American Mineralogist, Volume 66, pages 70-86, l98l

Petrology of contact metamorphosed argillite from the Rove Formation,Gunflint Trail. Minnesota

THeonone C. Lesorre. J. J. PAprKE. D. T. VeNnaaN'

Department of Earth and Space SciencesState University of New York at Stony Brook

Stony Brook, New York I 1794

exo G. B. Monnv

M innes ot a Ge olo gic al Surv eySt. Paul, Minnesota 55108

Abstract

The contact metamorphic efects of the Duluth Complex on the argillaceous Rove Forma-tion, northeastern Minnesota, are confined within a few hundred meters of the contact. Theargillite has recrystallized from the low-grade assemblage chlorite + muscovite + quartz toassemblages characterized by cordierite + biotite + microcline + muscovite + quartz. Cor-dierite coexists with biotite over a wide range in Mgl(Mg + Fe) from 0.90 to less than 0.50.Andalusite is restricted to iron-rich rocks, although it occurs in one unusual magnesium-richassemblage. Locally, corundum + microcliae and cordierite * hypersthene assemblages oc-cur at the contact with the Duluth Complex. The pressure, P"oria, of metamorphism is esti-mated to have been 1500 bars. The temperature near the contact ranged from -500o to600oC, based on the partitioning of Na and K between coexisting muscovite and microcline.The equilibrium constant for the continuous reaction muscovite + phlogopite * quartz :cordierite * microcline + HrO is calibrated and used to determine "/n,o attending meta-morphism. Results indicate that P",o ranges from P"",,o to jP"",,o. Calculation of the composi-tion of COH fluid for a graphite-bearing sample iadicates that the remainder of the fluid ismostly COr. The Rove Formation does not show the extensive mineralogic eflects of contactmetamorphism that the iron-formation shows, and the differences reflect the large thermaland compositional stability field for coexisting cordierite + biotite in pelitic rocks and the rel-atively small stability fields of the iron-rich minerals in the iron-formation.

Introduction

The -l Gy Duluth Complex in northeastern Min-nesota intruded an -2 Gy sequence of sedimentaryrocks that includes banded iron-formation (Gun-flint-Biwabik) and argillite and greywacke (Rove-Virginia Formations). The contact metamorphic ef-fects of the Duluth Complex on the iron-formationhave been extensively studied by French (1968),Bonnichsen (1968, 1969), and Morey et al. (1972)(Biwabik Iron Formation), and by Simmons et al.(1974) and Floran and Papike (1975, 1978) (GunflintIron Formation). These iron-rich rocks are sensitive

rPresent address: Los Alamos Scientific Laboratorv. LosAlamos, New Mexico 87545.0003 -o0/.x / 8 | / 0 I 02-0070$02.00

to changes in metamorphic grade, and rocks as faraway as 10 km (along strike) show effects of contactmetamorphism. The zones of metamorphism recog-nized by French (1968) and Floran and Papike(1978) are low-grade zones characterized by green-alite or minnesotaite, intermediate-grade zones de-fined by grunerite- and fayalite-bearing assemblages,and a high-grade zone characteitzed by ferrohyper-sthene or inverted pigeonite.

The Rove Formation consists predominantly ofcarbonaceous argillite and quartz-rich greywacke,and the petrology of these rocks has been determinedin order to compare the response of the more K- andAl-rich rocks to that of the iron-formation underlow-pressure metamorphic conditions. In this r€port,the mineral assemblages and mineral compositions in

70

LABOTKA ET AL.: ROVE

the Rove Formation are presented so that the distri-bution of elements among coexisting minerals in low-pressure, pelitic schists can be characterized. We usethe mineral reactions that are inferred to have oc-curred to estirnate the conditions of metamorphism.

Geologic setting

The Rove Formation is part of a sequence of Pro-terozoic (-2Gl sedimentary rocks assigned to theAnimikie Group. The group unconformably overlies2.7 Gy Archean rocks and consists of two units, theGunflint lron-Formation and the overlying RoveFormation. The stratigraphy and sedimentary pe-trography of the Rove Formation have been de-scribed by Morey (1969), and the general geology ofnortheastern Minnesota is shown in Figure l.

FORMATION, MINNESOTA

The Rove Formation consists of a lower argilliteunit, a middle transition unit, and an upper thin-bed-ded greywacke unit. The lower argillite unit is ap-proximately 150 m thick and consists of thin-bedded,fissile, fine-grained greywacke, silty argillite, andgraphitic argillite. Irregular limestone lenses and cal-careous concretions also occur in the lower part ofthe unit. Beds of greywacke are abundant in the tran-sition unit, and greywacke dominates the upper unit.The complete thickness of the two upper units isabout 900 m, but the upper parts are generally trun-cated by the Duluth Complex in the region aroundGunflint Lake.

The Duluth Complex, a composite mafic intrusion,truncates the rocks of the Animikie Group with slightangular discordance. Consequently, the complex is in

7 l

Fig. l. General geology of the Gunflint Lake area, northeast Minnesota. Sample localities are indicated by the letters KT (KakakebicTrail), CR (Cross River), GT (Gunflint Trail), LL (Loon Lake), GP (Gunflint Palisades), ML (Mayhew Lake), SRL (South RoundLake), RL (Rose Lake), and DL (Daniel's Lake).

LIIBOTKA ET AL.: ROVE FORMATION. MINNESOTA

contact with progressively higher units of the Ani-mikie Group from west to east. Gabbroic sills anddikes assigned to the Logan Intrusions (Weiblen etal., 1972) pervasively invade the Animikie Group inthe vicinity of Gunflint Lake. The Logan Intrusionsappear to be both older and younger than rocks ofthe Duluth Complex and are part of the same intru-sive complex.

Methods

The Rove Formation is not well exposed; outcropsgenerally only occur beneath the more resistant Lo-gan sills or beneath the Duluth Complex adjacent tothe contact. Samples were collected at sill contactsand in sequences away from the sills to determine theregional thermal environment and perturbations im-posed by the sills.

The rocks were examined in thin section and theidentification of some minerals was aided by X-raydiffractometry. Selected samples were analyzed withan ARL-EMX automated microprobe using simple sili-cate standards. Data were reduced on-line by themethod of Bence and Albee (1968) and the conec-tion factors of Albee and Ray (1970). Precision wasmonitored by analyzing minerals of known composi-tion and is -2Vo for major elements and -ll%o forminor elements. Unknown minerals were analyzed intwo or three areas on a standard thin section, andmineral compositions have ranges within the statedprecision over the area of a thin section.

The structural state of potassium-feldspar has notbeen determined, and the use of the term "micro-cline" does not imply that K-feldspar has the triclinicstructure.

Mineral assemblages in pelitic rocks

Sample locations are shown on Figure l, and themineral assemblages in analyzed samples are givenin Table l. Rocks that exhibit only incipient recrys-tallization and retain detrital textures are termed"low grade." Rocks that have recrystallized to fine-grained hornfels are termed "medium grade," andthose that show extensive interaction with the DuluthComplex are called "high grade."

The assemblages in quartz-bearing rocks can berepresented in the system KrO-FeO-MgO-AlrOr-HrO. HrO is assumed to be a boundary-value com-ponent, and because either muscovite + quartz ormicrocline + quartz assemblages occur, mineralcompositions may be projected onto the plane FeO-MgO-AlrO, from either of these points to show theeffects of Fe-Mg partitioning. These projections may

be used as phase diagrams if the chemical potentialsof HrO and each additional component are constantfor all assemblages in each projection (Thompson,1957).

