SHALLOW LEVEL EMPLACEMENT MECHANISMS OF THE …...2007 Geological Society of America Rocky Mountain...

SHALLOW LEVEL EMPLACEMENT MECHANISMS OF THE MIOCENE IRON AXIS LACCOLITH GROUP,

SOUTHWEST UTAH

2007 Geological Society of America Rocky Mountain Section Annual Meeting St. George, Utah

May 10, 2007

FIELD TRIP LEADERS

David B. Hacker, Kent State University Michael S. Petronis, New Mexico Highlands University

Daniel K. Holm, Kent State University John W. Geissman, University of New Mexico

The Miocene Pine Valley laccolith caps the south side of the Pine Valley Mountains just north of St. George. The red-colored cliffs consist of the Mesozoic Kayenta Formation and Navajo Sandstone.

D.B. Hacker, M.S. Pertronis, D.K. Holm, and J.W. Geissman Utah Geological Association Publication 35

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SHALLOW LEVEL EMPLACEMENT MECHANISMS OF THE MIOCENE IRON AXIS LACCOLITH GROUP,

SOUTHWEST UTAH

2007 Geological Society of America Rocky Mountain Section Annual Meeting St. George, Utah

May 10, 2007

David B. Hacker Department of Geology, Kent State University, Kent, Ohio 44242

Michael S. Petronis

Department of Natural Sciences, New Mexico Highlands University, Las Vegas, NM 87701

Daniel K. Holm Department of Geology, Kent State University, Kent, Ohio 44242

John W. Geissman

Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131

ABSTRACT

The Iron Axis consists of a NE-trending belt of early Miocene (~22-20 Ma) calc-alkaline

hypabyssal plutons whose trend is partly controlled by older NE-striking, SE-verging Sevier-age thrust faults. Intrusions were forcibly emplaced within 3.0 to 0.2 km of the surface and formed structural laccoliths of high relief, with some roofs failing and producing large gravity slides and erupting ash-flow tuffs and lava flows. Our examination of many of these laccoliths, using a combination of geologic mapping and structural, petrographic, anisotropy of magnetic susceptibility (AMS), and geochronology data, shows that individual plutons are homogeneous in composition, lack evidence of chemical or pertrographic zoning, and lack visible internal contacts; all of which suggests rapid emplacement of magma. We recognize two laccolith emplacement scenarios: 1) lateral emplacement associated with older NE-striking, NW-dipping Sevier-age (Late Cretaceous to early Tertiary) thrust faults, and 2) vertical emplacement associated with NE-striking subvertical dikes. The model of laccolith development is one of continuum from thin tabular sill emplacement (either laterally or vertically emplaced) through vertical inflation by subsequent injections of magma.

Magmatism in the Iron Axis occurred in distinct phases. The first phase occurred in the Escalante Desert and produced the quartz monzonite Lookout Point pluton at 22.51 Ma. This was followed by the main phase of quartz monzonite laccoliths emplaced along the older Sevier-age thrust faults. Concordant dates from seven exposed laccoliths show rapid emplacement between 22.0 and 21.76 Ma (<0.09 Ma duration). These laccoliths formed immediately following the eruption of the compositionally identical Harmony Hills Tuff (22.03 Ma) from an unidentified caldera, and may represent resurgent magma that intruded along thrust faults outside the caldera. The third phase (after a 1.3 Ma hiatus) included the quartz monzonite Pine Valley mega-laccolith (>300 km2) at 20.46 Ma and the gabbro-diorite Iron Peak laccolith at 20.2 Ma. The last phase included the granitic Mineral Mountain laccolith at 12.1±1.9 Ma, and the associated Ox Valley Tuff with compositions closer to Caliente caldera complex magmas but is also associated with an older Sevier-age thrust fault.


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INTRODUCTION

The purpose of this one-day field trip is to examine intrusive features of five Iron Axis laccoliths (Pine Valley, Iron Peak, Three Peaks, Granite Mountain, and Stoddard Mountain) in southwest Utah (figures 1 and 2), that collectively represent two different shallow level emplacement styles (lateral vs. vertical) for intrusions in this area. We will also evaluate field and analytical evidence bearing on their rapid emplacement and growth, and their link to local crustal anisotropies in the region.

Felsic magma transport and emplacement mechanisms needed to create large volume intrusions in the middle to upper crust remain controversial and highly debated; despite the importance of plutons as fundamental building blocks of the continents (see Pitcher, 1979; Hutton, 1988; and Miller and others, 1988 for detailed reviews). The growing recognition that many of these plutons are domal or tabular in shape has renewed the interest in laccoliths as a mechanism for producing “space” or “room” for large plutons, some the size of batholiths (Clemens and Mawer, 1992; Petford, 1996; Morgan and others, 1998; Aranguren and others, 2003). However, laccoliths are normally considered small plutons with typical sizes of 1 to 10 km in diameter and less than a kilometer thick; although it has been suggested they can range up to batholith-size (i.e., > 100 km2 ) (Corry, 1988). Despite the study of laccoliths for over a century (Gilbert, 1877; Pollard and Johnson, 1973; Johnson and Pollard, 1973; Corry, 1988; Jackson and Pollard, 1988; Roman-Berdiel and others, 1995; Henry and others, 1997), their mode of emplacement and magma dynamics remain poorly understood. A two-stage growth process, whereby initial sill emplacement is followed by upward inflation and roof lifting, is now largely accepted as the mechanism by which laccoliths form (Pollard and Johnson, 1973; Johnson and Pollard, 1973; McCaffrey and Petford, 1997; de Saint-Blanquat and others, 2001; Rocchi and others, 2002).

However, the timing and magma flow processes of laccolith development remain poorly constrained. For example, recent studies are beginning to provide new information about magma flow patterns and emplacement dynamics that are shedding new light on pluton development by single magma injections versus magma sheeting (de Saint-Blanquat and others, 2001; Morgan and others, 2001; Horsman and Tikoff, 2002; Petronis and others, 2004). Studies of timing and rate of magma emplacement are showing that laccoliths and large plutons can be emplaced rapidly as single or multipulse systems in under 100,000 years (Clemens and Mawer, 1992; Petford and others, 2000; de Saint-Blanquat and others, 2001) or incrementally over millions of years by episodic injections of magma separated by periods of quiescence (Henry and others, 1997; Brown and McClelland, 2000; Coleman and others, 2004).

This field guide concentrates on laccoliths that intruded into flat-lying stratigraphy and therefore displacements of the wall rocks resulted directly from the forceful intrusion of magma and not from concurrent tectonic deformation. Therefore, the fabrics and structures within the intrusions reflect the original magma flow during emplacement and lack any tectonic overprinting. The individual laccoliths lack significant variations in chemistry and petrography, and show no internal layering. Due to the lack of physical flow fabrics, anisotropy of magnetic susceptibility (AMS) studies have been valuable for magma-flow-fabric information that can test laccolith emplacement models. At present we recognize two laccolith emplacement scenarios for the Iron Axis laccoliths: 1) lateral


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20 mi

N

20 mi

N

20 mi20 mi

NN

Stop 1

Stop 7

Stop 8

Stop 2

Stop 4

Stop 6

Stop 5

Stop 9

20 mi

N

Stop 3

Stop 1

Stop 7

Stop 8

Stop 2

Stop 4

Stop 6

Stop 5

Stop 9

20 mi

N

20 mi20 mi

NN

Stop 3

Figure 1. Location of field trip stops.


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emplacement associated with NE-striking, NW-dipping Sevier thrust faults, and 2) vertical emplacement associated with NE-striking subvertical dikes. To explain the field and analytical observations, we envision a model of laccolith development as one of a continuum from initial thin sill emplacement (either laterally or vertically fed) through vertical inflation by continuous magma injections.

GEOLOGIC SETTING

The Iron Axis laccolithic group consists of a series of early Miocene calc-alkaline hypabyssal laccoliths and associated volcanic rocks just west of the present Colorado Plateau in southwest Utah (figure 2). The Iron Axis igneous rocks are part of the general middle Cenozoic calc-alkaline igneous sequence that spans much of the western United States. This magmatism is mostly considered to be associated with oblique convergence during subduction of oceanic lithosphere beneath western North

Figure 2. Map of southwest Utah showing intrusions of the Iron Axis: B - Big Mountain; BV - Bull Valley; D - The Dairy; G - Granite Mountain; H - Hardscrabble Hollow; I - Iron Peak; IM - Iron Mountain; LP - Lookout Point; MM - Mineral mountain; PP - Pinto Peak; PV - Pine Valley; SM - Stoddard Mountain; T - Three Peaks. General trend of Sevier age thrust faults shown as dashed lines with saw-teeth. Towns: CC - Cedar City; SG - St. George.


