Geochronology of Intrusive and Metamorphic Rocks in the Pilot Range, Utah and Nevada, and Comparison with Regional Patterns

Home , Pilot Range

Geochronology of intrusive and metamorphic rocks in the Pilot Range, Utah and Nevada, and comparison with regional patterns

iiiCMrw^P I U-S- Geological Survey, 345 MiddlefleldRoad, Menlo Park, California 94025 WENDY C. HlLLHOUofc, f ROBERT E. ZARTMAN U.S. Geological Survey, Denver Federal Center, Denver, Colorado 80225 MARVIN A. LANPHERE U.S. Geological Survey, 345 Middlefleld Road, Menlo Park, California 94025

ABSTRACT tonism; these ambiguities have resulted in diver- clude Archean gneiss, upper Proterozoic to gent views of the regional metamorphic and lower Mesozoic sedimentary rocks that are K-Ar and U-Pb isotopic studies demon- tectonic development. These views range from unmetamorphosed or metamorphosed to green- strate middle Mesozoic metamorphism and assigning the metamorphism entirely to the schist and amphibolite facies, Mesozoic granit- plutonism followed by Eocene plutonism in Mesozoic (Misch, 1960; Armstrong and Hansen, oids, and Cenozoic igneous and sedimentary the Pilot Range, northeastern Great Basin. 1966) to emphasizing the Cenozoic metamor- rocks. Geologic relations in mountain ranges in Combined with field relations, the age con- phism (Compton and others, 1977; Miller and which amphibolite-facies rocks crop out gener- straints indicate that local amphibolite-facies others, 1983) and primarily attributing the at- ally demonstrate convincingly that Cenozoic and widespread greenschist-facies metamor- tendant deformation to either shortening or ex- metamorphism, ductile deformation, and low- phism peaked between 165 and 150 Ma and tension modes. The divergent interpretations angle normal faulting occurred (Compton and that plutons were emplaced early in the de- seem to require models intermediate between others, 1977; Miller and others, 1983; Snoke formational and metamorphic history, be- the extremes (Armstrong, 1982; Miller and oth- and Lush, 1984), which has led to application of tween 165 and 155 Ma. Cenozoic plutons and ers, 1987; Snoke and Miller, 1987), but geologic the term "metamorphic core complex" for these widespread dikes that were emplaced at relations presenting convincing evidence for areas (Davis and Coney, 1979). Despite the about 40 Ma cut folds and faults, including a both Mesozoic and Cenozoic metamorphism are strong overprint of Cenozoic deformation, some major detachment fault that placed unmeta- few (Allmendinger and others, 1984). Studies in of the metamorphic core complexes retain evi- morphosed on metamorphosed rocks. the Pilot Range were begun with the hope that dence for Mesozoic metamorphism (Armstrong, The Mesozoic and early Cenozoic history analyzing the variably developed metamor- 1976; Compton and others, 1977; DeWitt, of metamorphism and plutonism in the Pilot phism, widespread deformation, and many ig- 1980; Snoke and Lush, 1984; Miller and others, Range is consistent with age data and field neous rocks would elucidate Mesozoic and 1987), supporting early proposals by Misch relations throughout the northeastern Great Cenozoic regional tectonics. (1960), Misch and Hazzard (1962), and Arm- Basin that indicate Late Jurassic plutonism This paper presents data from U-Pb and K-Ar strong and Hansen (1966) of widespread Meso- and metamorphism followed by Eocene and geochronologic studies and describes the con- zoic metamorphism (Armstrong, 1982). Oligocene plutonism and local metamor- straints that they place on timing of metamor- Geologic relations in ranges of the eastern phism. In many areas, rocks that were meta- phism, intrusion, and deformation in the Pilot Great Basin in which greenschist-facies rocks morphosed during Late Jurassic time were Range. The new data and interpretations sup- crop out have been less studied recently than cooled through Ar blocking temperatures for port the concepts of Mesozoic and Cenozoic re- those in ranges in which amphibolite-facies micas between 85 and 60 Ma. The wide- gional intrusive and metamorphic episodes in rocks occur. In many places, the greenschist spread occurrenc e of these mica ages suggests the eastern Great Basin. metamorphism is confined to Precambrian and a regional mechanism for cooling. Cambrian strata, and these lower grade meta- REGIONAL GEOLOGIC SETTING morphic rocks bear few of the characteristics of INTRODUCTION metamorphic core complexes. Lee and others Metamorphic rocks crop out widely in the (1970). Smith (1982), and Allmendinger and The Pilot Ranj;e is one of several mountain Pilot Range and several other mountain ranges others (1984) showed that the structural devel- ranges in the northeastern Great Basin that con- in the eastern Great Basin (Fig. 1). These ranges opment and metamorphism of some of these tains a rock record of complexly overlapping comprise a part of the discontinuously exposed low-grade areas are Mesozoic in age. Mesozoic and Cenozoic metamorphism, defor- Cordilleran metamorphic belt (Miller, 1980) Imposed upon the Mesozoic Cordilleran oro- mation, and plutonism. Ambiguities for the that extends the length of the Cordillera and genic belt was Cenozoic upper crustal extension, Mesozoic tectonic history of the region persist forms the western part of the Mesozoic and early resulting in complex low-angle fault slices, despite detailed studies because Mesozoic struc- Cenozoic Cordilleran orogenic belt (Drewes, normal-faulted tilted panels, and the present tures are difficult to directly date and in places 1978). Within the northeastern Great Basin, pattern of fault-block ranges separated by allu- have been modified by pervasive Cenozoic tec- rocks exposed within this metamorphic belt in- vial valleys. Extension in the northeastern Great

Geological Society of America Bulletin, v. 99, p. 866-879, 9 figs., 4 tables, December 1987.

866

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Basin may have begun as early as Late Creta- sic and during late Eocene into Oligocene time areas, an additional Cretaceous episode oc- ceous but certainly was evident by latest Eocene (Moore and McKee, 1983). Limited data sug- curred (Coats and others, 1965; Armstrong, (Miller and others, 1983). gest that the same two episodes occurred in 1970; Armstrong and Suppe, 1973; Roberts and Previous geochronologic studies of granitoid northeastern Nevada (Coats and others, 1965; others, 1971; Lee and others, 1980). The Ceno- plutons in central and northern Utah indicated Armstrong, 1970), but farther south in the Ely zoic episode in places is divisible into latest two episodes of intrusion during the Late Juras- area and west in the Eureka and Mountain City Eocene and late Oligocene parts. Upper Eocene

114° 112°

40"

TO COMPILE e FIGURE 8 O

e EXPLANATION

Low-grade metamorphic «e rocks a High-grade * o

c <0 O 50 km h L- _J O

Figure 1. Map of northeastern Great Basin, showing locations of mountain ranges discussed in text and generalized outcrop pattern of metamorphic rocks and plutons. Outlined area shows region sampled for data analysis shown in Figure 8. Willard-Paris thrust dotted where inferred.

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Figure 2. Bedrock geologic map of the Pilot Range, showing major tectonostrati- graphic blocks, plutons, and locations of samples for isotopic studies.

plutons are sparsely spread through the region, but upper Oligocene plutons appear to be confined to metamorphic core complexes.

GEOLOGY OF THE PILOT RANGE

The Pilot Range is a north-trending fault block in which structures and strata are gently warped about an east-trending axis, dipping south and north away from a central window of metamorphosed rocks. Rocks in this window are structurally ssparated from overlying unmetamorphosed rocks by a low-angle normal (detachment) fault, the Pilot Peak décollement (Fig. 2). Strata above and below the detachment fault are cut by faults nearly parallel to bedding, and those above the detachment are cut by a younger set of normal faults that have rotated tilt blocks into the detachment. The over-all picture is one of Mesozoic metamorphism and ductile deformation, succeeded by detachment faulting and tilting of fault blocks as the upper crust thinned.

