Dilles 1988 The chronology of early Mesozoic arc magmatism.pdf

Dilles 1988 The chronology of early Mesozoic arc magmatism.pdf

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  The chronology ofearly Mesozoic arc magmatism in the Yerington district of western Nevada and its regional implications JOHN H DILLES* 1 TAXjmc ' U7DTruT Department of Geology, Stanford University, Stanford, California 94305 J A M L o c . W K l C j n 1 f ABSTRACT The Yerington district, west-central Nevada, is underlain by arc volcanic, sedimentary, and plutonic rocks ranging in age from Mid- dle(?) Triassic to Middle Jurassic. Twenty-three U-Pb radiometric dates from eight igneous units have been determined for zircon from both plutonic and volcanic rocks in the district, which establish two distinct periods of voluminous intermediate to silicic, high-K calc- alkaline magmatism. The earlier period is constrained by zircon dates of 232 and 233 Ma on a quartz porphyry and a metadiorite, respec- tively, which intrude and are possibly comagmatic with andesitic to rhyolitic volcanic rocks. The second period of arc magmatism is con- strained by 5 concordant zircon dates that define an approximately 4-m.y. period (169-165 Ma) in the Middle Jurassic during which as much as 4 km of volcanic and volcaniclastic strata accumulated and two major batholiths were emplaced. Middle Jurassic magmatism began with eruption of the volcanics of Artesia Lake. They were intruded by the probably comagmatic 250-km2 Yerington batholith, which was differentiated, emplaced, and crystallized within approxi- mately 1 m.y. (169.4-168.5 Ma). Granite porphyry dikes (168.5 Ma) are the latest phase of the Yerington batholith and are synchronous with porphyry and skarn copper mineralization. Disconformably overlying the Artesia volcanics is a second volcanic suite consisting of as much as 2 km of latitic volcanics of Fulstone Spring (166.5 Ma). The volcanics of Fulstone Spring were subsequently intruded by quartz monzodiorite porphyry dikes (>165 Ma) and the Shamrock batholith (165.8 Ma). The Triassic through Lower Jurassic strati- graphic column was contact metamorphosed and folded adjacent to, and at least in part contemporaneously with emplacement of, the Yerington and Shamrock batholiths (169.4-165.8 Ma). We interpret the early, 232-233 Ma period of magmatism to represent a minimum age for the establishment of a west-facing Trias- sic volcanic arc within west-central Nevada following the Permian- Triassic Sonoma orogeny. The timing of Middle Jurassic arc magmatism and deformation in the Yerington district and surrounding regions is strikingly similar to the timing of magmatic and tectonic events documented in the Klamath Mountains region of northwestern California. We suggest that the similar record of arc magmatism and deformation in these two regions is not fortuitous but reflects a com- mon tectonic evolution within a single, west-facing arc constructed along the western edge of North America. *Present address: Department of Geology, Oregon State University, Corvallis, Oregon 97331-5506. INTRODUCTION The Yerington district (Fig. 1) is in west-central Nevada approxi- mately 80 km east of the Sierra Nevada. The district lies within the Basin and Range province and has undergone a complex Miocene to Recent extensional tectonic history that resulted in the tilting of pre-early Mio- cene-age rocks 50°-90° to the west and produced greater than 100% east- west extension across the district (Proffett, 1977). As a result of this extensional tectonic history, the pre-Tertiary geologic record is magnifi- cently exposed, essentially in cross section, within several major tilted fault blocks (Fig. 1). The bulk of the pre-Tertiary rocks exposed within the district include Upper Triassic to Middle Jurassic plutonic, volcanic, and sedimentary rocks that have been the subject of numerous detailed field and petrologic studies, primarily because of the occurrence of economic porphyry copper and copper-iron skarn mineralization (Einaudi, 1977, 1982; Proffett and Dilles, 1984; Dilles, 1984, 1987). Detailed geologic mapping summarized by Proffett and Dilles (1984) established an accurate relative chronology of magmatism, sedimentation, and deformation within the area. Existing radiometric age data, primarily K-Ar (Schilling, 1971; Bingler and others, 1980), however, were inconclusive in establishing an accurate radiometric chronology; for example, K-Ar dates of the Yering- ton batholith ranged from 143 to 173 Ma for hornblende and 91 to 143 Ma for biotite (Anaconda Co., unpub. data, cited in Proffett and Dilles, 1984). This study was started to establish an absolute chronology for early Mesozoic magmatism in the Yerington district, particularly the timing of magmatism related to porphyry copper and skarn mineralization. Eight samples of igneous rocks were collected from carefully mapped exposures and were dated radiometrically by the U-Pb zircon method. These geo- chronologic data (Tables 1 and 2) establish the age of two distinct major periods of arc magmatism, separated by a period of carbonate and fine- grained clastic deposition with only sporadic volcanic input. The earlier, Middle or earliest Late Triassic period is dated by U-Pb zircon dates of 232 and 233 Ma on a dike and a pluton, respectively, which intrude arc volcanics. The second and younger period is constrained by 5 U-Pb dates (17 zircon fractions), which define an approximately 4-m.y. period in the late Middle Jurassic (~ 169-165 Ma), during which as much as 4 km of volcanics was erupted and two batholiths were emplaced. The stratigraph- ic and chronologic relations of these two periods of arc magmatism are summarized in a generalized columnar section (Fig. 2). As an outgrowth of this study, the radiometric ages of igneous rocks in the Yerington district allow the comparison of magmatic and deformational events in the west- ern Nevada area with similar, well-dated events elsewhere in the western United States Cordillera, such as in the Klamath Mountains. In this paper, Geological Society of America Bulletin, v. 100, p. 644-652, 4 figs., 2 tables, May 1988.
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  Figure 1. Simplifiedgeologic map of the Yerington district, west-central Nevada, also showing localities that were sampled for U-Pb geochronology. The geology is modified from Proffett and Dilles (1984, and references therein), Bingler (1978), J. H. Dilles and J. M. Proffett (unpub. data), Hudson and Oriel (1979), Castor (1972), and Moore (1969).