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America that produced large fluxes of mantle-derived basaltic magma that intruded into the overlying continental lithosphere (Johnson, 1991; Nelson and Davidson, 1998; Rowley and others, 1998).

With the onset of calc-alkalic volcanism, prior to Iron Axis laccolith magmatism, a sequence of regional Oligocene and Miocene calc-alkaline andesite to rhyolite ash-flow tuffs of the Wah Wah Springs Formation (30 Ma), Isom Formation (27 Ma), and Quichapa Group (24 to 22.5 Ma) were spread over the area from sources to the northwest and west (mostly from the Indian Peak and Caliente caldera complexes; see Best and others, 1989 and Rowley and others, 1995) (figure 3). This sequence of pre-Iron Axis volcanic rocks overlie fluvial and lacustrine sedimentary rocks of the upper Paleocene-Oligocene Claron Formation, that in turn, unconformably overlie Cretaceous and Jurassic sedimentary rocks deformed during the Sevier orogeny (Late Cretaceous to early Tertiary) that produced east and southeast verging thrusting.

During Iron Axis magmatic activity, ascending magma from an inferred batholith complex intruded along one or more northeast-striking Sevier-age thrust faults before being emplaced as bulbous laccoliths within Mesozoic and Tertiary sedimentary strata (Mackin, 1960; Blank and Mackin, 1967; Blank and others, 1992; Rowley and others, 1998; Rowley, 1998) (figures 4 and 5 ). More than a dozen exposed intrusions have been mapped within the magmatic province and others are inferred from structures in the cover-rock and aeromagnetic signatures, or known from drilling and seismic data. Thus the intrusions are in a northeast-trending belt that follows the trend of the Sevier orogenic front. These intrusions were forcibly emplaced into sedimentary rocks at depths ranging mostly between 2.5 and 0.25 km from the surface and deformed their roofs by upward folding and faulting. The intrusions in the eastern Bull Valley Mountains (Bull Valley, Hardscrabble Hollow, and Big Mountain) and the Iron Springs mining district (Iron Mountain, Granite Mountain, and Three Peaks) intruded into limestone strata of the Jurassic Carmel Formation or the Temple Cap Formation and produced major replacement magnetite and hematite ore bodies. Only three plutons in the central part of the Iron Axis, making up the Iron Springs mining district, were extensively mined and constitute the largest iron ore production in the western United States (Mackin, 1960; Blank and Mackin, 1967; Barker, 1995). Iron solutions were derived from deuteric breakdown of ferromagnesian minerals in the outer selvage-joint phase of the intrusions (Mackin and Ingerson, 1960; Rowley and Barker, 1978; Barker 1995).

Plutons of the central part of the Iron Axis are quartz monzonite to granodiorite porphyries characterized by phenocrysts of plagioclase (andesine-labradorite), biotite, hornblende and/or pyroxene (diopsidic augite), and magnetite in a groundmass (1/3 to 1/2 total volume) of very fine-grained quartz and potassium feldspar. The only exceptions to this composition occurs in the Mineral Mountain intrusion at the southwest end of the, which is a granite porphyry (Morris, 1980; Adair, 1986), and the Iron Peak intrusion at the northeast end, which is a gabbro-diorite porphyry (Spurrney, 1984).

Structural and topographic relief was produced by the emplacement of the Iron Axis laccoliths. The nature of the igneous intrusions in the Iron Axis province has been historically controversial, in part because the current level of erosion makes it difficult to distinguish the intrusions as stocks or laccoliths. Only the Pine Valley and Iron Peak intrusions have been eroded to expose their planar floors. However, geophysical and


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Figure 3. Tectonic map of southwest Utah, southeast Nevada, and northwest Arizona showing trends of major structures (modified from Blank and Kucks, 1989 and Scott and Swadley, 1995).


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Figure 4. Generalized geology of southwest Utah showing cross section A-B of figure 5 through the Lookout Point pluton and the Three Peaks Iron Axis laccolith. M-Modena, E-Enterprise, N-Newcastle, CC-Cedar City, SG-St. George (modified from Hintze, 1980, and Shubat and Siders, 1988).


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re 5

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A-B

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Sevi

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drill-hole data, together with detailed field mapping, confirmed that the intrusions have nearly planar floors and irregular but concordant roofs (Van Kooten, 1988; Hacker, 1998; Hacker and others, 2002). Cross-sections show that most laccoliths have a somewhat typical plano-convex shape (Hacker, 1998) but are not circular in map view. During the emplacement of the laccoliths, several (Pine Valley, Bull Valley-Big Mountain, Pinto Peak, Stoddard Mountain, and Iron Mountain) were partially deroofed by catastrophic gravity sliding and vented to the surface (except Iron Mountain) forming ash-flow tuffs and minor lava flows that buried the gravity slide masses (Mackin, 1960; Blank and others, 1992; Hacker, 1995, 1998; Hacker and others, 2002).

Following Iron Axis magmatic activity (post-20 Ma), the area once again received additional regional calc-alkaline ash-flow deposits from the Caliente caldera complex to the west. Beginning at about 14-15 Ma, a change from calc-alkaline to bimodal (basalt and high silica rhyolite) magmatism began in the region and has persisted sporadically throughout the late Cenozoic and was accompanied by regional basin-range extensional faulting. Bimodal magmatism produced the extensive basaltic lava flows and associated cinder cones found in southwest Utah (figure 4), but their total volume is much less than that of the earlier calc-alkaline volcanic rocks (Rowley and Dixon, 2001). The basin-range style tectonic extension imprinted a dominantly northerly striking fault pattern upon previously formed structures. Regional crustal extension in the Basin and Range occurred through a complex combination of displacements along low-angle normal, high-angle normal, and strike-slip fault systems (Moores and others, 1968; Anderson, 1971; Armstrong, 1972; Wernicke and others, 1988; Anderson and Barnhard, 1993; Rowley and Dixon, 2001). Extension in the region occurred principally during the last 20 Ma (Anderson and Mehnert, 1979; Wernicke and others, 1988; Rowley and Dixon, 2001) and appears to have taken place in two episodes with two slightly different extension directions.

The prominent style of extension that produced the present topography of this eastern part of the Basin and Range Province was high-angle, basin-range block faulting (horst and graben style) that began in the late Miocene about 10 to 8 Ma (Anderson and Mehnert, 1979; Rowley and others, 1979; 1981; Shubat and Siders, 1988; Rowley and Dixon, 2001). Steep, north-to-northeast-trending, high-angle normal faults are abundant in the Basin and Range Province and synextensional sediments eroded from the uplifted blocks were deposited in the adjacent developing basins that now generally occupy more area than the ranges. Many basins have accumulated very thick sections of basin fill, such as the Newcastle graben west of the Antelope Range in the southern Escalante desert. In contrast, the Colorado Plateau to the east remained an undeformed stable highland. The boundary between the Basin and Range Province and Colorado Plateau is not sharp in southwest Utah; instead they are separated by a structural transition zone that is approximately 32 km wide (figure 3). The area of transition is characterized by high-angle faults similar to those in the Basin and Range, but generally they are less abundant and have less displacement (Anderson and Rowley, 1975; Rowley and others, 1979, 1998; Scott and Swadley, 1995; Rowley 1998; Rowley and Dixon, 2001), and the intervening basins occupy less area than the ranges and are filled with a thinner sequence of alluvial fill.


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STRATIGRAPHIC AND IGNEOUS UNITS

The rock section of the Iron Axis region can be divided into four distinct sequences based on their associated origin before, with, or following Iron Axis magmatic activity (figure 6). The pre-Iron Axis rocks include a lower sequence of in-place sedimentary rocks, that host the Iron Axis intrusions, and an upper sequence of mostly regionally deposited volcanic rocks that later became part of allochthonous gravity-slide masses derived from the collapsed roofs of some laccoliths (see Hacker and others, 2002). The Iron Axis rocks include a sequence of volcanic and intrusive igneous rocks related to magmas that formed laccolithic structures. The post-Iron Axis rocks include regionally and locally derived volcanic rocks and local sedimentary rocks that formed contemporaneously with post-Iron Axis extensional structures.