Structural Blocks and Metamorphism

Strata in the Pilot Range lie in three structural blocks. Metamorphic rocks in the central window of the range, underlying the Pilot Peak décollement, comprise upper Proterozoic and Lower and Middle Cambrian metamorphosed clastic and carbon ate rocks. Structurally overlying the metamoiphic rocks are unmetamorphosed miogeoclinal Upper Cambrian to Upper Permian carbonate rocks. Unmetamorphosed Cambrian strata are distinguished from structurally underlying metamorphosed Cambrian strata by different ages and depositional environments (McCollumandMcCollum, 1984; Miller, 1984) as well as metamorphic grade. The structurally highest block is composed of Oligocene and Miocene volcanic; and terrigenous sedimentary rocks and occupies a position structurally above unmetamorphosed Paleozoic rocks. Compre- hensive descriptions of strata in the Pilot Range were given by Miller (1984). Rocks beneath the Pilot Peak décollement contain greenschist- and amphibolite-facies metamorphic assemblages. Rocks containing retrograded staurolite and probable garnet occur within an area efist of Pilot Peak (Fig. 2), but elsewhere, rocks are typically metamorphosed to biotite or chlorite zones of greenschist facies.

The higher grade rocks show a crude spatial Three sets of folds and at least two sets of association with small foliated muscovite-biotite low-angle faults in rocks of the lower structural granite bodies east of Pilot Peak. Numerous block (Table 1) record east- and southeast- small hornblende-biotite granodiorite bodies directed structural transport (Fig. 3A). Textures in the southern Pilot Range heated adjacent in these metamorphic rocks indicate that strain metamorphic and sedimentary rocks and accompanied recrystallization. Strata that are caused retrograde and contact metamorphism, metamorphosed to chlorite zone are broadly respectively. folded and moderately cleaved at low angles to Although the sedimentary rocks above the bedding. Rocks in the Bettridge Creek area (Fig. 2) that contain biotite, and locally garnet, are Pilot Peak décollement show no signs of re- Middle Jurassic gional metamorphism, color alteration values more tightly folded and penetratively cleaved. for conodonts in these rocks indicate tempera- East of Pilot Peak (Fig. 2), marble and schist of tures ranging from 170 to >400 °C. Wide amphibolite facies are complexly folded and fo- contact-metamorphic aureoles developed in liated. A thrust fault (Fig. 3A) is folded by first- these rocks near granodiorite plutons. phase folds (Table 1), and rocks near the thrust fault are generally recrystallized and unfrac- Structural History tured. Metamorphic isograds cross the thrust fault, consistent with pétrographie relations indi- During the middle Mesozoic, rocks in the cating metamorphic mineral growth during first lower structural block were faulted parallel to and second phases of folding. bedding, folded, intruded, and metamorphosed. Unmetamorphosed rocks of the middle block By latest Mesozoic time, metamorphism had are cut by low-angle faults nearly parallel to waned, and by the Oligocene, unmetamor- bedding that cut out strata in many cases. These phosed strata of the middle block were broken may be kinematically related to geometrically Early Oligocene by a multitude of normal faults, tilted, and em- similar Mesozoic faults of the lower block. The placed on the metamorphic rocks along the Pilot bedding-plane faults are cut by north-striking Figure 3. Diagrammatic cross sections of Peak décollement. Later (Neogene) extensional high-angle normal faults (Table 1) that generally the Pilot Range, illustrating tectonic evolu- faulting is recorded by a detachment fault sys- dip west and have rotated strata (Fig. 3B). The tion. A. Intrusion of granite during Late Ju- tem that emplaced the upper structural block of tilted panels dip into and are cut by the Pilot rassic following bedding-plane faulting and volcanic rocks as young as Miocene. Upper Eo- Peak décollement. The décollement lies gener- folding. B. Intrusion of uppermost Eocene to cene to lower Oligocene plutons intruding the ally parallel to bedding in underlying Cambrian lower Oligocene granodiorite dikes and plu- Pilot Peak décollement are cut by low-angle metamorphic rocks and discordantly beneath tons into Pilot Peak décollement, its meta- faults in the detachment system that emplaced moderately dipping Cambrian and Ordovician morphosed autochthon, and unmetamor- the upper block. limestone. Deformation adjacent to the dé- phosed tilt-block faulted allochthon.

TABLE 1. SUMMARY OF STRUCTURES IN THE PILOT RANGE

Structural Lithology Cleavage Minor folds Major folds* Low-angle faults High-angle faults block (location)

Lower— Chlorite-zone None None Parallel to bedding, Reverse, north Metamorphic quartzite small displacement, strike, down to east rocks beneath Chlorite-zone One, at low Rare, north- attenuates strata Pilot Peak metasiltstone angle to bedding trending décollement Biotite- to garnet- One parallel to Synmetamorphic: Synmetamorphic: zone phyllite and bedding second F| = east trend Northeast trend, parallel to bedding; quartzite in some fold F2 = northeast overturned to attenuate and duplicate (Bettridge Creek) hinges trend southeast strata Late metamorphic Late metamorphic: Minor F3 = north trend moderate angle to bedding, attenuate strata

Garnet- to staurolite- One parallel to Synmetamorphic: Synmetamorphic: zone schist, marble, bedding second Fj = east trend parallel to bedding: and quartzite axial plane to F2 = northeast attenuate and duplicate (east of Pilot Peak) second folds trend strata None Late metamorphic: Late metamorphic: F-j = north trend parallel to bedding, attenuate strata

Middle- Dolomite, limestone, None Locally adjacent Bedding-plane faults East strike; north Li nmetamorphosed sandstone to low-angle moved east, attenuated to northeast strike, Paleozoic rocks faults and duplicated strata; and down to west later low-angle faults

Upper— Tuff, siltstone, None None Local, north- Parallel to and at Down-to-basin Tertiary rocks sandstone trending moderate angles to faults bounding bedding; hanging-wall range strata dip east

•North-trending arch and broad east-trending warp affect rocks in entire range.

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collement generally was brittle in the hanging the data of Hoggatt and Miller (1981), are here- centration was determined by isotope dilution wall and ductile in the footwall, but the last in reported. Results for the ages of mineral on an aliquot of the sample removed after bomb movement breed,ited both hanging-wall and formation in three plutons, several dikes, and digestion but otherwise subjected to parallel footwall rocks in a zone < 1 m wide. Bedding three metamorphic rock samples are grouped chemical treatment. A combined U and Th and cleavage in rocks of the footwall were below by geologic unit. tracer was added to the entire sample prior to folded into parallelism with the fault where it digestion, and U and Th were separated together cut across strata, and drag folds indicate that Analytical Methods on a nitrate-form anion-exchange resin from the some movement of the hanging wall was ap- elutriant off the lead-isotope composition col- proximately eastward with respect to the foot- K-Ar. Potassium analyses were performed by umn. Lead was loaded with H3PO4 and silica wall. In contrast, tilted hanging-wall rocks and flame photometry using lithium metaborate fu- gel on a rhenium filament and analyzed by a 30 offset depositional facies suggest major west- sion, the lithium serving as an internal standard cm, computer-controlled mass spectrometer ward translation. The décollement cut most (Ingamells, 1970). Argon was analyzed by providing automated data reduction. Uranium faults and folds in hanging-wall and footwall standard isotope-dilution and mass-spectrome- and thorium were analyzed using a triple fila- strata but was intruded by Paleogene granodio- try techniques described by Dalrymple and ment configuration by a 15 cm mass spectrome- rite sills that show only minor local brecciation. Lanphere (1969). Uncertainties in the ages re- ter. Element concentrations are considered Oligocene to Miocene strata in the upper ported are based upon the empirical approach accurate to ±1%, and the following uncertainties structural block dip east as a result of normal described by Tabor and others (1985). are assigned to the measured lead-isotope ratios: faulting above and along a system of flat faults 40Ar/39Ar. Samples for 40Ar/39Ar analysis 206pb/:204pb) ±2%. 207pb/206pb) ±Q l%. an(j (Table 1). Faults in the upper block cut strata as were sealed in air in flat-bottomed fused silica 208pb/206pbi ±0.2%. Common lead correction young as 8 Ma (Miller, 1984) and have been vials and irradiated at 1 megawatt in the core of has been made by first subtracting 0.2 ng of reactivated(?) as range-bounding faults as re- the U.S. Geological Survey TRIGA reactor for nominal analytical blank and then assuming the remaining cently as the Pleistocene (Miller and Schneyer, 24 hr, receiving a neutron dose of approximately 204pb 1985). 2.4 x 1018 nvt. Details of the reactor flux char- to be that specified by Stacey acteristics, the flux monitor mineral, and the cor- and Kramers (1975) for lead of the appropriate Igneous Rocks rections for interfering K- and Ca-derived Ar age. isotopes are given in Dalrymple and others Result and Interpretations Granitoid rock.1; in the Pilot Range are divided (1981). into two groups on the basis of mineralogy, age, Irradiated samples were fused by induction McGinty Monzogranite. The McGinty and textures. Foliated older granite and heating, and the Ar was purified in a standard Monzogranite is herein named for exposures in pegmatite (Miners Spring Granite), largely extraction line. Samples were held for 30 min at its herein-designated type area in sec. 26, T. 6 synchronous with metamorphism and ductile each temperature during incremental-heating N., R. 19 W., along McGinty Ridge in the deformation in the footwall of the Pilot Peak experiments. Ar analyses were done with a Crater Island NW 7.5' quadrangle, Utah (Fig. décollement, contain biotite and scattered mus- 22.86-cm-radius, 90°-sector, multiple-collector 2). It was previously referred to as the "grano- covite. This granite and pegmatite is widespread mass spectrometer using digital data acquisition diorite of Patterson Pass" by Hoggatt and Miller east of Pilot Peak:, occurring as numerous dikes (Stacey and others, 1981). (1981). The pluton crops out over -18 km2 and small bodies that lack chilled margins. U-Th-Pb. Sized fractions of zircon for U-Th- along the east side of the northern Pilot Range Younger granite and granodiorite form dikes Pb analysis were dissolved using a modified and is exposed at two points along the west side and plutons (McGinty Monzogranite, Bettridge Krogh (1973) method of HF-HNO3 bomb di- of the range. It is texturally and compositionally Creek Granodiorite) that typically contain bio- gestion, after which lead was purified by homogeneous, composed of unfoliated porphy- tite and hornblende. These igneous rocks occur bromide-form anion-exchange resin. Lead con- ritic biotite monzogranite to granodiorite. Pink in a broad area in the footwall of the décolle- ment, where they cut ductile structures and most faults, and also in a pluton in the northern Pilot TABLE 2. K-Ar ANALYTICAL DATA FROM THE PILOT RANGE. UTAH AND NEVADA Range and as scattered small bodies in the hang- 40 40 Rock unit Sample no. Material K20 »Ar •Ar^/Ar Age ing wall. The two largest bodies of granodiorite 10 <*) (IO' mol/gm) (S) (Ma)t are enveloped by metamorphic aureoles, where- as dikes and irregularly shaped small bodies McGinty M79PR-1I6® Biotite 8.58 4.565 84.2 36.6 ± 0.5 have chilled rims and hypabyssal textures. Monzogranite Bettridge Creek M79PR-85§ Biotite 8.96 3.518 68.0 27.1 ± 0.5 Granodiorite M79PR-85§ Hornblende 0.898 1.210 67.5 91.2 ± 1.5 M8IPR-82 Hornblende 0.890 1.269 62.9 96.4 ± 2.9 GEOCHRONOLOGY Miners Spring M79PR-17l5 Muscovite 10.62 8.729 79.2 56.2 ± 0.8 Granite

