Magmatism, Ash-Flow Tuffs, and Calderas of the Ignimbrite Flareup in the Western Nevada Volcanic Field, Great Basin

The 36–18 Ma southern Great Basin, USA, ignimbrite province and ﬂ areup themed issue

Magmatism, ash-fl ow tuffs, and calderas of the ignimbrite fl areup in the western Nevada volcanic fi eld, Great Basin, USA

Christopher D. Henry1 and David A. John2 1Nevada Bureau of Mines and Geology, University of Nevada, Reno, Nevada 89557, USA 2U.S. Geological Survey, Menlo Park, California 94025, USA

ABSTRACT Channelization and westward fl ow of most refl ect proximity to (depth of) large resurgent tuffs in paleovalleys allowed them to travel intrusions. With the exception of the giant The western Nevada volcanic fi eld is the great distances, many as much as ~250 km Round Mountain epithermal gold deposit, western third of a belt of calderas through (original distance) to what is now the west- few known caldera-related hydrothermal Nevada and western Utah. Twenty-three ern foothills of the Sierra Nevada, which was systems are strongly mineralized. Major calderas and their caldera-forming tuffs not a barrier to westward fl ow of ash fl ows at middle Cenozoic precious and base metal are reasonably well identifi ed in the western that time. At least three tuffs fl owed eastward mineral deposits in and along the margins of Nevada volcanic fi eld, and the presence of at across a north-south paleodivide through the western Nevada volcanic fi eld are mostly least another 14 areally extensive, apparently central Nevada. That tuffs could fl ow sig- related to intrusive rocks that preceded cal- voluminous ash-fl ow tuffs whose sources nifi cant distances apparently uphill raises dera-forming eruptions. are unknown suggests a similar number of questions about the absolute elevation of the undiscovered calderas. Eruption and caldera region and the elevation, relief, and location INTRODUCTION collapse occurred between at least 34.4 and of the paleodivide. 23.3 Ma and clustered into fi ve ~0.5–2.7-Ma- Calderas are equant to slightly elongate, Huge volumes of dominantly rhyolitic ash- long episodes separated by quiescent periods at least 12 km in diameter, and as much as fl ow tuff that erupted from numerous calderas of ~1.4 Ma. One eruption and caldera col- 35 km in longest dimension. Exceptional in the Great Basin, mostly during the middle lapse occurred at 19.5 Ma. Intermediate to exposure of two caldera complexes that Cenozoic, constitute the ignimbrite flareup silicic lavas or shallow intrusions commonly resulted from extensional faulting and tilt- (Fig. 1; defi ned by Coney, 1978, and recognized preceded caldera-forming eruptions by ing show that calderas subsided as much and documented by McKee, 1971; Lipman 1–6 Ma in any specifi c area. Caldera-related as 5 km as large piston-like blocks; caldera et al., 1972; Noble, 1972; Best et al., 1989; Best as well as other magmatism migrated from walls were vertical to steeply inward dipping and Christiansen, 1991). The ignimbrite fl areup northeast Nevada to the southwest through to depths ≥4–5 km, and topographic walls in the Great Basin was part of a larger rhyolite time, probably resulting from rollback of formed by slumping of wall rock into the cal- ignimbrite province that includes calderas from the formerly shallow-dipping Farallon slab. dera were only slightly outboard (≤1 km) of Idaho through the Sierra Madre Occidental of Calderas are restricted to the area northeast structural margins. western Mexico (Cather et al., 2009; Henry of what was to become the Walker Lane, Most calderas show abundant post-col- et al., 2010, 2012b [Challis volcanic fi eld: although intermediate and effusive magma- lapse magmatism expressed as resurgent Fisher et al., 1992; Janecke et al., 1997] [South- tism continued to migrate to the southwest intrusions, ring-fracture intrusions, or ern Rocky Mountain volcanic fi eld: Lipman across the future Walker Lane. intracaldera lavas that are closely related et al., 1970; McIntosh and Chapin, 2004; Lipman Most ash-fl ow tuffs in the western Nevada temporally (~0–0.5 Ma younger) to caldera and McIntosh, 2008] [Mogollon-Datil volcanic volcanic fi eld are rhyolites, with approxi- formation. Granitoid intrusions, which were fi eld: McIntosh et al., 1992; Chapin et al., 2004] mately equal numbers of sparsely porphyritic emplaced at paleodepths ranging from <1 to [Trans-Pecos Texas: Henry and McDowell, (≤15% phenocrysts) and abundantly por- ~7 km, are compositionally similar to both 1986; Henry et al., 1994] [Sierra Madre Occi- phyritic (~20–50% phenocrysts) tuffs. Both intracaldera ash-fl ow tuffs and post-caldera dental: McDowell and Clabaugh, 1979; Ferrari sparsely and abundantly porphyritic rhyo- lavas. Therefore in the western Nevada vol- et al., 2007; McDowell, 2007; McDowell and lites commonly show compositional or petro- canic fi eld, erupted caldera-forming tuffs McIntosh, 2012]). This overall ignimbrite prov- graphic evidence of zoning to trachydacites commonly were the upper parts of large ince was the largest Cenozoic magmatic event or dacites. At least four tuffs have volumes magma chambers that retained considerable in western North America (Henry et al., 2010, greater than 1000 km3, with one possibly as volumes of magma after tuff eruption. 2012b). The most intense period of ash-fl ow much as ~3000 km3. However, the volumes of Several calderas in the western Nevada eruptions in the Great Basin was between ca. 37 most tuffs are diffi cult to estimate, because volcanic fi eld hosted large hydrothermal sys- and 22 Ma, but voluminous ash-fl ow eruptions many tuffs primarily fi lled their source cal- tems and underwent extensive hydrothermal both preceded and followed, from ca. 45 to 8 Ma deras and/or fl owed and were deposited alteration. Different types of hydrothermal (Sawyer et al., 1994; Henry et al., 2010). in paleovalleys, and thus are irregularly systems (neutral-pH alkali-chloride and acid Papers in this themed issue separate the middle distributed. or low-pH magmatic-hydrothermal) may Cenozoic calderas of the Great Basin into the

Geosphere; August 2013; v. 9; no. 3; p. 951–1008; doi:10.1130/GES00867.1; 27 ﬁ gures; 4 tables; 6 supplemental ﬁ les. Received 29 September 2012 ♦ Revision received 20 March 2013 ♦ Accepted 22 March 2013 ♦ Published online 13 June 2013

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42° 120° Oregon116° Idaho 112°

Warner Range California Nevada Utah Northeastern ? Nevada (45–40 Ma)

? ?

? Western Nevada Volcanic Field ? (calderas: Caetano 34.4–19.5 Ma) Thomas Range 40° (~37–32 Ma) ?

Stillwater ? ?

complex ort ? ? Nevada caldera belt This rep (~36–35 Ma)

Walker Lane Central Nevada Indian Peak Volcanic Field (~32–27 Ma) (~36–18.4 Ma)

(2013a) Marysvale Best et al. 38° Q (~27–19 Ma) Caliente Caldera Henry et al. (~24–14 Ma) Sierra Nevada Volcanic field (2012 boundaries

(Best et al., 2013a) a) Paleovalley, flow direction Kane Springs Wash (Henry et al., 2012a) (~15–14 Ma) Southwestern Proposed Nevada (~15–7.6 Ma) paleodividesPaleodivide Caldera age transects 0 500 km

Figure 1. Index map of the Great Basin showing middle Cenozoic calderas, caldera groups, and their age ranges. This report focuses on the western Nevada volcanic ﬁ eld. Also shown are interpreted middle Cenozoic paleovalleys and faults of the Walker Lane. Not shown are middle Miocene calderas of the initial Yellowstone hotspot in northwestern Nevada and the Long Valley and Little Walker calderas of the Sierra Nevada. Northeast-striking lines are caldera age transects (Fig. 4). The Kane Springs Wash calderas are not part of the Caliente caldera complex. Q—Quinn Canyon Range.

Indian Peak–Caliente, central Nevada, and west- Recent geologic mapping, geochronology, 135 previously unpublished dates; Supplemen- ern Nevada volcanic fi elds (Fig. 1; Best et al., study of mineral deposits, and other research tal Tables 11 and 22), (3) regionally correlating 2013a). Division between the Indian Peak– have greatly expanded the understanding of ash-fl ow tuffs, including correlation of many Caliente and central Nevada volcanic fi elds is calderas, magmatism, and tectonic history of to source calderas and recognizing the wide based on the absence of calderas between the northern and western Nevada (Proffett and Prof- two fi elds. Division between the central Nevada fett, 1976; Ekren et al., 1980; Ekren and Byers, 1Supplemental Table 1. 40Ar/39Ar ages of ash-fl ow and western Nevada volcanic fi elds is based on 1985, 1986a, 1986b, 1986c; Boden, 1986, 1992; tuffs in the western Nevada volcanic fi eld. If you are the apparent restriction of most ash-fl ow tuffs Shawe, 1995, 1998, 1999a, 1999b, 2002; Shawe viewing the PDF of this paper or reading it offl ine, erupted from the western fi eld to areas west and Byers, 1999; Shawe et al., 1986; Shawe please visit http://dx.doi.org/10.1130/GES00867.S1 or the full-text article on www.gsapubs.org to view of a proposed paleodivide (Henry, 2008; Best et al., 2000; John, 1992a, 1995, 2001; Faulds Supplemental Table 1. et al., 2009; Henry and Faulds, 2010; Henry et al., 2005a, 2005b; Colgan et al., 2008; John 2Supplemental Table 2. Analytical data and prob- et al., 2012a). The outlines of the three fi elds et al., 2008; Henry and Faulds, 2010; Muntean ability plots of 40Ar/39Ar ages of ash-fl ow tuffs in the shown in Figure 1 are drawn around known and et al., 2011; Henry et al., 2012a). The greatest western Nevada volcanic fi eld. If you are viewing the PDF of this paper or reading it offl ine, please visit inferred calderas and surrounding areas within strides have come in (1) identifying calderas, http://dx.doi.org/10.1130/GES00867.S2 or the full- 40 39 20–50 km underlain by temporally equivalent (2) establishing a precise Ar/ Ar chronology text article on www.gsapubs.org to view Supplemen- effusive and intrusive rocks. of ignimbrite activity (including 250 total and tal Table 2.

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distribution of many across the region, (4) rec- a transitional zone rifted in the Neoproterozoic, 0.7049 to 0.7055 (John, 1992a). In contrast, four ognizing that ash-fl ow tuffs commonly fl owed into a region composed of accreted Phanerozoic analyses from the Caetano caldera and underly- through and were deposited in paleovalleys, oceanic and island arc terranes (Fig. 2). Most ing andesite lava fl ows above transitional crust (5) comprehensively delineating the geometry calderas of the western Nevada volcanic fi eld range from 0.7070 to 0.7076 (B. Cousens, 2012, of several calderas, and (6) understanding the developed in the accreted terranes, but several personal commun.). relationship of major ore deposits to magmatism. calderas in the northern and eastern part of the Calderas of the Great Basin are restricted to The understanding of geochemistry and western Nevada volcanic fi eld are in the transi- areas of present-day thick crust (Fig. 2). Notably,

petrology of the western Nevada volcanic fi eld is tional zone. The Sri = 0.706 isopleth, commonly the gap between pre–40 Ma and post–37 Ma less advanced than for the Indian Peak–Caliente used to estimate the edge of the Precambrian caldera regions correlates with an east-west zone and central Nevada volcanic fi elds, which have basement (Kistler, 1990; Tosdal et al., 2000), of thin crust, as well as with the Uinta-Cortez been intensively studied by M.G. Best, E.H. approximately coincides with the paleodivide axis and western end of the Archean-Protero- Christiansen, and colleagues (Best et al., 2013b, proposed by Best and colleagues (Best et al., zoic suture (Roberts et al., 1965; Presnell, 1998; 2013c). Uncertainty in correlation of many tuffs, 2009, 2013a) in central Nevada and slightly Tosdal et al., 2000; Rodriguez and Williams, the plethora of locally assigned stratigraphic west of our published interpretation (Henry, 2008). However, correlation between present formation names, and a tendency to combine 2008; Henry et al. 2012a). thick crust and middle Cenozoic calderas prob- multiple regional and/or local ash-fl ow tuffs and The few isotopic data for the western Nevada ably only indicates that the caldera belt resisted lavas into composite formations are signifi cant volcanic fi eld are consistent with the regional late Cenozoic extension (Henry et al., 2012b), problems. trends. Twenty-eight initial Sr ratios from the so crust there has not been appreciably thinned. This paper is the fi rst comprehensive treat- Stillwater caldera complex developed in oceanic The amount and distribution of pre–, syn–, ment of middle Cenozoic volcanism in west- crust range from 0.7047 to 0.7057, with nearly and post–ignimbrite fl areup extension have ern Nevada during the peak of the ignimbrite all values between 0.7050 and 0.7054 (John, been intensely studied and debated. A major epi- fl areup . It (1) provides a state-of-the-art sum- 1995; this study). Eleven analyses from ash-fl ow sode of extension began in the middle Miocene mary of ignimbrite chronology, composition, tuffs and interbedded lava fl ows in the Paradise across most of northern Nevada, although the caldera sources, and associated lavas and intru- Range also overlying oceanic crust range from amount of extension varied greatly across the sions, (2) documents the character of caldera- forming ignimbrite volcanism developed on the western slope of the Sevier orogenic plateau and 42° 120° OR ID 116° 112° commonly on oceanic crust west of Precambrian CA NV UT basement, (3) includes numerous new certain ? Mostly Northeastern and probable correlations of widely scattered Oceanic Nevada outcrops of tuffs based on stratigraphy, 40Ar/39Ar 208Pb/204Pb = 39.7 ages, phenocryst assemblages, and composi- 208Pb/204Pb = 38.8 Precambrian basement tions, (4) reinterprets two proposed calderas to iSr = 0.706 show that they probably do not exist, and (5) uses extraordinary three-dimensional exposures of the 40° iSr = 0.704 Caetano caldera and the Stillwater caldera com- ? plex (Fig. 1) to document fundamental charac- Transitional- teristics of calderas that are rarely exposed else- Rifted where. The understanding provided by geologic Western Crustal Thickness ? mapping and other studies of these exceptionally (km); data from Gilbert (2012) Central exposed calderas can be applied worldwide and 23 Indian Peak can further constrain models of ash-fl ow caldera Marysvale genesis. Finally, this paper complements similar 38° 33 Caliente reports that comprehensively document the cen- ? tral Nevada and Indian Peak–Caliente volcanic ? fi elds, which developed on a high Sevier plateau 43 overlying thicker, Precambrian crust (Best et al., Southwestern AZ 2013b, 2013c). The combined three papers are Nevada Precambrian the fi rst comprehensive description of the Great 53 basement Basin ignimbrite province and allow for detailed 0 500 km comparisons in the character of volcanism across the province (Best et al., 2013a). Figure 2. Crustal thickness and crustal isotope provinces of Nevada. Crustal thickness data from Gilbert (2012; contoured in ArcMap [www.esri.com]). Pb lines in blue and Sr lines in CRUSTAL-TECTONIC SETTING AND black from Tosdal et al. (2000). Calderas of the ignimbrite fl areup appear to be restricted EVOLUTION OF WESTERN NEVADA to areas of thicker crust (generally ≥34 km), possibly indicating that caldera belts resisted mid- to late Cenozoic extension and their crust has therefore not been thinned as much as Calderas of the Great Basin developed in a in adjacent regions. Calderas also extend from intact Precambrian basement on the east, wide range of crustal settings. Isotopic data indi- across a transition zone rifted in the Neoproterozoic, into a region composed of accreted cate that the caldera province extends westward Phanerozoic oceanic and island arc terranes on the west. Calderas of the western Nevada from undisturbed Precambrian basement, across volcanic fi eld extend from rifted basement into the accreted terranes.

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region (Proffett, 1977; Best and Christiansen, basins adjacent to, the Clan Alpine and Desa- several calderas at ca. 25 Ma and the 19.5 Ma 1991; Surpless et al., 2002; Colgan et al., 2008, toya Mountains. At least four, mostly older Fairview Peak caldera. 2010; Henry, 2008; Colgan and Henry, 2009; calderas lie northeast of the main belt. The Along the eastern transect, the ca. 35.4 Ma Henry and Faulds, 2010; Henry et al., 2011). Nine Hill caldera is postulated to underlie the caldera for the Pancake Summit Tuff of the cen- Many geologists interpret major pre– and/or Carson Sink (Deino, 1985, 1989; Best et al., tral Nevada volcanic fi eld is at the northeastern syn–ignimbrite fl areup extension (Gans et al., 1989; A.L. Deino, 2012, written commun.). Cal- end. The 32.9 Ma Northumberland caldera is the 1989; Seedorff, 1991; Smith, 1992; McGrew deras are absent southwest of a distinct line that next to the southwest. The numerous calderas of and Snee, 1994; Dilles and Gans, 1995; Mueller marks the northeastern margin of the Walker the Toquima caldera complex are in the middle. et al., 1999; McGrew et al., 2000; Druschke et al., Lane (Fig. 1). The youngest calderas, the 24.8 Ma Manhattan, 2009). However, the lack of angular unconfor- Based on identifi ed calderas and the num- 23.3 Ma Toiyabe, and 24.5 Peavine, are at the mities between Cenozoic tuffs, continuity of ber of voluminous ash-fl ow tuffs whose erup- southwest end. ash-fl ow tuffs in paleovalleys across areas of tion likely induced caldera collapse, at least The area of the transects has undergone varia- major extension, and the paucity of early or 37 calderas are probably present in the west- ble amounts of middle Miocene and younger middle Cenozoic sedimentary deposits in the ern Nevada volcanic fi eld (Fig. 1; Tables 1 extension, as much as 100% across the Caetano caldera belt indicate that pre– or syn–ignimbrite and 2). Twenty-three calderas are reasonably caldera and the southern Stillwater Range (John, fl areup extension was minor and/or exceed- well identifi ed on the basis of thick (≥500 m) 1995; Hudson et al., 2000; Colgan et al., 2008) ingly local (Best and Christiansen, 1991; Henry, intracaldera ash-fl ow tuff, commonly contain- but much less across all other calderas. Also, 2008; Colgan and Henry, 2009; Long, 2012). ing megabreccia, or the presence of caldera extension was west-northwest, nearly perpendic- Most calderas in the western Nevada vol canic walls. Detailed to reasonably thorough recon- ular to the northeast-oriented transects, so correc- fi eld are little tilted and therefore probably little naissance mapping exists for many calderas as tion of north-south distances or southward rates extended. Correlation between calderas and thick documented below. Most calderas are ~20 km for extension are not critical. Caldera magmatism crust is a manifestation of their lack of extension. in diameter. The largest known calderas are migrated southwestward at ~20 km/Ma between However, two calderas or caldera clusters at tran- those for the tuff of Elevenmile Canyon (~25 × ca. 34 and 29 Ma along the western transect, then sitions to thinner crust underwent major exten- 35–40 km), the tuff of Campbell Creek (~14– slowed to no more than ~4 km/Ma between ca. sion. The ~100% extension of the 34 Ma Caetano 20 × 35 km), and the lower tuff of Mount Jef- 29 and 19 Ma, and ended at the edge of the future caldera took place mostly between 16 and 10 Ma ferson (~20 × 28 km). Walker Lane (Fig. 1). Migration was ~4 km/Ma (John et al., 2008; Colgan et al., 2008). A similar Another ~14 calderas, dependent on possible along the eastern transect the entire time. magnitude of extension in the 29–25 Ma Still- correlations, are inferred in the northwestern water caldera complex was interpreted to have part of the western Nevada volcanic fi eld based Fish Creek Mountains Anomaly been mostly at 25–24 Ma (John, 1992b; Hudson on the widespread distribution of ash-fl ow tuffs et al., 2000). However, new, unpublished apatite in that part of the fi eld and westward to western Formation of the 24.9 Ma Fish Creek Moun- fi ssion-track dating by Joseph Colgan and David Nevada and the eastern Sierra Nevada (Faulds et tains caldera is an anomalously young event John indicates that major uplift of the east side of al., 2005a, 2005b). Prominent examples are the in an area that otherwise underwent extensive the southern Stillwater Range was ca. 16–12 Ma, very widespread Nine Hill Tuff (Deino, 1985; magmatism between ca. 40 and 33 Ma, includ- so timing of major extension is equivocal. Best et al., 1989) and tuffs of Axehandle Can- ing the 34 Ma Caetano caldera (John et al., yon, Rattlesnake Canyon, and Sutcliffe (Faulds 2008; Cousens et al., 2011). The Fish Creek CALDERA DISTRIBUTION et al., 2005a, 2005b; Brooks et al., 2008; Henry Mountains caldera fi ts temporally with the AND TIMING and Faulds, 2010). Many calderas may be hid- 26–24 Ma calderas of the Stillwater Range and den beneath basins. At least one tuff, the tuff of Clan Alpine Mountains, ~100 km to the south- Major ash-fl ow tuffs and their known or sus- Miller Mountain of the Candelaria Hills, which west at the end of the transect (Fig. 4). pected calderas in the western Nevada volcanic correlates with the Monotony Tuff, erupted fi eld range from 34 to 19.5 Ma, although only from a caldera in the central Nevada volcanic Rollback Origin of Cenozoic Magmatism one is younger than 23.3 Ma (Table 1; Fig. 3). fi eld (Best et al., 2013c). Intrusive and effusive magmatism began Southwesterly migration of caldera-forming as early as 40 Ma, and small- to probably Southwesterly Migration of Caldera- eruptions is part of a larger pattern of south- moderate-volume tuffs whose eruptions were Forming Magmatism westerly migration of all magmatism (Fig. 5). unlikely to have induced caldera collapse are That pattern is the primary evidence for relat- as old as 35.8 Ma. Major episodes of ash-fl ow Northeast-to-southwest transects across the ing magmatism to the removal (rollback?) of eruptions occurred at 34.4–34.0 Ma (3 cal- western and eastern parts of the western the formerly shallowly dipping Farallon slab, deras), 32.9–32.6 Ma (2), 31.5–28.8 Ma (13), Nevada volcanic fi eld illustrate the southwest- a mechanism proposed by many (Coney and 27.4–26.8 Ma (8), 25.4–23.3 Ma (20), and erly migration of caldera-forming magmatism Reynolds, 1977; Best and Christiansen, 1991; one event at 19.5 Ma. The western and central through time as well as one major anomaly Christiansen and Yeats, 1992; Humphreys, Nevada volcanic fi elds form a continuous belt (Fig. 4). Calderas are spread over a greater dis- 1995; Humphreys et al., 2003; Dickinson, of calderas from the Stillwater Range on the tance in the west than in the east, but the oldest 2006; Henry et al., 2009a, 2010; Colgan et al., northwest to the Quinn Canyon Range on the calderas are at the northeast end of both tran- 2011a). The dip of the subducting Farallon slab southeast (Fig. 1). Calderas are recognized in sects. Along the western transect, these are the shallowed between ca. 90 Ma, at the end of every range in this belt. The distribution of sev- well-established ca. 34 Ma Caetano and Hall the Cretaceous assembly of the Sierra Nevada eral widespread ash-fl ow tuffs suggests addi- Creek calderas and the less certainly located, batholith, and 50 Ma. Related magmatism tional cal deras especially in the northwestern 34.4 Ma caldera for the tuff of Cove Mine. The migrated as far east as Colorado (Dickinson and part of the belt in, and possibly buried beneath younger, southwestern end of the transect has Snyder, 1978; Lipman, 1992; Dickinson, 2006).

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Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/9/4/951/3346482/951.pdf by guest on 29 September 2021 Ignimbrites of the western Nevada volcanic fi eld ) d a e r b continued e ( t i h W 0 ; 7 a 9 2 1 9 , 9 m 1 a , h n n h et al., 1988 1986b, 1986c; Ekren et al., 1980; Hardyman 1992a 1992a and Dilles, 1984; Ekren and Byers, 1985, 1986a, et al., 1988; Dilles and Gans, 1995 Whitebread and John, 1992; John, 1992a et al., 2011 1992a 1989 Faulds et al., 2005a, 2005b Henry et al., 2004, 2009b; o o J Shawe, 1999a Brem et al., 1991; John, McKee and Conrad, 1987 B Brem et al., 1991; John, Bingler, 1978a; Proffett 1978a; Proffett Bingler, Brem et al., 1991; Henry, 1996a, 1996b Henry, Ekren et al., 1980; John, McKee, 1970; Cousens Deino, 1989; Best et al., John, 1995, 1997; this study John, 1995; Deino, 1985; ow d ow e b ) r 3 0 o 7 s 9 e r 1 nite and one ( y l e g e n K o of 310 km estimated volume phenocrysts nearby thickness exposed ash-fl probable ash-fl quartz phenocrysts of tuff and of tuff geophysical data feeder vents tuff vent tuff caldera has several original east-west of tuff; most of tuff; in widespread tuff Great Basin original east-west on distribution extent unknown extent unknown r c t M Anorthoclase One defi Highly extended; S Highly extended; e t i s e d ows and ow fl n a e t i o l t o ows overlie y e t ll caldera i h l r o s y age (Ma) Comments References u h r n n o l n w w i a o o r m e n n e u k n k v l lavas and intrusions; ca. 24 (K-Ar) domes and andesite lavas; and partly fi domes intrude margin of 19.3–18.6 Railroad Ridge caldera lava domes and dikes; ca. part of caldera (25.1); small-volume ash fl rhyolite lavas in south part of caldera (undated) Hercules Canyon in north 25 to 20 Post-caldera volcanism; n n o e o U Rhyolite Sphene phenocrysts; Rhyolite Sphene Andesite lava fl S Abundant rhyolite to dacite N Tuffs of Lee Canyon and Tuffs V Abundant rhyolite and dacite U ows overlie age (Ma) n n w w o o e n n Pre-caldera volcanism; k n k ows, and pluton IXL margin caldera formed by eruption of Campbell Creek of tuff 24.5 Ma tuff of Peavine 24.5 Ma tuff Canyon and underlie tuff near caldera of Toiyabe domes; 20.3–20.1 Canyon caldera formation lower cooling unit of tuff lower cooling unit of tuff of Poco Canyon erupted shortly prior to Elevenmile fl ca. 29 Ma tuff of Job ca. 29 Ma tuff Canyon, intermediate lava n n o U N Nested ca. 25 Ma calderas; U ? 0 6 1400 2 ≥ ) 3 ? (km e t ? l e l a 0 g r Tuff volume Tuff a r 3 e a 1 350 Large Nested ca. 25 Ma calderas; m d Large Large 1050 L Large Unknown Unknown Inferred source based S o ≤ ≤ Minimum Maximum M 0 5 0 ) 3 3 4 230 n 2 1 1 1 ≥ w ; ; ; o 3 6 0 n 1 1 1 k n × × × U (km; km 2 2 5 Caldera size 1 1 12 × 17; 160 190 Nested inside ca. 29 Ma 1 13 × 22; 12 × 18; 170 Large? Unknown Unknown Location inferred from 175 (assuming 350 (assuming ≥ circular caldera) 100% extension) 20 (north-south) × 35–40 (east-west); n n n a t w w t o o a n n h k k n Ridge Valley n n a Canyon Railroad for tuff of for tuff U U Desatoya Mountains Mountains Mountains Mountains M in southern Clan Alpine Reese River (Figs. 10, 11) Carson Sink? Source caldera TABLE 1. CORRELATION, AGE, SOURCE, AND VOLUME OF MAJOR ASH-FLOW TUFF UNITS IN THE WESTERN NEVADA VOLCANIC FIELD THE WESTERN NEVADA TUFF UNITS IN ASH-FLOW AND VOLUME OF MAJOR AGE, SOURCE, 1. CORRELATION, TABLE 8 3 . . 4 4 4 Age (Ma) 2 23.3 Toiyabe 14 × 20; 220 Large Andesite lava fl 2 25.0 Shoshone 25.1 25.2 Poco Canyon 15 (north-south); 25.4 Buried beneath 25.4– 2 n o i t a m r o F k c o R d n u Goldyke of Copper Mountain – tuff of Copper Mountain – tuff of Toiyabe tuff of Poco Canyon – tuff Tuff New Pass of Gabbs Valley ? tuffs in ? tuffs of Gabbs Valley the Paradise Range ? tuff of Railroad Ridge Clipper Gap ? tuff Tuff Bluestone Mine Ione Canyon Mountain Tuff o Blue Sphinx Tuff – tuff of – tuff Tuff Blue Sphinx Fish Creek Mountains TuffFish Creek Mountains 24.9 Fish Creek Tuffs of Sheep Canyon and Tuffs Santiago Canyon Tuff – tuff – tuff Tuff Santiago Canyon Tuff of Peavine CanyonTuff 24.5 Peavine 15 × 18; 215 R Tuff of Desatoya PeakTuff 24.7 Central Tuff of Chimney Spring – Tuff Tuff of Fairview PeakTuff 19.5 Peak Fairview 160 18 × 11; 115 640 Sparse dacite to rhyolite Tuff of Arc Dome of Tuff 25.2 Probably buried Underdown Tuff – tuff of – tuff Tuff Underdown ? Tuff Eureka Canyon Nine Hill Tuff – unit D, Bates Tuff Nine Hill Tuff (names, west to east) Tuff Tuff of Elevenmile CanyonTuff 25.1 Elevenmile

