than about 25meters in diameter can be uplifted. The eclogiteblocks in the Blue Ridge range in size fromcentimeter-scale blocks to continuous layers upto 200 meters thick and a kilometer long(Adams et al., 1995). The size of the largestblocks seems to prohibit uplift by Cloos’mechanism. Mechanisms which involve pairedthrust and normal faults (e.g. Platt, 1986) havebeen proposed to explain uplift of large

eclogite facies terranes, but as of this writingwe have no evidence for large-displacementnormal faults in the AMS. We do not know theextent of eclogite facies metamorphism withinthe AMS although Adams et al. (1995) sug-gested that high pressure metamorphism mayhave been widespread (see also Abbott andRaymond, this volume).

Collision of the Piedmont terrane withLaurentia occurred by the Middle Ordovician(Figure 3; Hatcher, 1989; Raymond andJohnson, 1994). The collision induced am-phibolite facies metamorphism in the under-thrust Laurentian margin and in the overlyingAMS. The amphibolite facies metamorphicfront propagated to the west as the collisionprogressed and affected rocks which wereultimately incorporated into the Pumpkin Patchthrust sheet. The eclogite bodies are locallyretrograded to amphibolite which probablyoccurred sometime during the Ordovician and

eclogite

AM

S protolith

10

20

30

40

50

60

70

80

90

100

Laur

entia

Piedmont

0 km

continental marginsediments

abyssal sediments

Early Ordovician

Figure 2. Ordovician convergence between Laurentia and Piedmont terrane. Subducted oceanic crust undergoes eclogitefacies metamorphism. Accretionary wedge is protolith for Ashe Metamorphic Suite (AMS).


25

Silurian as the eclogite was brought to shal-lower levels.

SILURO-DEVONIAN STRIKE-SLIP FAULT-ING

Following the collision of the Piedmontterrane with Laurentia, the Taconic sutureevolved into a dextral strike slip fault called theBurnsville fault (Adams et al., 1995; Figure 4).The presence of post-Taconic, dextral motionon the suture has been recognized at localitiesfrom Alabama to Newfoundland (Adams et al.,1995). This may have been a discrete event(Acadian orogeny?) which occurred tens ofmillions of years after the end of the Taconiccollision or it could have been the final phaseof a protracted event which began in the Or-dovician and ended in the Devonian. Existinggeochronologic data cannot discriminatebetween these two models (Goldberg andDallmeyer, 1997).

The Spruce Pine Plutonic Suite wasemplaced between 390 and 410 Ma (Kish,1983). Spruce Pine pegmatites intrudemylonites within the Burnsville fault zone andare also sheared, indicating that shearingoccurred sometime during the Siluro-Devo-nian. The part of the Burnsville fault zonecurrently exposed was active under amphibo-lite facies conditions (Adams et al., 1995;Burton, 1996).

The rocks of the Spruce Pine thrustsheet were initially juxtaposed against therocks of the Pumpkin Patch thrust sheet duringthe Ordovician. Movement between these twoterranes stopped by the end of the Devonian.

DEVONIAN-MISSISSIPPIAN UPLIFT ANDCOOLING

Goldberg and Dallmeyer (1997) report40Ar/39Ar muscovite ages which indicateregional cooling of the Blue Ridge thrust

Ashe Metamorphic Suite

10

20

30

40

50

60

70

80

90

100

0 km

eclogite

Piedmont

amphibolite

greenschist

Laur

entia

Taconic suture

Middle to Late Ordovician

amphibolite facies metamorphism induced in AMSand Laurentian basement

Figure 3. Taconic orogeny resulting from Middle to Late Ordovician collision between Piedmont and Laurentia.Underthrust Laurentian margin undergoes amphibolite facies metamorphism. Eclogite body is incorporated into AsheMetamorphic Suite.


26

complex during the Devonian and Mississip-pian (Figure 5). Rocks metamorphosed underamphibolite and eclogite facies during the

Ordovician through Devonian were upliftedand the amphibolite metamorphic front mayhave rotated to a steeper orientation.

AMSPiedmont

10

20

30

40

50

60

70

80

90

100

0 km

Spruce Pine pegmatites

Siluro-Devonian

amphibolite

greenschist

Burnsville fault

Figure 4. Taconic collision evolves into Siluro-Devonian dextral strike-slip shear zone (Burnsville fault). Spruce Pinepegmatites intrude fault zone, eclogite, AMS, and Laurentian basement rocks.

AMS

eclogite

Piedmont

10

20

30

40

50

60

70

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90

100

0 km

Devonian - Mississippian uplift and cooling

Burnsville fault

uplift and erosion of eclogite and relictOrdovician-Devonian amphibolite facies

metamorphic rocks

amphibolite

Figure 5. Uplift and cooling during Devonian - Mississippian time.


27

MISSISSIPPIAN-PENNSYLVANIAN(ALLEGHANIAN) THRUSTING

Thrusting during the Alleghanianprogressed from the southeast towards thenorthwest. The thrust below the compositeSpruce Pine/Pumpkin Patch thrust sheet cutsthrough the part of the Laurentian crust thathad experienced Ordovician - Devonian am-phibolite facies metamorphism (Figure 6). Ifthe thrust sheet had originally included lowergrade Laurentian rocks, they have since beenremoved by erosion.

Northwest transport of the compositeSpruce Pine/Pumpkin Patch thrust sheets wasprobably accompanied by movement of thePiedmont thrust sheet (Figure 7). This out-of-sequence thrust would have decapitated theTaconic suture and transported it to the north-west (see Rankin et al., 1991, for a similarinterpretation). In addition, the relict Ordovi-cian - Devonian amphibolite front would have

been overridden such that later thrusts wouldcut Laurentian crust which had not experiencedhigh grade Paleozoic metamorphism (Figure8).

In our model, the pervasive Alleghaniangreenschist metamorphism in the Beech Moun-tain thrust sheet was induced by emplacementof the composite Spruce Pine/Pumpkin Patchthrust sheet. The weak Alleghanian metamor-phism present in the Pardee Point thrust sheetwas induced by emplacement of the BeechMountain thrust sheet along the Linville Fallsfault. All of the thrust sheets were folded byduplexing of basement rocks beneath theGrandfather Mountain window (Figure 9;Rankin et al., 1991; Schedl et al., 1997).

10

20

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60

70

80

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100

0 km

AMSPiedmont

Burnsville fault

future trace of thrust below Pumpkin Patchand Spruce Pine thrust sheets

Mississippian

edge of Ordovician-Devonianamphibolite facies

metamorphic front in Laurentian margin

amphibolite

Figure 6. First Alleghanian thrust cuts through Taconic suture and into Ordovician - Devonian amphibolite grade rocksin the Laurentian basement.


28

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0 km

Taconic suture below InnerPiedmont thrust sheet

AMS Piedmont

Burnsville fault

Spruce Pine thrust sheetPumpkin Patch thrust sheet

future trace of Linville Fallsfault below Beech Mountain

thrust sheet

greenschist facies metamorphisminduced in Beech Mountain

thrust sheet

Mississippian - Pennsylvanian

Ordovician-Devonian amphibolitefacies metamorphic front

overridden by later thrusts

amphibolite

10

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0 km

AMS Piedmont

Burnsville fault

Spruce Pine thrust sheetPumpkin Patch thrust sheet

future trace of Linville Falls fault

future trace of thrust belowPardee Point thrust sheet

future trace of thrusts belowGrandfather Mountain window



Figure 7. Composite Spruce Pine/Pumpkin Patch thrust sheet induces greenschist facies metamorphism in adjacentBeech Mount thrust sheet rocks. Decapitated Taconic suture in footwall is beneath Piedmont thrust sheet.

Figure 8. Later Alleghanian thrusts propagate into Laurentian basement which has not been affected by Ordovi-cian - Devonian events.


29

SUMMARY

The four thrust sheets of the Blue Ridgethrust complex contain a fairly complete recordof at least two and possibly three distinctorogenic episodes. The structurally highestSpruce Pine thrust sheet contains eclogite andamphibolite facies metamorphic rocks associ-ated with the Ordovician Taconic orogeny.This event sutured the Spruce Pine thrust sheetto the Laurentian rocks of the Pumpkin Patchthrust sheet and induced amphibolite faciesmetamorphism within the Pumpkin Patchsheet. Siluro-Devonian transpression, dueeither to a separate event (Acadian orogeny?),

or a prolonged transpressional collision initi-ated during the Taconic orogeny, is recorded bythe dextral strike-slip Burnsville fault. Devo-nian amphibolite facies rocks in the SprucePine and Pumpkin Patch sheets are a record ofthis event.

Northwest thrusting during theAlleghanian orogeny transported the compositeSpruce Pine/Pumpkin Patch thrust sheet overthe Laurentian rocks of the Beech Mountainthrust sheet. This induced pervasivegreenschist facies metamorphism within theBeech Mountain sheet, which in turn was thrustover the adjacent Pardee Point thrust sheetalong the Linville Falls fault. The lack of

10

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90

100

0 km

AMS

Piedmont

Burnsville fault



Linville Falls fault




eclogiteB

revard fault

future location ofGrandfather Mountain

window

Figure 9. Duplexing beneath Grandfather Mountain window domes thrust sheets. Later erosion creates GrandfatherMountain window.


30

significant Paleozoic metamorphism in thePardee Point thrust sheet suggests that thethickness of the thrust stack above the PardeePoint sheet was significantly less than abovethe Beech Mountain sheet.

ACKNOWLEDGMENTS

Work presented in this study waspartially supported by NSF grant EAR9316033. Many of the ideas in this paper arosefrom discussions with J.R. Butler, R.A.Willard, F.H. Burton, and S.A. Goldberg.Additional financial assistance to CHT andMGA came from the University of NorthCarolina at Chapel Hill Department of Geol-ogy, Sigma Xi, the Geological Society ofAmerica, and the North Carolina GeologicalSurvey.

REFERENCES

Abbott, R.N., Jr., and Raymond, L.A., 1984, The AsheMetamorphic Suite, northwest North Carolina:metamorphism and observations on geologic history:American Journal of Science, v. 284, p. 350-375.

Adams, M.G., 1990, The geology of the Valle Crucis area,northwestern North Carolina: Masters Thesis,University of North Carolina at Chapel Hill, 95 p.

Adams, M.G., Stewart, K.G., Trupe, C.H., and Willard,R.A., 1995, Tectonic significance of high-pressuremetamorphic rocks and dextral strike-slip faulting inthe southern Appalachians: in Hibbard, J., van Staal,C.R., Cawood, P. and Colman-Sadd, S., editors,Current Perspectives in the Appalachian-Caledonianorogen, Geological Association of Canada SpecialPaper 41, p. 21-42.

Burton, F.H., 1996, Kinematic study of the Taconicsuture, west-central North Carolina: Masters Thesis,University of North Carolina at Chapel Hill, 114 p.

Butler, J.R., 1991, Metamorphism, in Horton, J.W.,Jr.,and Zullo, V.A., editors, The geology of the Caroli-nas: Carolina Geological Society Fiftieth Anniver-sary Volume: Knoxville, The University of Tennes-see Press, Carolina Geological Society, p. 127-141.

Cloos, M., 1982, Flow melangés: Numerical modellingand geologic constraints on their origin in theFranciscan subduction complex: Geological Societyof America Bulletin, v. 93, p. 330-345.

Goldberg, S.A., Butler, J.R., Mies, J.W., and Trupe, C.H.,1989, The southern Appalachian orogen in northwest-ern North Carolina and adjacent states: InternationalGeological Congress Field Trip T365 Guidebook,American Geophysical Union, Washington, D.C., 55p.


Hatcher, R. D., Jr., 1989, Tectonic synthesis of the U. S.Appalachians, p. 511-535, in Hatcher, R. D., Jr.,Thomas, W. A., and Viele, G. W., eds., The Appala-chian-Ouachita orogen in the United States: Boulder,Colorado, Geological Society of America, thegeology of North America, v. F-2, 767 p.

Kish, S.A., 1983, A geochronological study of deforma-tion and metamorphism in the Blue Ridge andPiedmont of the Carolinas: Ph. D dissertation,University of North Carolina at Chapel Hill, 220 p.

Mies, J.W., 1990, Structural and petrologic studies ofmylonite at the Grenville basement - Ashe Formationboundary, Grayson County, Virginia to MitchellCounty, North Carolina, Ph.D. dissertation, Univer-sity of North Carolina at Chapel Hill, 330 p.

Platt, J.P., 1986, Dynamics of orogenic wedges and theuplift of high-pressure metamorphic rocks: Geologi-cal Society of America Bulletin, v. 97, p. 1037-1053.

Rankin, D.W., Dillon, W.P., Black, D.F.B., Boyer, S.E.,Daniels, D.L., Goldsmith, R., Grow, J.A., Horton,J.W.,Jr., Hutchinson, D.R., Klitgord, K.D.,McKowell, R.C., Miltop, D.J., Owens, J.P., Phillips,J.D., with contributions by Bayer, K.C., Butler, J.R.,Elliott, D.W., and Milici, R.C., 1991, CentennialContinent/Ocean Transect #16, E-4, Central Ken-tucky to the Carolina Trough, The Geological Societyof America, 41 p.

Raymond, L.A., and Johnson, P.A., 1994, The Mars HillTerrane: An enigmatic southern Appalachian terrane:Geological Society of America Abstracts withPrograms, v. 26, n. 4, p. 59.

Raymond, L.A., Yurkovich, S.P., and McKinney, M.,1989, Block-in-matrix structures in the NorthCarolina Blue Ridge belt and their significance forthe tectonic history of the southern Appalachianorogen, in Horton, J.W., Jr., and Rast, N., editors,Melangés and Olistostromes of the U.S. Appala-chians: Geological Society of America Special Paper228, p. 195-215.

Schedl, A., Fullagar, P.D., and Valley, J.W., 1997, Implica-tions of geochemical and gravity data for the natureof strata beneath the Blue Ridge - Piedmont thrust


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sheet, USA: Earth and Planetary Science Letters, v.146, p. 165-179.

Schedl, A., McCabe, C., Montanez, I., Fullagar, P.D., andValley, J., 1992, Alleghenian regional diagenesis: Aresponse to the migration of modified metamorphicfluids derived from beneath the Blue Ridge-Piedmontthrust sheet: Journal of Geology, v. 100, p. 339-352.

Trupe, C.H., 1989, Microstructural analysis of mylonitesfrom the Blue Ridge thrust complex, northwesternNorth Carolina: M.S. Thesis, University of NorthCarolina at Chapel Hill, 101 p.



33

ABSTRACT

The Blue Ridge thrust complex in thevicinity of the Grandfather Mountain windowconsists of at least four thrust sheets with differ-ent metamorphic and deformational histories.The lower three thrust sheets contain Grenville-age amphibolite to granulite grade metamorphicrocks that were variably affected by subsequentPaleozoic tectonothermal events. In the lowestsheet, the Pardee Point thrust sheet, Paleozoicmetamorphism was no higher than chloritegrade, and Grenville metamorphic assemblagesare not significantly retrograded. Grenvillebasement and Neoproterozoic intrusive rocks ofthe Beech Mountain thrust sheet were perva-sively deformed during greenschist gradeAlleghanian faulting, but evidence for earlierPaleozoic metamorphism is equivocal. ThePumpkin Patch thrust sheet contains Grenvillebasement gneisses and NeoproterozoicBakersville intrusive rocks that were affected byamphibolite facies Paleozoic metamorphism.The uppermost tectonic unit, the Spruce Pinethrust sheet, consists of metasedimentary andmetavolcanic rocks of the Ashe and AlligatorBack Metamorphic Suites. The occurrence ofeclogite at the base of the Spruce Pine thrustsheet documents early high-pressure metamor-phism. Pelitic schists and amphibolites of theAshe Metamorphic Suite record kyanite grade

peak regional metamorphism followed bycooling and decompression along a clockwiseretrograde P-T path. Both Ordovician andSiluro-Devonian radiometric ages are recordedin metamorphic rocks from the Spruce Pine andPumpkin Patch thrust sheets. Dextral strike-slipfaulting along the Spruce Pine-Pumpkin Patchcontact occurred under amphibolite-gradeconditions, and was broadly contemporaneouswith intrusion of the Spruce Pine pegmatites(~390-400 Ma). Late Paleozoic greenschist-grade deformation records assembly and em-placement of the Blue Ridge thrust complexalong top-to-northwest thrust faults.

INTRODUCTION

The Blue Ridge thrust complex west andnorthwest of the Grandfather Mountain window(Figure 1) consists of a series of crystallinethrust sheets bounded by shear zones up to akilometer thick (Goldberg et al., 1986a, 1989;Butler et al., 1987). The complex forms thehanging wall of the Linville Falls fault, whichframes the Grandfather Mountain window, andis bounded on the west by the Stone Mountain-Iron Mountain-Holston Mountain fault systemand on the east by the Brevard fault zone.Component thrust sheets display differences inP-T and deformation histories, and generallyshow a decrease in Paleozoic regional metamor-phic grade with increasing structural depthwithin the complex (Goldberg et al., 1989). TheBlue Ridge thrust complex thus preserves aninverted metamorphic gradient due to combinedstructural and metamorphic processes.

CONDITIONS AND TIMING OF METAMORPHISM IN THE BLUE RIDGE THRUSTCOMPLEX, NORTHWESTERN NORTH CAROLINA AND EASTERN TENNESSEE

Mark G. Adams1 and Charles H. Trupe2

Department of GeologyUniversity of North CarolinaChapel Hill, NC 27599-3315

Present addresses:1 Appalachian Resources, 205 Providence Rd., ChapelHill, NC, 28605 and Dept. of Geology, Appalachian StateUniv., Boone, NC 286082 2619 Mt. Gilead Church Rd., Pittsboro, NC, 27312

34

The structurally highest sheet, theSpruce Pine thrust sheet (Toe terrane ofRaymond et al., 1989; Jefferson terrane ofHorton et al., 1989) contains high-grademetavolcanic and metasedimentary rocks (Asheand Alligator Back Metamorphic Suites;Rankin, 1970; Abbott and Raymond, 1984;Raymond et al., 1989), Paleozoic graniticintrusive rocks, and ultramafic bodies, and lacksrecognizable Grenville-age basement rocks.The three thrust sheets structurally below theSpruce Pine thrust sheet are characterized by thepresence of Grenville basement rocks,Bakersville Suite mafic dikes and gabbro bod-ies, and Crossnore and Beech Suite peralkalinegranitic plutons. The Pumpkin Patch thrust

sheet (Cullowhee terrane of Raymond et al.,1989; Mars Hill terrane of Bartholomew andLewis, 1992), occurs immediately below theSpruce Pine thrust sheet, bounded by theBurnsville fault (Adams et al., 1995a). Beneaththe Pumpkin Patch thrust sheet is the BeechMountain thrust sheet (Sherwood terrane ofRaymond et al., 1989), which contains theNeoproterozoic Crossnore-Beech intrusive suite.The lowermost thrust sheet is the Pardee Pointthrust sheet, in which Cambrian ChilhoweeGroup sedimentary rocks unconformably overlieGrenville basement rocks.

The Blue Ridge thrust complex recordsmultiple tectonothermal events. Basementthrust sheets were affected by Grenville-age

Figure 1. Generalized tectonic map of the Blue Ridge thrust complex of northwestern North Carolina and easternTennessee. Modified after Adams et al. (1995a).

Grandfather

Mountain

window

Linville

Gossan-Lead fa

ult

Burnsville fault


Unaka M

ountain fault

Iron

Mou

ntain

fault

Holston M

ountain fault

Long

Brevard Fault z

one






0 5 10

Kilometers

NCTN

N

Boone

Banner Elk

Linville Falls

Burnsville

Bakersville

Falls fault

Ridge

fault

Area oflarge map

Adams and Trupe

35

(1000-1200 Ma) metamorphism and deforma-tion. Most regional syntheses of the Paleozoictectonic evolution of the southern Appalachiansbegin with an Ordovician tectonothermal eventcorrelated with the Taconic orogeny, followedby Acadian metamorphism and deformation,and major thrusting during the Alleghanianorogeny (e.g. Butler, 1973; Hatcher, 1987;Rankin et al., 1989). In the following discus-sion, age ranges used for these tectonothermalevents are as follows: Taconic, 510-460 Ma;Acadian, 410-360 Ma; and, Alleghanian, 325-265 Ma (Butler, 1991).

SPRUCE PINE THRUST SHEET

The Spruce Pine thrust sheet is theuppermost unit in the Blue Ridge thrust com-plex, and contains rocks of the NeoproterozoicAshe and Alligator Back Metamorphic Suites.Bryant and Reed (1970) and Abbott andRaymond (1984) postulated that the contactbetween the Ashe Metamorphic Suite (AMS)and underlying Cranberry Gneiss is a fault. Thecontact between the Spruce Pine and PumpkinPatch thrust sheets was defined as a northwest-directed, post-metamorphic thrust fault(Goldberg et al., 1986a; Butler et al., 1987), andsubsequent studies have verified these relation-ships adjacent to and northeast of the Grandfa-ther Mountain window (Mies, 1988; 1990;Mies and Trupe, 1989). Along strike to thesouthwest, the contact between the AMS andbasement is a syn- to post-peak-metamorphicdextral strike-slip fault that predates the thrustfault adjacent to the window (Trupe et al., 1991;Willard et al., 1991; Mallard et al., 1994;Willard and Adams, 1994; Adams et al.,1995a). Adams et al. (1995a) named this faultthe Burnsville fault. Field relations indicate thatthe strike slip faulting is overprinted in thevicinity of the Grandfather Mountain windowby Alleghanian shearing and thrust faulting(Adams et al., 1995a). Farther to the southwest,other workers have stated that the presumably

equivalent contact is a pre-metamorphic thrustfault, correlative with the Hayesville fault(Hatcher, 1987; Merschat and Wiener, 1988;Eckert et al., 1989; Hatcher and Goldberg,1991).

The Spruce Pine thrust sheet consistspredominantly of amphibolite faciesmetasedimentary rocks including metapeliticschist, metasandstone, and mica gneiss. Alsopresent are abundant amphibolite, minor dunite,altered ultramafic rocks, and Siluro-Devoniangranitoid intrusions of the Spruce Pine PlutonicSuite (Rankin et al., 1973). Eclogite occurs atthe base of the thrust sheet in the Bakersville,North Carolina area (Willard and Adams, 1994;Adams et al., 1995a). The association ofmetasedimentary rocks, amphibolite, and ultra-mafic rocks has been interpreted by severalworkers to represent a combination of metamor-phosed oceanic crust and sediments formed eastof the Laurentian margin, possibly as an accre-tionary mélange containing fragments of dis-membered ophiolite (Hatcher, 1978; Abbott andRaymond, 1984; Horton et al., 1989; Raymondet al., 1989; Willard and Adams, 1994; Adamset al., 1995a). Geochemical analyses of maficand ultramafic rocks in the eastern Blue Ridgegenerally support this interpretation (Hatcher etal., 1984; Misra and Conte, 1991).

GRADE AND TIMING OF METAMOR-PHISM

The Spruce Pine thrust sheet preservesevidence of several metamorphic events.Eclogite facies rocks occur near the base of thesheet, suggesting an early episode of high-pressure metamorphism. The metasedimentaryand metavolcanic rocks of the Ashe Metamor-phic Suite (AMS) contain amphibolite faciesassemblages (kyanite grade). These rocks weresubsequently affected by shearing in theBurnsville fault that deformed, but did notsignificantly retrograde, the amphibolite faciesassemblages. To the northeast, near the Grand-

Conditions and Timing of Metamorphism

36

father Mountain window, rocks of the AMSwere overprinted by greenschist facies retro-gression associated with northwest-directedthrust faulting.

Peak metamorphic assemblages in theAMS indicate amphibolite facies conditions ofmetamorphism. Published P-T estimates areconsistent with conditions demonstrated bymineral assemblages (Table 1). Abbott andRaymond (1984) described evidence for mul-tiple metamorphic events in the AMS rocksnorth of the Grandfather Mountain window, andestimated peak conditions of 7-10 kbar and 700°C. McSween et al. (1989) presented estimatesfor metamorphic temperatures and pressures ofin the AMS of 600-650 °C and 7.5 kbar.Satterfield (1992) reported estimates of 650 °C

at a minimum pressure of 7.5 kbar for peakmetamorphic conditions in AMS rocks north ofthe Grandfather Mountain window. Althoughthese workers interpreted metamorphic condi-tions to represent Taconic metamorphism, noneof these studies presented geochronologicconstraints for the time of regional metamor-phism.