Geochronologic studies in the Pilot Range Granodiorite M79PR-2I§ Biotite 9.13 3.978 69.8 30.1 ±0.5 dikes M80PR-28 Biotite 2.15 1.172 23.8 37.5 ± 2.6 provide new data on the timing of igneous and M82PR-79 Hornblende 0.838 0.669 48.3 54.6 ± 1.6

metamorphic events. Initial conventional K-Ar Cambrian P80PR -14 Muscovite 9.38 11.524 93.4 83.4 ± 2.5 studies pointed to Mesozoic metamorphism and schist M80PR-26 Biotite 8.67 8.109 80.5 63.9 = 1.9 M80PR-26 Hornblende 0.385 9.889 72.3 170.3 ± 5.1 plutonism, followed by Oligocene plutonism Precambrian M80PR-43 Hornblende 0.260 0.706 60.7 179.7 ± 9.0 (Hoggatt and Miller, 1981), but these events schist were imprecisely defined. Continued conven- 10 10 4 tional K-Ar studies and selected studies by U- tConstants: + X,' = 0.581 x 10 yr' kß = 4.962 * l

zoned potassium feldspar phenocrysts typically 116, Table 2) is from a rock within which the Nevada-Utah. Although the body is small, crop- <3 cm in diameter are set in a pale gray biotite is in places oxidized to a red color and 5% ping out over ~ 1 km2, it displays critical cross- groundmass of coarse-grained plagioclase, po- to 10% of the flakes are partially altered to chlo- cutting relations and is unusually precisely tassium feldspar, quartz, and biotite. Accessory rite. Other samples of the pluton examined dated, warranting its formal naming. Dikes and minerals include hornblende, zircon, sphene, petrographically also contained chloritized bio- small bodies surrounding the pluton are particu- apatite, and xenotime(?). Pink aplite and peg- tite. This alteration probably affected the K-Ar larly abundant in the canyon drained by Bet- matite dikes, generally <50 cm wide, are com- isotopic system, and thus, the date of 36.6 Ma tridge Creek (Fig. 4). The pluton is composed mon. Inclusions, typically 10 to 20 cm in may not accurately reflect the age of cooling of of fairly homogeneous, foliated, medium- to diameter, are composed of biotite-hornblende the pluton following crystallization. Separation coarse-grained hornblende-biotite granodiorite. monzodiorite and hornblende diorite. The plu- procedures eliminated all but traces of the al- Apatite and zircon are common accessory min- ton cuts most structures affecting adjacent rocks tered biotite prior to analysis, however. The ap- erals. Hornblende is dark green with shredded in lower and middle structural blocks and forms parently ubiquitous biotite alteration also may borders and reaction rims of biotite. Equant a contact aureole in these rocks. The fault at the be reflected in the date for the sample analyzed hornblende is also included within potassium base of the upper structural block cuts the by Coats and others (1965) because alteration of feldspar. Abundant zircon and rare pyroxene in- pluton. this type tends to produce a decrease in conven- clusions occur in hornblende. Biotite is green to A biotite K-Ar date of 36.6 ± 0.5 Ma for the tional K-Ar dates. The 36.6 Ma date is consid- brown and in some samples is slightly altered to McGinty Monzogranite was reported by Hog- ered a minimum for the crystallization age, and chlorite. Foliation is weakly to moderately de- gatt and Miller (1981); no further isotopic work so the McGinty pluton probably intruded veloped but inconsistently oriented and there- has been performed. As noted in the previous somewhat prior to 37 Ma. The McGinty Mon- fore probably primary; it is defined by elongate work, the date is compatible with a previous zogranite is herein considered to be Eocene or quartz and aligned mafic minerals. Aplite dikes determination by the Pb-a method of 30 ± Oligocene in age. 2 to 7 cm thick are locally abundant parallel to 10 Ma (Coats and others, 1965) but not with a Bettridge Creek Granodiorite. The Bettridge foliation, but pink pegmatite dikes are rare. The previous biotite K-Ar date of 32 ± 1.6 Ma Creek Granodiorite is herein named for expo- pluton contains abundant xenoliths <8 cm in (Coats and others, 1965; revised using new sures in its herein-designated type area in the diameter that are approximately hornblende decay constants). The biotite date reported by canyon north of Bettridge Creek, sec. 16, T. 4 diorite in composition. In many cases, the xeno- Hoggatt and Miller (1981) (sample M79PR- N., R. 19 W., in the Pilot Peak 7.5' quadrangle, liths are nearly disaggregated and identifiable

ii4°or EXP LAN ATION Surficial deposits (Quaternary)

Bettridge Creek Granodiorite (Tertiary)

Prospect Mountain Quartzite (Cambrian and Late Proterozoic)

McCoy Creek Group (Late Proterozoic)-Consists of:

Unit G

Unit F

Unit E

Contact-Dotted where concealed High-angle "fault-Bar and ball on downthrown side Low-angle fault-Teeth on upper plate Anticline Syncline Strike and dip of foliation Strike and dip of bedding Inclined Vertical Overturned Sample location

Figure 4. Geologic map of the Bettridge Creek area, southeast Pilot Range (location shown in Fig. 2). Locations of geochronology samples indicated; M80PR-19 sampled at same location as M79PR-171.