AGE RELATIONS

Magmatism in the Iron Axis occurred in distinct phases as recognized by 40Ar/39Ar and K-Ar ages (table1; figures 7 and 8). The first phase occurred in the Escalante Desert and produced the quartz monzonite Lookout Point pluton at 22.51 Ma. Only a small portion of the pluton is exposed, but drilling and seismic surveys have delineated more in the subsurface (see figure 5). This was followed by the main phase of quartz monzonite laccoliths emplaced along older Sevier-age thrust faults. Concordant dates from seven exposed laccoliths show rapid emplacement between 22.0 and 21.76 Ma. The difference in analytical error between the oldest laccolith (Pinto Peak) and youngest (Three Peaks) shows the duration of the main phase could have been be as little as 90,000 years which supports rapid emplacement of these intrusions. The main phase of laccoliths formed immediately following the eruption of the compositionally identical Harmony Hills Tuff (22.03 Ma) from an unidentified caldera to the west, which may represent resurgent magma that intruded along thrust faults outside the caldera (Hacker and others, 2005). The third phase (after a 1.3 Ma hiatus) included the quartz monzonite Pine Valley mega-laccolith (>300 km2) at 20.46 Ma and the gabbro-diorite Iron Peak laccolith at 20.2 Ma. These two intrusions were emplaced by dike systems instead of laterally emplaced along thrust faults. The last phase included the granitic Mineral Mountain laccolith at 12.1±1.9 Ma and the associated Ox Valley Tuff. This new age (recorded in Biek and others, 2007) and composition are closer to Caliente caldera complex magmas, but is also associated with an older Sevier-age thrust fault which is why it has historically been included with the Iron Axis laccoliths. The affinity of these laccoliths to caldera-forming eruptions probably reflects large power inputs from sources below, thus resulting in rapid emplacement of the shallow plutons.


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AGE

(Ma)

FORMATION MEMBER

Thickness

(m)

HOLOCENE- PLEISTOCENE

Surficial deposits-alluvial, mass wasting

0-46

Post Iron Axis Volcanic and Sedimentary Rocks

Basaltic lava flows

0-61

1.6

Boulder deposits

0-61

PLEISTOCENE- PLIOCENE

Eight Mile Dacite

0-152

Older piedmont-slope deposits

0-91

PLIOCENE

Older basaltic lava flows

0-61

MIOCENE

Older alluvial deposits

0-91

19

Racer Canyon Tuff

0-91

20.5

Pine Valley laccolith

Pine Valley Latite Timber Mountain flow

0-610

Iron Axis Rocks

Rencher Peak flow

0-457

Page Ranch Formation

0-122

The Dairy laccolith

Stoddard Mountain laccolith

Volcanic rocks of

Comanche Canyon

Lava flow member

0-152 Ash-flow tuff member

0-61

21.5

Rencher Formation (Derived from Bull Valley Intrusion)

Upper ash-flow tuff member

0-46 Lower ash-flow tuff member

0-107

21.9

Pinto Peak

laccolith

Rocks of Paradise Sandstone member

0-7.6

Upper lava flow member

0-183

Lower lava flow member

0-122

Ash-flow tuff member

0-91

22.5

Q

uich

apa

Gro

up

Harmony Hills Tuff

30-99

Pre-Iron AxisVolcanic Rocks

23

Condor Canyon Formation Bauers Tuff Member

24-61

Swett Tuff Member

0-9

24

Leach Canyon Formation

46-152

OLIGOCENE

27-26

Isom Formation Hole-in-the-Wall Tuff Mbr

12

Baldhills

Tuff Member

Associated lava flows and sedimentary rocks

15-30

30

Wah Wah Springs Fm., Needles Range Group

0-9

OLIGOCENE- PALEOCENE

Claron Formation

137-213

Pre-Iron AxisSedimentary Rocks

CRETACEOUS

Iron Springs Formation

1067-1280

Dakota Conglomerate

0-15

Carmel Formation 91-137 Temple Cap Formation

61

Navajo Sandstone

610

Kayenta Formation 244

Figure 6. Composite stratigraphic (sedimentary and volcanic) and lithodemic (igneous intrusive) units in the Pine Valley Mountains area.


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Laccolith Emplacement Ages

PV

LP

IP

TP

SM

IMBVBMHHPP

GM

20

21

22

23

Age

(Ma)

LP - Lookout Point

PP - Pinto Peak

HH – Hardscrabble Hollow

BM – Big Mountain

BV – Bull Valley

IM – Iron Mountain

SM – Stoddard Mountain

TP – Three Peaks

GM – Granite Mountain

PV – Pine Valley

IP – Iron Peak

Laccolith Emplacement Ages

PV

LP

IP

TP

SM

IMBVBMHHPP

GM

20

21

22

23

Age

(Ma)

LP - Lookout Point

PP - Pinto Peak

HH – Hardscrabble Hollow

BM – Big Mountain

BV – Bull Valley

IM – Iron Mountain

SM – Stoddard Mountain

TP – Three Peaks

GM – Granite Mountain

PV – Pine Valley

IP – Iron Peak

Figure 7. Graph of laccolith ages from table 1.

Table 1. Summary of Iron Axis laccolith ages

Symbol Laccolith Method Age (Ma) Emplacement Phase

MM Mineral Mountain 40Ar/39Ar 12.10 ± 1.9 4 IP Iron Peak K-Ar 20.20 ± 0.05 3 PV Pine Valley 40Ar/39Ar 20.46 ± 0.05 TP Three Peaks 40Ar/39Ar 21.76 ± 0.06

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GM Granite Mountain K-Ar 21.50 ± 0.4 SM Stoddard Mountain 40Ar/39Ar 21.86 ± 0.09 IM Iron Mountain 40Ar/39Ar 21.85 ± 0.31 BV Bull Valley 40Ar/39Ar 21.98 ± 0.10 BM Big Mountain 40Ar/39Ar 22.00 ± 0.09 HH Hardscrabble Hollow 40Ar/39Ar 22.02 ± 0.11 PP Pinto Peak 40Ar/39Ar 21.96 ± 0.11 LP Lookout Point 40Ar/39Ar 22.51 ± 0.09 1 Summary data from Rowley and others (2006) and Biek and others (2007)


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MODEL FOR EMPLACEMENT OF THE IRON AXIS LACCOLITHS

The observations we present here in the field guide support a model involving initial sill formation that was either laterally fed by transport of magma along pre-laccolith thrust faults or vertically through feeder dikes, followed by subsequent inflation of the sill by forcible intrusion of continuous or multi pulses of magma. The progression of stages of emplacement and growth is viewed here as a continuum as suggested by Corry (1988) for the genesis of most laccoliths in general.

Initial Sill Emplacement

The magma that formed the different laccoliths was a half-crystallized mush at the time of emplacement (as evidenced by the large percentage of phenocysts in the porphyritic rocks) and spread as a thin sill within the sedimentary strata of the Temple Cap, Iron Springs, and Claron Formations at depths of approximately less than 3 km to

Figure 8. Distribution of laccoliths grouped by emplacement phase listed in table 1.


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as little as 200 m. The actual thickness of the sills is unknown but Corry (1988) suggests that the sills (or protolaccoliths) in most laccoliths were less than 30 m thick regardless of the overall diameter of the laccolith. The magma is assumed to have spread to its full length as a sill prior to vertical growth. Initial sills for the laccoliths were fed either by magma ascending older thrust faults and subsequently laterally emplaced in one direction, or fed vertically by a feeder dike system. Most of the Iron axis laccoliths were laterally emplaced with the exception of the Pine Valley and Iron Peak laccoliths. Although feeder dikes are not exposed beneath the Pine Valley laccolith (even though its floor is exposed over its entire southern flank), a dike system with a northeast-southwest trend would explain the anomalous length of the laccolith along this line. Feeder dikes are present below the Iron Peak laccolith, which is the only other intrusion in the Iron Axis group with an exposed floor. The Iron Peak intrusion also intruded into the Claron Formation, and erosion has exposed a tightly clustered swarm of feeder dikes that average approximately 2 m thick and cut up through the nearly horizontal Claron beds at an angle of 40 to 90 degrees (Spurney, 1984).