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  646 DILLES ANDWRIGHT TABLE 1. URANIUM-LEAD ISOTOPIC DATA Samplet Wt. (mg) U 206p,,. Measured ratios^ Atomic ratios Apparent ages** (Ma) (ppm) (ppm) Mipb 2 °?Pb 208pb 206 Pb* 2°?Pb' 2 <"pb* 207pb. 207pb. 204pb 206pb 2 0 6 ^ w 235u 206pb. w 235u 206P b . Y-l>200++ 19.3 693.5 21.68 3922 0.05457 0.16344 0.03639 0.25509 0.05083 230.4 230.7 233 ± 5 Y-l>200 16.2 694.8 21.58 3220 0.05534 0.16884 0.03616 0.25322 0.05079 229.0 229.2 231 ± 5 Y-l <200 18.3 852.7 26.12 3208 0.05538 0.17572 0.03567 0.24985 0.05081 225.9 226.4 232 ± 5 W-35 >200 19.6 1812 56.66 5362 0.05357 0.20233 0.03638 0.25503 0.05084 230.4 230.6 233 ± 4 W-35 <200 16.2 1940 54.52 2397 0.05693 0.21492 0.03272 0.22914 0.05082 207.5 209.6 233 ± 6 W-35 <200 mg 22.9 2469 51.28 6468 0.05311 0.21496 0.02417 0.16949 0.05085 154.0 159.0 234 ± 4 Y-767 >100+t 22.4 284.5 6.50 1781 0.05742 0.14750 0.02658 0.18141 0.04950 169.1 169.3 172 ± 6 Y-767 >100 22.4 319.0 7.31 1808 0.05767 0.13877 0.02666 0.18216 0.04955 169.6 169.9 174 ± 5 Y-767 >100 14.7 321.0 7.33 1474 0.05947 0.14304 0.02658 0.18147 0.04951 169.1 169.3 172 ± 11 Y-767>100 17.3 317.1 7.27 1711 0.05812 0.13924 0.02668 0.18221 0.04954 169.7 169.9 173 ± 6 Y-767>100 19.9 317.8 7.28 1869 0.05733 0.13650 0.02665 0.18182 0.04948 169.5 169.6 171 ± 5 Y-767>100 17.1 316.8 7.24 1739 0.05794 0.13992 0.02662 0.18169 0.04950 169.4 169.5 172 ± 9 Y-767 >200 26.3 327.5 7.50 1536 0.05898 0.14321 0.02663 0.18155 0.04944 169.4 169.4 169 ± 7 Y-767 <325 23.3 352.7 8.07 2381 0.05562 0.13213 0.02664 0.18163 0.04945 169.5 169.5 169 ± 9 Y-781 >100tt 21.0 343.6 7.82 3030 0.05438 0.17804 0.02648 0.18089 0.04954 168.5 168.8 173 ± 11 Y-781<200 25.1 486.3 11.06 4924 0.05251 0.18600 0.02648 0.18085 0.04953 168.5 168.8 173 ± 6 BK-45 >100++ 18.2 504.4 11.33 2857 0.05473 0.15959 0.02616 0.17884 0.04959 166.4 167.0 175 ± 12 BK-45 <200 16.5 599.0 13.47 4308 0.05284 0.17238 0.02617 0.17840 0.04944 166.6 166.7 168 ± 8 BK-63 <200 23.7 547.6 12.20 6667 0.05166 0.15348 0.02593 0.17683 0.04946 165.0 165.3 170 ± 6 Y-818>100++ 27.2 530.2 11.88 1550 0.05890 0.15088 0.02607 0.17775 0.04943 166.0 166.1 168+ 8 Y-818 >100 19.9 484.8 10.84 1607 0.05859 0.14907 0.02603 0.17752 0.04946 165.7 165.9 170 ± 9 Y-818 <200 20.3 1005 22.41 1525 0.05902 0.14948 0.02594 0.17672 0.04940 165.1 165.2 167 ± 8 BK-38 <200 23.0 518.3 10.75 6197 0.05187 0.16783 0.02415 0.16488 0.04950 153.9 155.0 172 ± 5 •Denotes radiogenic Pb, corrected for common Pb using the isotopic composition of ^ P b / ^ P b = 18.6 and 2 ^Pb/2 f i 4 Pb = 15.6. Sample dissolution and ion exchange chemistry modified fromKrogh(1973). Total Pb processing blanks ranged from 150-500 picograms. +> 100, <200 and so on, refer to size fractions in mesh. All size fractions handpicked to greater than 99.9% purity. All analyzed zircon fractions were nonmagnetic at 1.7 amp with a 20° forward and a 1° side tilt on the Frantz magnetic separator, except for W-35 <200 mg, which was magnetic with a side tilt of 3°. §See Wright and Fahan (1988) for details of mass spectrometer procedures and mass fractionation corrections. Uncertainties in the 2 0 8 Pb/2 0 6 Pb and 2 0 7 Pb/2 0 6 Pb measured ratios are on the order of 0.03% to 0.10%. 2 0 6 Pb/2 0 4 Pb errors range from about 0.7% to 5%, depending upon the ratio (all uncertainties are at the 2 a level). "Ages calculated using the following constants: decay constants for 2 3 5 U and 2 3 8 U = 9.8485E-10yr_1 and 1.55125E-10yr-', respectively; ^ U / 2 3 ^ = 137.88. Precisions of2 0 6 Pb*/2 3 8 U ratios are on the order of 0.2% to 0.3% based upon the replicate analyses of Y-767 > 100 and on another "standard" zircon fraction. The 2 a uncertainties in the 2 0 7 Pb*/2 0 6 Pb* ages were calculated on the combined uncertainties in mass spectrometry (chiefly the uncertainty in the 2 0 6 Pb/2 0 4 Pb measured ratio) and an assumed uncertainty of ±0.2 in the 2 ^ P b / 2 ^ P b ratio used for the common-Pb correction. ttZircon fraction abrasion similar to the method described by Krogh (1982). we use the time scale of Harland and others (1982). Plutonic rocks are named using the International Union of Geological Sciences system (Streckeisen, 1976), which differs from the previous use of the classifica- tion system of Williams and others (1955) in Einaudi (1977,1982), Prof- fett (1979), and Proffett and Dilles (1984). U-Pb ANALYTICAL RESULTS The oldest dated Jurassic igneous unit is the Yerington batholith (Figs. 1 and 2). We have analyzed eight zircon fractions from the early quartz monzodiorite phase of the batholith, including five replicate anal- yses of Y-767 >100, and two size fractions of the youngest phase (granite porphyry dikes) of the batholith (Y-767 and Y-781, Table 1). All analyzed zircon fractions from Y-767 are low in uranium and thus are relatively unsusceptible to recent loss of radiogenic lead; the 206 Pb*/238 U dates are analytically indistinguishable from one another, based on the replicate analyses of Y-767 >100, and the average 206 Pb*/238 U date is 169.4 Ma with a 2a standard deviation (precision) of ±0.4 Ma (Table 1). Because there is no evidence for any loss of radiogenic lead from the coarse to fine fractions, we interpret these analyses as concordant at 169.4 ± 0.4 Ma. For a detailed discussion of the type of error analysis used