Goldberg et al. (1992a, 1992b), Adamset al. (1995a), and Trupe (1997) reported P-Testimates for staurolite-kyanite-garnet micaschists and garnet amphibolites of the AMS.The assemblage in the schists constrains P-Testimates to the stability field ofkyanite+staurolite+garnet+biotite+muscovite(Goldberg et al., 1992b). Fibrolite and chlorite

a Abbott and Raymond, 1984; b McSween et al., 1989; c Goldberg et al., 1992a; d Satterfield, 1992; e Adams etal., 1995a; f Trupe, 1997; g Willard and Adams, 1994; h Adams and Stewart, 1993; i Monrad and Gulley, 1983 ; j

Gulley, 1985; k Adams et al., 1995b.* Grenville metamorphism; ** Paleozoic metamorphism

Adams and Trupe

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that occur along grain boundaries of majorminerals in the schists are interpreted to be latephases that grew after the peak of metamor-phism. These authors interpreted these observa-tions to indicate that final equilibration occurrednear the kyanite-sillimanite phase boundary,indicating decompression and cooling. P-Testimates utilizing several geothermobarometersfor core-rim assemblages are consistent withpetrographic observations (Figure 2). Estimatesfor peak P-T conditions in metapelitic schistsare approximately 7-9 kbar and 640-700 °C.Estimates from mineral rim compositions are 5-6.5 kbar and 550-610 °C. P-T estimates forgarnet amphibolites are 8.5-11 kbar and 675-705°C.

Eclogite bodies occur at the base of theAMS within a sheared metapelitic matrix alongthe Burnsville fault near Bakersville, NorthCarolina. The peak eclogite assemblage con-sists of garnet, omphacite, quartz and rutile.Retrograde metamorphism variably affected theeclogite; the most retrogressed rocks have beenaltered to amphibolite. Willard and Adams(1994) reported minimum pressures of 13-17kbar at 625-790 °C for the eclogite faciesassemblage, and retrograde amphibolite faciesconditions of 8.5-12 kbar and 650-730 °C.Adjacent sheared metapelitic rocks of the AMSrecord P-T estimates of 6-8 kbar and 580-640°C for peak assemblages, and 5.5-6.5 kbar at565-610 °C for final equilibration (Adams andStewart, 1993; Willard and Adams, 1994).

Published isotopic ages for AMS rockssuggest that they experienced both Taconic andAcadian metamorphism (Table 2). Goldberg etal. (1993) reported Sm-Nd ages of 456-462 Mafrom hornblende and epidote in the AMS. Rb-Sr mineral isochrons from amphibolites andmetapelites yield dates of 395-419 Ma(Goldberg et al., 1992a; 1993). Goldberg andDallmeyer (1997) reported ages for metamor-phic minerals of the Spruce Pine and PumpkinPatch thrust sheets. Garnet and hornblende ages(Rb-Sr, Sm-Nd, and 40Ar/39Ar) yield bothOrdovician and Devonian ages. These authorsinterpreted the two groups of ages to reflectdiscrete Taconic (Ordovician) and Acadian(Siluro-Devonian) metamorphic events.

Eclogite facies rocks from theBakersville area give Silurian to Ordovicianages. Sm-Nd garnet - whole-rock ages from theeclogites range from approximately 410 to 450Ma, with older ages recorded by abraded garnetcores (Adams et al., 1995b). Eclogite mineralseparates of garnet and clinopyroxene yield aSm-Nd mineral isochron age of 410±6 Ma (2σ)(Goldberg et al., 1994).

The Spruce Pine Plutonic Suite (Rankinet al., 1973) consists of pegmatites and alaskitebodies that intrude the AMS. Butler (1973)

Figure 2. Petrogenetic grid showing P-T estimatesfor eclogite and associated rocks in the AMS(modified after Willard and Adams, 1994 andAdams et al., 1995a). P-T estimates suggest that therocks record a continuous retrograde P-T path.Aluminum silicate phase boundaries afterHoldaway (1971). Omphacite + quartz = plagio-clase reaction boundary calculated from microprobeanalyses of omphacite (Jd30) and plagioclase(An30) using the Holland (1980, 1983) jadeite-albite-quartz barometer. And=andalusite;Ky=kyanite; Omp=omphacite; Plag=plagioclase;Qtz=quartz; Sil=sillimanite.

Eclogite facies metamorphism(minimum P)

Schist adjacent to eclogite("peak" conditions)

Retrograde eclogite and AMSamphibolite

AMS schist ("peak" conditions)

AMS schist (final equilibration)

P-T Estimates

10

20

5

15

25

And

Ky Sil

200 400 600 800 1000

Omp + Qtz

Plag

Pre

ssur

e (k

bar)

Temperature (°C)

Possible P-T path


38

stated that these intrusions were closely relatedin time to amphibolite-grade metamorphism,based on their occurrence almost entirely in thekyanite and higher metamorphic zones, and theabsence of contact metamorphic aureoles inAMS rocks. The pegmatites show both concor-dant and discordant relationships relative to themain metamorphic foliation. Rb-Sr whole-rockages for the pegmatites fall in the range of 390-410 Ma (Kish, 1983; see summary by Butler,

1991). These dates therefore represent a mini-mum age for high-grade metamorphism of theAMS.

The Spruce Pine pegmatites also placeconstraints on the timing of deformation at thebase of the Spruce Pine thrust sheet. Along theBurnsville fault, pegmatites intrude myloniticrocks, and are themselves sheared, indicatingthat dextral strike-slip faulting was contempora-neous with intrusion of the pegmatites (Adams,

a - Goldberg and Dallmeyer (1997); b - Goldberg, Dallmeyer, and Su (1993); c - Lesure (1968); d - Kish(1983); e - summary by Butler (1991); f - Adams et al. (1995b); g .- Goldberg et al. (1994); h - Dallmeyer(1975); i - Monrad and Gulley (1983).

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1995; Adams et al., 1995a). Additionally, peakmetamorphic minerals are deformed, but notretrograded, in these mylonites. Near theGrandfather Mountain window, AMS rocks areretrograded and Spruce Pine pegmatites aredeformed by greenschist-grade northwest-directed thrust faulting (Goldberg et al., 1986a;Mies, 1987). These observations suggest thatthe Burnsville fault is Acadian in age, and thrustfaulting near the Grandfather Mountain windowoccurred later, presumably during Alleghanianthrusting.

PUMPKIN PATCH THRUST SHEET

Southwest of the Grandfather Mountainwindow, the Pumpkin Patch thrust sheet under-lies the Spruce Pine thrust sheet and overlies theBeech Mountain thrust sheet (Figure 1). TheSpruce Pine sheet-Pumpkin Patch sheet contactis represented by the Burnsville fault. ThePumpkin Patch sheet-Beech Mountain sheetcontact is represented by an unnamed thrustfault. Northeast of the Grandfather Mountainwindow, the Pumpkin Patch thrust sheet isapparently absent. The lithology of the PumpkinPatch thrust sheet is dominated byMesoproterozoic gneisses and NeoproterozoicBakersville intrusive rocks.


The rocks of the Pumpkin Patch thrustsheet show evidence of multiple metamorphicevents. The Mesoproterozoic basement rockshave a gneissic layering containing amphiboliteto granulite facies assemblages (Gulley, 1985;Adams et al., 1995a). Additionally, these base-ment gneisses are migmatitic, indicating tem-peratures sufficiently high for anatexis. Thegneissic layering in these basement rocksexhibits refolded folds, indicating multipledeformation. This gneissic layering is cross-cutby metadiabase dikes of the Bakersville Intru-

sive Suite. Thus, cross-cutting relationships ofthe Bakersville dikes indicate that the basementgneisses were metamorphosed prior to theintrusion of the dikes. The fact that the dikes arealso metamorphosed provides evidence ofmultiple metamorphic events in the PumpkinPatch thrust sheet. Evidence of retrograde,greenschist facies metamorphism is rare in thePumpkin Patch sheet.

Monrad and Gulley (1983) and Gulley(1985) documented granulite facies conditions(6.5 - 8 kbar at 750 - 847°C) for the Cloudlandand Carvers Gap gneisses in the Pumpkin Patchthrust sheet. These authors also reported Meso-to Neoproterozoic, Rb-Sr whole rock ages forthese gneisses (ca. 1.8 Ga for the Carvers Gapgneiss; ca. 800 Ma for the Cloudland gneiss).These authors suggested that the 1.8 Ga agedemonstrates the existence of continental crustprior to this date and the 800 Ma age representslimited isotopic equilibration of granulite faciesassemblages during the waning stages of theGrenville orogeny. Monrad and Gulley (1983)also reported P-T estimates for a subsequentamphibolite facies metamorphic event (10 - 12kbar at 680 - 760°C). They interpreted theseconditions to record metamorphism during theTaconic orogeny.

Adams et al. (1995b) reported P-Testimates for the Meadlock Mountain gneiss.Microprobe analyses of cores from the peakassemblage yield estimates of ~ 13 kbar at 725°C, and rims give estimates of ~9 kbar at 660° C(Figure 3). These authors interpreted theseestimates to represent “peak” and final equili-bration conditions of metamorphism during aPaleozoic orogeny (either the OrdovicianTaconic or Siluro-Devonian Acadian orogeny).

Goldberg and Dallmeyer (1997) docu-mented radiometric ages for metamorphicminerals from Bakersville metagabbro in thePumpkin Patch thrust sheet. They interpretedthe Sm-Nd and most of the Rb-Sr isochrons torecord Ordovician metamorphism. These au-thors also reported one Rb-Sr hornblende age


40

and several 40Ar/39Ar hornblende ages thatrecord Devonian dates. These authors suggestthat the data indicate multiple metamorphicevents during the Paleozoic, but temperaturesduring the Devonian metamorphism may havebeen below the closure temperature for Nd inhornblende.

BEECH MOUNTAIN THRUST SHEET

The Beech Mountain thrust sheet di-rectly overlies the Grandfather Mountain win-dow and is bounded on the southeast by theLinville Falls fault and on the northwest by theStone Mountain shear zone (Figure 1). Thelithology of the thrust sheet is dominated byMesoproterozoic gneisses, Neoproterozoicgranitic plutons, and minor Neoproterozoicmafic dikes. The Mesoproterozoic gneisses

include rocks that were metamorphosed orcrystallized during the Grenville orogeny be-tween approximately 1177 to 950 Ma. (Fullagarand Bartholomew, 1983). The Neoproterozoicgranitic plutons include those of the Crossnore-Beech intrusive suite with crystallization ages ofapproximately 741 Ma. (Su et al., 1994). TheNeoproterozoic mafic rocks include those of theBakersville intrusive suite dated at approxi-mately 734 Ma. (Goldberg et al., 1986b).


Locally the rocks of the Beech Mountainthrust sheet show evidence of multiple meta-morphic events. The Mesoproterozoic, Grenvilleage gneisses contain layers of garnet amphibo-lite, suggesting early metamorphic conditions ofamphibolite facies. Additionally, migmatites areubiquitous in the Cranberry Mine layered gneiss(Bartholomew and Lewis, 1984) or the ValleCrucis gneiss (Adams, 1990), indicating thermalconditions at least high enough to induce partialmelting. Virtually all of the rocks in the Beechsheet show a late overprint of greenschist faciesmetamorphism evidenced predominantly bypetrographic observations (greenschist faciesequilibrium mineral assemblages) and operativedeformation mechanisms in deformed rocks(Trupe, 1989; Adams, 1990; Adams and Su,1996). Table 3 summarizes P-T estimates forrocks from the Beech Mountain thrust sheet.

Timing of metamorphism is constrainedby cross-cutting relations and documentedradiometric isotope dates. Crossnore-Beechgranitic rocks and mafic dikes of the Bakersvillesuite intrude and cross cut gneissic layering inGrenville basement rocks. These intrusive rocksdo not show evidence of the same high grademetamorphism that affected the basement rocks.Thus, the amphibolite facies metamorphism thataffected the basement rocks must be older thanthe 741 to 734 Ma rocks of the Crossnore-Beechand Bakersville intrusive suites. Additionally,

Figure 3. Petrogenetic grid showing P-T estimatesof Adams et al. (1995b) for the Meadlock Mountaingneiss. Aluminum silicate phase boundaries afterHoldaway (1971). P-T estimates are similar tothose for amphibolite and retrogressed eclogite ofthe AMS, and the lower range of P-T estimates foreclogite as shown in Figure 2.

P-T Estimates

10

20

5

15

25

And

Ky Sil

200 400 600 800 1000

Pre

ssur

e (k

bar)

Temperature (°C)

Meadlock Mtn. gneiss (rims)

Meadlock Mtn. gneiss (cores)

Adams and Trupe

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Dallmeyer (1975), Fullagar and Bartholomew(1983) and Goldberg and Dallmeyer (1997)interpreted isotopic dates to represent metamor-phic ages for basement rocks in the Beech sheetat approximately 1.0 Ga.

Timing of the greenschist facies meta-morphism in the Beech sheet is constrained byfield relations and radiometric dates. Fieldrelations suggest that the pervasive metamor-phic foliation in the Beech sheet is related toshearing along fault zones within and borderingthe thrust sheet (Adams and Su, 1996).Mylonites within the Linville Falls shear zone,Long Ridge shear zone, and other minor shearzones within the Beech sheet record isotopicages around 300 Ma (Van Camp and Fullagar,1982; Schedl et al., 1992; Adams and Su, 1996).Goldberg and Dallmeyer (1997) report isotopicages for mylonites from the shear zone along thePumpkin Patch-Beech sheet contact at around

325 Ma. These data indicate that the PumpkinPatch and Beech sheets were probably juxta-posed during early stages of the Alleghanianorogeny and the composite Blue Ridge thrustcomplex was transported along the LinvilleFalls fault during later stages of the Alleghanian.

Evidence for earlier Paleozoic metamor-phism in the Beech sheet is somewhat ambigu-ous. Goldberg and Dallmeyer (1997) reported aRb-Sr hornblende age of 451 Ma and 40Ar/39Arage of 383 for a sample from the Beech sheet(their Sample 1). Geologic mapping (Adams,unpublished data) demonstrates that theirSample 1 is well within the Pumpkin Patchsheet (which is more consistent with the rest oftheir data). Dallmeyer (1975) also reported abiotite 40Ar/39Ar age of 518 Ma from a rockthat may reside within the Beech sheet; how-ever, analysis of hornblende from the same rockyields an age of 893 Ma. The 518 Ma biotite age

1. Evaluation of operative deformation mechanisms. 2. Applying average geothermal gradient and average lithostaticpressure. 3. Phengite geobarometer. 4. Two feldspar thermometry. 5. Regionally restored cross sections. 6. Cation-exchange thermometry. 7. Petrographic observations (equilibrium mineral assemblage). 8. Fluid inclusions. 9. Biotite-muscovite-chlorite barometry.a. Adams, 1990. b. Trupe, 1989. c. Goldberg and Dallmeyer, 1997. d. Schedl et al., 1992. e. Wojtal and Mitra, 1988.f. McCrary, 1997. g. Butler, 1991 and personal com.*Mesoproterozoic conditions. **Paleozoic conditions.


42

is considered suspect for several reasons: 1)because of the discrepancy between the biotiteand hornblende ages; 2) the 518 Ma biotite agedoes not appear to be related to any well-docu-mented thermal event in the Blue Ridge: 3) Kish(1989) warns that biotite from the western BlueRidge may contain excess 40Ar and, thus, maybe unreliable. Existing, unequivocal data sug-gest that the only Paleozoic metamorphic eventthat affected the rocks of the Beech sheet wasthe Alleghanian orogeny.

PARDEE POINT THRUST SHEET

The Pardee Point thrust sheet underliesthe Beech Mountain thrust sheet along theUnaka Mountain-Stone Mountain fault system(Figure 1). As illustrated by King and Ferguson(1960), the Pardee Point thrust sheet is includedin the Mountain City window. Crystalline rocksin this sheet include Mesoproterozoic basementgneiss, Neoproterozoic Bakersville IntrusiveSuite, and minor pegmatites. Nonconformablyoverlying the crystalline rocks are basal clasticrocks of the Chilhowee Group and other Paleo-zoic sedimentary rocks (Shady and RomeFormations).


Quantitative estimates of metamorphicconditions for rocks in the Pardee Point thrustsheet are sparse (Table 3). However, the crystal-line rocks of the Pardee Point thrust sheetcontain garnet amphibolite within the gneissiclayering. Additionally, migmatization is promi-nent throughout these gneisses. These observa-tions indicate that metamorphic conditionsreached at least amphibolite facies and thattemperature conditions were sufficient to induceanatexis. McCrary (1997) reported garnet-biotite temperatures for Pardee Point gneiss of675-775 °C, interpreted as cooling temperaturesfollowing high-grade (granulite) Grenville

metamorphism. Butler (1991) noted that thinsections indicate that the amphibolite faciesminerals in the gneisses do not show significantretrogression by greenschist facies metamor-phism.

Neoproterozoic Bakersville mafic rockscross-cut the gneissic layering in the rocks ofthe Pardee Point sheet, indicating that themetamorphism and formation of the layeringpredates the intrusion of the 734 Ma Bakersvillerocks. Virtually all of the documented radiomet-ric ages from minerals from the gneisses arePrecambrian (Long et al., 1959; Dallmeyer,1975; McCrary, 1997), suggesting that therocks of the Pardee Point sheet were not af-fected by Paleozoic metamorphism, or at leasttemperatures were not high enough to reset theisotope systematics in those minerals.

Locally, basal deposits of the Chilhoweegroup (Neoproterozoic ? or Lower Cambrian)rest unconformably on the crystalline basementrocks of the Pardee Point thrust sheet. Higher inthe stratigraphic sequence, conglomerates abovethe Middle Ordovician unconformity in theValley and Ridge province contain clasts ofearlier Paleozoic sedimentary rocks (Kellbergand Grant, 1958), providing evidence for upliftand erosion during the Taconic orogeny. Upliftand erosion of rocks correlative with those inthe Pardee Point sheet suggest that this thrustsheet was not deeply buried during the Taconicorogeny and, thus, the lack of evidence forOrdovician-age metamorphism is not surprising.

The Pardee Point thrust sheet was buriedby overthrusting of the Beech sheet during theAlleghanian orogeny. As the rocks of theChilhowee Group are anchimetamorphic (But-ler, 1991), metamorphic conditions resultingfrom this burial probably was no greater thanchlorite-grade. The data indicate that the PardeePoint thrust sheet was subjected to only verylow grade metamorphism during the latest(Alleghanian) Paleozoic orogenic event.

Adams and Trupe

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SUMMARY

The earliest metamorphism recorded bybasement thrust sheets of the Blue Ridge thrustcomplex resulted from the MesoproterozoicGrenville orogeny. Mineral assemblages, thepresence of migmatitic gneisses, and limited P-T data indicate upper amphibolite to granulitefacies conditions of metamorphism. The timingof this metamorphism is constrained by Precam-brian isotopic ages and ages of intrusive rocks(Neoproterozoic Crossnore-Beech andBakersville suites) that cross-cut layering andfolds in the gneisses. Grenville-age metamor-phism affected the Pumpkin Patch, BeechMountain, and Pardee point thrust sheets, buthas not been recognized in the Spruce Pinethrust sheet.

Thrust sheets of the Blue Ridge thrustcomplex were variably affected by multiplePaleozoic tectonothermal events. The PardeePoint thrust sheet experienced only very low-grade metamorphism during Alleghanian thrust-ing, and Grenville metamorphic assemblageswere not significantly retrogressed (Butler,1991). The Beech Mountain thrust sheet waspervasively deformed during greenschist-gradeAlleghanian thrusting. Isotopic ages for thesefault-related rocks range from ~300-330 Ma(Van Camp and Fullagar, 1982; Goldberg andDallmeyer, 1991; Schedl et al., 1992; Adamsand Su, 1996; Goldberg and Dallmeyer, 1997).However, evidence for earlier Paleozoic meta-morphism is somewhat ambiguous in the BeechMountain thrust sheet.

Isotopic ages for metamorphic mineralsfrom the Spruce Pine and Pumpkin Patch thrustsheets record both Ordovician and Siluro-Devonian metamorphism. Bakersville maficintrusive rocks are metamorphosed, placing alower age limit on post-Grenville metamor-phism in the Pumpkin Patch sheet. High-pressure metamorphism is recorded by eclogitefacies rocks and relatively high pressure esti-mates in the AMS and Meadlock Mountain

gneiss. Amphibolite facies assemblages suggesta retrograde P-T path during cooling and de-compression of the AMS. High-temperaturedextral strike-slip shearing deformed but did notretrograde amphibolite-facies assemblages, andwas broadly contemporaneous with intrusion ofthe Spruce Pine pegmatites. The similarity inmetamorphic grade and isotopic ages in theSpruce Pine and Pumpkin Patch sheets suggeststhat they were juxtaposed no later than theDevonian. The greenschist-grade retrogressioncommon in late Paleozoic shear zones of theBlue Ridge thrust complex is lacking in thePumpkin Patch and Spruce Pine sheets south-west of the Grandfather Mountain window.However, near the window, high-grade assem-blages are overprinted by greenschist-gradeshearing associated with Alleghanian northwest-directed thrusting.

The available data are consistent withthe following aspects of the Paleozoic tectonicevolution of the southern Appalachian BlueRidge. Closure of an ocean basin and subse-quent continental or arc-continent collisioncaused the Ordovician Taconic orogeny result-ing in metamorphism of at least part of the AMSand formation of eclogite in the associatedsubduction zone complex. Subsequent orcontinued crustal thickening caused Siluro-Devonian amphibolite facies metamorphism.Dextral strike-slip faulting along the Burnsvillefault occurred shortly after peak regional meta-morphism, and the final juxtaposition of theAMS and underlying basement of the PumpkinPatch thrust sheet occurred by the Devonian.The magnitude of displacement during thisfaulting is unknown. A southwestward progres-sion of Devonian clastic wedges has been notedby several workers (e.g. Dennison, 1985; Ferrilland Thomas, 1988; Ettensohn, 1995). Theabsence of a large Devonian clastic wedge in thesouthern Appalachians and the southwestwardprogression of Devonian clastic wedges may beexplained by Acadian dextral transpression(Ferrill and Thomas, 1988).


44

The late Paleozoic history of the BlueRidge thrust complex was dominated by north-west-directed faulting. Thrust sheets within thethrust complex are typically separated bygreenschist-grade mylonite zones with top-to-northwest sense of movement (e.g. Bryant andReed, 1970; Trupe, 1989; Goldberg et al.,1989, 1992b; Trupe et al., 1990; Adams, 1990).Isotopic ages of mylonitic rocks from theLinville Falls and related shear zones are ~300Ma (Van Camp and Fullagar, 1982; Schedl etal., 1992; Adams and Su, 1996), and representfinal emplacement of the assembled thrustcomplex during the Alleghanian orogeny.

ACKNOWLEDGEMENTS

Some of the work discussed in this paper wasdone by the authors and colleagues duringgraduate study at the University of North Caro-lina-Chapel Hill. We thank many co-workersfor useful discussions over the years, particu-larly J.R. Butler, K.G. Stewart, S.A. Goldberg,and R.A. Willard. Financial assistance for partof this research was provided by NationalScience Foundation Grants EAR 9316033 and8518463, the University of North Carolina-Chapel Hill Geology Department, Sigma Xi, theGeological Society of America, and the NorthCarolina Geological Survey.

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Goldberg, S.A., Adams, M.G., Trupe, C.H., Stewart, K.G.,and Butler, J.R., 1994, Sm-Nd isotopic constraints ona Blue Ridge eclogite, northwestern North Carolina:Geological Society of America Abstracts withPrograms, v. 26, p. 196.

Goldberg, S.A., Butler, J.R., and Fullagar, P.D., 1986b,The Bakersville dike swarm: Geochronology andpetrogenesis of Late Proterozoic basaltic magmatismin the southern Appalachian Blue Ridge: AmericanJournal of Science, v. 286, p. 403-430.

Goldberg, S.A., Butler, J.R., and Mies, J.W., 1986a,Subdivision of the Blue Ridge thrust complex,western North Carolina and adjacent Tennessee:Geological Society of America Abstracts withPrograms, v. 18, p. 616-617.

Goldberg, S.A., Butler, J.R., Mies, J.W., and Trupe, C.H.,1989, The southern Appalachian orogen in northwest-ern North Carolina and adjacent states: InternationalGeological Congress Field Trip T365 Guidebook,American Geophysical Union, Washington, DC, 55 p.