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Mesh size Concentration (ppm) Isotopie composition of lead* Age (Ma)

U Th Pb 204Pb 206Pb 2roPb 208Pb ^Pb ^Pb ^Pb ^Pb

238U 235u 206ft 232^

M81PR-82 Bettridge Creek Granodiorite

-150+200 1.8S7 487 12.09 0.0257 86.69 4.888 8.397 41.1 45.5 287 41.8 -250+325 2,295 715 14.68 0.0585 84.06 5.025 10.85 39.6 41.9 174 40.2 -400 2.8É6 1397 18.58 0.0178 82.06 4.192 13.74 39.6 40.4 93 39.4

M80PR-19 Miners Spring Granite -100+150 5,728 1,544 133.2 0.0041 88.09 4.442 7.468 152 154 183 143 -150+200 4,528 1,478 105.9 0.0035 85.83 4.307 9.862 149 150 175 147 -200+250 4.3É9 1,326 104.7 0.0024 87.11 4.339 8.551 155 155 167 151 -250+325 4.4É5 1,457 107.6 0.0040 86.45 4.332 9.219 154 155 168 152 -325+400 4,6/1 1,658 113.3 0.0025 85.98 4.280 9.737 154 155 164 149

Note: decay constants: 231iU = 1.55125 x HT10 yr"1; 235U = 9.8485 * 10"'° yr1; 235Th = 4.9475 x 10"11 yr1; 238U/235U = 137.88. Isotopie composition of common lead assumed to be 204Pb: 206Pb: 207Pb: 208Pb = 1:18.4:15.638.3. •Atom percent.

Bettridge Creek Granodiorite Miners Spring Granite M81PR-82 M80PR-19

\ 0.24 > A a a

'Pb/"5U

Figure S. Concordia plots of U-Pb data for samples from the Pilot Range. A. Sample M81PR-82, Bettridge Creek Granodiorite. B. Sample M80PR-19, Miners Spring Granite; three size fractions with nearly identical isotopic data are superposed.

TABLE 4. "°Ar/39Ar ANALYTICAL DATA FOR HORNBLENDE FROM GRANODIORITE AND SCHIST, PILOT RANGE, UTAH AND NEVADA

37 39 36 39 39 36 3 Temperature «Ar/^Ar Ar/ Ar* Ar/ Ar Ar A'Ca 'A'Ca «ArK Apparent K/Ca Calculated age^ (°C) (% total) TO (%) m (mol/mol) (Ma)

M81PR-82 hornblende; J = 0.0048855

Recalculated total-fusion age = 94.9 Ma 650 ;78.8 0.6908 0.3445 1.83 0.05 0.04 43.1 0.003 0.75 576 ± 5.4 700 43.32 0.2860 0.1155 1.08 0.07 0.02 21.2 0.014 1.82 79.3 ± 2.7 775 20.19 0.2396 0.0384 0.93 0.17 0.02 43.9 0.029 2.17 76.4 ± 3.0 830 72.10 1.229 0.1961 0.94 0.17 0.08 19.8 0.008 0.42 121 ± 3.2 920 70.98 2.693 0.2017 0.88 0.36 0.17 16.3 0.008 0.19 99.5 ± 3.4 965 75.18 2.602 0.2132 0.54 0.33 0.16 16.5 0.008 0.20 106 ± 5.3 1005 31.10 7.080 0.0446 7.65 4.32 0.45 59.4 0.019 0.07 157 ± 1.6 1050 18.84 6.419 0.0145 18.02 12.06 0.41 80.0 0.032 0.08 129 ± 1.3 fused 8.504 6.053 0.00563 68.13 29.25 0.38 86.1 0.070 0.09 63.6 ± 0.6

M80PR-26 hornblende; J = 0.004885®

Recalculated total-fusion ag e = 157 Ma 650 .170.5 7.419 0.9875 1.11 0.20 0.47 21.4 0.002 0.07 593 ± 9.9 700 109.6 19.69 0.3508 0.72 1.53 1.25 6.8 0.005 0.03 65.7 ± 10.2 770 25.37 8.498 0.0688 1.35 3.36 0.54 22.6 0.023 0.06 50.0 ± 5.0 850 69.41 6.248 0.2155 1.31 0.79 0.40 9.0 0.009 0.08 54.2 ± 5.4 892 87.50 9.423 0.2600 1.56 0.99 0.60 13.1 0.007 0.06 98.5 ± 4.6 960 39.93 14.34 0.0583 14.15 6.69 0.91 59.7 0.015 0.04 200 ± 2.0 1010 42.55 14.00 0.0900 49.60 4.23 0.89 40.1 0.014 0.04 146 ± 1.5 1070 26.21 14.06 0.0333 22.48 11.49 0.89 66.8 0.023 0.04 149 ± 1.5 fused 29.57 14.48 0.0434 7.72 9.06 0.92 60.5 0.020 0.04 153 ± 1.7

Note: subscripts in columns 6-9 indicate radiogenic (R), calcium-derived (Ca), and potassium-derived (K) argon. 37 •Corrected for Ardeciy(t05 = 35.1 days). - fx^ + At' = 0.581 x 10"yr"'; k^ = 4.962 x 10"'® yr '. The plus-or-minus valuts are estimates of the standard deviation of analytical precision, is a function of the age of the monitor mineral and of the integrated fast-neutron flux. The monitor mineral, SB-3 biotite, has an age of 161.4 Ma.

only as patches of hornblende-rich granodiorite. The Bettridge Creek Granodiorite and its related dikes intruded low- and high-angle faults and 600 folds (Fig. 4) and retrograded upper(?) green- BETTRIDGE CREEK GRANODIORITE schist-facies rocks in the footwall of the Pilot M81PR-82 HORNBLENDE Peak décollement to lower greenschist facies. <0 Hoggatt and Miller (1981) considered the ë 400 H Bettridge Creek Granodiorite to be Creta- 0 O) ceous(?) on the basis of highly discordant ra conventional K-Ar dates of 27.1 ± 0.5 and 91.2 c 0 ±1.5 Ma on coexisting biotite and hornblende, rak a 200 respectively (Table 2, sample M79PR-85). Sub- a < sequent K-Ar study of another sample apparently confirmed the earlier results with a conventional K-Ar date for hornblende of 96.4

± 2.9 Ma (Table 2, sample M81PR-82). _L J_ _L U-Pb results for zircon separated from sample 10 20 30 40 50 60 70 80 90 100 M81PR-82 give a considerably younger date 39 Ar released, cumulative percent than do hornblende K-Ar results. Three analyzed size fractions of zircon plot as a linear array on the concordia diagram, defining a chord with upper and lower intercepts of 2350 B +1120/—650 and 38.9 ± 0.9 Ma, respectively CAMBRIAN SCHIST (Table 3, Fig. 5A). The upper intercept age can 600 f be associated with an inherited component of M80PR-26 HORNBLENDE zircon, probably derived from underlying Pre- cambrian basement. The Pilot Range lies near a boundary between upper Archean basement 400 149±0.9 0 rocks to the north and middle Proterozoic o> basement to the south (Condie, 1969). Within CO the broad error limits of the upper intercept, both of the terranes are permitted as possible ra a. 200 1070 FUSE a sources of the inherited zircon in the magma. < The component of zircon giving the lower intercept we interpret as yielding the crystallization I/ age of the rock. During magma solidification, a I I I J 1 I L- _J I L. second generation of zircon probably formed as 10 20 30 40 50 60 70 80 90 100

overgrowths on xenocrystic cores and as newly 39 Ar released, cumulative percent nucleating crystals. Because of the dominance of the overgrowth component, this crystallization event is determined quite precisely. Hornblende from sample M81PR-82 was CAMBRIAN SCHIST M80PR-26 also analyzed by the 40Ar/39Ar method and USED IN FIT shows an Ar release pattern of generally decreas- NOT USED IN FIT ing age of gas increments with increasing 2500 0.004 temperature (Table 4, Fig. 6A). This pattern is typically interpreted as the product of excess Ar 2000 - retention. The recalculated total fusion date of 0.003 < < 94.9 Ma, calculated by recombining data from to O CJ 1500 - individual gas increments, is in good agreement ^ 0.002 with the conventional K-Ar date of 96.4 Ma < < o 1000 The gas release pattern from incremental heating

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sample M79PR-85 could reflect either slow Miners Spring Granite. The Miners Spring characteristically crops out as dikes and pods cooling of the pluton to the temperature at Granite is herein named for exposures in its that intrude Cambrian schist and marble along which biotite retains argon or differential loss of herein-designated type area east of Miners an area of ~6 km2 of the eastern Pilot Range 40Ar from biotite after emplacement of the Spring, sec. 17, T. 36 N, R. 70 E., Miners Can- (Fig. 7). The granite is white to light gray, fine- pluton. yon 7.5' quadrangle, Nevada-Utah. The granite to medium-grained, equigranular, moderately