Low grade

The rocks from the Rove Formation are character-ized by detrital textures except near the Duluth Com-plex and adjacent to some sills. The clastic grains insiltstones and greywackes have angular to sub-rounded shapes, and the matrix consists of fine-(<0.25 nm) to very fine-grained (<0.0J mm) mi-caceous minerals. The micaceous minerals chlorite.muscovite, and biotite have recrystallized from thepresumably original clay matrix. The textures de-pend strongly on the rock type. The graphite-rich ar-gillite near the base of the formation contains angu-lar quartz and plagioclase grains, very fine-grainedbiotite and chlorite, and comparatively large (>0.5mm) muscovite grains that have an idioblastic formand appear to be recrystallized detrital muscovite. Inmany samples, chlorite and muscovite are finely in-tergrown in the matrix. Albite grains are irregular inshape and contain fine-grained inclusions of epidote("saussurite"). In some rocks, epidote occurs as dis-tinct grains.

The observed mineral assemblages include quartz+ muscovite + chlorite * biotite * plagioclase andquartz * muscovite + biotite * microcline + plagio-clase. Rocks that were collected from Rose Lake andfarther east, near Lake Superior, do not contain bio-tite. The diagnostic assemblage quartz * muscovite+ chlorite + biotite + microcline + plagioclase oc-curs in sample 4ls from South Round Lake. Graph-ite is very abundant in rocks from the lower part ofthe formation. near the western end of GunflintLake. All assemblages contain ilmenite or spheneand commonly pyrrhotite which is locally altered tomarcasite. The chlorite-free assemblage quartz *muscovite + biotite * cordierite + plagioclase occursin the Rove adjacent to the sill contact at SouthRound Lake.

Figure 2 illustrates the compositions of mineralsthat occur in muscovite + quartz-bearing assem-blages from low-grade rocks of the Rove Formation.Chlorite + biotite coexist over a range in Mgl(Mg +Fe) from 0.65 to 0.50, biotite is slightly more iron-rich than coexisting chlorite, and microcline coexistswith magnesium-rich chlorite. The low-grade sample4le contains cordierite + biotite and indicates thatnear the sill chlorite has broken down at the compo-sition Mg/(Mg + Fe) : 0.50.

LABOTKA ET AL: ROVE FORMATION. MINNESOTA

Table l. Analyzed samples, Rove Formation

73

Sample Quartz Muscovite Microcline Biotite Chlorite Cordierite Amphibole Hypersthene Al2SiO5l Plagioclase Others2

LOV GRADE

Cross River3 X43d X43t X

South Rounci Lake4 I e X4 l s X

Daniels Lake42c X

MEDIUM GRADE

Cunllint Trail22a X23? 6 X

Mayhew Lake35a X35t X35q X

Loon Lake39a X396 X)9c X39d X39\ X39m X39r X

Cunllint Palisades

xx

X i ,gx i , tX ep,sph

X i ,sX rrs

XX X

X

X

XXX

XX

XXX

XX

X

X

X

X

X

XXX

X

X

X X

X X

XX XX X

X

xX

X iX cpx,sph,sX gr i rs

Xx

XXXX

XXXxXXx

l r S

l r S

I ' s

l r S

l r s

l r s

I ' s

I ' s

8rs

8,rl r 8

t r S r S

r ,g

l, s,cPXcor,gah,r,s

HIGH GRADE

Kakakebic Traill 6 al 5 bl 6 it6jl 8 al 8 bl 8 cl 8 e

X

XXXX

XXXXX

X

Y

X XX

X XX X

x

x

lAnd = Anda lus i te , S i l = S i l l iman i te2 i = i l m e n i t e , r : r u t i l e r s p h = s p h e n e , g = g r a p h i t e , s = s u l p h i d e , c p x = d i o p s i d e , c o r = c o r d u n d u m , g a h = g a h n i t e , e P = e P i d o t e

Medium gradeAt the localities of Mayhew Lake, Loon Lake, and

Gunflint Trail adjacent to the Duluth Complex,rocks from the Rove Formation are completely re-crystallized to fine-grained hornfels. All aspects of asedimentary protolith, except compositional layering,have been obliterated. The most common mineral as-semblage is quartz * muscovite + microcline * bio-tite + cordierite + plagioclase. The grain sizes of mostminerals range from <0.10 mm to 0.25 mm, but mus-covite occurs as porphyroblasts up to I mm in size.

At both Loon Lake and Mayhew Lake, closelyspaced sequences of samples taken progressively far-ther from the contact show that the grain sizes ofmuscovite and biotite decrease from I mm to lessthan 0.25 mm within a distance of 20 m from thecontact.

Cordierite occurs as ellipsoidal poikiloblasts, -0.5

mm in diameter, that contain numerous quaftz andfeldspar inclusions. In many samples cordierite is al-tered to a yellowish chlorite. Plagioclase, microcline,and quartz are fine-grained (-0.1 mm), equigranular,

A lzS iOs

LABOTKA ET AL; ROVE FORMATION. MINNESOTA

Microc/rne

Fig. 2. Compositions of coexisting minerals inmuscovite + quartz-bearing rocks.

and cannot easily be distinguished in thin sectionfrom one another. Pyrrhotite and ilmenite are addi-tional minerals in all rocks, and graphite is locallypresent.

Because microcline occurs in most assemblages,the compositions of coexisting minerals are repre-sented in Figure 3 in a projection from KAISi.O, andSiOr. Muscovite compositions contain minoramounts of Mg and Fe2* (Table 2), but the range istoo small to show on Figure 3; thus, muscovite isplotted at the apex of the triangle. The assemblagemuscovite + cordierite + biotite occurs in the twosuites of samples collected adjacent to sills at May-hew Lake (35) and Loon Lake (39). Samples col-lected adjacent to the sills are labeled a, and samplescollected progressively farther from the sills are la-beled b, c, and so on. Samples from the Loon Lakelocality are coarser-grained and closer to the high-grade region at Kakakebic Trail than the samplesfrom the Mayhew Lake locality.

The compositions of biotite and cordierite in thelow-grade

Andolusite

Microc/ine

Muscovite

Antiteoarho-l$t- t - - f - - '

roo_ !_ _ _ Jt- /s MsoMicroc/ine Amphibole

Fig. 3. Compositions of coexisting minerals in medium-grade microcline * quartz-bearing rocks. Distribution of Na and K inmuscovite + microcline + andalusite assemblage 48f also shown. Points labeled by letter only are samples 39. Biotite + cummingtoniteassemblages do not contain microcline.

I"./IBOTKA ET AL: ROVE iORMATION, MINNESOTA

Table 2. Representative mineral compositions

75

MUSCOVtTE

35t )5q J9a 39r 4EI

BIOTITE

351 359 )9a

CHLORITE

19r 48f 3

CORDIERITE

J5t 359 39a 39t 481t 5 bl6b

S i O 2 4 6 . 4 4 7 . t 4 6 . 0 4 7 . 7 4 7 . 2 4 7 . 1T i o 2 0 . I 0 . I 0 . 0 0 . 5 o . 3 0 . 5AI2O3 )5 .4 36 ,7 34 .8 35 .6 37 .6 34 .9M g O . 0 . 2 0 , 6 2 . 4 0 . 7 1 . 0 0 . 8F e O r 2 . 2 0 . 9 3 . O 0 . 8 0 . 7 0 . 1M n O 0 . 0 0 . 0 0 . 0 0 . 0 0 , 0 0 . 0C a O 0 , 0 0 . 0 0 . 0 0 . 0 0 . 0 0 . 0N a 2 O 0 . ) 0 . 3 0 . 2 0 . 6 O . 2 0 . 3K 2 o ^ 9 . E 1 0 . I 9 . 4 1 0 . 3 E . 4 1 0 . 7H 2 O t 5 . 6 3 . 5 4 . 2 3 . E 4 . 6 5 . 6T o t a l 1 0 0 . 0 1 0 0 . 0 1 0 0 . 0 1 0 0 . 0 1 0 0 . 0 1 0 0 . 0