Goldberg, S.A., Butler, J.R., Trupe, C.H., and Adams,M.G., 1992b, The Blue Ridge thrust complexnorthwest of the Grandfather Mountain window,North Carolina and Tennessee, in Dennison, J.M.,and Stewart, K.G., editors, Geologic field guides toNorth Carolina and vicinity: Chapel Hill, NorthCarolina, Department of Geology, University of

North Carolina at Chapel Hill, Geologic GuidebookNo. 1, p. 213 - 233.

Goldberg, S.A., and Dallmeyer, R.D., 1991, Rb-Sr and40Ar/39Ar mineral ages from the Blue Ridge west ofthe Grandfather Mountain window: evidence for LateMississippian thrusting and Devonian metamor-phism: Geological Society of America Abstracts withPrograms, v. 23, p. 35.


Goldberg, S.A., Dallmeyer, R.D., and Su, Q., 1993,Chronology of Paleozoic metamorphism and defor-mation in the southern Appalachian Blue Ridge basedon Sr, Nd, Pb, and Ar isotopic systematics: EOS, v.74, p. 122.

Goldberg, S.A., Trupe, C.H., and Adams, M.G., 1992a,Pressure-temperature-time constraints for a segmentof the Spruce Pine thrust sheet, North Carolina BlueRidge: Geological Society of America Abstracts withPrograms, v. 24, p. 17.

Gulley, G.L., Jr., 1985, A Proterozoic granulite-faciesterrane on Roan Mountain, western Blue Ridge belt,North Carolina - Tennessee: Geological Society ofAmerica Bulletin, v. 96, p. 1428-1439.

Hatcher, R.D., Jr., 1978, Tectonics of the western Pied-mont and Blue Ridge, southern Appalachians:Review and speculation: American Journal ofScience, v. 278, p. 276-304.

Hatcher, R.D., Jr., 1987, Tectonics of the southern andcentral Appalachian internides: Annual Review ofEarth and Planetary Sciences, v. 15, p. 337-362.

Hatcher, R.D., Jr., and Goldberg, S.A., 1991, The BlueRidge geologic province: in Horton, J.W., Jr., andZullo, V.A., editors, The geology of the Carolinas:Carolina Geological Society Fiftieth AnniversaryVolume: Knoxville, The University of TennesseePress, Carolina Geological Society, p. 11-35.

Hatcher, R.D., Jr., Hooper, R.J., Petty, S.M., and Willis,J.D., 1984, Structure and chemical petrology of threesouthern Appalachian mafic-ultramafic complexesand their bearing upon the tectonics of emplacementand origin of Appalachian ultramafic bodies: Ameri-can Journal of Science, v. 284, p. 484-506.

Holdaway, M.J., 1971, Stability of andalusite and thealuminum silicate phase diagram: American Journalof Science, v. 271, p. 97-131.

Holland, T.J.B., 1980, The reaction albite = jadeite +quartz determined experimentally in the range 600-1200°C: American Mineralogist, v. 65, p. 129-134.

Holland, T.J.B., 1983, The experimental determination ofactivities in disordered and short-range ordered


46

jadeitic pyroxenes: Contributions to Mineralogy andPetrology, v. 82, p. 214-220.

Horton, J.W., Drake, A.A., Jr., and Rankin, D.W., 1989,Tectonostratigraphic terranes and their boundaries inthe central and southern Appalachians: in Dallmeyer,R.D., editor, Terranes in the Circum-Atlantic Paleo-zoic orogens: Geological Society of America SpecialPaper 230, p. 213-245.

Kellberg, J.M., and Grant, L.F., 1958, Coarse conglomer-ates of the Middle Ordovician in the southernAppalachian valley, Geological Society of AmericaBulletin, v. 67, p. 697-716.

King, P.B., and Ferguson, H.W., 1960, Geology ofnortheasternmost Tennessee with Description of thebasement rocks by Warren Hamilton, United StatesGeological Survey Professional Paper 311, 160 p.

Kish, S.A., 1983, A geochronological study of deforma-tion and metamorphism in the Blue Ridge andPiedmont of the Carolinas: Ph. D. Dissertation,University of North Carolina at Chapel Hill, 220 p.

Kish, S.A., 1989, Igneous and metamorphic history of theeastern Blue Ridge, southwestern North Carolina: K-Ar and Rb-Sr studies: in Fritz, W.J., Hatcher, R.D.,Jr., and Hopson, J.L., editors., Geology of the easternBlue Ridge of northeast Georgia and the adjacentCarolinas: Atlanta, Georgia, Georgia GeologicalSociety, Inc., Georgia Geological Society Guide-books, v. 9, no. 3, p. 41-74.

Lesure, F.G., 1968, Mica deposits of the Blue Ridge inNorth Carolina: United States Geological SurveyProfessional Paper 577, 124 p.

Long, L.E., Kulp, J.L., and Eckelmann, F.D., 1959,Chronology of major metamorphic events in thesoutheastern United States, American Journal ofScience, v. 257, 585-603.

Mallard, L.D., Adams, M.G., and Stewart, K.G., 1994,Kinematic analysis of a possible suture in thesouthern Appalachians, northwestern North Carolina:Geological Society of America Abstracts withPrograms, v. 26, p. 25-26.

McCrary, C.R., III, 1997, Pressure-temperature-timehistory of Grenville basement gneisses in the PardeePoint thrust sheet near Hampton, Tennessee: Geo-logical Society of America Abstracts with Programs,v. 29, p. 57.

McSween, H.Y., Jr., Abbott, R.N., and Raymond, L.A.,1989, Metamorphic conditions in the Ashe Metamor-phic Suite, North Carolina Blue Ridge: Geology, v.17, p. 1140-1143.

Merschat, C.E., and Wiener, L.S., 1988, Geology of theSandymush and Canton quadrangles, North Carolina:North Carolina Geological Survey Bulletin 90, 66 p.

Mies, J.W., 1987, An investigation of the Proterozoicbasement-Ashe Formation boundary west of the

Grandfather Mountain window, northwestern NorthCarolina: Masters Thesis, University of NorthCarolina at Chapel Hill, 74 p.

Mies, J.W., 1988, A major thrust at the base of the AsheFormation in the Blue Ridge of northwestern NorthCarolina: Geological Society of America Abstractswith Programs, v. 20, p. 280.

Mies, J.W., 1990, Structural and petrologic studies ofmylonite at the Grenville basement - Ashe Formationboundary, Grayson County, Virginia to MitchellCounty, North Carolina: Ph. D. Dissertation,University of North Carolina at Chapel Hill, 330 p.

Mies, J.W., and Trupe, C.H., 1989, Kinematic analysis inthe Blue Ridge thrust complex, northwestern NorthCarolina and adjacent states: Geological Society ofAmerica Abstracts with Programs, v. 21, p. 50.

Misra, K.C., and Conte, J.A., 1991, Amphibolites of theAshe and Alligator Back Formations, North Carolina:Samples of Late Proterozoic - early Paleozoicoceanic crust: Geological Society of AmericaBulletin, v. 103, p. 737-750.

Monrad, J.R., and Gulley, G.L., 1983, Age and P-Tconditions during metamorphism of granulite-faciesgneisses, Roan Mountain, NC-TN, in Lewis, S.E.,editor, Geologic investigations in the Blue Ridge ofnorthwestern North Carolina: Carolina GeologicalSociety Field Trip Guidebook, 29 p.

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Rankin, D.W., Espenshade, G.H., and Shaw, K.W., 1973,Stratigraphy and structure of the metamorphic belt innorthwestern North Carolina and southwesternVirginia: A study from the Blue Ridge across theBrevard fault zone to the Sauratown Mountainsanticlinorium: American Journal of Science, v. 273-A, p. 1-40.

Raymond, L.A., Yurkovich, S.P., and McKinney, M.,1989, Block-in-matrix structures in the NorthCarolina Blue Ridge belt and their significance forthe tectonic history of the southern Appalachianorogen: in Horton, J.W., Jr., and Rast, N., editors,Melangés and Olistostromes of the U.S. Appala-

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chians: Geological Society of America Special Paper228, p. 195-215.

Satterfield, M.E., 1992, Investigation of the metamor-phosed massive sulfide deposit at the Ore Knob mine,Ore Knob, North Carolina: Masters Thesis, Univer-sity of North Carolina at Chapel Hill, 110 p.

Schedl, A., McCabe, C., Montanez, I., Fullagar, P.D., andValley, J., 1992, Alleghenian regional diagenesis: Aresponse to the migration of modified metamorphicfluids derived from beneath the Blue Ridge-Piedmontthrust sheet: Journal of Geology, v. 100, p. 339-352.

Su, Q., Goldberg, S.A., and Fullagar, P.D., 1994, PreciseU-Pb zircon ages of Neoproterozoic plutons in thesouthern Appalachian Blue Ridge and their implica-tions for the initial rifting of Laurentia: PrecambrianResearch, v. 68, p. 81-95.

Trupe, C.H., 1989, Microstructural analysis of mylonitesfrom the Blue Ridge thrust complex, northwesternNorth Carolina: M.S. Thesis, University of NorthCarolina at Chapel Hill, 101 p.

Trupe, C.H., 1997, Deformation and metamorphism inpart of the Blue Ridge thrust complex, northwesternNorth Carolina: Ph. D. Dissertation, University ofNorth Carolina at Chapel Hill, 176 p.

Trupe, C.H., Adams, M.G., Butler, J.R., and Willard,R.A., 1991, Geology of the Burnsville, NC quad-rangle: Basement-cover relationships in the NorthCarolina Blue Ridge: Geological Society of AmericaAbstracts with Programs, v. 23, p. 140.

Trupe, C.H., Butler, J.R., Mies, J.W., Adams, M.G., andGoldberg, S.A., 1990, The Linville Falls fault andrelated shear zone: Geological Society of AmericaAbstracts with Programs, v. 22, p. 66.

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Willard, R.A., Trupe, C.H., Adams, M.G., and Butler,J.R., 1991, Transition in the character of the Ashe-Basement contact west of the Grandfather Mountainwindow: Geological Society of America Abstractswith Programs, v. 23, p. 150.

Wojtal, , S., and Mitra, G., 1988, Nature of deformation insome fault rocks from Appalachian thrusts, in Mitra,G., and Wojtal, S., editors, Geometries and Mecha-nism of Thrusting, with Special Reference to theAppalachians: Geological Society of AmericaSpecial Paper 222, p. 17-33.


49

ABSTRACT

The Linville Falls fault frames theGrandfather Mountain window, and separatescrystalline thrust sheets of the Blue Ridge thrustcomplex from basement and metasedimentaryrocks within the window. The fault is named foran exposure at Linville Falls, NC, where base-ment rocks overlie Paleozoic metasedimentaryrocks of the Tablerock thrust sheet. Previousworkers have described the fault at this expo-sure as a meter-scale mylonite zone, and haveattempted to explain thickness variations be-tween Linville Falls and exposures north of theGrandfather Mountain window. Detailedmapping shows that in the vicinity of LinvilleFalls, a ductile shear zone several hundredmeters thick occurs immediately above themeter-scale mylonite zone. Field relationshipsindicate that the thin mylonite zone exposed atLinville Falls is a small part of the fault zone ingeneral, and that there is no significant variationin shear zone thickness between this exposureand exposures north of the Grandfather Moun-tain window.

At Linville Falls, the hanging wall rockabove the meter-scale shear zone is a slice ofalkali feldspar granite structurally overlain bybasement-derived mylonite of the Linville Falls

shear zone. Shear zone mylonites record crys-tal-plastic deformation under greenschist-gradeconditions, and kinematic indicators are consis-tent with top-to-northwest shear sense. Immedi-ately below shear zone mylonites, cataclasticdeformation overprints ductile fabric within thegranite. The presence of cataclastic rocks in theLinville Falls shear zone indicates that move-ment continued under brittle conditions ofdeformation as the Blue Ridge thrust complexreached relatively shallow crustal levels.

INTRODUCTION

The Blue Ridge geologic province of thesouthern Appalachian orogen is one of theworld’s largest crystalline thrust terranes. Innorthwestern North Carolina, the crystallinethrust complex has been partially eroded to formthe Grandfather Mountain window. TheLinville Falls fault occurs at the base of the BlueRidge thrust complex, and defines the border ofthe Grandfather Mountain window (Goldberg etal., 1992). The region adjoining the windowprovides a cross-section through the Blue Ridgethrust complex, and has been the focus ofseveral important studies of faulting and fault-related rocks (e.g. Boyer and Elliott, 1982;Boyer and Mitra, 1988; Boyer, 1992; Schedl etal., 1992). However, most of these studies havebeen based upon relatively old, small-scale,rather than detailed, geologic mapping.

STRUCTURAL RELATIONSHIPS IN THE LINVILLE FALLS SHEAR ZONE, BLUERIDGE THRUST COMPLEX, NORTHWESTERN NORTH CAROLINA

Charles H. Trupe*

Department of GeologyUniversity of North Carolina Chapel Hill, NC 27599-3315

_______________________________________*Present address:2619 Mt. Gilead Church Rd.Pittsboro, NC 27312

50

Arthur Keith (1903, 1905) conducted thefirst mapping in the vicinity of Linville Fallsand the Grandfather Mountain window on theCranberry and Mt. Mitchell 30 minute quad-rangles, but this did not include the exposure ofthe fault at Linville Falls. The GrandfatherMountain window was first interpreted as awindow on the “Geologic Map of the UnitedStates” (Stose and Ljungstedt, 1932), and wassubsequently named the Grandfather Mountainwindow by Stose and Stose (1944).

Mapping of 15 minute quadrangles wasundertaken by the United States GeologicalSurvey from 1956 to 1962, and resulted inseveral short reports and maps of the area inwhich the Linville Falls and Tablerock faultswere named and described (see references inBryant and Reed, 1970). These studies culmi-nated in the publication of United States Geo-logical Survey Professional Paper 615 (Bryantand Reed, 1970), which has since been regardedas the definitive geologic mapping in the area.Subsequent topical studies in the Linville Fallsarea have relied upon the 1:62,500-scale geo-logic map of Bryant and Reed (1970) as a basisfor more detailed structural and petrologicstudies (e.g. Boyer, 1978, 1992; Boyer andElliott, 1982; Schedl, 1985, 1988; Wojtal andMitra, 1988; Boyer and Mitra, 1988; Schedl etal., 1992; Newman and Mitra, 1993), but thesestudies did not present any new large-scalegeologic mapping.

Detailed geologic mapping in the vicin-ity of Linville Falls fault has revealed relation-ships more complex than previously recognized.There are three main points that arise from thisresearch. First, above the meter-scale LinvilleFalls fault zone (as originally defined, theLinville Falls fault sensu stricto), crystallinebasement rocks are pervasively sheared tomylonite and ultramylonite (classification ofSibson, 1977) in a zone up to one kilometerthick. Trupe et al. (1990) termed this shear zonethe Linville Falls shear zone, and described it asa thick, mixed-rock mylonite zone that every-

where lies just above the Linville Falls fault asoriginally mapped. Second, the hanging wall ofthe meter-scale shear zone at Linville Fallsconsists of a tectonic block (Berkland et al.,1972) of alkali feldspar metagranite similar inmineralogy and chemistry to NeoproterozoicCrossnore-Beech Suite granites. Third, the baseof the Linville Falls shear zone is characterizedby mylonitic rocks that are overprinted bycataclastic deformation. Cataclasite zones aremost prominent in the lowest few meters of thehanging wall. These features are similar tothose noted in exposures of the Linville Fallsshear zone north of the Grandfather Mountainwindow (Trupe et al., 1990; Adams, 1990a,1990b; Trupe and Adams, 1991; Goldberg etal., 1992; Adams and Su, 1996).

The present investigation is the result ofdetailed geologic mapping in the Linville Fallsarea. Preliminary reports have been presentedby Trupe et al. (1990), Trupe and Adams (1991),and Goldberg et al. (1992), in which the natureof the Linville Falls shear zone and its relation-ship to the Linville Falls fault were discussed.The present paper constitutes the first publica-tion of detailed new mapping in the LinvilleFalls area.

The purpose of this paper is to presentnew field, structural, and petrologic data in thevicinity Linville Falls. These data demonstratethe nature of structural relationships and defor-mation within the Linville Falls fault zone,which should be considered in interpretations offault-related rocks in the vicinity of the Grand-father Mountain window.

GEOLOGIC SETTING

The Blue Ridge thrust complex ofnorthwestern North Carolina (Figure 1) is astack of crystalline thrust sheets separated bythick mylonite zones and characterized bydifferent P-T and deformation histories (Butler,1973; Abbott and Raymond, 1984; Goldberg etal., 1986, 1989, 1992; Butler et al., 1987). The

Trupe

51

base of the Blue Ridge thrust complex is markedby shear zones of the Linville Falls fault, a faultthat defines the Grandfather Mountain window.Mylonitic rocks within and bounding the lowerthrust sheets (which consist predominantly ofMesoproterozoic basement rocks) record top-to-northwest sense of movement and greenschistfacies conditions (Goldberg et al., 1986, 1989,1992; Mies et al., 1987; Mies and Trupe, 1989;Trupe, 1989a, 1989b; Adams, 1990b). Isotopic

ages for these mylonitic rocks are ~300-330 Ma(Van Camp and Fullagar, 1982, Rb-Sr whole-rock age; Schedl et al., 1992, Rb-Sr whole-rockage; Goldberg et al., 1993, Rb-Sr and 40Ar/39Ar mica ages; Su, 1994, Rb-Sr whole-rockage; Su and Fullagar, 1995, Rb-Sr whole-rockage; Adams and Su, 1996, Rb-Sr whole-rockages). The boundary between the uppermostunit and adjacent Mesoproterozoic basement,however, is marked locally by a syn-regional-

Virginia

Tennessee 4050000 mN

4500

00 m

E

4000000 mN

Virginia

North Carolina

Banner Elk

Boone

Grandfather Mountain Formation

Mount Rogers Formation

Bakersville Intrusive Suite (latePrecambrian rift-related basalt,diabase and gabbro)

Crossnore Plutonic Suite (latePrecambrian rift-related alkali andperalkalic granitic rocks)

Spruce Pine thrust sheet (includes LateProterozoic (?) Ashe MetamorphicSuite) amphibolite grade

Pumpkin Patch thrust sheet(predominantly Grenville-age graniticrocks and mafic gneiss) amphiboliteto granulite grade

Pardee Point thrust sheet(predominantly Grenville-age graniticgneiss with Paleozoic cover rocks)subgreenschist to chlorite zone

Beech Mountain thrust sheet(predominantly Grenville-agegranitic rocks) greenschist grade

Thrust Sheets

Spruce Pine Plutonic Suite (Siluro-Devonian alaskites and pegmatites)

Paleozoic metasedimentary rocks

Grenville-age basement rockswithin Grandfather Mountainwindow

AREA OF LARGE MAP

0 5 10

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GRANDFATHERMOUNTAINWINDOWBurnsville fault

EXPLANATIONTe

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Figure 1. Generalized geologic and tectonic map of part of the Blue Ridge thrust complex in the vicinity of the Grandfa-ther Mountain window, modifed from Adams et al., (1995). LFSZ - Linville Falls shear zone. TRF - Tablerock fault.LF - Linville Falls. Neoproterozoic Crossnore-Beech suite plutons indicated by B (Beech Granite), BM (Brown Moun-tain Granite), and C (Crossnore Granite). Registration coordinates are for Universal Transverse Mercator grid, zone 17.

Linville Falls Shear Zone

52

metamorphic, dextral strike-slip fault formedunder amphibolite grade conditions (Trupe etal., 1991; Willard et al., 1991; Mallard et al.,1994; Adams et al., 1995). Near the Grandfa-ther Mountain window, this strike-slip deforma-tion is overprinted by post regional-metamor-phic mylonitization associated with the LinvilleFalls shear zone (Adams et al., 1995).

Within the Grandfather Mountain win-dow, Neoproterozoic Grandfather MountainFormation sedimentary and volcanic rocksoverlie Mesoproterozoic basement gneisses,which are intruded by the NeoproterozoicBrown Mountain Granite (Bryant and Reed,1970; Fetter and Goldberg, 1995). TheTablerock thrust sheet (Figure 1) is a tectonicslice of Cambrian Chilhowee Groupmetaquartzites and Shady Dolomite that occursbetween the Blue Ridge thrust complex and theunderlying Precambrian rocks of the Grandfa-ther Mountain window (Bryant and Reed,1970). The crystalline rocks overlying theTablerock thrust sheet were mapped as Cran-berry Gneiss by Bryant and Reed (1970), andconsist of layered granodioritic gneiss, horn-blende gneiss, biotite schist, and amphibolite.

LINVILLE FALLS SHEAR ZONE

The Linville Falls fault (sensu stricto) isnamed for an exposure on the west side of theLinville River, approximately 100 metersupstream of the National Park Service upperfalls overlook. Bryant and Reed (1970, p. 163)described the fault as “marked by 6 to 18 inchesof white to green finely laminatedblastomylonite which is parallel to bedding inthe quartzite and to foliation in the gneiss”(Figure 2). The “blastomylonite” separates“Cranberry Gneiss” in the hanging wall fromChilhowee Group quartzite of the Tablerockthrust sheet in the footwall. Bryant and Reed(1970) noted extensive shearing in basementrocks above the fault, and described slices ofquartzite, amphibolite, and mica schist interca-

lated with mylonitic gneisses along the fault atseveral locations around the Grandfather Moun-tain window.

New mapping in the vicinity of LinvilleFalls reveals that the hanging wall of theLinville Falls fault (sensu stricto) consists of athick mylonite zone, the Linville Falls shearzone (Figure 3). The base of the shear zone isdefined by the Linville Falls fault (sensustricto). The Linville Falls shear zone is ap-proximately 800 meters thick near LinvilleFalls, and is well-exposed along the Blue RidgeParkway west of the Linville River. The shearzone consists of mylonite and ultramylonitederived from basement gneisses (biotite gneiss,hornblende gneiss, and granitic gneiss), andcontains tectonic blocks or slices of magnetite-hornblendite, amphibolite, and metagranite.The magnetite-hornblendite is similar to rocksof the Cranberry magnetite deposits that occurin the Beech Mountain thrust sheet along thenorthwestern margin of the Grandfather Moun-tain window (W. Ussler, personal communica-tion). Mylonitic foliation in the shear zone

Figure 2. Exposure of the Linville Falls fault at LinvilleFalls. This is the exposure originally described by Bryantand Reed(1970). Hammer rests on sheared ChilhoweeGroup sericite-quartz mylonite; handle points northeast.Rock in the hanging wall is Linville Falls granite. Late,open folds with northeast-trending axes can be seen in thequartzite and laminated mylonite.

Trupe

53

wraps around these slices, suggesting tectonicemplacement of boudins or slices during thrust-ing.

This new mapping also demonstratesthat the rock in the hanging wall of the LinvilleFalls fault (sensu stricto) is a tectonic slice ofalkali feldspar metagranite, referred to hereafteras the Linville Falls granite. The“blastomylonite” of Bryant and Reed (1970)occurs between the granite and underlyingChilhowee Group quartzites and arkosic quartz-ites of the Tablerock thrust sheet. The graniteconsists of perthitic alkali feldspar, quartz, andminor plagioclase, with biotite, hornblende,fine-grained white mica, epidote, zircon, fluo-rite, apatite and opaque minerals (Table 1). TheLinville Falls granite has not been isotopicallydated, but is similar in outcrop appearance to theBeech Granite. It may be a tectonic slice of thelatter unit. The Linville Falls granite is alsosimilar in chemistry to the Beech, Brown Moun-tain, and Crossnore Granites (Table 2). Petro-graphically, the Linville Falls granite resemblesthe Beech Granite, as both contain abundantperthitic feldspar and accessory fluorite. TheLinville Falls granite apparently lacks aegerine-augite, which is characteristic of the CrossnoreGranite (Bryant and Reed, 1970; Su et al.,

1994). The granite is variably sheared, rangingfrom weakly deformed to ultramylonitic; duc-tile fabric in the granite has been overprinted bycataclastic fabric. The granite is up to 50 metersthick, and extends 0.5 kilometers south and 1kilometer north of the Linville Falls exposure.Basement-derived mylonite and ultramylonite ofthe Linville Falls shear zone structurally overliethe Linville Falls granite and locally occurbelow it. The contact between the granite andoverlying mylonites may be seen just north ofthe National Park Service Linville Falls Camp-ground, where a second slice of granite is

* Includes hornblende, zircon, apatite, fluorite, andcataclastic material too fine-grained to identify. Insample 8970, this is predominantly cataclastic material.