EXPLANATION

Surficial deposits (Quaternary)

Granodiorite (Tertiary)

lljgjl! Miners Spring Granite (Jurassic)

Limestons and shale (Permian to Mississip- pian)

Carbonati; strata (Ordovician and Cambrian)

Marble and schist (Cambrian)

Prospect Mountain Quartzite (Cambrian and Late Proterozoic)

- Contact

•High-angle fault-Dashed where approximately located, tar and ball on downthrown side

•Low-angle fault-Dashed where approximately located; dotted where concealed; double barbs on upper plate (Cenozoic)

• Low-angle fault-Dashed where approximately located; dotted where concealed; teeth on upper pice (Mesozoic)

ii4° r

Figure 7. Geologic map of an area east of Pilot Creek, southern Pilot Range (location shown in Fig. 2). Locations of geochronology samples indicated.

foliated, muscovite-biotite syenogranite to mon- nor hypabyssal textures were produced. The M80PR-28 taken from the adit gave a conven- zogranite. Biotite is generally altered to chlorite, granite was emplaced in neighboring parts of tional K-Ar date of 37.5 ± 2.6 Ma on an impure and muscovite in some samples has grown from several low-angle fault blocks (Fig. 7), suggest- separate. Low K2O and radiogenic argon for or reacted with plagioclase. Accessory minerals ing that it intruded the faults. The wall rocks this sample make interpretation of the data diffi- are garnet, sphene, zircon, and apatite. Foliation may have been metamorphosed prior to intru- cult. Hornblende from a dike filling the dé- parallels that in wall rocks and is formed by the sion and certainly were metamorphosed and de- collement 1 km northwest of Miners Spring preferred orientation of biotite, muscovite, and formed subsequent to intrusion. Either the rocks (sample M82PR-79) yielded a conventional K- recrystallized quartz. Micas generally total less did not cool to the blocking temperature for Ar date of 54.6 ± 1.6 Ma (Table 2). than 3% by volume, but muscovite content is argon in muscovite until 56 Ma, or a partial loss In light of the complex thermal and structural locally as high as 20%. Dikes locally contain of argon occurred after 56 Ma. history of the area and the spread of apparent calcite and(or) hornblende, indicating that wall Granodiorite Dikes. Dikes and pods <150 ages yielded by the three samples, it is not possi- rocks in places were incorporated into the gran- m2 in area are common in the region from Bett- ble to establish the crystallization age of the ite. A locally persistent rock type of this rock ridge Creek south to Miners Spring and west granodiorite dikes. Because biotite from the unit is white, plagioclase-potassium feldspar- from Miners Spring (Figs. 2,7). Mineralogically Bettridge Creek pluton yielded conventional K- quartz pegmatite that forms pods having fine- and texturally, these dikes are all broadly similar Ar dates younger than the age of crystallization grained borders. The Miners Spring Granite in that they have a tri-modal grain size formed determined by U-Th-Pb methods, the biotite shares at least the last two of the three fold sets by phenocrysts, fine-grained minerals, and an dates for the granodiorite dikes probably repre- affecting the enclosing schist. Although outcrops aphanitic matrix. Potassium feldspar pheno- sent minimum ages for crystallization. The of the Miners Spring are known only from the crysts about 1 cm in diameter occur in a matrix hornblende date possibly represents excess argon area of amphibolite facies rocks east of Pilot of subequigranular quartz, plagioclase, potas- (also analogous to the Bettridge Creek pluton) Peak (Fig. 7), rare boulders of muscovite-biotite sium feldspar, biotite, and hornblende. Acces- and cannot be interpreted. On the basis of com- granite are found on the west side of Pilot Peak, sory minerals are sphene, zircon, pyroxene, and positional and isotopic similarity, however, the suggesting that the granite locally intruded apatite. Medium gray aphanitic matrix to the dikes may be a part of a 39 Ma intrusive event chlorite- and biotite-zone rocks exposed on the crystals is commonly present and constitutes as that included the Bettridge Creek Granodiorite. west side of the range. much as 40% of the rock. Variations of the tex- Dikes and small plutons crop out sparsely in Hoggatt and Miller (1981) reported that the ture and mineralogy include: (1) potassium much of the hanging wall of the Pilot Peak Miners Spring Granite yielded a conventional feldspar is not always phenocrystic, (2) biotite décollement in the southern Pilot Range. On the K-Ar date for muscovite of 56.2 ± 0.8 Ma locally forms 1-cm books, (3) quartz content basis of textural and compositional similarity (Table 2, sample M79PR-171). U-Th-Pb data varies from a few percent to 25%, and (4) horn- with dikes east of Pilot Peak, the best estimate (sample M80PR-19, Table 3, Fig. 5 B) for zir- blende is locally absent. In many dikes, horn- for the age of the southern Pilot Range dikes cons collected from the same dike, however, are blende and biotite are highly altered, chiefly to and small plutons is latest Eocene or early best interpreted as indicating a crystallization chlorite and clays. Plagioclase is typically par- Oligocene. age between 155 and 165 Ma. The five analyzed tially sericitized or propylitized, and potassium Metamorphic Rocks. Metamorphic biotite size fractions of zircon display a slight discord- feldspar is chalky white in many outcrops, indi- and chlorite are widespread in rocks in the ance that is a function of grain size. If the small cating some alteration to clays. Hornblende is footwall of the Pilot Peak décollement. Mica- discrepancy between 206Pb/238U and 207Pb/ green-brown where unaltered, and biotite is ceous and calcareous rocks near Bettridge Creek 206Pb dates is interpreted as due to a very small green and rarely contains cores of light green and east of Pilot Peak contain relics of higher amount of older xenocrystic zircon, then the calcic amphibole. The dikes and pods have grade minerals such as wollastonite, tremolite, prominent chilled margins and only rarely are 206Pb/238u dates of about 155 Ma should actinolite, garnet, staurolite, and hornblende. foliated or lineated. They were in many cases closely approximate the time of crystallization. Oriented pseudomorphs of garnet and staurolite intruded into high- and low-angle fault zones Alternatively, the unusually high uranium con- contain unoriented retrograde assemblages such but only rarely were themselves fractured. Dikes tent of the zircon could have caused sufficient as epidote, white mica, chlorite, and biotite. intruded into the Pilot Peak décollement cut radiation damage to also allow minor loss of Miller and Lush (1981) attributed the amphibo- brecciated and highly cleaved wall rocks but radiogenic lead. In that case, and if the three lite-facies minerals to localized heating around show only minor fracturing. Rocks near the finer grain-size fractions are free of inheritance, plutons and the retrogression to a later middle dikes are retrograded to assemblages bearing their tightly grouped 207Pb/206Pb dates of about Tertiary event. Garnet and staurolite in musco- hornblende, actinolite, epidote, and chlorite. No 165 Ma might better indicate the time of crystal- vite schist, however, show a strongly developed discernible change in the dikes occurs near the lization. If radiogenic lead were lost relatively preferred orientation, indicating that regional de- Bettridge Creek pluton, where their lithologie recently, such a disturbance would have lowered formation accompanied the amphibolite-facies similarity with the pluton suggests that all dikes the 206Pb/238U but not the 207Pb/206Pb dates. metamorphism. In addition, amphibolite-facies are related to that 39-m.y.-old granodiorite. The two explanations for the discordance en- minerals and synkinematic quartz-feldspar veins compass the range in uncertainty for the crystal- Three granodiorite dikes were sampled for K- occur over wider areas than indicated by Miller lization age of the Miners Spring Granite, which Ar analysis (Table 2). Sample M79PR-21, re- and Lush and, although best developed in the is considered to be between 155 and 165 Ma ported by Hoggatt and Miller (1981), cuts a area of exposure of the Miners Spring Granite, (Jurassic). bedding-plane fault (Fig. 7) between metamor- are not spatially restricted to the pluton expo- The K-Ar and U-Pb data are interpreted as phosed Cambrian strata; it yielded a conven- sures. Hornblende locally is a retrograde product indicating that the network of dikes and small tional K-Ar date for biotite of 30.1 ± 0.5 Ma. A adjacent to granodiorite dikes and small plutons bodies of the Miners Spring Granite was in- dike intruding the Pilot Peak décollement was and the Bettridge Creek Granodiorite, suggesting truded between 155 and 165 Ma at mesozonal penetrated by an adit near Miners Spring (Fig. that Tertiary pluton emplacement and diking conditions under which neither chilled margins 6). Books of oxidized brown biotite from sample caused retrogression.