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Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/9/4/951/3346482/951.pdf by guest on 29 September 2021 Henry and John ) ; ; ; 7 6 6 8 7 7 9 9 9 1 1 1 , , , continued t t d t t ( a e e f f r f f n o o r r o P P C d d d n n n a a a t t t t e e e e f f f f K Brooks et al., 2008; Henry et al., 2012 Hardyman et al., 1988 Boden, 1986, 1987, 1992, 1994; Shawe, 1995, 2002; Henry et al., 1996; Henry, 2010 1994; Shawe, 1999b, 1997; Henry and Henry, Faulds, 2010 1997; Henry and Faulds, 2002; Henry et al., 1996; et al., 2012 Ekren et al., 1980 Henry et al., 1996; Henry, Henry et al., 1996; Henry, 1997; Henry and Faulds, 1994; Shawe, 2002; Shawe et al., 2000; Henry 1997; et al., 1996; Henry, Henry and Faulds, 2010 Robinson and Stewart, 1984; Petronis et al., 2009; Best et al., 2013c 1994; Shawe, 1999b, 2002; Shawe et al., 2000; 2010 Shawe et al., 2000; Henry 1997; et al., 1996; Henry, Henry and Faulds, 2010 Brooks et al., 2008; Henry 1994; Shawe, 2002; o o c r r Boden, 1986, 1987, 1992, John, 1995 M Riehle et al., 1972; P P Boden, 1986, 1987, 1992, Boden, 1986, 1987, 1992, McKee and Conrad, 1987; Speed and Cogbill, 1979; Boden, 1986, 1987, 1992, ) continued original east-west extent unknown widespread tuff in widespread tuff Great Basin quartz phenocrysts; second-most Highly extended; Strongly resorbed ll a r e d ow l a c k e e 1 km diameter; r ≤ C age (Ma) Comments References l l n n e w w b o o p n n 1 km diameter along ring m k k breccias up to 2 km thick fi caldera; ca. 29 domes along margin of Deep Canyon caldera 26.95 ± 0.03, n = 5 rhyolites fracture; 27.30 ± 0.01, n = 4 tuff; 28.84 ± 0.05, 28.83 tuff; ≤ compositionally related to relationship to caldera remnant; 27.51 ± 0.25, 27.49 ± 0.10 ± 0.04 Post-caldera volcanism; a n n C Dacite-andesite lavas and U Numerous ring-fracture U Numerous rhyolite fl Numerous rhyolite domes Rhyolite domes of uncertain ow dome ows and age (Ma) n w o n Pre-caldera volcanism; k dacite lava fl breccias, dikes, and sills (ca. 30–29) complex (ca. 30? Ma); thick sequence of andesite- Campbell Creek eruption Toquima complex from Toquima 32.6, especially 27.4 to 26.9 Toquima complex from Toquima 32.6, especially 27.4 to 26.9 m.y. older than caldera m.y. Toquima complex from Toquima Toquima complex from Toquima 32.6, especially 27.4 to 26.9 complex from Toquima 32.6, especially 27.4 to 26.9 32.6, especially 27.4 to 26.9 n U e g r a 4700 L ) 3 (km Tuff volume Tuff 240 Large Large rhyolite fl 360 3000 Andesite lavas, age unknown Dacite to rhyolite intrusions Large E probably predates Tuff Minimum Maximum ) n 2 w 35; 600 o ≥ n k n caldera caldera) U (km; km Overprinted Large? magmatism in Ongoing Caldera size 13 × 22; 200 300 Unknown magmatism in Ongoing 15 × 20; 240 240 960 Hornblende andesite lavas ~6 20 × 28; 400 900 1600 Ongoing magmatism in eruption from 3 × remnant Large Ongoing magmatism in Probably early Campbell Creek 14–20 × (assuming circular a eld y o t a s Alpine e Mount Range Range Nevada possibly complex Toquima Toquima Toquima caldera? remnant, in central Jefferson Jefferson D Mounains Mounains of Ryecroft Job Canyon80 (north-south); 10 emnant, part volcanic fi (Figs. 10, 11) Range caldera Source caldera 1 . 9 Age (Ma) 29.4 26.9 Round Mountain 3 × 4 remnant Large Ongoing magmatism in 2 27.0 Upper Mount 27.1 Unknown, 27.3 lower 27.4 Caldera 27.6 Pancake 27.4 Caldera r 28.9 Desatoya TABLE 1. CORRELATION, AGE, SOURCE, AND VOLUME OF MAJOR ASH-FLOW TUFF UNITS IN THE WESTERN NEVADA VOLCANIC FIELD ( THE WESTERN NEVADA TUFF UNITS IN ASH-FLOW AND VOLUME OF MAJOR AGE, SOURCE, 1. CORRELATION, TABLE eld ow E f f tuff of Mount Jefferson tuff Heights Member) Mountain of Guild Mine Member) – of Mount lower tuff of Guild Mine Member) – of Ryecroft Canyon tuff Jefferson tuff of Corcoran Canyon tuff unit C, Bates Mountain Tuff unit C, Bates Mountain from central Nevada fi Monotony Tuff; outfl Monotony Tuff; u Tuff of Job CanyonTuff ca. Singatse Tuff – tuff of Round – tuff Tuff Singatse T Lenihan Canyon Tuff – upper Tuff Lenihan Canyon (Weed Tuff Mickey Pass Tuff (names, west to east) Tuff Tuff of Deep CanyonTuff 29.0 Clan Mickey Pass Tuff (main part Tuff Mickey Pass Mickey Pass Tuff (basal part Tuff Mickey Pass Tuff of Miller Mountain – Tuff Tuff of Logan Spring – upper Tuff Tuff of Campbell Creek – Tuff

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Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/9/4/951/3346482/951.pdf by guest on 29 September 2021 Ignimbrites of the western Nevada volcanic fi eld , e w ; ; ; ; ; ; n a 0 9 9 9 9 9 9 a h 1 0 0 0 0 0 0 g l S 0 0 0 0 0 0 0 o ; 2 2 2 2 2 2 2 C 2 , , , , , , , s ; 9 4 4 4 4 4 4 d 8 9 0 0 0 0 0 0 l 0 1 0 0 0 0 0 0 u 0 , 2 2 2 2 2 2 a 2 6 , , , , , , F ...... l l l l l l , 8 . l d a a a a a a 9 a t t t t t t n 1 t e e e e e e a , e n y y y y y y y r r r r r r r e n n n n n n n n d h et al., 1996; Henry and Faulds et al., 2005a, 2005b 2005b 2005b 2005b Faulds et al., 2005a, 1994; Shawe, 2002; Henry Faulds, 2010 et al., 2008, 2011 Faulds et al., 2005a, Faulds et al., 2005a, Faulds et al., 2005a, 1999a; McKee, 1976; Henry et al., 1996 2005b Faulds et al., 2005a, b et al., 2008, 2011 J.P. Colgan and C.D. J.P. Henry, unpub. mapping Henry, e e e e e e e o o H H Boden, 1986, 1987, 1992, H H H McKee, 1974, 1976 John et al., 2008; Colgan Stewart and McKee, 1968; B H H J ) continued ow tuff oor locally ash-fl ~3.5 km; caldera fl exposed so area uncertain dike exposed for Highly extended; Moderately extended age (Ma) Comments References n n n n n n n n n w w w w w w w w w o o o o o o o o o n n n n n n n n n k k k k k k k k k tuff; 33.95 ± 0.05, n = 3 tuff; compositionally related to 33.51 ± 0.03 Post-caldera volcanism; n n n n n n n n n U U U Several rhyolite intrusions U U U U U U age (Ma) n n n n n n n n w w w w w w w w o o o o o o o o n n n n n n n n Pre-caldera volcanism; k k k k k k k k 35.69 ± 0.06, n = 13. Minor andesite lavas; 35.21 ± 0.18 n n n n n n n n U U U U U U U U e e e t g g r r a e e e e r a a g g g g l l e r r r r d a a a a y y r r o L L L L e e to large ) M 3 V V e (km t a r Tuff volume Tuff e d Large Several rhyolite intrusions, o ≤ Minimum Maximum M d ) n n n n n n n e 2 t w w w w w w w n i o o o o o o o r n n n n n n n p r k k k k k k k e n n n n n n n v U U U U U U U (km; km O Caldera size 15 × 15; 180 180 720 4 × 3 remnant Moderate 2.5 × 7 remnant Large? area area area area? area? area? Valley? complex complex Toquima Desatoya Mountains Clan Alpine Clan Alpine Clan Alpine Clan Alpine Clan Alpine Clan Alpine Desatoya – Desatoya – Desatoya – Desatoya – Desatoya – Desatoya – ?Stillwater – Reese River (Figs. 10, 11) early Toquima early Toquima Source caldera Toquima Range Toquima Age (Ma) 31.4 Stillwater – 30.1 ?Stillwater – 32.6 Dry Canyon, 30.4– 30.6 30.3 ?Stillwater – 30.0 Early 31.2 Stillwater – 29.4 Probably TABLE 1. CORRELATION, AGE, SOURCE, AND VOLUME OF MAJOR ASH-FLOW TUFF UNITS IN THE WESTERN NEVADA VOLCANIC FIELD ( THE WESTERN NEVADA TUFF UNITS IN ASH-FLOW AND VOLUME OF MAJOR AGE, SOURCE, 1. CORRELATION, TABLE ?—uncertain correlation. Edwards Creek Tuff Edwards Creek Edwards Creek Tuff – unit Tuff Edwards Creek Tuff B, Bates Mountain Canyon Note: Canyon part of Edwards Creek A, Bates – unit Tuff Mountain Tuff Hoodoo Canyon Edwards Creek Tuff Edwards Creek tuff of McCoy Mine tuff Tuff of Hardscrabble Tuff Tuffs of Dry Canyon and Tuffs Tuff of Mine Canyon – part Tuff Tuff of Axehandle Canyon of Tuff 31.5 Stillwater – Tuff of Sutcliffe – part of of Sutcliffe Tuff Tuff of Cove Spring – part Tuff Northumberland Tuff 32.9 Northern Lower tuff of Corcoran Lower tuff Tuff of Rattlesnake Canyon – Tuff Caetano TuffCaetano of Hall CreekTuff 34.0 Caetano 34.0 × 10–18; 280 20 Hall Creek 1100 × 10; ~150 20 Abundant rhyolite dikes; Tuff (names, west to east) Tuff Tuff of Dogskin Mountain – Tuff Tuff of Cove MineTuff 34.4 Beneath

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18 West Correlation of Major Ignimbrites East (Eastern Sierra Nevada of the Western Nevada Volcanic Field (Approximate Paleodivide and and Western Nevada) (Definite =, Probable -, and Possible ?) Boundary: Western – Central (arranged diagrammatically west to east; Nevada Volcanic Fields) names in bold are preferred)

19.5 tuff of Fairview Peak (19.48±0.02; 2)

tuff of Redrock Canyon (24.1±0.8) Santiago Canyon Tuff (23.27±0.05) ======tuff of Copper Mountain ======tuff of Toiyabe (23.31±0.05) tuff of Perry Canyon (23.53±0.26) tuff of Gallagher Pass == Poinsettia tuff member, Hu-Pwi Rhyodacite (23.62±0.04)

Age, Ma Candelaria Junction Tuff (23.76±0.13) Belleville Tuff (24.25±0.29) 22 tuff of Sheep Canyon (24.1±0.7) - tuff of Ione Canyon Blue Sphinx Tuff (24.30±0.01; 2) ? ? ? tuff of Goldyke (~23) tuff of Peavine Canyon (24.53±0.08) tuff of Desatoya Peak (24.74±0.06) Round Rock Formation (24.80±0.14) 23.3 Fish Creek Mountains Tuff (24.91±0.03; 3) Underdown Tuff (24.96±0.05) ======tuff of Clipper Gap (24.87±0.10) Eureka Canyon Tuff (25.06±0.07) tuff of Brunton Pass (25.12±0.06) Bluestone Mine Tuff ? ? tuff of Gabbs Valley (25.15±0.06) 24 tuff of Railroad Ridge (25.15±0.06) tuff of Arc Dome (25.18±0.06) tuff of Lee Canyon (25.00±0.06) == tuff of Hercules Canyon (25.05±0.06) tuff of Chimney Spring (25.26±0.05; 4) == tuff of Poco Canyon (25.26±0.07) ======New Pass Tuff (25.32±0.06) tuff of Painted Hills (25.06±0.01; 2) ? ? ? ? ? tuff of Elevenmile Canyon (25.06±0.01; 3) tuff of Camel Spring (25.31±0.07) 25.4 lower tuff of Poco Canyon Nine Hill Tuff (25.39±0.07; 9) ======tuff of Green Springs (25.29±0.07) ======unit D, Bates Mountain Tuff (25.44±0.07) 26 Metallic City Tuff (25.81±0.12)

upper cooling unit of Singatse Tuff ? Petrified Spring Tuff 26.8 Singatse Tuff (26.85±0.02) ------tuff of Round Mountain (26.86±0.03; 5) Lenihan Canyon Tuff (26.94±0.06) ------upper tuff of Mt Jefferson (27.01±0.05; 4) Mickey Pass Tuff (Weed Heights member, 27.12±0.02; 2) ------Mickey Pass Tuff (Guild Mine mbr, units 2-4; 27.24±0.04; 8) = lower tuff Mt Jefferson (27.25±0.04; 4) = tuff Moores Creek (27.28±0.06) 27.4 Mickey Pass Tuff (Guild Mine member, unit1 1; 27.42±0.09; 2) - tuff of Logan Spring (27.38±0.08) - tuff of Ryecroft Canyon (27.43±0.09) 28 upper tuff of Corcoran Canyon (27.52±0.05) tuff of Miller Mountain (27.65±0.05) ======Monotony Tuff (27.58±0.05; 2)

28.8 tuff of Winnemucca Ranch (28.75±0.10; 2) ------tuff of Deep Canyon (28.98±0.11) tuff of Campbell Creek (29.01±0.03; 4) === Campbell Creek rhyolite (28.97±0.03; 2) ======unit C, Bates Mountain Tuff (28.84±0.09) tuff E (29.19±0.04; 6) tuff of Job Canyon (29.43±0.29) 30 tuff of Dogskin Mountain (29.40±0.10 - 29.68±0.21; 4) ------tuff of McCoy Mine lower tuff of Corcoran Canyon (30.01±0.08) tuff of Mine Canyon (30.12±0.06; 6) ======part of Edwards Creek Tuff ======tuff of Cove Spring (30.31±0.05; 10) ======part of Edwards Creek Tuff (30.29±0.09) ======tuff of Sutcliffe (30.56±0.07; 11) ======part of Edwards Creek Tuff (30.51±0.08) ==== unit B, Bates Mountain Tuff (30.61±0.06; 2) 31.5 tuff of Rattlesnake Canyon (31.24±0.05; 9) = part of Edwards Creek Tuff (31.20±0.06; 2) == unit A, Bates Mountain Tuff (31.23±0.07) tuff of Hardscrabble Canyon (31.46±0.07; 3) ------unnamed (31.7±0.09) ------tuff of Axehandle Canyon (31.47±0.05; 13) ======unnamed (31.54±0.09) ------part of tuff of Bottle Summit (31.44±0.04) 32

32.6 unnamed (32.46±0.07) ? ? ? tuff of Hoodoo Canyon (32.57±0.08) = tuff of Dry Canyon (32.60±0.13) 32.9 Northumberland Tuff (32.91±0.05)

34 ======Caetano Tuff (34.00±0.05; 14) 34.0 tuff of Hall Creek (33.92±0.02; 3) 34.4 tuff of Cove Mine (34.44±0.02; 4) tuff of Dry Creek (34.66±0.03)

tuff of Mount Lewis (35.81±0.29) 36 Figure 3. Defi nite (=), probable (-), or possible (?) correlations of major ash-fl ow tuffs of the western Nevada volcanic fi eld. Position of names and extension of symbols beyond names (e.g., Caetano Tuff) diagrammatically indicate west-east distribution of tuffs. Names in bold are accepted formal names or best names based on dominant use. All but the tuffs of Perry Canyon, Gallagher Pass, and Mount Lewis are defi nite or probable caldera-forming units. The Monotony Tuff and probably the tuff of Dry Creek, which crops out only at the eastern edge of the western Nevada fi eld, erupted from calderas in the central Nevada fi eld (shown in italics). The Monotony, Nine Hill, and Underdown Tuffs and the tuff of Camp- bell Creek extend far into the central Nevada fi eld. All ages (in Ma) are sanidine 40Ar/39Ar based on Fish Canyon Tuff sanidine at 28.201 Ma (Kuiper et al., 2008), except mostly plagioclase on tuffs of Dogskin Mountain and Mount Lewis and U-Pb zircon on tuff of Job Canyon, and are followed by the number of dated samples (n = 1 if not specifi ed). Age of Underdown Tuff is from Best et al. (2013c).

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Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/9/4/951/3346482/951.pdf by guest on 29 September 2021 Ignimbrites of the western Nevada volcanic ﬁ eld ) continued ( Source of data Cousens et al. this study (2011); this study 1986a,1986b, Ekren et al. (1980); this study Whitebread et al. 2002); this study study 1996b); this study Byers (1985, 1986c) (1988) This study Bingler (1978a); Ekren et al., 1980 John (1992a) John (1992a); John (1992a) This study Shawe (1999a, McKee (1970); Bonham (1970); this John (1992a) John (1992a) Bingler (1978a); John (1992a) Henry (1996a, Ekren and z e t d r t s e y a n a r l l u a e e c t q l s d o a n n y h t e u k r e t quartz amme r b o pumice sphene sphene sphene ﬁ o Notable n I a Abundant Abundant A Abundant n biotite-rich Vermicular Vermicular m small black phenocrysts phenocrysts phenocrysts phenocrysts R A S Smoky quartz characteristics fayalitic olivine (sieve-textured) resorbed quartz # r r t z t x h , x z p p z, x p (z) Smoky quartz, p c s p all, z Smoky quartz a c ap, z) to 1.3 Other 0 to tr cpx–tr (all, ap, phenos cpx, sph) sph, (cpx, (all, ap, sph) § 0.4 0.4 0.2 0.8 0.5 0.2 (tr to 1.3) Opaques (0.1 to 0.8) (0.2 to 1.0) (0.1 to 0.2) (0.1 to 1.0) (0.1 to 0.3) § 7 . 1 2 o t r r o 1 t t t 0.4 0.1 1.1 0.1 0.3 0.0 5 . 8 . 0 0 0.0 to 2.9) (0.0 to 1.5) (0.0 to 0.4) (0.0 to 1.4) (0.0 to 0.3) (0.0 to 0.1) Hornblende s c i t s § i 3 9 r r . 2 1 t e o 1 8 t t r o o . o 2 t t t t c o 0 1.3 1.1 1.9 0.5 2.6 0.0 5 t r r a . t t 0 r Biotite 2 1 a (0.9 to 1.7) (0.2 to 2.1) (0.5 to 5.8) (0.1 to 1.0) (0.0 to 0.1) (0.9 to 3.1) h c e c s i § 3 a h . l p c 1 8 2 a 1 o r o o h 5 3 8 o t t g t t 9.7 2.5 4.0 4.9 r 12.8 20.3 13.1 o 5 1 r o 3 t . n (15 to 29) e 0 (2.2 to 3.1) (1.8 to 7.8) K-feldspar a (1.6 to 13.9) (0.0 to 10.3) P (11.2 to 15.4) (11.2 (11.8 to 14.4) (11.8 7 § 8 . 3 3 0 5 1 3 1 o o o 2 1 1 6 t t o t t 9.8 4.5 0.4 11.3 11.6 18.3 3 2 3 7 . 1 (<1 to 2) 1 (1.6 to 7.0) (0.0 to 0.9) 1 (6.7 to 15.7) (3.3 to 28.1) (6.8 to 32.1) (7.1 to 16.2) Plagioclase 7 § . 1 4 3 7 5 5 o . o o o 3 t t t t 1 0 6.5 6.3 3.4 3.1 5.1 12.3 14.5 r 1 1 5 t . 0 (10 to 19) (5.2 to 8.7) (0.1 to 9.0) (1.0 to 8.7) (2.7 to 4.3) (2.1 to 11.3) (9.5 to 15.3) Quartz § 5 5 5 5 2 1 5 1 4 0 2 o o o o o t t t t t 1 3 Total Total 4 2 4 0 5 2 to 27 2 1 1 2 (26 to 50) (5.9 to 7.3) (9.4 to 36.4) phenocrysts (28.0 to 35.3) (13.9 to 44.3) (28.9 to 32.0) (14.4 to 36.2) 2 2 2 66.5 66.5 7 30.6 1 2 436 436 1 21 28.9 14 33.0 19 28.0 29 29.1 samples Number of † 2 0.11 0.12 0.120.25 11 to 6 4 to 3 0.6 0.2 to 1.5 1.5 to 4.4 to 0.3 0 to tr 0 0 to 0.3 0.23 0.34 0.14 0.29 0.13 0.54 TiO zoned; 0.18 to 0.22 to 0.12 to 0.14 to 0.25 to 0.13 to 0.08 to 0.23 to 0.34 (top) 0.16 (base), n o i t i s o † 2 p 1 m 7 74 77 75 75 76 73 76.8 76.1 77.1 o 72 (top) C 74 (base), R Rock name* SiO TABLE 2. COMPOSITIONAL AND PETROGRAPHIC CHARACTERISTICS OF ASH-FLOW TUFFS IN THE WESTERN NEVADA VOLCANIC FIELD THE WESTERN NEVADA TUFFS IN ASH-FLOW AND PETROGRAPHIC CHARACTERISTICS OF 2. COMPOSITIONAL TABLE 7 5 3 3 . to . R-HSR 72 to R-HSR 72 HSR 73.8 to HSR 73.8 HSR 76.9 to HSR 76.9 HSR 74.4 0.28 28 to 44 1 to 11 9 to 22 5.5 to 13.2 1.7 to 2.5 0.3 to 1.8 to 0.9 0.3 sph, HSR 75.8 to HSR 75.8 . . 4 4 4 3 Age (Ma) 2 25.4 2 25.1 25.0 HSR 75.6 to 2 2 <24.3 R-TD 68 to Tuff Tuff Tuff Pass Lower HSR 77to 0.09 Tuff of Tuff of Tuff of Tuff of Tuff of Eureka Copper Toiyabe Canyon Goldyke Brunton Santiago Mine Tuff Mountain Bluestone Canyon(?) cooling unit Underdown Clipper Gap Blue Sphinx Canyon Tuff and subunits Other names n o y n a C e n i v a e P f o f tuff of Railroad Ridge(?) tuff tuff of Brunton Pass(?), tuff of Gabbs Valley(?), tuff of Camel Spring(?), tuff of Menter Canyon(?)), tuff Clipper Gap Goldyke Bluestone Mine Tuff(?), Tuff(?), Bluestone Mine of Copper Mountain – tuff of Copper Mountain – tuff of Toiyabe f u Tuff of Desatoya PeakTuff Lower Santiago Canyon Tuff – tuff – tuff Tuff Santiago Canyon Tuff of Arc Dome of Tuff Upper 25.2 R to 73 Tuff of Fairview PeakTuff 19.5 HSR 75 to Tuff of Sheep CanyonTuff of Ione Tuff Round Rock Formation 24.8 of – tuff Tuff Underdown R zoned; Blue Sphinx Tuff – tuff of – tuff Tuff Blue Sphinx Tuff (names, west to east) Fish Creek Mountains TuffFish Creek Mountains 24.9 HSR 74 to – Tuff Eureka Canyon T

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Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/9/4/951/3346482/951.pdf by guest on 29 September 2021 Henry and John ) continued ( Source of data this study (1985, 1986c); this study Hardyman et al. this study (1976, 1985, (1985,1986c); (1988); this study this study this study this study Deino (1985); this study this study Ekren and Byers (1985) 1986b); this study this study Ekren and Byers John (1992a) This study Ekren and Byers Ekren and Byers John (1992a) John (1992a) Riehle et al. (1972); John (1995); John (1995); This study John (1995, 1997); John (1995); Bingler (1978a); Deino (1985); John (1992a) Ekren et al. (1980); John (1995); Notable sanidine, sanidine, sanidine, sanidine, rich pumice rich pumice plagioclase, anorthoclase smoky quartz smoky quartz smoky quartz characteristics Three feldspars: c ﬁ # a 1 6 0 m o x o d t t (z) p e r all, z Iridescent ap, z Coarse biotite- (cpx) ) c z (all) Iridescent cpx tr ap, z, to 0.5 2 Other r t (cpx, z) phenos e (all, cpx) 0.8 to 1.4 t l ap, z, (cpx) Coarse biotite- a § 7 . continued 0 1 . tr o 0.3 0.5 0 0.7 0.7 0.5 0.4 t r t (tr to 0.8) (tr to 0.4) (tr to 0.1) Opaques (0.1 to 1.2) (0.1 to 1.5) (0.1 to 1.3) (0.3 to 1.0) (0.1 to 0.8) § 0 . 1 00.2 00.2 o 0.3 0.0 0.4 0.4 0.0 0.0 0.0 t 0 (altered) (0.0 to tr) (0.0 to 1.8) (0.0 to 1.3) (0.0 to 0.8) (0.0 to 0.2) (0.0 to 0.1) (0.0 to 0.2) Hornblende s c i t 6 4 s . . § i 1 2 r . . 4 0 e 0 0 1 t . o o t t c o o 0.0 2.0 3.4 0 2.5 0.4 0.1 0.4 t t a 8 2 r r r . . Biotite t t a (tr to 0.1) 1 0 (0.4 to 6.4) (0.1 to 5.3) (0.0 to 0.1) (1.4 to 3.1) (0.0 to 0.2) (0.0 to 1.7) (0.0 to 1.3) h c c i § h p 5 4 5 5 a 1 1 r 3 . o o 2tr 2tr t t g o o 6.1 t t 2.5 7.8 3 8.2 3.0 18.6 20.3 o 0 1 r 8 8 t e (1.2 to 5.3) (2.4 to 3.7) K-feldspar (1.2 to 14.0) (3.1 to 12.2) (0.7 to 15.1) (7.9 to 28.9) P (10.2 to 30.6) 5 § 6 1 2 2 1 3 r o . o t o t t t 0.4 3.9 5.6 2 4.4 1.8 23.2 19.0 17.9 r 0 t 6 1 (0.0 to 0.7) (0.4 to 9.1) (1.4 to 2.4) (5.5 to 36.8) (3.9 to 47.3) (0.0 to 10.2) Plagioclase (0.1 to 10.4) (15.4 to 22.5) ? ? § 7 5 5 . 9 . . 2 3 0 0 . o t o 2.3 0.2 3.7 0.5 4.0 0.8 2 o o t 13.6 15.7 t t 5 0 r r t t (tr to 0.6) (0.0 to 9.2) (2.0 to 8.9) (0.5 to 1.0) (0.0 to 2.0) (0.0 to 13.7) (8.1 to 19.4) (9.5 to 19.7) Quartz § 3 8 5 1 3 3 6 o o o o 8 t t t t Total Total 0 2 2 0 2 1 2 (1.3 to 6.4) (4.7 to 6.5) (0.3 to 10.8) phenocrysts (11.1 to 55.5) (11.1 (14.5 to 58.1) (30.9 to 53.5) (26.8 to 40.3) (22.6 to 49.1) 4 to 11 0.4 to 3.5 0.8 to 4.6 1.3 to 5.1 tr to 1.0 to 1.0 0 tr 31t11tr 13415tr171 86.3 86.3 53.2 53.2 1 39.6 18.3 0.0 21.0 0.0 0.0 0.4 all, z Iridescent 10 34.4 48 36.4 24 37.0 15 41.3 114 32.3 samples Number of † 2 0.11 0.28 0.16 0.44 0.67 0.21 0.67 0.44 0.21 0.18 0.16 0.35 0.13 0.21 TiO 0.08 to 0.21 to 0.10 to 0.13 to 0.35 to 0.32 to 0.16 to 0.06 to 0.27 to 0.09 to 0.14 to 0.13 to 0.07 to 0.09 to n o i t i s o † 2 p m 70 71 78 78 74 72 76 78 75 78 77.4 76.1 76.8 76.1 o C Rock name* SiO TABLE 2. COMPOSITIONAL AND PETROGRAPHIC CHARACTERISTICS OF ASH-FLOW TUFFS IN THE WESTERN NEVADA VOLCANIC FIELD ( THE WESTERN NEVADA TUFFS IN ASH-FLOW AND PETROGRAPHIC CHARACTERISTICS OF 2. COMPOSITIONAL TABLE R 70.3 0.4 1 25 5 12 8 2 ? HSR 75 to HSR 75 HSR 74 to HSR 74 HSR 75.5 to HSR 75.5 R-HSR 71.2 to R-HSR 71.2 R-HSR 73.5 to R-HSR 73.5 R-TD 64 to R-TD 64 HSR 75.8 to HSR 75.8 HSR 75 to HSR 75 R 73.7 0.18 3 5.9 HSR 74.8 to HSR 74.8 HSR 74 to HSR 74 R-TD 65 to R-TD 65 HSR 76.5 0.09 2 1.1 to 1.2 0.2 to 0.3 to 0.8 0.6 0.2 0 0 tr Age (Ma) 25.4 R 71 to 25.2 R-HSR 71.8 to 25.1 R 69 to Tuff Ridge (GV1) (GV3) (GV2) Tuff of Tuff of Tuff of Tuff of Tuff of Valley, Valley, Spring Range (GV1?) Springs Canyon Canyon Canyon Canyon Railroad Canyon, Canyon, Hercules Paradise Chimney lower unit lower unit upper unit upper unit New Pass Elevenmile Tuff of Poco Tuff Tuff of Poco Tuff Lower tuff of Lower tuff Upper tuff of Upper tuff and subunits Middle tuff of Middle tuff Other names Tuff of Green Tuff Gabbs Valley Gabbs Valley Gabbs Valley Gabbs Valley Gabbs Valley Gabbs Valley Nine Hill Tuff, Tuff, Nine Hill Nine Hill Tuff, Tuff, Nine Hill Tuff of Gabbs Tuff Camel Spring Tuff of Menter Tuff f f u T g ) n i r p S d e continued Bates Mountain Tuff Bates Mountain Bluestone Mine Tuff(?), Tuff(?), Bluestone Mine of Brunton Pass(?), tuff of Gabbs Valley(?), tuff of Poco Canyon – New Pass Tuff tuff of Camel Spring(?), tuff of Menter Canyon(?)), tuff of Railroad Ridge(?) tuff ( Nine Hill Tuff – unit D, Tuff Nine Hill Petriﬁ Tuff Eureka Canyon Tuff – Tuff Eureka Canyon Tuff of Elevenmile CanyonTuff of Lee Tuff (names, west to east) Tuff of Chimney Spring – tuff of Chimney Spring – tuff Tuff