Spreading as a sill explains the pattern of intrusion for the Dairy and Stoddard Mountain laccoliths. Both intrusions apparently emanated from a common conduit north of Pinto Peak (most likely the same conduit that fed the Pinto Peak intrusion). Finding the area to the south blocked by the already congealed Pinto Peak intrusion, the magma migrated in sills to the east (for Stoddard Mountain intrusion) and west (for the Dairy intrusion). The thick lava flows from the Pinto Peak intrusion provided greater lithostatic pressure and also acted as a barrier to sill migration. Therefore, the sills migrated east and west (Richie Flat and Pinto anticlines) before migrating to the south to their fullest extent.

Vertical Growth and Roof Deformation

Following emplacement of the sill to its full lateral extent, bending of the entire

roof overburden began as magma was continually added to the base of the laccolith to forcibly thicken the intrusion vertically. As the intrusion inflated, the overlying host rocks were gently rotated and arched into doubly hinged flexures around the periphery. Due to the shallow emplacement (thin overburden), extension over the upflexed area was accommodated by brittle fracturing and high-angle normal faulting of the roofs. Although the peripheral flexures of the exposed laccoliths are only partially preserved, and the crestal extensional faults have presumably been eroded, these described features are consistent with those found preserved in portions of the uneroded crests of the Dairy and Stoddard Mountain laccoliths (e.g., Richie Flat anticline). The geometry of the laterally fed laccoliths is often asymmetrical in cross section with the over steepened flank of the laccolith occurring on the side farthest from the source area, which corresponds to where the sill stopped. Thus the laccolith is thickest here where magma piled up, so to speak, from continued lateral injections.

We envision the magma to be emplaced continuously or in rapid successive pulses, but there are no preserved internal contacts to record how many. In either case, the magma was from a single batch generated in the source area which explains the homogeneous nature of the intrusions.


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Gravity Sliding and Volcanism

An unusual aspect of laccolith growth in the Iron Axis group is the catastrophic collapse of some of the laccoliths flanks and subsequent venting of ash flows and lava flows. As the intrusions continued their vertical growth, the limbs of the overlying peripheral flexure steepened as the hinges tightened. Extensional faulting at the hinge crest most likely reduced the lateral support of the limb on an otherwise already steepened and unstable slope. The resulting slabs of roof rock detached within the sedimentary beds of the Iron Springs or Claron Formations and slid intact onto the former land surface below (see Hacker and others, 2002). The sudden loss of peripheral overburden greatly reduced the lithostatic pressure that was essentially holding the roof “down” in advance of the upward loading forces applied by the thickening magma. This sudden release of overburden by gravity sliding most likely resulted in immediate frothing of the magma due to massive pressure release, as with the 1980 Mount St. Helens eruption (Lipman and Mullinax, 1981). Laccoliths that produced gravity slides and volcanism include Pinto Peak, Bull Valley, Stoddard Mountain, and Pine Valley Mountain. The Iron Mountain laccolith also produced gravity slides, but did not vent volcanic units.

ROAD LOG Inc. Cum. Description Mileage Mileage ______________________________________________________________________ 0.0 0.0 This road log begins at the intersection East St George Boulevard and

Interstate 15 (I-15) which is exit 8. Travel north on I-15 to the town of Leeds.

11.0 11.0 We are traveling along strike of the west limb of the Virgin anticline

where Mesozoic sedimentary units dip westward toward the younger Pine Valley laccolith visible on the skyline to the left.

11.0 22.0 Exit 22 (Leeds/Siler Reef), leave I-15 and travel north on State Route

(SR) 228 (S. Main St.) off of exit ramp. Proceed through town of Leeds and follow signs leading to historic Silver Reef.

1.6 23.6 Turn left (west) on Silver Reef Road which passes beneath the I-15

overpass. Remain on Silver Reef Road as it winds through the community of Silver Reef.

1.2 24.8 Silver Reef Road veers to the left into the historic mining district but we

will remain traveling straight on Oak Gove Road. The historic Silver Reef mining district is noted for its production of ore-grade silver chloride from sandstone first discovered in 1866, with the main silver mining activity lasting from 1876 to 1888 (Proctor, 1953; Proctor and


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Brimhall, 1986). About 8 million ounces of silver were produced from the district prior to 1910. The main ore horizons are contained within the Springdale Sandstone Member of the Moenave Formation. The Springdale has been repeated by thrust faults on the Virgin anticlines northwest flank to form prominent ridges known as “reefs” with names like White, Buckeye, and Butte Reefs.

0.4 25.2 Paved part of Oak Grove Road stops at the Dixie National Forest

boundary sign. Reset odometer and continue on gravel Forest Road 031 (still Oak Grove Road). From here we will be traveling up section through Mesozoic sedimentary rock units that dip gentle to the NE and make up the west limb of the Virgin anticline (refer to figure 6). Red cliffs straight ahead are composed of sandstones and siltstones of the Jurassic Kayenta Formation, which grades into the overlying Jurassic Navajo Sandstone that makes up the beautiful scenery of Zion National Park.

1.1 1.1 Navajo Sandstone on left and right. The massively cross-bedded

sandstones of this world renowned coastal and inland paleodune field are clearly visible.

0.5 1.6 Forest sign to St. George and Oak Grove Campground. Continue to

the right to the Oak Grove Campground. 1.0 2.6 Jurassic Temple Cap and Carmel Formations to the right. Units

consist of fossiliferous limestones, red mudstones and sandstones, and gypsum deposited in a variety of environments ranging from shallow marine, shoreline, and sabka. Limestones of the Carmel are hosts to some of the major iron deposits surrounding the Three Peaks, Granite Mountain, and Iron Mountain laccoliths we will visit on this trip.

0.6 3.2 Cretaceous Iron Springs Formation from here to campground. Unit

consists of ledgy slopes of interbedded sandstones, siltstones, and mudstone forming part of an east thickening clastic wedge. Sediments were derived from erosion of Sevier orogeny uplifts to the west and deposited in braided stream and flood-plain environments.

3.2 4.2 Debris-flow and alluvial deposits of mostly Pleistocene age containing

large boulder igneous clasts derived from erosion of the Pine Valley laccolith.

2.5 6.7 STOP 1. DISCUSSION OF PINE VALLEY LACCOLITH. Turn right

on small dirt road that leads to a trailhead parking area and park next to horse coral.


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This stop provides a good spot to view cross-sectional features of the Pine Valley laccolith that have been exposed by erosion. The Pine Valley laccolith is the largest intrusion of the Iron Axis group, but its origin has long been debated. A laccolithic structure was first proposed by Cook (1957); however, most interpretations have favored an extrusive origin in the form of lava flows (Dobbin, 1939; Gardner, 1941), a single thick lava dome (Mattison, 1972), or a thick lava flow intruded by a sill-like pluton (Grant, 1995). Much of the confusion stems from the nature of the rock itself, which resembles glassy volcanic porphyry, and the presence of a similar volcanic sequence (Pine Valley Latite) to the north of the laccolith.

The main exposed area of the Pine Valley laccolith trends northeast-southwest and measures 30 km long and 11km wide in map view (figure 9), and has a remaining maximum thickness of 900 m. The shape is rather irregular due to erosion that has fortuitously exposed the laccoliths floor along the entire south and west sides. Field relations show that the Pine Valley intrusion was emplaced laterally into the upper sedimentary rocks of the Claron Formation, and that the maximum thickness of the cover was only 150 to 200 m. In cross section (figure 9b) the laccolith is thicker to the south. Variations in roof thickness resulted in changes in lithostatic load and thus played a role in the vertical inflation of the laccolith. Where the roof thickness was locally greater, like in the Grass Valley area to the north, vertical growth was more restricted. Several large gravity slides (up to ~10 km2) of mostly Claron and Quichapa rocks were mapped on the north side of the laccolith (figure 9a) (Hacker, 1998, Hacker and others, 2002) as well as on the southeast side (Hurlow and Biek, 2003). All of these gravity slide masses are overlain by extrusive latite porphyry derived from the laccolith

The laccolith mass consists of monzonite porphyry that is remarkably homogeneous in its mineralogy, chemical composition, and texture throughout its horizontal and vertical section. Therefore the laccolith has not been found to be compositionally zoned, nor does it show any recognizable magmatic flow fabric except at the very top. Petrographically, the intrusive rock is a crystal-rich porphyry containing 40-50% phenocrysts of plagioclase (60-70%), pyroxene (8-15% total; 1-2% hypersthene, 7-13% orthopyroxene), sanidine (2-5%), biotite (3-10%), and Fe-Ti oxides (<2%; mostly magnetite). The phenocrysts range in size from 1 to 15 mm (sanidine and plagioclase being the largest) and are set in a holocrystalline microgranular to granophyric groundmass containing microlites of feldspar and quartz.