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Three metamorphic rocks were collected to has an apparent age of 200 Ma, suggest that tions. The Bettridge Creek Granodiorite crystal- constrain the timing of metamorphism. Sample there may be a small amount of excess 40Ar in lized at about 39 Ma, after it and nearby dike M80PR-26 (Fig. 7) is Cambrian calcareous the hornblende. swarms were emplaced, probably at a shallow schist containing hornblende both as oriented Sample P80PR-14 (Fig. 7) is Cambrian crustal level. The pluton may have cooled crystals and as sparse unoriented aggregates after garnet-staurolite-muscovite schist that contains slowly until the Ar blocking temperature for an unknown porphyroblast, as well as biotite, muscovite formed in strongly oriented aggre- biotite was reached at 27 Ma or differential loss calcite, quartz, treinolite, and epidote. Biotite is gates parallel to foliation. Muscovite in this of 40Ar in biotite occurred after 27 Ma. Because strongly oriented and therefore is a product of sample yielded a conventional K-Ar date of 83.4 the epizonal pluton probably underwent rapid dynamothermal metamorphism. Conventional ± 2.5 Ma (Table 2). Sample M80PR-43 (Fig. 4) heat loss, after emplacement, the second alterna- K-Ar dates for bic tite and hornblende are 63.9 is Proterozoic amphibole schist consisting of tive is more probable. Granodiorite dikes south ± 1.9 and 170.3 i 5.1 Ma, respectively (Table hornblende, plagioclase, and quartz; all minerals of the Bettridge Creek pluton probably are re- 2). The recalculated 40Ar/39Ar total fusion date are oriented with a well-developed schistosity. lated to the pluton; available K-Ar data point to of 157 Ma for this hornblende is somewhat The conventional K-Ar date on hornblende is emplacement of the dikes before 30 to 37 Ma. younger than the conventional date (Table 4). 179.7 ± 9.0 Ma (Table 2), in close agreement The McGinty Monzogranite also was emplaced The 40Ar/39Ar age spectrum (Fig. 6B) indicates with the conventional K-Ar date of hornblende and crystallized somewhat before 36 Ma. That that the mineral has been geologically disturbed from sample M80PR-26 calcareous schist. all intrusive rocks dated by K-Ar and U-Pb to a slight degree; however, the hornblende has a The Late Cretaceous K-Ar dates obtained for methods as 30 to 39 Ma belong to a single well-defined plateau for the three highest tem- biotite and muscovite can be interpreted either intrusive episode is supported by commonalities perature Ar release increments in which nearly as recording times when the minerals cooled in textures, pointing to their shallow crustal em- 39 80% of the Ar was released. The weighted from peak metamorphic temperatures to their placement, and their metaluminous composi- mean age of this plateau is 149 ± 1 Ma; the blocking temperatures or as representing differ- tions of generally hornblende-biotite grano- 40 39 Ar/ Ar isochron age for the plateau incre- ential loss of argon during retrogression or some diorite. Further, voluminous pre-Miocene tuffs ments is 152 ± 6 Ma (Fig. 6C). These data meet other event. In either case, the dates are minima preserved nearby are about 37 Ma (Compton, 40 39 the criteria for a meaningful Ar/ Ar incre- for mineral growth during metamorphism. Al- 1983; Miller, 1984, 1985); no record of older mental heating age (Lanphere and Dalrymple, though the K-Ar data for hornblende may be Tertiary volcanism is known in this part of 1978). None of the 5 gas increments released distorted by minor inherited 40Ar, the 39Ar/ northeastern Nevada. between 650 and 892 °C contain as much as 2% 40 Ar data suggest that the distortion is small and On the basis of structural data, the regional of the total 39Ar released, and these increments that metamorphic hornblende cooled to the metamo rphism and ductile deformation was en- yield ages significantly different from the plateau blocking temperature at about 150 Ma. tirely Mesozoic; it primarily postdated intrusion age. Dalrymple and Lanphere (1974) suggested of the Miners Spring Granite and entirely pre- that such small amounts of gas may be isotopi- SYNTHESIS dated intrusion of the Bettridge Creek Grano- cally fractionated during heating and that it is diorite. Retrograde metamorphic minerals near unwise to attribute geological significance to the the Oligocene intrusions indicate that the rocks resulting apparent ages. If the first 5 gas incre- The geochronologic and geologic data indi- had cooled below their peak metamorphic ments are pooled, then the apparent age of the cate intrusive episodes in Late Jurassic and latest temperatures and were locally reheated at about first 6% of the 39Ar released is 165 Ma. This Eocene into early Oligocene times. The Miners 39 Ma. Metamorphic hornblende from schist "increment" plus the 960 °C increment, which Spring Granite crystallized between 165 and 155 Ma after intrusion under mesozonal condi- provides a 150 Ma minimum age for peak high-

METHOD IB] U-Pb on zircon B o> gl Rb-Sr B o [M] B B M to K-Ar TJ B B B M B B B B M B B B B M ® 13 B B B M a 5 3 1 E 1Ä 13 Lb" B B B ? M B B 3 • 13 * B • ? M M B J JEL S0 /A- 180 170 160 150 120 110 90 80 70 Date (Ma)

Figure 8. Histogram of published dates on intrusive rocks in northeastern Nevada and northwestern Utah. Note that the scales change at 50 Ma. Circle denote» age too low; asterisk, age too high, both on the basis of inspection of other data from same intrusive body. Symbols for K-Ar: B = biotite, M = muscovite, H = hornblende, X = mixture, ? = data not available. Data are from references cited in the text, plus Allmendinger and Jordan (1984), Crittenden and others (1973), Lee and Marvin (1981), Kistler and others (1981), Slack (1974), and D. M. Miller and J. K. Nakata (unpub. data). All data are revised with new decay constants. Data limited to region bounded by lat 40°N, ilong 115°30'W, (at 42°30'N, and long 111°30'W (see Fig. 1). Note that the Miners Spring Granite is represented by a single box from 165 to 155 Ma.