54

3982000mN

32C- c

3981

40

A'

3980

23

A

3979

27

26

413000mE

28

415

38

Zgmf

414

26

416

20

Zams

417

16

Zlfg

418

20

35° 57' 30"

16

Zlfg

81°

57' 3

0"

14

81°

55' 0

0"

35

Tablerockfault

48

23

Ybg

01/21 1 Mile

31

Linvi lle Fallsfault

01/21 1 Kilometer

5

Contour interval 40 feet

14

Geology of Part of the Linville Falls ShearZone, Linville Falls Quadrangle, NC

UTM Grid zone 17 and 1995Magnetic north declination at

center of sheet

26

Ybg

3978

23

Ybg

C- c

41

C- c

14

C- s

18

5.5°

3310

MN

27

29

6

15

29

21

919

38

23

19

24

27

26

31

20

8

1313

18

17

12

18

25

1015 5

12

24

25

16

2

11

9

2021

5

29

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27

2536

30

18

18

35

11

15 6

21

3

3

10

1418

5

20

18

18

19

4

7

18

26

22

30

21

20

20 2521

9

820

14

23

17

1210

5

10

15

16

12

7

20

18

32

a

g

hb

Type locality ofLinvi lle Falls fault

Figure 3. Geologic map and interpretive cross-section of part of the Linville Falls shear zone. The base map is acomputer scan of part of the Linville Falls, NC 71/2 minute quadrangle. Portions of the Shady Dolomite and AsheMetamorphic Suite contacts, and the southernmost part of the Linville Falls fault (above the Shady Dolomite) aremodified after Bryant and Reed (1970).

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a

4000

3000

2000

1000

0

Zgmf

Ybg

C- s

A A'

C- c

Zlfg

Interpretive Cross Section from A to A'

Zams

Linville Falls faul t Tabl erock fault

3

18

5

10

Contact, dashed where approximate

Thrust fault, dashed where approximate

Trend and plunge of crenulation axis

Trend and plunge of mineral stretching line

Trend and plunge of minor fold axis

Strike and dip of mylonitic foliation

g

hb

Amphibolite

Alkali feldspar granite

Magnetite-hornblendite

Map Symbols

ZgmfGrandfather Mountain Formation: Medium-grained arkose, massive, weakly foliated.Low greenschist-grade metamorphism.

ZamsAshe Metamorphic Suite: Kyanite-garnet mica schist and amphibolite. Sheared andretrograded (greenschist grade) near fault contact with basement rocks in the LinvilleFalls shear zone.

ZlfgLinville Falls granite: Medium- to coarse-grained alkali feldspar metagranite.Sheared to mylonite and ultramylonite, and overprinted by cataclasis.

Ybg

C- s

C- c

Shady Dolomite: Massive, buff to light gray, fine-grained siliceous dolomite.Cataclastic deformation is prevalent near contact with the Linville Falls shear zone.

Chilhowee Group, undivided: Meta-arkose, feldspathic quartzite, and quartzite,biotite grade. Typically sheared to mylonite and ultramylonite. Compositionallayering generally parallel to mylonitic foliation.

Neoproterozoic

Biotite gneiss: Layered to nonlayered biotite gneiss and granitic gneiss, with minor hornblendegneiss, amphibolite, and epidote-biotite schist. Includes bodies of alkali feldspar granite andmagnetite hornblendite. Strongly sheared to mylonite and ultramylonite in the Linville Fallsshear zone.

Paleozoic

Mesoproterozoic

EXPLANATION

Form lines (cross section), closely spacedin areas of intense shearing

Dip direction (cross section)

Linville Falls shear zone

Linville Falls S

hear Zone

56

exposed (Figure 4). The Linville Falls granitepinches out to the north and south, where rocksof the Tablerock thrust sheet are juxtaposedagainst basement-derived mylonite of theLinville Falls shear zone along the Linville Fallsfault (sensu stricto).

Structural data for the Linville Fallsshear zone are summarized in Figure 5.Mylonitic foliation in the shear zone generallydips gently to moderately west and southwest.Mineral stretching lineations are very consistent,and plunge gently to the northwest and south-east. Foliation and lineation data for theLinville Falls granite and Chilhowee Grouprocks of the Tablerock thrust sheet are similar tothose for the Linville Falls shear zone, andsuggest that they all experienced a commondeformation history, as noted by Bryant andReed (1970). Minor fold axes range fromperpendicular to parallel to mineral stretchinglineations. Bryant and Reed (1969, 1970) andMies (1991) suggested that minor fold hingelines were rotated during shearing toward

parallelism with mineral stretching lineations.Axes to crenulation are perpendicular to mineralstretching lineations, and either post-date thrust-ing (Hatcher and Butler, 1986), or are related tolate-stage movement in the Linville Falls shearzone (Bryant and Reed, 1970), possibly due tothrust imbrication and duplex formation withinthe Grandfather Mountain window (Boyer,1992).

Cataclasite occurs within the LinvilleFalls granite, but has not been observed inChilhowee quartzites. Mylonites near the baseof the Linville Falls shear zone, immediatelyabove the Linville Falls granite, have a minorcataclastic overprint confined to the first fewmeters above the granite. Cataclastic fabric inthese mylonites consists of randomly sized andoriented angular fragments of mylonitic materialin a very fine-grained matrix of quartz, feldspar,and mica. These cataclasite zones are predomi-nantly normal microfaults. Cataclasites are alsofound at the base of the Linville Falls shear zone

Figure 4. Contact between the Linville Falls granite(bottom) and ultramylonite of the Linville Fallsshear zone (top). Hammer rests on the contact.Photograph taken facing west. Note platy weather-ing of finely laminated ultramylonite.

Figure 5. Equal area, lower hemisphere projections ofstructural data from the Linville Falls shear zone. Boxes= poles to mylonitic foliation; dots = mineral stretchinglineations; crosses = minor fold axes; open circle =crenulation axis. a) Linville Falls shear zone; b) LinvilleFalls granite; c) Chilhowee Group metaquartzites.

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north of the Grandfather Mountain window nearBanner Elk and Bowers Gap, NC (Trupe andAdams, 1991; Goldberg et al., 1992), and arealso present in the Long Ridge, Unaka Moun-tain, and Stone Mountain fault systems (Kingand Ferguson, 1960; Bryant and Reed, 1970;Diegel and Wojtal, 1985; Adams and Trupe,1991; O’Hara, 1992; Adams and Su, 1996). Asat Linville Falls, the cataclasites overprintmylonitic fabrics, and occur at the base of theshear zone.

MICROSTRUCTURES AND CONDITIONSOF DEFORMATION ASSOCIATED WITHTHE LINVILLE FALLS SHEAR ZONE

The Linville Falls shear zone consists ofprotomylonite, mylonite and ultramylonitederived from a variety of basement rocks. Thepredominant rock type is feldsparporphyroclastic mylonite (Figure 6). Most areL-S mylonites, with well-developed foliationdefined by compositional layers of quartz andfeldspar alternating with layers of fine-grainedwhite mica. Lineation is defined by elongatequartz-feldspar aggregates or biotite streaks.

Feldspar in the mylonites is deformed primarilyby cleavage-controlled fracture, with minorrecrystallization along cracks and grain bound-aries. Porphyroclast tails consist of fine-grainedquartz, mica, and feldspar. In many basement-derived mylonites, plagioclase is the predomi-nant feldspar, and it is typically sausseritic andaltered to epidote, white mica, and quartz. Thisalteration suggests that hydrolysis of feldsparswas an important mechanism in the reduction offeldspar porphyroclast size and the formation ofquartz-mica matrix material.

Quartz is deformed by crystal-plasticmechanisms in all Linville Falls shear zonemylonites. Quartz forms ribbons or composi-tional layers of dynamically recrystallizedgrains that range from 10 µm to 100 µm in sizeand average ~50 µm. Grains range from highlystrained with undulose extinction and irregular,serrated grain boundaries, to clear, polygonal,and strain-free with straight grain boundaries.These fabrics suggest deformation by disloca-tion creep accommodated by dynamic recrystal-lization and recovery (e.g. White, 1976, 1977;Kerrich and Allison, 1978; Urai et al., 1986).

LINVILLE FALLS GRANITE

In weakly sheared Linville Falls granite,a penetrative foliation did not develop. Thepredominant deformation mechanism wascleavage-controlled brittle fracture of feldspars,with minor recrystallization in quartz. Plagio-clase deformed by crystal-plastic mechanisms asevidenced by bent or kinked twin lamellae andundulatory extinction. In more strongly shearedsamples, feldspar grain-size is reduced, and afine-grained matrix of quartz, mica, and recrys-tallized feldspar form a well-developedmylonitic foliation. In such mylonitic LinvilleFalls granite, quartz layers consist of aggregatesof recrystallized grains averaging ~50 µm indiameter. K-feldspars exhibit minor recrystalli-zation along fractures and grain boundaries.Mylonitic foliation is defined by fine-grained

Figure 6. Photomicrograph (plane light) of basement-derived, feldspar porphyroclastic mylonite from theLinville Falls shear zone. Field of view is ~6.5 mm wide.σ-type asymmetric porphyroclasts give top-to-northwestsense of shear.


58

phyllosilicates and quartz-feldspar ribbons.Near the Linville Falls fault (sensu stricto),fluid-rock interaction is suggested by an in-crease in phyllosilicate content, and feldspargrain-size reduction apparently enhanced byhydrolysis. Previous studies have suggestedthat H2O-rich fluids played an important role inthe formation of Linville Falls shear zonemylonites (e.g., Bryant and Reed, 1970; Wojtaland Mitra, 1988; Schedl et al., 1992; Trupe,1989b; Newman and Mitra, 1993).

Mylonitic foliation in the Linville Fallsgranite is cut by cataclasite zones (Figure 7).These zones consist of randomly orientedfragments of mylonitic granite in a very fine-grained matrix of quartz, epidote, chlorite, andopaque minerals. Much of the matrix materialis dark red-brown to black, and apparentlyconsists of very fine-grained iron oxide.Cataclasite zones do not appear to have beenoverprinted by subsequent ductile deformation.This sequence of deformation contrasts withrelationships between brittle and ductile defor-mation in the Long Ridge fault and related shearzones north of the Grandfather Mountain win-dow. Adams and Su (1996) noted that cataclasiswas overprinted by ductile deformation in theseshear zones, and suggested that these relation-ships could be explained by alternating episodes

of brittle and ductile deformation enhanced byhigh fluid pressures within the Long Ridge fault.

CHILHOWEE GROUP QUARTZITE

Quartzites and arkosic quartzites of theChilhowee Group were strongly sheared in theLinville Falls shear zone to form feldspar-quartz-sericite mylonites. Mylonitic foliation isdefined by fine-grained white mica and layers ofrecrystallized quartz. Opaque minerals (pre-dominantly magnetite) are concentrated alongthe foliation. Alkali feldspar (microcline) formssmall, rounded porphyroclasts with tails ofquartz and white mica. Recrystallized quartzgrains range from 10 µm to 100 µm, averagingabout 30 µm.

There are some quartzites between theLinville Falls shear zone and the Tablerock faultthat are not mylonitic. These rocks possess low-grade regional metamorphic fabric with weaklydefined foliation and lineation. Relict sedimen-tary features are preserved in some of these less-deformed rocks. Shearing within the Tablerockthrust sheet is most intense near its boundingfaults (Figure 3).

SUMMARY OF MICROSTRUCTURES ANDCONDITIONS OF DEFORMATION IN THELINVILLE FALLS SHEAR ZONE

Mylonitic rocks in the Linville Falls areaall exhibit similar deformation features. K-feldspars were deformed predominantly bybrittle fracture with minor recrystallizationalong cracks and grain boundaries. Plagioclasegrains possess bent or kinked twin lamellae andundulose extinction. These features indicatethat feldspars deformed by a combination ofmicrocataclasis and intracrystalline slip (Tullis,1983; Tullis and Yund, 1987). Quartz deformedby crystal-plastic mechanisms, with microstruc-tures suggestive of deformation by dislocationcreep accommodated by dynamic recrystalliza-tion (White, 1976, 1977; Urai et al., 1986). The

Figure 7. Photomicrograph (plane light) of myloniticLinville Falls granite cut by cataclasite zones. Field ofview is ~6.5 mm wide.

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features noted above are consistent with mid- toupper greenschist facies conditions (~350-450°C) of deformation in the mylonitic rocks of theLinville Falls shear zone (e.g. Voll, 1976;Tullis, 1983; Simpson, 1985). Additionally, themineral assemblages present in the mylonitesreflect greenschist facies metamorphism. Simi-lar estimates for conditions of deformation inthe Linville Falls fault have been reported bySchedl (1985, 1988), Wojtal and Mitra (1988),and Trupe (1989b).

In weakly sheared Linville Falls granite,the abundant K-feldspars may have inhibitedpenetrative foliation development. Alterna-tively, the lack of penetrative mylonitic foliationin the less deformed granite may be the result oflow strain within the granite block, whilemylonitization was more intense along marginsof the block. In any case, the majority of thegranite is strongly mylonitic.

Cataclasite zones in the Linville Fallsgranite suggest deformation under differentconditions than those noted above. Cataclasiscould have been initiated by an influx of fluids

into the Linville Falls shear zone, thus reducingthe effective pressure (Hubbert and Rubey,1959; Etheridge et al., 1984)). Alternatively,cataclastic deformation may represent overprint-ing by brittle deformation as the Blue Ridgethrust complex reached relatively shallowcrustal levels during late stages of thrusting.

KINEMATIC ANALYSIS

Bryant and Reed (1970) interpreted theLinville Falls fault as a top-to-northwest thrustfault, and interpreted mineral stretching linea-tions in the sheared rocks to represent the direc-tion of tectonic transport. Sense of movementderived from microscopic and mesoscopickinematic indicators verifies this interpretation.

Examples of microscopic kinematicindicators from the Linville Falls shear zone andmylonitic Chilhowee Group quartzites areshown in Figure 8. Asymmetric porphyroclastsare present, and yield top-to-northwest shearsense. Dimensional preferred orientation inrecrystallized quartz grains and composite

a bFigure 8. Kinematic indicators in the Linville Falls shear zone. Field of view is ~6.5 mm wide in both photomicro-graphs. a) Photomicrograph (crossed polars) of Chilhowee-derived mylonite (sample 8968) 1 meter below the LinvilleFalls granite. Sample lies below granite slice shown in Figure 4. Top-to-northwest shear sense is shown by σ-typefeldspar porphyroclast at bottom of photograph, composite planar fabric, and dimensional preferred orientation ofrecrystallized quartz grains. b) Photomicrograph of ultramylonite from the Linville Falls shear zone (sample 90-009) 1meter above the Linville Falls granite contact shown in Figure 4. Top-to-northwest shear sense is shown by σ-typefeldspar porphyroclasts. A small brittle microfault in upper left corner cuts mylonitic foliation and gives top-to-north-west sense of shear.


60

planar fabric (shear bands, S-C fabric) also givetop-to-northwest sense of shear.

Quartz c-axes were measured in samplesfrom the Linville Falls shear zone, Linville Fallsgranite, and Chilhowee-derived mylonite (Fig-ure 9). Quartz c-axes in mylonite from theLinville Falls shear zone one meter above theLinville Falls granite describe a single girdledistribution with maxima near the pole tofoliation, and parallel to the Y-direction. Thequartz c-axis fabric is consistent with basal andprism slip in the <a> direction (Lister andDornsiepen, 1982; Schmid and Casey, 1986). Asample of Linville Falls granite yields a diffusecrossed-girdle pattern. The fabric skeleton is

consistent with top-to-northwest shear sense, asare other kinematic indicators within the granite(e.g., asymmetric porphyroclasts, dimensionalpreferred orientation in quartz, composite planarfabrics). Chilhowee-derived mylonite one meterbelow the Linville Falls granite exhibits aweakly defined pattern of c-axes similar to theshear zone mylonite above, with point maximanear the pole to foliation, consistent with top-to-northwest sense of shear. Additional analyses ofquartz c-axis orientations in the Linville Fallsshear zone and Tablerock thrust sheet (Trupe,1989b, 1997) also indicate top-to-northwestshear sense.

Figure 9. Equal area, lower hemisphere projections of quartz c-axes from the Linville Falls shear zone. All are XZsections, plotted relative to vertical east-west foliation and horizontal mineral stretching lineation (boxes). Contourinterval is 1% / 1% area. Sample 90-010 is Linville Falls shear zone mylonite 2 meters above the Linville Fallsgranite. Sample 94-02 is Linville Falls granite; sample occurs approximately midway between Linville Falls shearzone mylonites and Chilhowee Group quartzites. Sample 8968 is Chilhowee mylonite shown in Figure 8a. Allgive top-to-northwest shear sense. Sample 94-02 yields a crossed-girdle distribution; fabric skeleton asymmetry isconsistent with interpreted sense of shear.

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DISCUSSION AND CONCLUSIONS

The base of the Blue Ridge thrust com-plex comprises a thick ductile shear zone thatcontains tectonic blocks or slices of variouslithologies in a matrix of sheared basementrocks. Mylonites were deformed undergreenschist facies conditions, and all containkinematic indicators consistent with top-to-northwest sense of shear. At Linville Falls, aslice of alkali feldspar granite overliesChilhowee Group metaquartzites of theTablerock thrust sheet. This granite is probablya slice of Neoproterozoic Beech Granite.Mylonites at the base of the Linville Falls shearzone were overprinted by cataclastic deforma-tion fabrics. Field observations depicted sche-matically by closely spaced form lines in thecross section of Figure 3 suggest that theTablerock thrust sheet is strongly sheared at itsbase and near the Linville Falls shear zone.Indeed, the volume of mylonitic rocks aboveand below the Linville Falls fault as originallydefined, and the similarities in structural andmicrostructural features in the Linville Fallsshear zone and underlying Tablerock thrustsheet may indicate that shearing associated withemplacement of the Blue Ridge thrust complexextends to the base of the Tablerock thrust sheet,at least in the vicinity of Linville Falls. Bryantand Reed (1970) interpreted the Tablerock thrustsheet as a tectonic slice incorporated duringthrusting of Blue Ridge crystalline rocks overrocks of the Grandfather Mountain window.

In previously reported studies, geologistsfailed to recognize the magnitude of the LinvilleFalls shear zone; hence, their interpretations arebased upon incomplete information. In particu-lar, studies that have attempted to explain thevariation in thickness of the Linville Falls faultfrom Linville Falls to exposures north of theGrandfather Mountain window (e.g. Wojtal andMitra, 1988; Newman and Mitra, 1993) areflawed, because they did not recognize theconsiderable width of the Linville Falls shear

zone. For example, Wojtal and Mitra (1988)described the Linville Falls fault at LinvilleFalls as an intensely deformed zone 10 to 15meters thick, with most of the displacementconfined to a 1-meter-thick mylonite zone alongthe fault. Newman and Mitra (1993) stated thata variation in fault thickness of >60 timesoccurs over a 20 kilometer distance from south-east to northwest along the Linville Falls fault,and attempted to explain this change by volumeloss and fluid-rock interaction during shearing.While fluid-rock interaction was undoubtedlyimportant in the deformation of the LinvilleFalls shear zone, a model that requires a largevolume loss is unnecessary in this case, as thereis no significant variation in thickness of theshear zone between Linville Falls and BannerElk. Additionally, Newman and Mitra (1993)assumed that the protolith for fault zonemylonites at Banner Elk was a quartz-dioritegneiss, which is exposed 500 meters above zonethey interpreted to be the fault. The quartz-diorite gneiss was shown to be an exotic slicewithin the Linville Falls shear zone by Adams(1990b) and Adams and Su (1996). Hence, atboth Linville falls and Banner Elk, Wojtal andMitra (1988) and Newman and Mitra (1993) notonly failed to recognize the significant thicknessof mylonite, they also overlooked the occur-rence of exotic slices in the Linville Falls shearzone. Thus their interpretations are based uponincomplete field data. The Linville Falls shearzone is hundreds of meters thick both west andnorth of the Grandfather Mountain window, andthe well-exposed laminated ultramylonite atLinville Falls is a high strain zone at the base ofthe shear zone. An alternate interpretation ofthis ultramylonite is that the ductility contrastbetween relatively competent mica-poor alkalifeldspar granite in the hanging wall and thequartz-sericite rocks in the footwall resulted in ahigh strain zone between the two units.

The conclusions about the features of theLinville Falls shear zone and their implicationsare as follows. (1) The base of the Blue Ridge


62

thrust complex comprises a thick mylonite zoneproduced under greenschist facies conditions,probably during the early Alleghanian orogeny.(2) Ductile fabrics of the mylonites have beenoverprinted by cataclastic fabrics. (3) Fluidflow perhaps contributed to mylonitization andcataclasis. Evidence for fluid involvement inthe Linville Falls shear zone includes hydrolysisof feldspars, retrogressive metamorphism ofsheared amphibolites and mylonitic hornblendegneisses, and local silicification of the LinvilleFalls granite. (4) The ductile fabrics probablyformed as the Blue Ridge thrust complex devel-oped at relatively great depth. (5) Near theGrandfather Mountain window, the LinvilleFalls shear zone appears to overprint earlierformed mylonite zones between thrust sheetswithin the complex (Goldberg et al., 1989).This suggests that the Linville Falls shear zonerepresents late movement of the Blue Ridgethrust complex along its base. (6) The largevolume of mylonitic rocks associated with theTablerock thrust sheet suggests that it accommo-dated high shear strain and essentially acted as alarge shear zone, or as part of the Linville Fallsshear zone. Mylonitization of is pervasivethroughout the Tablerock thrust sheet, butcataclasites have not been observed in theserocks or in the Tablerock fault at the base of thethrust sheet (Trupe, 1997). Boyer (1992)suggested that the southern portion of theLinville Falls fault was active early in thedeformation history, and that the Tablerockthrust formed later under generally brittleconditions of deformation and cut up-section tomerge with the northern Linville Falls fault. Inthis interpretation, the Tablerock fault is an out-of-sequence thrust, and is not truncated by theLinville Falls shear zone. In contrast, Bryantand Reed (1970) state that the Tablerock fault iscut by the Linville Falls fault, and is thereforeolder. This issue remains unresolved. Theexistence of a Tablerock shear zone, however,could explain Boyer’s interpretation that the twofaults merged during their deformation history.

Tectonic blocks within the Linville Fallsshear zone consist of Chilhowee Groupmetasandstone, granite bodies, and exotic rocks(e.g., Adams, 1990a, 1990b; Trupe et al., 1990;Adams and Trupe, 1991; Adams and Su, 1996).(7) The Linville Falls granite is one of thesetectonic blocks, and may be a slice of BeechGranite, or possibly the Crossnore or BrownMountain granites. The Beech and Crossnoregranites both occur in the hanging wall of theLinville Falls fault. The Stone Mountain faultsystem has been interpreted to represent emer-gence of the Linville Falls fault north of theBeech Granite (e.g., Bryant and Reed, 1970;Harris et al., 1981; Boyer and Elliott, 1982),thus permitting the Beech Granite to locallydefine the base of the Blue Ridge thrust com-plex and to provide blocks to the shear zone.The Brown Mountain Granite is in the footwallof the Tablerock fault, which makes it geometri-cally difficult for it to have provided slices tothe Linville Falls shear zone. However, if thebase of the Tablerock thrust sheet is simply asheared unconformity rather than a thrust faultwith significant displacement (an explanationrejected by Bryant and Reed, 1970, p. 106), theBrown Mountain Granite could provide footwallslices to the Linville Falls shear zone. Alterna-tively, if the Brown Mountain Granite wasoriginally farther southeast than its presentlocation, the Linville Falls granite could be afootwall slice of Brown Mountain Granitecarried along the base of the Linville Falls shearzone prior to emplacement of the Tablerockthrust sheet.

During Alleghanian thrusting, (8) theBlue Ridge thrust complex was emplaced alongthe Linville Falls shear zone. During the lateAlleghanian orogeny, cataclastic fabrics over-printed ductile fabrics in mylonites at the baseof the complex. Cataclastic deformation couldresult from an influx of fluids leading to reduc-tion of effective stress (e.g., Hubbert and Rubey,1959; Etheridge et al., 1984), or a change instrain rate at ductile depths (e.g., Hobbs et al.,

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1986). Alternatively, cataclastic deformationcould be due to movement of the Blue Ridgethrust complex into relatively shallow crustallevels. Cataclasite zones were themselvesoverprinted by ductile deformation in other~300 Ma faults north of the Grandfather Moun-tain window (Adams and Su, 1996), suggestingfluctuation of conditions (e.g., fluid pressure,temperature, strain rate) during later stages ofmovement. The ~300 Ma Rb-Sr whole-rockdate for the Linville Falls fault (Van Camp andFullagar, 1982; Schedl et al., 1992) indicatesthe age of closure of the Rb-Sr isotopic system,probably near the end of mylonite formation.Cataclasis may have been coeval with closure,or slightly younger. Adams and Su (1996)obtained similar ~300 Ma ages on mylonitesoverprinted by cataclastic fabrics in the LongRidge fault, which they interpreted to be anAlleghanian feature associated with the LinvilleFalls shear zone. (9) The significance of theLinville Falls fault (sensu stricto) is that itdefines the base of the Linville Falls shear zone,and records the last stages of movement of thecomposite Blue Ridge thrust complex.