960 Geosphere, August 2013

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/9/4/951/3346482/951.pdf by guest on 29 September 2021 Ignimbrites of the western Nevada volcanic ﬁ eld ) continued ( Source of data Faulds (2010); Hardyman et al. D.A. John, this study this study (1976) (1976); Ekren et al. (1980); this study 1994); Henry and this study 1994); Henry and this study (1976); this study 1994); this study 2008); Henry et al. (2012a) (1988) 2008); this study unpub. data) (1971); Henry et al. (2004); this study Shawe (1995); Boden (1986, 1992); this study this study Faulds (2010); Boden (1986) Boden (1986, 1992); Proffett and Proffett and Proffett Proffett and Proffett Proffett Boden (1986, 1992, Boden (1986, 1992, and Proffett Proffett Boden (1986, 1992, Brooks et al. (2003, Riehle et al. (1972); Brooks et al. (2003, John (1995); McKee and Stewart Boden (1986, 1992); Bingler (1978a); John (1995); quartz Notable characteristics (sieve-textured) # 3 , l 1 l , a 0.5 x (cpx) Vermicular ) to 0.5 Other p z, (ap) z, (ap) cpx 0.6 phenos c (0 to 1.2), all § continued 0.4 0.5 Opaques (0.0 to 0.9) (0.1 to 1.3) § r t o 1 tr t 0.4 0.4 0.0 0 sparse hypersthene (0 to 1.3) (0.0 to 1.3) (0.0 to 0.2) (0.0 to 0.4) (0.6 to 1.6) Hornblende s c i t s § i r e 5 t , 3 1 1 3 c 0.3 0.5 1.5 0.2 2 a r Biotite (2 to 3) (2 to 5) (tr to 3) (tr to 2) (tr to 1) 0.5 (top) a 1 (base) to (0.0 to 1.0) (0.6 to 2.9) (0.0 to 1.3) h c c i § h p a 1 r , 4 9 4 6 6 g 6.8 2.1 7.5 2 o 0 to 1 2.5 r 7 (top) (1 to 4) (1 to 7) t (4 to 12) (2 to 10) (3 to 10) e 5 (base) to K-feldspar (2.8 to 15.7) (1.8 to 15.0) P § 2 2 0 , 3 4 8 1 17 2.8 5.3 5 13.6 2 (1 to 5) (3 to 7) (2 to 17) (1.5 to 4) (11 to 23) (11 (12 to 25) to 3–5 (top) (4.7 to 27.0) (0.9 to 13.3) Plagioclase 12–15 (base) § 1 , 9 5 4 4 2 10 0.5 0.6 0 trace 20 (1 to 9) (2 to 8) (1 to 4) (7 to 13) (1.5 to 8) 3–4 (top) (0.0 to 1.6) (0.0 to 5.1) 1–2 (base) to Quartz § 8 3 6 3 , to 1 2 0 Total Total 3 (9 to 41) (7 to 22) (4 to 10) (15 to 30) (21 to 45) (15 to 28) 12–17 (top) (7.8 to 33.9) (4.0 to 26.9) phenocrysts 20–25 (base) 20–25 5 23.2 5 3 28 to 35 1 to 3 17 to 24 3 to to 3.5 3 to 1.5 1 cpx 0.2, opx 1 2 25 21.8 10 21 15 32 18 24 15 7.5 29 13.9 samples Number of † 2 0 1 . 6 . 0 0.52 0.44 0.50 0.21 0.58 0.14 0 TiO zoned; zoned; zoned; 0.39 to 0.63 to 0.12 to 0.31 to 0.09 to 0.14 to 0.20 (top) 0.26 (top) 0.47 (top) 0.50 (base), 0.15 (base), 0.18 (base), n o i t i s o † 2 7 p . 6 3 m 69 76 78 78 77 6 69.6 7 o 76 (top) 74 (top) 67 (top) C 75 (base), 77 (base), 72 (base), D R T Rock name* SiO HSR-R 72 to 0 1 TABLE 2. COMPOSITIONAL AND PETROGRAPHIC CHARACTERISTICS OF ASH-FLOW TUFFS IN THE WESTERN NEVADA VOLCANIC FIELD ( THE WESTERN NEVADA TUFFS IN ASH-FLOW AND PETROGRAPHIC CHARACTERISTICS OF 2. COMPOSITIONAL TABLE . R-TD 68 to R-TD 68 . TD 64 0.63 1 43 1.7 29 5.6 4.5 0.4 1.7 9 0 Age (Ma) 29.4 2 29.0 R 71.5 0.29 3 26.9 HSR-R zoned; 27.4 R-TD? 73 0.27 3 15 to 30 1 to 3 8 to 25 2 to 8 to 3 2 to 0.5 tr tr cpx, 29.4to R-TD 66.6 27.1 R-HSR zoned; 68 to 68 27.1 R-HSR zoned; 27.3to HSR 75 28.9to HSR 75 27.0 R-TD zoned; Tuff Tuff of Spring Canyon Canyon Canyon Ryecroft Mountain Upper tuff Upper tuff of Corcoran Intracalderato HSR-D 68 Mickey Pass and subunits Other names Singatse Tuff 69 0.47 1 38 5 23 7 2.5 Tuff of Logan Tuff Tuff of Round Tuff E f Canyon Round Mountain Canyon of Guild Mine Member) – of Mount lower tuff upper tuff of Mount upper tuff Jefferson unit C, Bates Mountain Tuff of McCoy Mine tuff of Guild Mine Member) – of Ryecroft Canyon – tuff of Logan Spring – tuff of Corcoran upper tuff Jefferson Heights Member) f u Tuff of Deep CanyonTuff of War Tuff of Corcoran Lower tuff T Singatse Tuff – tuff of – tuff Tuff Singatse Mickey Pass Tuff (main part Tuff Mickey Pass of Campbell Creek – Tuff of Dogskin Mountain – Tuff Lenihan Canyon Tuff – Tuff Lenihan Canyon Tuff (names, west to east) Mickey Pass Tuff (Weed (Weed Tuff Mickey Pass (basal part Tuff Mickey Pass of Job CanyonTuff Extracaldera ca.

Geosphere, August 2013 961

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/9/4/951/3346482/951.pdf by guest on 29 September 2021 Henry and John Source of data Boden (1992); this study (1969); Henry this study this study this study this study 1976); this study this study this study this study this study this study et al. (2004); this study Henry et al. (2004); Henry et al. (2004); Sargent and McKee Henry et al. (2004); Henry et al. (2004); McKee (1976); McKee (1974, John et al. (2008); John et al. (2008); John et al. (2008); John et al. (2008); John et al. (2008) Henry et al. (2004); Notable Smoky quartz Smoky quartz characteristics # r t x x p p c all, z all, z ) c Other 0 to tr 0 to tr 0 to tr phenos all, ap, z Smoky quartz all, ap, z Smoky quartz (ap, opx) (ap, opx) all, z (ap) Smoky quartz § continued 0.4 0.3 0.2 (tr to 1.1) (tr to 0.5) Opaques (0.0 to 2.0) (0.1 to 0.7) (0.0 to 0.4) § r t 0.8 0.1 0.3 0 to tr cpx 0 to tr cpx 0 to tr 02. 0 to 0.4 0.4 (0.0 to 0.3) (0.0 to 2.0) (0.0 to 2.3) Hornblende s c i t 5 s . § i 1 1 5 1 r r . 2 t e o o 2 o 5 t t t r r t . o o t t t t c o 0 6.2 1.8 2.4 2.1 1.6 1.8 2.4 1.6 5 5 t 1 a . . 5 0 r . < Biotite (0 to 3) 0 0 1 a (tr to 2.5) 1 (2.7 to 7.8) (0.6 to 4.2) (1.0 to 3.5) (1.2 to 3.5) (0.5 to 2.5) (0.6 to 3.3) h c c i § h 3 p 3 4 7 4 6 6 a o r t o o o o o o 5 t t t t t t g 6.1 4.4 4.1 5.4 3.9 5 11.7 13.3 10.5 10.4 o . 1 1 4 2 3 3 r (2 to 7) (2 to 8) (1 to 6) t (tr to 8) 2 e K-feldspar (2.5 to 14.9) (7.6 to 14.5) (6.6 to 14.2) (6.2 to 15.7) (9.2 to 14.9) P § 5 0 3 . 8 4 7 3 1 1 2 o o o o 3 t t t t o o o t t 6.8 t 21.7 12.7 13.6 16.9 16.8 6 2 4 2 8 4 (3 to 7) 2 (1 to 15) (2 to 15) (2 to 10) (8.0 to 19.4) (5.5 to 20.8) Plagioclase (14.3 to 23.3) (13.6 to 32.7) (12.3 to 20.9) § r r 4 2 t t r r r r o o o o t t t t tr 4.3 tr 7.2 tr t t t t 7.8 9.6 10.4 10.3 12.9 1 1 0 0 (0 to 1.5) (0 to trace) 7 (2.8 to 9.4) (4.9 to 16.7) (4.1 to 15.7) (6.4 to 17.9) (5.7 to 18.9) Quartz § 2 5 5 1 1 1 1 7 8 1 1 7 2 . o o o o o 8 o t t t 2 t o t t t 1 Total Total 0 0 5 6 1 8 9 1 1 1 (5 to 18) (5 to 21) (8 to 15) (5 to 12) phenocrysts (24.9 to 47.2) (33.2 to 48.0) (30.9 to 50.0) (31.7 to 43.0) (36.3 to 49.1) 2 6 3 3 3 25 to 2 1 to 20 16 1 to 3 to 4 2 0.1 to 0.2 cpx, opx 7 2 12513tr12tr 78.4 78.4 2 18 43.1 16 13.4 16 38.2 18 39.9 10 40.3 12 13.7 15 41.7 samples Number of † 2 0.45 0.33 0.26 0.41 0.17 0.25 0.27 0.23 0.15 0.30 0.18 TiO 0.25 to 0.28 to 0.10 to 0.20 to 0.20 to 0.05 to 0.10 to 0.14 to 0.18 to 0.48 and 0.16 and n o i t i s o † 2 p m 73 71 73 76 77 75 78 75 75 76 75 o C Rock name* SiO HSR-R 74 to 3 1 4 TABLE 2. COMPOSITIONAL AND PETROGRAPHIC CHARACTERISTICS OF ASH-FLOW TUFFS IN THE WESTERN NEVADA VOLCANIC FIELD ( THE WESTERN NEVADA TUFFS IN ASH-FLOW AND PETROGRAPHIC CHARACTERISTICS OF 2. COMPOSITIONAL TABLE . HSR 76 to HSR 76 . R72 to to R72 . to 0 0 1 Age (Ma) 3 3 3 30.6 r r r r r r e e e e e e p p p w w w 32.6and TD-R 67 30.4 31.2and HSR-R 73 p p p o o o U L L U U Lower L Upper lower unit lower unit Upper unit R to 72 and subunits Other names n o y n a C n e g l o n b y i r b n p a a r S C c s e e d v n r i o Numbers in italics are visual estimates from thin sections. a M C H f f f o o o Weight percent, major elements normalized to 100% volatile free. Weight trace. Mean and (range) in volume percent; tr, all—allanite; ap—apatite; cpx—clinopyroxene; opx—orthopyroxene; sph—sphene; tr—trace; z—zircon. f f Hoodoo Canyon **Feldspar phenocrysts are mostly or entirely anorthoclase. Edwards Creek Tuff – unit Tuff Edwards Creek Tuff B, Bates Mountain † § # part of Edwards Creek A, Bates – unit Tuff Mountain Tuff Note: TD, trachydacite. *D, dacite; HSR, high-silica rhyolite; R, f f f f u u u tuffs of Dry Canyon and tuffs Tuff of Cove MineTuff 34.4 R to 69 T Tuff of Axehandle Canyon of Tuff 31.5 RNorthumberland Tuff 68.5 to 32.9 HSR-R 74 to of Hall CreekTuff 34.0 HSR-R 72 to T Tuff of Rattlesnake Canyon – Tuff Tuff of Sutcliffe – part of of Sutcliffe Tuff Tuff (names, west to east) Caetano Tuff Extracaldera 34.0 HSR-R 71 to t

962 Geosphere, August 2013

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18 continued, eruption of voluminous silicic ash- Fairview Peak Northwest end of western fl ow tuffs resumed at ca. 37 Ma (Henry et al., 20 Nevada volcanic field 2010). Ignimbrite eruptions either initiated or 22 increased greatly at ca. 37 Ma in the southern Elevenmile Canyon Rocky Mountain, Mogollon-Datil, Trans-Pecos 24 Poco Canyon Desatoya Peak Fish Creek Mts Texas, and Sierra Madre Occidental fi elds 26 Nine Hill? Railroad Ridge (Lipman, 1992; Lipman and McIntosh, 2008; Chapin et al., 2004; McIntosh and Chapin, 2004; 28 Campbell Creek McDowell and McIntosh, 2012; Henry et al., Caldera Age (Ma) Job Canyon 30 2012b). However, major ash-fl ow volcanism Clan Alpine? began earlier, as early as 49 Ma in the Challis 32 volcanic fi eld of Idaho (Fisher et al., 1992; Hall Creek Caetano Janecke et al., 1997) and 45 Ma in the Sierra 34 Cove Mine? Madre Occidental (McDowell and McIntosh, 36 2012), and both the rhyolitic and more interme- Southwest Northeast diate magmatism in the Great Basin are proba- 20 Southeast end of western bly related to slab rollback (Henry et al., 2009a, Nevada volcanic field 2010). Ash-fl ow magmatism ended or paused at 22 Toiyabe least by 22 Ma in all fi elds and resumed only 24 Peavine in the Great Basin and Sierra Madre Occidental Manhattan (Henry et al., 2010; McDowell and McIntosh, 26 Round Mtn Upper Mt Jefferson 2012). Southwestward to westward migration of 28 Ryecroft Lower Mt Jefferson the front of magmatism continued to the pres-

Caldera Age (Ma) ent in the Great Basin, with establishment of Corcoran Canyon 30 the ancestral Cascades arc in western Nevada and the Sierra Nevada at least by ca. 20 Ma 32 Dry Canyon Northumberland (Cousens et al., 2008; Henry et al., 2009a; Vikre 34 and Henry, 2011; John et al., 2012) and as early Pancake Summit as 40 Ma in the Warner Range of northeastern 36 0100200California (Colgan et al., 2011a). Present-day Distance Perpendicular to Caldera Belt (km) Despite the continuity of magmatism, major caldera-forming eruptions were restricted to Figure 4. Ages of calderas plotted against distance along two areas northeast of the Walker Lane, a north- northeast-striking transects perpendicular to the caldera belt (see west-striking zone of late Cenozoic dextral slip Fig. 1 for location). Caldera magmatism migrated to the southwest (Fig. 1; Stewart, 1988; Faulds and Henry, 2008). through time along both transects but over a greater total distance Late Cenozoic faulting obviously could not along the northwest transect. Dashed lines show uncertainly located have infl uenced the location of middle Cenozoic calderas. calderas. However, the Walker Lane is interpreted to be inherited from a zone of Mesozoic strike-slip faults (Albers, 1967; Oldow, 1984, Magmatism during shallowing of the slab is rep- have dehydrated (English et al., 2003) and 1992; Stewart, 1988; Schweickert and Lahren, resented by scattered Late Cretaceous plutons in whether the great volumes of magma erupted 1990; Hardyman and Oldow, 1991), although northern Nevada (Barton, 1996). During this during rollback would require sources in addi- this interpretation is also contested (Proffett “fl at-slab subduction,” the slab is interpreted tion to mantle lithosphere, i.e., asthenospheric and Dilles, 2008). These older structures may to have followed the base of the lithosphere mantle (Farmer et al., 2008). have precluded caldera formation in the Walker instead of dipping into the asthenosphere. The Despite the relatively regular progression of Lane by weakening or otherwise modifying the relatively cool temperature of the shallow slab magmatism, composition and eruptive style var- crust so that mantle-derived magmas traversed and the absence of an asthenospheric wedge ied considerably across the Great Basin in space or accumulated in it differently. Although large- between slab and overlying mantle lithosphere and time (Henry et al., 2009a, 2010). Ceno- volume rhyolite magmatism and caldera collapse precluded melting. However, slab dewatering zoic magmatism began in northeastern Nevada stopped at the future northeastern boundary of is interpreted to have intensely hydrated (meta- ca. 46 Ma and was dominated by andesite and the Walker Lane, predominantly intermediate somatized) the mantle lithosphere, “priming” it dacite lavas and their intrusive equivalents magmatism characterized by stratovolcanoes for future magma generation (Humphreys et al., (Brooks et al., 1995; Henry and Ressel, 2000; and lava domes continued to migrate to the 2003) and mineralization (Richards, 2009; Theodore, 2000; Ressel and Henry, 2006). Only southwest, across the Walker Lane and into the Muntean et al., 2011). As the shallow slab rolled about fi ve voluminous, rhyolitic ash-fl ow tuffs Sierra Nevada (Dilles and Gans, 1995; Busby back and sank, asthenosphere welled up behind erupted between 45 and 40 Ma in northeastern et al., 2008; Cousens et al., 2008; Busby and it, heated the overlying, fertile, hydrated litho- Nevada (Castor et al., 2003; Henry, 2008). Putirka, 2009; Colgan et al., 2011a; Vikre and sphere, and generated voluminous magmas. Following a hiatus of ~3 Ma during which Henry, 2011; John et al., 2012). Questions about this overall mechanism include southwestward migration and intermediate- Although the greatest number and volume of the degree to which the cold, shallow slab would composition lava fl ow and dome magmatism rhyolite ignimbrite eruptions occurred between

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42° Warner Range 120° OR116° ID 112° 2012a) and the Nine Hill Tuff (Deino, 1989; CA 45? NV UT Best et al., 1989). The Weed Heights Member ? of the Mickey Pass Tuff apparently is reversely zoned from silicic dacite to high-silica rhyo- 34 lite (Table 2; Proffett and Proffett, 1976; Ekren 16 et al., 1980). 42 Phenocryst assemblages in rhyolitic ignimbrites of the western Nevada volcanic fi eld vary 40° 40 little (Table 2); quartz, plagioclase, and sanidine ? in variable proportions generally form >90% 38 of the total phenocryst populations. Biotite and lesser hornblende are the principal mafi c silicate 36 ? minerals, with minor clino- and orthopyrox- 34 ene present in some tuffs. Fe-Ti oxide miner- 32 als are ubiquitous. Anorthoclase is present in 30 a few tuffs, most notably the Nine Hill Tuff, 38° 26 which has three feldspar phenocrysts, sanidine, 24 anortho clase, and plagioclase. Sphene is a notable accessory phase in the Santiago Canyon Tuff Caldera (45–7.8 Ma) 16 Age contours (Ma): and its correlative units, the tuffs of Toiyabe and initiation of magmatism 12 Copper Mountain. Zircon, apatite, and allanite based on all rollback- are other common accessory phases. related magmatism 0 500 km Notably sparse in the western Nevada vol canic fi eld are “Isom-type” trachydacite ignimbrites that contain a higher-temperature, anhydrous Figure 5. Contours of age of initial, locally sourced magmatism showing southwesterly phenocryst assemblage of plagioclase, pyrox- migration of magmatism. Intermediate to silicic intrusive and effusive activity generally ene, and Fe-Ti oxides. Tuffs with Isom-type preceded caldera-forming ash-fl ow eruptions by 2–6 Ma in any area. For example, forma- characteristics but unknown sources that are tion of the 34 Ma Caetano caldera followed ~6 Ma of nearby, semi-continuous andesitic to probably in the western Nevada fi eld include non-explosive rhyolitic activity (John et al., 2009). Development of the 32.9 Ma Northum- (1) the Belleville Tuff in the Candelaria area berland caldera was preceded by andesitic and rhyolitic intrusions at 35.4 Ma (McKee, (Speed and Cogbill, 1979) and (2) the tuff of 1974, 1976; our unpublished data). Crow Springs in the Royston Hills (Whitebread and Hardyman, 1987). In contrast, ignimbrites of this type are widespread and voluminous in

37 and 22 Ma, major caldera-forming eruptions CO2. In this section, we describe the general the volcanic fi elds to the east and include tuffs of continued to ca. 14 Ma in the Caliente cal- characteristics of the ignimbrites and briefl y the Isom Formation that were erupted from the dera complex (Snee and Rowley, 2000) and to compare their characteristics to ignimbrites in Indian Peak–Caliente fi eld (Best et al., 2013b, 8 Ma in the southwestern Nevada volcanic fi eld the central Nevada and Indian Peak–Caliente 2013c). (Fig. 1; Sawyer et al., 1994). fi elds (Best et al., 2013b, 2013c). Geochemical Ignimbrites in the western Nevada fi eld are and petrographic features of ignimbrites related overwhelmingly rhyolite using the IUGS (Inter- OVERVIEW OF IGNIMBRITES to the Stillwater and Caetano calderas are dis- national Union of Geological Sciences) clas- cussed in more detail in the section about these sifi cation system (Le Maitre, 1989), although Composition and Phenocryst Assemblage complexes. a few tuffs are zoned to trachydacite or dacite Chemical analyses and petrographic data for (Fig. 6A). Most notable of the zoned tuffs is the Ignimbrites of the western Nevada vol canic most major ignimbrites in the western Nevada tuff of Eleven mile Canyon, whose silica content field are characterized by 460 whole-rock volcanic fi eld defi ne two major rock types: ranges from 64.3 to 74.5 wt%. Other zoned tuffs chemical analyses, including 98 new analyses phenocryst-rich (>20 vol%) and phenocryst- and those that consist of dacite or trachydacite and additional trace element data for 87 pub- poor (<15 vol%) rhyolites (Figs. 6A and 7; include the Nine Hill Tuff and tuffs of Sheep Can- lished analyses (Supplemental Table 33), and Table 2). Both types of rhyolitic tuff may be nor- yon, Ryecroft Canyon, Logan Spring, Corcoran by petrographic and modal data summarized in mally zoned upward from rhyolite or high-silica Canyon, Cove Mine, and Job Canyon and the Table 2. An additional ~100 chemical analyses rhyolite to trachydacite, although a distinct sub- upper unit of the tuff of Mount Jefferson (Fig. were discarded due to hydrothermal alteration, type of phenocryst-rich rhyolite, which includes 6A). Five analyses of the correlative tuffs of Dog- most commonly manifested by alkali gains the Caetano Tuff and the tuff of Poco Canyon, is skin Mountain and McCoy Mine, a widespread,

(K2O) and losses (Na2O) and/or addition of characterized by great intracaldera thicknesses large-volume, plagioclase- and biotite-rich unit, (>2000 m) of relatively homogeneous crystal- extend from trachydacite and dacite to low- 3 Supplemental Table 3. Chemical analyses of ash- rich (30–50 vol% phenocrysts) high-silica silica rhyolite with 66.6%–69.6% SiO2 (Fig. 6). fl ow tuffs in the western Nevada volcanic fi eld. If you rhyolite (see discussion of the Caetano and Still- The tuff of Dogskin Mountain–McCoy Mine is are viewing the PDF of this paper or reading it offl ine, please visit http://dx.doi.org/10.1130/GES00867.S3 water calderas). Phenocryst-poor rhyolites form the only ignimbrite of the western Nevada fi eld or the full-text article on www.gsapubs.org to view several exceptionally widespread units, includ- that could be a “monotonous intermediate” Supplemental Table 3. ing the tuff of Campbell Creek (Henry et al., (Hildreth , 1981). The tuff of Perry Canyon and

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11 11 A B I Indian Peak-Caliente tuffs n = 466 I C Central Nevada tuffs n = 433 I 10 Rhyolite 10 I I Trachydacite CI I CI I I I I I II I C I II II I I I I I I I I II I I I I C C I I I I III I I I I I I I II I I I II C II I C I I I C IC I I I I I I C C I I 9 9 I C I I C ICI I I C I I I L C C CI C C I CI C CI CC C G C C I C I CC C C C I I I I I C I II CC CC C C C C C IC I C C CCC C CI C I IC C CC C CI I IC CC G CC C I C C C C C C CCC I CCC CCC I C I I CC I C R CI I C CC C I CI I I C CCI CCCCRC IC RCC O (wt. %) O (wt. %) I I II I C C CCCCI C CC I CIRC 2 2 I I CI I I I C C I C IC C C CCC C C 8 8 I C CI C C I CICCCC C CCC I I C I C C CCC C C CCC C C II I CC C L I CI C CC CI C C C C C I I I C I I CCI C C I C C CCCCC C II I I C C I C C CCRCC I C CCCC C I I I II IC II CC C C C C C C Trachy- I I II I C I C I C I IC CC C I C O + K O + K I I C I CI C C C C I I C C C C 2 2 I I II I I C I C I CC CC C CC I I I CI II I C I CI I C I CC C I andesite I II II I I I I C CI C C I I I C I I II L I C C Na 7 Na 7 C I III I C I I I I I I C I C C C C I I I C I I II CI IIICI C I I C C C C C I I I I II I II I CI I I II I C C CC I CI II II II I II C I C C C C I I I I I I II I I I I I CC C I I II IIII I I I I I II II I CI IC C C I I I I III II I III I I I C I C II I I I II C I II I I I II I CC C I III I I I I I I I I I I I I C I I C I I I II I I I I CI C C I I I II I CI I I II CI I II C 6 Andesite Dacite 6 I I II I I I I C I I I II I CI C C C I I I I C C I I I I I I I I I I C C I 5 5 60 65 70 75 80 60 65 70 75 80

SiO2 (wt. %) SiO2 (wt. %) 7 10 C D

6 8 Alkalic 6 5 Shoshonitic Alkali-calcic O)-CaO (wt. %)

2 4 O (wt. %) 2

K 4 O+K 2

(Na 2 High-K Calc-alkalic 3

0 Medium-K Calcic 2 60 65 70 75 80 60 65 70 75 80

SiO2 (wt. %) SiO2 (wt. %) 1.1 E Western Nevada Volcanic Field tuffs (n=459) Other WNVF tuffs 1.0 Isom-type tuffs Ferroan (tholeiitic) Elevenmile Canyon caldera tuffs tuff of Campbell Creek 0.9 Nine Hill Tuff tuff of Dogskin Mountain tuff of Cove Mine 0.8 Caetano Tuff tuff of Job Canyon

FeO*/(FeO*+MgO) (wt. %) tuff of Sheep Canyon 0.7 tuff of Corcoran Canyon Poinsettia Tuff Magnesian (calc-alkaline) tuff of Perry Canyon 0.6 tuff of Gabbs Valley 60 65 70 75 80

SiO2 (wt %)

Figure 6. Chemical variation diagrams for whole-rock samples of ignimbrites in the western Nevada volcanic ﬁ eld (WNVF). All analyses normalized to 100% volatile free. Plotted data are tabulated in Supplemental Table 3 [see footnote 3]. (A) Total alkali-silica diagram. Field boundaries from Le Maitre (1989). (B) Total alkali-silica diagram for ignimbrites in the central Nevada and Indian Peak–Caliente ﬁ elds

from Best et al. (2013b, 2013c). (C) K2O-SiO2 diagram using classifi cation of Le Maitre (1989) and Ewart (1982). (D) Modifi ed alkali-lime index diagram using classifi cation of Frost et al. (2001). (E) Ferroan-magnesian classifi cation of Frost et al. (2001). FeO* is total Fe as FeO.