Where the Pine Valley intrusion has been dissected by erosion, it exhibits massive horizontal color banding when viewed from a distance, which is prominently visible on the steep southern face of the intrusion. Cook (1957) divided these color zones from bottom to top into a dark brown zone, a brown zone, a white zone, and a purple zone. Even though the zones appear as distinct layers, the contacts between them appear gradational and it is often difficult to distinguish the passage from one zone to the other in the field. The basal "dark brown zone" of the unit is about 15 to 25 m thick, is a very resistant cliff former, and has a pseudocolumnar joint pattern. Its groundmass is microcrystalline to glassy, and the base of the zone usually has a dark brown to black, vitrophyric chilled layer, about 20 cm to 1 m thick. The overlying "brown zone" is thicker, about 230 to 350 m, but less resistant than the basal zone. The zone is characterized by a vertical joint pattern, which is accentuated on weathered surfaces. The rock weathers reddish-brown but is greenish-gray on fresh surfaces and has a


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mostly microgranular groundmass. Above the brown zone is the mostly light-to-medium-gray rock of the "white zone" that ranges from about 30 to 110 m thick. The rock commonly contains miarolitic cavities up to 1 cm in diameter. The mafic minerals appear more oxidized and lighter in color, and along with the gray groundmass, give the

Figure 9. (A) Simplified geologic map and (B) north-south cross sections showing spatial relations and geometry of the Pine Valley laccolith and Pine Valley latite, as well as adjacent Miocene Iron Axis laccoliths and their structures and stratigraphy. Landmarks include: GV-Grass Valley; NH-New Harmony; PV-Pine Valley; RP-Rencher Peak; CC-Comanche Canyon.


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rock an overall bleached or white appearance. The groundmass appears slightly coarser and more granophyric than the lower zones. The overlying "purple zone" is about 300 to 500 m thick and locally appears to have a sharp contact with the white zone below, but this contact is also lithologically gradational. It only appears sharp because of the color contrast provided by the reddish-purple groundmass, but this color contact can be seen cutting across phenocrysts as well. The mafic minerals appear distinctly fresher in hand samples, but still show some alteration in thin sections. The groundmass is also finer grained (more microgranular) than the white zone and resembles that of the brown zone.

The origin of these color bands or zones was studied in some detail by Cook (1957), who believed the banding was due in part to gravitational crystal fractionation as well as differential deuteric alteration. However, Mattison (1972) and McDuffie (1991) conducted more detailed modal and chemical studies of samples taken along vertical transects and found no statistically significant concentrations or depletions of minerals or major chemical elements in any zone. Mattison believed that the color bands were due partly to the secondary oxidation of iron as well as partly due to slight variations in groundmass texture due to devitrification of the original glassy groundmass.

The only internal structures observed in the igneous mass are in the form of platy partings representing magmatic flow layering present near peripheral areas where the laccolith erupted or in places within the purple zone at the very top. The flow layers in the purple zone are tilted and sometimes broken into blocks that dip in multiple directions, and locally dikes of the same composition cut the flow layering.

These observations collectively suggest that the Pine Valley laccolith accumulated from a single voluminous batch of magma that, once fully emplaced, cooled slower in the middle to form a coarser groundmass but cooled faster on the top and bottom. Data suggest that the magma inflated the laccolith from the bottom up by injecting new magma to the bottom of the intrusion and lifting the magma above it. The purple zone apparently was not as restricted in its movements, forming flow layers as the confining pressure dropped either due to gravity sliding or breaking through its roof. The more brittle purple zone (following cooling) apparently was jostled around by the underlying magma suggesting the roof rocks where broken apart.

The exposed size of the laccolith covers an area of ~300 km2; however, the estimated original dimension was closer to ~800 km2 based on locations of erosional outliers, structures, and sedimentary deposits derived from the intrusion (figure 10). This would show the laccolith originally at ~45 km long and ~20 km wide. The northern boundary is defined by preserved contacts with the country rocks and in the interpreted buried portions beneath the Grass Valley anticline and portions of the Pine Valley latite. The western and southern boundary is delineated by erosional outliers. The eastern boundary is more problematic, but is interpreted to have extended beyond the present location of the Hurricane fault as is suggested by: 1) the presence of monzonite porphyry outcrops in the Hurricane fault zone, and 2) large boulder debris deposits containing clasts of monzonite porphyry on Kolob Terrace (Anderson and Mehnert, 1979) and more recently mapped by Bob Biek on Miner Peak (Biek, 2002) (see figure 10). Since the Hurricane fault post-dates the laccolith, there is no evidence that the distribution of the laccolith was limited by faulting. The Pine Valley “mega-laccolith”, at


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45 by 20 km and ~350 km3, greatly exceeds the typical size of recorded laccoliths and yet, remarkably, was intruded at an extremely shallow depth of less than 200 m. The

Figure 10. Map showing estimated original dimensions of the Pine Valley laccolith and trend of buried possible feeder dike system. Stars=debris flow deposits containing large clasts of Pine Valley intrusive rocks; crosses=outcrops of Pine Valley laccolith in Hurricane fault zone (data from Biek and others, 2007).


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lack of visible and chemical internal zoning in the Pine Valley laccolith compares to other laccoliths in the Iron Axis group that also show no internal zoning and are interpreted to be single shot injections of magma (Hacker and others, 2002).

An emplacement model for the Pine Valley laccolith based on the above observations and arguments is presented here in figure 11. The model involves

Figure 11. Schematic model for evolution of the Pine Valley laccolith. Cross section is north-south through laccolith. Growth is envisioned to be a continuum from one stage to the next. (A) Stage 1 – ascent of magma through en echelon dike system followed by lateral migration of sill to its fullest extent. (B) Stage 2 – laccolith formation by vertical growth. (C) Stage 3 - gravity sliding from laccolith and continued growth. (D) Stage 4 - lateral expansion by overturning remaining peripheral flanks and extruding onto surface.


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transport of magma through feeder dikes, initial formation of a horizontal sill, subsequent vertical inflation of the sill by forcible intrusion along with gravity sliding and subsequent extrusion of magma. Return to vehicle. 8.3 8.3 Return to the highway at Leeds, reset odometer and turn left on I-15

and travel north toward Cedar City. 5.5 5.4 Erosional outliers of Pine Valley laccolith forming prominent knolls on

right and left side of highway. Lower elevations of outcrops are due to down to the east normal faulting that displaces the Pine Valley laccolith.

12.6 18.0 Exit 40 to the Kolob section of Zion National Park to the east. 2.0 20.0 Exit 42 to town of New Harmony in distance due west (left) with Pine

Valley latite flows forming the ridge behind the town and grading into the Pine Valley laccolith to the south. Stoddard Mountain laccolith forms high ridges to the north of the Pine Valley latite.

17.0 37.0 Exit 59 (Cedar City), leave I-15, exit to right and at end of ramp reset

odometer and turn right (west) and travel west on SR 56. 4.7 4.7 Turn right on Iron Springs Road. 4.1 8.8 STOP 2. PRE-LACCOLITH SEVIER-AGE STRUCTURES. Pull off

on right side of road at wide gravel area.

The emplacement of the main phase of Iron Axis laccoliths was partially controlled by NE-striking, SE-verging Sevier-age (Late Cretaceous to early Tertiary) thrust faults. At the Iron Springs mining district (where we are now located), Iron Mountain, Granite Mountain, and Three Peaks laccoliths were emplaced along the Iron Springs Gap thrust fault. The Iron Springs Gap thrust fault is stratigraphically within the Carmel and Temple Cap formations, or between the Carmel and the Navajo Sandstone, and displaced these units (i.e., Temple Cap and Carmel Formations) over the Iron Springs Formation. Ascending magma tended to follow this older thrust fault, which provided access for the magma to flow upward through the massive Navajo Sandstone (Mackin, 1960), from where it spread southeastward as sills before inflating. The floors of the laccoliths are not exposed, but a petroleum well (figure 12), near Iron Springs, intersected the top and bottom contacts of the Three Peaks laccolith at 700 and 1,500 m depth, respectively (Van Kooten, 1988). From this vantage point we can view the emergence of the Iron Springs Gap thrust where it displaces units of the Jurassic Temple Cap and Carmel Formations over upturned or overturned units of the Cretaceous Iron Springs Formation (figures 13 and 14). Return to vehicle.