ACKNOWLEDGEMENTS

Financial support for this research was providedby a University of North Carolina-Chapel HillDepartment of Geology Martin Fellowship,grants from the Geological Society of America,and NSF Grant EAR-9316033. The U.S. Na-tional Park Service is acknowledged for permis-sion to collect samples and for field support.Structural data were plotted using Stereonet 4.9(R. W. Allmendinger). Geochemical analyseswere performed by XRAL Laboratories.Samples of Beech, Brown Mountain, andCrossnore Granites for geochemical analysiswere provided by Qi Su. I wish to thank J.R.Butler, K.G. Stewart and M.G. Adams forhelpful discussions, and L.A. Raymond and J.Hibbard for thorough reviews of this manu-script.

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Keith, A., 1903, Description of the Cranberry quadrangle,North Carolina-Tennessee: United States GeologicalSurvey Geologic Atlas, Folio 90.

Keith, A., 1905, Description of the Mount Mitchellquadrangle, North Carolina-Tennessee: United StatesGeological Survey Geological Atlas, Folio 124.

Kerrich, R., and Allison, I., 1978, Flow mechanisms inrocks: Microscopic and mesoscopic structures, andtheir relation to physical conditions of deformation inthe crust: Geoscience Canada, v. 5, p. 109-118.

King, P. B., and Ferguson, H. W., 1960, Geology ofnortheasternmost Tennessee: United States Geologi-cal Survey Professional Paper 311, 160 p.

Kretz, R., 1983, Symbols for rock-forming minerals:American Mineralogist, v. 68, p. 277-279.

Lister, G. S., and Dornsiepen, U. F., 1982, Fabric transi-tions in the Saxony granulite terrain: Journal ofStructural Geology, v. 4, p. 81-92.

Mallard, L. D., Adams, M. G., and Stewart, K. G., 1994,Kinematic analysis of a possible suture in thesouthern Appalachians, northwestern North Carolina:Geological Society of America Abstracts withPrograms, v. 26, p. 25-26.

Mies, J. W., 1991, Planar dispersion of folds in ductileshear zones and kinematic interpretation of fold-hinge girdles: Journal of Structural Geology, v. 13, p.281-297.

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Mies, J. W., and Trupe, C. H., 1989, Kinematic analysis inthe Blue Ridge thrust complex, northwestern NorthCarolina and adjacent states: Geological Society ofAmerica Abstracts with Programs, v. 21, p. 50.

Mies, J. W., Butler, J. R., and Goldberg, S. A., 1987,Kinematic analysis of mylonite zones in the BlueRidge, northwestern North Carolina and adjacentTennessee: Geological Society of America Abstractswith Programs, v. 19, p. 118.


O’Hara, K., 1992, Major- and trace-element constraintson the petrogenesis of a fault-relatedpseudotachylyte, western Blue Ridge province, NorthCarolina: Tectonophysics, v. 204, p. 279-288.

Schedl, A., 1985, Deformation processes in the LinvilleFalls fault zone: Geological Society of AmericaAbstracts with Programs, v. 17, p. 134.

Schedl, A., 1988, Possible geochemical implications ofsynthrusting growth of feldspar in the Linville Fallsfault, North Carolina: Geological Society of AmericaAbstracts with Programs, v. 20, p. 180.

Schedl, A., McCabe, C., Montanez, I., Fullagar, P. D., andValley, J., 1992, Alleghanian regional diagenesis: Aresponse to the migration of modified metamorphicfluids derived from beneath the Blue Ridge-Piedmontthrust sheet: Journal of Geology, v. 100, p. 339-352.

Schmid, S. M., and Casey, M., 1986, Complete fabricanalysis of some commonly observed quartz c-axispatterns, in Hobbs, B. E., and Heard, H. C., editors,Mineral and Rock Deformation, Laboratory Studies:American Geophysical Union Geophysical Mono-graph 36, p. 263-286.

Sibson, R.H., 1977, Fault rocks and fault mechanisms:Journal of the Geological Society of London, v. 133,p. 191-213.

Simpson, C., 1985, Deformation of granitic rocks acrossthe brittle-ductile transition: Journal of StructuralGeology, v. 7, p. 503-511.

Stose, G. W., and Ljungstedt, O. A., 1932, Geologic mapof the United States: United States GeologicalSurvey, scale 1:2,500,000.

Stose, G. W., and Stose, A. J., 1944, The ChilhoweeGroup and the Ocoee Series of the southern Appala-chians: American Journal of Science, v. 242, p. 367-390, 401-416.

Su, Q., 1994, Geochronological and isotopic studies ofLater Proterozoic Crossnore-type plutons in thesouthern Appalachian Blue Ridge, North Carolinaand Tennessee: Ph.D. Dissertation, University ofNorth Carolina at Chapel Hill, 158 p.

Su, Q., and Fullagar, P.D., 1995, Rb-Sr and Sm-Ndisotopic systematics during greenschist faciesmetamorphism and deformation: Examples from thesouthern Appalachian Blue Ridge: Journal ofGeology, v. 103, p. 423-436.

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Trupe, C. H., 1989a, Kinematic analysis and deformationmechanisms in mylonites from the Blue Ridge thrustcomplex, northwestern North Carolina: GeologicalSociety of America Abstracts with Programs, v. 21, p.62.

Trupe, C. H., 1989b, Microstructural analysis ofmylonites from the Blue Ridge thrust complex,northwestern North Carolina: MS Thesis, Universityof North Carolina at Chapel Hill, 101 p.

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Trupe, C. H., and Adams, M. G., 1991, Cataclasticdeformation in the Linville Falls shear zone, westernNorth Carolina Blue Ridge: Geological Society ofAmerica Abstracts with Programs, v. 23, p. 141.

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ABSTRACT

Ultramafic rock bodies are scatteredthroughout the Ashe Metamorphic Suite, bothnortheast and southwest of the GrandfatherMountain Window in North Carolina. Of sevenbodies examined northeast of the window andsix bodies exposed southwest of the window,only two lack the older anhydrous, Eclogite (?)or Granulite (?) Facies olivine-pyroxene-chromite mineral assemblage recognized inmany of these rock bodies. Two amphibolitefacies assemblages characterized by hydrousamphibole and phyllosilicate phases, such astremolite, anthophyllite, and chlorite, are recog-nized in all of the bodies; and one or two retro-grade Greenschist Facies assemblages arereflected by serpentine-dominated assemblagesin more than half of the bodies. A greenschistfacies assemblage consisting of antigorite +chlorite + magnetite may reflect cooling afterthe peak of regional metamorphism, but themeaning of the textural relationship betweenthis assemblage and an apparently youngeranthophyllite-bearing assemblage and thethermodynamic implications of that relationshipare unresolved. Overall, the sequence of assem-blages reflects decreasing T with time. Theparticular sequence of phase assemblagesdeveloped during retrograde metamorphism isdetermined by the specific paths through P-T-aH2O-time space.

All of the ultramafic bodies describedherein occur in the Toe Terrane (= Spruce PineThrust Sheet), which lies within the Blue Ridgesection of the Piedmont Zone of the SouthernAppalachian Orogen. The structural settings,contact relations, and metamorphic histories ofthe ultramafic rock bodies indicate that theywere emplaced tectonically into the enclosingrocks and were then metamorphosed and de-formed during Taconic high pressure, Amphibo-lite Facies events. Later these rocks underwentretrograde metamorphism during AcadianAmphibolite Facies and Alleghenian (andperhaps younger) Greenschist Facies events.The structural setting, shape, distribution, andassociation with other exotic rock masses,including eclogite, at the western edge of thePiedmont Zone, suggest that these alpine ultra-mafic rocks represent mantle and ophiolitefragments within an accretionary, tectonicmelange that marks the Taconic suture.

INTRODUCTION

Alpine ultramafic rock bodies are widelydistributed, although not abundant, in the East-ern Blue Ridge Belt of North Carolina (Pratt andLewis, 1905; Hess, 1955; Larrabee, 1966; Misraand Keller, 1978; Abbott and Raymond, 1984;Raymond, 1995, p. 667ff.). Rocks in thesebodies contain several assemblages of minerals,most of which represent retrograde metamor-phic events that occurred over Phanerozoictime. The oldest assemblages are dominated by

PETROLOGY AND TECTONIC SIGNIFICANCE OF ULTRAMAFIC ROCKS NEAR THEGRANDFATHER MOUNTAIN WINDOW IN THE BLUE RIDGE BELT, TOE TERRANE,

WESTERN PIEDMONT ZONE, NORTH CAROLINA

Loren A. Raymond and Richard N. Abbott, Jr.Department of Geology

Appalachian State UniversityBoone, NC 28608

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olivine and orthopyroxene and contain Cr-spinel. Younger assemblages contain hydrousphases such as tremolite, anthophyllite, talc, andchlorite and the youngest assemblages, typi-cally, are dominated by serpentine minerals.

The ultramafic rock bodies occur in twoterranes within the Blue Ridge Belt (Fig. 1).The eastern terrane is called the Toe Terrane(Raymond, 1987; Raymond et al., 1989) or theSpruce Pine Thrust Sheet (Goldberg et al., 1989;Adams et al., 1995), and is included in theJefferson Terrane of Horton et al. (1989; Rankinet al., 1993)(Figure 1). The western belt isreferred to as the Cullowhee Terrane (Raymond,1987; Raymond et al., 1989), the Mars HillTerrane (Bartholomew and Lewis, 1988), or thePumpkin Patch Thrust Sheet (Goldberg et al.,1989; Adams et al., 1995)(Figure 1). Thewestern belt has also been included in theJefferson Terrane of Horton et al. (1989; Rankinet al., 1993), and both terranes are assigned tothe western part of the Piedmont Zone of

Hibbard and Samson (1995). Herein, we referto the the eastern and western belts as the Toeand Cullowhee terranes, respectively.

Near the Grandfather Mountain Window,both to the northeast and the southwest, ultrama-fic rock bodies occur in the Toe Terrane. Farthersouthwest, ultramafic rock bodies also occur inthe Cullowhee Terrane. We have reviewed theliterature on, examined, and here report onseven representative bodies northeast and sixbodies southwest of the Grandfather MountainWindow, all in the Toe Terrane, in order tocharacterize the petrography, petrologic history,structure, and tectonic significance of the ultra-mafic rocks in this region.

LOCATIONS, STRUCTURAL RELA-TIONS, AND ROCK TYPES

The locations of the 13 bodies of ultrama-fic rock selected for this study are shown inFigure 2. All of the ultramafic rock bodies

Lit holo gic Belt sof Brown et al.,1 98 5

Terranes of Raymond ,1 98 7; Raymond etal., 19 89

Terrane & Massifsof Bart holomewandLewis, 19 88

----

Thrust Sheet s ofGold berg et al. , 1 98 9;Adams et al., 1 99 5

Terrane & Massif ofHort on et al.,19 8 9;Hort on et al., 1 99 1

Zones of Hib bardand Samson, 1 99 5

Alligat or Back Fm.,Ashe Met amorphicSuite, & related rocks

To e Terrane Spruce PineThrust Sheet

Jef f erson Terrane

Laurent ia, inc ludingFrench Bro ad Massif

Cumberland Zone(Laurent ian rocks)

West ern Part oft he Piedmont Zone

Granodiorit ic Gneissand associated ro cks

Migmat it ic Bio t it e-Hornblende Gneiss &Biot it e Gneiss(migmat it ic)

Biot it e Granit ic Gneiss& associated ro cks(including GreatSmoky Group)

(= Laurent ianBasement rocks)

----

Cullowhee Terrane

Sherwood Terrane

Wat aug a Massif

Mars Hill Terrane

Elk River Massif

Pard ee PointThrust Sheet

Pumpkin Pat chThrust Sheet

Beech Mount ainThrust Sheet

Figure 1. A comparison of recently assigned names for various structural and lithologic belts of rock within theNorth Carolina Blue Ridge Belt.

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occur in the Ashe Metamorphic Suite of the ToeTerrane, although earlier some were assignederroneously to the Cullowhee Terrane (shown asmigmatitic biotite and migmatitic biotite-hornblende gneiss) on the Geologic Map ofNorth Carolina (Brown et al., 1985). Bodiesnorth of the Grandfather Mountain Windowinclude the Edmonds Ultramafic Body, on theNorth Carolina-Virginia border; the NathansCreek, Warrensville, and Todd bodies exposedin or near communities of those names; and theGreer Hollow and two Rich Mountain bodies

exposed at those geographic locations. To thesouthwest, ultramafic rocks include the Frank,Spruce Pine, and Newdale bodies, near commu-nities of those names, and the Blue Rock Road,Woody, and Day Book bodies (Appendix Acontains locations of both northern and southernbodies).

In most cases, the contacts of the ultrama-fic bodies are concealed by soil, but in a fewcases, the contacts are exposed. Typically, rockson one or both sides of inferred contacts areexposed within a few meters of the contact. In

Figure 2. Generalized geologic map of northwestern North Carolina showing the structural belts bounded by majorthrust faults and the locations of the ultramafic rock bodies described in this report (base map after Brown et al., 1985;and Adams et al., 1995). BZ = Brevard Fault Zone; CT = Cullowhee Terrane; GMW = Grandfather Mountain Win-dow; pCa = Ashe Metamorphic Suite; pCab = Alligator Back Metamorphic Suite; ST = Sherwood Terrane; TT = ToeTerrane. Ultramafic bodies are BRR = Blue Rock Road; DB = Day Book; E = Edmonds; F = Frank; GH = GreerHollow; RM = Rich Mountain; N = Newdale; NC = Nathans Creek; SPN = Spruce Pine North; T = Todd; W = Woody;WV = Warrensville.

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no case is there preserved evidence of contactmetamorphism, such as higher grade mineralassemblages, anhydrous phase assemblages, orlocalized migmatitic rocks adjacent to theultramafic bodies in the surrounding countryrocks. Where contacts are exposed, they aretypically sharp and are marked by lower grade,hydrated mineral assemblages in the enclosingrocks. In some cases, schists with hydrousphase assemblages that suggest metasomaticreaction between ultramafic rock and adjacentschist and gneiss occur as a “blackwall” orreaction zones at the contact. No apophyses ordikes are known to extend from the ultramaficbodies into the surrounding rocks.

Each of the bodies selected for this studyreveals some features typical of BlueRidgeultramafic rock bodies. General geologic mapsof selected ultramafic bodies are shown inFigure 3 and others have been illustrated else-where (e.g., Stose and Stose, 1957; Swanson,1981).

The Edmonds Ultramafic Body, mappedby Stose and Stose (1957) as a body of ultrama-fic rock complexly interfingered with rocks ofthe Lynchburg Gneiss (now included in theAshe Metamorphic Suite), occurs at the bound-ary between the Ashe Metamorphic Suite andthe Alligator Back Metamorphic Suite(Espenshade et al., 1975: Abbott and Raymond,1984). The body contains serpentinite, chlorite-talc schist, chlorite-tremolite schist,anthophyllite-tremolite schist, actinolite-chloriteschist, and chlorite dunite (Stose and Stose,1957; Scotford and Williams, 1983; this report).The details of internal structure and composi-tional variations are unmapped, but the bodyappears to contain a foliation parallel to theregional foliation. In addition, local folds in thefoliation indicate post-metamorphic deformationof the body. Outcrops of this body occur alongsmall road cuts in the vicinity of Edmonds onHighway 18 near the Virginia border, as well aslocally to the north between Edmonds, NorthCarolina and Blue Ridge Mills, Virginia.

The Nathans Creek Ultramafic Body (=Shatley Springs body of Scotford and Williams,1983) has a clearly folded map pattern (Fig. 3a;Rankin et al., 1972), indicating post-emplace-ment deformation. Foliation generally parallelsthat in the surrounding country rocks, which aremica schist, mica gneiss, and hornblende schistand gneiss. The body itself is composed ofchlorite-tremolite schist and only locally con-tains remnant olivine (Scotford and Williams,1983; this report). Good exposures of theNathans Creek body occur northeast ofJefferson, North Carolina, along Highway 221at Nathans Creek, as well as on private proper-ties to the north.

Petrographically, the Warrensville Ultra-mafic Body is similar to the Nathans Creekbody, in that chlorite-tremolite schists dominate(Scotford and Williams, 1983; this report).Local porphyroclasts of olivine occur in somesamples. The body and the foliation within itgenerally parallel the foliation in the enclosingrocks, which consist of hornblende schist andgneiss (Jones, 1976). No internal structuralfeatures or petrographic variations have beenmapped to date. The most easily accessibleoutcrop of the Warrensville Ultramafic Bodyoccurs in a road cut, below a private yard,across from the park in the community ofWarrensville, North Carolina, located on High-way 88, west of West Jefferson, North Carolina.

The Todd Ultramafic Body, like theNathans Creek body, is folded with the sur-rounding rocks (Rankin et al., 1972). The bodylies within a part of the Ashe Metamorphic Suitedominated by hornblende gneiss and schist, butcontaining some mica schists. Retrogradedmetadunite occurs locally within this body,harzburgite is reported by Larrabee (1966), andtalc-tremolite schist is present, but the dominantrock type is chlorite-tremolite schist (Scotfordand Williams, 1983; this report). As is the casewith other bodies in this region, no detailedmaps of internal structural or petrologic featureshave been published. The body is exposed in

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Figure 3. Maps of selected ultramafic rock bodies (note varying scales). A - Greer Hollow Ultramafic Body. Darkunit at southern end is magnetite-garnet-chlorite schist (in part, based on Raymond, 1995, p. 672). B - NathansCreek Ultramafic Body (after Rankin et al., 1972). C - Hoots Ultramafic Body, Rich Mountain. Opx =orthopyroxenite bands. D - Frank Ultramafic Body . Pd = peridotite bands (modified from Vrona, 1979).


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minor outcrops along Highway 194 and onadjoining roads, between Todd, North Carolinaand the Baldwin and Fleetwood communities.

At Greer Hollow, the ultramafic bodycontains foliations and folds that are extensionsof the same structures in the enclosing rocks(Raymond et al., 1988; Raymond, 1995; thisreport; Fig. 3b). Anthophyllite, tremolite, andchlorite schists dominate in the body, but amappable mass of porphyroblastic magnetite-garnet-chlorite schist occurs at the southern endof the body. Minor porphyroclasticorthopyroxene-olivine-tremolite-anthophyllite-chlorite schist contains harzburgitemicrolithons. Late serpentinite veins cut thenorthern end of the body. Enclosing rocks arepredominantly plagioclase-hornblende rocks,including gneiss, schist, and semi-schist. At thesoutheastern edge of the body, a small mass ofpelitic schist crops out, separated from theultramafic body by an approximately one meter-thick layer of actinolite-talc schist anddiablastite. The Greer Hollow Ultramafic Bodyoccurs near Todd, North, Carolina, on privateproperty near Greer Hollow Road.

Two of several ultramafic bodies depictedby Pratt and Lewis (1905) and Larrabee (1966)in the Rich Mountain area north of Boone,North Carolina were later described by Hearn etal. (1977), Callahan et al. (1978), and Swanson(1980) as the Rich Mountain Ultramafic Bodies.These two bodies, the Hoots Ultramafic Bodyand the McNeil Ultramafic Body, occur withinhornblende schist and gneiss and are locatednear outcrops of retrograded eclogite describedin this volume by Abbott and Raymond (1997).Local float blocks of granitoid pegmatite andmetadiorite suggest the presence of such rockswithin the surrounding gneisses and schists(Swanson, 1980).

The Hoots Ultramafic Body is a small(96m X 98m) mass of dunite with local, discon-tinuous layers of orthopyroxenite and associatedharzburgite (Fig. 3c). Minor veins of talc-amphibole diablastite and serpentine cut the

dunite. The contacts are concealed by soil,colluvium, and block streams, but outcrops ofhornblende schist and gneiss occur a few metersfrom them. Foliation and layering in the bodyare approximately parallel to the foliation in thesurrounding hornblende schist and gneiss whichhave an average foliation trend of N30°W (Fig.3c).

The McNeil Ultramafic Body is a verypoorly exposed body composed primarily ofdunite and represented in the field by a fewoutcrops and float blocks (locally piled up bythe landowner). Minor harzburgite is reportedhere (Swanson, 1980), but the relationshipbetween the harzburgite and dunite is notknown. A “blackwall” alteration zone com-posed of tremolite-anthophyllite-talc schist isexposed in a pond bank at the north end of thisultramafic body. All other contacts are entirelyconcealed by soil. The surrounding countryrocks consist of hornblende schist and gneiss.The approximate north-south trend of theMcNeil Ultramafic Body parallels the generallynorth-south trend of foliation in the hornblendeschist and gneiss. The body is located onprivate property near Junaluska Road, east ofZionville, North Carolina and permission mustbe obtained to visit the body.

South of the Grandfather MountainWindow, the Frank Ultramafic Body is exposedin the community of that name (Brobst, 1962;Bluhm, 1976). The body forms a folded massthat trends generally east-west (Fig. 3d). Bluhm(1976) mapped the contacts of the body as thrustfaults. Foliation and layering within the bodyare not parallel to either the lithologic contactsin the country rock or the foliation in that rock,but Bluhm (1976) demonstrated that earlierfabrics in the ultramafic body have been over-printed by later fabrics produced by subsequentdeformation of both the country rock and theultramafic body. The east-west trend of thebody roughly parallels the N80°E trend of adominant foliation. Petrographically, the FrankUltramafic Body consists of dunite, five

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“layers” of lherzolite, and associated retrogrademetamorphic assemblages (Bluhm, 1976; thisreport). At the margins of the body, along veinsand locally throughout the body, the dunite andlherzolite have been retrogressively metamor-phosed to hydrous phase assemblages contain-ing phases such as anthophyllite, tremolite, talc,chlorite, and serpentines. The enclosing rocks incontact with the ultramafic body consist ofhornblende gneiss, garnet-hornblende schist,hornblende-biotite schist, garnet-muscoviteschist, and muscovite schist of the Ashe Meta-morphic Suite (Bluhm, 1976). A quarry fordunite has been developed and periodicallyactivated within the center of the Frank body,and permission to visit the quarry must beobtained from the current mine owners. Out-crops of both of the principal rock types, duniteand lherzolite, and common metamorphicoverprint assemblages, exist along the slopes ofthe North Toe River channel below Highway19E at the road junction in Frank.

The Spruce Pine North Ultramafic Bodyis the northern of two bodies of ultramafic rockexposed north of a shopping center located insouthern Spruce Pine on Highway 226 (Brobst,1962). The body contains at least three retro-grade mineral assemblages overprinted on anearly dunite assemblage of chromite + olivine.The various retrograde assemblages are charac-terized by minerals such as anthophyllite,tremolite, talc, chlorite, magnesite, and serpen-tine. The body lies within a unit of the AsheMetamorphic Suite dominated by mica schistand gneiss, and containing minor hornblendegneiss; and it occurs just north of a body ofgranitoid igneous rock of the Spruce PinePlutonic Suite (Brobst, 1962; McSween et al.,1991; Wood, 1996). The long dimension of thebody parallels the general ENE trend of thelocal foliation in the enclosing rocks. Goodexposures of this ultramafic body occur lessthan 100m east of Highway 226 on a side road

leading east, but they are now part of the land-scaping of a residence that was constructed onthe body. Sampling is not recommended.