herein, see Mattin- son (1987). The two size fractions analyzed from the youngest phase of the Yerington batholith (Y-781, Table 1) yielded identical 206 Pb*/238 U dates of 168.5 ± 0.4 Ma (all analytical precisions for 206pb*/238u ^ ^ a r e based upon the reproducibility of the 206 Pb*/238 U ratio from Y-767 and another "standard" zircon fraction; Wright and Fahan, 1988). The granite porphyry dike is thus constrained to be younger than the 169.4 Ma date of the early quartz monzodiorite phase and has a minimum date of 168.5 ± 0.4 Ma. The Shamrock batholith is constrained from field relations to post- date emplacement of the Yerington batholith (Figs. 1 and 2; Y-818, Table 1). The isotopic analyses of the two coarse zircon fractions indicate a minimum emplacement date of 165.8 ± 0.4 Ma. The small size fraction, dated at 165.1 Ma, contains significantly more uranium than do the coarse fractions that may have lost a small amount of radiogenic lead. The Ful- stone volcanics (Figs. 1 and 2; BK-45, Table 1) are constrained by field relations to be younger than the 169.4 ± 0.4 Ma date of the early phase of the Yerington batholith and older than the 165.8 ± 0.4 Ma date of the Shamrock batholith. There is no evidence from the BK-45 isotopic data for any loss of radiogenic lead from the two size fractions analyzed; the average 206 Pb*/238 U date is 166.5 ± 0.4 Ma, which overlaps within analytical error with the date of the Shamrock batholith. The 165.0 ± 0.4 TABLE 2. SAMPLE DESCRIPTIONS AND LOCATIONS Y-l Quartz porphyry dike cutting McConnell Canyon volcanics; lat 38°56'32" N„ long 119°12'14" W. W-35 Wassuk biotite-hornblende metadiorite intruding silicic ignimbrites; Ut 38°59'01" N„ long 118°54W W. Y-767 Hornblende quartz monzodiorite of Yerington batholith; lat 38°57'34" N, long 119°13'59* W. Y-781 Hornblende-biotite granite porphyry dike of the Yerington batholith; lat 38°58'23* N, long U9°13'50" W. BK-45 Hornblende-biotite quartz latite flow dome with sparse quartz phenocrysts and K-feldspar megacrysts from volcanics of Fulstone Spring; lat S g - i M ' ^ N., long 119°22'22" W. BK-63 Biotite-hornblende quartz monzodiorite porphyry dike with biotite-rich groundmass and sparse quartz phenocrysts and K-feldspar megacrysts; lat 39°04'30" N„ long 119°24'00" W. Y-818 Biotite-hornblende granite of the Shamrock batholith; lat 38°56'05" N„ long 119°15'48" W. BK-38 Hornblende-biotite porphyritic granite of Sunrise Pass, Pine Nut Range; Ut 39°05"04" N, long 119°25'16' W.
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  EARLY MESOZOIC ARCMAGMATISM, WESTERN NEVADA 647 COMPOSITE MESOZOIC COLUMNAR SECTION T 7 K M - MIDDLE </> CO < OL Z) F U L S T O N E VOLCANICS 166.5 Ma . LOWER UPPER o en < UPPER CE OR H MIDDLE ? NORIAN U. CARMAN f M c C O N N E L L CANYON V O L C A N I C S SHAMROCK B A T H O L I T H 165.8 Ma Q U A R T Z MONZODIORITE s P O R P H Y R Y DIKES 165 Ma GRANITE "PORPHYRY DIKES 168.5 Ma - Q U A R T Z MONZONITE • P O R P H Y R I T I C GRANITE QUARTZ •MONZODIORITE 169.4 Ma Q U A R T Z PORPHYRY 2 3 2 Ma > < m o 2 ce LÜ >- - - 6 - - 5 - - 4 N O R T H E R N W A S S U K V O L C A N I C S WASSUK METADIORITE 2 3 3 Ma - " 3 - - 2 -LO SEDIMENTARY AND VOLCANIC UTHOLOGIC SYMBOLS CONGLOMERATE CALCAREOUS ARGILLITE DOLOMITE SILICIC IGNIMBRITE QUARTZ LATITE PORPHYRY o LATITE PORPHYRY V-C" TUFF AND TUFFACEOUS SANDSTONE VOLCANIC BRECCIA RHYOLITE ANDESITE Figure 2. Generalized composite column of Mesozoic volcanic, plutonic, and sedimentary rocks from the Yerington district. The column is compiled from Einaudi (1982), Dilles (1984 and unpub. data), and Proffett and Dilles (1984). Ma date determined on one size fraction from a quartz monzodiorite porphyry dike (Figs. 1 and 2; BK-63, Table 1) can be only considered as a minimum age, but the dike cannot be significantly older than 165 Ma because it intrudes the 166.5 ± 0.4-m.y.-old Fulstone volcanics. The Sunrise Pass pluton (Figs. 1 and 2; BK-38, Table 1) has a minimum age of 154 Ma based on the 206 Pb*/238 U date on a single size fraction, but we interpret that this date reflects recent loss of radiogenic lead and that the true age is -172 ± 5 Ma based on the 207 Pb*/206 Pb* date. The isotopic data for the Wassuk metadiorite and the quartz por- phyry dike from McConnell Canyon (Figs. 1 and 2; W-35, Y-l, Table 1) clearly indicate loss of variable amounts of radiogenic lead from the small zircon size fractions. On the concordia diagram (Fig. 3A), the three size fractions of the Wassuk metadiorite define a highly linear data array with an emplacement date of 232.7 ± 2.9 Ma and a lead loss trajectory indicat- ing recent loss of radiogenic lead. The data for the quartz porphyry dike from McConnell Canyon (Fig. 3B) do not have enough age spread to allow a precise calculation of the age and errors from the analytical data alone. A force fit of a chord through the three data points and an assumed lower concordia intercept of 0 ± 15 Ma, however, yields an upper inter- cept date of 232.2 ± 2.3 Ma (Fig. 3B). The force fit through the lower intercept seems justifiable because the three size fractions have indistin- guishable 207 Pb*/206 Pb* ratios, and the data for the Wassuk metadiorite indicate a recent loss of radiogenic lead. Regardless of these uncertainties, the age data clearly indicate a minimum date of ca. 230 Ma.