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9 Average Volume of Phenocrysts in Ignimbrites Western Nevada ignimbrites have several of the Western Nevada Volcanic Field (38 units) distinct characteristics relative to their eastern 8 contemporaries, most notably the aforemen- 7 tioned dearth of dacite and Isom-type compositions that characterize large parts of the volcanic 6 fi elds to the east (Fig. 6B; see Best et al., 2013b, 5 2013c). Western Nevada volcanic fi eld ignimbrites are relatively enriched in alkalis, whereas 4 calcic compositions are common in tuffs in the 3 other two fields (K-rich Isom-type tuffs in the eastern fi elds, which have alkali-calcic to Number of Ignimbrites 2 shoshonitic compositions, are notable excep- 1 tions). In addition, a larger proportion of the western Nevada ignimbrites are ferroan with 0 0–5 5–10 10–15 15–20 20–25 25–30 30–35 35–40 40+ higher FeO*/(FeO* + MgO) than typical rhyo- Total Phenocrysts (volume %) lites in the eastern fi elds (Fig. 6E). In contrast, in the central Nevada fi eld more than half of Figure 7. Histogram showing average volume percent of total pheno- the ignimbrites are magnesian and in the Indian crysts in major ignimbrite units in the western Nevada volcanic Peak–Caliente fi eld virtually all but the Isom fi eld. Diagram constructed using modal data in Table 2. Formation tuffs are magnesian. The western Nevada ignimbrites, including sparse Isom-type units, are notably enriched in Pb and U and have the Poinsettia Tuff Member of the Hu-pwi Rhyo- and Poco Canyon calderas; this variation likely lower Th and Th/U than the eastern ignimbrites dacite are small-volume dacitic ignimbrites prob- refl ects crystal fractionation. The tuffs have a (Fig. 9). These characteristics also likely refl ect ably related to intermediate lava domes. large range in Ba/Nb from ~1–180 that generally differences in underlying types of crust. The western Nevada ignimbrites mostly form decreases with increasing silica content and sug-

a high-K series based on their K2O and SiO2 gests fractionation of alkali feldspar in the more Volumes contents (Fig. 6C). Several units, including the silicic rocks, which also are notably depleted in trachy dacitic parts of tuffs related to the Eleven- Ba and Sr. The samples with lowest Ba/Nb values At least four ignimbrites in the western mile Canyon caldera and the Nine Hill Tuff, (<5) include the lower unit of the Nine Hill Tuff Nevada volcanic ﬁ eld (Caetano Tuff, tuffs of are shoshonitic. On the basis of Na, K, and Ca and the most highly evolved parts of the Caetano Campbell Creek and Elevenmile Canyon, and contents, most western Nevada volcanic ﬁ eld Tuff and tuffs related to the Poco Canyon caldera the correlative Mickey Pass Tuff [main part ignimbrites are calc-alkalic to alkali-calcic (Fig. (Supplemental Table 3 [see footnote 3]). of the Guild Mine Member] and lower tuff of 6D). The Job Canyon and Elevenmile Canyon The western Nevada ignimbrites are generally Mount Jefferson), have estimated volumes caldera tuffs and the Nine Hill Tuff are alkali- similar compositionally to rhyolite ignimbrites >1000 km3 and qualify as resulting from super-

calcic. The Nine Hill Tuff is Na2O rich and in the central Nevada and Indian Peak–Caliente eruptions (de Silva, 2008; Miller and Wark, CaO poor, whereas the Caetano Tuff is CaO ﬁ elds to the east. All have high concentrations of 2008) (Tables 1 and 2). However, based on their

rich (Fig. 8). Other tuffs with trachydacite or K2O and are calc-alkalic to alkali-calcic. How- wide distributions, several other tuffs, including dacite compositions are calc-alkalic. The west- ever, ignimbrites in the western Nevada fi eld the Nine Hill Tuff, the tuffs of Axehandle and ern Nevada volcanic fi eld ignimbrites are nearly have relatively high total alkali contents com- Rattlesnake Canyons, and the correlative tuffs of evenly split between ferroan (tholeiitic) and pared to contemporary ash-fl ow tuffs elsewhere Dogskin Mountain and McCoy Mine, likely also magnesian (calc-alkaline) compositions using in the Great Basin (Figs. 6A and B). Because have volumes >1000 km3. The volumes of many

their Fe-Mg contents (Fig. 6E). Several units, overall concentrations of K2O vary little through- ignimbrites and the total volume of the ignim- including the Elevenmile Canyon caldera tuffs, out the Great Basin ignimbrite province with brites in the western Nevada volcanic fi eld can- have trachydacitic compositions with ferroan the exception of K-rich Isom-type ignimbrites, not be estimated with available data due to large characteristics and rhyolitic compositions with the increased alkalinity of the western Nevada uncertainties in caldera dimensions, amount magnesian characteristics. volcanic fi eld ignimbrites relative to their east- of collapse, and thickness of intracaldera tuff; Primitive mantle-normalized trace element ern counterparts is due to their generally higher channelization of outfl ow tuff by paleotopog-

patterns for western Nevada tuffs generally show Na2O contents (Fig. 8), a trend also mimicked in raphy; and uncertainty in regional correlations. notable depletions in high-fi eld-strength ele- lava fl ows in the western Nevada fi eld (see sec- ments (HFSE, Ti, Nb, Zr) and P suggestive of tion below on lava fl ows and domes), and may DESCRIPTION OF MAJOR fractionation of Fe-Ti oxides, zircon, and apa- refl ect different types of crust (accreted oceanic IGNIMBRITES AND THEIR CALDERAS tite that are characteristic of subduction-related and transitional crust) underlying the west- magmas (Fig. 9; e.g., Wood et al., 1979; Gill, ern Nevada fi eld. With the exception of highly This section briefl y summarizes character- 1981; Pearce et al., 1984; Hildreth and Moor- evolved, high-silica rhyolite parts of several istics of individual tuffs, known and suspected bath, 1988). Large-ion lithophilic elements (LIL, ignimbrites in the western Nevada volcanic fi eld, correlations, and what is known about calderas, Rb, Ba, K, Pb, U, Th, Sr) are variably enriched, most of the rhyolites in all 3 fi elds have trace organized by age groups beginning with the old- although trace element contents vary signifi - element patterns and ratios of LIL/HFSE (e.g., est. Many published geologic maps assigned cantly within zoned ignimbrites, such as the Ba/Nb > 15) similar to those characteristic of local stratigraphic names to tuffs before cor- Caetano Tuff and tuffs related to the Elevenmile subduction-related rhyolites elsewhere. relation was possible between different areas.

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19 5

4 17

3 (wt. %)

3 15 O 2

Al 2 FeO* (wt. %)

13 1

11 0 2 5

CaO (wt. %) 2 MgO (wt. %)

0 0 5 0.8

0.7

0.6 4 0.5

0.4 (wt. %) O (wt. %) 2 2 TiO Na 0.3 3 0.2

0.1

2 0.0 0.3 60 70 80 SiO2 (wt. %) Western Nevada Volcanic Field tuffs (n=459) Other WNVF tuffs 0.2 Isom-type tuffs Elevenmile Canyon caldera tuffs tuff of Campbell Creek (wt. %) 5 Nine Hill Tuff O 2

P tuff of Dogskin Mountain 0.1 tuff of Cove Mine Caetano Tuff tuff of Job Canyon tuff of Sheep Canyon tuff of Corcoran Canyon 0.0 Poinsettia Tuff 60 70 80 tuff of Perry Canyon

SiO2 (wt. %) tuff of Gabbs Valley

Figure 8. Harker variation diagrams for whole-rock samples of western Nevada volcanic ﬁ eld (WNVF) ignimbrites. All analyses normalized to 100% volatile free. Plotted data are tabulated in Supplemental Table 3 (see footnote 3). FeO* is total Fe as FeO.

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1000 1000

Indian Peak-Caliente-Central Nevada ignimbrites 100 100

10 10 Rock/Primitive Mantle 1 1 Caetano Tuff upper unit upper lower unit Tuff of Campbell Creek lower lower unit .1 .1 Rb Ba Th U K Nb La Pb Ce Sr Nd P Sm Zr Ti Y Rb Ba Th U K Nb La Pb Ce Sr Nd P Sm Zr Ti Y

1000 1000

100 100

10 10 Paradise Range tuffs tuff of Toiyabe

Rock/Primitive Mantle tuff of Sheep Canyon tuff of Goldyke 1 Stillwater caldera complex 1 lower white tuff Poco Canyon caldera biotite-lithic tuff Elevenmile Canyon caldera tuff of Gabbs Valley Job Canyon caldera .1 .1 Rb Ba Th U K Nb La Pb Ce Sr Nd P Sm Zr Ti Y Rb Ba Th U K Nb La Pb Ce Sr Nd P Sm Zr Ti Y

1000 1000

100 100

10 10 Rock/Primitive Mantle 1 1

Isom-type tuffs Nine Hill Tuff

.1 .1 Rb Ba Th U K Nb La Pb Ce Sr Nd P Sm Zr Ti Y Rb Ba Th U K Nb La Pb Ce Sr Nd P Sm Zr Ti Y

Figure 9. Primitive mantle-normalized trace element diagrams for whole-rock samples of tuffs in the western Nevada volcanic ﬁ eld. Shaded outline shows all tuffs in the central Nevada, Indian Peak, and Caliente ﬁ elds from Best et al. (2013b, 2013c). Primitive mantle composition from Sun and McDonough (1989). Plotted data are tabulated in Supplemental Table 3 (see footnote 3).

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For that reason, the number of names greatly voluminous tuff of this episode is the ~1100 km3, chemically similar to low-silica parts of the exceeds the number of separate voluminous 34.0 Ma Caetano Tuff (John et al., 2008). Most of Caetano Tuff, is exposed locally north and tuffs. Tables 1 and 2 summarize compositional, this phenocryst-rich rhyolite accumulated within west of the Caetano caldera. The possible petrographic, age, and volume data. Figures its caldera, where it is as much as 4 km thick and location of the Cove Mine caldera is based on 10 and 11 show tuff distributions and calderas normally zoned from high- to low-silica rhyolite the distribution of the tuff and an exploration divided into 34.4–26.8 Ma and 25.4–19.5 Ma (see section on the Caetano and Stillwater cal- drill hole that penetrated more than 300 m of segments, respectively. Supplemental Table 1 deras). Outfl ow tuff is found ~5 km to the east Caetano-like tuff in Reese River Valley north (see footnote 1) provides 40Ar/39Ar ages for in the Cortez Mountains, 50 km to the west in of the Caetano caldera (P.J. Dobak, Barrick most tuffs. the Tobin Range, and 40 km to the south inside the Exploration Inc., 2012, personal commun.). Hall Creek caldera (Gonsior, 2006; Gonsior and This thick tuff may lie inside a Cove Mine 34.4–34.0 Ma Dilles, 2008; John et al., 2008; this study). caldera because (1) the tuff of Cove Mine was originally considered to be the Caetano Tuff, Caetano Tuff and Caldera (34.0 Ma) Tuff of Cove Mine (34.4 Ma) (2) the drill hole is in the middle of the distri- The oldest ignimbrites of the western Nevada The 34.4 Ma, low-silica rhyolite tuff of bution of the tuff of Cove Mine, and (3) out- fi eld are in its northeastern part (Fig. 10). The most Cove Mine, which is petrographically and fl ow Caetano Tuff is generally no more than

120° 118° Cove Mine? 116° 0 100 km ? ? Sh tuff of Campbell Creek 120 km to northeast Caetano Sierra Nevada Nevada ? California F 40° e

? d

v i

d St ? o e ? l ? Job a Canyon CA P Hall Creek ~ ? ? no known outflow Reno ? Deep CCanyon B ? ? D Dogskin V Campbell Mtn? Creek S

Te To ? 39° C ? Northumberland ? Si Lower Mount Jefferson P Dry Upper GV Canyon Corcoran ? W G Canyon ? Round T Mtn Ryecroft Western, Distal Outflow = Intracaldera or Proximal Outflow Canyon Petrified Spring Tuff Ro 26.9 Singatse Tuff = tuff of Round Mountain E 27.0 Lenihan Canyon Tuff = upper tuff of Mount Jefferson ? 27.1 Mickey Pass Tuff (Weed Heights Mbr) Pa 27.3 Mickey Pass Tuff (upper Guild Mine Mbr) = lower tuff of Mount Jefferson 27.4 Mickey Pass Tuff (lower Guild Mine Mbr) = tuff of Ryecroft Canyon ? 28.838° tuff of Winnemucca Ranch 29.0 tuff of Deep Canyon CH 28.9 tuff of Campbell Creek = C unit, Bates Mtn Tuff 29.2 tuff E = unnamed R 29.4 tuff of Job Canyon 29.5 tuff of Dogskin Mtn = tuff of McCoy Mine 30.0 lower tuff of Corcoran Canyon 32.6 unnamed tuff = tuff of Dry Canyon = tuff of Hoodoo Canyon 30.1 tuff of Mine Canyon = part of 30.3 tuff of Cove Spring = 32.9 Northumberland Tuff Edwards Caetano Tuff 30.5 tuff of Sutcliffe = B unit, Bates Mountain Tuff Creek Tuff 34.0 31.2 tuff of Rattlesnake Canyon = A unit, Bates Mountain Tuff 34.0 tuff of Hall Creek 31.4 tuff of Hardscrabble Canyon = unnamed 34.4 tuff of Cove Mine 31.5 tuff of Axehandle Canyon = unnamed 34.7 tuff of Dry Creek

Figure 10. Calderas and tuffs formed during the 34.4–34.0, 32.9–32.6, 31.5–28.8, and 27.4–26.8 Ma intervals. Known locations of outﬂ ow deposits are shown as colored symbols, and approximate margins of associated calderas use the same color. Intracaldera tuff crops out extensively in all calderas. Calderas not erupting during the time intervals covered by this ﬁ gure are in light red. Blue lines and arrowheads are paleovalleys as in Figure 1. B—Bates Mountain; C—Carson Range; CA—Clan Alpine Mountains; CH—Candelaria Hills; D—Desa- toya Mountains; E—Excelsior Mountains; F—Fish Creek Mountains; G—Gillis Range; GV—Gabbs Valley Range; P—Paradise Range; Pa—Pancake Range; R—Reveille Range; Ro—Royston Hills; S—Shoshone Mountains; Sh—Shoshone Range; Si—Singatse Range; St— Stillwater Range; T—Toquima Range; Te—Terrill Mountains; To—Toiyabe Range; V—Virginia Range;W—Wassuk Range.

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~50km 120° 118° 116° 0 100 km ? Sh Fish Creek Mts

Sierra Nevada Nevada California F 40° e

? d

v i

d St ? o e ? l

? a Poco CA P ~ Nine Hill ? Canyon Reno ? Railroad ? Ridge B ? D Elevenmile Desatoya Peak V Canyon ? no known outflow Fairview Underdown Peak S ~70km

Te no known outflow ? To ? 39° C ? Arc Dome ? ~150km

Si P ? Toiyabe W G GV T

Manhattan Ro Peavine Western, Distal Outflow = Intracaldera or Proximal Outflow no known no known outflow 19.5 tuff of Fairview Peak E outflow 23.3 Santiago Canyon Tuff = tuff of Copper Mountain = tuff of Toiyabe 24.3 Blue Sphinx Tuff ? tuff of Goldyke Pa 24.5 tuff of Peavine Canyon 24.738° tuff of Desatoya Peak 24.8 Round Rock Formation (Manhattan caldera) CH 24.9 Fish Creek Mts Tuff 24.9 Underdown Tuff = tuff of Clipper Gap R 25.1 tuff of Brunton Pass 25.2 tuff of Arc Dome 25.1 Eureka Canyon Tuff = part of tuff of Gabbs Valley ? tuff of Railroad Ridge 25.2 tuff of Gabbs Valley (Paradise Range) tuff of Camel Spring 25.3 tuff of Menter Canyon 25.1 tuff of Painted Hills ? tuff of Elevenmile Canyon 25.3 tuff of Chimney Spring = tuff of Poco Canyon = New Pass Tuff 25.4 Nine Hill Tuff = D unit, Bates Mountain Tuff

Figure 11. Calderas and tuffs formed during the 25.4–23.3 and 19.5 Ma intervals. Explanation and location abbreviations are given in Figure 10.

250 m thick (in Golconda Canyon) and vir- 32.9 and 32.6 Ma Smoky Valley—we estimate the caldera had an tually unknown north of the Caetano caldera area of ~180 km2. Neither the amount of collapse (John et al., 2008). Northumberland Tuff and Caldera (32.9 Ma) nor thickness of intracaldera tuff are known, but Two rhyolitic tuffs erupted at 32.9 and 32.6 collapse of at least 1 km seems a minimum, so Tuff of Hall Creek and Caldera (34.0 Ma) Ma from calderas in the Toquima Range (Fig. the erupted volume is probably at least 180 km3. The other defi nitely caldera-forming unit in 10). The 32.91 ± 0.05 Ma Northumberland Tuff this group is the sparsely porphyritic, 34.0 Ma is phenocryst-rich, high-silica rhyolite contain- Tuff of Dry Canyon and Caldera (32.6 Ma) tuff of Hall Creek. The tuff of Hall Creek is cur- ing abundant quartz and sanidine and minor The 32.60 ± 0.13 Ma tuff of Dry Canyon rently only known within its caldera (Stewart biotite and plagioclase (McKee, 1974, 1976). erupted from a caldera that is largely overprinted et al., 1977), where it is at least 600 m thick, Although McKee (1976) did not map any out- by younger calderas of the Toquima caldera contains megabreccia along its southern and fl ow Northumberland Tuff, the tuff of Stone- complex (Figs. 10 and 12; Boden, 1992; Shawe, northern caldera margins, and is cut by two berger Canyon, which he mapped as occupying a 2002). Phenocrysts in the fi ne-grained, moder- sets of rhyolite domes of distinctly different >250-m-deep paleovalley 7–15 km northeast of ately porphyritic tuff are mostly plagioclase and age: one is ca. 33.5 Ma and possibly related the caldera, is probably correlative based on its biotite with lesser quartz, sanidine, hornblende, to the magma erupted as tuff, and the other ca. similar coarse-grained phenocryst assemblage, and clino- and orthopyroxene. The exposed 25 Ma and clearly unrelated (J.P. Colgan and both tuffs being high-silica rhyolites, close spa- intracaldera tuff contains abundant blocks up to C.D. Henry, unpub. reconnaissance mapping). tial association, and its coarsely lithic character 300 m in diameter, mostly of Paleozoic sedimen- The 10 × 20 km size of the caldera and ≥600 m that suggests a proximal deposit. Based on the tary and Cretaceous granitic rocks, and pinches thickness of intracaldera tuff suggest a mini- exposed eastern half of the Northumberland out abruptly against Paleozoic rocks at its east- mum volume of 100 km3. caldera—the western half is buried beneath Big ern caldera wall (Boden, 1992; Henry, 1997).

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H95-120 27.31±.07 P 0 5 10 km Q x H95-121 27.18±.09 036 mi P lower Mount Jefferson caldera H94-13 Tmj H94-14 x x 27.38±.08 Q Q 27.29±.08 H94-39 Trc 26.93±.05 Tl H94-15 x27.28±.06 Moores Creek H94-38 Tj 27.31±.10 Tmj Q H94-36 xH94-37 26.96±.08 27.25±.11 upper Mount Jefferson H95-113 caldera 27.51±.25 Figure 12. Simpliﬁ ed geologic H94-35 Tj 26.91±.10 map of the Toquima caldera Tj x H94-4 x F04-134 complex showing recognized 26.94±.08 27.04±.05 Tc H94-33 and probable calderas (from 26.96±.09 Boden, 1986, 1992; Shawe, 38° 45' Dry Canyon H94-3 Tj 1995, 1998, 1999a, 1999b, 2002; H94-2 27.03±.12 caldera 32.60±.13 Shawe and Byers, 1999; Shawe Round x Mountain Td x H95-116 et al., 2000; Henry et al., 1996; caldera H94-26 26.99±.07 27.04±.10 x Trc Trc this study). Note that (1) Ceno- H95-118 Tg 27.30±.08 zoic magmatism started with Tr H94-31 intrusion of granodiorite ~4 Ma 26.86± 27.30±.10 Corcoran 0.02 (5) x Canyon caldera before any caldera-forming x Tmj Tc H95-114 ash-ﬂ ow eruptions, (2) caldera- Kg 30.01±.08 H94-28 forming magmatism lasted 27.25±.08 P H94-29 Tr Ryecroft Q ~8 Ma, and (3) ring-fracture 27.49±.10 caldera x H94-30 intrusions of both the lower and P 27.43±.09 upper Mount Jefferson cal deras are indistinguishable in age Kg Q Quaternary undivided from their intracaldera tuffs. Trc tuffs of Road Canyon 22-26? Ma Tm Round Rock Fm, Manhattan Caldera 24.80±0.11 Ma Manhattan Tr tuff of Round Mountain 26.86±0.02 Ma (n=5) caldera Tj upper tuff of Mount Jefferson 27.01±0.05 Ma (n=4) Tmj lower tuff of Mount Jefferson 27.25±0.04 Ma (n=5) Tm Tl tuff of Logan Spring 27.38±0.08 Ma (n=1) H94-43 x Tr tuff of Ryecroft Canyon 27.43±0.09 Ma (n=1) 24.66±.07 Tc tuff of Corcoran Canyon 30.01±0.08 Ma (n=1) Td tuff of Dry Canyon 32.60±0.13 Ma (n=1) H94-42 Tg Granodiorite and related dikes 34-37 Ma x 24.80±.11 Ring-fracture intrusions upper Mount Jefferson caldera 26.95±0.03 (n=5) lower Mount Jefferson caldera 27.30±0.01 (n=4) Caldera margin P Kg Cretaceous granite P Q P Paleozoic rocks High-angle normal fault 38° 30' Kg 117° 00' x Sample location and age

The tuff of Hoodoo Canyon of McKee (1976), to the west may also be outfl ow (Figs. 3 and 10). the Stillwater–Clan Alpine–Desatoya region which crops out in and around the Northumber- Given the incomplete exposure of the caldera, no in the western part of the volcanic fi eld (Fig. land caldera to the north, is probable outfl ow tuff estimates of area or volume are possible. 10). These are the tuffs of Axehandle Canyon, correlative with the tuff of Dry Canyon based Hardscrabble Canyon, Rattlesnake Canyon, on indistinguishable 40Ar/39Ar age (32.57 ± 31.5–28.8 Ma Sutcliffe, Cove Spring, Mine Canyon, Dogskin 0.08 Ma), sanidine K/Ca, and phenocryst assem- Mountain, Job Canyon, tuff E, Campbell Creek, blage. A petrographically similar tuff also with At least eleven major rhyolitic tuffs and Deep Canyon, and Winnemucca Ranch. All an indistinguishable age (32.46 ± 0.07 Ma; one trachydacitic tuff erupted between 31.5 except the tuffs of Job Canyon and Deep Can- H01-70) and sanidine K/Ca in the Wassuk Range and 28.8 Ma, probably all from calderas in yon were fi rst mapped as outfl ow deposits in

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western Nevada and the eastern Sierra Nevada (Garside et al., 2005; Henry and Faulds, 2010; Monotony caldera in the Pancake Range (Sup- (Brooks et al., 2003, 2008; Henry et al., 2004, Henry et al., 2012a). plemental Fig. 1 [see footnote 4]; Ekren et al., 2009b; Faulds et al., 2005a, 2005b; Hinz et al., The ca. 30.5 Ma tuff of Sutcliffe and ca. 1974; Best et al., 1989) and spread mostly east- 2009) and then correlated eastward into the cal- 29.5 Ma tuff of Dogskin Mountain–McCoy ward but also westward at least to the Cande- dera belt on the basis of stratigraphy, phenocryst Mine are composite units consisting of petro- laria Hills, where it was mapped as the 27.65 ± assemblage, age, sanidine K/Ca, and for some graphically similar tuffs that were initially 0.05 Ma tuff of Miller Mountain (Petronis et al., of the units, composition and paleomagnetic mapped as single tuffs in western Nevada 2009; Page, 1959; Speed and Cogbill, 1979; data (Gromme et al., 1972; Hinz et al., 2009; (Henry et al., 2004). The large age range, varia- Stewart, 1979). Correlation is based on similar- C.S. Gromme, 2010, written commun.). Com- tion in phenocryst assemblage and abundance in ity in phenocryst assemblage and abundance, positions of glass shards further supported the the tuff of Sutcliffe (although some variation is a and compositions of biotite and hornblende phe- correlations and extended the range of the tuffs result of zoning), and, most importantly, separa- nocrysts (Best et al., 2013c, their fi gure 33). Ash of Axehandle Canyon, Rattlesnake Canyon, tion of cooling units by sedimentary sequences fl ows of the Monotony Tuff also spread south- and Campbell Creek farther west in the Sierra up to 10 m thick in western Nevada (Henry et westward into what is now a restricted military Nevada (Cassel et al., 2009a). The tuff of Job al., 2009b) indicate that the tuffs of Sutcliffe and range (Ekren et al., 1971). Fridrich and Thomp- Canyon is the oldest recognized caldera-form- Dogskin Mountain were multiple eruptions pos- son (2011) identifi ed a tuff in the Funeral Moun- ing tuff in the Stillwater caldera complex (John, sibly over longer time spans than most caldera- tains in California at about 36°37′N, 116°45′W, 1995; Hudson et al., 2000), and the tuff of Deep forming units. as Monotony based on petrographic character- Canyon is an intracaldera deposit in the Clan Sources in the Clan Alpine–Desatoya region istics. The voluminous (~4700 km3; Best et al., Alpine Mountains (Hardyman et al., 1988). are interpreted mostly on the basis of recogni- 2013c), crystal-rich Monotony Tuff is dacite to Important correlations include those of tion of several defi nite calderas (for the tuffs of low-silica rhyolite that contains mostly plagio- the tuffs of Rattlesnake Canyon, Sutcliffe, Job Canyon, Campbell Creek, and Deep Can- clase and biotite, with minor sanidine and and Campbell Creek with the A, B, and C yon), recognition of probable calderas (for the quartz. It is the only tuff erupted from the cen- units, respectively, of the Bates Mountain tuff of Dogskin Mountain and tuff E), and tuff tral Nevada volcanic fi eld known to have spread Tuff (Fig. 3; Gromme et al., 1972; John et al., distribution. The tuffs are restricted to the east- widely into western Nevada. 2008; Henry et al., 2012a). Additionally, the west belt of paleovalleys, which indicates they tuffs of Sutcliffe, Cove Spring, and Mine Can- must have erupted from sources in that belt, 27.4–26.85 Ma yon probably comprise at least parts of the which can only be in the western part of the undivided Edwards Creek Tuff (McKee and western Nevada volcanic fi eld. Numerous voluminous rhyolitic to trachy- Stewart , 971; Stewart et al., 1977). McKee Volumes of most of these tuffs are poorly dacitic ash-fl ow tuffs erupted from the Toquima and Stewart (1971) noted similarities between known but probably all are large. Based on the caldera complex between 27.4 and 26.85 Ma in parts of the Edwards Creek and Bates Mountain area of its caldera and probable amount of col- one of the most intense outpourings of tuff in Tuffs. The tuff of Bottle Summit mapped at the lapse, the tuff of Campbell Creek probably had the western Nevada volcanic fi eld (Figs. 3, 10, south end of Bates Mountain (McKee, 1968) is a volume of at least 1200 km3 and possibly as and 12). The younger calderas are well exposed, the easternmost known outcrop of the tuff of much as 3000 km3 (Henry et al., 2012a). The but these calderas and Basin and Range faulting Axe handle Canyon. Based on their distinctive tuff of Job Canyon has a minimum volume of largely overprint the older calderas. As with the pheno cryst assemblages of abundant plagio- 240 km3 but could be much larger because the tuffs of the 31.5–28.8 Ma episode, these tuffs clase and biotite, the tuffs of Dogskin Mountain estimate is based on only one slice through crop out far to the west. However, because the and McCoy Mine also correlate (McKee and the caldera. caldera sources were farther south, their outfl ow Stewart, 1971; Mark Hudson, 2002, personal The only tuff in the 31.5–28.8 Ma age range tuffs are similarly to the south. Outfl ow tuffs of commun.; our data). not erupted from the Clan Alpine region is the the two episodes mostly do not overlap. Notable about most of these tuffs is their wide 30.01 ± 0.08 Ma lower tuff of Corcoran Canyon Figure 13 shows our proposed correlation distribution from known or probable sources in the Toquima Range (Boden, 1992; Shawe of tuffs from the Toquima caldera complex to ~200 km westward to the Sierra Nevada and et al., 2000; Shawe, 2002). This tuff is as much the westernmost outfl ow. Uncertainties remain, variably eastward toward and, at least for the as 2 km thick and contains megabreccia in its and some proposed correlations undoubtedly tuff of Campbell Creek, across the interpreted ~15 km2 outcrop area, which is probably a rem- will prove incorrect. Many of the tuffs of this paleo divide (Fig. 10; Supplemental Fig. 14; nant of its source caldera mostly overprinted by episode were mapped in considerable detail in Henry et al., 2012a). The tuffs fl owed, were younger calderas of the Toquima caldera com- the Toquima caldera complex (Boden, 1992; deposited, and are preserved in paleovalleys plex (Boden, 1992; Shawe et al., 2000). The tuff Shawe et al., 2002), in the Gillis and Gabbs as much as 1.2 km deep in western Nevada has not been recognized outside this area, and Valley Ranges (Hardyman, 1980; Ekren and no outfl ow tuff is known. Given the very incom- Byers, 1985, 1986a, 1986b, 1986c; Ekren et al., 4Supplemental Figure 1. Distributions of the tuff plete exposure of the caldera, no estimates of 1980), and from the Wassuk Range westward of Campbell Creek, Monotony Tuff (only its west- area or volume are possible. to the Singatse Range (Proffett and Proffett, ern extent; see Best et al. [2013c] for full extent), 1976; Bingler, 1977; Proffett, 1977; Proffett and Nine Hill Tuff, and Underdown Tuff–tuff of Clipper Gap and their known or suspected calderas. These Monotony Tuff from Central Nevada Dilles, 1984; Dilles and Gans, 1995) to western- tuffs have wider distributions than other tuffs, and (27.6 Ma) most Nevada (Bingler, 1978a, 1978b). The out- all crossed the paleodivides proposed by Best et al. fl ow tuffs were relatively well correlated in these (2013a) and Henry et al. (2012a). If you are view- The 27.58 ± 0.05 Ma (n = 2, Supplemen- early works, but their sources in the Toquima ing the PDF of this paper or reading it offl ine, please visit http://dx.doi.org/10.1130/GES00867.S4 or the tal Table 1 [see footnote 1]; 27.57 ± 0.04 Ma caldera complex have only recently been rec- full-text article on www.gsapubs.org to view Supple- [n = 9], Best et al., 2013c) Monotony Tuff of the ognized and understanding of the complex is mental Figure 1. central Nevada volcanic fi eld erupted from the still evolving (Henry and Faulds, 2010). The