The Newdale Ultramafic Body is locatedabout 0.6km north of Highway 19E alongHighway 80 in the Newdale community(Brobst, 1962; Vrona, 1979; this report). Roadcuts, a water-filled quarry on the east, and aborrow pit on the west side of Highway 80provide numerous exposures of the main rock,dunite, and retrograde veins cross-cutting it(permission is required to enter the quarry). Theveins and associated zones of replacementminerals contain various assemblages composedof talc, anthophyllite, tremolite, chlorite, magne-site, serpentine, and magnetite. The ultramaficbody is folded and shows three phases of defor-mation, including one that predates the twodeformation events present in the surroundingrocks (Vrona, 1979). The long dimension ofthis elliptical ultramafic body parallels thegeneral N60°E trend of the foliation in theenclosing rocks, but internal fabrics in thedunite show no relationship to this foliation(Vrona, 1979). Contacts with the enclosingrocks are not well exposed, but Vrona (1979)shows the contact as a fault that cross-cuts localstructural and lithologic trends. The ultramaficbody is entirely enclosed in hornblende gneissand schist (amphibolite), but is in contact withan anthophyllite schist layer also enclosedwithin the hornblende gneiss and schist (Vrona,1979).

The Blue Rock Road Ultramafic Body isexposed south of Newdale in a road cut on BlueRock Road, just south of Blue Rock Branch,near a hill top east of the South Toe River. Thebody occurs within a mica schist and gneiss unitof the Ashe Metamorphic Suite and is cut at oneend by a pegmatite of the Spruce Pine PlutonicSuite (Brobst, 1962). The rock here isanthopyllite-tremolite-talc-chlorite schist. Thenortheast trend of the long dimension of thebody parallels the trend of the local foliation.


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The Woody Ultramafic Body is one ofseveral bodies in the Thunderstruck Knob areaof Yancey County, North Carolina and is locatedabout 4.5 miles (7.2 km) north of Micaville(Brobst, 1962; Kingsbury and Heimlich, 1978;Raymond, 1995, p. 667ff.). The body is a 300mlong, elliptical mass composed of dunite,harzburgite, retrograded lherzolite, various+anthophyllite+tremolite+talc-chlorite schists,and serpentinites. Mineralogical layering is notevident in the body, in spite of the presence ofperidotites within the dunite (Kingsbury andHeimlich, 1978). The long dimension of theelliptical body parallels the northeasterly trendof foliation in the enclosing rock, but the ends ofthe body are discordant to that foliation(Kingsbury and Heimlich, 1978). The enclosingrock consists of two mica-quartz-feldspargneiss. Southeast of the ultramafic body, agranitoid pegmatite has intruded country rocksparallel to the foliation (Kingsbury andHeimlich, 1978). Inasmuch as the WoodyUltramafic Body is on private property, permis-sion to examine the body must be obtained fromthe owner.

The Day Book Ultramafic Body is argu-ably the most studied ultramafic body in theNorth Carolina Blue Ridge Belt (e.g., Hunter,1941; Kulp and Brobst, 1954; Brobst, 1962;McCormick, 1975; Swanson and Raymond,1976; Swanson, 1981; Swanson et al., 1985;Raymond, 1995, p. 667ff., Swanson, 1996). Thebody is located north of Burnsville in the com-munity of Day Book. The Day Book dunite hasbeen mined for many years and the map patternof the contacts has changed somewhat as miningprogressed. In the 1970s, two bodies of rock,one substantially smaller than the other, existedend-to-end (Swanson, 1981), but recentlydeveloped exposures suggest that these bodiesare connected at depth. At 1970s depths ofexposure, the Day Book Ultramafic Bodyconsisted of dunite, orthopyroxene-bearingdunite, chromite dunite, olivine chromitite, andchromitite. Layers of chromitite, some of which

are folded, are the most recognizable layers inthe dunite, but they are discontinuous and aninternal structural and lithologic map of thebody(ies) has not been produced. At minelevels in the mid-1990s, orthopyroxene wasabundant enough in some quarry bottom expo-sures to make the exposed rock a harzburgite. Atabular granitoid pluton bounds the Day BookUltramafic Body on the northwest and dikesextend from this pluton into and across theultramafic body. Especially along these dikemargins, but also in zones extending into thedunites and harzburgites, hydrous retrogrademetamorphic assemblages containing vermicu-lite, phlogopite, talc, magnesite, anthophyllite,tremolite, chlorite, antigorite, lizardite, andother minerals replace the anhydrous minerals.The enclosing rocks at Day Book are micagneiss and schist, hornblende gneiss and schist,and pegmatitic granodiorite, but the ultramaficbody is only in contact with the first and last ofthese rock types (Swanson, 1981). Foliations inthe mica schists and gneiss trend northeast, butlocally wrap around the margins of the ultrama-fic body, the long dimension of which trendsnortheast parallel to the average foliation of theschist. Attitudes of veins in the dunite areunrelated to this northeast trend (Swanson,1981). Permission to enter the Day Bookquarries is required by the company that ownsthe body.

In summary, the ultramafic rock bodiesoccur in contact with hornblende schists andgneisses, mica schists, mica gneisses, relatedsemi-schists, and granitoid igneous rocks. Insome cases in the Toe Terrane, notably at DayBook, the granitoid rocks intrude the ultramaficrocks, constraining the emplacement of theultramafic rocks to a pre-Taconic age (> 395m.y.) (Swanson,1981; Kish, 1989; McSween etal., 1991). No evidence of magmatic emplace-ment of the ultramafic rock bodies exists anddetailed structural studies (Bluhm, 1976;Kingsbury and Heimlich, 1978; Vrona, 1979)indicate that the ultramafic bodies were

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emplaced as solid masses, bringing with themrelict structures and textures. The long dimen-sions of the bodies, in map views, generallyparallel the foliation direction within the sur-rounding schists and gneisses. At Edmonds,Warrensville, Rich Mountain, Greer Hollow,Frank, and Newdale, it is clear that majorfoliation development, folding events, or bothoccurred after emplacement of the ultramaficbodies, because parallel structures occur in thesurrounding rocks and the ultramafic rocks.

PETROLOGY

Previously published studies of BlueRidge ultramafic rocks in North Carolina arenumerous (e.g., Pratt and Lewis, 1905; Carpen-ter and Phyfer, 1969; Neuhauser and Carpenter,1971; Dribus et al., 1976; Yurkovich, 1977;Kingsbury and Heimlich, 1978; Misra andKeller, 1978; Swanson, 1980; 1981; 1996; 1997;Raymond and Swanson, 1981; Huneycutt andHeimlich, 1980; Astwood et al., 1982;McElhaney and McSween, 1983; Scotford andWilliams, 1983; Hatcher et al., 1984; Abbottand Raymond, 1984; Raymond and Abbott,1985; Raymond et al., 1988; Raymond, 1995,Ch. 31; Tenthorey et al., 1996). Previouslypublished syntheses of the data available on theultramafic and associated rocks (e.g., Abbottand Raymond, 1984; Raymond, 1995, p. 571ff.,p. 667ff.) suggest that at least five recrystalliza-tion events are represented by the variousmineral assemblages and textures in the ultra-mafic rocks and at least three of these eventshave affected the associated schists andgneisses. Three major problems lack a consen-sus: (1) the sequence and nature of crystalliza-tion events; (2) the timing, nature, and signifi-cance of serpentinization; and (3) the nature ofthe tectonic history implied by the petrologicdata.

PETROGRAPHIC OBSERVATIONS

The petrographic data from the 13 ultra-mafic bodies described briefly above reveal fivemineral associations:

(A-1) olivine + pyroxene(s) + chromite;(A-2) olivine + pyroxene(s) + chromite +

tremolite + chlorite;(A-3) + tremolite + anthophyllite +

chlorite + talc + phlogopite +magnesite + magnetite + garnet;

(A-4) antigorite + magnetite + chlorite +talc + tremolite; and

(A-5) lizardite + chrysotile + magnetite +chlorite + talc + tremolite +magnesite.

The age sequence of these assemblages gener-ally appears to be A-1 = oldest to A-5 = young-est. Some textural data suggest, however, thatan A-4 association, consisting of antigorite +chlorite + magnetite, formed locally between theformation of assemblages A-2 and A-3 (e.g.,Swanson et al., 1985; Raymond et al., 1988;Swanson, 1996). The significance of thesetextural relationships is controversial (seebelow).

Table 1 lists the associations found in eachof the bodies described above. Some bodies,such as the Nathans Creek Ultramafic Body,contain mineral assemblages representing onlyone of the associations, whereas others, such asthe Day Book Ultramafic Body contain mineralassemblages representing all five associations.Northeast of the immediate vicinity of theGrandfather Mountain Window; anthophyllite,tremolite, and chlorite bearing assemblages areapparently more abundant, reflecting morepervasive, lower grade, fluid-richer metamor-phism of the ultramafic rocks in this region.

The ultramafic rocks described herein,and in the original studies on which this reviewis based, show petrofabrics and textural relation-ships like those in many other deformed ultra-mafic rocks (e.g., Evans and Tromsdorff, 1974;Ave Lallement, 1976; O’Hanley, 1996), ultra-


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mafic xenoliths (Basu, 1977; Pike andSchwarzman, 1977), and experimentally de-formed ultramafic rocks (e.g., Carter and AveLallement, 1970; Zeuch and Green, 1984). Thegeneral sequence of deformation in the ToeTerrane rocks, as revealed by rock textures, isalso consistent with sequences in those de-formed ultramafic materials. The texturalsequence, from oldest to youngest textures, is:

(T-1) coarse porphyroclastic texture; replacedby

(T-2) equigranular-mosaic and equigranular-tabular texture; replaced by

(T-3) fine porphyroclastic and equigranular tabular texture; cross-cut andreplaced by

(T-4) lepidoblastic, nematoblastic-foliated,and diablastic textures of several genera-tions.

TABLE 1. OBSERVED METAMORPHIC MINERAL ASSOCIATIONS IN TOE TERRANEULTRAMAFIC ROCK BODIES________________________________________________________________________________________________

ASSOCIATIONSLOCALITIES A-1 A-2 A-3 A-4 A-5 SOURCES

EDMONDS X X X Scotford & Williams (1983);Raymond (1995)

NATHANS X Scotford & Williams (1983); This paper CREEK

WARRENSVILLE X X Scotford & Williams (1983); This paper

TODD X X X Scotford & Williams (1983); This paper

GREER HOLLOW X X X X Raymond et al. (1988); Raymond (1995)

RICH MOUNTAIN McNeil X X X X Swanson (1980); This paper Hoots X X X X Swanson (1980); This paper

FRANK X X X Bluhm (1976); This paper

SPRUCE PINE X X X X This paper

NEWDALE X X X (?) X Vrona (1979); This paper

BLUE ROCK ROAD X This paper

WOODY X X X X Kingsbury & Heimlich (1978);Raymond (1995); This paper

DAY BOOK X X X X X Swanson (1981; and others, 1985);Raymond (1995)

___________________________________________________________________________________________

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The first three textures are exhibited by rockswith association A-1 and A-2 mineral assem-blages, whereas rocks of the fourth texture type(T-4) contain one or more mineral assemblagesof the associations A-3, A-4, or A-5.

As noted above, the assemblage antigorite+ magnetite + chlorite (of A-4) appears to haveformed in some rocks (e.g., at Day Book) beforean anthophyllite-bearing assemblage of A-3.Texturally, the antigorite-rich A-4 assemblage inthese rocks occurs as rims on and along frac-tures in olivine grains that occur inequigranular-mosaic texture in dunite. Limitedreplacement of the olivine along grain bound-aries and fractures results in a microscopicmesh-texture. The anthophyllite in these rocksoccurs in both apparently compatible relation-ships and as cross-cutting grains with the ser-pentine that forms the mesh-texture. The ambi-guity of the textural relations allows two likelypossibilities: (1) that the anthophyllite and themesh-textured assemblage serpentine + chlorite+ magnetite formed under approximately thesame conditions, which would require eitherthat the fields of stability of antigorite andanthophyllite overlap (1a), or that anthophyllitecrystallized metastably outside of its field ofstability (1b); or (2) that the mesh-texturedassemblage serpentine + chlorite + magnetiteformed after the development of theanthophyllite, but did not affect the grain mar-gins of the anthophyllite in a noticeable way.Thermodynamic considerations make veryunlikely a third alternative, (3) that theantigoritic mesh-textured assemblage developedat a lower temperature than the anthophyllite-bearing assemblage, and was overprinted by theanthophyllite-bearing assemblage at a highertemperature (Swanson et al., 1985). The issueis important, because, if the antigoritic mesh-textures did form at lower T than theanthophyllite-bearing assemblage, the thermalhistory of the mountain belt would include atleast one cycle of cooling and reheating, whichhas significant tectonic implications.

Clearly, ambiguities and complexitiesinvolving associations A-3 and A-4 exist, as dosome involving association A-5. The issue ofthe appearance of antigorite-bearing mesh-textured assemblages will be discussed furtherin the section below on phase relationships.Association A-3 has a complex set of mineralsand the order of appearance of these mineralsvaries, depending on particular local conditions.Association A-4 is characterized by the appear-ance of serpentine minerals cross-cutting tremo-lite and other earlier formed minerals.Nematoblastic to lepidoblastic veins of serpen-tine minerals may cut one another. AssociationA-5 includes — in addition to the more commonMg-silicate phases — diablastic, nematoblastic,radial-planar, and lepidoblastic veins containingminerals such as aragonite and garnierite. It isclear from the available data, including thenumber of cross-cutting and partial replacementtextures, that textural equilibrium was onlyattained locally.

PHASE RELATIONSHIPS AND THE PETRO-GENETIC GRID

The phase relationships in the systemMgO-SiO2-H2O have been debated and studiedextensively since the pioneering work of Bowenand Tuttle (1949). Works by Chernosky et al.(1985) and Day et al. (1985) among others,have increased our present level of understand-ing with regard to this system. Reactions involv-ing the stability of serpentine and anthophylliteare of particular importance to the ultramaficrock bodies described herein. Important contro-versies involving the MgO-SiO2-H2O systemrelate to the shape and size of the stability fieldsof anthophyllite and antigorite in P-T space(Day and Halback, 1979; Day et al., 1985;Chernosky et al., 1985; Spear, 1993, Ch. 13;O’Hanley, 1996). At issue are the field-limitingreactions:


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talc + enstatite = anthophyllite(reaction 1)

andantigorite = olivine + talc + water(reaction 2) (Fig. 4).

Depending on the locations of these reactions,the stability fields of anthophyllite and serpen-tine may or may not overlap. In the simplesystem MgO-SiO2-H2O, fields of stability forantigorite and anthophyllite do not overlap (Fig.4).

The occurrence of serpentine andanthophyllite together in apparent equilibriumassemblages in some hydrated dunites suggeststhat an overlap of stability fields is possible(Sanford, 1977). Such an overlap provides onepossible explanation for the ambiguousanthophyllite and mesh-texture antigoritecombinations noted above. The possibility ofoverlapping stability fields is supported by somecalculations and experiments (Delaney andHelgeson, 1978; Day et al., 1985), but not byothers, especially those for the simple system(Spear, 1993, Ch. 13; see Fig. 4).

Figure 4. Petrogenetic grid for ultramafic rocks showing selected reaction curves involving either antigorite (Atg)or anthophyllite (Ath). Other minerals are brucite (Brc), enstatite (En), forsterite olivine (Fo), quartz (Qtz), andtalc (Tlc). Solid lines for the pure (“simple”) system. Dashed lines show changes resulting from the addition ofiron to the system (also indicated by an (Fe) after the Ath symbol.)(Curves from Evans and Guggenheim, 1988;Spear, 1993).

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Additional components in the system andvariables affecting the system (e.g., the presenceof Fe or Cl, or a low aH2O) increase the likeli-hood of overlapping stability fields ofanthophyllite and antigorite at high pressures.Changing phase associations resulting fromchanges in XH2O have not yet been worked out,but the predicted effect of adding a relativelysmall amount of iron to the system is to reducethe temperature of reaction 1 by up to 140°C,creating an overlap in the stability fields atpressures of about 0.03 Gpa to 0.9 Gpa (Fig. 4;Evans and Guggenheim, 1988; O’Hanley,1996). Other components that could have anaffect on antigorite stability are chlorine andfluorine. Clearly, bulk composition and thecomposition of the fluid phase during metamor-phism exercise significant control over mineralstability.

That anthophyllite can exist outside itsfield of stability at low T is obvious — it per-sists at the Earth surface. From a thermody-namic standpoint, metastable persistence iseasily explained as a consequence of there beingan inadequate amount of heat during uplift (andcooling) to drive the breakdown reaction. Incontrast, from a thermodynamic perspective, itis unreasonable to expect antigorite to persist athigher T than its field of stability as defined byreaction 2 (alternative (3) above), insofar asexcess heat would be abundant and readilyavailable to drive the reaction during a progradethermal event.

Considering the textural evidence and thethermodynamic considerations, it is likely thatone or another of the possibilities (1a) — stablecrystallization, (1b) — metastable crystalliza-tion, or (2) — sequential crystallization withoutalteration of anthophyllite grain margins, pro-vides an explanation for the presence ofanthophyllite in antigorite-bearing rocks. Inas-much as anthophyllite persists at the Earthsurface in euhedral crystals (i.e., there are noobvious signs of instability), the only certaintyis that anthophyllite grew somewhere on a path

of declining P-T, between the high T stabilitylimit of anthophyllite and very low T at Earthsurface conditions.

DISCUSSION and CONCLUSION

The stability fields of anthophyllite andserpentine are important in unraveling themetamorphic and tectonic history of the ultra-mafic rocks and the terranes in which theyoccur. Two hypotheses for the metamorphichistory of the Toe Terrane in the eastern BlueRidge Belt have been offered in recent years.One hypothesis posits alternating thermal peaksand valleys over time, with each thermal peakrepresenting an orogenic (deformational andmetamorphic) episode, specifically the Taconic,Acadian, and Alleghenian orogenies (Butler,1973; Hatcher et al, 1979; McSween andHatcher, 1985; Swanson et al., 1985; Goldbergand Dallmeyer, 1997). The alternative hypoth-esis suggests that there was monotonic coolingover time from a single thermal peak (reachedduring the Taconic Orogeny) in the OrdovicianPeriod, with each mineral association markingthe passage (in space) of the rocks through areaction boundary reflecting specific P-T condi-tions (Abbott and Raymond, 1984; Raymond,1995, p. 572ff.). To date, no definitive P-T-tcurves have been determined that would allowdiscrimination between these possibilities.

If the stability fields of serpentine(antigorite) and anthophyllite for the eventsaffecting the Toe Terrane were known, thepetrologic data available would allow discrimi-nation between the hypotheses. If it could beshown that the stability fields of antigorite andanthophyllite did not overlap and that develop-ment of association A-4 at lower temperaturesoccurred before development of association A-3 at higher temperatures, then the hypothesis ofButler (1973) would be supported. Thermody-namically, this scenario is quite unlikely. Ifinstead, the stability fields of anthophyllite andantigorite did overlap due to a combination of


80

factors such as the presence of iron, a lowaH2O, or the presence of F or Cl in the fluidphase, or if anthophyllite grew metastably atlower T than its stability field would suggest forequilibrium conditions, then the monotoniccooling hypothesis is favored. The textural andthermodynamic data favor the latter possibilitiesat this time.

In both the Butler (1973) and Abbott andRaymond (1984) hypotheses, association A-1and texture T-1 may represent mantle or lowercrustal, pre-Taconic (pre-Ordovician) metamor-phism and deformation, whereas mineral asso-ciation A-2 and textures T-2 and T-3 wouldlikely represent Taconic metamorphism anddeformation. In the Butler hypothesis, mesh-textured association A-4 would represent a post-Taconic cooling of the terrane below tempera-tures of Devonian Acadian metamorphism. Inthe Abbott and Raymond hypothesis, post-Taconic cooling below the P-T level of Acadianmetamorphism did not occur. Acadian metamor-phism (association A-3) and deformation (repre-sented by T-4 textures) at higher T is followedby lower T, post-Taconic cooling in the Butlerhypothesis, whereas in the Abbott and Raymondhypothesis, Acadian metamorphism and defor-mation fall on an overall cooling trend from theTaconic thermal high. Alleghenian metamor-phism and deformation are represented byassociations A-4 and A-5 and textures of the T-4type in both hypotheses.

Clearly, discrimination between thehypotheses requires an unequivocal demonstra-tion that the rocks either did or did not coolbelow the temperatures of Acadian metamor-phism prior to that event and after the peakTaconic metamorphic event. While the thermo-dynamic argument mitigates against such acooling event, no direct evidence in support ofeither hypothesis is available.

The multiple textures and mineral assem-blages in the ultramafic rocks record events notas clearly recorded in rocks less sensitive tomineralogical changes induced by a fluid phase.

If the textures and mineral assemblages can belinked definitively to specific events, we canbetter understand the tectonic history of theSouthern Appalachian Orogen. In particular,discrimination between the Butler (1973) andAbbott and Raymond (1984) hypotheses willallow us to understand whether the Acadianevent in the Southern Appalachian Orogen wasa more passive uplift event marked by localplutonism and reequilibration of metamorphicrocks or a more active tectonic event involvingregional deformation accompanied by reheatingto new, post-Taconic thermal high.

Together, the structural data, metamor-phic histories of the ultramafic rocks, and thescattered distribution of ultramafic rock bodiesand rare eclogites (Adams et al., 1995; Abbottand Raymond, 1997-this volume), marbles (e.g.,Stose and Stose, 1957; Brobst, 1962), andmetaconglomerates (Rankin et al., 1972) withina terrain dominated by hornblende schist,hornblende gneiss, mica schist and gneiss, andquartzo-feldspathic semischist suggest that thepremetamorphic condition of the Toe Terranewas that of a tectonic melange (Raymond andSwanson, 1981; Abbott and Raymond, 1984;Raymond et al., 1989). The ultramafic rocksand some mafic rocks experienced metamor-phism before or during emplacement into thesurrounding rocks, but before the TaconicOrogeny and its associated amphibolite faciesmetamorphism (Vrona, 1979; Abbott andRaymond, 1984; Adams et al., 1995; Tenthoreyet al. 1996). Acadian amphibolite facies meta-morphism and Alleghenian greenschist faciesmetamorphism and deformational events associ-ated with both of these later orogenies left theirimprint on the rocks, locally resulting both inthe total replacement of earlier formed fabricsand minerals and in folding and foliation devel-opment. Fluids injected by plutonism during theAcadian orogeny caused extensive replacementlocally.

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The series of tectonic and metamorphicevents implied by the data obtained near theGrandfather Mountain Window and elsewherein the Toe and Cullowhee terranes is consistentwith collisional tectonic models for the southernAppalachian Orogen (e.g., Odom and Fullagar,1973; Hatcher, 1978; Abbott and Raymond,1984; Adams et al., 1995; Tenthorey et al.,1996). Pre-Taconic subduction and metamor-phism of ophiolitic ultramafic and mafic oceaniccrust in an accretionary complex, followed byTaconic emplacement, deformation, and meta-morphism is consistent with both the metamor-phic and the structural histories of the ultramaficrocks. The presently exposed accretionarymelange of ultramafic rocks, eclogites, andassociated rocks marks the Taconic suture.Acadian events were dominated by thermal andfluid induced changes, and there appears to havebeen local fabric development, as well. Finally,the Alleghenian Orogeny deformed and upliftedrocks across much of the orogen, while retro-gressively metamorphosing rocks in the BlueRidge core of the orogen.

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Hibbard, J.P., van Staal, C.R., and Cawood, P.A.(eds.), Current Perspectives in the Appalachian-Caledonian Orogen: Geological Association ofCanada Special Paper 41, p. 191-205.

Horton, J.W., Drake, A.A., and Rankin, D.W., 1989,Tectonostratigraphic terranes and their Paleozoicboundaries in the central and southern Appalachians:in Dallmeyer, R.D. (ed.), Terranes in the Circum-Atlantic Paleozoic orogens: Geological Society ofAmerica Special Paper 230, p. 213-245.

Horton, J.W., Drake, A.A., Jr., Rankin, D.W., andDallmeyer, R.D., 1991, Preliminarytectonostratigraphic map of the central and southernAppalachians: U.S. Geological Survey Map I-2163.

Huneycutt, F.M. and Heimlich, R.A., 1980, Petrology ofthe Balsam Gap Dunite, Jackson County, NorthCarolina: Southeastern Geology, v. 21, p. 251-260.

Hunter, C.E., 1941, Forsterite olivine deposits of NorthCarolina and Georgia: North Carolina Division ofMineral Resources Bulletin Number 41, 117p.

Jones, T.Z., 1976, Petrography, structure, and metamor-phic history of the Warrensville and JeffersonQuadrangles, southern Blue Ridge, northwesternNorth Carolina: Ph.D. Dissertation, Miami Univer-sity, Oxford, Ohio, 130 p.

Kingsbury, R.H., and Heimlich, R.A., 1978, Petrology ofan ultramafic body near Micaville, Yancey County,North Carolina: Southeastern Geology, v. 20, p. 33-46.