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  648 DILLES ANDWRIGHT MIDDLE OR EARLIEST LATE TRIASSIC MAGMATISM The oldest exposed rocks within the Yerington district are Middle or earliest Late Triassic-age volcanic and plutonic rocks exposed in the Sin- gatse and Wassuk Ranges (Fig. 1). They include a 1,400-m andesite- rhyolite sequence in the Singatse Range named the "McConnell Canyon volcanics" (Proffett and Dilles, 1984) and'probably correlative andesites and rhyolites underlain by silicic ignimbrites in the northern Wassuk Range (Figs. 1,2; Dilles and others, 1983; Bingler, 1978). The following summary of the geology of the McConnell Canyon volcanics is taken largely from unpublished data of J. M. Proffett and M. T. Einaudi. The McConnell Canyon volcanics are predominantly andesitic flows and brec- cias that are in turn overlain by rhyolitic domes, flows, and breccias with minor conglomerate and volcanic sandstone (Fig. 2). They are earliest Late Triassic or older, as late Carnian ammonoid fossils (Silberling, 1984) have been recovered from the limestone unit that disconformably overlies the volcanic section (Fig. 2). Quartz porphyry dikes and small plugs intrude the McConnell Canyon section but are not known to intrude above the level of the disconformity between the Carnian limestone and the volcanic rocks. These quartz porphyry dikes are compositionally sim- ilar to rhyolitic rocks of the McConnell Canyon volcanics, and therefore, we interpret that they could be comagmatic. The 232.2 ± 2.3 Ma zircon date (Fig. 3B) from a quartz porphyry dike thus places a minimum age on the McConnell Canyon volcanics and may closely approximate the age of the rhyolitic part of the section. In the northern Wassuk Range, a possibly correlative volcanic section consists of andesite and rhyolite that overlie silicic ignimbrite. The Wassuk metadiorite, which has a U-Pb zircon concordia date of 232.7 ± 2.9 Ma (Fig. 3A), intrudes the silicic ignimbrite and an andesite that may be in the base of the andesite-rhyolite section (Figs. 1 and 2). The metadiorite may be comagmatic with and therefore date the andesite that it intrudes. None- theless, our data place a minimum age of 232.7 ± 2.9 Ma on the silicic ignimbrite in the Wassuk Range. 0 0 3 6 1 1 i i • 1 ' I 0 . 0 3 4 w-- 3 5 o s? - ^ 0 . 0 3 2 00 IO 0 . 0 3 0 • O Q_ g 0 . 0 2 8 CM - S i / / O / / y * / / / - 0 0 2 6 - / ./' y I N T E R C E P T S AT - 0 . 0 2 4 2 3 2 . 7 ± 2 . 9 a n d ~ 3 = M 0 Ma ( M S W D = . 9 7 8 ) - 0 0 2 2 i 1 I . I . I / 2°7pb / 235 u 2 0 7 p b / 2 3 5 y LATE TRIASSIC THROUGH EARLY JURASSIC SEDIMENTATION AND MINOR VOLCANISM Following the Middle or earliest Late Triassic period of voluminous magmatism, volcanic activity waned, and a thick section of carbonate, fine-grained epiclastic rocks, and minor volcanic rocks that range in age from late Carnian through the Early Jurassic were deposited over the district (Figs. 1, 2). In the McConnell Canyon and Ludwig areas (Figs. 1, 2), this section contains as much as 1,800 m of interbedded siltstone, black argillite, limestone, tuffaceous sandstone, tuff, tuff breccia, gypsum, and quartzose sandstone, which can be broadly correlated with the Oreana Peak, Gardnerville, and Preachers Formations of Noble (1962) in the Pine Nut Range to the west (Proffett and Dilles, 1984). The Preachers Forma- tion is late Toarcian (latest Early Jurassic) or younger on the basis of fossil ages from the underlying Gardnerville Formation in the Pine Nut Range (Noble, 1962; Silberling, 1984). MIDDLE JURASSIC MAGMATISM Following accumulation of the Triassic-Jurassic sedimentary section, a major period of intermediate to silicic, high-K calc-alkaline magmatism occurred throughout the Yerington district, as documented by Noble (1962), Castor (1972), Bingler (1978), and Proffett and Dilles (1984) (Figs. 1, 2). Five U-Pb zircon dates on plutonic and volcanic rocks estab- lish that the magmatism was restricted to a short interval of approximately 169-165 Ma in the Middle Jurassic (Table 1). The volcanic rocks of Artesia Lake, which overlie quartz sandstone of the Preachers Formation, were the first Middle Jurassic magmas (Figs. 1, 2). They constitute as Figure 3. Concordia diagram of zircon size fractions from the Wassuk metadiorite (W-35, Fig. 3A) and the McConnell Canyon quartz porphyry (Y-l, Fig. 3B). Intercepts and 2a errors were calcu- lated with the program of Ludwig (1984). Stars are shown instead of error ellipses in 3A because ellipses are too small to be visible at the scale of the concordia. much as 2 km of andesite, dacite, and basalt flows; minor silicic pyroclastic rocks; volcaniclastic sedimentary rocks; and small shallow-level intrusions (Hudson and Oriel, 1979; Proffett and Dilles, 1984; Dilles, 1984). They are primarily exposed within the Buckskin Range but also occur as pen- dants within the Yerington batholith in the Singatse Range (Proffett and Dilles, 1984; Fig. 1). The thickest sections of the Artesia Lake volcanics are found both above and intruded by the Yerington batholith and have undergone widespread intense hydrothermal alteration to sericitic and advanced argillic assemblages (Hudson, 1983; Proffett and Dilles, 1984). The close spatial association of the volcanics with the batholith, and the similar ranges in chemical composition, Sr isotopic composition (basalt of Artesia Lake, 0.7039; Yerington batholith, average of 9 analyses, 0.7040; Dilles, 1987), and hydrothermal alteration styles of the volcanics and batholith, suggests that extrusive and intrusive rocks are a single magma suite (Proffett and Dilles, 1984). The U-Pb dates and field data require that the Artesia Lake volcanics entirely predate the overlying 166.5 Ma Ful- stone volcanics, probably entirely predate 168.5 Ma granite porphyry dikes that crosscut them, and partly predate 169.4 Ma quartz monzodiorite of the Yerington batholith, which intrudes their basal exposures (Fig. 2,
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  EARLY MESOZOIC ARCMAGMATISM, WESTERN NEVADA 649 Table 1). Because the Artesia Lake volcanics are thought to be comag- matic with the Yerington batholith, we interpret that their basal exposures do not greatly predate 169 Ma. The Yerington batholith, with an outcrop area of-250 km2 after the effects of Cenozoic Basin and Range extension are removed (Proffett and Dilles, 1984), ranges from fine-grained biotite-hornblende quartz monzo- diorite to medium-grained hornblende-biotite granite. The batholith con- sists of three successively emplaced units (Fig. 2), each volumetrically smaller and more deeply and centrally emplaced within the batholith: (1) quartz monzodiorite, (2) "Bear" quartz monzonite, and (3) porphyritic granite (Proffett and Dilles, 1984). The porphyritic granite forms a stock with cupola-like apices from which comagmatic granite porphyry dike swarms emanate. Although the dikes crosscut the stock apex, they grade texturally downward into porphyritic granite and are interpreted to have tapped deeper levels of the magma chamber (Dilles, 1984). At both the Yerington Mine and the Ann-Mason area (Fig. 1), porphyry copper min- eralization occurred synchronously with emplacement of granite porphyry dike swarms (Proffett, 1979; Dilles, 