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0.5 9.3 Turn vehicle around and head back SE along Iron Springs Road for 0.5 miles. Turn left (north) on Sage Hills Drive (gravel). Road will pass over railroad tracks.

0.5 9.8 Remain on Sage Hills Drive to right. Road to left leads to an

operating open pit mine where intrusive rocks are being extracted and crushed for aggregate. Turn left on Ainsworth Drive after passing mining road.

0.3 10.1 STOP 3. THREE PEAKS LACCOLITH. Park in the open area on

the left across from the gate to Ainsworth Ranch. Hike the four wheel trail north along the fence to where it ends at the overlook of the open pit mine.

The Three Peaks laccolith is a representative example of the three Iron Springs

mining district intrusions (i.e., Iron Mountain, Granite Mountain, and Three Peaks). The huge ore bodies of hematite and magnetite which rim the intrusive margins of the three laccoliths made the Iron Springs mining district the largest iron producing area in the western United States. Most of the economically significant iron ore occurs as strata-bound bodies in the adjacent limestone units of the Carmel Formation and locally as minor veins cutting quartz monzonite.

Within these laccoliths Mackin (1947) recognized three significant quartz monzonite porphyry zones. The interior zone, making up the majority of the laccoliths, consists of altered, darker weathering, friable rock which forms grus-covered lower slopes, especially in The Three Peaks laccolith. The peripheral shell zone is the outermost zone in contact with wall rocks, and consists of relatively unaltered, resistant,

Figure 12. Seismic line showing Arco Three Peaks well penetrated the laccolith (modified from Van Kooten, 1988).


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light-gray rock characterized by platy jointing. The peripheral shell is 10 to 65 m thick and represents the chilled outer margin of the intrusion that grades into the interior. In many places, a third zone intervenes between the peripheral shell and interior zones. This intermediate zone is known as the zone of selvage joints (selvage joint zone) is distinguished by single sets of subparallel or slightly radiating and closely spaced subvertical joints. The selvage (or bleached) area consist of a more resistant, light-colored rock extending from 1 cm to nearly 1 m on each side of the joint (figure 15).

North 3 Miles 3 Km

Figure 13. Geologic map of Granite Mountain and Three Peaks laccoliths. See figure 14 for unit symbols (modified from Rowley and others, 2006).


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Figure 14. Stratigraphic column of Iron County area of SW Utah (modified from Rowley and others, 2006).


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The selvage rock generally forms ribs separated by darker, less resistant rock that is identical to, and grades downward into, the interior zone (Barker, 1995). The bleaching of the selvages is the result of all amphibole and nearly all biotite being replaced by secondary aggregate and calcite. The fractures in the zone of selvage joints provided conduits for the escape of volatile phases during alteration of the interior with some iron precipitated as magnetite along joints. However, most of the iron came from the bleached selvages and escaped into the overlying limestones of the Carmel Formation to form strata-bound bodies.

The selvage joints are attributed to structural readjustment of the roof rocks or to continued injection of magma below, and are not due to thermal contraction. They are absent in areas where the intrusive contact is planar or gently curved (more than 2 km of radius of curvature) (Barker, 1995). These joints indicate that as the intrusion was vertically inflating and doming the roof rocks, the outer part of the interior zone underwent brittle failure within marginal roof flexures (such as monoclines) with radius of curvatures less than 2 km, while the interior was still weak, to produce the zone of selvage joints below the peripheral shell. The peripheral shell was apparently more strongly bonded to the wall rock than the interior and responded to stress by faulting at low angles to the intrusive contact, and by brecciation, rather than by forming joints as in the interior below it. The peripheral shell, therefore, failed in a brittle manner before

Figure 15. Close-up of selvage joint in Three Peaks laccolith.


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intrusion was complete, behaving more nearly like the overburden than the ductile rock or magma below. The selvage joints formed after the peripheral shell had become decoupled from the interior, which is why the joints did not penetrate the peripheral shell in any sizable numbers.

The different zones of the laccoliths lack any distinguishable internal contacts that would indicate separate intrusions, and the kind and proportions of phenocrysts are the same throughout the laccoliths.

Preliminary study of the AMS fabrics of the upper exposed levels of The Three Peaks and Granite Mountain laccoliths (figure 16) mimic the vertical growth stage of the intrusions.

Hike north along the four wheel trail next to the fence leading into The Three Peaks laccolith. Observe the peripheral shell zone and the selvage joint zone. Selvage joints are clearly visible in the outcrops along and in the trail. The trail ends at the overlook of an open pit mine where ore was extracted from a small faulted strata-bound body. 0.6 10.7 Return to vehicle and drive back to Iron Springs Road and turn left

(SE) on Iron Springs Road. 0.6 11.3 Turn right (west) on Desert Mound Road and reset odometer. 4.7 4.7 Pavement ends at railroad tracks; proceed on gravel. Desert Mound

mine to the right. Sweet Hills to the left consist mostly of Quichapa Group and capped by ash flow tuffs of the volcanic rocks of Comanche Canyon erupted from Stoddard Mountain laccolith.

2.2 6.9 STOP 4. NECK OF THE DESERT. Intersection with Comstock

Road (also gravel).

From this stop we can get a view and feeling of the scale of the three laccoliths (Iron Mountain, Granite Mountain, and Three Peaks) that make up the Iron Springs mining district. The Iron Mountain laccolith is to the SW with the abandoned Comstock mine visible on its NE flank (figure 17). Granite Mountain is visible to the NE with the Desert Mound mine in the foreground. The Three Peaks laccolith is visible beyond Granite Mountain to the NE. These three laccoliths crop out over an area of approximately 35 km2, and at least three times as much area is underlain at shallow depths as indicated by drilling and by seismic, gravity, and magnetic surveys (Blank and Makin, 1967; Barker, 1995). Emplacement of these laccoliths was at minimum depths of 1 to 2.3 km (Barker, 1995). Mounds of rock cuttings from exploration boreholes are abundantly visible at this stop. Return to vehicle. 1.9 8.8 Turn left (SW) on Comstock Road. Mining road straight ahead leads

to abandoned Comstock mine on NE flank of Iron Mountain. Stay to left on Comstock Road.

2.9 11.7 Railroad tracks.


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Figure 16. AMS fabric data from Granite Mountain and Three Peaks laccoliths. (A) Magnetic foliations tend to strike parallel to the laccolith margin and dip outwards towards the margin of the intrusion. Lineations do not yield an obvious pattern. (B) Lower hemisphere equal area projections of site AMS data. K1 axis=square, K2 axis=triangle, K3 axis=circle. AMS foliations overall dip to the northwest, indicating possible magma emplacement from NW to SE.

A


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B

Gra

nite

Mou

ntai

n Th

ree

Peak

s


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3.8 15.5 STOP 5. PRE-LACCOLITH STRUCTURES AND GRAVITY SLIDES

FROM IRON MOUNTAIN LACCOLITH. Pull off on right side of road.

From this stop we can view part of the Iron Springs Gap thrust fault to the west

and the Iron Mountain gravity slide to the east. The low hill to the west is capped with highly fractured limestones of the Carmel Formation displaced eastward over the Iron Springs Formation exposed below. The Iron Mountain laccolith appears to have intruded into the Temple Cap Formation below the main thrust fault and most likely only used the fault as a subvertical conduit through the Navajo Sandstone, but the magma became arrested in the shales of the Temple Cap instead of continuing upward along the fault. In plan view, the Iron Mountain laccolith trends NE and is 6 km long by 4 km wide, with country rock on its SE side being vertical to overturned. This over steepened SE side, with a topographic relief of about 1 km, produced collapse of part of its flank resulting in the Iron Mountain gravity slide.