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Yerington and Wassuk Range Nine Hill Paleovalley Gillis and (Proffett and Proffett, 1976; Toquima Caldera and western Nevada Bingler, 1978b; Gabbs Valley Ranges (Bingler, 1977, 1978a; Proffett and Dilles, 1984; (Hardyman, 1980; Paradise Range Complex Henry and Faulds, 2010) Dilles and Gans, 1995) Ekren et al., 1980) John (1992a) (Boden, 1992; Santiago Shawe, 2002; Canyon Tuff not present? tuff of tuff of Toiyabe Henry et al., 1996) Copper Mountain (23.31±0.05) (23.27±0.05) ? tuff and breccia of Hu-pwi Rhyodacite Gallagher Pass tuff of Brunton Pass Blue Sphinx Tuff Blue Sphinx Tuff (25.12±0.06) upper (24.29±0.05) tuff of Arc Dome lower (24.31±0.05) (25.18±0.06) tuff of ? dacite tuff Gabbs Valley tuff of Gabbs Valley Red tuff (25.16±0.03) ? (25.15±0.06) Lower white tuff (25.22±0.02) tuff of Camel Spring tuffs of (25.31±0.07) Eureka Canyon Tuff Road Canyon Eureka Canyon Tuff ? tuff of Menter Canyon (25.06±0.06) Eureka Canyon Tuff (25.42±0.07) Nine Hill Tuff tuff of Green Springs Member Three tuff of Chimney Spring Nine Hill Tuff (25.29±0.07) unit 10 Petrified Springs Tuff Mine Tuff ? Nine Hill Tuff Bluestone tuff of (25.49±0.06) Singatse Tuff ? Round Mountain unit 9 (26.84±0.06) Singatse Tuff (26.86±0.02; n=5) unit 8 (26.86±0.05)

unit 7 tuff and correlates with upper tuff of ? sediments Lenihan Weed Heights Canyon Tuff Mount Jefferson (27.01±0.05; n=4) unit 6 Member ? (27.11±0.06 27.12±0.06) Lenihan Canyon Tuff unit 5 (26.94±0.06) Heights Mbr Weed sediments Guild Mine lower tuff of Member Mount Jefferson unit 4 (27.25±0.04; n=5) upper unit (27.26±0.05) (27.29±0.07)

Mickey Pass Tuff Mickey Pass unit 3 Mickey Pass Tuff Mickey Pass (27.30±0.05)

unit 2

(27.30±0.06) Benton Springs Group Guild Mine Mbr

Mickey Pass Tuff Mickey Pass ? ? present, also tuff of Logan Spring tuff of lower unit unit 1 not mapped (27.48±0.06) (27.35±0.06) and upper tuff of Ryecroft Canyon separately Corcoran Canyon (27.43±0.09)

Figure 13. Proposed and possible correlations of ash-ﬂ ow tuffs between western Nevada and the Toquima caldera complex. See text for comprehensive discussion. 40Ar/39Ar dates in regular type from Henry et al. (1996), Henry and Faulds (2010), and this study. 40Ar/39Ar dates in italics from Dilles and Gans (1995).

number, distribution, and sources of tuffs in the and variably revises their studies. However, vol- (2000) reported an age of 27.52 ± 0.05 Ma on the Toquima caldera complex remain incompletely canic stratigraphy, structure, and evolution in the upper tuff of Corcoran Canyon. Both tuffs are understood. Further mapping, chemical analy- Toquima Range are complex, and this summary thick, megabreccia-containing units in the south- ses, and paleomagnetic data to test correlation is far from the last word. eastern part of the complex, almost certainly are needed. in caldera remnants. Shawe and Byers (1999) The Toquima caldera complex was indepen- Tuffs of Ryecroft Canyon, Corcoran Canyon, and Shawe et al. (2000) reasonably interpreted dently mapped in two monumental efforts by and Logan Spring (27.4 Ma) a Ryecroft Canyon caldera to extend beneath Boden (1986, 1992) and D.R. Shawe and col- The oldest tuffs of the Toquima caldera com- Monitor Valley immediately east of these out- leagues (Shawe, 1995, 1998, 1999a, 1999b, plex crop out in small areas that are mostly crops (Fig. 12). The 27.38 ± 0.08 Ma tuff of 2002; Shawe and Byers, 1999; Shawe et al., remnants of their older calderas (Fig. 12). The Logan Spring is a thin, presumably outﬂ ow 2000). Named units are almost identical between lower tuff of Ryecroft Canyon (Boden, 1992; deposit that underlies the lower tuff of Mount the two projects, but their maps differ consider- Shawe and Byers, 1999) has sanidine 40Ar/39Ar Jefferson in the northeastern part of the complex ably in many areas. Our work (Henry et al., ages of 27.43 ± 0.09 Ma (this study) and 27.48 ± (Boden, 1992; this study). All three tuffs are 1996, 1997; Henry, 1997; this study) builds upon 0.03 Ma (Shawe and Byers, 1999). Shawe et al. petrographically similar , crystal-rich, low-silica

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rhyolites or trachydacites containing abundant 0.02 Ma, n = 5), sanidine K/Ca, and phenocryst many calderas in the western Nevada volcanic plagioclase and biotite, with lesser sanidine, assemblage of the quartz- and sanidine-rich fi eld (Henry, 1996a, 1996b; John et al., 2008). quartz, and other mafi c minerals (Table 2). None outfl ow tuff. Chemical analyses and paleomag- We interpret the Lenihan Canyon Tuff of of the three tuffs occur together in a stratigraphic netic data to test the correlation would be use- westernmost Nevada to be the correlative out- section, so all could be the same tuff. ful. The caldera has an area of ~400 km2. With fl ow tuff (Fig. 13). Correlation is based on a We interpret the lowermost part of the Mickey an exposed thickness of 1300 m of intracaldera similar phenocryst assemblage, age (26.94 ± Pass Tuff of western Nevada (Gillis, Gabbs Val- tuff, the erupted volume is at least 520 km3. Total 0.06 Ma, n = 1), and sanidine K/Ca. However, ley, Singatse, and Virginia Ranges) to be outfl ow caldera collapse of 3–4 km, typical of calderas Lenihan Canyon Tuff has not been recognized tuff correlative with one or more of the Toquima in Nevada and the western U.S. (Lipman, 1984, elsewhere over a distance of ~200 km between plagioclase-biotite tuffs (Figs. 10 and 13). 2007; John, 1995; John et al., 2008), suggests a western Nevada and the Toquima Range (Fig. This correlation is based on the indistinguish- volume of 1200–1600 km3. This large volume is 10). About half of the intervening area has been able age, sanidine K/Ca, and plagioclase- and consistent with the great extent and thickness of mapped in suffi cient detail that the tuff might biotite-rich, quartz- and sanidine-poor pheno- outfl ow tuff (Fig. 10). be recognized, but much of the mapping com- cryst assemblage of all the tuffs. Given the very bined many tuffs into units with local names incomplete exposure of the caldera(s), no esti- (Fig. 12). For example, the tuffs of Royston Weed Heights Member of the Mickey Pass mate of area is possible. However, the great out- Hills and Cedar Mountains in the Royston Hills Tuff (27.1 Ma) fl ow distribution suggests at least one eruption are phenocryst-rich rhyolites with K-Ar ages The Weed Heights Member is outfl ow tuff was voluminous. between 29 and 26 Ma (Whitebread and Hardy- recognized only in the Singatse, Wassuk, Gillis , man, 1987). These tuffs could correlate with and Gabbs Valley Ranges (Proffett and Proffett, Lower Tuff of Mount Jefferson and Caldera almost any of the tuffs of the Toquima caldera 1976; Ekren et al., 1980; Proffett and Dilles, (27.3 Ma) complex. In the Paradise Range, the 25.4 Ma 1984). Ages from near the base and near the What we term the lower tuff of Mount Jeffer- Nine Hill Tuff, there called the tuff of Green top in the Singatse Range are 27.12 ± 0.06 and son and its lower Mount Jefferson caldera are the Springs, is the oldest exposed Tertiary volcanic 27.11 ± 0.06 Ma, respectively (this study). The most voluminous tuff and largest caldera of the unit in the paleovalley that crosses the central Weed Heights Member is as much as 180 m Toquima caldera complex (Fig. 12; Table 1). Our part of the range (Fig. 13; John, 1992a). thick in the Singatse Range and 120 m thick in lower tuff of Mount Jefferson combines the tuff The caldera for the upper tuff of Mount Jef- the Gillis Range. The tuff is apparently reversely of Moores Creek, the lower tuff of Mount Jeffer- ferson is irregularly nested inside the lower zoned, as indicated by both phenocryst assem- son, the lower tuff of Trail Canyon, and the upper Mount Jefferson caldera (Fig. 12). Total area is blage and three chemical analyses, from low- tuff of Ryecroft Canyon of Boden (1992) and ~200 km2. Cross sections by Boden (1992) indi- silica rhyolite at the base to high-silica rhyolite parts of the tuffs of Ryecroft Canyon and Antone cate a probable intracaldera thickness of 1.5 km, near the top (Proffett and Proffett, 1976; Ekren Canyon and megabreccia of Meadow Canyon so a minimum erupted volume of 300 km3. et al., 1980). The source is unknown but could and possibly part of the principal member of be in the Toquima Range as indicated by its tem- the tuff of Mount Jefferson of Shawe (1999b; Tuff of Round Mountain, Correlative Singatse poral tie to other Toquima units (Fig. 13). Shawe et al., 2000). This correlation is based Tuff, and Caldera (26.9 Ma) on distribution of thick (≥1300 m), petrographi- The tuff of Round Mountain fi lls the eastern cally similar, breccia-rich, intracaldera tuff with Upper Tuff of Mount Jefferson and Caldera part of its caldera in the southwestern part of the indistinguishable ages (27.25 ± 0.04 Ma, n = 5) (27.0 Ma) Toquima caldera complex (Fig. 12). The tuff and association with rhyolite domes, also with The upper tuff of Mount Jefferson encom- hosts one of the world’s largest epithermal gold- indistinguishable ages (27.30 ± 0.01 Ma, n = 4), passes the upper and capping members of the silver deposits (Ferguson, 1921; Tingley and along the caldera margin (Fig. 12). The domes tuff of Mount Jefferson of Boden (1986, 1992) Berger, 1985; Shawe et al., 1986; Mills et al., were emplaced almost immediately after ash- and the principal member of the tuff of Mount 1988; Sander and Einaudi, 1990; Henry et al., fl ow eruption and caldera collapse and pre- Jefferson of Shawe (1999b, 2002) and Shawe 1996). Mining and drilling reveal two cooling sumably tapped the same magma chamber that et al. (2000). Correlation is based on similar units, a lower one more than 600 m thick that erupted tuff. zoned phenocryst assemblage, composition, wedges out abruptly against pre-Cenozoic rocks The lower tuff of Mount Jefferson contains age (27.01 ± 0.05 Ma, n = 4), and association against the eastern caldera topographic wall, ~25% phenocrysts dominated by quartz and with rhyolite domes of indistinguishable age and an upper one ~390 m thick that laps onto the sanidine with lesser plagioclase and biotite (26.95 ± 0.03 Ma, n = 5) along its caldera mar- caldera wall; both contain megabreccia (Boden, (Table 2). It is high-silica rhyolite that is not gin (Fig. 12). The upper tuff of Mount Jeffer- 1992; Henry et al., 1996). Phenocrysts in both obviously compositionally zoned but basal parts son is irregularly zoned upward from less to units consist of subequal amounts of plagio- are not exposed and exposed sections have not more abundantly porphyritic, with increases in clase, sanidine, and quartz, with lesser biotite, been suffi ciently examined to evaluate zoning. plagioclase, biotite, hornblende, and clinopy- Fe-Ti oxides, zircon, apatite, and, locally, horn- The lower tuff of Mount Jefferson is distinct in roxene and decreases in quartz and sanidine, blende. The upper unit is zoned upward in phe-

phenocryst assemblage, composition, and age and from rhyolite with 75% SiO2 to trachydacite nocryst abundance, which increases from ~10%

from the older plagioclase-biotite tuffs and the with 67% SiO2 (Table 2; Boden, 1986, 1987, to 40%, and from high-silica rhyolite (77 wt%

younger upper tuff of Mount Jefferson. 1994). The uppermost parts of the upper tuff of SiO2) at its base to rhyolite (73 wt% SiO2) at its The main part of the Guild Mine Member of Mount Jefferson (capping members of Boden, top (Boden, 1986). 40Ar/39Ar dates on sanidine the Mickey Pass Tuff of western Nevada is prob- 1992) consist of several cooling units separated from both units are indistinguishable and aver- ably the outﬂ ow equivalent of the lower tuff of by tuffaceous sedimentary deposits. The implied age 26.86 ± 0.02 Ma (n = 5). Mount Jefferson (Figs. 10 and 13). Correlation change from continuous to apparently small, We interpret the Singatse Tuff, which is is again based on indistinguishable age (27.28 ± discontinuous eruptions is characteristic of widespread in western Nevada (Singatse to

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Gabbs Valley Ranges), to be correlative outﬂ ow neously because they form a single cooling unit A date on the upper tuff of Poco Canyon is tuff (Figs. 10 and 13). Correlation is based on in most places. Also, the lower, sparsely por- 25.26 ± 0.07 Ma, and on the New Pass Tuff at similar phenocryst assemblage and identical phyritic phase commonly contains abundantly New Pass is 25.32 ± 0.06 Ma. The New Pass 40Ar/39Ar ages on two samples from the north- porphyritic pumice. However, both phases have Tuff nicely illustrates the distinctly elliptical ern Wassuk Range (26.84 ± 0.06 Ma on a lower a basal vitrophyre, indicating a slight cool- distribution, preferentially west of the caldera, cooling unit, and 26.86 ± 0.05 Ma on the main ing break, at Haskell Peak, a distal location in of many western Nevada tuffs (Fig. 11). upper cooling unit). The only chemical analy- the eastern Sierra Nevada (Brooks et al., 2003, sis of Singatse Tuff, from the Singatse Range, 2008). The Nine Hill Tuff is unusual among Tuff of Elevenmile Canyon and Caldera

has 69% SiO2 (Proffett and Proffett, 1976), middle Cenozoic (36–18 Ma) cooling units in (25.1 Ma) less silicic than analyses of the upper unit at the Great Basin, because it contains phenocrysts The phenocryst-rich tuff of Elevenmile Can- Round Mountain, which may indicate that less- of three feldspars—plagioclase, sanidine, and yon erupted from the Elevenmile Canyon cal- evolved parts were not preserved or sampled anorthoclase (Deino, 1985). Its high concen- dera in the southern Stillwater Range (John, in the Toquima Range. The Petrifi ed Spring trations of both Zr and Nb are also unusual for 1995). Additional mapping since 1995 indicates Tuff, a local unit in the Gabbs Valley and Gillis tuffs in the Great Basin ignimbrite province. that the Elevenmile Canyon caldera extends Ranges (Ekren et al., 1980), may be an upper Deino (1985, 1989) demonstrated that the Nine eastward into the southern Clan Alpine Moun- cooling unit of the Singatse Tuff, equivalent to Hill Tuff and the D unit of the Bates Mountain tains, thus making it one of the largest calderas the upper cooling unit east of Yerington. The Tuff (Sargent and McKee, 1969; Gromme et al., in the western Nevada volcanic fi eld (Fig. 11; Petrifi ed Spring Tuff everywhere occurs with 1972) in central Nevada are the same, based on John, 1997; this study). Intracaldera tuff is more and almost everywhere rests on Singatse Tuff distinctive composition, age, and paleomagnetic than 3400 m thick in the Stillwater Range and (Fig. 10), but intermediate lavas separate the direction. Because further data have strength- as much as 2000 m thick with abundant mega- two in a few places. The Petrifi ed Spring Tuff ened the correlation of these two units, the breccia in the southwestern Clan Alpine Moun- has a similar phenocryst assemblage as the Sin- formal Nine Hill Tuff name (Bingler, 1978a) tains. The tuff of Lee Canyon, which overlies gatse Tuff, but neither an age nor a chemical should be used. Its source has not been veri- the tuff of Elevenmile Canyon in the Stillwater analysis is available. fi ed but is interpreted to be concealed beneath Range, and the tuff of Hercules Canyon, which So little of the Round Mountain caldera is the Carson Sink based on its distribution, thick- overlies Elevenmile Canyon in the Clan Alpine exposed that no estimates of size or volume are ness, and facies variations (Deino, 1985; Best Mountains, are late eruptions probably of the possible. Proffett and Proffett (1976) reported a et al., 1989). Elevenmile Canyon magma system (John, 1995, volume of 3500 km3 for the Singatse Tuff but 1997). All three tuffs are petrographically simi- provided no supporting data. Regardless, the New Pass Tuff and Correlative Tuffs of Poco lar (Table 2). The volume of erupted tuff from great outfl ow distance suggests a moderately Canyon and Chimney Spring, and Poco the Elevenmile Canyon caldera is probably voluminous tuff. Canyon Caldera (25.3 Ma) ≥1050–1400 km3. The 25.27 ± 0.05 Ma (n = 7) New Pass Intracaldera tuff of Elevenmile Canyon in 25.4–23.3 Ma Tuff is a petrographically distinctive, high- the southern Clan Alpine Mountains is 25.07 ± silica rhyolite that is widespread in western 0.06 Ma, and the tuff of Hercules Canyon there Nevada (Fig. 11). The tuff was recognized and is 25.05 ± 0.06 Ma. The tuff of Elevenmile Ash-fl ow eruption and caldera collapse named from New Pass in the northern part of Canyon in the Stillwater Range is altered and resumed in the western Nevada volcanic fi eld the Desatoya Mountains (McKee and Stewart, undated, but the overlying tuff of Lee Canyon is at 25.4 Ma, following a hiatus of ~1.4 Ma, and 1971; Stewart et al., 1977). Deino (1989) cor- 25.00 ± 0.06 Ma. proceeded essentially continuously to 23.3 Ma related the tuff of Chimney Spring of west- A major stratigraphic and 40Ar/39Ar age (Fig. 3). Calderas formed along the entire ernmost Nevada with the New Pass Tuff, and conundrum remains about the New Pass Tuff northwest-southeast length of the belt and, with John (1992b, 1995) correlated both with the and tuff of Elevenmile Canyon. In the Still- one exception, are along its southwestern edge upper tuff of Poco Canyon, the intracaldera water Range, the sequence consists of lower (Fig. 11). tuff in the Poco Canyon caldera in the Still- tuff of Poco Canyon, tuff of Elevenmile Can- water Range. Intracaldera tuff of Poco Canyon yon, megabreccia of Government Trail Canyon, Nine Hill Tuff (25.4 Ma) is as much as 2500 m thick and is underlain by upper tuff of Poco Canyon (New Pass Tuff), and The 25.38 ± 0.08 Ma (n = 12; Supplemen- a megabreccia unit of high-silica rhyolite (the tuff of Lee Canyon (John, 1995). At New Pass, tal Table 1 [see footnote 1]) Nine Hill Tuff, the megabreccia of Government Trail Canyon) up the New Pass Tuff consists of a single cooling most widespread tuff of the Great Basin and one to 1800 m thick that is inferred to be geneti- unit that is overlain by an undated tuff that is of the world’s most widely distributed tuffs, led cally related to the tuff of Poco Canyon (John, petrographically similar to the tuffs of Eleven- off this episode. The Nine Hill Tuff crops out 1995; see section below on Caetano and Still- mile Canyon, Lee Canyon, and Hercules Can- from near Ely on the east to the western foot- water calderas). Correlation of the three named yon. In western Nevada, the tuff of Chimney hills of the Sierra Nevada on the west (Deino, tuff units is based on distinctive phenocryst Spring, correlative with the New Pass Tuff, is 1989; Best et al., 1989; Fig. 11; Supplemental assemblage (abundant adularescent sanidine also a single cooling unit and is overlain by the Fig. 1 [see footnote 4]). The Nine Hill Tuff con- and smoky quartz), composition (74–78% tuff of Painted Hills. Henry et al. (2004) inter- 40 39 sists of two petrographic and chemical phases, SiO2), Ar/ Ar age, sanidine K/Ca, and paleo- preted the tuff of Painted Hills to correlate with a lower, sparsely porphyritic high-silica rhyolite magnetic direction (Deino, 1989; John, 1995; the tuff of Elevenmile Canyon on the basis of that defi nes its full distribution and an upper, Hudson et al., 2000; Henry and Faulds, 2010). similar phenocryst assemblage, 40Ar/39Ar dates moderately porphyritic low-silica rhyolite that is Five 40Ar/39Ar dates in western Nevada and the (25.05 ± 0.06 and 25.06 ± 0.06 Ma, on two sam- restricted to the central part (Table 2). The two eastern Sierra Nevada average 25.26 ± 0.05 Ma ples of tuff of Painted Hills), and sanidine K/Ca. phases were probably emplaced almost simulta- (Brooks et al., 2008; Henry and Faulds, 2010). Alternatively, the tuff of Painted Hills could

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correlate with either of the tuffs of Lee Canyon and 24.70 ± 0.05 Ma, determined at the New front of the Shoshone Mountains, suggesting the or Hercules Canyon. The apparent contradiction Mexico Tech and Berkeley Geochronology labs caldera is at least that wide. The Bonita Canyon between fi eld relations and 40Ar/39Ar ages indi- (A.L. Deino, 2012, written commun.), respec- Formation, a >500-m-thick sequence of epi- cates the need for further study. tively. The match in ages demonstrates the age clastic rocks interbedded with thin air-fall and discrepancy is not analytical and that the two ash-fl ow tuffs and local megabreccia blocks of Eureka Canyon Tuff, Bluestone Mine Tuff, tuffs are of different age. The ages are also con- pre-Tertiary rocks, overlies the Underdown Tuff, Tuff of Gabbs Valley, Paradise Range Tuffs, sistent with stratigraphic relations in the north- extends 7 km farther north, and lithologically and Railroad Ridge Caldera (25.4–25.1 Ma) ern Shoshone Mountains as the younger tuff resembles caldera-fi ll sequences elsewhere, pos- A group of sparsely porphyritic rhyolite tuffs overlies the 25.0 Ma Underdown Tuff. The Arc sibly suggesting an even larger caldera. with similar ages (ca. 25.4–25.1 Ma) and pheno- Dome source caldera is located on the basis of cryst assemblages crops out extensively from increasing unit thickness, lithologic diversity, Fish Creek Mountains Tuff and Caldera western Nevada to the Paradise Range and Sho- hydrothermal alteration, a negative gravity (24.9 Ma) shone Mountains (Figs. 3 and 11). From west anomaly, and the presence of nearby dikes and The 24.91 ± 0.03 Ma (n = 3) Fish Creek to east, these include the Eureka Canyon Tuff rhyolite domes (Brem et al., 1991). The tuff of Mountains Tuff crops out mostly inside its (Bingler, 1978a; Henry and Faulds, 2010), tuffs Arc Dome was probably moderately volumi- caldera in the northeastern part of the western in the heterogeneous Bluestone Mine Tuff near nous, but its limited westward distribution sug- Nevada fi eld, where it is anomalously young Yerington (Proffett and Proffett, 1976; Proffett gests a smaller volume than many other tuffs. compared to surrounding magmatism (McKee, and Dilles, 1984), tuffs in the heterogeneous tuff 1970; Cousens et al., 2011; Figs. 4, 5, and 11). of Gabbs Valley (Ekren et al., 1980), the tuffs Underdown Tuff and Caldera and Tuff of Outfl ow tuff has been found in a paleo valley of Menter Canyon, Camel Spring, Gabbs Valley, Clipper Gap (25.0 Ma) ~10 km west of the caldera (Gonsior and Dilles, and Brunton Pass in the Paradise Range (John, The 24.96 ± 0.05 Ma (Best et al., 2013c), 2008). The abundantly porphyritic, high-silica 1992a; Fig. 13), and the tuff of Union Canyon sparsely porphyritic Underdown Tuff is recog- rhyolite contains abundant smoky quartz and in the Shoshone Mountains (Whitebread et al., nized only as an intracaldera deposit along the sanidine, minor plagioclase, and a trace of 1988). Specifi c correlations between these tuffs eastern edge of the Shoshone Mountains (Fig. biotite (Table 2). The caldera is distinctly non- are incompletely resolved. The Nine Hill Tuff is 11; Bonham, 1970). However, several outfl ow resurgent; gently inward-dipping tuffaceous part of this suite, and its presence in the Blue- tuffs are probable or possible equivalents. Based sediments occupy an intracaldera basin. The stone Mine Tuff, in the tuff of Gabbs Valley, on analyses of three intracaldera samples (Sup- caldera is also remarkably untilted and therefore and as the tuff of Green Springs in the Paradise plemental Table 3 [see footnote 3]), the Under- unextended, occupying a domain of low exten- Range is now certain. The Eureka Canyon Tuff down Tuff is high-silica rhyolite with distinctive sion separating two highly extended domains to is defi nitely present throughout much of the high Y (36–42 ppm) and Nb (28–31 ppm) con- the west and east (Colgan et al., 2008; Fosdick region, but its greater phenocryst abundance tents. The tuff of Clipper Gap, widely distrib- and Colgan, 2008; Gonsior and Dilles, 2008; makes correlation with any of the tuffs in the uted east of the caldera (Fig. 11; Supplemental Colgan and Henry, 2009; see section on the Paradise Range uncertain. Henry and Faulds Fig. 1 [see footnote 4]), is probably correla- Caetano caldera). McKee (1970) estimated a (2010) suggested correlation of the Eureka Can- tive outfl ow tuff based on similar age (24.87 ± total volume of 310 km3. yon Tuff with the Railroad Ridge caldera in the 0.10 Ma, Supplemental Table 1 [see footnote 1]; Clan Alpine Mountains, but the differences in 24.95 ± 0.07 Ma, Best et al., 2013c), phenocryst Round Rock Formation and Manhattan distribution and phenocryst abundance make assemblage, and chemical composition, includ- Caldera (24.8 Ma) this correlation unlikely, despite the age match. ing unusually high Nb (Best et al., 2013c; this The 24.80 ± 0.14 Ma Round Rock Forma- If none of these tuffs correlate with the tuff of study). The “red tuff” of John (1992a) in the tion and its source Manhattan caldera are the Railroad Ridge, then that caldera has no recog- Paradise Range may also be correlative based youngest ignimbrite and caldera identifi ed in nized outfl ow tuff. The distribution of this group on similar phenocryst assemblage and chemi- the Toquima Range (Fig. 11; Shawe, 1999a). of tuffs suggests source calderas in or around the cal composition, especially high Y (37 ppm) The tuff is normally zoned rhyolite contain- Toiyabe Range. and Nb (35 ppm). Two other tuffs have simi- ing sparse phenocrysts of plagioclase, quartz, lar sparse phenocryst assemblages, have high sanidine, biotite, and hornblende that increase Tuff of Arc Dome and Caldera (25.2 Ma) Y and Nb, and are in reasonable stratigraphic in abundance upward (Shawe, 1999a). No cor- The 25.18 ± 0.06 Ma, crystal-rich, rhyolitic positions but have reported 40Ar/39Ar ages that relative outfl ow facies tuff has been identifi ed. tuff of Arc Dome is characterized by distinc- appear slightly too old. Dilles and Gans (1995) Geophysical modeling indicates the intracaldera tive bipyramidal smoky quartz phenocrysts. tentatively correlated a 25.15 ± 0.03 Ma tuff ignimbrite is 1.0–1.4 km thick in the 11 × 17 km The tuff is normally zoned from high- to low- with high Y (34 and 45 ppm) and Nb (35 and 36 caldera (Shawe and Snyder, 1988), which indi- silica rhyolite (Table 2; John, 1992a). The tuff ppm) in the Wassuk Range with the “red tuff”. cates a minimum tuff volume of ~135 km3. of Arc Dome blankets much of the area around The 25.12 ± 0.03 Ma tuff of Brunton Pass in the an interpreted caldera buried beneath the south Paradise Range also has high Y (42 and 63 ppm) Tuff of Desatoya Peak and Caldera (24.7 Ma) end of Reese River Valley between the Toiyabe and Nb (34 and 35 ppm). Further work is needed The tuff of Desatoya Peak is a crystal-rich Range and Shoshone Mountains (Fig. 11; to fully evaluate possible correlations. The com- rhyolite known only from its own caldera in the Brem et al., 1991; John, 1992a). However, a bined Underdown Tuff–tuff of Clipper Gap is Desatoya Mountains (Fig. 11). The tuff con- sample petrographically and compositionally younger than but appears to be a continuation of sists of two, petrographically similar cooling indistinguishable from the tuff of Arc Dome the 25.4–25.1 Ma pulse of sparsely porphyritic units separated by up to 100 m of air-fall tuff, (John, 1992a) from the large outcrop area in tuffs (Fig. 3). tuffaceous sedimentary rocks, and lava fl ows the northern Shoshone Mountains gives dis- Probable intracaldera Underdown Tuff is (McKee and Conrad, 1987). The upper unit is at tinctly younger 40Ar/39Ar ages of 24.72 ± 0.05 exposed for ~23 km along the eastern range least 600 m thick, and the lower unit greater than