1984; Carten, 1986). Copper skarn formation is also broadly contemporaneous with intrusion of granite por- phyries (Knopf, 1918; Einaudi, 1982; Harris and Einaudi, 1982). Our zircon data on the batholith (Table 1) indicate that the early quartz mon- zodiorite phase has a concordant date of 169.4 Ma and that a late granite porphyry dike has a concordant date of 168.5 Ma. The field and petrologic data and zircon dates thus collectively support the conclusion that all three units of the batholith form a comagmatic suite emplaced within approxi- mately 1 m.y. The 168.5 Ma date of granite porphyry establishes that porphyry and skarn copper mineralization is Middle Jurassic. The volcanic rocks of Fulstone Spring (166.5 Ma; Table 1) form the next major Middle Jurassic magmatic unit and crop out in the Buckskin and northern Pine Nut Ranges, where they disconformably overlie the volcanics of Artesia Lake (Figs. 1, 2). The name "volcanics of Fulstone Spring" has been informally applied by Proffett and Dilles (1984) to rocks also called the "Cretaceous hornblende dacite porphyries" and the "Churchill Canyon sequence" by Hudson and Oriel (1979) and the "Cretaceous porphyry series" by Castor (1972). They may also correlate with lithologically similar conglomerate and volcanic breccia of the Veta Grande Formation and dacite flows and tuffs of the Gold Bug Formation of Noble (1962) in the southern Pine Nut Range near the southern contact of the Mount Siegel batholith. The volcanics of Fulstone Spring are a locally thick section (as much as 2,400 m) of latite and quartz latite porphyry domes, breccias, and flows with lesser amounts of latite ignim- brites, andesites, and conglomerates (Fig. 2). Volcanic porphyries contain the phenocryst assemblage plagioclase + hornblende + biotite ± quartz ± K-feldspar megacrysts. They locally overlie a hematite-rich paleosol(?) and are incipiently altered to propylitic assemblages (hematite, chlorite, and calcite), which suggests a subaerial origin. J. M. Proffett (1982, personal commun.) noted that granite porphyry dikes of the Yerington batholith are petrographically similar to some Fulstone porphyries and that the two could be cogenetic. Field relations, however, give ambiguous age relations (Dilles, 1984). Compositional data (Castor, 1972; Dilles, 1984) indicate that Si02 values of the Fulstone volcanics overlap with those of the Shamrock batholith but are approximately 5 wt% lower than those of the granite porphyries of the Yerington batholith. The 166.5 Ma date of a quartz latite porphyry of the Fulstone volcanics is younger than and ana- lytically distinct from the 168.5 Ma date of granite porphyry of the Yering- ton batholith, but it is analytically indistinguishable from the 165.8 Ma date of the Shamrock batholith. The zircon data, therefore, support our preliminary interpretation that the Fulstone volcanics may be cogenetic with the Shamrock batholith. Two intrusive units, quartz monzodiorite porphyry dikes and the Shamrock batholith, were emplaced at 165-166 Ma (Table 1) and proba- bly are the youngest Middle Jurassic magmatic rocks in the district. Biotite-hornblende quartz monzodiorite porphyry dikes intrude the Ful- stone volcanics and all older rock units (Figs. 1, 2). They contain sparse K-feldspar and quartz phenocrysts set in a fine-grained, biotite-rich groundmass. In the Singatse Range, they have been inferred to be cut by and therefore predate the Shamrock batholith (Proffett and Dilles, 1984), although no contact relations are exposed. The dikes commonly occupy major (> 1 km displacement), east-west, graben-forming faults that bound and downdrop a structural block containing the Yerington batholith on its north and south sides (Fig. 1) in the Yerington district (Proffett and Dilles, 1984) and in the northern Wassuk Range (Bingler, 1973,1978). Prior to Tertiary tilting, the faults trended east-west and were nearly vertical. A minimum date of 165 Ma was obtained from a single zircon fraction from a quartz monzodiorite porphyry dike intruding the Fulstone volcan- ics and establishes that faults intruded by the dikes must have been active prior to 165 Ma. The Shamrock batholith (165.8 Ma) forms a large pluton (>250 km2 area) of medium-grained, hornblende-biotite granite lying to the south of the Yerington batholith in the Singatse Range (Fig. 1). Proffett and Dilles (1984) have tentatively correlated the Shamrock batholith with a petrographically similar granite pluton in the northern Pine Nut Range, called the "Mount Siegel batholith" by Stewart and Noble (1979, Fig. 1). In the Pine Nut Range, the Mount Siegel (Shamrock?) batholith intrudes quartz latite breccias of the Fulstone volcanics at its northern contact. A concordant zircon date of 165.8 Ma (Table 1) for the Shamrock batholith is analytically indistinguishable from the 165 Ma minimum date of a quartz monzodiorite porphyry dike and suggests the possibility that the two units are genetically related. As suggested above, the Fulstone volcan- ics could be early extrusive magmas that were subsequently intruded by comagmatic intrusive rocks consisting of the granodiorite porphyry dikes and the Shamrock batholith, but resolution of this question awaits more detailed study. Another Jurassic pluton occurs in the Sunrise Pass area of the north- ern Pine Nut Range (Fig. 1) (Castor, 1972). It consists principally of hornblende-biotite porphyritic granite. Its age relative to the Shamrock batholith, quartz monzodiorite porphyry dikes, and Fulstone volcanics is uncertain owing to ambiguous field relations and the discordant U-Pb zircon date (Table 1). As previously discussed, we tentatively interpret the age of this pluton to be ~ 172 ± 5 Ma. MIDDLE JURASSIC DEFORMATION AND METAMORPHISM In the Yerington district, folding and contact metamorphism oc- curred near Middle Jurassic batholiths, and elsewhere, low-grade regional metamorphism affected pre-Middle Jurassic rocks. In the Wassuk Range, the 232.7 ± 2.9-m.y.-old Wassuk metadiorite and associated volcanic rocks are in part regionally metamorphosed to biotite-bearing schistose rocks and are crosscut by the early quartz monzodiorite phase (169.4 Ma) of the Yerington batholith, which has no metamorphic fabric. This defor- mation is therefore constrained to be post-233 Ma and pre-169.4 Ma. In the Singatse Range, the McConnell Canyon volcanics and the overlying Upper Triassic through Lower Jurassic sedimentary and volcanic strata are exposed in the McConnell Canyon and Ludwig pendants as locally schist- ose metamorphic rocks within an overturned anticline between the Yerington and Shamrock batholiths (Fig. 1). The fold's axial plane strikes N80°W, and its axis plunges vertically. The fold, however, is upright and trends west-northwest after the effects of Tertiary and late Mesozoic west tilting are removed (Geissman and others, 1982; Proffett and Dilles, 1984). Contact metamorphic and metasomatic minerals in the pendants are zoned relative to the quartz monzodiorite of the Yerington batholith (Einaudi, 1977, 1982; Harris and Einaudi, 1982). Normal faults, which cut the fold and which predate copper skarn formation, are occupied by granite por- phyry dikes of the Yerington batholith (Knopf, 1918; Harris and Einaudi,
• 7.