FORMULA PROPORTIONS3

3 5 . 8 3 7 . 2 3 5 . 5 ) 4 . 7 3 E . I 3 6 , 1 4 1 . 21 , 5 5 , O 3 . 8 4 , 0 3 . E 2 . 3 0 . 8

t 9 . 3 1 5 . 2 1 8 . 5 t 9 . 0 1 7 . 9 1 9 . 9 1 5 . 99 . 7 l J , 2 6 . 7 7 . 4 9 . 4 9 . 5 2 3 . 1

1 7 . l t 6 . 4 2 2 . 5 2 l . l 1 9 . l 1 9 . 2 2 . 80 , t 0 . 1 0 . 0 0 . 0 0 . 0 0 . 0 0 . 00 . 0 0 . 0 0 . 0 0 . 0 0 . 0 0 . 0 0 , 00 . 0 0 . I 0 . 0 0 . 0 0 . 0 0 . 0 0 . 09 . 2 9 . 3 9 . 9 9 . 5 9 . 1 9 . 4 9 . 97 . 3 3 . 5 1 . O 4 . J 2 . 6 3 . 6 6 . J

1 0 0 . 0 1 0 0 . 0 1 0 0 . 0 1 0 0 . 0 1 0 0 . 0 1 0 0 . 0 1 0 0 . 0

2 . 7 5 2 , 7 6 2 . 6 9 2 . 6 5 2 . 7 9 2 . 6 9 2 , 9 21 . 7 5 1 . 1 3 1 . 6 6 t . 7 t 1 . 5 5 t . 7 5 t . 3 10 . 0 9 0 . 2 8 0 . 2 2 0 . 2 3 0 . 2 1 0 . 1 3 0 . 0 4t . t 2 1 . 4 6 0 . 7 5 0 . E 4 1 , 0 2 t . 0 5 2 . 4 4l . l 0 . l . 0 l t . 4 J t . 3 5 L t 7 1 . 2 0 0 . 1 70 . 0 1 0 . 0 0 0 . 0 0 0 . 0 0 0 . 0 0 0 . 0 0 0 . 0 00 . 0 0 0 . 0 0 0 . 0 0 0 . 0 0 0 . 0 0 0 . 0 0 0 . 0 00 . 0 0 0 . 0 1 0 . 0 0 0 . 0 0 0 . 0 0 0 . 0 0 0 . 0 00 . 9 0 0 . t 7 0 . 9 6 0 . 9 3 0 . 8 5 0 . 8 9 0 . 9 0

2 6 . 3 4 9 . 6 4 t . E 4 7 . 9 4 9 . 3 4 E . E J 0 . 20 . 4 0 . 0 0 , 0 0 . I 0 , 0 0 . 0 0 . 0

2 2 . 5 ) 4 . 2 3 2 . 7 ) 4 . 6 3 2 . 9 ) 4 . 1 3 5 , 1t 6 . 3 9 . 6 6 . 6 6 . 6 7 . 3 7 . 2 1 2 . 32 2 . 1 6 . 8 l l . 3 1 0 . 9 1 0 . 0 1 0 . 2 2 . 2

0 . 2 0 . 0 0 . 0 0 . 1 0 . 1 0 . 1 0 . 10 , 0 0 . I 0 . 0 0 . 0 0 . 0 0 . 0 0 , 00 . 1 0 . 1 o . 2 0 . 0 0 . 1 0 , 1 0 . 00 . 0 0 . 0 0 . 0 0 . 0 0 . 0 0 . 0 0 . 0

l l . 9100.0 t00 .4 99 .6 t00 .2 99 .7 100.5 99 ,9

s i 3 . u 3 . 1 0A l 2 . 7 9 2 . 8 1T i 0 . 0 1 0 , 0 1M B 0 . 0 2 0 . 0 6F e O . l 2 0 . O 5M n 0 . 0 0 0 . 0 0C a 0 , 0 0 0 . 0 0N a 0 . 0 4 0 . 0 4K 0 . E 3 0 . 8 4

3 , 0 5 3 . t i 3 , 0 7 i . t 42 . 7 2 2 . 7 5 2 . 8 9 2 . 7 40 . 0 0 0 . 0 2 0 . 0 1 0 . 0 30 . 2 4 0 , o 7 0 . 0 9 0 , 0 80 . l 7 0 . 0 4 0 . 0 4 0 , 0 10 . 0 0 0 . 0 0 0 . 0 0 0 . 0 00 . 0 0 0 . 0 0 0 . 0 0 0 . 0 0o , o 2 0 . 0 7 0 , 0 3 0 . 0 40 . 8 0 0 . t 6 0 . 7 0 0 . 9 1

2 . 7 | 4 . 9 7 5 . 0 02 . 7 ) 4 . 0 1 3 . 9 60 . 0 3 0 . 0 0 0 . 0 02 . 5 0 1 . 4 3 l . 0 tl . 9 l o . 5 7 0 . 9 70 . 0 1 0 . 0 0 0 . 0 00 . 0 0 0 . 0 r 0 . 0 00 . 0 1 0 . 0 1 0 , 0 40 . 0 0 0 . 0 0 0 . 0 0

4 . 9 0 5 , 0 4 4 . 9 5 4 . 9 44 . t 7 1 . 9 7 4 . 0 t 4 . 0 50 . 0 0 0 . 0 0 0 . 0 0 0 . 0 0l . 0 t l . I I 1 . 0 9 1 . 8 00 . 9 3 0 . 8 5 0 . 8 7 0 . l t0 . 0 0 0 . 0 0 0 . 0 1 0 . 0 10 . 0 0 0 . 0 0 0 . 0 0 0 . 0 00 . 0 0 0 . 0 1 0 . 0 2 0 , 0 00 . 0 0 0 . 0 0 0 . 0 0 0 . 0 0

assemblage quartz * microcline + muscovite + cor-dierite + biotite range from relatively Mg-rich com-positions for Loon Lake samples, in which biotite hasan Mg/(Mg * Fe) ratio of about 0.50 and cordieritea ratio of about 0.65, to more Fe-rich compositions

for Mayhew Lake samples in which biotite has anMgl(Mg + Fe) ratio of 0.35 and cordierite a ratio ofabout 0.50. Because this assemblage quartz * micro-cline + muscovite + cordierite + biotite is invariantunder constaat T, P, and &u,o conditions, the ob-

Table 2. (continued)

HYPERSTHENE

Samplel l6b

MICROCLINE

)5t 35q )9a 39r 4E fl 6b

PLAGIOCLASE

l6b 35f 35q )9a

SiO2Ti02Al2o3MeoFe-o2MnOCaONa20K e Oejo3Total

5 t . 2n )3 . 4

1 8 . 02 6 . 70 . Jo . 20 . 00 . 0

a a (

6 5 . 1 6 6 . 8 6 4 . E 6 5 . 8 6 5 . 9 6 5 . 50 . 0 0 . 0 0 . 0 0 . 0 0 . 0 0 . 0

1 9 . 3 1 8 . 8 1 9 . 8 1 8 . 7 1 9 . 7 t 8 . 70 . 0 0 . 0 0 . 0 0 . 0 0 . 0 0 . 50 . 2 0 . 2 0 . 2 0 . 2 0 . 4 0 . 00 . 0 0 . 0 0 . 0 0 . 0 0 . 0 0 . 00 . 2 0 . 0 0 . 0 0 . 0 0 . 0 0 . 11 . 7 2 . t 1 . 9 1 . 8 t . J 1 . 8