To the east is the area of chaotically juxtaposed slide blocks of the Iron Mountain slide complex that detached from the uplifted roof of the Iron Mountain laccolith. The Iron Mountain slide covered an area of at least 30 km2 and moved at least 6 km to the east (Rowley and others, 2001). Detachment zones along which the cover rocks slid formed in the Iron Springs Formation, and carried rocks of the Iron Springs and Claron Formations, and volcanic units of the Quichapa Group, eastward. At the base of the high cliff (known as the Swett Hills) is the Woolsey Ranch fault, a northwest-striking, left-lateral gravity tear fault that bounds the north side of the Iron Mountain slide and displaces to the east the allochthonous rocks relative to the autochthonous rocks of the Swett Hills. The slide blocks resemble small thrust sheets stacked into duplexes in the more distal parts. The Swett Hills are capped with smaller slide blocks overlain by ash-

Figure 17. View looking southwest at Iron Mountain with Comstock Mine on left and Stoddard Mountain to the far left.


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flow tuffs derived from the Stoddard Mountain laccolith. Return to vehicle and continue south on Comstock Road. 6.4 21.9 Turn right (west) on SR 56 and reset odometer. 0.3 0.3 Turn left (SW) on Pinto Road (gravel). 0.4 0.7 Highly fractured Claron Formation in hills to left and right. 1.1 1.8 Crossing over the approximate western edge of Iron Mountain slide

complex. On the left are autochthonous Claron and Iron Springs rocks that are overturned to the north by the Stoddard Mountain intrusion, which forms the high gray cliffs directly to the south (left). The laccolith intruded the Iron Springs Formation at a shallower level than the other intrusions of the Iron Springs mining district, and did not form any strata-bound iron deposits.

0.8 2.6 Near vertical units of Claron on right (north) and Iron Springs on left

(south). Quartz monzonite of the Stoddard Mountain intrusion exposed in cliff to the south. Hill to the north consists of Quichapa rocks that have been thrusted to the north over the Harmony Hills Tuff by the shouldering action of the Stoddard Mountain intrusion.

1.0 3.6 Hill to the right is capped by a small slide mass from the Stoddard

Mountain intrusion (see cross section in figure16) composed of Claron, Leach Canyon, Bauers, and Harmony Hills units transported northward, and which rests on the former land surface consisting of Harmony Hills Tuff and a local andesitic lava flow (local volcanic rocks of Hacker, 1998) derived from fissures and deposited above the Harmony Hills before Iron Axis magmatic activity. Iron Springs country rock on the left, adjacent to Stoddard Mountain intrusion.

1.1 4.7 STOP 6. STODDARD MOUNTAIN LACCOLITH. Pull off to right

and view road cut in Stoddard Mountain laccolith on south side of road.

The exposed portion of the laccolith has a roughly elliptical shape, and is

approximately 9 km long N-S and 6 km wide E-W, and about 56 km2 in area. The unexposed portion of the laccolith extends eastward at least another 5 km beneath the Richie Flat anticline (figure 18) as delineated by drill cores (Cook, 1957). It intruded clastic sedimentary rocks of the Cretaceous Iron Springs Formation, which, along with Tertiary volcanic rocks, dip steeply away from the exposed portion of the intrusion and more gently over the Richie Flat anticline. In the northeast part of the laccolith the country rock is steeply overturned. Although erosion has partially removed some of the country rock to expose the upper crest and flank of the intrusion (the laccolith is well


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exposed over a topographic relief of about 700 m), the floor is not exposed. Based on geologic mapping and cross section construction, the intrusion has a remaining

Figure 18. (A) Simplified geologic map showing inferred flow paths of magma emplacement (modified from Petronis and others, 2004).


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thickness of about 1.5 km where exposed and 1.0 km beneath Richie Flat. The laccolith consists of homogeneous medium-grained quartz monzonite

porphyry found in two recognizable gradational zones. Marking the outermost 20 to 60 m of the intrusion is a chilled margin zone (peripheral shell zone), which is exposed in the outcrop here and consists of resistant, light-gray, pink, and light-yellow unaltered porphyry. This zone contains approximately 45% phenocrysts (approximately 29% plagioclase, 2% clinopyroxene, 1% biotite, trace amphibole, 1% Fe-Ti oxides, and 14% aggregates of plagioclase, sanidine, clinopyroxene, chlorite, and calcite as replacements of amphibole and biotite phenocrysts) in a fine granular groundmass of quartz and alkali feldspar (Barker, 1995). The phenocrysts reach lengths of 8 mm and show no preferred magma orientation fabric. The interior zone, which makes up the majority of the intrusion, consists of a more crumbly to moderately resistant, light-red, pink, light-grayish-green, and tan, deuterically altered porphyry. The interior zone consists of the same minerals as the peripheral chill zone except that the groundmass is slightly coarser and the minerals are more highly altered. This zone contains abundant irregular miarolitic cavities suggestive of shallow emplacement depth. Return to vehicles and continue southwest on Pinto road, crossing Little Pinto Creek. 0.3 5.0 Historic Page Ranch on right. Junction with gravel road on left (Dixie

National Forest Road 029) that leads southeast to New Harmony. Continue southwest on Pinto road.

Figure 18. (B) Cross sections of the Stoddard Mountain laccolith showing estimated extent, vertical thickness, and intrusive and extrusive features.


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0.1 5.1 Crest of low rise, entering Richie Flat. Richie Flat is floored by the Iron Springs Formation that has been arched upward by the unexposed part of the Stoddard Mountain intrusion to form a northeast-southwest trending anticline, which the Pinto road follows. This anticlinal valley is due to the brittle nature of the overlying roof rocks that were extensionally broken along crestal faults and fractures during uplift that made them more susceptible to erosion.

0.2 5.3 Paradise ridge on left contains tilted (by Stoddard Mountain intrusion)

units of the Claron Formation, Quichapa Group, and volcanic “rocks of Paradise.” The rocks of Paradise (Hacker, 1998) consist of an ash-flow tuff unit (white layer toward the top) overlain by two lava flow units (capping the ridge) that vented from the Pinto Peak intrusion. The lava flow units were originally mapped as the Paradise intrusion by Cook (1957), who believed it connected with the Stoddard Mountain intrusion.

0.7 5.0 Kane Point ridge on the right contains northward tilted (also by

Stoddard Mountain intrusion) units of the Claron Formation and Quichapa Group and is capped by the Racer Canyon Tuff (not visible from here). Hill on near right is a Quaternary slide mass made up of pink and white Claron rocks and darker colored Quichapa units that slumped southward from the cliff. Detachment is in the Iron Springs Formation.

5.0 7.9 STOP 7. RICHIE FLAT ANTICLINE AND STODDARD MOUNTAIN

LACCOLITH EMPLACEMENT. Junction with road on left (Dixie National Forest Road 014) leading south to Pinto Spring. Turn left and park before cattle guard.

Kane Point ridge to the north, with a Racer Canyon Tuff cliff on top, and Paradise

ridge on the south help define the scale of the anticline. Figure 19 shows a view along the anticline axis. From here, the Pinto Peak laccolith is to the south and The Dairy laccolith is to the SW. These three laccoliths were emplaced at higher stratigraphic levels (Iron Springs and Claron Formations) than those in the Iron Springs mining district and the Bull Valley Mountains to the SW (all within the Temple Cap and Carmel Formations). Magma for these laccoliths apparently broke out from underneath the Iron Springs Gap trust fault and stepped up section into shale units of the younger formations. This would explain why these laccoliths lie south of the buried trend of the Iron Springs thrust.

Field evidence shows that the Pinto Peak and Iron Mountain laccoliths were emplaced at about the same time and prior to Stoddard Mountain emplacement. Both of these intrusions produced gravity slides, plus the Pinto Peak intrusion vented a thick sequence of ash-flow tuff and lava flows. Stoddard Mountain and The Dairy laccoliths are interpreted to have been emplaced at relatively the same time since they both


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deformed volcanic units vented from the Pinto Peak laccolith. The Stoddard Mountain laccolith also vented ash-flow tuffs and lava flows following several gravity sliding events from its flank. At least two vent areas are interpreted along its flank, one on the east side where the intrusion bulges out from the main pluton (see figure 18), and on the southwest side where lava flows grade downward into the intrusion (Hacker, 1998; Rowley and others, 2006).