  650 DILLES ANDWRIGHT 1982). Deformation and metamorphism adjacent to and attributable to the Shamrock batholith are at most weakly developed (Dilles and others, 1983; M. T. Einaudi, 1986, personal commun.). Collectively, these rela- tions strongly suggest that most deformation and metamorphism accom- panied emplacement of the Yerington batholith (169.4-168.5 Ma) in the Singatse Range. The strict age relations, however, require that all deforma- tion must be Middle Jurassic because it postdates the uppermost Lower to lowermost Middle Jurassic Preachers Formation and predates the 165.8 Ma Shamrock batholith. REGIONAL IMPLICATIONS AND CONCLUSIONS The Permian-Triassic Sonoma orogeny (Silberling and Roberts, 1962) ended with the emplacement of the Golconda allochthon in the Early Triassic as the result of the closure of a back-arc basin (Burchfiel and Davis, 1972; Miller and others, 1984) or the collision of the exotic arc terrane, Sonomia, with the continental margin (Speed, 1979). Postdating the Sonoma orogeny, an early Mesozoic, west-facing Andean-type arc was constructed along the western margin of North America (see, for example, Burchfiel and Davis, 1972). These arc rocks are exposed from the Sonora province, Mexico, northward through Arizona and the Mojave Desert to the eastern Sierra Nevada at lat 38°-39° N. (Fig. 4). This portion of the arc was built on or adjacent to cratonal North America as shown by Precam- brian crystalline basement or by sedimentary ties to the craton, such as interbedded Aztec/Navajo/Nugget sandstone in the Lower Jurassic part of the section. Within time-equivalent metavolcanic and plutonic rocks of the western Sierra Nevada foothills, the remainder of the Sierra north of approximately lat 39° N., northwestern Nevada, and the Klamath Moun- tains, such links to continental North America are generally absent or speculative. Nearly coincident with this transition at lat 39° N. are the change in 87 Sr/86 Sr initial ratios from >0.706 to <0.706 (Kistler and Peterman, 1978; Fig. 4), an increase in the proportion of mafic to felsic igneous rocks, and a transition from predominantly subaerial to predomi- nantly submarine conditions (for example, Schweickert, 1978,1981). The early Mesozoic, Andean-type arc, to a first approximation, was thus con- structed upon Precambrian craton or Paleozoic-early Mesozoic continen- tal shelf sediments to the south of lat 39° N. and on young, thin, immature crust to the north. The latter appears to have been morphologically part of the North American continent in the early Mesozoic (Speed, 1979) and by inference was composed of a combination of Paleozoic oceanic island- arc rocks, oceanic crust, and minor oceanic and terrigenous sediments (Stewart, 1972; Burchfiel and Davis, 1972; Speed, 1979; Miller and oth- ers, 1984). The Yerington district is north and west of the inferred edge of Precambrian craton as indicated by the 0.706 initial Sr isopleth (Fig. 4). Lower Mesozoic rocks of the district are inferred to be underlain princi- pally by older volcanic-arc rocks (Speed, 1978) that constitute the Sono- mia terrane of Speed (1979). These lower Mesozoic rocks were probably contiguous with strata deposited on or adjacent to North American craton, as shown by the following review. Speed (1978) subdivided what he called the "early Mesozoic marine province" of the western Great Basin into three principal paleogeographic subprovinces: (1) a volcanic arc terrane, (2) a basinal terrane, and (3) a shelf terrane (Fig. 4). The shelf terrane was deposited near the inferred edge of Precambrian craton, whereas the volcanic arc terrane, which in- cludes the Yerington district, was located an unknown distance farther west (Fig. 3). Speed (1978) and Proffett and Dilles (1984), however, have broadly correlated the Upper Triassic to upper Lower or lower Middle Jurassic sedimentary and volcanic section of the Yerington district (Fig. 2) to the Oreana Peak, Gardnerville, and Preachers Formations of Noble (1962) in the southern Pine Nut Range (Fig. 1). Noble (1962), Stanley (1971), Stewart (1980), and Silberling (1984) have tentatively correlated Figure 4. Distribution of well-dated Upper Triassic to Upper Jurassic (215-144 Ma) plutons of the western United States, shown in black, based on a compilation by C. Karish (unpub. data). The 0.704 and 0.706 Sr isopleths for Mesozoic igneous rocks are generalized from Kistler and Peterman (1978) and Kistler (1983). The basinal, volcanic arc, and shelf terranes of the early Mesozoic marine province are generalized from Speed (1978). The Luning-Fencemaker thrust system is taken from Oldow (1983,1984). The northern Sierra (NS) and Jackson Mountains (JM) areas discussed in text are shown. the Oreana Peak and Gardnerville Formations (and parts of the Yerington section) on the basis of fossil ages and lithology with the Luning, Gabbs, and Sunrise Formations of Muller and Ferguson (1936, 1939), which extend eastward into the shelf terrane of Speed (1978). Despite subsequent tectonic shuffling by Mesozoic thrust and strike-slip faults (Oldow, 1978) and Cenozoic strike-slip and normal faults, the available evidence suggests that the volcanic arc terrane was in complex facies relation with the shelf terrane during the Late Triassic and early Early Jurassic (for example, Speed, 1978; Silberling, 1984; Oldow, 1984). This part of the shelf terrane is allochthonous (Paradise subterrane of Silberling and Jones, 1984) and is bounded by the Mesozoic Luning-Fencemaker thrust system (Oldow, 1984; Fig. 4) and therefore cannot be directly correlated to rocks on the autochthonous continental shelf to the east (for example, Silberling and Jones, 1984). The lithology and age of these rocks, however, are similar to shelf-terrane strata that are autochthonous to North America 100-200 km north (Fig. 4), and we concur with Speed (1978), who suggested that thrust-bounded rocks of the southern shelf terrane also accumulated in proximity to the relict Paleozoic continental margin. Beginning in the late Early to early Middle Jurassic, a change in paleogeography to predomi- nantly subareal conditions occurred, and quartz-rich sandstone (belonging
• 8.