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400 m with no base exposed. McKee and Conrad Canyon consists of four cooling units as much view Peak was followed ~0.2–0.4 Ma later by (1987) reported a partial chemical analy sis that as 600 m thick, contains abundant, coarsely por- emplacement of a series of rhyolite to dacite

indicates the tuff is rhyolite with ~71% SiO2. phyritic biotite-rich pumice, and is noteworthy lava domes around the ring fracture of the cal- The lower unit has a sanidine 40Ar/39Ar date of for its variable and relatively mafi c composition dera. The caldera has an area of ~160 km2; mini- 24.74 ± 0.06 Ma. Reconnaissance observations (Fig. 6; John, 1992a). The possibly correla- mum erupted volume is ~115 km3 and could suggest a ~12 × 17 km caldera that has an area tive tuff of Ione Canyon consists of two cool- be several times that. Despite their young age, of 160 km2 (Henry et al., 2012a). Minimum ing units several hundred meters thick that are Fairview Peak caldera rocks have an arc geo- volume based on the 1.2 km thickness of intra- megascopically identical to the tuff of Sheep chemical signature and are petrographically and caldera deposits is ~190 km3. Canyon (Whitebread et al., 1988). The source(s) compositionally like older tuffs. of these tuffs is unknown. Tuff of Peavine Canyon and Peavine Caldera Previously Interpreted but Doubtful (24.5 Ma) Santiago Canyon Tuff and Correlative Calderas The 24.53 ± 0.08 Ma tuff of Peavine Can- Tuffs of Toiyabe and Copper Mountain, and yon crops out only(?) in the relatively unstudied Toiyabe Caldera (23.3 Ma) Ash-Flow Tuffs of the Candelaria Hills Peavine caldera of the southeastern Toiyabe The Santiago Canyon Tuff is the young- A sequence of twelve, ca. 27.6–23.3 Ma ash- Range (Fig. 11; Brem and Snyder, 1983; Brem est widespread ash-fl ow tuff of the ignimbrite fl ow cooling units, individually ranging from et al., 1991). The sparsely porphyritic tuff is fl areup (Fig. 11). The abundantly porphyritic about 20 to 130 m thick, in the Candelaria Hills distinctive in having anorthoclase rather than tuff is distinctive in having abundant sphene has been interpreted (1) to have been erupted sanidine phenocrysts (Table 2). Tuff that crops in addition to the usual assemblage of quartz, from an east-trending, fault-bounded trough that out in the Manhattan caldera that was consid- sanidine, plagioclase, biotite, and hornblende was subsiding incrementally during the erup- ered possible outfl ow tuff of Peavine Canyon is (Table 2). The presence of sphene and its young tions, possibly as a result of magma withdrawal, petrographically dissimilar to the tuff of Peavine age (23.27 ± 0.05 Ma) provide certain correla- i.e., a “slow caldera” (our term) (Robinson Canyon and is therefore not correlative (Shawe, tion with the tuff of Copper Mountain in the and Stewart, 1984) or (2) to fi ll an east-north- 1999a). The Peavine caldera is partly covered Gillis and Gabbs Valley Ranges (Ekren et al., east–elongated tectonic trough resulting from by the younger Toiyabe caldera but covers an 1980; Hardyman, 1980) and the tuff of Toiyabe approximately north-south extension, i.e., a vol- area of at least 215 km2 (Table 1). (23.31 ± 0.05 Ma) in the Paradise and Toiyabe cano-tectonic trough (Speed and Cogbill, 1979; Ranges (Brem et al., 1991; John, 1992a; Henry Petronis et al., 2004; Petronis and Geissman, Blue Sphinx Tuff (24.3 Ma) and Possibly and Faulds, 2010). Outfl ow tuff of Toiyabe 2009). A caldera is inconsistent with the lack

Correlative Tuff of Goldyke is normally zoned from 75% to 72% SiO2 of any observable caldera-like structures, espe- The 24.30 ± 0.02 Ma (n = 2) abundantly (Table 2; John, 1992a). The source caldera is in cially the thin deposits that lack megabreccia, porphyritic Blue Sphinx Tuff crops out in the the southern Toiyabe Range, where reconnais- and an east-northeast–elongated tectonic trough Wassuk , Gillis, and Gabbs Valley Ranges (Fig. sance mapping identifi ed thick intracaldera tuff is inconsistent with the lack of a map pable struc- 11; Bingler, 1978b; Ekren et al., 1980; Hardy- of Toiyabe , part of a caldera wall, and several ture of that orientation in the Candelaria Hills man, 1980; Proffett and Dilles, 1984). The tuff ash-fl ow tuff feeder vents (Whitebread et al., and of similarly oriented extensional basins is distinctive for its variety of phenocryst phases 1988; Brem et al., 1991; John, 1992a). The San- anywhere else in the Great Basin (Fig. 14). On and especially for its intensely resorbed quartz tiago Canyon Tuff has not been recognized in the basis of published data and our examination, and plagioclase phenocrysts (Table 2). The pos- the Singatse or Wassuk Ranges (Proffett and we interpret the tuffs to fi ll approximately west- sibly correlative tuff of Goldyke, which crops Dilles, 1984; Bingler , 1978b). draining paleovalleys, similar to those to the out south of the Paradise Range, is petrographi- north (Garside et al., 2002; Henry and Faulds, cally similar and has 23.0 ± 0.9 and 23.6 ± Tuff of Fairview Peak and Caldera 2010; Henry et al., 2012a), and to have erupted 0.7 Ma K-Ar biotite ages (John, 1992a). How- (19.5 Ma) from mostly unknown sources to the east. ever, the tuff of Goldyke underlies some minor Geologic maps of the area show that the tuffs that may correlate with two tuffs in the The 11 × 18 km Fairview Peak caldera tuffs fi lled a region with nearly 800 m of topo- Wassuk Range that are dated at around 25.2 Ma (Henry, 1996a, 1996b), source of the 19.48 ± graphic relief and wedge out in depositional (John, 1992a; Dilles and Gans, 1995). The tuff 0.03 Ma tuff of Fairview Peak, is the youngest contact with pre-Cenozoic rocks (Fig. 14; see

of Goldyke has 74–75% SiO2; no analyses are caldera in the western Nevada volcanic fi eld Stewart [1979, 1981] and Stewart et al. [1983] available for the Blue Sphinx Tuff. Unlike the (Figs. 3, 4, and 11) and is almost 4 Ma younger for wedge-out of individual tuffs). Tuff stratigra- previous calderas with no known outflow, than the next-youngest caldera. The sparsely phy varies tremendously laterally, as individual the Blue Sphinx and Goldyke are outfl ow tuffs porphyritic, high-silica rhyolite contains pheno- tuffs pinch and swell. Most contacts between with no known source caldera, which is likely to crysts of plagio clase, sanidine, quartz, and bio- tuff and pre-Cenozoic rock were mapped as be near the Toiyabe Range on the basis of their tite. All tuff is either intracaldera, where it is depositional. The distribution of tuffs and the distribution. greater than 700 m thick, or no more than ~5 km geometry of contacts do not support either a outside the caldera topographic wall. An east- caldera or an extensional basin. Attitudes of the Tuffs of Sheep Canyon and Ione Canyon striking dike of vertically foliated tuff of Fair- tuffs vary tremendously; many dip away from (ca. 24 Ma) view Peak that cuts through subhorizontal tuff basement rocks, probably refl ecting primary dip The tuffs of Sheep Canyon and Ione Canyon near the approximate northeastern margin of the where they compacted against slopes, e.g., north comprise multiple cooling units of crystal-rich caldera is a probable ash-fl ow vent (39.2054°N, and east of Miller Mountain in the south-central rhyolite, dacite, and trachydacite in the southern 118.0469°W). The dike is 200 m long, up to part of Figure 14 (Henry and Faulds, 2010). Paradise Range and central Shoshone Moun- 20 m wide, and has a vitrophyric margin and Rock fragments in the tuffs are less than 2 cm tains, respectively. The ca. 24 Ma tuff of Sheep a devitrifi ed core. Eruption of the tuff of Fair- in diameter, except in one unit where fragments

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Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/9/4/951/3346482/951.pdf by guest on 29 September 2021 Henry and John Q 118° MzPz 3 23 MzPz 25 MzPz Q Q Tt MzPz 30 5 Tl Tl MzPz Tt 20 Tl Tt Q Attitude of compaction foliation in ash-flow tuff Possible paleovalley and drainage direction Quaternary undivided Quaternary basalt Mafic to intermediate lavas and volcaniclastic rocks (Pliocene-late Miocene?) of the Candelaria tuffs Hills (~27.6-23.3 Ma) Pre-Cenozoic rocks undivided 15 ed (Robert C. Speed Unpublished Tt Tl Pz Tt 15 Tl 10 z Q Tt 25 Tl Tt Qb 0 2.5 5 km 10 Tt M MzPz 25 25 25 Q 27 MzPz Tt Q Q 5 Tl 5 Tl 5 20 Tl 15 10 10 Tt Tt 15 Tl Tt 33 Tl Tt Qb 26 15 8 Q 7 Tt Tl Q 10 Tt Tt 5 10 Q 8 20 35 MzPz Tt 28 Q MzPz 10 Q Q 5 12 10 25 MzPz Q Tt 20 Tt Tl 16 15 38 Tt 20 22 Tt 26 Q ′ 10 Tt 20 15 7 Tl Q MzPz 118° 10 118° 30 15 20 Q 45 MzPz Tt Q 18 MzPz 15 Q Tt MzPz Tl 48 20 Tl 40 25 Tt Tl Tl MzPz Tt Tl Tt Tt Tl MzPz 10 8 20 5 Tl MzPz Q 10 Tt 18 Q Tl ′ Q 118° 15 118° ′ ′ Tl Q 5 Tt Tt 17 38° 10 38° 05 5 Tl Figure 14. Geologic map of the Candelaria Hills, from Stewart (1979, 1981), et al. (1983), unpublished maps by R.C. Spe 14. Geologic map of the Candelaria Hills, from Figure and this study. Geologic Maps Collection; http://www.nbmg.unr.edu/Departments/GBSSRL/Collections.html),

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are up to 10 cm in diameter (Robinson and 0120.5 km Stewart, 1984). Sedimentary deposits including coarse conglomerates with well-rounded pre- Q Quaternary undivided Q Cenozoic clasts up to at least 60 cm in diam- Tml Mafic to intermediate lava Tsc Santiago Canyon Tuff (23.3 Ma) KJg eter both underlie the oldest tuff and separate 38° 56’56′ individual tuffs. All these characteristics seem Tuffs of Gabbs Valley Tgv3 Unit 3 30 more consistent with deposition of distal tuffs in Tgv2u Unit 2 (with upper, welded Q paleovalleys, which has been widely recognized Tgv2l and lower, nonwelded parts) Tml 36 Tgv2 Tgv2 in the Great Basin (Eckberg et al., 2005; Henry Tgv1 Unit 1 (including 25.4 Ma and Faulds, 2010; Henry et al., 2012a). Nine Hill Tuff) 57 Petronis and Geissman (2009) and Petronis KJg Cretaceous/Jurassic granodiorite Q 40 39 et al. (2009) reported ten Ar/ Ar ages rang- TR Triassic rocks undivided ing from 27.65 ± 0.05 to 23.31 ± 0.17 Ma from Fig. 16D

the tuff sequence. Major tuffs are, from oldest 70 to youngest, the Metallic City Tuff (25.73 ± TR 30 0.12 Ma), the Belleville Tuff (24.24 ± 0.29 Ma), Tml and the Candelaria Junction Tuff (23.76 ± TTR 38° 55′ 25 0.13 Ma), the second youngest of the sequence. Tsc As discussed above, the biotite-rich tuff of Tgv2 Tgv2 Miller Mountain, one of the oldest in the Can- Q TR delaria Hills (27.65 ± 0.05 Ma; Petronis et al., 30 Tml 2009), is distal outfl ow Monotony Tuff from the central Nevada volcanic fi eld. Other pos- TR 25 Q Tgv3 sible correlations are between two units of the 35 25 35 tuff of Volcanic Hills (27.29 ± 0.16 and 27.07 ± Tgv1 30 0.12 Ma; Petronis et al., 2009) and the 27.3 and TR Tsc 27.0 Ma lower and upper tuffs of Mount Jeffer- Q son in the Toquima caldera complex (Fig. 3). Tgv1 30 Fissure Vent and Proposed Caldera in 38° 54′ Tml Gabbs Valley Fig. 16A Ekren and Byers (1976, 1986c) interpreted a Tgv1 caldera source for one or more of the tuffs of Tgv2u 89 80 Q Gabbs Valley at Fissure Ridge in Gabbs Val- Tgv1 37 ley (Figs. 11, 15, and 16). Their interpretation 22 30 35 39 seems to be based mostly on an interpreted ash- Tgv3 Tsc Tml Q fl ow fi ssure vent in the western wall or bound- Tgv2l ary of a caldera. However, we fi nd little or no 25 evidence for a caldera and suggest the fi ssure Tgv2u vent is a narrow canyon eroded into older tuffs. The fi ssure vent is an ~400-m-deep cut into Tgv1 two older units of the tuff of Gabbs Valley (Tgv1 118° 12′ 118° 11′ 118° 10′ and Tgv2 of Ekren and Byers, 1986c) fi lled with Figure 15. Geologic map of the Fissure Ridge area, Gabbs Valley, modifi ed from Ekren and the youngest unit (Tgv3). White, poorly welded Byers (1976, 1986c) based on this study. tuff up to ~3 m thick and overlying black, dense vitrophyre up to ~5 m thick are developed all along the walls of the fi ssure, and welding foliation parallels the walls at the walls (Figs. The interpreted fi ssure vent is the strongest topography developed on pre-Cenozoic rocks 15, 16A, and 16B). The vent widens upward evidence for a possible source at this loca- north of the vent. Farther north, Tgv1 is absent, from ~60 m to ~460 m at which point vent rock tion. Ekren and Byers (1976) discounted that and Tgv2 fi lls a more than 300-m-deep val- merges with initially horizontal Tgv3 outside it was a canyon fi lled by tuff in part because ley in the pre-Cenozoic rocks. These relation- the vent. Ekren and Byers (1986c) mapped the they found no evidence for signifi cant erosion ships require that Tgv1 either not be deposited Cretaceous or Jurassic Granodiorite (KJg)-Tgv2 between the different tuffs. However, the area or be completely eroded before deposition of contact in the north and most tuff-tuff contacts has no other evidence for a caldera. The tuffs Tgv2. Similarly, all tuffs of Gabbs Valley and except near the fi ssure as caldera-boundary of Gabbs Valley contain no megabreccia typical the overlying Santiago Canyon Tuff (their tuff faults, with down-to-the-east and/or left-lateral of intracaldera tuffs, and none of them, includ- of Copper Mountain; Fig. 3) thicken and thin displacement. They further interpreted eastward ing Tgv3 in the fi ssure, are noticeably lithic or along Fissure Ridge, which Ekren and Byers thickening of Tgv1 and Tgv2 to ~300 m each contain coarse lithic fragments (our observa- interpreted to be a result of caldera faulting. ~8 km to the north along the interpreted caldera tions). The geologic map of Ekren and Byers However, basal poorly welded zones and vitro- boundary to be into the caldera. (1986c) shows Tgv1 depositionally on irregular phyres occur continuously along these contacts,

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A B

43 33 37 55 77 30 62 42 80 32 89 54 22 79 56 26 57 3745 38 35 39

0 100 200 m

C D

KJg

Tgv2 KJg Tgv2

0 200 400 m

Figure 16. Photographs and Google Earth (www.google.com/earth/index.html) images of the Fissure Ridge area. (A) Photo looking southeast at the tuff-fi lled fi ssure. The fi ssure is ~400 m wide at its widest in this photo. (B) Google Earth image of the fi ssure showing strikes and dips in the wall rock (in red) and fi ssure-fi lling tuff (in black; Tgv3; Fig. 15). Steeply to moderately inward-dipping, white, poorly welded tuff and dense, black vitrophyre are developed all along the contact with wall rock. However, dips decrease inward and were fl at in the middle of the fi ssure before extensional faulting that tilted rocks ~30° to the southeast. (C) Google Earth image of the contact between Tgv2 and Cretaceous or Jurassic granodiorite (KJg). Although interpreted as a down-to-the-east normal fault (Ekren and Byers, 1976, 1986c), we reinterpret the contact to be depositional. White, poorly welded tuff overlain by black, dense vitrophyre are developed all along the contact. Attitudes in the tuff parallel the east-dipping contact (Fig. 15). (D) Photo looking northeast along the contact between Tgv2 and Cretaceous or Jurassic granodiorite showing gentle to moderate east dips of the tuff and contact.

notably between Tgv2 and KJg, which we rein- tuffs maintaining constant thicknesses eastward Miocene to Pliocene sedimentary rocks (Esmer- terpret to be a depositional contact (Figs. 16C into the caldera. However, all tuffs are buried alda Formation; Stewart et al., 1999) overly- and 16D). An alternative interpretation is that beneath an extensional basin fi ll east of Fissure ing probably older Miocene lavas to a total the tuffs were being alternately deposited and Ridge, so thicknesses to the east are unknown. depth of 5060 feet (1540 m). The eastern well eroded in a paleovalley, typical of paleovalley Sedimentary sections encountered in two oil encountered a nearly 6000-ft-thick (1830 m) relationships elsewhere in Nevada (Henry and exploration wells in Gabbs Valley ~7 and 12 km section of Esmeralda Formation to the bottom Faulds, 2010). All tuffs, including the overlying, east-northeast of the fi ssure and within the inter- of the hole at a depth of 7707 ft (2349 m). Any ~2 Ma-younger Santiago Canyon Tuff, are tilted preted caldera are consistent with Gabbs Valley caldera-related rocks would have to be at still 25° to 30° southeastward into Gabbs Valley. being a late Cenozoic basin-and-range graben greater depths. Despite interpreting a caldera, Ekren and but provide no evidence regarding a possible An alternative explanation of the fi ssure is Byers (1986c) in their cross sections show all caldera. The western well went through middle that it is a narrow canyon eroded into outfl ow

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Tgv1 and Tgv2 in a paleovalley that was subse- et al., 2009a, 2009b, 2012; Druschke et al., and relief is especially high in the Toquima quently fi lled by outfl ow Tgv3. Tgv3 generally 2009; Molnar, 2010; Henry et al., 2011, 2012a). Range–Monitor Range area. On the basis of has a poorly welded zone and dense vitrophyre In northern and central to western Nevada and analogy to the modern Andes, Best et al. (2009; (Figs. 16A and 16B) along the wall, similar to eastern California, the Eocene and Oligocene 2013a) proposed that a western side with a depositional contacts of Tgv3 outside the fi s- tuffs fl owed, were deposited, and are preserved variable slope progressed up to a broad, gentle sure and of all other tuffs. The paucity of lithic primarily in deep (as much as 1.2 km) but wide divide and an eastern plateau slightly less than 1 fragments in Tgv3 in the fi ssure suggests that (8–10 km) paleovalleys (Proffett and Proffett, km below the divide (Fig. 17). Such a low divide wall rock was not removed by eruption of tuff 1976; Henry, 2008; Henry and Faulds, 2010; would seem to be an insuffi cient topographic from the fi ssure and allows that it was removed Cassel et al., 2012; Henry et al., 2012a). The barrier to greatly limit eastward dispersion of by pre-fi ssure erosion. The fi ssure closes and tuffs were able to fl ow great distances, as much ash fl ows. Eruptions from low on the west side vitrophyre dipping 35° eastward is continuous as ~250 km corrected for extension (Henry and would likely spread preferentially to the west, across the lowest exposed, western end, which Faulds, 2010), down these valleys because they but eruptions from near the divide should have would allow the western end to be the base of a were channelized, did not disperse, and did not been able to spread both east and west. The paleovalley. Devitrifi ed rock in the middle of the mix with air as much as would tuffs that spread abundance of calderas through central Nevada fi ssure also fl attens inward, and both rock types more radially. The distribution of most of the means that many of them must have been on or in the middle of the fi ssure were approximately tuffs in the western Nevada volcanic fi eld sug- within a few tens of kilometers of the divide. horizontal before the eastward, extensional tilt- gests they spread radially only within a few tens That, with the few noted exceptions, tuffs appar- ing. Before extensional tilting, the slope of fi s- of kilometers of their source. ently did not spread more symmetrically down sure walls ranged from steep, even vertical, to Most tuffs of the western Nevada volcanic both sides of the paleodivide requires further as little as ~35°. Moderate to steep slopes in the fi eld are recognized in, and probably restricted investigation. walls of canyons cut into modern, welded or to, areas west of the interpreted paleodivide. even non-welded tuffs are common. A canyon A topographic barrier along the divide may LAVA FLOWS AND DOMES IN THE as much as 33 m deep commonly with near- have restricted their fl ow to the east. However, WESTERN NEVADA VOLCANIC FIELD verti cal walls was cut into the tuff of the Val- the absolute elevation, location, and relief of a ley of Ten Thousand Smokes, Alaska, within paleodivide are not fully resolved. The volumi- Lava fl ows and domes are a widespread 10 years of the tuff’s deposition (Griggs, 1922); nous Nine Hill Tuff and tuff of Campbell Creek, but relatively small-volume component of the canyon walls remain near vertical today. There- and the Underdown Tuff–tuff of Clipper Gap of western Nevada and adjacent central Nevada fore, we conclude that no caldera is present in unknown volume, derived from sources at or fi elds (Fig. 18; Supplemental Text5). They com- Gabbs Valley, and the fi ssure vent could be a north of 39°N, fl owed far into eastern Nevada monly are the earliest manifestation of Tertiary narrow paleocanyon. (Figs. 10 and 11; Supplemental Fig. 1 [see foot- magmatism within an area and often presage note 5]; Deino, 1985; Best et al., 1989; Henry caldera-forming ignimbrite eruptions from the IMPLICATIONS OF TUFF et al., 2012a; Best et al., 2013c; A.L. Deino, same area, but they commonly are hundreds of DISTRIBUTION FOR EOCENE– 2012, written commun.). Whether the topo- thousands to several million years older than OLIGOCENE TOPOGRAPHY graphic barrier did not exist to the north, spe- associated calderas (Fig. 18 and Table 1). Lava cial eruption dynamics allowed these ash fl ows fl ow sequences and domes range in composition Geologic mapping of Cenozoic ash-fl ow to travel especially far, or other factors were tuffs in northern and western Nevada demon- involved is unknown. 5Supplemental Text. Lava fl ows and domes in the strates that they erupted onto a surface with Our current view is that the divide shown by western Nevada volcanic fi eld. If you are viewing the PDF of this paper or reading it offl ine, please considerable relief—the paleovalleys of Fig- Henry et al. (2012a) south of ~40°N probably visit http://dx.doi.org/10.1130/GES00867.S5 or the ure 1 (Proffett and Proffett, 1976; Proffett and should be closer to that proposed by Best et al. full-text article on www.gsapubs.org to view the Sup- Dilles, 1984; Dilek and Moores, 1999; Faulds (2009, 2013a) (Fig. 1). Uncertainty in location plemental Text. et al., 2005a, 2005b; Garside et al., 2005; Henry, 2008; Cassel et al., 2009a, 2009b; Henry and Faulds, 2010; Henry et al., 2012a). The region Sierra Caldera belt was an erosional highland, commonly referred Nevada to as the Nevadaplano, which was drained by 4 Cassel et al. (2012) Ignimbrite fill major west- and east-trending rivers, with a ? north-south paleodivide through east-central 2 Paleodivide Best et al. (2009) (Henry et al., 2012a) Nevada. The highland developed at least by ca. 20X vertical exaggeration 50 Ma, probably as a result of crustal thicken- Elevation (km) 0 ing during Mesozoic–early Cenozoic shorten- 0 100 200 300 400 500 ing, Mesozoic batholith emplacement in the Distance from Pacific Ocean in mid-Cenozoic (km) Sierra Nevada, and possibly a later contribu- tion from slab removal (e.g., Humphreys et al., Figure 17. Interpreted paleotopography from the former Pacifi c Ocean eastward across the 2003; DeCelles, 2004; Dickinson, 2006; Best caldera belt and Nevadaplano, from Best et al. (2009, their fi gure 17) and Cassel et al. (2012, et al., 2009; Ernst, 2010; Mix et al., 2011). The their fi gure 8). Profi le from Best et al. also shows ignimbrite fi ll that would have smoothed absolute elevation and topographic-extensional pre-volcanic topography east of their paleodivide. Cassel et al. interpreted that the Sierra evolution of this highland in the mid-Cenozoic Nevada was at its present elevation in the late Eocene–early Oligocene and the area to the remain highly controversial (Wakabayashi and east a relatively fl at plateau. Best et al. interpreted a slightly shallower gradient to between Sawyer, 2001; Mulch et al., 2006, 2008; Cassel 200 and 250 km east of the Pacifi c Ocean but a similar maximum elevation.