Kish, S.A., 1989, Paleozoic thermal history of the BlueRidge in southwestern North Carolina — Constraintsbased upon mineral cooling ages and the ages ofintrusive rocks: Geological Society of AmericaAbstracts with Programs, v. 21, No. 3, p. 45.

Kulp, J.A., and Brobst, D.A., 1954, Notes on the duniteand the geochemistry of vermiculite at the Day Bookdeposit, Yancey County, N.C.: Economic Geology, v.49, p. 211-220.

Larrabee, D.M., 1966, Map showing distribution ofultramafic and intrusive mafic rocks from northernNew Jersey to eastern Alabama: U.S. GeologicalSurvey Map I-476.

McCormick, G.R., 1975, A chemical study ofKammererite, Day book body, Yancey County, NorthCarolina: American Mineralogist, v. 60, p. 924-927.

McElhaney, M.S., and McSween, H.Y., Jr., 1983, Petrol-ogy of the Chunky Gal Mountain mafic-ultramaficcomplex, North Carolina: Geological Society ofAmerica Bulletin, v. 94, p. 855-874.

McSween, H.Y., Jr. and Hatcher, R.D., Jr., 1985,Ophiolites (?) of the southern Appalachian BlueRidge: in Woodward, N.E. (ed.), Field Trips in theSouthern Appalachians: University of Tennessee,

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Department of Geological Sciences, Studies inGeology 9, p. 144-170.

McSween, H.Y., Jr., Speer, A., and Fullagar, P.D., 1991,Plutonic rocks, in Horton, J.W., and Zullo, V.A.(eds.), The Geology of the Carolinas: The Univer-sity of Tennessee Press, Knoxville, p. 109-129.

Misra, K.C., and Keller, F.B., 1978, Ultramafic bodies inthe southern Appalachians: A Review: AmericanJournal of Science, v. 278, p. 389-418.

Neuhauser, K.R., and Carpenter, J.R., 1971, A structuraland petrographic analysis of the Bank’s Creek (NorthCarolina) serpentine: Southeastern Geology, v. 13, p.151-165.

Odom, A.L. and Fullagar, P.D., 1973, Geochronologic andtectonic relationships between the Inner Piedmont,Brevard Zone, and Blue Ridge Belts, North Carolina:American Journal of Science, v. 273-A, p. 133-149.

O’Hanley, D.S., 1996, Serpentinites: Oxford UniversityPress, New York, 277p.

Pratt, J.H. and Lewis, J.V., 1905, Corundum and thePeridotites of North Carolina: North CarolinaGeological Survey, v. 1, 464p.

Pike, J.E.N., and Schwarzman, E.C., 1977, Classificationof textures in ultramafic xenoliths: Journal ofGeology, v. 85, p. 49-61.

Rankin, D.W., Espenshade, G.H., and Neuman, R.B.,1972, Geologic map of the west half of the Winston-Salem Quadrangle, North Carolina, Virginia, andTennessee: USGS Map I-709-A.

Rankin, D.W., Drake, A.A., Jr., and Ratcliffe, N.M., inRankin, D.W. (ed.), 1993, Proterozoic rocks east andsoutheast of the Grenville Front: in Reed, J.C., Jr,Bickford, M.E., Houston, R.S., Link, P.K., Rankin,D.W., Sims, P.K., and van Schmus, W.R. (eds.),Precambrian: Conterminous U.S.: GeologicalSociety of America, The Geology of North America,v. C-2, p. 378-422.

Raymond, L.A., 1987, Terrane amalgamation in the BlueRidge Belt, Southern Appalachian Orogen: Geologi-cal Society of America Abstracts with Programs, v.19, p. 125.

Raymond, L.A., 1995, Petrology: The Study of Igneous,Sedimentary, and Metamorphic Rocks: Wm.C.Brown Publ., Dubuque, 742p.

Raymond, L.A., and Abbott, R.N., Jr., 1985, Metamor-phism of alpine ultramafic rocks, North CarolinaBlue Ridge: I - Observed metamorphic histories:Geological Society of America Abstracts withPrograms, v. 17 , p 695.

Raymond, L.A., Leatherman, L.E., Vance, K., Cook, T.and Abbott, R.N., Jr., 1988, The Greer HollowUltramafic Body, eastern Blue Ridge Province, NorthCarolina: Petrography, field relations, and implica-tions for metamorphic history of the Blue Ridge

Province: Geological Society of America Abstractswith Programs, v. 20, No. 4, p. 310.

Raymond, L.A., and Swanson, S.E., 1981, Deformationhistory of ultramafic rocks, Blue Ridge Belt, southernAppalachian Mountains: Correlations with Appala-chian tectonic events: Geological Society of AmericaAbstracts with Programs, v. 19, p. 125.

Raymond, L.A., Yurkovich, S.P., and McKinney, M.,1989, Block-in-matrix structures in the NorthCarolina Blue Ridge belt and their significance forthe tectonic history of the southern AppalachianOrogen, in Horton, J.W., Jr. & Rast, N. (eds.),Melanges and Olistostromes of the U.S. Appala-chians: Geological Society of America Special Paper228, p. 195-215.

Sanford, R.F., 1977, The coexistence of antigorite andanthophyllite in ultramafic rocks: Geological Societyof America Abstracts with Programs, v. 9, p. 1154-1155.

Scotford, D.M., and Williams, J.R., 1983, Petrology andgeochemistry of metamorphosed ultramafic bodies inportion of the Blue Ridge of North Carolina andVirginia: American Mineralogist, v. 68, p. 78-94.

Spear, F.S., 1993, Metamorphic Phase Equilibria andPressure-Temperature-Time Paths: MineralogicalSociety of America, Washington, D.C., 799p.

Stose, A.J. and Stose, G.W., 1957, Geology and mineralresources of the Gossan Lead District and adjacentareas in Virginia: Virginia Division of MineralResources, Bulletin 72, 233p.

Swanson, S. E., 1980, Petrology of the Rich Mountainultramafic bodies and associated rocks, WataugaCounty, North Carolina: Southeastern Geology, v.21, p. 209-225.

Swanson, S.E., 1981, Mineralogy and petrology of theDay Book Dunite and associated rocks, westernNorth Carolina: Southeastern Geology, v. 22, p. 53-77.

Swanson, S.E., 1996, Metamorphic history of ultramaficrocks from the Spruce Pine Thrust Sheet, WesternNorth Carolina: Geological Society of AmericaAbstracts with Programs, v. 28, no. 2, p. 46.

Swanson, S.E., 1997, Mineral compositions from ultrama-fic rocks from the Spruce Pine Thrust Sheet, westernNorth Carolina: Geological Society of AmericaAbstracts with Programs, v. 29, No. 3, p. 72.

Swanson, S.E., Naney, M.T., and Sturnick, M.A., 1985,Polyphase metamorphism in the Blue Ridge ofwestern North Carolina: Evidence from the DayBook Dunite Geological Society of America Ab-stracts and Programs, v. 17, p. 138.

Swanson, S.E. and Raymond, L.A., 1976, Alteration ofthe Day Book dunite, Spruce Pine District, western


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North Carolina: Geological Society of AmericaAbstracts with Programs, v.8, p. 282.

Tenthorey, E.A., Ryan, J.G., and Snow, E.A., 1996,Petrogenesis of sapphirine-bearing metatroctolitesfrom the Buck Creek ultramafic body, southernAppalachians: Journal of Metamorphic Petrology, v.14, p. 103-114.

Vrona, J.P., 1979, Structure of the Newdale ultramafite,Yancey County, North Carolina: M.S. Thesis,Southern Illinois University, 115 p.

Wood, P.A., 1996, Petrogenesis of the Spruce PinePegmatites, North Carolina: M.S. Thesis, VirginiaPolytechnic Institute and State University, 99 p.

Yurkovich, S.P., 1977, The Corundum Hill Dunite, MaconCounty, North Carolina: Southeastern Geology, v. 19,p. 55-68.

Zeuch, D.H., and Green, H.W., II, 1984, Experimentaldeformation of a synthetic dunite at high temperatureand pressure. I. Mechanical behavior, opticalmicrostructure and deformation mechanisms:Tectonophysics, v. 110, p. 233-262.

APPENDIX A. LOCATIONS OF ULTRA-MAFIC BODIES

Edmonds Ultramafic Body. Cumberland Knob7 1/2' Quadrangle, VA–NC. Small out-crops are present in road cuts along Hwy18, within 0.5 km, both east and west ofEdmonds and south of the NC/VA stateline. Also, ultramafic rock underlies theBig Spring Church vicinity northeast ofEdmonds on VA Road 613. Road cutsprovide access, but much of the body lieson private property.

Nathans Creek Ultramafic Body. Jefferson, NC7 1/2' Quadrangle. The unincorporatedcommunity of Nathans Creek is located inthe east central part of the JeffersonQuadrangle on Highway 221. TheNathans Creek Ultramafic Body is ex-posed approximately 0.6 km north of theHwy 221 – Nathans Creek School Roadjunction on Hwy 221. Small outcropsoccur on the east side of the road and boldoutcrops occur on the west at the spot

where a Nathans Creek tributary and Hwy221 start to parallel one another. The roadcuts provide access, but much of the bodylies on private property.

Warrensville Ultramafic Body. Warrensville,NC 7 1/2’ Quadrangle. The WarrensvilleUltramafic Body is best exposed in a roadcut across from the park in the towncenter of Warrensville, which is located atthe Hwy 88–194 bifurcation approxi-mately 9 km northwest of West Jefferson,NC.

Todd Ultramafic Body. Todd, NC 7 1/2' Quad-rangle. The body is exposed in road cutsand field outcrops in Ashe County, north-east of the community of Todd, located onHwy 194. Exposures occur east of LaurelKnob Church at BM 3247 on Hwy 194and between Laurel Knob Gap and MillCreek Church.

Greer Hollow Ultramafic Body. Todd 7 1/2'Quadrangle. The Greer Hollow Ultrama-fic Body underlies a small stream valleyand the adjoining hill between the Ashe–Watauga County line and Greer HollowRoad, which leads north from Highway194 approximately 1 km west of the Toddexit from Hwy 194. This body is onprivate property and should not be visitedwithout permission of the landowner orlocal overseer.

Rich Mountain Ultramafic Bodies. Zionville,NC 7 1/2' Quadrangle.The McNeil Ultramafic Body is exposedin a pasture, on private property, in thedrainage depression immediately east ofBuckeye Knob on Cove Creek Ridge inthe SW 1/4 SW 1/4 of the quadrangle.The site is approximately 1 km south of

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the Junaluska Road – Potato Hill Roadjunction, located at approximately 36° 16'40" North latitude.

The Hoots Ultramafic Body is locatedsouth of the saddle between Wolf Ridgeand Cove Creek Ridge at about 36° 15’46" North latitude. Access is restrictedand requires permission to pass over atleast two parcels of private property.Recently, access has been denied by one ofthose two property owners.

Frank Ultramafic Body. Carvers Gap 7 1/2'Quadrangle, NC–TN. The Frank Ultra-mafic Body is located at the west edge ofthe Carvers Gap Quadrangle, south andwest of Hwy 19E, southwest of the com-munity of Frank, on the north end ofCopperas Bald, and along the North ToeRiver Banks. Copperas Bald and the minethere are on private property.

Spruce Pine North Ultramafic Body. SprucePine 7 1/2' Quadrangle, NC. The SprucePine North Ultramafic Body occurs on thehill immediately east of the junction ofGraveyard and Grassy creeks, east of Hwy226, in southern Spruce Pine, NorthCarolina. The body is exposed in roadcuts <100 m east of Hwy 226 on the roadthat leads to Carter Ridge.

Newdale Ultramafic Body. Micaville 7 1/2'Quadrangle, NC. The Newdale Ultrama-fic Body straddles Hwy 80 in the NewdaleCommunity, about 0.6 km north of theHwy 80–Hwy 19E junction. Exposuresoccur along the highway, in a pit west ofthe highway, and in a restricted access,water filled quarry east of the highway.

Blue Rock Road Ultramafic Body. Micaville 71/2' Quadrangle, NC. This small body islocated just north of the crest of the hill on

Blue Rock Road, south of Blue RockBranch, a few kilometers south of Hwy19E. The rock is exposed in a road cut onthe grade <100 m north of the crest of thehill.

Woody Ultramafic Body. Micaville 7 1/2'Quadrangle, NC. The Woody UltramaficBody is located at an elevation of about2700'+ 100’ on the northeast flank ofChestnut Mountain in the NW 1/4 of theMicaville Quadrangle. The body, and anadjoining body, cap the hills flanking asmall north draining creek line at 35° 58'30" N latitude. The property is privatelyowned and permission is required foraccess.

Day Book Ultramafic Body. Burnsville 7 1/2'Quadrangle, NC. The Day Book Ultrama-fic Body is located on Mine Fork, about1.5 km southeast of Day Book. The bodyhas been mined extensively and an obvi-ous mill is on the site. The body straddlesthe road here. Mine workings are locatednorthwest of the road along Mine Forkand a mine office is located east of theroad. Permission is required to enter themine.


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ABSTRACT

Pelitic and mafic rocks dominate the Asheand Alligator Back Metamorphic Suites north ofthe Grandfather Mountain window. New infor-mation on phase relationships in pelitic rocksallows re-interpretation of metamorphic gradi-ents in P and T. The gradient in P suggests thatthe highest metamorphic pressures wereachieved near the base of the Ashe MetamorphicSuite, immediately southwest and immediatelynorth of the Grandfather Mountain window.

The location of previously describedeclogites (ECG1) in the Ashe MetamorphicSuite southwest of the Grandfather Mountainwindow and the location of recently discovered,retrograded eclogite (ECG2) in the Ashe Meta-morphic Suite north of the Grandfather Moun-tain window are consistent with the highest-pressure zone, inferred from relationships in thepelitic rocks. We report here a preliminarydescription of the retrograded eclogite (ECG2)north of the Grandfather Mountain window.ECG2 is in the same structural position asECG1, i.e., near the base of the Ashe Metamor-phic Suite. ECG2 differs from ECG1, in twoimportant ways: (1) the presence of epidote inECG2, and (2) the sequence of retrogradechanges recognized in the mineral assemblage.Samples examined so far indicate thatomphacite in ECG2 has been completely re-placed by symplectitic diopside+plagioclase.The original (highest grade) mineral assem-blages in ECG2, however, are believed to have

been omphacite+garnet+quartz andomphacite+garnet+epidote+quartz. Most likelythe presence of epidote indicates a higherfugacity for O2 than in ECG1.

Textural relationships suggest the follow-ing retrograde paragenesis for ECG2:

(a) Omp(I) + Gar + Qtz, and Omp(I) +Gar + Epi + Qtz;

(b) Omp(II) + Gar + Epi + Qtz,(c) (Dio + Pla) + Gar + Epi + Qtz, where

(Dio + Pla) is symplectite,(d) (Dio + Pla) + Gar + Epi + Hnb + Qtz,(e) Pla + Gar + Epi + Hnb + Qtz.

Distinctive plagioclase + quartz coronas aroundgarnet were presumably formed by reaction ofgarnet with omphacite during the change fromassemblage (b) to assemblage (c). Similarcoronas around garnets in pyroxene-free horn-blende schist (e) suggest that large areas of themafic rocks in the Ashe Metamorphic Suite mayhave been retrograded from eclogite.

Observations support a growing body ofevidence for the following general conclusions:(1) The Ashe Metamorphic Suite has anensimatic origin. (2) The Ashe MetamorphicSuite is a subduction-related accretionarymelange. (3) This melange marks the Taconicsuture between the North American Craton andthe Inner Piedmont.

In a palinspastic reconstruction the thrustfault at the structural base of the Ashe Metamor-phic Suite appears to have intercepted thegreatest depths (i.e., the highest-P metamorphicrocks) beneath parts of the Ashe and AlligatorBack Metamorphic Suites now exposed imme-

PETROLOGY OF PELITIC AND MAFIC ROCKS IN THE ASHE ANDALLIGATOR BACK METAMORPHIC SUITES, NORTHEAST OF THE

GRANDFATHER MOUNTAIN WINDOW

Richard N. Abbott, Jr. and Loren A. RaymondDepartment of Geology

Appalachian State UniversityBoone, NC 28608

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diately north and immediately southwest of theGrandfather Mountain window. The greatestvolume of mafic rock is in these same parts ofthe Ashe Metamorphic Suite. We offer thepossibility that the nascent, subduction-related,basal thrust fault was deflected downward quitesimply by an obstacle, in the form of an iso-lated, mafic volcanic edifice on oceanic crust —a seamount.

INTRODUCTION

The Ashe Metamorphic Suite (AMS), theAlligator Back Metamorphic Suite (ABMS),

and their inferred lithostratigraphic equivalentsin the Blue Ridge Belt of the southern Appala-chian Orogen (Figure 1), were metamorphosedto the amphibolite facies of regional BarrovianFacies Series metamorphism during the TaconicOrogeny (Goldberg and Dallmeyer, 1997). TheAMS is exposed northeast and southwest of theGrandfather Mountain window (Rankin et al.,1973; Goldberg et al., 1989), where it forms aprincipal Neoproterozoic to Early Paleozoic (?)sequence of sedimentary and volcanic rocks inthe southern Blue Ridge Belt. The AMS struc-turally overlies Mesoproterozoic basement(including the Cranberry Mines Gneiss) and is

Figure 1. Metamorphic zones in the eastern Blue Ridge (modified from Drake et al., 1989). Bold lines are major thrustfaults separating the Blue Ridge Belt from the Valley and Ridge Province (V&R) to the northwest and the Inner Pied-mont (IP) to the southeast. Thrust faults within the Blue Ridge Belt separate the western Blue Ridge (stippled) from theeastern Blue Ridge, and also form the borders of the Grandfather Mountain window (GMW). The Ashe and AlligatorBack Metamorphic Suite are the principal constituents of the eastern Blue Ridge Belt. Three pseudo-invariant points areindicated by filled circles, where the garnet, staurolite, and kyanite zones meet.

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structurally overlain by the ABMS (Rankin etal., 1973; Fullagar and Odom, 1973; Fullagarand Bartholomew, 1983; Abbott and Raymond,1984). The Ashe and Alligator Back Metamor-phic Suites are confined to the Spruce Pinethrust sheet of Goldberg et al. (1989). TheCranberry Mines Gneiss is confined to theBeech Mountain thrust sheet of Goldberg et al.(1989). These thrust sheets structurally overliea deeper, basement-cover sequence of lowermetamorphic grade. The latter sequence in-cludes the Neoproterozoic Grandfather Moun-tain Formation and underlying MesoproterozoicBlowing Rock and Wilson Creek Gneisses,which are exposed in the Grandfather Mountainwindow (Figure 1; Bryant and Reed, 1970).

Major rock types in the AMS includehornblende schist and gneiss, pelitic schist, andquartzofeldspathic schist and gneiss. Thehornblende schist and gneiss are interpreted tobe metabasalt (e.g., Rankin, 1970; Misra andConte, 1991). Minor components of the suiteinclude eclogite (Willard and Adams, 1994;Adams et al., 1995) and ultramafic rocks. Theultramafic rocks are variously interpreted to beresidual mantle blocks, ophiolite fragments, orintrusions (e.g., Rankin et al., 1973; Abbott andRaymond, 1984; McSween and Hatcher, 1985;Wang and Glover, 1991). The ABMS is domi-nated by pelitic rocks, but also contains horn-blende schists and rare ultramafic rocks (Rankinet al., 1973; Conley, 1987).

Well to the southwest of the GrandfatherMountain window, the Mesoproterozoic (?)basement gneisses and schists are structurallyoverlain by a different group of rocks—gneissesand schists of the Neoproterozoic (?) GreatSmokey Group (Hadley and Nelson, 1971;Brown et al., 1985). These rocks are domi-nantly pelitic to quartzofeldspathic schists andgneisses, but slates and metaconglomerates alsooccur in the unit.

The allochthonous Neoproterozoic rocks,especially the AMS and ABMS, have beenaffected by at least three metamorphic and

deformational events (e.g., Butler, 1972, 1973;Abbott and Raymond, 1984; Adams et al.,1995). The major prograde event has generallybeen assigned to the Ordovician Taconic Orog-eny (Abbott and Raymond, 1984; Miller et al.,1997; Goldberg and Dallmeyer, 1997). Incom-plete retrograde recrystallization is widelydeveloped.

The objectives of this paper are: (1) toreview the metamorphic petrology of pelitic andmafic rocks in the AMS and ABMS northeast ofthe Grandfather Mountain window, (2) todiscuss new constraints on metamorphic condi-tions (P,T), and regional metamorphic gradientsin P and T, (3) to describe previously unre-ported, retrograded eclogites in the mafic rocksof the AMS, northeast of the Grandfather Moun-tain window, (4) to re-evaluate the P-T-t(t=time) path of the AMS, and (5) to discuss thelikely origin of the AMS.

PELITIC ROCKS

General statement. North of the Grandfa-ther Mountain window, pelitic rocks in the AMSare mainly muscovite schists. The essentialminerals are muscovite, quartz, and plagioclase.Where quartz and plagioclase dominate, therocks are properly called quartzofeldspathicschists or gneisses. Depending on the grade ofmetamorphism and bulk composition, otherminerals include various combinations ofbiotite, garnet, chlorite, staurolite, and kyanite.Accessory mineral included Fe-oxides, apatite,and tourmaline. The distribution of mineralassemblages is consistent with a BarrovianMetamorphic Facies Series.

Regional and detailed metamorphic maps(Figure 1) of the central Blue Ridge Belt(Hadley and Nelson, 1971; Espenshade et al.,1975; Brown et al., 1985; Abbott and Raymond,1984; McSween et al., 1989; Abbott et al., 1991;Butler, 1991) reveal three occurrences of anAFM (A=Al 2O3-Na2O-K2O-CaO, F=FeO,M=MgO; Thompson, 1957) pseudo-invariant

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point (intersection of isograds). Pseudo-invari-ant points of the type described here have notbeen recognized elsewhere, although Labotka(1981) has predicted their existence in theory,and thermodynamic calculations (Spear et al.,1995) indicate reasonable crustal conditions (P,T) for their existence. Thus, the southern BlueRidge Belt affords an opportunity to character-ize a possibly unique, relatively high-pressureregime of amphibolite facies metamorphism.

Each of the pseudo-invariant points islocated at the intersection of the three mappedisograds. The three isograds that define thepoint of intersection, correspond to the AFMreactions

Gar + Chl = Bio + Sta, [Kya]Sta = Bio + Kya + Gar, [Chl]

and Gar + Chl = Bio + Kya. [Sta]

Mineral abbreviations are explained in Table 1.Mapping of reaction isograds suggests invari-ance (with respect to P and T) only within the

limits of resolution permitted by the spatialdistribution of relevant, observed mineralassemblages. Spear et al. (1995) have shownthat conditions at such an intersection of reac-tions depend on, among other factors, the CaOand MnO content of the rock. Indeed, while thepoint of intersection of the reaction isogradscannot actually be invariant, the point of inter-section has characteristics of invariance in thecontext of a limited range of bulk compositionsin pelitic rocks. That the intersection can beidentified at all on the basis of the distributionof mineral assemblages suggests that the CaOand MnO contents, among other bulk composi-tional factors, are more or less uniform inrelevant rock types in the AMS and ABMS.Theoretically, two other reactions,

Sta = Gar + Kya + Chl, [Bio]and Sta + Chl = Bio + Kya, [Gar]

Table 1. Abbreviations used for minerals, components,and metamorphic zones.

_________________________________________________________________________mineral (components)

And andalusiteBio biotiteChl chloriteDio diopside (Di = CaMgSi2O6, Hd = CaFeSi2O6)Epi epidoteGar garnetHnb hornblendeKya kyaniteOmp omphacite (Di = CaMgSi2O6, Hd = CaFeSi2O6, Jd = NaAlSi2O6, Ts = CaAl2SiO6)Pla plagioclase (Ab = NaAlSi3O8, An = CaAl2Si2O8)Qtz quartz (In Figure 6, this is 3SiO2)Sil sillimaniteSta staurolite_________________________________________________________________________

metamorphic zones (Figures 1 and 4)G garnet zoneSt staurolite zoneK kyanite zoneSi sillimanite zone_________________________________________________________________________

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are involved at each intersection, but corre-sponding isograds apparently were not pro-duced, probably because of inappropriate bulkcompositions. As noted above, P-T conditionsfor this kind of intersection remain poorlyconstrained, for reasons just noted (Spear et al.,1995). Based on determinations near one of theinvariant points in the AMS (McSween et al.,1989) northeast of the Grandfather Mountainwindow, P and T are estimated to be ~0.75 GPaand 600-650 °C.