  EARLY MESOZOIC ARCMAGMATISM, WESTERN NEVADA 651 to the Boyer Ranch, Dunlap, and Preachers Formations and equivalents) was deposited across the shelf, volcanic arc, and basinal terranes (Speed and Jones, 1969; Speed, 1978). Furthermore, quartz-rich sandstones within the Dunlap Formation are thought to be derived from the craton because they are petrographically similar to the Navajo sandstone of the Colorado Plateau (Speed and Jones, 1969; Stanley, 1971). Collectively, these relations suggest that lower Mesozoic rocks at Yerington and else- where in the volcanic arc terrane can be stratigraphically linked to conti- nental North America, probably by the Late Triassic and certainly by early Middle Jurassic. The Triassic-Lower Jurassic section at Yerington records volcanism and sedimentation in the volcanic arc terrane of Speed (1978), which postdates the Permian-Triassic Sonoma orogeny. Although the McConnell Canyon volcanics contain no intercalated sedimentary rocks that can be correlated to autochthonous North American sequences, we suggest that they also accumulated within the volcanic arc terrane near continental North America. They were deposited in apparent stratigraphic continuity with the overlying sediments, which have stratigraphic ties to the shelf and craton, as previously discussed. Stratigraphic continuity is also supported by the lack of evidence for deformation or significant erosion of the volcanics before deposition of the overlying limestone (J. M. Proffett, 1985, personal commun.) and by the short time between volcanism (at >230 Ma) and limestone deposition (late Carman, ca. 225 Ma; Harland and others, 1982) (Fig. 2). We thus believe that the McConnell Canyon volcanics are part of the volcanic arc terrane and that the 232.2 ± 2.3 and 232.7 ± 2.9 Ma dates of possibly cogenetic intrusions within the volcanics are minimum ages for establishment of early Mesozoic arc magmatism in western Nevada after the Sonoma orogeny. Following the accumulation of the Upper Triassic-Lower Jurassic volcanic and sedimentary section, a major period of Middle Jurassic magmatism occurred in the Yerington district, which establishes the age of a younger period of North American arc magmatism in western Nevada. The field and isotopic data document eruption of as much as 4 km of volcanic rocks and the emplacement of two batholiths between about 169 and 165 Ma. Within this arc, deformation is constrained by field and isotopic age relations in the Singatse Range to postdate deposition of the quartz sandstone of the Preachers Formation (post-late Toarcian or latest Early Jurassic; Noble, 1962; Silberling, 1984) and to predate the em- placement of the Shamrock batholith (165.8 Ma). Contact metamorphism and metasomatism in the Ludwig and McConnell Canyon pendants (Fig. 1), however, is synchronous with the emplacement of the 169.4- to 168.5-m.y.-old Yerington batholith and could be synchronous with deformation. Deformation at Yerington may thus be related to batholith emplacement rather than to regional compression. Oldow (1983, 1984) assigned the Triassic and Jurassic rocks of the Yerington district to what he termed the "Pine Nut assemblage" and ascribed north-northwest-trending folds within the area, including the large fold exposed in the McConnell Canyon and Ludwig pendants (J. S. Oldow, 1983, personal commun.), to regional deformation of probable Nevadan age. Our data, however, re- quire that deformation of the McConnell Canyon and Ludwig pendants occurred prior to 165.8 Ma, in the Middle Jurassic, and thus distinctly predates the Nevadan orogeny, which is currently dated at ~ 150-157 Ma (Harper and Wright, 1984; Schweickert and others, 1984a; Sharp, 1985). From a regional perspective, the Middle Jurassic age of deformation within the arc at Yerington overlaps with the age of initiation of regional compressive deformation that resulted in the formation of the Luning- Fencemaker fold and thrust belt of Oldow (1983). As discussed in detail by Speed (1978) and Oldow (1983, 1984), the basinal, arc volcanic, and shelf terranes east of the Yerington district were complexly folded and thrust faulted, which were interpreted to be the result of closure of a back-arc basin that lay between the continental shelf and the axis of Mesozoic arc magmatism near the present California-Nevada border (Fig. 3). Timing constraints on the evolution of the fold and thrust belt suggest that it began in the Middle Jurassic and may have continued into the Early Cretaceous (Speed, 1974, 1978; Speed and Kistler, 1980; Oldow, 1983,1984). Speed and Jones (1969), Stanley (1971), and Speed (1978) have suggested that the inception of deformation is recorded by the encroachment of the post-latest Early Jurassic-age quartz-rich sandstones of the Boyer Ranch, Dunlap, and Preachers Formations and equivalents. Speed (1978), however, suggested that much of the deformation is Middle Jurassic, as supported by the approximately 165 Ma date of the Humboldt lopolith (Willden and Speed, 1974), which both is cut by strands of the Fencemaker thrust system and contact metamorphoses the upper and lower plates of other strands of the thrust system (Speed, 1974,1976). The record of early Mesozoic magmatism and deformation in west- ern Nevada is very similar to that of coeval magmatic-arc rocks that lie north of lat 39° N. In the western Klamath Mountains province (Fig. 4), recent field and geochronologic investigations (Wright and Sharp, 1982; Fahan and Wright, 1983; Harper and Wright, 1984; Wright and Fahan, 1988) have documented a major period of Middle Jurassic arc magmatism between about 177 and 159 Ma. Deformation in the Middle Jurassic arc involved regional west-vergent thrust faulting and metamorphism that occurred in the interval of -170-164 Ma (Wright and Fahan, 1988). The timing of Middle Jurassic arc magmatism and deformation in the Yering- ton area of Nevada and in the western Klamath Mountains are thus strikingly similar. In other areas north of lat 39° N. such as northwestern Nevada and the northern Sierra Nevada, the ages of magmatism and deformation are not as well constrained but could be similar. In the northern Sierra Nevada, Mesozoic arc volcanic rocks and volcanogenic sediments range from middle(?) Early Jurassic to latest Middle Jurassic (Callovian) and were thrust faulted and deformed, possibly both synchronously with Mid- dle Jurassic plutonism and during the Late Jurassic Nevadan orogeny (McMath, 1966; Schweickert, 1978; Harwood, 1983; Schweickert and others, 1984b). In the Jackson Mountains of northwestern Nevada, subal- kaline arc volcanic and intrusive rocks of Late Triassic (?) to Early Jurassic (?) age were thrust faulted and deformed in the Middle Jurassic (?) and overlain by syn- and post(?)