1 4 . 0 1 4 . 2 1 4 . I l 3 . E 1 4 . 8 t 3 . 2

68.7 60 .9 66 .7 62 .0 67 .6 69 .90 . 0 0 . 0 0 . I 0 . 0 0 . 0 0 . 0

l 9 . l 2 5 . 8 2 t . 5 2 5 . 5 2 2 . 6 2 1 . 00 . 0 0 . 0 0 . 0 0 . 0 0 . 0 0 . 00 . 2 0 . 2 0 . 3 0 . 2 0 . I 0 . 20 . 0 0 . 0 0 . 0 0 . 0 0 . 0 0 . 00 . 3 7 . 3 2 . 9 5 . 5 2 . 7 0 . 7

l l . 5 7 . 4 1 0 . 8 8 . 8 1 0 . 9 t 0 . 70 . 1 0 . 2 0 . 3 0 . 2 0 . 2 0 . 2

100.7 102. I 100.8 101.3 t02 .1 99 .8 99 .9 I01 .8 102.6 102.2 104.1 102.7

FORMULA PROPORTIONS'

SiAIT,iM gFeM n

NaK

t . 9 40 . 1 50 . 0 01 . 0 20 . 8 30 . 0 10 . 0 l0 . 0 00 . 0 0

2 . 9 7 3 . 0 0 2 . 9 5 3 . 0 11 . 0 4 0 . 9 9 l 0 6 0 . 9 90 . 0 0 0 . 0 0 0 . 0 0 0 . 0 00 . 0 0 0 . 0 0 0 . 0 0 0 . 0 00 . 0 1 0 . 0 1 0 . 0 1 0 . 0 10 . 0 0 0 . 0 0 0 . 0 0 0 . 0 00 . 0 I 0 . 0 0 0 . 0 0 0 . 0 00 . l 5 0 . 1 8 0 . 1 7 0 . 1 60 . 8 1 0 . 8 2 0 . E 2 0 . 7 9

) q 7 ? n l

1 . 0 4 l . 0 l0 . 0 0 0 . 0 00 . 0 0 0 , 0 30 . 0 1 0 . 0 00 . 0 0 0 . 0 00 . 0 0 0 . 0 00 . 1 2 0 . 1 60 . 8 5 0 . 7 8

3 . 0 0 2 . 6 7 2 . E 8 2 . 7 0 2 . 8 6 2 . 9 70 . 9 9 t . 1 3 1 . 0 9 1 . 3 t l . 1 3 1 . 0 50 . 0 0 0 . 0 0 0 . 0 0 0 . 0 0 0 . 0 0 0 . 0 00 . 0 0 0 . 0 0 0 . 0 0 0 . 0 0 0 . 0 0 0 . 0 00 . 0 1 0 . 0 1 0 . 0 1 0 . 0 1 0 . 0 0 0 . 0 00 . 0 0 0 . 0 0 0 . 0 0 0 . 0 0 0 . 0 0 0 . 0 00 . 0 1 0 . 3 4 0 . t 4 0 . 2 6 0 . t 2 0 . 0 )0 . 9 8 0 . 6 2 0 . 9 0 0 . 7 4 0 . 8 9 0 . 8 80 . 0 0 0 . 0 1 0 . 0 2 0 . 0 1 0 . 0 1 0 . 0 1

I all Fe as FeO2 tl2o by differencer based on total positive charges of 12 (hypersthene), 15 (microcline and plagioclase), 22 (muscovite

and biotite), 24 (chlorite), and 36 (cordierite)

L/IBOTKA ET AL.: ROVE FORMAT'ION, MINNESOTA

served range in mineral compositions indicates thatthe rocks containing this assemblage equilibratedover a range in metamorphic grade. The positions ofthe three-phase fields on the AFM diagram in Figure3 indicate the relative grades of metamorphism underwhich the various muscovite + cordierite + biotiteassemblages formed. Displacement of the field iscaused by the continuous dehydration reaction

muscovite + biotite + quartz : cordierite+ microcline + HrO

An increase in 7 or a decrease in ,rH,o drives this re-action to the right, and because biotite is more iron-rich than coexisting cordierite, the three-phase fieldcordierite * biotite * muscovite is displaced to moreiron-rich compositions. Hence, the samples fromMayhew Lake (35f,q) formed under higher-gradeconditions than the samples from Loon Lake (39a-r).

The relative difference in grade between the LoonLake and Mayhew Lake localities is reversed fromthe anticipated difference based on the grain sizesand the distance from the high-grade KakakebicTrail locality. An independent estimate of the rela-tive grade is based on the partitioning of Na and K

between coexisting muscovite and microcline (Fig.4). Microcline is more Na-rich than coexisting mus-covite, but the partition coefficient Ko is not con-stant. Ko is a function of temperature and composi-tion, but because the range in Na/K for thecoexisting phases is limited to K-rich compositions,Ko is essentially a function of temperature. BecauseKo approaches 1.0 as temperature increases, samplesfrom Mayhew Lake equilibrated at lower temper-atures than the samples from Loon Lake. On thebasis of compositions of coexisting cordierite andbiotite, the lower-temperature Mayhew Lake samplesformed at higher grade than the higher-temperatureLoon Lake samples; thus variations in fluid composi-tion strongly influenced the equilibrium muscovite +phlogopite + quartz : cordierite * microcline *HrO.

The only andalusite-bearing rock (sample 48f) wascollected from the Gunflint Palisades locality (Fig. l)and contains the assemblage quartz + andalusite +cordierite + biotite * muscovite + microcline + il-menite + graphite. Andalusite occurs as relativelylarge (l-2 mm) crystals with chiastolite habit in agraphite-rich matrix. The assemblage is unusual be-

K A lS isOe

NoAlSisOePlogioclasc CoAlzSizOe)

(No/K)Microcrnc

Fig. a. (A) Compositions of coexisting feldspars, and (B) distribution of Na and K between coexisting microcline and muscovite.

BA

o

; o.2oo6J

=\<o

: O. l

o.?o . l

L./IBOTKA ET AL.: ROVE FORMATION. MINNESOTA

cause the composition of biotite is very Mg-rich, andbecause the coexisting cordierite is more Fe-rich thanbiotite. The compositions of these minerals are givenin Table 2 and shown on Figure 3. This reversal inFe-Mg partitioning suggests that the assemblagemay not have been in equilibrium, but there is notextural evidence for disequilibrium, and the compo-sitional relationship between cordierite and biotite isconstant over the area of a thin section. Mg-rich as-semblages in pelitic schists are not well documentedin the literature for comparison, and the assemblagein sample 48f is tentatively accepted as an equilib-rium assemblage. Because plagioclase is absent, thisassemblage indicates that in the calcium-free systemthe reaction muscovite + albite * quartz : andalu-site + microcline * HrO has occurred. In the me-dium-grade rocks at Loon Lake muscovite coexistswith plagioclase as sodic as An,r; therefore andalusiteand microcline coexist only in very Na-rich, Ca-poorrocks.

The partitioning of Fe and Mg between coexistingcordierite and biotite in medium-grade and high-grade samples is illustrated in Figure 5a. In all sam-ples except number 48f biotite is more Fe-rich thancoexisting cordierite, but the magnitude of the distri-bution coefficient seems to depend on composition.

Guidotti et al. (1975a) find no dependence of Ko oncomposition for coexisting cordierite + biotite +muscovite (no microcline) in very magnesium-richpelitic schists, and Guidotti et al. (1975b) also find norelation between'IAl and Mgl(Mg + Fe) in biotite.Figure 5b shows an inverse correlation between Mg/(Mg + Fe) and '"Al which is a gauge of the amountof substitution of cations other than Fe and Mg inbiotite. Herein lies the reason for the compositionaldependence of Ko. If the Al content of biotite is con-stant, then the partition coefficient may be directlyrelated to the equilibrium constant for the exchangereaction

aMg-biotite + BFe-cordierite : aFe-biotite+ BMg-cordierite

where a and B are the numbers of exchangeable cat-ions in cordierite and biotite, respectively. The equi-librium constant is (a."-o,",,,")'( a*"-.",u)B / (a*s-biotit )"(aF"-."d)8. If the Fe-Mg solutions are ideal, then the equi-librium constant equals (Ko)"u (Albee, 1972) and K"is independent of composition. If, however, some ofthe Fe in biotite is associated with octahedral Al,then this Fe cannot exchange with Mg in cordierite,and Ko is not related in a simple way to the equilib-rium constant for the exchange reaction. Instead of

B.

aq

o.

o.

o.