Characterizing the point of intersectionnortheast of the Grandfather Mountain window.The pseudo-invariant point, immediately northof Boone, North Carolina, has already beendescribed by us in considerable detail (Abbottand Raymond, 1984; McSween et al., 1989;Abbott et al., 1991). We defined three metamor-phic zones surrounding the psedu-invariantpoint as Ip, IIp, and IIIp (Figure 2; Abbott andRaymond, 1984; McSween et al., 1989), forwhich the mineral assemblages, coexisting withquartz and muscovite, are given in Figure 2.Diagnostic assemblages are Gar+Bio+Chl inzone Ip, Gar+Sta+Bio and Gar+Sta+Chl in zoneIIp, and Gar+Bio+Kya in zone IIIp. Rankin etal. (1972, 1973) showed that the staurolite zone(hatched region, Figure 2), which is bounded tothe southeast and northwest by garnet andkyanite zones respectively, ceases to be recog-nizable southwest of a point (marked ‘i’, Figure2) where the staurolite and kyanite isogradsconverge. Southwest of the point, garnet-graderocks are juxtaposed with kyanite-grade rocks.The relationships might otherwise be explainedby a fault, separating the kyanite and garnetgrade rocks south of point i, but there is noindication of what should be a fairly obviouspost-metamorphic fault.

Northeast of the point of intersection, thestaurolite isograd is due to the AFM reaction[Kya] (see above), and the kyanite isograd isdue to the reaction [Chl]. Hence, kyaniteappears in response to the disappearance ofstaurolite. These reactions are common in other

terranes (e.g., Guidotti, 1970; Abbott, 1979;Thompson and Norton, 1968; Albee, 1968).The five minerals involved in these two reac-tions are also involved in the equilibrium wherethe two isograds meet. Interpreted as an “in-variant point,” three other four-phase AFMequilibria must be involved at the invariantpoint, one each for the non-involvement ofstaurolite, biotite, and garnet. UsingSchreinemakers method, it is a straightforwardmatter to determine the missing reactions and tolocate them properly around the invariant point.The reaction for the non-involvement of stauro-

Figure 2. Metamorphic zones in pelitic rocks of theAMS and ABMS, north of the Grandfather Mountainwindow. Open circles indicate AFM mineral assem-blages in the garnet zone (Ip). The common assem-blage is Gar+Bio+Chl. The shaded region is thestaurolite zone (IIp). The diagnostic AFM asemblagesare Sta+Bio+Gar and Sta+Bio+Chl. Filled circlesindicate mineral assemblages in the kyanite zone(IIIp). The common assemblage is Gar+Bio+Kya.Isograds bounding the staurolite zone meet at thepseudo-invariant point marked “i.” Southwest of “i,”garnet zone (Ip) assemblages are juxtaposed with thekyanite zone (IIIp) assemblages. General direction ofincreasing P and direction of increasing T are indicatedby arrows. See Figure 3.


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lite, [Sta], must necessarily extend southwestfrom the point of intersection, and is presumablyresponsible for the appearance of kyanite. Theremaining two reactions, [Bio] and [Gar],though theoretically possible, are not observedbecause of inappropriate bulk rock composi-tions.

Relationships around the point of intersec-tion are shown schematically in Figure 3. P-Tslopes of the various reactions are consistentwith calculations by Spear et al. (1995). Signifi-cantly, the slope (dP/dT) of reaction [Kya] ispositive, but very steep to nearly vertical, whilethe slope of reaction [Chl] is negative.

In Figure 2, we have superimposed sche-matically on the mapped relationships an arrowfor increasing P (at constant T) and an arrow forincreasing T (at constant P) by direct analogywith the relationships shown in Figure 3. The

pressure of metamorphism, as recorded andpreserved by the distribution of the mineralassemblages, increases toward the west, and thetemperature of metamorphism increases towardthe north.

Two other pseudo-invariant points. Theregional metamorphic map, shown here asFigure 4 (from Drake et al., 1989), illustratesrelationships that can be found on various moredetailed maps (Hadley and Nelson, 1971;Rankin et al., 1972; Espenshade et al., 1975).On this map, we have marked, by means of thearrows for increasing P and increasing T, theprobable locations of two other pseudo-invariantpoints involving the same isograds.

The northeasternmost of the points isapproximately on the boundary between thegeologic maps for the west and east halves ofthe Winston-Salem 2° sheet, in the vicinity ofthe town of Devotion, NC. The pseudo-invari-ant point is implicit in the mineral assemblagesreported on these geologic maps by Rankin etal. (1972) and Espenshade et al. (1975). Unfor-tunately, neither detailed mapping nor petrogra-phy has been performed to verify the preciselocation of the intersection and the precisetrends of the traces of the reaction isograds.

The southwesternmost (third) pseudo-invariant point, some 100 miles SW of theGrandfather Mountain window, appears near thetown of Cove Creek on the metamorphic map ofthe geologic map of North Carolina (Brown etal., 1985), which is based on the earlier work ofHadley and Nelson (1971). Here the pseudo-invariant point appears to lie within rocks of theGreat Smoky Group. Again, neither detailed

Figure 3. Schematic phase relationships in pelitic rocks(fine lines) and mafic rocks (bold line) in AMS andABMS. Slopes of AFM reactions (pelitic rocks) areconsistent with calculations by Spear et al. (1995) forconditions at pseudo-invariant point of P = 7.5 kbars, T =640 °C, XMn(Gar) = 0.25. The shaded region is thestaurolite zone of a typical Barrovian Metamorphic FaciesSeries.

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mapping nor petrography has been performed todetermine the precise location of the inferredpseudo-invariant point and the precise trends ofthe traces of the reaction isograds.

If the relationships just described are evenqualitatively correct, important conclusionsnaturally follow:1. Inferred gradients in P (at constant T),Figure 4) indicate that the highest pressure ofmetamorphism in the AMS and ABMS shouldbe recorded near the base of the Spruce Pinethrust sheet, close to the Grandfather Mountainwindow (stippled areas in Figure 4). Signifi-cantly, the locations of eclogites southwest ofthe Grandfather Mountain window (Willard andAdams, 1994) and northeast of the Grandfather

Mountain window (this report) support thisgeneral interpretation (Figure 4).2. These areas, with evidence for the highestpressures of metamorphism, do not coincidewith the areas that experienced the highesttemperatures. This follows from inferredgradients in P and T.3. The geothermal gradient, at the time ofmetamorphism, varied systematically fromplace to place laterally across the volume ofcrust affected by the metamorphism. In thehighest-pressure regions (close to GrandfatherMountain window), the temperatures werecomparatively low; thus, the geothermal gradi-ent was comparatively steep (i.e., a coolgeotherm). To the northeast and to the south-

Figure 4. Direction of increasing metamorphic pressure and direction of increasing metamorphic temperature in theEastern Blue Ridge Belt at each of the pseudo-invariant points. Inferred regions of high pressure metamorphism arestippled. Eclogite locales are indicated by X’s. Eclogite southwest of Grandfather Mountain window has been describedby Willard and Adams (1994). Retrograded eclogite immediately north of Grandfather Mountain window is described inthe report. (modified from Drake et al., 1989.)


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west, away from the Grandfather Mountainwindow, comparable temperatures correspond tolower pressures; thus, the geothermal gradientwas comparatively shallower (i.e., a hotgeotherm).

METABASITES

Hornblende schist and gneiss. The essen-tial minerals in the mafic rocks of the AMS andABMS are hornblende, quartz, and plagioclase.Varieties of amphibolite are distinguished byvarious combinations of garnet, biotite, epidote-ziosite, and magnetite. Locally, extreme vari-ants, dominated by epidote (epidotites) ordominated by garnet (garnetites), occur aslenses (dm-scale) and thin (mm- to cm-scale)layers, parallel to the foliation. Accessoryminerals include sphene, apatite, ilmenite,zircon, and iron sulfides. Late replacementminerals include white mica, calcite, chlorite,and ferric oxides.

Abbott and Raymond (1984) identifiedtwo metamorphic zones in the mafic rocks ofthe AMS-ABMS northeast of the GrandfatherMountain window (Figure 5). In terms of CFMminerals (C=CaO+K2O+Na2O-Al2O3, F=FeO-Fe2O3, M=MgO; Abbott, 1982), the zones areseparated by a reaction isograd,

Bio + Epi = Gar + Hnb.Quartz, plagioclase, and Fe-oxide are involvedin the reaction. The lefthand side correspondsto the low-grade (zone Im), where the commonCFM mineral assemblage is Hnb+Bio+Epi(coexisting with quartz, plagioclase, Fe-oxide).Garnet and hornblende are not compatible. Onthe high-grade side (zone IIm), diagnostic CFMassemblages are Gar+Hnb, Gar+Hnb+Epi, andGar+Hnb+Bio. The isograd passes practicallythrough the AFM pseudo-invariant point, insuch a way that the P-T slope (dP/dT) of thereaction is constrained to be negative (Figure 3).

Gar-Hnb and Gar-Bio geothermometry isconsistent with Gar-Bio geothermometry for thepelitic rocks (McSween et al., 1989). Tempera-

tures range from 530 to 622°C in zone Im; andfrom 611 to 729 °C in zone IIm. The mapdistribution of estimated temperatures(McSween et al., 1989) is consistent with thegeneral direction of the increasing T, inferredfrom relationships in the pelitic rocks.

Figure 5. Metamorphic zones in mafic rocks northof the Grandfather Mountain window (from Abbottand Raymond, 1984; McSween et al., 1989). Filledcircles indicate mafic assemblages with Gar + Hnb;the common, diognostic CFM assemblage isHnb+Gar+Epi. Open circles indicate maficassemblages with Epi + Bio (Hnb and Gar areincompatible). The common, diognostic CFMassemblage is Hnb+Bio+Epi. Non-eclogitic maficassemblages containing clinopyroxene (diopside,mentioned in McSween et al., 1989) are indicted by+’s. Eclogite sites are indicated by X’s:

NCA 5: lat. N 36° 15' 34", long. W 81° 43' 22",Junaluska Rd.

NCA 170: lat. N 36° 16' 44.2", long. W 81° 43'41", Howard Creek Rd., 50-100 m NE ofjunction with Tater Hill Rd.

(No sample): lat. N 36° 16' 46.6", long. W 81° 43'38", Howard Creek Rd., 250 m NE ofjunction with Tater Hill Rd. Boulders fromupslope, to north.

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Eclogite. This is the first description ofretrograded eclogites from northeast of theGrandfather Mountain window. There areactually three sites (Figure 5, precise locationsgiven in caption), which are close to each otherand in nearly the same structural position, closeto the base of the AMS, at its westernmost edge.It is not possible to know, at this time, if thethree sites are parts of a contiguous area ofretrograded eclogite, because mapping is incom-plete. The locations are coincident, however,with the area of highest metamorphic pressuresin the AMS, as inferred from the general direc-tion of increasing P in the pelitic rocks.

The retrograded eclogites occur as thin(cm-scale), granoblastic layers in otherwisetypical amphibolite. The essential minerals aresymplectic intergrowths of diopside and plagio-clase (representing former omphacite), generallyeuhedral to subhedral garnet (<1mm), polygonalepidote, and quartz. Symplectic grains retainsimple polygonal shapes, and are related to oneanother in such a way that the mosaic texture ofthe original omphacite is recognizable. Bound-aries between layers of retrograded eclogite andamphibolite are gradational, showing a progres-sive replacement of the eclogite by hornblendeschist. Locally the vermicular texture of theoriginal symplectite is preserved in the horn-blende. Plagioclase occurs only in symplectiteand with quartz in coronas around garnets.Presumably the coronas formed in response to aretrograde reaction between garnet and sur-rounding symplectite (Adams et al., 1995). Thedistinctive coronas are very common in thesymplectite-free hornblende schist, especiallynear the base of the AMS, along the southernand western margin of the thrust sheet. Inter-preted as relict features inherited from an ear-lier, higher-grade condition of the rock, thecoronas suggest that a significant volume of theamphibolite in the AMS is retrograded(amphibolitized) eclogite (Adams et al., 1995).

These inferred eclogites northeast of theGrandfather Mountain window are distinguished

from those described by Willard and Adams(1994) by the presence of epidote. In somesamples epidote occurs in thin (mm-scale),nearly monomineralic layers or lenses, boundedby the symplectic diopside+plagioclase andgarnet. Individual grains of epidote are free ofinclusions. Aggregates contain variableamounts of garnet, grains of symplectic diop-side-plagioclase, or both. By itself or withlesser amounts of other minerals—mainlygarnet and symplectic diopside+plagioclase—the epidote forms aggregates that show a well-devoloped, equigranular mosaic texture. Asecond type of epidote (with quartz) occursbetween garnet and symplectic diopside-plagio-clase in some samples, as crude coronas, sug-gesting that it resulted from an early retrogradereaction involving garnet and omphacite. Lo-cally, euhedral garnets are embedded insymplectite (diopside+plagioclase) with onlyminor quartz, no epidote, and no hornblende,suggesting an original metamorphic assemblageof omphacite and garnet.

Hornblende seems not to have any specialsite for nucleation. Grains of hornblende occuralong every kind of grain boundary involvingcombinations of garnet, epidote, andsymplectite. Hornblende also develops atboundaries between grains of symplecticdiopside+plagioclase, cutting across the ver-micular texture of the symplectite.

Based on textural relationships, the fol-lowing sequence of assemblages (excludingaccessory minerals) is envisioned, starting withthe earliest, highest-grade assemblage,

(a) Omp(I) + Gar + Qtz, and Omp(I) +Gar + Epi + Qtz,

(b) Omp(II) + Gar + Epi + Qtz,(c) (Dio + Pla) + Gar + Epi + Qtz, where

(Dio + Pla) is symplectite,(d) (Dio + Pla) + Gar + Epi + Hnb + Qtz,and(e) Pla + Gar + Epi + Hnb + Qtz.


96

Reactions relating assemblage (a) to (b),and assemblage (b) to (c) can be convenientlyrepresented in the compositional space definedby principal components of omphacite (Omp)plus quartz, that is Jd = NaAlSi2O6, Ts =CaAl2SiO6, Di-Hd = Ca(Mg,Fe)Si2O6, and Qtz= 3 SiO2 (Figure 6). Assuming the rock systemis open with respect to H2O and O2, epidoteplots in the same place as garnet, in this simpli-fied representation. The appearance of epidoteis thought to be controlled mainly by the avail-ability of O2 and H2O, according to one or bothof the following reactions involving componentsof an original, slightly tschermakitic omphacite,Omp(I), components of garnet, or both.

CaFeSi2O6 + CaAl2SiO6 + 0.5 H2O +Hd and Ts in Omp(I)

0.25 O2 = Ca2FeAl2Si3O12(OH) Epi

Ca2FeAl2Si3O12 + 0.5 H2O + 0.25 O2in Gar

= Ca2FeAl2Si3O12(OH) Epi

The Ts-content of most omphacites (Deer et al.,1992; Cameron and Papike, 1980) is very low,typically much less than 10%. Omphacitesanalyzed by Willard and Adams (1994) from

eclogite in the AMS contain less than 9% Ts-component. Presumably, production of epidotefrom omphacite by the first reaction is lessimportant than production of epidote fromgarnet by the second reaction. The reader willnote that the component of garnet in the secondreaction is chemically equivalent to combinedchemical components of Omp(I) in the firstreaction. The first reaction has the effect ofchanging the composition of the omphacitetoward the (Di-Hd)-Jd join, directly away fromepidote. In the context of Figure 6, the reactionmay be written simply,

Omp(I) + H2O + O2 = Omp(II) + Epi.

The bulk composition of the rock is in thetriangle Qtz-Gar(Epi)-Omp(II). The absence ofepidote from the paragenesis of the eclogitesouthwest of the Grandfather Mountain window(Willard and Adams, 1994) may be due simplyto lower fugacity of O2.

Based on optical properties and X-raydiffraction characteristics, the clinopyroxene inthe symplectite is diopside, close to the Di-Hdapex in Figure 6; the plagioclase is oligoclase(~An20). Volumetric proportions of oligoclaseand diopside in the symplectite suggest that theJd-content of original omphacite was at least35%. A line connecting the diopside and the

Omp(II) Dio

Gar, Epi

Qt z

Omp(I)

Ts

Di-HdJd

Ab

An

CaAl2SiO6

Ca(Mg,Fe)Si2O6NaAlSi2O6

Qt z = 3SiO2

plag

iocl

ase

x

Figure 6. Eclogite tetrahedron (designed by one of us,RNA), defined by components, Jd = NaAlSi2O6, Di-Hd =Ca(Mg,Fe)Si2O6, Ts = CaAl2SiO6, and Qtz = 3SiO2.The Qtz component is taken as 3 units of SiO2, so that allcompositions in the tetrahedron are normalized to 6oxygen atoms. In this way, all compositions representapproximately the same volume; hence, modal relation-ships are preserved (approximately). The bulk composi-tion of typical eclogite is in the shaded plane, e.g.,composition marked X. Oxidation of the initialomphacite Omp(I) and compositionally equivalentcomponents in Gar produce epidote, driving the composi-tion of the omphacite to Omp(II). Symplectic Dio+Plaforms by the reaction Om(II) + Qtz + Gar (or Epi) -> Pla+ Dio. The final mineral assemblage is Dio + Pla + Gar/Epi + Qtz.

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plagioclase of the symplectite pierces the Qtz-Gar(Epi)-Omp(II or I, ~Jd35) plane. Hence, thereaction producing the symplectite necessarilytakes the form,

Omp(II or I) + Gar(or Epi) + Qtz = Dio + Pla,

defining the compatibility tetrahedron Dio-Pla-Qtz-Gar(Epi), within which the bulk composi-tion resides. Admittedly, the amount of Gar(orEpi) involved in the reaction may be small.

At this writing, chemical analyses arebeing performed by electron microprobe (J.Greenwood, Univ. of Tennessee, Knoxville) onthe minerals in the eclogite. Results are not yetavailable. Using mass balance calculations onthe analyses of symplectic diopside and plagio-clase, we hope to recover the composition of theoriginal omphacite. Using relevantgeothermometers and geobarometers, we thenhope to obtain quantitative estimates of theoriginal P and T.

Hornblende cannot be plotted in Figure 6.However, assemblage (c), Dio + Pla + Gar + Epi+ Qtz, is amenable to a CFM representation(Abbott, 1982), wherein the appearance ofhornblende can at least be understood qualita-tively. The relationships relevant to assemblage(c) are shown schematically in Figure 7a. Ahypothetical bulk composition is indicated in theGar+Dio+Epi field. Upon uplift, cooling, andhydration, the 2-phase Gar-Epi field rotates

clockwise, shifting the Gar-Cpx-Epi fieldtoward the CF sideline and increasing the areaof the Hnb-Gar-Epi field, such that in Figure 7bthe same bulk composition resides in the latterfield. Relevant multivariant CFM reactionsoperating on the bulk composition could be,

Cpx (+ H2O) = Gar + Epi, first,

followed by,

Gar + Epi (+ H2O) = Hnb.

H2O necessarily appears on the left side ofeach reaction; hence, the reactions are driven bythe availability (activity) of H2O. Of course,the combined reaction,

Cpx (+ H2O) -> Hnb,

is permissable if, for reasons of disequilibrium,any diopside should persist when conditions areappropriate for the relationships in Figure 7b.

In the eclogites southwest of the Grandfa-ther Mountain window, described by Willardand Adams (1994), hornblende starts to form ina different way, earlier in the paragenesis, beforethe breakdown of omphacite to symplecticDio+Pla. Willard and Adams (1994) suggest thereaction,

3 Gar + 7 Omp + 2 H2O = 2 Hnb + 4 Pla.

Gar Gar

Dio Dio

Epi Epi

Hnb Hnb

C C

F FM M

a. b.

+Qtz +Pla +Qtz +Pla

Figure 7. CFM diagrams (C = CaO+Na2O+K2O-Al2O3, F = FeO-Fe2O3, M = MgO, Abbott, 1982),illustrating schematically the retrograde conversion ofeclogite to amphibolite. A hypothetical bulk composi-tion is indicated by the filled circle, expressed at highgrade (a) by symplectic (Dio+Pla) + Gar + Epi + Qtz(see Figure 6). At lower P-T conditions (b) in thepresence of H2O, the same bulk composition is ex-pressed by Hnb + Gar + Epi + Pla + Qtz.


98

DISCUSSION

A hypothetical retrograde P-T path isgiven in Figure 8. The stippled region enclosesP and T conditions estimated by McSween et al.(1989) for amphibolites and pelitic schists in theAMS. Without knowing precise compositionsfor omphacite (I or II) and garnet in the originaleclogite, P can be constrained only crudely. Asnoted earlier, a conservative estimate of the Jd-content of the omphacite (I or II) is at least 35%.The reaction Omp(Jd30) + Qtz = Pla(An20)(Holland, 1980, 1983) places a lower limit on Pfor omphacite (I or II). Lacking any meaningfulconstraints on T, but assuming a retrograde P-Tpath of positive slope (dP/dT > 0) passingthrough the AFM pseudo-invariant point i, theminimum T would be approximately 600 °C,and the minimum P would be approximately 1.3Gpa. These conditions are comparable to thoseestimated for the eclogites in the AMS, south-west of the Grandfather Mountain window(Willard and Adams, 1994; Adams et al., 1995).

The distribution of eclogites in the AMS(immediately southwest and northeast of theGrandfather Mountain window) is consistentwith the metamorphic pressure gradients pre-

served in the pelitic rocks. Parts of the AMSthat experienced the greatest uplift; hence,originated at the greatest depths; are adjacent tothe Grandfather Mountain window. In apalinspastic reconstruction, this means the basalthrust fault intercepted the greatest depthsbeneath parts of the AMS now exposed immedi-ately north and immediately southwest of theGrandfather Mountain window. The greatestvolume of mafic rock is in these same parts ofthe AMS. Early during subduction (Abbott andRaymond, 1984; Willard and Adams, 1994;Adams et al., 1995) along what would becomethe basal thrust fault of the AMS, downwarddeflection of the surface of the fault wouldexplain the interception of the highest-P parts ofthe AMS. Downward deflection of the nascentfault suggests that the effected parts of the AMSwere different from other parts of the easternBlue Ridge (i.e, further away from the generalvicinity of the Grandfather Mountain window)with regard to physical properties of the crust atthe time. Perhaps the most significant, distin-guishing feature of the AMS-ABMS adjacent tothe Grandfather Mountain window is the greatvolume of mafic rocks. We offer the possibilitythat the nascent thrust fault was deflecteddownward quite simply by an obstacle, in theform of what was originally an isolated maficvolcanic edifice on oceanic crust — a seamount.Trace element and REE geochemistry of AMS-ABMS amphibolites (Misra and Conte, 1991)are generally consistent with this interpretation.Misra and Conte (1991) describe three composi-

Figure 8. P-T diagram illustrating schematically theretrograde path of eclogites in the AMS. Phase relation-ships from Figure 3 are positioned such that point i isconsistent with conditions estimated by McSween et al.(1989). Slopes of reactions are consistent with calcula-tions by Spear et al. (1995). Omp + Qtz = Pla reaction(Omp, Jd30; Pla, An20) is from Holland (1980, 1983).Al2SiO5 polymorphic transformations, involving And,Kya, and Sil, are from Holdaway (1971). Stippled regionencloses range of P-T conditions in AMS, as estimated byMcSween et al. (1989).

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tional groups of basalts. The paleotectonicsetting for Group I is back-arc or plume; forgroup II, spreading center (N-MORB); and forgroup III, transitional between spreading centerand plume (T-MORB).

CONCLUSIONS

With regard to the origin of the AMS-ABMS, the findings reported here support thethree main conclusions of Willard and Adams(1994) and Adams et al. (1995), for the samereasons. Summarizing their conclusions:

1. The discovery of eclogites in theeastern Blue Ridge helps to resolve controversyregarding the origin of this part of the southernAppalachian Orogen.

2. The collected body of evidencestrongly supports an ensimatic origin for theAMS, as a subduction-related melange. Sub-duction was most likely toward the east, suchthat the melange accumulated along the westernedge of Piedmont terrane.

3. The AMS marks the suture between theancient North American Craton and the Pied-mont terrane.

We offer an additional, fourth conclusion:4. The Grandfather Mountain window

may mark the site where a seamount was incor-porated into the subduction melange.

ACKNOWLEDGEMENTS

We appreciate the helpful comments fromMark Adams and Michael Roden.

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