grade metamorphism. The metamorphic event McGinty Monzogranite and probably continu- latest Eocene into early Oligocene and the other possibly began somewhat before 165 Ma be- ing in several episodes to Quaternary time, low- late Oligocene in age. Plutons with 43 to 30 Ma cause the Miners Spring Granite intruded rocks angle faults in the northern Pilot Range dates are sparse but widespread in northeast that may have been already folded. Most, if not displaced Paleozoic and Tertiary strata across Nevada and northwest Utah (Moore and all, of the metamorphism, however, is con- the McGinty Monzogranite (Miller and others, McKee, 1983). As in the Pilot Range, they are strained to between 165 and 150 Ma by evi- 1982; Miller, 1985). K-Ar data for biotite indi- generally metaluminous hornblende-biotite dence of ductile deformation and metamorphic cate that either last cooling or a disturbance, granodiorite and are epizonal. Some of these assemblages in the granite. Although metamor- such as low-grade metamorphism, occurred dur- plutons have a sizable component of lower phic biotite and muscovite possibly experienced ing this episode. crustal Precambrian lead (Stacey and Zartman, differential loss of ^Ar, as did micas in intrusive 1978). Coeval volcanic rocks occur in many rocks, they give minimum ages for achieving Ar REGIONAL PATTERNS areas (Compton, 1983; Moore and McKee, retention at appropriate blocking temperatures. 1983; Miller, 1984). Although emplaced at shal- Thus, the rocks had cooled to about 350 °C Intrusive Episodes low levels, the plutons mainly occur in ranges (Purdy and Jäger, 1976) by 83 Ma as recorded exposing metamorphic rocks or deeply buried by muscovite and to 260 to 180 °C (Harrison Geochronologic data for intrusive rocks in the strata. This relation supports a model whereby and others, 1979) by 64 Ma as recorded by Pilot Range fit well into a regional pattern of the magma was generated in loci undergoing biotite. Discordance between muscovite K-Ar Late Jurassic and Eocene into Oligocene pluton- upper crustal extension and emplaced into once dates (56 Ma, metamorphosed Miners Spring ism in northeastern Nevada and northwestern deeply buried rocks as they were exhumed Granite; 83 Ma, schist) indicates that either dif- Utah (Fig. 8). As pointed out by Moore and (Miller and others, 1983). 40 ferential loss of Ar affected the metamorphic McKee (1983), plutonic rocks with documented The late Oligocene intrusive episode is pri- rocks or metamorphism was complex and var- Cretaceous ages occur south and west of this marily restricted to metamorphic core com- ied over short distances. region but are apparently absent within it. plexes and is represented in the area used to The Mesozoic and early Cenozoic history of Mesozoic K-Ar dates for plutons in the area compile Figure 8 by two plutons in the Grouse metamorphism, deformation, and intrusion in scatter from 127 to 174 Ma, with only one date Creek and Albion Mountains and by the Little the Pilot Range is best constrained by relations older than 160 Ma. In cases in which multiple Cottonwood stock in the Wasatch Mountains. exhibited in the footwall of the Pilot Peak dé- data exist for a pluton, wide ranges of dates are The former two plutons are late kinematic collement. Penetrative structures and brittle to duc- reported, such as K-Ar biotite dates of 127,130, (Armstrong, 1968; Compton and others, 1977; tile faults that are parallel to bedding formed and 152 Ma on the Silver Zone Pass pluton Todd, 1980) and contain minor muscovite in during metamorphism in the Late Jurassic and (references in Miller, 1984). Thus, the broad biotite granite and granodiorite. Similar plutons Early Cretaceous. Peraluminous granite, in- peak of K-Ar dates on plutonic rocks between in other Great Basin metamorphic core com- truded between 165 and 155 Ma, was meta- 125 and 160 Ma (Fig. 8) may include a "tail" of plexes are about the same age (Best and others, morphosed and folded. Metamorphic isograds dates that resulted from slow cooling or differen- 1974; Snoke and Lush, 1984). 40 crudely outline the area of outcropping Miners tial loss of Ar. Based on petrologic and geo- Spring Granite, suggesting that high-grade logic studies of many plutons in the area, we Metamorphic Episodes metamorphism may have been caused by lo- suspect that most plutons with early Early Cre- cally higher isotherms resulting from heat from taceous K-Ar ages will prove to be Late Jurassic The times and durations of metamorphic the intrusion. Probably in the late Early Cre- in age. In our interpretation, the data in Figure 8 events in the eastern Great Basin are poorly con- taceous, and certainly by 83 Ma, metamorphic point to a widespread intrusive event between strained. In places, minimum ages for metamor- temperatures had declined to about 350 °C. Fol- 155 and 165 Ma. More plutons of this age are phism are given by crosscutting structures or lowing and possibly partly during metamor- known to the south and west (Lee and others, plutons, but maximum ages are difficult to estab- phism, the Pilot Peak décollement displaced 1970; Coats and others, 1965; Roberts and oth- lish. Region-wide, the best estimate for begin- little-metamorphosed, but faulted, strata in the ers, 1971). Late Cretaceous and earliest Tertiary ning of metamorphism is that it must postdate hanging wall laterally at least several tens of dates shown in Figure 8 may represent rare Cre- deposition of Upper Triassic or Lower Jurassic kilometres along a detachment fault system, jux- taceous plutons, but in many cases, these dates nonorogenic sediments (Misch, 1960; Arm- taposing Cambrian strata of dramatically differ- represent excess Ar in hornblende or cooling strong and Hansen, 1966). Amphibolite facies ent depositional facies. By about 40 Ma, all ages for muscovite. Cretaceous plutons are not rocks exposed in metamorphic core complexes faults in the structural block below the décolle- firmly established in northeastern Nevada and were cooled during the late Cenozoic in many ment, and the décollement itself, had ceased northwestern Utah. cases, but Mesozoic metamorphic episodes may movement and were intruded by dikes and The Upper Jurassic plutons of northeast have been a major factor in creating the meta- plutons. Nevada and northwest Utah in many cases in- morphic fabrics (Armstrong, 1982; Snoke and Strata in the hanging wall of the Pilot Peak truded lower Paleozoic or older rocks and their Miller, 1987). Greenschist facies rocks exposed décollement were cut by three sets of faults prior wide metamorphic aureoles are suggestive of in ranges between, or along the fringes of, the to last movement along the décollement before mesozonal conditions for emplacement. Interest- metamorphic core complexes give better records 40 Ma. Bedding-plane faults in these strata may ingly, these plutons range widely in chemistry for Mesozoic metamorphism, judging from stud- be the same age as kinematically similar Meso- from peraluminous to metaluminous (Lee and ies in the northern Albion Mountains (Arm- zoic bedding-plane faults in the footwall of the others, 1981; Miller, 1984) and in composition strong, 1976), the Pilot Range, the southern Pilot Peak décollement. High-angle faults prob- from granite to diorite. Evidently, widely differ- Snake Range (Lee and others, 1970), the eastern ably formed in conjunction with the Pilot Peak ing source rocks were melted. Raft River Mountains (Armstrong and Hansen, décollement. The group of Cenozoic dates (Fig. 8) proba- 1966; Compton and others, 1977), and the Following late Eocene intrusion of the bly represents two distinct intrusive pulses, one Black Pine Mountains (Smith, 1982).

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200 Date (Ma)

Figure 9. Cumulative frequency plot of metamorphic conventional K-Ar dates (revised with new decay constants) reported for metasedimen- tary and metaigneous rocks from a selected region in the northeastern Great Basin. Northern Albion Mountains data are for the predominantly greenschist-facies rocks north of lat 42°15'N. Pilot Range data are reported in this paper. Data for all low-grade terranes include those from southern Snake Range and northern Albion Mountains, as well as scattered data throughout region.

A cumulative frequency plot (Fig. 9) of K-Ar from amphibolite facies rocks in metamorphic CONCLUSIONS dates for greenschist-facies metamorphic rocks core complexes (Fig. 9, high-grade terranes) from the northern Albion Mountains (Arm- contrast sharply with data from low-grade ter- Plutons in the Pilot Range were intruded dur- strong, 1976) demonstrates that >40% are Late ranes in demonstrating negligible Cretaceous ing two episodes, one in the Late Jurassic and Cretaceous, apparently representing cooling cooling but pronounced Neogene cooling. the other in the latest Eocene into early Oligo- from earlier metarnorphism. Dates from 77 to About 80% of the dates are younger than 40 Ma, cene, and thus belong to episodes recognized 111 Ma determined by whole-rock K-Ar on and 56% are younger than 30 Ma. These late elsewhere in northeast Nevada and northwest greenschist-facies argillite from the Black Pine Eocene to early Miocene dates indicate the time Utah. The plutons of each episode are distinctive Mountains (Smith, 1982), and our results from of cooling caused by upper crustal extension and with respect to style of intrusion and igneous the Pilot Range (Fig. 9), roughly accord with the attendant uplift, but metamorphic peaks were rock chemistry. Cretaceous plutons are rare or Albion data. Data from the southern Snake Mesozoic in age in some places and Cenozoic in absent in this region but occur widely to the Range (Lee and others, 1970,1980) bear strong age in others. For instance, in the southern Al- south and west. similarities to the Albion data, although Cre- bion Mountains and northern Grouse Creek Metarnorphism in the Pilot Range is con- taceous intrusions heating the rocks in the Snake Mountains, mylonite development, amphibolite- strained to Late Jurassic and Early Cretaceous Range doubtless contributed to this pattern. facies metarnorphism, and ductile faulting are time on the basis of relations with isotopically Overall, 38% of the K-Ar dates from low-grade closely related in time to emplacement of upper dated plutons and by conventional K-Ar and terranes fall between 88 and 62 Ma. This wide- Oligocene plutons (Armstrong, 1968, 1982; 40Ar/39Ar dates on metamorphic minerals. The spread and synchronous cooling of greenschist- Compton and others, 1977; Todd, 1980). reasonably firm lower boundary at early Late facies rocks during Late Cretaceous time sug- Furthermore, in the Ruby-East Humboldt Jurassic time for beginning of metarnorphism is, gests a mechanism such as uplift along a thrust, Mountains, Oligocene plutons are deformed and to our knowledge, the first such boundary for ramping basement rocks east over shallower metamorphosed (Snoke and Lush, 1984). In the timing of Mesozoic metarnorphism in the rocks (Armstrong, 1982), or upper crustal ex- both of these high-grade metamorphic terranes, eastern Great Basin. Evidence for cooling of tension by detachment faulting, causing rapid distinct Mesozoic and Cenozoic metamorphic metamorphic rocks by Late Cretaceous time in uplift and attendant cooling. events are documented (Armstrong, 1968,1976; the Pilot Range is seen elsewhere in the eastern Cumulative frequency plots of K-Ar data Snoke and Lush, 1984). Great Basin, suggesting a regional mechanism