(Du-+ctl

=

c'l

o

.9EoC)

LL

ql

Q'lo

t.

o.?

r6ba*"&'::

a35f

t .o t .2 1.3rvA I

t .5t .4

I og (ttttg Z Fe ) g; ot iteFig. 5. (A) Distribution of Fe and Mg between coexisting cordierite and biotite in medium-grade and high-grade rocks. Cordierite is

generally more Mg-rich than biotite exccpt for the Mg-rich sample 48f. The lines Ko : 1.00 and Ko : 1.78 are shown for reference. (B)Inverse correlation between Mg/(Mg + Fe) and IVAI content in biotite.

LABOTKA ET AL: ROVE FORMATION. MINNESOTA

two components related by the substitution of Fe forMg, biotite from the Rove Formation contains addi-tional components that relate an increase in Al andTi to an increase in Fe. In this case, XBl" includes Fefrom both the annite and the other components, andK, will be different from the equilibrium constantfor the exchange reaction in which only the molefraction of annite is considered.

The assemblage quaftz + biotite * cummingtonite* plagioclase occurs in two samples from Loon Lake(39) and one from Mayhew Lake (35). The amphi-bole is very fine-grained and is intergrown with thequaftz + plagioclase matrix. Microcline does notcoexist with cummingtonite, but the compositions ofcoexisting biotite and amphibole are shown on Fig-ure 4 to illustrate the partitioning of Fe and Mg be-tween them.

High grade

Rocks from the Kakakebic Trail show the greatestdegree of recrystallization, and some show extensiveinteraction with the Duluth Complex during em-placement. Here the Duluth Complex intruded thelower, carbonaceous part of the Rove Formation sothat graphite has recrystallizgd to 0.25 to 0.50 mmgrains. Cordierite forms spherical porphyroblastswith no inclusions and unusual twin patterns. Plagio-clase grains are -0.25 mm in diameter and havepolygonal forms.

Several samples show a variety of textures withinsmall areas and contain unusual assemblages that in-dicate assimilation and reaction of the Rove with thegabbro. There are regions in sample l6b that containtabular, igneous plagioclase, and adjacent regions inwhich the plagioclase forms a polygonal aggregate.The shapes of hypersthene grains range from sub-hedral with few inclusions to very irregular poikilo-blasts.

Pelitic rocks near the contact with the DuluthComplex contain the muscovite-free assemblageq\aftz + biotite * cordierite + K-feldspar + plagio-clase + rutile + graphite * pyrrhotite. The composi-tions of coexisting cordierite * biotite in K-feldspar-bearing rocks are shown in Figure 6.

Most of the Rove Formation that crops out onKakakebic Trail occurs as inclusions in the contactzone with the gabbro. The assemblage in these hy-brid rocks is quartz + biotite * K-feldspar + hyper-sthene + plagioclase * ilmenite + graphite + pyrrho-tite. The compositions of coexisting biotite andhypersthene are shown on Figure 6. Many peliticrocks adjacent to the gabbro are deficient in quartz

FeO,/ - .- .< \ MqoHyperslhene

Fig. 6. Distribution of Fe, Mg, and Al among minerals in high-grade rocks. Samples l8c and l6b do not contain quartz. Samplcsl6a and l6i do not contain microcline.

and suggest that the silica-rich country rock has re-acted with the silica-poor Duluth Complex. Assem-blages in two of the analyzed rocks are cordierite +biotite * hypersthene * K-feldspar + plagioclase +itnenite + graphite + pyrrhotite (l6b) and cordierite+ sillimanite * corundum + K-feldspar * Zn spinel(gahnite) (l8c). The compositions of these quartz-free assemblages are also shown in Figure 6. Peliticschists have interacted with the gabbro through thereactions

(a) biotite + SiO, : cordierite + hypersthene+ K-feldspar + HrO

(b) muscovite * quartz : sillimanite+ K-feldspar + HrO

and

(c) muscovite : corundum * K-feldspar * HrO.

The activity of SiO, in sample 18c was buffered bythe reaction

sillimanite : corundum + SiOr.

In sample l6b the ratio f ,,o/a"io, was determined bythe compositions of coexisting biotite + cordierite +hypersthene + K-feldspar through reaction (a).

AKFM facies series

In well-documented studies of contact- or low-pressure regional metamorphism of pelitic schists(Santa Rosa, Nevada, Compton, 1960; Steinach, Ba-

LABOTKA ET AL.: ROVE FORMATION, MINNESOTA

varia, Okrusch, 1969; Alsace, Bosma, 1964; Buchan,Read, 1952; Abukuma, Miyashiro, 1958) biotite +andalusite is a very common assemblage, and La-botka (1978) demonstrated for the Panamint Moun-tains that rocks with Mgl(Mg + Fe) in the range 0.35to 0.56 contain andalusite. The rarity of andalusite-bearing assemblages in the Rove Formation was puz-zling in light of these studies. However, in the RoveFormation muscovite + quartz + cordierite is stableover a large range in Mg/(Mg + Fe) from 0.09 to0.50, and the biotite that coexists with the most iron-rich cordierite has Mg/(Mg + Fe) : 0.35. Microclinealso coexists with this biotite * cordierite + mus-covite assemblage, and because the four minerals arerelated by the continuous reaction

muscovite + biotite * quartz : cordierite +microcline + HrO

muscovite cannot coexist with magnesium-rich bio-tite. Thus, andalusite in pelitic schists is restricted toiron-rich compositions. These phase relations aresimilar to those reported by Tilley (1924) for the con-tact metamorphism of pelitic schists in the Comriearea, Scotland.

The facies series for progressive contact meta-morphism of the Rove Formation is shown in Figure7a-b. This series progresses from the low-grade as-semblage muscovite + quartz + chlorite + biotite +microcline to the medium-grade assemblage mus-covite + quartz + biotite * cordierite * microcline.The reaction sequence is based on the observed orderof increasing Fe content in coexisting minerals: cor-dierite, chlorite, biotite. The unusual Mg-rich assem-blage in sample 48f is neglected. The order of thegeneralized reactions assumes that H'O is evolvedwith increasing grade:

(l) chlorite + microcline : biotite * muscovite *qaartz + HrO

(2) chlorite + andalusite + quartz : cordierite +HrO

(3) muscovite + chlorite + quartz : andalusite +biotite + H,O

(4) chlorite * muscovite * quartz: cordierite + mi-crocline + HrO

(5) chlorite * microcline + quartz : cordierite *biotite * muscovite + HrO

(6) chlorite + muscovite l quartz: cordierite + an-dalusi te*biot i te+HrO

(7) biotite * muscovite * quartz : cordierite + mi-crocline + HrO

Figure 7c illustrates a slightly different sequence of

reactions if reaction I goes to completion over the en-tire range in Mgl(Mg * Fe) before reaction 4 occurs.

Figure 7 predicts a variety of assemblages thatought to have formed in rocks with compositions onthe plane chlorite * muscovite * quartz but are notobservedt vrz. chlorite * cordierite * microcline andchlorite + cordierite + biotite. Low-grade assem-blages in pelitic rocks include chlorite, chlorite *biotite, or rarely chlorite * biotite * microcline, andthese react to chlorite-free cordierite * biotite assem-blages with no apparent intermediate steps. Eitherthe intermediate reactions that limit the stability ofchlorite occur over a very narrow temperature inter-val, or the intermediate reactions were oversteppedbecause of the rapid temperature rise during em-placement of the gabbro complex. The total masstransfer recorded in these rocks is the sum of the in-termediate reactions: chlorite + muscovite f quartz--+ biotite + cordierite * microcline + HrO. The co-efficients in this mass-balance relation depend on thebulk-rock composition.