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Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/9/4/951/3346482/951.pdf by guest on 29 September 2021 Henry and John eld. Pluton eld. Kilometers Middle Cenozoic plutons (~40 to 25 Ma) Rhyolite lavas, domes, and intrusions Andesite-dacite lavas Caldera, location certain Caldera, location approximate Caldera, concealed Paleodivide Approximate age range (Ma) of lava flows and domes 15 39–35 0 30 60 90 120 BD, Bruner district; Mountain; BM, Bates CA, Clan Alpine BD, Range; Mountains; Stillwater CSR, central DM, Desatoya Creek FCM, Fish Canyon; FC, Fye Pass; Emigrant Mountains; EP, Range; LM, Valley GVR, Gabbs Peak; Fairview Mountains; FP, NSR, northern Mountains; ML, Mt. Lewis; Louderback Range; PR, Paradise Toiyabe Shoshone Range; NTR, northern SPR, caldera complex; Stillwater Hills;Range; SCC, RH, Royston Tobin TBR, Range; Toiyabe Range; STR, southern Simpson Park Butte Vigus VB, Range; Toiyabe TR, Range; Toquima TQ, Range;

a) al., 2012 (Henry et

38–36 Paleodivide EP Garden Pass Mt Hope ca 36 FC 36–35 SPR Walti Canyon Shoshone Range (Granite Mountain, Hilltop, Tenabo) 39–35 BM Volcanic Field Volcanic NTR Central Nevada ML NSR Dry Canyon >34, 25 Caetano intrusions TR Battle Mountain (Copper Canyon, Elder Creek, etc.) 37–34,27 TQ ? 34–29 Jett VB ca 34

Canyon

ows and plutons in immediately adjacent to the western Nevada volcanic ﬁ

FCM 2013a) al., et (Best STR Paleodivide 34–33 McCoy-Cove 24–23 TBR ? RH Jersey Canyon BD ca 33 ca 20 DM 29–26 CA PR 35–25? 26–24 23–20 ca 30 Granite Mountain LM

CSR ? FP ? John (1995), Crafford (2007), and Colgan et al. (2011b). ed from 31–29 GVR >31 >27, 23 White Cloud Canyon SCC 25–20 ? Chalk Mountain distribution modiﬁ Figure 18. Map showing the distribution of lava ﬂ Figure Stillwater (IXL and Stillwater (IXL Freeman Creek) Volcanic Field Volcanic Western Nevada Western 119°N 118°N 117°N 116°N 40°N 39°N 38°N

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from high-silica rhyolite to mafi c andesite, with rhyolites especially common in the central part Lava Flows and Domes in and adjacent to the Western Nevada Volcanic Field of the fi eld in the southern Stillwater Range and Clan Alpine Mountains (Figs. 18 and 19). 12 Most larger volumes and areas underlain by lava Tephri- Trachyte fl ows and domes are >5–10 km distant from phonolite known calderas and appear unrelated to caldera- 10 Phono- Tephrite Trachydacite forming volcanism. Post-caldera, intermediate- Trachy- composition lava fl ows are present in some andesite calderas, notably in the Job Canyon caldera in 8 Basaltic Rhyolite the Stillwater Range and in the northwest part trachy- of the Caetano caldera. Silicic (rhyolite) domes andesite 6 Trachy- and related lava fl ows are present in most cal- basalt O (weight percent)

deras and represent resurgent magmas emplaced 2 Dacite shortly after caldera formation. Basaltic Andesite O+K 4 Chemical analyses of 166 lava-ﬂ ow and lava- 2 Basalt andesite

dome samples scattered among these expo- Na sures indicate a wide range of compositions 2 from high-silica rhyolite to basaltic andesite,

although analyzed samples with <57% SiO2 are scarce and restricted to the undated lavas north and south of Fairview Peak (Fig. 19). 7

The rocks are K2O rich and most form a high-K calc-alkaline series. Early rhyolites that under- 6 lie the Stillwater caldera complex and post–

Caetano caldera andesites are notably K2O rich. Samples from the western centers (Stillwater, 5 Fairview Peak, Paradise Range) generally are more alkaline than samples collected farther 4 east (Emigrant Pass, Fye Canyon, Bates Moun-

tain), mostly as a consequence of higher Na2O Shoshonitic contents. 3 O (weight percent) 2 K DACITE TUFFS RELATED TO 2 High-K LAVA DOMES Medium-K Several, small- to possibly moderate-volume 1 Low-K dacitic ash-ﬂ ow tuffs are more likely related to dacite lava domes, some possibly forming from 0 dome collapse, than to caldera collapse. All 45 50 55 60 65 70 75 80 SiO (weight percent) known examples are outside of or at the edge 2 of the caldera ﬁ eld. They are better considered Post-caldera rhyolite, Stillwater Range (~25 to 20 Ma) Post-Caetano caldera andesites (~33 to 29 Ma) part of the contemporaneous lavas than related Clan Alpine Mountains (~35 Ma) to the calderas. SW Paradise Range (~26 to 24 Ma) Fairview Peak (~30? to 21 Ma) Pre-Caetano caldera andesites (35.2 Ma) Dacite of Mount Lewis (35.8 Ma) Post-Job Canyon caldera, Stillwater Range (~29 Ma) Cortez dikes (~unaltered only; 35.7 Ma)

The 35.81 ± 0.29 Ma, small-volume, dacitic Pre-Job Canyon caldera, Stillwater Range (~30 Ma) Volcanic rocks of Fye Canyon (35.7 Ma) tuff of Mount Lewis erupted from an unknown Older rhyolite, Stillwater Range (~30 Ma) Bates Mountain (~36 Ma) but almost certainly non-caldera source probably in the northern Shoshone Range at 35.8 Ma. Tobin Range (33-25 Ma) Emigrant Pass (~38 to 36 Ma) The tuff is spatially associated with a similar- age dacite intrusion and several aphyric rhyolite intrusions and is mostly confi ned to an east-west Figure 19. Total alkali-silica and K2O-silica diagrams for lava fl ows of the western Nevada paleovalley across the range. The tuff is ~30 km volcanic fi eld. Sources of data: Stillwater Range, John (1995) and this study; Fairview Peak north of the 34 Ma Caetano caldera. The tuff is area, Henry (1996a, 1996b) and this study; Tobin Range, Gonsior and Dilles (2008); Clan moderately porphyritic, containing 20%–30%, Alpine Mountains, Riehle et al. (1972); Caetano, this study; Cortez, this study; Fye Canyon, fi ne-grained phenocrysts of plagioclase, horn- this study; Bates Mountain, this study; Emigrant Pass, Ressel and Henry (2006) and this blende, biotite, less-abundant quartz, and sparse study. Classifi cation from Le Maitre (1989) and Ewart (1982). K-feldspar and abundant small fragments of

Geosphere, August 2013 983

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/9/4/951/3346482/951.pdf by guest on 29 September 2021 Henry and John n n o Paleozoic sedimentary rocks. It is generally o s s ; l l , 2 n ; e e e 9 o propy litically altered and no chemical analyses 8 K K s 9 w 0 l ; ; 1 a 0 e 5 5 , h

are available. 2 s 6 6 K n , S e 9 9 ; h n ; c 1 1 7 7 o a 6 n , , 0 0 J g 8 e s s 0 0 l r 9 e e d 2 2 o t t Tuffs in the Hu-Pwi Rhyodacite (23.6 Ma) e 1 n f , , a a C . . , a l l e . l G G d a a R d a t t n d d a t e e a n n e e r a a y y The tuff and breccia of Gallagher Pass in k b e c a a i y y r r e l l w t d i u u B the northern Wassuk Range and the time-cor- B a l l s et al., 2008 and Eng, 1995; du Bray et al., 2007 du Bray et al., 2007 2007; this study 2007; et al., 2008 1995 et al., 2008; this study et al., 2011b; this study et al., 2011b; et al., 2007 2007; this study 2007; et al., 2007 1973; du Bray et al., 2007 this study et al., 2011b; du Bray et al., 2007 l l h i i h o u u John, 1992c, 1995; this study F S G Brooks et al., 1991; Emmons John, 1992c, 1995; this study G d d John, 1992c; du Bray et al., John, 1992c; du Bray et al., W John et al., 2008, 2009; Colgan relative Hu-pwi Rhyodacite in the Gillis and Theodore et al., Roberts, 1964; John et al., 2008, 2009; Colgan Gabbs Valley Ranges consist predominantly of intermediate lavas with abundant phenocrysts of plagioclase, biotite, and pyroxene (Proffett

and Proffett, 1976; Ekren et al., 1980; Hardy- s t n

man, 1980; Proffett and Dilles, 1984; Dilles and e m

Gans, 1995). Numerous cooling units of crystal- m o

rich tuff having the same phenocryst assem- C blage are interbedded with the lavas and have been mapped as the lower Nugent Tuff Mem- caldera aplitic groundmass Canyon and Elevenmile calderas caldera ber and the trachydacitic upper Poin settia Tuff caldera Resurgent intrusion in Job Canyon

Member in the Gillis and Gabbs Valley Ranges. u A s Dilles and Gans (1995) reported a biotite e n p i n y e t

40 39 o i - v

Ar/ Ar age of 23.60 ± 0.04 Ma on what they t n i a c l e e e i e e l z r l i n n n n n l

interpreted to be the Poinsettia Tuff Member a a o o o o o a t C r e N N N None Several small stocks and dikes et al., 1992; du Bray Theodore N N skarn , in the northern Wassuk Range. The combined e n m n Au skarn, i r y l replacement a Porphyry Cu, M o

group of tuffs crops out in a west-northwest– k s polymetallic veins P disseminated Au-Ag trending belt ~70 km long, which was probably u A

shorter before strike-slip faulting of the Walker d 4 o e ) t 3 t 4 4 2 . . . a a 6 Lane, but the distribution of individual tuffs is o . t 1 9 0 d M Age 9 3 38.5 3 39.1 34.0 None Resurgent intrusion in Caetano 37.7 3 38.5 ( n 7 ca. 25 None Composite pluton underlying Poco 3 3

unknown. Nevertheless, the tuffs are certainly U more voluminous than the dacite of Mount 7 9 7 5 . . . . 6 9 5 9 Lewis or the tuff of Perry Canyon. 3 6 6 6 6 7 o o o o o t t t t t 1 2 2 5 4 . . . . to 67.1 6 Tuff of Perry Canyon (23.5 Ma) (altered) 5 8 7 9 (wt% SiO2) 61.4 to 68.5 29.1 Polymetallic veins and 6 6 6 67.2 to 71.4 38.2 Porphyry Cu Several small stocks and dikes et al., 1973; du Bray Theodore 6 Composition * The 23.53 ± 0.26 Ma tuff of Perry Canyon e t i n r o i o t i e e e e i in western Nevada north of Reno consists of - t t t t d i i i i s e r r r r t o i o e e o o o o t t z i i i i i i n two cooling units of crystal-rich, plagioclase > p n d d d d n n o m o o o o z o a a r r o n n n n hornblende > biotite > quartz > opaque miner- n m c a a a a o G G l r r r r z t a M

als dacite tuff (Garside et al., 2003). A single r G G G G d monzogranite a to granodiorite o TABLE 3. PLUTONS IN THE WESTERN NEVADA VOLCANIC FIELD THE WESTERN NEVADA IN 3. PLUTONS TABLE chemical analysis indicates 65% SiO (Fig. 6). u 2 M Q The tuff crops out over an area of ~50 km2 and is spatially and temporally associated with dacite s n i e e e a

to rhyolite lava domes. e g g t g g n n n n n a a u a a o R R e n R i R g r M e e a e t n

MIDDLE CENOZOIC PLUTONS IN THE e e n n g k t n n a b o o n e a o u a i R h h a e t o

WESTERN NEVADA VOLCANIC FIELD w y r s s cation (Streckeisen, 1976). l i a a l R i o o M o C c t t m h h T o i s S e h l S S L u a t n s t i n r q E n n r a F e r r Granitic plutons intruded approximately coeval o e h B T e e n t h t r h h u t t u e r with the ignimbrite fl areup are scattered across the r o t o o o s S S e N western Nevada volcanic fi eld and the northwest- N ern corner of the adjacent central Nevada volcanic W fi eld (Fig. 18; Table 3). Plutons are exposed both in the cores of highly extended calderas (Still-

water and Caetano calderas) and in areas that n k o i c s o t lack nearby exposures of coeval vol canic rocks u r s n t s k n o n i i t e (i.e., Eocene plutons near Battle Mountain). c k a u o i l n t t d o p s n k

These plutons were emplaced between ca. 40 and y u c k n k n n o c o o e o t a n o y y e s M 25 Ma, and their compositions range from granite t o r C n n t s o e C a a u t y l i p b e r e C to granodiorite with ~77–65 wt% SiO (du Bray C p n o

2 a e s t t m l a n r t y *International Union of Geologic Sciences classiﬁ L l d r r i a l e e e

et al., 2007; this study). X Freeman Creek pluton Southern Stillwater Range Granite; granodiorite 71.3 to 74.2; 66.2 I T H J White Cloud Canyon pluton Stillwater Range Granite 74.2 to 77.0 ca. 25 None Coarse K-feldspar phenocrysts with Brown stock Northeastern Fish Creek Mountains Tonalite-granodiorite 65.0 to 67.7 40 to 39 Au skarn, distal G Carico Lake pluton Carico Lake valley Granite 71.3 to 73.2 34.0 None Resurgent intrusion in Caetano Long Canyon intrusions Battle Mountain Granodiorite-granite 66.9 to 71.5 to 39.3 D Granite Mountain pluton Northern Shoshone Range Granodiorite 66.4 to 69.6 ca. 39 to Chalk Mountain pluton Chalk Mountain Granodiorite-granite 69.5 to 73.1 ca. 25 Polymetallic Copper Canyon stockRedrock Canyon intrusion Battle Mountain Carico Lake valley Granodiorite-granite 66.9 to 73.1 39.4 to Granite 74.1 to 75.2 E N J

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The largest plutons include the 34.0 Ma IMPLICATIONS ABOUT the Caetano Tuff (John et al., 2008; Moore and Carico Lake and Redrock Canyon plutons in the DEVELOPMENT OF IGNIMBRITE Henry, 2010; Colgan et al., 2011b). center of the Caetano caldera and the 29.1 Ma CALDERAS AS INDICATED FROM The crystal-rich Caetano Tuff (typically 35%– IXL and ca. 25 Ma composite Freeman Creek THE CAETANO AND STILLWATER 45% phenocrysts) is zoned upward from high-

plutons in the Stillwater caldera complex (dis- MAGMATIC SYSTEMS silica rhyolite (76%–78% SiO2), which forms cussed in detail in the section on the Caetano most of the lower unit, to rhyolite (72%–74%

and Stillwater calderas). Other large plutons The Caetano caldera and the three calderas SiO2) that forms the upper part of the lower unit include the Granite Mountain, Hilltop, and of the Stillwater caldera complex in the west- and ash fl ows in the upper unit (John et al., 2008). Tenabo stocks in the northern Shoshone Range ern Nevada volcanic fi eld are exceptionally Intrusive rocks related to the Caetano caldera (all ca. 39 Ma), the Copper Canyon, Elder well-exposed examples of single and multi- include the Redrock Canyon and Carico Lake Creek, and Long Canyon stocks in the Battle cycle (nested) calderas, respectively, formed plutons in the center of the caldera, smaller Mountain area (ca. 39–38 Ma), the 40–39 Ma by large-volume ignimbrite eruptions. Large- rhyolite porphyry ring-fracture plugs, and a Brown stock in the northern Fish Creek Moun- magnitude (≥100%) crustal extension formed composite tuff feeder dike along an outer ring tains, the ca. 31 Ma Granite Mountain pluton tilted fault blocks that expose extraordinary, fracture (Fig. 20; John et al., 2008; Colgan in the East Range, the ca. 25 Ma White Cloud 5–10-km-thick sections through the calderas et al., 2011b). The Redrock Canyon intrusion is Canyon pluton in the Stillwater Range, and the and underlying granitic plutons. In this section, composed of medium-grained granite porphyry ca. 25 Ma Chalk Mountain pluton. we review salient points of published studies of containing ~30% phenocrysts in a devitrifi ed Of particular signifi cance to the regional these calderas and add new data to establish the felsite to aplitic (0.01–0.10 mm) quartz-feldspar tectonic history, several plutons, notably ca. magmatic, structural, and hydrothermal nature groundmass. The Carico Lake pluton consists of 40–38 Ma intrusions at Battle Mountain and of these unique calderas (Table 4). Special 55–65 vol%, medium-grained phenocrysts in a in the northern Shoshone Range and northern emphasis is given to their structure and how this microcrystalline (0.05–0.10 mm) groundmass Fish Creek Mountains, are not spatially associ- contrasts markedly with current popular models of quartz and feldspar. The Carico Lake and ated with coeval volcanic rocks. However, these of caldera subsidence, as invoked, for example, Redrock Canyon intrusions are compositionally intrusions are locally unconformably overlain for the two largest calderas in the Indian Peak– similar to the upper unit of the Caetano Tuff. by late Eocene volcanic rocks (the 34.4 Ma tuff Caliente fi eld to the east (Best et al., 2013b) and of Cove Mine or 35.8 Ma tuff of Mount Lewis; the possible genetic relationship between large- Stillwater Caldera Complex, Seedorff, 1991; Doebrich, 1995; Johnston et al., volume ignimbrites and granitic plutons. West-Central Nevada 2008; this study), which indicates rapid and signifi cant uplift (several kilometers) and erosion Caetano Caldera, North-Central Nevada The Stillwater caldera complex in the southern between ca. 38 and 35.8–34.4 Ma, just before part of the Stillwater Range and the southwest- major caldera-forming ignimbrite eruptions The 34.0 Ma Caetano caldera near the ern part of the Clan Alpine Mountains along the began in the region. Several of these intru- northeast end of the western Nevada volcanic northwest margin of the western Nevada vol- sions also are noteworthy for signifi cant geneti- fi eld formed during eruption of >1100 km3 canic fi eld comprises three partly overlapping, cally related base and precious metal deposits of Caetano Tuff (Figs. 1, 10, and 20; Table 4; Oligocene ash-fl ow calderas and subjacent plu- (e.g., ca. 39–38 Ma Copper Canyon and Elder John et al., 2008; Colgan et al., 2011b). Most tonic rocks (Figs. 1 and 21; Table 4; John, 1995). Creek porphyry copper deposits and Fortitude of the tuff is a thick intracaldera sequence; little In the southern Stillwater Range, the north half gold skarn deposit at Battle Mountain and the outfl ow tuff is preserved. Multiple resurgent of the caldera complex was tilted 70°–90° west ca. 39 Ma Cove-McCoy gold-silver skarn and granitic plutons were emplaced into the center and the south half tilted 55°–75° east during distal disseminated deposits in the Fish Creek of the caldera within 100 k.y. after caldera for- large-magnitude crustal extension in the latest Mountains). mation. Middle Miocene extensional faults cut Oligocene (John, 1992b, 1995; Hudson et al., Middle Cenozoic plutons associated with sig- and tilted the originally 20 × 12–18 km caldera 2000) and/or middle Miocene (J.P. Colgan, nifi cant mineral deposits in the western Nevada into a set of fault blocks dipping ~40° east that 2011, personal commun.). The calderas and volcanic fi eld are older than nearby calderas, expose paleodepths of as much as 5 km; the fault underlying plutons are exposed in cross section although resurgent plutons formed large, but blocks include the entire caldera-fi ll sequence over an ~10 km depth range. Our ongoing map- apparently unmineralized, hydrothermal systems and the upper parts of resurgent plutons (Colgan ping indicates that the Elevenmile Canyon cal- in the Caetano and Job Canyon calderas (see sec- et al., 2008). dera extends east and southeast into the southern tion on the Caetano and Stillwater calderas). The Rocks related to the Caetano caldera include Clan Alpine Mountains (Fig. 11). most important mineral deposits in the western two units of intracaldera Caetano Tuff, two large The ca. 29 Ma Job Canyon caldera consists Nevada volcanic fi eld are the 35.7 Ma Cortez granite porphyry plutons, and smaller rhyolite of two structural blocks (Figs. 22 and 23). The and Cortez Hills Carlin-type gold deposits and and rhyolite porphyry dikes and plugs (John 6-km-wide, 70°–90°-west-dipping north block is the 26.1 Ma Round Mountain epithermal gold et al., 2008; Colgan et al., 2011b). The Caetano the least structurally deformed part of the Still- deposit. The Cortez and Cortez Hills gold depos- Tuff is crystal rich and zoned upward from water caldera complex. It consists of as much its are spatially and temporally associated with high-silica rhyolite to lower-silica rhyolite. It as 1500 m of pre-collapse intermediate-compo- rhyolite porphyry dikes, sills, and fl ow domes consists of the lower compound cooling unit as sition lava fl ows and breccias (older dacite and (the Cortez dikes; Wells et al., 1969; Muntean much as 3000 m thick separated by a complete andesite unit) overlain by 2000 m of relatively et al., 2011). The Round Mountain deposit is not cooling break from the upper unit, a thinner crystal-poor rhyolite intracaldera ash-fl ow tuff associated with exposed intrusive rocks, but it (500–2000 m thick) sequence of thin ash fl ows (tuff of Job Canyon) locally interbedded with formed along the structural margin of the Round and interbedded lacustrine and fl uvial sedimen- thick sequences of caldera-collapse breccia, Mountain caldera within 0.5 Ma of caldera for- tary rocks. Zones and lenses of megabreccia and overlain in turn by up to 2500 m of intermediate mation (Henry et al., 1996, 1997). mesobreccia are abundant in the upper part of lava fl ows and minor lacustrine and fl uvial

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sedimentary rocks (younger dacite and andesite in 2 a ow r unit). The 3-km-wide south block of the cal- e a r d l e

a dera consists of thinner sequences (total thick- d l c a f ≤ c o ness 2500 m) of intermediate lava ﬂ ows and r n a e o t overlying intracaldera tuff of Job Canyon with y M n n e 5 a c local interbedded collapse breccia; post–caldera 2 f C . o a e

l collapse lava ﬂ ows are absent in this structural i c g n m i

n block. The north part of the caldera is intruded m e lled by sedimentary tuff unit lled by sedimentary tuff o v d 3 e by the compositionally zoned granodiorite– l e ); compositional zoning uncertain l E 2 b

i quartz monzodiorite IXL pluton. Outﬂ ow tuff of s s and south walls; slide blocks in tuff as and south walls; slide blocks in tuff much as 6 km from south wall SiO forming moat in southern third of caldera ﬁ plutonic rocks in extrusive rocks; 66%–67% SiO unit (?), tuff of Elevenmile Canyon, unit (?), tuff granodiorite phase of Freeman Creek at north end fault at least 3 km deep; north wall— volcanic rocks, high-angle Tertiary fault obscured by intrusive rocks tuff extends 25–30 km east and tuff Alpine Mountains and north end Clan of Fairview Peak pluton grained biotite-hornblende granodiorite porphyry end—Oligocene volcanic rocks; continuous for as much 5 km along strike southeast to Louderback and southern

o Job Canyon is widely distributed to the south and P >600 km along north Blocks and lenses in tuff (~61%–74% to rhyolite tuff Trachydacite >3 km in south and central parts, <1 Regional distribution unknown; outﬂ Tuff of Lee Canyon, sedimentary tuff of Lee Canyon, sedimentary tuff Tuff Freeman Creek pluton—medium- South wall—Mesozoic rocks, subvertical South end—Mesozoic rocks; north southeast (Fig. 10; John, 1995, 1997; this study). The ca. 25 Ma Poco Canyon and Elevenmile Canyon calderas and underlying Freeman Creek

; granite pluton overlap in time and space. Poco Canyon 2 a r e 2

d caldera-related deposits comprise two cooling l a a c units of intracaldera crystal-rich rhyolite and M n o 5

y high-silica rhyolite tuff (lower and upper units of 2 n . a a the tuff of Poco Canyon) separated by a unit of c C o

c crystal-poor high-silica rhyolite tuff and caldera- o P

t collapse breccia (megabreccia of Government n 3 e

d Trail Canyon) and by a thick unit of crystal-rich ), weak normal zoning; Freeman i 2 v ow tuff regionally widespread—New ow tuff e rhyolite and trachydacite ash-ﬂ ow tuff related to t tuff of Poco Canyon; 1.8-km-thick mega- tuff breccia unit beneath upper cooling and tuff of Chimney Spring(?) in western and tuff end granite porphyry SiO porphyry—~71%–75% SiO north end Pass Tuff in central Nevada (60 km east) Tuff Pass Nevada (150 km west) megabreccia of Government Trail Canyon, Trail megabreccia of Government granite phase of Freeman Creek pluton, Creek pluton—72%–74% SiO grained granite; granite porphyry ring- fracture dike—porphyroaphanitic rhyolite porphyry (shallow) to medium-grained granite porphyry (deep) and older rhyolite unit); high-angle fault at much as 4 km along strike o Blocks in upper cooling unit of (intracaldera) N >4 km along north margin; >1.5 at south >200 km Rhyolite to high-silica rhyolite tuff (73%–77% Rhyolite to high-silica rhyolite tuff Outﬂ Tuff of Poco Canyon (2 cooling units), Tuff Tertiary volcanic rocks (Job Canyon caldera Tertiary Oligocene volcanic rocks; continuous for as Freeman Creek pluton—medium- to coarse- the Elevenmile Canyon caldera (tuff of Eleven- , 2 mile Canyon) (Figs. 22 and 23). Coarse, locally tuffaceous breccias and sandstones containing ow(?)

2.5 km abundant detritus derived from the lower tuff of ≤

a Poco Canyon locally overlie this unit near the r e d l south margin of the caldera (sandstone and sedi- a a c M mentary breccia unit, Fig. 22; John, 1993). Total n 9 o 2 y . 1 km; south block— thickness of caldera-related deposits in the Poco n ≤ a a c

C Canyon caldera is locally >4500 m. The New Pass b c zoning from roof downward o ne- to medium-grained, Tuff to the east and the tuff of Chimney Spring to J ); IXL pluton—~60%–68% SiO ); IXL 2

3 the west are outfl ow tuffs that correlate with the ), rhyolite to silicic dacite tuff (68%– ), rhyolite to silicic dacite tuff 2 intracaldera tuff of Poco Canyon (Fig. 3). and south side of north block; blocks in near south end intracaldera tuff SiO felsic to mafi tuff extends 30 km southeast to tuff Louderback and southwestern Clan Alpine Mountains piston-like uplift Job Canyon, older dacite and andesite pluton (lavas), IXL biotite-hornblende granodiorite to quartz monzodiorite 75% SiO penetrating at least 5 km deep continuous for as much 3 km along strike not evident In the Stillwater Range, the Elevenmile Can- Mesozoic rocks; sub-vertical fault Lenses in intracaldera tuff along north wall Lenses in intracaldera tuff Younger dacite and andesite (lavas), tuff of dacite and andesite (lavas), tuff Younger IXL pluton—fi IXL >200 km to rhyolite (~55%–75% Trachyandesite North block—uncertain, with possible Regional distribution unknown; outfl Oligocene volcanic rocks where preserved; yon caldera is fi lled by >3000 m of crystal-rich rhyolite to trachydacite ash-fl ow tuff (tuff of ;

2 Elevenmile Canyon), locally overlain in the southern and central parts of the caldera by a thin sequence of probable caldera-lake deposits (sedimentary tuff unit), and in the northern and 2 a r central parts of the caldera by the locally thick e d l a

a (as much as 1100 m) rhyolitic tuff of Lee Can- M c , zoned upward from high- 0 o 2

. yon. The total thickness of Elevenmile Canyon n 4 a 3 t

e caldera deposits in the Stillwater Range locally a

C exceeds 4000 m (Fig. 22). In the eastern part ) intrusions: Redrock Canyon 2 of the caldera in the southwestern Clan Alpine

ow tuffs and sedimentary rocks), Redrock ow tuffs Mountains, intracaldera tuff of Elevenmile Can- lled paleovalley extending 40 km west; yon is as much as 2000 m thick and is overlain 3 1 km outside ring-fracture fault ; Redrock Canyon intrusion 74%–75% SiO 2 ≤ ow tuff extends 35 km south ow tuff by as much as 1100 m of the trachydacitic to TABLE 4. SUMMARY OF MAJOR FEATURES OF THE CAETANO CALDERA AND CALDERAS IN THE STILLWATER CALDERA COMPLEX, NEVADA CALDERA THE STILLWATER AND CALDERAS IN CALDERA THE CAETANO OF OF MAJOR FEATURES 4. SUMMARY TABLE ow tuff fi in upper part of lower tuff unit and upper in upper part of lower tuff silica rhyolite to rhyolite; Carico Lake pluton 71%–73% SiO (60°–90°) extending >4 km deep; local topographic of thin ash-fl Canyon intrusion, Carico Lake pluton, small ring- fracture intrusions ring-fracture plugs 70.5%–73% SiO andesite; continuous for as much 4 km along strike rim outfl intrusion—medium-coarse grained biotite-hornblende granite porphyry (~30% phenocrysts); Carico Lake pluton—biotite-(hornblende) granite porphyry (50%–60% phenocrysts); porphyro-aphanitic rhyolite plugs along ring fractures of caldera rhyolitic tuff of Hercules Canyon (Fig. 22; John, Caetano Tuff 78%–72% SiO Tuff Caetano Lenses in intracaldera tuff near caldera walls especially Lenses in intracaldera tuff Paleozoic rocks; vertical to steep inward dipping fault Outfl 1997; this study). The composite Freeman Creek pluton ll Up to 4 km North block, ±5 km; south intrudes the central and northern parts of the Poco Canyon and Elevenmile Canyon calderas (Figs. 21 and 23). It consists of an older grano-

oor Paleozoic rocks, Eocene gravel deposits, diorite porphyry phase probably related to the Elevenmile Canyon caldera and a younger por- ow facies tuff and ow facies tuff e mesobreccia related rocks and regional distribution compositional zoning bounding structures

g phyritic granite phase probably related to the A Caldera-related units (lower compound cooling unit, upper unit Tuff Caetano Megabreccia and Intrusive rocks large (25 km Two Caldera host rocks and Thickness of caldera fi of ignimbritesVolume km 1100 Composition of caldera- Outfl Caldera fl Caldera resurgence Carico Lake pluton forcibly emplaced and domed center

986 Geosphere, August 2013

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N S N T AI M OU

E Z R T ( O ( ( ( C ( ( ( ( ( ( ( ( ( ( (

( ( ( ( (

( E Y E

L (

Mount Tenabo

L x

EUREKA CO EUREKA

A A

Cortez Hills mine CO LANDER

l l

T T

l l

N N

l l

A l

Cortez mine

C l

l l

Buried

caldera wall

RAN

Tuff dike Tuff l

Wenban Spring Wenban Mount Caetano

Caldera floor

l B

Pipeline mine

x l

Caldera wall

l l

(

Caldera wall

(

Caldera floor

(

( (

(

Pre-caldera lava flows and gravel deposits (Eocene) Cortez dikes (Eocene) Sedimentary and granitic rocks (Mesozoic Paleozoic) Megabreccia Caldera wall Hydrothermal alteration

05 km 05

V A L L E Y E L L A V

location. 1 for See Figure John et al. (2008) and Colgan (2011b). ed from

Red

L A K E K A L Mountain

Carico Lake C A R I C O C I R A C

Horse Mountain

Caetano caldera rocks (Eocene)

l l

117.00°W 116.75°W

ed geologic map of the Caetano caldera. Modiﬁ

l l

Surficial deposits and mine dumps (Quaternary) Basaltic andesite lavas and dikes (Miocene) Sedimentary rocks (Miocene) and lava flows (Oligocene) Post-caldera sedimentary rocks, tuffs, Ring fracture intrusions Carico Lake pluton Redrock Canyon intrusion Tuff Upper unit of Caetano Tuff Lower unit of Caetano Tuff Outflow Caetano

R I V E E V I R

20. Simpliﬁ Figure

R R

R E E S E S E E R A

L L

E Y Y

40.25°N 40.125°N

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118°15′W Figure 23

39°40′N ? 80

? 72 IXL pluton Carson 75 Sink ? 81

80 85 65

83 85 Job Peak 39°35′N 75 87 70

Figure 21 (legend on following 65 Dixie Valley fault page). Generalized geologic map Freeman Creek pluton of the southern Stillwater Range showing major structural fea- 85 55 tures of the Stillwater caldera

complex. Modified from John Table Mountain (1995). Box shows outline of Fig- ure 23. See Figure 1 for location. 77 Dixie Valley 58

61 63 39°30′N 65 25 35 70 75

65 55 60 42

55 20

2 km 35

7 25

39°25′N 31

Poco Canyon caldera. A 7-km-long dike zoned Comparison of the Caetano and structural features that offer unique insights into from porphyro-aphanitic rhyolite on the west to Stillwater Calderas caldera formation (Table 4). Many features are granite porphyry on the east intruded the north similar to those described in typical well-studied edges of these calderas and may be a ring-frac- The Caetano and Stillwater calderas pro- caldera systems, including thick intracaldera ture dike related to the Poco Canyon caldera vide unusual exposures of a broad spectrum of tuff, megabreccia, and resurgent intrusions, but (Figs. 21 and 23). volcanic, subvolcanic, and plutonic rocks and other features, as described below, are unique.