for cooling. Our data for Mesozoic meiamor- Best, M. G., Armstrong, R. L., Graustein, W. C., Embree, G. F., and Ahlborn, northwest Utah, southern Idaho and northeast Nevada: Utah Geological R. C., 1974, Mica granites of the Kern Mountains pluton, eastern White Association Publication 13, p. 45-63. phism reinforce early reports ascribing meta- Pine County, Nevada: Remobilized basement of the Cordilleran 1985, Geologic map of the Lucin quadrangle, Box Elder County, Utah: miogeosyncline?: Geological Society of America Bulletin, v. 85, Utah Geological and Mineral Survey Map 78, 10 p., scale 1:24,000. morphism in the eastern Great Basin to a p. 1277-1286. Miller, D. M., and Lush, A. P., 1981, Preliminary geologic map of the Pilot Mesozoic orogeny. Coats, R. R., Marvin, R. F., and Stem, T. W., 1965, Reconnaissance of mineral Peak and adjacent quadrangles, Elko County, Nevada, and Box Elder ages of plutons in Elko County, Nevada, and vicinity: U.S. Geological County, Utah: U.S. Geological Survey Open-File Report 81-658,18 p., The geochronologic data permit assigning Survey Professional Paper 525-D, p. D11-D15. scale 1:24,000. Compton, R. R., 1983, Displaced Miocene rocks on the west flank of the Raft Miller, D. M., and Schneyer, J. D., 1985, Geologic map of the Tecoma quad- ages to structural events in the Pilot Range. River-Grouse Creek core complex, Utah, in Miller, D. M., Todd, V. R., rangle, Box Elder County, Utah, and Elko County, Nevada: Utah Geo- and Howard, K. A., eds.. Tectonic and stratigraphic studies in the logical and Mineral Survey Map 77,8 p., scale 1:24,000. Penetrative ductile deformation and bedding- eastern Great Basin: Geological Society of America Memoir 157, Miller, D. M., Lush, A. P., and Schneyer, J. D., 1982, Preliminary geologic plane faulting during Middle(?) and Late Juras- p. 271-280. map of Patterson Pass and Crater Island NW quadrangles. Box Elder Compton, R. R., Todd, V. R., Zartman, R. E., and Naeser, C. W., 1977, County, Utah, and Elko County, Nevada: U.S. Geological Survey sic time were followed by high-angle faulting, Oligocene and Miocene metamorphism, folding and low-angle faulting Open-File Report 82-834,24 p., scale 1:24,000. in northwestern Utah: Geological Society of America Bulletin, v. 88, Miller, E. L., Gans, P. B., and Garing, J., 1983, The Snake Range décollement: probably during crustal extension, at or before p. 1237-1250. An exhumed mid-Tertiary ductile-brittle transition: Tectonics, v. 2, 40 Ma. By 37 Ma, a major detachment fault had Condie, K. C., 1969, Geological evolution of the Precambrian rocks in northern p. 239-263. Utah and adjacent areas: Utah Geological and Mineral Survey Bulletin Miller, E. L., Gans, P. B., Wright, J, E., and Sutter, J. F., 1987, Metamorphic juxtaposed strata with different sedimentological 82, p. 71-95. history of the east-central Basin and Range province: Tectonic setting and metamorphic histories. Crittenden, M. D., Jr., Stuckless, J. S., Kistler, R. W., and Stern, T. W., 1973, and relationship to magmatism, in Ernst, W. G., ed., Metamorphism Radiometric dating of intrusive rocks in the Cottonwood area, Utah: and crustal evolution of the western United States: Englewood Cliffs, U.S. Geological Survey Journal of Research, v. 1, p. 173-178. New Jersey, Prentice-Hall, in press. Dalrympie, G. B., and Lanphere, M. A., 1969, Potassium-argon dating: San Misch, Peter, 1960, Regional structural reconnaissance in central northeast ACKNOWLEDGMENTS Francisco, W.H. Freeman and Company, 253 p. Nevada and some adjacent areas: Observations and interpretations, in 1974,40Ar/39Ar age spectra of some undisturbed terrestrial samples: Geology of east-central Nevada: Intermountain Association of Petro- Geochimica et Cosmochimica Acta, v. 38, p. 715-738. leum Geologists, Annual Field Conference, 11th, Guidebook, p. 17-42. Dalrympie, G. B., Alexander, E. C., Jr., Lanphere, M. A., and Kraker, G. P,, Misch, Peter, and Hazzard, J. C., 1962, Stratigraphy and metamorphism of late We thank J. L. Wooden, R. L. Armstrong, 1981, Irradiation of samples for *°Ar/39Ar dating using the Geological Precambrian rocks in central northeastern Nevada and adjacent Utah: R. W. Allmendinger, and P. B. Gans for Survey TRIGA reactor: U.S. Geological Survey Professional Paper American Association of Petroleum Geologists Bulletin, v. 46, 1176, 55 p. p. 289-343. discussions that helped formulate ideas pre- Davis, G. H., and Coney, P. J., 1979, Geologic development of the Cordilleran Moore, W. J., and McKee, E. H., 1983, Phanerozoic magmatism and minerali- metamorphic core complexes: Geology, v. 7, p. 120-124. zation in the Tooele 1° x 2° quadrangle, Utah, in Miller, D. M., Todd, sented herein. We also thank D. H. Rusling and DeWitt, Ed, 1980, Comment on 'Geologic development of the Cordilleran V. R., and Howard, K. A., eds., Tectonic and stratigraphic studies in the staff of the U.S. Geological Survey Reactor metamorphic core complexes': Geology, v. 8, p, 6-7. eastern Great Basin: Geological Society of America Memoir 157, Drewes, Harald, 1978, The Cordilleran orogenic belt between Nevada and p. 183-190. Facility for fast-neutron irradiation of samples, Chihuahua: Geological Society of America Bulletin, v. 89, p. 641-657. Purdy, J. W., and Jäger, E., 1976, K-Ar ages of rock-forming minerals from the Harrison, T. M., Armstrong, R. L., Naeser, C. W., and Harakel, J. E., 1979, central Alps: institute of Geology and Mineralogy, University of Padua, S. E. Koffman for assistance with the argon mea- Geochronology and thermal history of the Coast Plutonic Complex Padua, Italy, Memoir 30,31 p. surements, L. M. Kwak for performing the zir- near Prince Rupert, British Columbia: Canadian Journal of Earth Roberts, R. J., Radtke, A. S., and Coats, R. R., 1971, Gold-bearing deposits in Sciences, v. 16, p. 400-410. north-central Nevada and southwestern Idaho: Economic Geology, con U-Th-Pb isotopic analyses, and Dennis Sorg Hoggatt, W. C., and Miller, D. M., 1981, K-Ar ages of intrusive rocks of the v. 66, p. 14-33. for consistently provided high-quality mineral Pilot Range, Nevada and Utah: Isochron/West, no. 30, p. 21-22. Slack, J. F., 1974, Jurassic suprastructure in the Delano Mountains, northeast- Ingamells, C. O., 1970, Lithium metaborate flux in silicate analysis: Analytica ern Elko County, Nevada: Geological Society of America Bulletin, separations. J. L. Wooden, F. K. Miller, Gunter Chimica Acta, v. 52, no. 2, p. 323-334. v. 85, p. 269-272. Kistler, R. W., Ghent, E. D., and O'Neil, J. R., 1981, Petrogenesis of garnet Smith, J. F., 1982, Geologic map of the Strevell 15-minute quadrangle. Cassia Faure, and an anonymous reviewer provided two-mica granites in the Ruby Mountains, Nevada: Journal of Geo- County, Idaho: U.S. Geological Survey Miscellaneous Investigations helpful comments on earlier versions of this physical Research, v. 86, p. 10591-10606. Map 1-1403, scale 1:62,500, Krogh, T. E., 1973, A low-contamination method for hydrothermal decompo- Snoke, A. W., and Lush, A. P., 1984, Polyphase Mesozoic-Cenozoic deforma- paper. sition of zircon and extraction of U and Pb for isotope age determina- tional history of the northern Ruby Mountains-East Humboldt Range, tion: Geochimica et Cosmochimica Acta, v. 37, p. 485-494. Nevada, in Lintz, J., Jr., ed.: Western Geological Excursions, v. 4, Lanphere, M. A., and Dalrympie, G. 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