Intensive Parameters

P re s sur e and t emperature

Pressure, temperature, and activities of volatilecomponents may be calculated from the composi-tions of coexisting phases in the assemblage quartz *biotite * muscovite + cordierite + microcline * il-menite + graphite * pyrrhotite which occurs at theGunflint Palisades, Loon Lake, and Mayhew Lakelocalities. Pressure estimates can be made from theestimated thickness of overburden at the time ofmetamorphism. The Duluth Complex was emplacednear the surface, and the aggregate stratigraphicthickness of the gabbroic rocks and overlying vol-canic rocks near Lake Superior was 15 krrr. However,Weiblen and Morey (personal communication, 1979)infer that both the plutonic and extrusive rocks thinnorthward to near zero thickness in the vicinity ofGunflint Lake. Simmons et al. (1974) estimated aminimum pressure of 2.0 kbar during metamorphismof the underlying Gunflint Iron Formation, a pres-sure that is based on the composition of iron-rich pi-geonite produced during metamorphism.

A calcsilicate rock (sample 23) which occurs at theGunflint Trail locality contains the assemblage diop-side + anorthite * sphene + sphalerite + pyrite +chalcopyrite, and an additional estimate of the at-tending pressure may be based on the FeS content ofsphalerite. Scott (1973, 1975) calibrated the composi-tion of sphalerite that coexists with pyrite and pyr-

atzsios

80 LABOTKA ET AL: ROVE FORMATION. MINNESOTA

Fc-Chlori tc Mg-Chlori lc

Fc-Chlori tc O.5 Me -Chlorilr

F

B i

M i c r

Fig. 7' Generalized facies series for the low-pressure, contact metamorphism of the Rove Formation. (A) Facies types for muscovite+ quartz-bearing rocks. Low-grade rocks are represented by Tr, (although the three-phase field biotite + chlorite + microcline occurs atmore Mg-rich compositions) and medium-grade rocks are represented by T5. High-grade rocks are not represented. (B) Pseudobinary Z-Xdiagram on the join chlorite + muscovite + qtrafiz for the facies series shown in A. (C) T-X diagramfor an alternate facies series inwhich the reaction chlorite + microcline : biotite + muscovite + quartz + H2O goes to completion over the entire range in Mgl(Mg +Fe) before chlorite begins to break down to cordierite + biotite.

A

T,

T2

T3

T.

B

T5

T.

T5T2

T,

T5

rhotite for pressures (independent of temperature ex-cept at high temperatures) down to 2.5 kbar.Pyrrhotite does not occur in sample 23, so that theFeS content in sphalerite indicates a maximum pres-

sure. The composition is ZnorrFeorrS and indicatesthat the maximum pressure was less than 2.5 kbar.Metamorphism of the Rove Formation probably oc-curred at a pressure of 1.5 kbar. This value is the

M i c r

Mic r

Mic r

LABOTKA ET AL: ROVE FORMATION, MINNESOTA 8 l

minimum pressure suggested by Simmons et al.(1974) minus 0.5 kbar, because of the higher strati-graphic position of the Loon and Mayhew Lake sam-ples relative to the Gunflint Iron Formation.

Temperatures may be calculated from the compo-sitions of coexisting plagioclase and alkali feldspar,

and the calibration of Powell and Powell (1977) hasbeen used for several localities that contain assem-blages with coexisting feldspars (Table 3). Thesefeldspar temperatures are generally consistent withmetamorphic grade, but their absolute values seemlow in comparison with estimates from the stability

Table 3 Temperatures calculated from coexisting K-Na-Ca phases

SA[,I PLE PLACIOCI,ASE

Xab Xun Xo.

r \T ICROCL INE

Xab X"n Xo.

N4I. lSCOVITE

X,.nrsc Xp..

Kil Tz Tt

o- o-

l 6 b

16j

1 8 a

l z a

26

35f

)5q

39a

)9 t

39c

39d

39r

4 )d

48 f

4 . 6 1

8 . 2 7

2 . 4 5

2 . 5 9

'1 7L

3 . 5 8

l ' 2 0

) q /

566

490

497

) T J

555

482

548 r1 0

4 8 4 t 1 0

620+5

6I i+7

627 +7

568t I 0

57 0+ I 0

544+ 12

0 . 5 4 0 0 . 3 5 0 0 . 0 I 0

0 . 5 5 0 0 . 3 4 0 0 . 0 1 0

0 . 7 5 2 0 . 2 ) 9 0 . 0 0 9

0.897 0 .096 0 .007

0 . 8 3 7 0 . 1 5 1 0 . 0 1 2

0 . 8 5 7 0 . t 2 9 0 . 0 r 4

0 . 7 3 3 0 . 2 5 3 0 . 0 1 4

0 . 8 7 0 0 . 1 1 9 0 . 0 1 1

0 . 9 5 4 0 . 0 3 3 0 . 0 I 3

0 . 9 0 0 0 . 0 7 2 0 . 0 2 8

0 . 1 5 6 0 . 0 0 8 0 . 8 3 6

0 . t 8 0 0 . 0 1 0 0 . 8 1 0

0 . 1 4 7 0 . 0 0 1 0 . 8 5 7

0 . t 9 ) 0 . 0 0 0 0 . E 0 7

0 . 2 6 1 0 . 0 l t 0 . 7 2 8

0 . r 8 2 0 . 0 0 1 0 . 8 1 7

0 . 1 6 9 0 . 0 0 I 0 . E 3 0

0 . 1 5 8 0 . 0 0 1 0 . 8 3 1

0 . \ 4 2 0 . 0 0 0 0 . i i 5 8

0 . 1 2 8 0 . 0 0 0 0 . 8 7 2

0 . 1 5 0 0 . 0 0 0 0 . 8 5 0

0 . I 2 0 0 . 0 0 0 0 . 8 E 0

0 . 0 8 1 0 . 0 0 2 0 . 9 1 7

0 . 1 1 5 0 . 0 0 0 0 . 8 8 4

0 . 9 5 4 0 . 0 4 6

0 . 9 7 6 0 . 0 2 4

0 . 9 2 4 0 . 0 7 6

0. r+( ) 0 .060

0 . 9 4 t 0 . 0 5 9

0 . 9 5 5 0 . 0 4 5

0 . 9 6 1 0 . 0 J 9

0 . 9 7 1 0 . 0 2 9

4 r 8379

_ m l c a m l c r .I x grylslq r

L ) m i ca m ic r(Xpar Xor )

2 Temperature calcu. lated f rom coexist ing fe ldspars (Powel l and Powel l , 1977).

m i c r 2 m i c r- ( xo .

- ) - l $30 * 0 .093p + 2x ; t - ( 1340 + o .0 l ep ) I

, . -Jnicr l micr 2 micrRrn |

^ *P^= I . ( *o . ) ( - \ . e t + 1 .54 X "6 )

I ptag I

I x a u )3 Temperature calculated f rom coexist ing muscovi te and microcl ine (Thompson, 1974).

Arr x/RT - 6.489q - Gtt9.54lT ' )

Au x - (ur6156- 1 lp3;) - (uor-ps6) = RTlnKp + (W6*r-W6oy) + 2(W6'a-2wcms)X$fs%

+ 3(w6, ' . -w6p"Xxf i j .%)2 - 2(w636-2w6o.)X#.. - 3(w5,61-v/6abxxTicr)2

W6"6 = 6326.7 + 0.0925p - ! .9??] I Waldbaum and Thompson (1969)

Vco r = 767 1 ,8 + O . l l 2 lP - 3 . 85657

VtcDa = 3082.1 + 0.0822p + 0.1598T Eu8ster et a l ' (1972)Wcms = 416).9 + 0.1259p + 0.39547

Errors represent a +10% error in the wt.% K2O in muscovl te

P = 1500 bars

LABOTKA ET AL.: ROVE FORMATION. MINNESOTA

of pigeonite in the iron-formation: 547oC fromsample l6b at the gabbro contact based on coexistingfeldspars, and 800"C from Simmons er al. (1974)based on pigeonite stability.