-tectonic volcanogenic conglomerates as young as Early Cretaceous (Russell, 1984; Maher and Saleeby, 1986). The similarities in the magmatic and deformational histories of the western Klamath Mountains and the Yerington area do not appear to be a coincidence, but rather, they suggest to us the need for an integrated tectonic model for the early Mesozoic. Paleomagnetic studies in the Klam- ath Mountains are consistent with this contention because they indicate that Middle Jurassic plutons and Permian to Jurassic sedimentary and volcanic rocks have been rotated clockwise but have not been translated latitudinally with respect to North America (Mankinen and others, 1982; Schultz, 1983; Schultz and Levi, 1983). In addition, from regional struc- tural, stratigraphic, and magmatic relations for Middle and Late Jurassic tectonic elements of the Klamath Mountains province, Harper and Wright (1984) concluded that Jurassic arc magmatism evolved there above an east-dipping subduction zone (that is, west-facing arc). We thus propose a model in which the Middle Jurassic igneous arc rocks of the Yerington and Klamath Mountains regions are northerly continuations of the west- facing North American arc (Fig. 4). Whether the North American arc extended north to the Klamath Mountains as early as Late Triassic is more problematic. The Yerington area was probably part of North America by the Late Triassic (Carnian- Norian), as discussed above. In the western Klamath Mountains, the Mid- dle Jurassic arc was constructed, in part, upon Upper Triassic and Lower Jurassic (?) volcanic, sedimentary, and plutonic rocks of the Rattlesnake Creek terrane (Wright and Fahan, 1988; Wright and Wyld, 1985). This terrane contains a section of arc-related, basaltic pillow lavas and breccias, the upper part of which is interbedded with Upper Triassic (at least in part
• 9.
  652 DILLES ANDWRIGHT Norian) radiolarian cherts (Irwin and others, 1985) and is overlain by arc-related volcaniclastic rocks, all of which are intruded by plutons dated at -215 to —192 Ma (Wright and Wyld, 1985). This Late Triassic to Early Jurassic magmatism broadly spans the same time interval as that of volcanism and volcanogenic sedimentation in the Yerington area, which extends from the McConnell Canyon volcanics (ca. 232 Ma or Middle/ Late Triassic) and to the Gardnerville Formation (Toarcian or late Early Jurassic). The lower Mesozoic magmatic rocks in the Klamath Mountains thus may also be part of the North American arc, although they were formed on more primitive crust than were arc rocks farther south. This correlation is conjectural, however, and must be tested by accurate geo- chronology, petrologic studies, and tectonic reconstructions. ACKNOWLEDGMENTS Field work by Dilles was conducted while he was a doctoral student at Stanford University under the supervision of Marco T. Einaudi. The geochronology was carried out in the laboratories of J. M. Mattinson and G. R. Tilton at the University of California, Santa Barbara, and at the laboratories of J. Wooden and J. Stacey at the U.S. Geological Survey, Menlo Park, California, and we gratefully acknowledge their generous support. Financial support was provided by the Anaconda Company and by National Science Foundation Grants EAR 81-07735 (to J. E. Wright), EAR 83-13733 (to J. E. Wright and E. L. Miller), and EAR-85-07264 (to M. T. Einaudi). We thank Marco Einaudi, John Proffett, Norm Silberling, Joe Wooden, Elizabeth Miller, R. C. Speed, and P. van der Heyden for helpful discussions and for their reviews of this manuscript. REFERENCES CITED Bingler, E. C., 1973, Major east-west lineament in west-central Nevada: Geological Society of America Abstracts with Programs, v. 5, p. 83. 1978, Geologic map of the Schurz quadrangle: Nevada Bureau of Mines and Geology Map 60. Bingler, E. C., Silberman, M. L., and McKee, E. H., 1980, K-Ar ages of volcanic and plutonic rocks in the northern Wassuk Range, central-western Nevada: Isochron/West v. 27, p. 13-16. Burchfiel, B. C., and Davis, G. A., 1972, Structural framework and evolution of the southern part of the Cordilleran orogen, western United States: American Journal of Science, v. 272, p. 97-118. Carten, R. B., 1986, Sodium-calcium metasomatism; chemical, temporal, and spatial relationships at the Yerington, Nevada, porphyry copper deposit: Economic Geology, v. 81, p. 1495-1519. Castor, S. G., 1972, Geology of the central Pine Nut and northern Buckskin Ranges, Nevada: A study of Mesozoic intrusive activity [Ph.D. dissert.]: Reno, Nevada, University of Nevada, 270 p. Dilles, J. 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B., and Rodgers, D., 1984, Stratigraphy and structure of the Schoonover sequence, northeastern Nevada: Implications for Paleozoic plate-margin tectonics: Geological Society of America Bulletin, v. 95, p. 1063-1076. Moore, J. G., 1969, Geology and mineral deposits of Lyon, Douglas, and Ormsby Counties, Nevada: Nevada Bureau of Mines and Geology Bulletin, v. 75, p. 45. Muller, S. W., and Ferguson, H. G., 1936,Triassic and Lower Jurassic formations of west-central Nevada: Geological Society of America Bulletin, v. 47, p. 241-252. 1939, Mesozoic stratigraphy of the Hawthorne and Tonopah quadrangles, Nevada: Geological Society of America Bulletin, v. 50, p. 1573-1624. Noble, D. C., 1962, Mesozoic geology of the southern Pine Nut Range, Douglas County, Nevada [Ph.D. dissert]: Stanford, California, Stanford University, 200 p. Oldow, J. S., 1978, Triassic Pamlico Formation; an allochthonous sequence of volcanogenic-carbonate rocks in west- central Nevada, in Howell, D. G., and McDougall, K. 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MANUSCRIPT RECEIVED BY THE SOCIETY MARCH 2 7 , 1 9 8 6 REVISED MANUSCRIPT RECEIVED OCTOBER 1 4 , 1 9 8 7 MANUSCRIPT ACCEPTED OCTOBER 1 5 , 1 9 8 7 Printed in U.S.A.