Mapping of the Stoddard Mountain and The Dairy laccoliths suggests that the intrusions were emplaced as sills, which spread laterally for several kilometers before inflating to laccolithic forms (Hacker, 1995, 1998; Hacker and others, 2002). Based on field relations, magma for both Stoddard Mountain and The Dairy intrusions likely emanated from a common buried conduit north of Pinto Peak near where we are parked. Finding the area to the south blocked by the earlier Pinto Peak intrusion, the magma migrated to the east (for the Stoddard Mountain intrusion) and west (for The Dairy intrusion). The sills, emplaced at about 1.2 km depth within the Iron Springs Formation, are inferred to have spread laterally, east and west, for several kilometers around the northern edge of the thicker Pinto Peak volcanic-intrusive complex. As The Dairy sill migrated to the west and southwest, in stepped up to a higher structural level within the Claron Formation. As magma for the Stoddard Mountain sill migrated eastward around the Pinto Peak volcanic-intrusive barrier, it turned southward before inflating to its full extent. The results of an AMS fabric study (figure 20) support the

Figure 19. View of the intrusive anticline in Richie Flat looking east along the axis of the anticline, with the Stoddard Mountain intrusion in the distance.


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lateral flow emplacement of the laccolith (Petronis et. al, 2004). Thus it appears that differences in lithostatic pressure caused by thickness variations of the overburden rock where influential in lateral magma emplacement of these laccoliths. Return to vehicle.

7.6 15.5 Return to SR 56 and turn right (NW), reset odometer. 1.3 1.3 Diamond Z Ranch driveway on right. Ribs of resistant Claron beds to

the left and right of the road. Duncan Mountain to the south (right) consists of allochthonous Quichapa rocks in the Iron Mountain slide. Continue on SR 56.

Figure 20. Summary of accepted AMS data from the Stoddard Mountain laccolith. (A) magnetic lineation (solid arrows), (B) magnetic foliation (solid line with triangle). Rejected stations are indicated by black circles. The AMS lineation data show NE trending lineations in the north and S-SE trending lineations in the south part of the intrusion. The overall lack of westerly directed and steep magnetic lineations argues against emplacement via a central axial feeder system. We interpret these data to indicate that the magma initially migrated laterally eastward at ~1 km depth as a sill before spreading laterally north-south where it inflated to ~1-1.5 km thickness to form the laccolith.


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1.2 2.5 STOP 8. ALLOCHTHONOUS CLARON ROCKS. Park in pull off on right side of road.

View of highly fractured and pulverized outcrop of red Claron Formation on north

side of road. These steeply dipping allochthonous rocks, along with others viewed from this vantage point are formed of resistant narrow ribs of Claron beds that form an arcuate pattern around the Iron Mountain intrusion. Beyond Mount Claron (NE of road) is a view of the Swett Hills bounded by a south-facing cliff. At the base of the cliff is the Woolsey Ranch fault, a northwest-striking, left-lateral tear fault (bounding slide fault) that displaces to the east the allochthonous rocks of the Iron Mountain slide, that we have been driving on, relative to the autochthonous rocks of the Swett Hills. Return to vehicles and continue east on SR 56. 2.1 3.3 Old Woolsey Ranch and Duncan Creek on right (south) and Mount

Claron on left (north). Mount Claron is the type locality of the Claron Formation, and consists of faulted and fractured red and white limestone, sandstone, and mudstone above red and tan sandstone and mudstone of the Upper Cretaceous Iron Springs Formation exposed at the base of the hill. The basal low-angle gravity slide plane formed in the upper incompetent rocks of the Iron Springs Formation. To the right (south), the large hills on the near horizon (Flat Top Mountain) consist of allochthonous Quichapa rocks and represent the farthest extent (about 6 km) of the slides from the Iron Mountain intrusion (Rowley and others, 2001).

0.6 3.9 Road cut on left of highly fractured red and yellow allochthonous

Claron Formation within the Iron Mountain slide. 0.5 4.4 To the left (north) are highly fractured, steeply east-dipping, ash-flow

tuffs above a low-angle detachment of the Iron Mountain slide. The road cut on the north side of the road contains the light-pink Leach Canyon Formation on the right (east), underlain by purple and brown ash-flow tuffs of the Isom Formation (27-26 Ma) to the left (west). Sedimentary rocks of the Eocene to Oligocene Claron Formation farther to the west underlie the Isom Formation. These allochthonous rocks were highly fractured and steeply tilted during sliding eastward from Iron Mountain, but the stratigraphic order of the units is preserved. Return to vehicles and continue east on SR 56.

0.4 4.8 Highly fractured Leach Canyon Formation on the left. 0.1 4.9 We just crossed the frontal slide fault of the Iron Mountain slide.

Outcrops to the left are east dipping autochthonous regional Tertiary ash-flow tuffs belonging to the Quichapa Group on the north side of road.


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0.1 5.0 Bauers Tuff Member of the Condor Canyon Formation on left. 0.2 5.2 Leaving Leach Canyon with outcrop of east-dipping Harmony Hills Tuff

on left. 0.7 5.9 Crest of low hill formed in older poorly consolidated Miocene to

Pleistocene fanglomerates. 3.2 9.1 Low area on right is Quichapa Lake that is normally a dry playa in

Holocene and Pleistocene valley bottom alluvial deposits (Mackin and others, 1976). Harmony Mountains to the right (south) consist mostly of faulted Tertiary ash-flow tuffs.

2.9 12.0 Iron Springs Road to the left. 4.5 16.5 Turn onto I-15 north. 23.0 39.5 Exit 82, leave I-15 and proceed south (right) on S R 271 toward the

town of Paragonah. Reset odometer. 0.7 0.7 Turn left (east) on 100 N Street (gravel). Road turns sharply to the

left (north) and becomes Little Creek Canyon Road and follows Little Creek eastward through the canyon.

4.5 5.2 STOP 9. FEEDER DIKE SYSTEM OF IRON PEAK LACCOLITH.

Park on left (north) side of road across from Little Creek.

At this stop we are about 125 m below the floor of the Iron Peak laccolith that was cut by Little Creek to expose it in cross section on the north and south side of the creek. This is the most mafic of the Iron Axis laccoliths (gabbro-diorite in composition), has a remaining outcrop area of 7.5 km2, and was emplaced within the Claron Formation at a depth of about 1.2 km (Spurney, 1984). The laccolith has a concordant exposed floor with a chilled margin, and a preserved thickness of 400 m. The laccolith contains numerous magnetite-hematite veins, some as wide as 3 m, with the majority less than 2 cm. The veins are often short and pinch out or splay along strike (usually within less than 3 m), and often form intersecting sets which create a boxwork pattern. Spurney (1985) found that the veins increased in number toward the roof areas (now eroded), which is similar to the laccoliths in the Iron Springs district, but did not produce selvages, indicating fracturing during continued magma emplacement into the growing laccolith.

Of interest here is the presence of the feeder system for the laccolith, which consists of a swarm of dikes trending NE and ranging in thickness between 0.25 and 8 m (averaging about 2 m) and forming fin-shaped landforms up to 5 m high (figure 21). The dikes form sharp contacts with the Claron Formation, which has been bleached and baked by the magma. The dikes parallel the trend of Basin and Range faults in the area, and are most likely associated with initial Miocene extension.


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An unusual aspect of the feeder system is that it consists of multiple dikes instead of one large dike as is usually portrayed beneath unexposed laccoliths. At this point in our study we are not sure if they were all operating at once to feed the laccolith, or whether they operated at different times, each injecting different pulses of magma. Our preliminary sampling of the laccolith for AMS fabrics show a good pattern of lineations within the base of the laccolith showing outward magma flow patterns from the dikes (figure 22). Return to vehicles. 5.2 10.4 Return to I-15, and travel south to St. George. Reset odometer. 74.0 74.0 End of log and field trip at exit 8.

Figure 21. Part of feeder dike swarm below the Iron Peak laccolith.


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Figure 22. Simplified geologic map of the Iron Peak laccolith based on the mapping of Spurney (1984), and summary of accepted AMS fabric data. Arrow with number indicates the trend and plunge of the mean K1 lineation and strike and dip symbol indicates the orientation of the K1-K2 foliation plane. Cross-hatched pattern, igneous rocks; dotted pattern, pre-laccolith rocks; bold black lines, trace of late Tertiary normal faults; dashed lines, mafic dikes; doted lines, elevation contours in feet. In set diagram shows AMS data from the mafic feeder dikes.


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