Thompson (1974) calculated the Na-K exchangepotential, Ap, for the reaction muscovite + albite :paragonite * orthoclase, and temperatures may alsobe calculated from the compositions of coexistingmuscovite and microcline. He calculated two ex-pressions for Ap. The first is derived from end-mem-ber dehydration curves, but predicts unrealisticallylow temperatures (--25oC) for the metamorphismof the Rove Formation. The second exchange poten-tial is derived from ion-exchange experiments andpredicts temperatures that are similar to but gener-ally higher than temperatures calculated from co-existing feldspars (Table 3). The muscovite-micro-cline temperatures are consistent with the geologicsetting of the samples. Sample 35f which is adjacentto the contact of a large sill gives a temperature of548oC, and sample 35q which"is 5 m from the contactgives a temperature of 484'C. Sample 39a which isvery coarse-grained gives a temperature of 620"C;this temperature is greater than that from sample 35fwhich is finer-grained and farther from the high-grade area near Kakakebic Trail.

The errors of +l0oC in the muscovite-microclinetemperature are calculated from a generous tl}Vo er-ror in the wt.Vo KrO in muscovite. Realistic errors inthe temperature estimates are difficult to evaluate be-cause of the lack of agreement between the two-feld-spar and muscovite-microcline values, and becausethe other calibration of the muscovite-microcline ex-change potential by Thompson Q97g gives unrealis-tic results. The muscovite-microcline temperaturesfor samples 39a,b,c are very close to the dehydrationof muscovite + quartz, which suggest that they aretoo high and that the associated errors must begreater than those derived from analytical uncer-tainty. The errors in temperatures calculated fromthe two-feldspar calibration of Stormer (1975) andPowell and Powell (1977) are probably also large be-cause the thermometer is relatively insensitive to thealbite content in plagioclase at temperatures less than600'C (Fig. 3 in Stormer, 1975). The actual temper-atures during metamorphism may have been muchlower than the muscovite-microcline value, but theywere probably higher than the two-feldspar value.

Vo latile-p hase c omp o sition

The medium-grade samples from Loon Lake (39)are coarse-grained, record higher muscovite-micro-

cline temperatures, and are closer to the high-graderegion than samples from Mayhew Lake (35), but thecompositions of coexisting biotite and cordierite inthese samples indicate that the Mayhew Lake sam-ples recrystallized at 3 "higher grade." Thus, regionalvariation in the composition of the fluid phase musthave affected the relation

muscovite + biotite * quartz : microcline *cordierite + HrO.

Portions of the system MgO-AlO,-KrO-SiOr-H,Ohave been experimentally investigated by Schreyer,Yoder, Seifert, and others, and the data have beensummarized by Schreyer and Seifert (1969). Schreyerand Seifert (1967) reported the results of experimentsfor the reaction muscovite + phlogopite + quartz :cordierite * microcline + HrO which seems to haveoccurred at I kbar, 550o+l5oc, 2kbar,600o+l5oC,3 kbar, 670o+15o, 4 kbal 690otl5o, and 5 kbar,7lOotlOoc (P",o : P'). These data were fitted to arelation 0 : (A/T) + B + (CP/T) * h.f",o, in whichC is the volume change of solid phases divided by thegas constant. The least-squares result is

0 : (1.126 x 104 / T) - 19.96 + (0.128P/ T) * ln./",o

The total varianc€, d: 2(predicted value - mea-sured value)'z/(degrees of freedom), is 358, whichpredicts temperatures within the error estimated forthe location of the reaction. This relation can then beextended to include the effects of solid solution:

0 : (9.004 x 104/T) - t59.7 + (1.022p/T) 4 tn IQo

where

ln I(-o : 8 ln afl" + 3 ln affJ9"*o+ 8 tn /H,o - 6 ln aHl,T" - 2 ln ar-'ii""

for the stoichiometry

6 muscovite * 2 phlogopite + 15 quartz: 8 orthoclase * 3 cordierite + 8 HrO

The fugacity of HrO for a given temperature andpressure may be determined from the compositionsof coexisting phases in the assemblage muscovite *microcline * cordierite + biotite + qvaftz, providedthat the activity-composition relations for the miner-als are known.

The activity-composition relations of Waldbaumand Thompson (1969) for alkali feldspars and ofEugster et al. (1972) for white micas have been used,and cordierite is considered to behave ideally (4r"re_"o.a: X""). The inverse correlation of Mg and ''Al con-tents in biotite from the Rove Formation (Fig. 5b)

LIIBOTKA ET AL.: ROVE FORMATION, MINNESOTA

Table 4. /n,o in medium-grade Rove Formation

83

SA[4PLE TI CORDIERITE BIOTITE MLISCOVITE2 MICROCLINE2 f i 14HZO HZO

rc X M 8 Ti VIn t V I -u M g

mlcaXr r - RT InYr v , 5

musc

feldRTInY

mlcrbars

35f.

35q

39a

39b

39c

)9d

)9r

48f

54E

484

620

6t3

627

568

570

544

n 5 l

. 0 . 5 2

0 . 5 6

0. 58

0 . 5 6

0 . 5 0

0 . 5 6

0 . 9 l

4 . 5 2

1 . 2 0

t2.78

7 . 8 2

/ . ) )4 . 3 2

t . z z

t . 7 6

45.82

4 7 . 4 2

2 6 . 9 9

L 7 . 4 3

12.)6

20.54

1 6 . 8 9

15. )2

252

83

903

tt26

954

422

423

379

0 . 2 2 0 . ) 5

0 . 2 3 0 . ) 6

0 . 2 0 0 . 3 0

0 . 0 9 0 . 4 4

0 . 2 0 0 . . 3 6

0 . t 7 0 . 4 3

0 . 1 3 0 . 4 4

0 . 0 4 0 . 2 5

0 . 2 5 0 . 7 5

0 . 2 2 0 . 8 4

0 . 3 0 1 . o i

0 . l 4 l . 1 9

0 . 2 6 1 . 0 2

0 . 2 5 | . 0 2

0 . 1 8 1 . 0 5

0 . 1 0 2 . 4 4

| . 4 3 0 . 2 5

t . J 5 ' 0 . 2 8

l . 1 7 0 . 3 4

r . 1 4 0 . 4 0

l . 1 6 0 . 1 4

l . l 1 0 . 1 4

| . 2 0 0 . 1 5

0 . t 7 0 . 8 1

I Temperature derived from coexisting muscovite and microcline, Table 3.

2 RTlnYl = ( l -Xl)2(Wcl+2(W6;2-W6 1)X1). X and W6 for muscovi te and microcl ine are given in Table 3.

, t n r 6 c a l c u l a t e d f r o m t h e e q u i l i b r i u m 6 m u s c o v i t e + 2 p h l o g o p i t e + l S q u a r t z = 3 c o r d i e r i t e + 8 m i c r o c l i n e + 8 H 2 O .

U fHzO calculated f rom the equi l ibr ium muscdvi te + quartz = andalusi te, + microcl ine + H2O (Chatter jee and Froese, 1975).

o = 1500 bars.

suggests that all Mg occurs as the phlogopite com-ponent. The activity of phlogopite in biotite is as-sumed to equal the cube of the mole fraction of Mgin the octahedral site.

The fugacities of HrO calculated from the aboveexpression for eight medium-grade samples are givenin Table 4. The temperatures used in the calculationare those derived from the muscovite-microclineequilibrium. These temperatures are maximum tem-peratures, and the fugacities calculated from themrepresent maximum possible values of fr,o duringmetamorphism of these samples.

Samples 39a,b,c from Loon Lake give ,fx.o valuesclose to (or exceeding) the values for pure HrO at1500 bars (Burnham et al.,1969). The samples fromMayhew Lake (35) and Gunflint Palisades (48f1 givevalues much lower than the pure-HrO value.

Figure 8 illustrates the ranges of possible values ofP, T, and fr,o for samples 35f, 39a, and 48f. Thesevalues are determined by the biotite-cordierite-mus-covite-microcline'equilibrium, and the temperaturesderived from the muscovite-microcline exchangeequilibrium are superimposed to show the error in/",o resulting from a tl5o error. The equilibrium be-tween muscovite-microcline-andalusite (Chatterjee

and Froese, 1975) for sample 48f is also presented toshow the relative correspondence between the twoequilibria. Figure 8 shows that the calculated valuesof fr,o depend very strongly on temperature, but noton pressure. For example, an error in 7 of + l5'C forsample 35f results in an error of t50 bars in/",o butan error ir P""'o of 500 bars results in an error of only+22 bars in fr,o.Figure 8 also shows that in sample48f for Pr,o - P"o,,o the biotite-cordierite equilibriumoccurs at higher temperatures tlan the muscovite-aluminositcate equilibrium. This topology violatesSchreinemakers' rules and indicates that the relativeerror between the two equilibria is at least 25oC.Thus, even if the temperature is well known, thevalue of fu,o at P"ora : 1500 bars is only precisewithin a range of 100 bars. For example, values of

/n,o calculated for sample 48f at 544"C are 379 barsand 483 bars from the cordierite-biotite and mus-covite-andalusite equilibria respectively (Table 4).

.The uncertainity is larger at greater rock pressures.The total uncertainty in the maximum /",o forsample 48f resulting from error in the maximum tem-perature and in calibration of the biotite-cordieriteequilibrium is *100 bars.

Sample 48f contains graphite and if Pa"ia : P"oria

LABOTKA ET AL: ROVE FORMATION. MINNESOTA

B 39o

600 500 600

T ("C)

Fig. 8. The equilibrium muscovite + biotite + quartz : cordierite + microcline * H2O for samples 35f (A), 39a (B), and 48f (C).Muscovite-microcline temperatur€s are given to show the effects of an error of tl}Vo on K2O in muscovite on the calculated /uro. Theequilibrium muscovite + quartz : andalusite + microcline + H2O for sample 48f is also shown in C to illustrate the relative errorbetween the equilibria for determining /""o. Broken lines in C correspond to the muscovite-andalusite equilibria.

35fA

oo-o

(L

700

and if C-O-H gas species are the only importantcomponents in the fluid phase, then the compositionof the fluid phase can be calculated by the methodreviewed by Eugster (1977). The results for the maxi-mum values of /",o are given in Table 5. At Po",o :1500 bars the maximum fraction of HrO in the fluidis 0.65+0.10, and most of the remainder of the fluidis CO,. The composition of the fluid at 500oC is alsogiven in Table 5 to show that, at lower values of /".o,the ratio f .ro/f .o, is smaller and /o, is larger than athigher values of _/",o. The maximum value of /o, forsample 48f is on the order of 10-23 at 550oC, which isclose to the value for the quartz-magnetite-fayalitebuffer. The graphite-free rocks from Mayhew Lake(samples 35) were probably metamorphosed undermore oxidizing conditions, and the coexisting fluidconsisted almost entirely of HrO and COr. Samples39a,b,c from Loon Lake appear to have equilibratedwith a relatively pure-HrO fluid. These samples werecollected near a fault zone that was active during theemplacement of the Duluth Complex, and the com-positions of the fluids percolating through the faultzone appear to have been very different from thecompositions of 'local' fluids.

The relatively large amount of CO, in the fluidphase and the prevailing f o,near the quartz-magne-

tite-fayalite buffer are consistent with the suggestionsof Floran and Papike (1978) and Simmons et al.(1974) for the composition of the fluid phase duringmetamorphism of the underlying iron-formation.The fluid phase may have communicated with alarge volume of rock during contact metamorphism;indeed, the assumption that Po",o : P*,,o may not bevalid for low-pressure contact metamorphism. Theeffect of permeability (Po",o < P*,.) on the calcu-lation of the fluid composition is to increasa X^,o,f "to/f "o,,

and /",, and to decrease./o,. For example,in sample 48f, if the fluid pressure was 1000 bars (in-stead of 1500 bars) X',o : 0.69, f .',: 125 bars, /.o,:287 barc, f o,: 4 x l0-'zo bars, /", : 9.5 bars, and/.o: 1.7 bars.

Table 5. Fluid composition for sample 48f

fxzo lcoz Jcsq fco tn2 loz XH2o xcoz

1.2 x lo -234J x rc-241.8 x l0 -23

3791 786 464ut 292 2oozoo2 t25o 8

2 , 8 5 . 7t , 7 t 2

0 . 5 4 0 . 4 40 . 7 6 0 . 1 60 . 3 1 0 . 6 9

I Range in the maximum values of I9.6 derived from maximum possible

^ temierature oI 544oC t"z Values calculated for a temperature of 500t to show effect of lower

temperature on f lu id composit ionp = 1500 bars

a+

F

LABOTKA ET AL.: ROVE FORMATION. MINNESOTA 85

Summary

The contact metamorphic effects of the DuluthComplex on the argillaceous Rove Formation areconfined to the region within a few hundred metersof the contact. In this region the low-grade assem-blage chlorite * muscovite + quartz reacted to formthe medium-grade assemblage cordierite * biotite +microcline. Locally adjacent to the Duluth Complexthe high-grade assemblages corundum + microclineand hypersthene * cordierite formed.

The compositions of the coexisting minerals in themedium-grade assemblage biotite + cordierite + mi-crocline provide estimates of the maximum values oftemperature and partial pressure of HrO at 1500bars. These values indicate that medium-grade as-semblages formed in the range 500o to 600'C andthat for most samples P",o = rhP"orio. The composi-tions of coexisting cordierite and biotite are sensitiveto variations in /,,,o. On the basis of texture, distancefrom high-grade area, and partitioning of Na and Kbetween muscovite and microcline, sample 35fformed at a lower temperature than sample 39a, butsample 35f contains a more dehydrated, higher-gradeassemblage than 39a.

The contact metamorphic effects of the DuluthComplex on the Rove Formation contrast sharplywith the effects on the Gunflint Iron Formation. Theiron-formation preserves prograde reactions as far asl0 km from the contact. The differences between theiron-formation and the Rove Formation are, in part,the result of the large number of iron silicates thatmay occur in rocks poor in aluminum and potassium.The pelitic Rove Formation lacks a variety of assem-blages and records few reactions because in low-pres-sure, contact metamorphic aureoles the assemblagebiotite * cordierite * microcline is stable over largeranges in Mel(Mg * Fe), in Z, and in -/",o.

The iron-formation appears to have attained tem-peratures as high as 800oC. Medium-grade RoveFormation records maximum temperatures of 600oC,but no reliable estimates of metamorphic temper-ature of the high-grade region can be made. There isno evidence of partial melting of the high-gradeRove Formation, but the minimum melting temper-ature of the assemblage quartz + biotite + plagio-clase * hypersthene under the condition P'zo ( P,oriais probably greater than 800oC (Luth, 1967). Becauseall other factors regarding the depth of emplacementand temperature of the magma at emplacement areequal, the differences in temperatures recorded bythe iron-formation and Rove Formation at equiva-lent distances from the contact must be due to differ-

ences in the thermal properties and permeabilities ofthe two rock types.

AcknowledgmentsResearch on the contact metamorphic effects of the Duluth

Complex was supported by NSF grant EAR 7'123150. We thankC. White for field assistance, R. Wintsch, C. Klein, and an anony-mous referee for critical reviews, and R. Spencer and L. Koh fortheir technical help on the assembly of the manuscript.

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Manuscript received, December 17, 1979;acceptedfor publication, September 19, 1980.