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EXPLANATION

Surficial deposits (Quaternary) Older gravel deposits (Quaternary and Tertiary) Post-caldera collapse Intermediate and mafic lava flows (Miocene) rocks and deposits Sedimentary rocks (Miocene) Post-caldera dikes Silicic intrusive rocks (Miocene and Oligocene) and domes Granite porphyry (Miocene or Oligocene) Freeman Creek pluton (Miocene or Oligocene) Tuff of Lee Canyon (Oligocene) Megabreccia of Government Trail Canyon (Oligocene) Sedimentary tuff (Oligocene) Tuff of Elevenmile Canyon (Oligocene) Rocks of the Stillwater Tuff of Poco Canyon (Oligocene) caldera complex Figure 21 (legend). Upper cooling unit Lower cooling unit IXL pluton (Oligocene) Younger dacite and andesite (Oligocene) Tuff of Job Canyon (Oligocene) Older dacite and andesite (Oligocene) Older rhyolite (Oligocene) Pre-caldera rocks Metamorphic and granitic rocks (Mesozoic) Contact Fault–dotted where concealed. Bar and ball on downthrown side Low-angle fault–double ticks on upper plate

87 Strike and dip of compaction foliation

72 Strike and dip of bedding

Regional and Pre–Caldera Collapse series of intermediate-composition lava fl ows the older rhyolite sequence form the fl oor of the Magmatism and breccias on the north side of the rhyolite Elevenmile Canyon caldera in the southwestern Caldera-forming ignimbrite eruptions in both dome complex (older dacite and andesite unit, Clan Alpine Mountains (Fig. 22). the Stillwater and northern Toiyabe Ranges were Fig. 23). Lava fl ows and a sill-and-dike com- The Caetano caldera was mostly built directly preceded by several million years of effusive plex underlying the south side of the caldera on Paleozoic sedimentary rocks, although it and shallow intrusive silicic magmatic activ- may have formed one or more small composite was preceded by ~6 Ma of regional, mostly ity. Cenozoic magmatic activity in the southern volcanoes. Following emplacement of the IXL silicic and mostly intrusive magmatism, includ- Stillwater Range began ca. 31(?) Ma with erup- pluton into the Job Canyon caldera a 3–4 Ma ing 35.7 Ma rhyolite dikes and fl ow domes tion of lava fl ows and minor tuffs of the older hiatus in magmatic activity preceded initial (Cortez dikes) emplaced just outside the north- rhyolite unit that probably formed a large lava eruptions that formed the Poco Canyon and east margin of the caldera (John et al., 2008, dome complex that extended east to the west Elevenmile Canyon calderas. Pre–caldera col- 2009; Moore and Henry, 2010). Only a very side of the Clan Alpine Mountains (Figs. 18, 21, lapse lava fl ows are not exposed beneath the small volume of andesitic lava erupted locally 23, and 24; Reihle et al., 1972; Hardyman et al., intracaldera tuffs of either the Poco Canyon or ~1.2 Ma prior to caldera formation. Major 1988; John, 1995, 1997; C. Henry and D. John, the Elevenmile Canyon calderas; these calderas pre-collapse doming of surrounding Paleozoic unpub. mapping). These rhyolites are geochemi- were developed in part on Mesozoic rocks, as rocks is not evident. cally distinct from, and probably genetically well as on the older rhyolite sequence and the unrelated to, the Job Canyon caldera system south block of the Job Canyon caldera (Figs. Caldera Subsidence and Caldera Floors (John, 1995). 21 and 23). Outfl ow tuff from the Job Canyon Collapse of the Caetano and Stillwater cal deras Magmatic activity related to the Job Canyon caldera, an uncorrelated tuff (rheomorphically occurred along subvertical to steeply inward- caldera system began with eruption of a thick fl owed tuff of John [1997]), and lava fl ows of dipping faults that penetrated to depths of >5 km.

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Elevenmile Canyon Poco Canyon caldera caldera

Louderback Mountains Stillwater Southwest Clan Alpine Range Mountains

Tuff of Lee Canyon Tuff of Poco Canyon, (0–1,100 m) upper cooling unit (2,500 m) Sedimentary tuff (0–600 m)

Tuff of Hercules Canyon (1,200 m) Rocks of Megabreccia blocks Elevenmile Megabreccia of Canyon Rocks of Poco Government Trail caldera Canyon caldera Canyon (1,800 m) Tuff of Elevenmile Tuff of Elevenmile Canyon (2,000 m) Canyon (3,400+ m) Sandstone and sedimentary breccia Rheomorphically (0–200 m) flowed tuff (0–60 m) Tuff of Poco Canyon, Outflow tuff of lower cooling unit Job Canyon 1,500 m) (300–600 m) Cretaceous granitic Older rhyolite rocks and Mesozoic Pre-caldera (1,000 m) metamorphic rocks Older rhyolite Pre-caldera rocks rocks (1500 m) Andesite and conglomerate Freeman Creek pluton Syncaldera Metamorphic plutons rocks Granite porphyry

Job Canyon caldera

North block

Younger dacite and 1,000 m andesite (2,500 m)

Volcaniclastic sedimentary rocks

Megabreccia blocks South block

Lacustrine sedimentary rocks (0–200 m) Younger dacite and andesite (0–500 m)

Rocks related to Tuff of Job Canyon (2,000 m) Job Canyon Tuff of Job Canyon Rocks related to caldera Job Canyon Megabreccia (1,000–2,000 m) caldera Megabreccia blocks Older dacite and andesite: Lava flows and flow Older dacite and breccias (1,100 m) andesite (800 m) Breccia and conglomerate (0–450 m) Older rhyolite Pre-caldera rocks IXL pluton (1,500 m)

Figure 22. Summary composite stratigraphic sections for the Stillwater caldera complex in the southern Stillwater Range, Louderback Mountains, and southwestern Clan Alpine Mountains.

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118°15'W Dixie Valley fault

Dixie 1 Valley IXL district 11

39°40'N 2 80 Carson 6 Sink 57

7 E Canyon ast Job 8 72 72 5 anyon C 10 W e s 75 t 7 b North block Jo ? 81 8

78 12 P o c 84 3 o Ca ny 7474 on 58 16 16 77 4 85 80 66 65 15 85 85 4 73 9

80 14 South block

83 80

85 Job Peak 39°35'N 75 13 87 65 70 2 km

EXPLANATION Surficial deposits IXL pluton Basalt and basaltic andesite Post-caldera rocks Younger dacite and andesite Silicic intrusive rocks and deposits Lacustrine sedimentary rocks Job Canyon Intermediate intrusive rocks Lava flows and flow breccias caldera Granite porphyry Megabreccia Freeman Creek pluton Tuff of Job Canyon Tuff of Poco Canyon Older dacite and andesite Upper cooling unit Poco Canyon Older rhyolite Lower cooling unit caldera Pre-caldera rocks Metamorphic rocks Megabreccia of Government Trail Canyon Contact Tuff of Lee Canyon Elevenmile Canyon Tuff of Elevenmile Canyon caldera Fault–dashed where concealed. Bar and ball on downthrown side Ring fracture–dotted where concealed 87 Strike and dip of compaction foliation

Caldera floor 72 Strike and dip of bedding 5 Structural features described in text

Figure 23. Geologic map of the Job Canyon and northern parts of the Elevenmile Canyon and Poco Canyon calderas (modifi ed from John, 1995). Numbered features: 1—Job Canyon caldera wall; 2—Mesozoic rocks underlying fl oor of Job Canyon caldera; 3—structural boundary between north and south blocks in Job Canyon caldera; 4—andesite sill and dike complex underlying south block of Job Canyon caldera; 5—feeder dike for younger dacite and andesite unit; 6—lacustrine sedimentary rocks in Job Canyon caldera; 7 and 8—subsidiary growth faults in interior of Job Canyon caldera; 9—early faults in south block of Job Canyon caldera; 10—roof of IXL pluton; 11—polymetallic skarn occurrences around IXL pluton (IXL district); 12—north ring fracture of Elevenmile Canyon and Poco Canyon calderas; 13—fault separating structural blocks in Elevenmile Canyon and Poco Canyon calderas and forming south margin of Job Canyon caldera; 14—fl oor of Elevenmile Canyon caldera; 15—fl oor of Poco Canyon caldera; 16—granite porphyry ring-fracture dike along north margin of Poco Canyon caldera.

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Figure 24. Photographs of fea- Caldera wall tures in the Caetano and Still- water calderas. (A) Northeast structural wall of the Caetano caldera in the Toiyabe Range. View looking west along the Caetano Tuff Slaven Chert Copper fault, which separates Devonian Slaven Chert from intracaldera Caetano Tuff. (B) Exposure of the northeast structural wall of Caetano caldera in the northern Toiyabe Range looking north (out- A B crop on ridgeline at the top of photo A). Wall rocks consist of brecciated, silicifi ed Slaven Chert with oblique west- plunging slickensides (yellow arrow). Compaction foliation Tuff of Elevenmile Canyon in Caetano Tuff dips ~40° east (with marble blocks) Megabreccia and restoration of tuff to horizontal results in approximately vertical-plunging slickensides. (C) View looking east at the southwest structural margin of the Elevenmile Canyon caldera in Elevenmile Canyon, C D southern Stillwater Range. The caldera margin consists of a IXL Dixie Valley silicifi ed fault contact between district intracaldera tuff of Eleven- Job Peak IXL Canyon mile Canyon containing sparse, large blocks of Mesozoic mar- Older Rhyolite Mesozoic IXL pluton ble and massive megabreccia rocks consisting mostly of blocks of hornfelsed Mesozoic argil- Freeman Creek pluton lite with minor tuff matrix. The fault is presently oriented approximately N60°W, 85°SW but untilting by rotation of E F compaction foliation in nearby tuff to horizontal restores fault to a steep north dip into the caldera. This fault apparently is a subsidiary fault subparallel to the caldera- bounding fault that lies ~150 m south in Elevenmile Canyon where the photograph was taken. (D) Caetano Tuff dike intruded into the outer ring fracture along northeast margin of the caldera. The dike in the left side of the photo is strongly fl ow banded with light-colored, semi-continuous, variably vesicular bands separated by darker bands. Fragments of Paleozoic rocks up to a few centimeters in diameter are common. Across a sharp transition zone in the middle of the photo, the light-colored bands have broken into individual pumice blocks. The rock on the right is a densely welded, pumice-rich ash-fl ow tuff in which the compaction foliation is irregularly parallel to the dike margin. Flow-banded rock and individual pumice blocks have compositions that are indistinguishable from the most highly evolved Caetano Tuff. Hammer handle is ~55 cm long. (E) View looking west at the west side of Dixie Valley and the east side of the Stillwater Range. The composite Freeman Creek pluton, which is inferred to represent unerupted residual magma from the Elevenmile and Poco Canyon calderas, forms light-colored outcrops in the lower part of the range. Most of the darker rocks in the upper part of the range are composed of an ~2-km-thick sequence of rhyolite fl ow domes (older rhyolite unit) that underlie all three calderas of the Stillwater caldera complex. The roof of the pluton dips steeply (60°–90°) west. On the left side of the photo, the total vertical relief through the pluton is ~800 m and Job Peak is ~1400 m above the range front. (F) View east down IXL Canyon from the top of the Stillwater Range showing the north margin of the IXL pluton. The pluton margin is mostly an approximately east-striking, subvertical intrusive contact into Mesozoic metasedimentary rocks that lies directly below (east of) the Job Canyon caldera wall. Mine workings in the IXL district are in small polymetallic skarns formed in limestone beds within the Mesozoic rocks.

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Thicknesses of intracaldera tuffs and post–cal- gins (Fig. 20; John et al., 2008; Moore and K/Ca indistinguishable from those of Caetano dera collapse rocks indicate that caldera sub- Henry, 2010; Colgan et al., 2011b). On the Tuff and chemical analyses of both rock types sidence exceeded 4 km for all four calderas. northeast margin, the greatest collapse (up to overlap with analyses of evolved Caetano Tuff. Caldera-collapse fault zones (structural mar- 4 km) occurred along an inner fracture that tran- The composite dike is interpreted as a feeder for gins) are narrow in the deeper parts of the cal- sitions upward from a discrete fault zone to a erupted Caetano Tuff that tapped sparsely and deras and transition upward to wall slump zones topographic wall. The topographic wall here fi nely porphyritic rhyolite—the most compo- (topographic margins) that contain large land- (and elsewhere in the caldera) is only ~1 km sitionally evolved part of the Caetano magma slide blocks of wall rocks; the structural and outboard of the structural margin, and mega- chamber—and subsequently more abundantly topographic margins are separated by ≤1 km. breccia constitutes no more than a few percent porphyritic rhyolite that was highly evolved but Exposed caldera fl oor segments are coherent of intracaldera fi ll. A smaller amount of collapse more typical of Caetano Tuff. Similar highly blocks as much as 4 km long and imply that the occurred along an outer ring fracture, which con- evolved, fi nely porphyritic tuff locally forms the calderas collapsed as large piston-like blocks. tains a 10–40-m-wide, fl ow-banded to lithic and basal few tens of meters of intracaldera Caetano Caldera subsidence in the Stillwater Range pumice-rich ash-fl ow tuff dike that fed a highly Tuff near Wenban Spring in the Toiyabe Range, dropped coherent wall-rock masses along sub- evolved phase of the eruption (Figs. 20 and 24; where it is overlain by >3000 m of crystal-rich vertical ring-fracture faults, thereby juxtapos- see section on vents and feeder dikes). Caldera (30%–45% phenocyrsts) rhyolite tuff. ing thick sequences of intracaldera ash-fl ow tuff structural-margin faults, including the outer ring Ash-fl ow tuff feeder dikes are not exposed against Mesozoic wall rocks and large landslide fracture, are locally exceptionally well exposed in the Stillwater Range. Several andesite dikes blocks sloughed off caldera walls forming cal- and dip 60°–70° inward to ~4–5 km depth (Fig. intrude intracaldera tuff of Job Canyon and dera-collapse breccia (Figs. 21 and 24; John and 20). Outward-dipping reverse faults required apparently fed post-collapse lava fl ows of the Silberling, 1994; John, 1995). Topographic walls to make space for caldera collapse and promi- Job Canyon caldera (John, 1995). These dikes outside the structural margins are not preserved, nently developed in analog models (Kennedy are truncated by the underlying IXL pluton. and along the north side of the Job Canyon cal- et al., 2004; Acocella, 2007; Howard, 2010) dera and the south side of the Eleven mile Canyon are not exposed but could be concealed beneath Late Intracaldera Magmatism caldera, the topographic walls nearly coincided intracaldera tuff. Both the Caetano caldera and calderas in the with the ring fractures. Each of the calderas col- The Caetano caldera fl oor subsided asymmet- Stillwater Range are underlain by large com- lapsed as two or more internally coherent blocks rically as apparently coherent block(s) during posite granitic plutons. Extensive data for the that had greatly different amounts of subsidence eruption of the Caetano Tuff. Early subsidence Caetano system argue strongly that the granitic (e.g., ~5 km and ~2 km in the Job Canyon cal- was greater in the eastern part of the caldera, bodies were residual magma from the reservoir dera, Figs. 21, 22, and 23). Subsidiary growth where lower intracaldera Caetano Tuff ponded that was the source of the Caetano Tuff. Pre- faults with relatively minor displacement cut and formed a single compound cooling unit as liminary data from the Stillwater calderas allow post–caldera collapse deposits within the Job much as 3 km thick. Subsequent subsidence was a similar interpretation. These plutons have steep Canyon caldera. A fault that separates the two greater in the western part of the caldera, where margins and fl at roofs, are exposed over several- structural blocks of the Job Canyon caldera later the upper intracaldera Caetano Tuff is as much kilometer depth ranges, and have faulted lower formed the north walls of both the Poco Canyon as 2 km thick but thins to <0.5 km in the eastern contacts, which indicate that they are large stocks and Elevenmile Canyon calderas. A second, part of the caldera. Where exposed mostly in the or the apical parts of batholiths (Figs. 20, 21, and long-active fault formed the south margin of the eastern part of the caldera, the caldera fl oor is a 24). The shallowly emplaced (≤1–2 km) resur- Job Canyon caldera and separated the Poco Can- planar surface subparallel to compaction folia- gent plutons in the Caetano caldera show evi- yon and Elevenmile Canyon cal deras into struc- tion in overlying tuff (Fig. 20). dence for both passive and forcible emplacement, tural blocks with distinctly different thicknesses whereas the more deeply emplaced (≥4–5 km) of caldera fi ll (Fig. 23). Vents and Feeder Dikes for the Caldera- plutons in the southern Stillwater Range appear The fl oors of the Elevenmile Canyon and Related Rocks to have been passively emplaced. Stoped wall- Poco Canyon calderas and of the south block A 10–40-m-thick composite dike, composed rock blocks are rare to absent in all plutons. and part of the north block of the Job Canyon of sparsely porphyritic rhyolite cut by abun- Eruption of the upper Caetano Tuff was caldera are mostly preserved in the Stillwater dantly porphyritic rhyolite tuff, is exposed along closely followed (≤0.1 Ma) by emplacement Range (Figs. 21 and 23). In addition, much an outer ring fracture on the northeast margin of of two large resurgent granitic intrusions in the of the fl oor of the Elevenmile Canyon caldera the Caetano caldera (Figs. 20 and 24; Moore and center of the caldera (Fig. 20; John et al., 2008; is preserved in the southwestern Clan Alpine Henry, 2010; Colgan et al., 2011b). The sparsely Colgan et al., 2011b). Both the 34.01 ± 0.05 Ma Mountains (John, 1997; this study). Topography porphyritic rhyolite is massive to locally fl ow Carico Lake and the 33.90 ± 0.05 Ma Redrock of caldera fl oors is characterized by relatively banded, whereas the tuff is strongly fl ow banded Canyon plutons intruded the upper unit of the fl at to undulatory surfaces several kilometers near its margins; the fl ow bands disaggregate Caetano Tuff indicating that they ascended to long, locally offset by small-displacement (hun- into individual fl attened pumice blocks inward. within <1–2 km of the surface. Sensitive high dreds of meters), steeply dipping faults that Both the fl ow-banded and disaggregated parts resolution ion microprobe (SHRIMP) U-Pb were active during or shortly following caldera of the tuff contain abundant to sparse fragments zircon and sanidine 40Ar/39Ar tuff eruption and collapse (Fig. 23). Compaction foliation in tuffs up to 25 cm in diameter of Paleozoic rocks pluton ages are indistinguishable, and the ages, and bedding in caldera-fi lling deposits are sub- similar to nearby wall rocks and of porphyritic whole-rock major, trace element, and rare earth parallel to caldera fl oors. rhyolite. Both sparsely porphyritic rhyolite and element (REE) geochemistry, mineral chem- Collapse of the Caetano caldera occurred tuff contain the same phenocryst assemblage istry, and oxygen isotope signatures (John dominantly along a single ring fracture, as Caetano Tuff, and the tuff has phenocrysts et al., 2008, 2009; Watts et al., 2012a, 2012b) although a composite ring fracture is preserved in the same abundance as the Caetano Tuff. indicate a genetic connection between the on the north-central and northeast caldera mar- Both phases have sanidine 40Ar/39Ar ages and Caetano Tuff and Carico Lake pluton (Fig. 25).

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Caetano Caldera Rocks 12 6 7.03 6.3 Tephri- Trachyte phonolite

10 Phono- 5 Tephrite Trachydacite Trachy- andesite 8 Rhyolitehy e 4 Basaltic Caetano Tuff, outflow trachy- Caetano Tuff pumice lump andesite Rhyolite 6 Trachy- 3 Caetano Tuff, upper unit basalt Caetano Tuff, top, lower unit O (weight percent) 2 Dacite Caetano Tuff, lower lower unit Basaltic Andesite O+K 4 2 2 CaO (weight percent) Ring-fracture intrusions Basalt andesite

Na Carico Lake pluton

2 1 Redrock Canyon intrusion Post-caldera andesites Pre-caldera andesites 0 0 45 50 55 60 65 70 75 80 55 60 65 70 75 80

SiO2 (weight percent) SiO2 (weight percent)

1000 60

50 800

40 600

Nb (ppm) 30 Sr (ppm) 400 20

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0 0 55 60 65 70 75 80 0 100 200 300 400 500 Zr (ppm) SiO2 (weight percent)

6000

5000 100 4000

Ba (ppm) 3000

Rock/Chondrites 10 2000

1000

1 0 55 60 65 70 75 80 La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

SiO2 (weight percent)

Figure 25 (on this and following three pages). Plots showing compositional variation of rocks related to the Caetano, Job Canyon, Poco

Canyon, and Elevenmile Canyon calderas. For each caldera system, total alkali-silica, CaO-SiO2, Sr-SiO2, Ba-SiO2, Zr-Nb, and rare earth elements are plotted for whole-rock samples. Major elements are normalized to 100% volatile free. Rare earth elements are normalized using chondrite values of Sun and McDonough (1989). (A) Caetano caldera, including pre- and post-caldera lava ﬂ ows, and pumice lumps.

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Job Canyon Caldera Rocks

12 6 Tephri- Trachyte phonolite

10 Phono- 5 Tephrite Trachydacite Trachy- andesite 8 4 Basaltic Rhyolite trachyandesite 6 Trachy- 3 basalt O (weight percent) 2 Dacite Andesite O+K 4 Basaltic 2 2 Basalt andesite CaO (weight percent) Na

2 1

0 0 45 50 55 60 65 70 75 80 55 60 65 70 75 80 SiO (weight percent) 2 SiO2 (weight percent)

1000 60

50 IXL pluton 800 Younger dacite and andesite Outflow tuff of Job Canyon 40 Tuff of Job Canyon 600 Older dacite and andesite 30 Sr (ppm) Nb (ppm) 400 20

200 10

0 0 55 60 65 70 75 80 0 100 200 300 400 500 Zr (ppm) SiO2 (weight percent)

6000

5000 100

4000

3000 Ba (ppm)

10 2000

1000

1 0 55 60 65 70 75 80 La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

SiO2 (weight percent)

Figure 25 (continued). (B) Job Canyon caldera.

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Elevenmile Canyon Caldera Rocks

12 6 Tephri- Trachyte phonolite

10 Phono- 5 Tephrite Trachydacite Trachy- andesite 8 4 Basaltic trachy- Rhyolite andesite 6 Trachy- 3 basalt O (weight percent) O (weight 2 Dacite

O+K Andesite 2 4 Basaltic 2 Basalt andesite CaO (weight percent) Na

2 1

0 0 45 50 55 60 65 70 75 80 55 60 65 70 75 80 SiO (weight percent) 2 SiO2 (weight percent)

1000 60 Sedimentary tuff unit Freeman Creek pluton 50 800 Tuff of Hercules Canyon Tuff of Lee Canyon 40 Tuff of Elevenmile Canyon 600 Tuff of Painted Hills

Nb (ppm) 30 Sr (ppm) 400 20

200 10

0 0 55 60 65 70 75 80 0 100 200 300 400 500 Zr (ppm) SiO2 (weight percent)

6000

5000 100 4000

Ba (ppm) 3000

Rock/Chondrites 10 2000

1000

1 0 55 60 65 70 75 80 La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

SiO2 (weight percent)

Figure 25 (continued). (C) Elevenmile Canyon caldera and tuff of Painted Hills.

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Poco Canyon Caldera Rocks

12 6 Tephri- Trachyte phonolite Granite porphyry 10 Phono- 5 Tephrite Trachydacite Trachy- Freeman Creek pluton andesite Upper tuff of Poco Canyon 8 4 Basaltic Rhyolite Megabreccia of Government Trail Canyon trachyandesite Lower tuff of Poco Canyon 6 Trachy- 3 Tuff of Chimney Spring basalt

O (weight percent) New Pass Tuff 2 Dacite Basaltic Andesite O+K 4

2 2 Basalt andesite CaO (weight percent) Na

2 1

0 0 45 50 55 60 65 70 75 80 55 60 65 70 75 80

SiO2 (weight percent)

1000 60

SiO2 (weight percent)

50 800

40 600

Nb (ppm) 30 550 Sr (ppm) 400 20

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0 0 55 60 65 70 75 80 0 100 200 300 400 500 Zr (ppm) SiO2 (weight percent)

6000

5000 100

4000

Ba (ppm) 3000

Rock/Chondrites 10 2000

1000

1 0 55 60 65 70 75 80 La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

SiO2 (weight percent)

Figure 25 (continued). (D) Poco Canyon caldera, New Pass Tuff, and tuff of Chimney Spring. Data from John (1995), John et al. (2008), and this study.

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To explain cessation of tuff eruption while simi- tem, whereas the granite phase likely is related greater than the thickness of intracaldera tuff, lar magma remained in the chamber, we specu- to the Poco Canyon caldera (John, 1995). which probably represents Caetano magma late that collapse along the exposed, upper, Available geochronologic data provide gen- erupted as extracaldera fallout and ash-ﬂ ow inward-dipping faults along with ponding of eral support for this interpretation, but as noted tuffs. Because of the extraordinary exposures of several kilometers of intracaldera Caetano Tuff previously, there are inconsistencies between intracaldera rocks, this estimate is more robust self-sealed eruption conduits. stratigraphic relations and sanidine 40Ar/39Ar than estimates for most other large-volume, The early Redrock Canyon intrusion steeply ages and between U-Pb and 40Ar/39Ar ages. caldera-forming tuffs, including those for the cuts and did not deform the surrounding tuff. In Weighted mean SHRIMP zircon U-Pb ages of Stillwater calderas. contrast, Caetano Tuff wall rocks of the slightly the granodiorite (25.01 ± 0.15 Ma) and granite John (1995) estimated volumes of intracal- younger Carico Lake pluton were rotated ~90° (24.83 ± 0.20 Ma) phases of the Freeman Creek dera rocks as ~200 km3 each for the Job Canyon along both the north and south sides of the pluton pluton are analytically indistinguishable, and and Poco Canyon calderas and ~600 km3 for the indicating forcible emplacement of the pluton. only slightly younger than sanidine 40Ar/39Ar Elevenmile Canyon caldera. These estimates The contrasting behavior of the two intrusions ages of 25.07 ± 0.06 Ma, 25.05 ± 0.06 Ma, and assumed circular calderas and used exposed might be related to variations in their crystal- 25.26 ± 0.07 Ma for the tuff of Elevenmile Can- widths of calderas and average thicknesses of linity; the Redrock Canyon intrusion contains yon, tuff of Hercules Canyon, and upper unit of caldera-related deposits to calculate volumes. <30–40 vol% phenocrysts in an altered felsite the tuff of Poco Canyon, respectively (Table 1). The volume estimated for the Elevenmile Can- groundmass, whereas the Carico Lake pluton However, these tuffs have weighted mean yon caldera did not consider continuation of contains ~60% phenocrysts in an aplitic ground- SHRIMP zircon U-Pb ages of 25.27 ± 0.25 Ma, the caldera east of Dixie Valley into the south- mass. Locally abundant xenoliths of high-grade 25.43 ± 0.19 Ma, and 25.70 ± 0.19 Ma, respec- western Clan Alpine Mountains. Subsequent metamorphosed pelitic rocks in the Carico Lake tively, which are somewhat older than their sani- mapping indicates that the tuff of Elevenmile pluton are interpreted as mid-crustal wall rocks dine 40Ar/39Ar ages. Tuff U-Pb zircon ages being Canyon is >2000 m thick and contains mega- of the Caetano magma chamber that were incor- older than 40Ar/39Ar eruption ages is similar to breccia lenses in this area, which suggest that the porated in the pluton during its emplacement. age relations characteristic of some other large- Elevenmile Canyon caldera is much larger and In the Stillwater Range, the large exposed volume silicic ignimbrites, such as in the Tim- the volume of erupted products (mostly the tuff thicknesses of the IXL pluton (4000–5000 m) ber Mountain–Oasis Valley caldera complex, of Elevenmile Canyon) was much greater than and the Freeman Creek pluton (~2000 m), both Nevada (Bindeman et al., 2006). The composite, 600 km3, probably at least 1050–1400 km3 (Fig. with faulted lower contacts, suggest that they porphyro-aphanitic–granitic ring-fracture dike 21; John, 1997; this study). The widespread dis- probably are the upper parts of batholiths. along the north margin of the Freeman Creek tribution of outﬂ ow tuff from the Poco Canyon The IXL pluton underlies and intrudes the pluton is the only exposed intrusion along a ring caldera (New Pass Tuff, tuff of Chimney Spring; lower part of the Job Canyon caldera (Fig. 23). fracture in the Stillwater calderas (Fig. 21). Fig. 11) suggests that the previous volume esti- It is zoned compositionally from a more felsic The absence of structural doming of the cal- mate for this caldera also is likely too small.

top with ~68 wt% SiO2 to ~59–60% SiO2 in its deras in the Stillwater Range during magma deepest exposures ~4.5 km below the pluton resurgence and pluton emplacement may be Compositions of the Caetano and roof (John, 1995). The deeper parts of the pluton due to relatively deep pluton emplacement Stillwater Tuffs are compositionally similar to the post-collapse and/or continued eruption of lava ﬂ ows during Both the Caetano Tuff and the upper tuff of lava ﬂ ow sequence in the Job Canyon caldera, magma resurgence. The top of the IXL pluton Poco Canyon have rhyolite to high-silica rhyo-

whereas the top of the pluton is compositionally was emplaced at a depth of ~4–5 km. The depth lite compositions (72%–78% SiO2) and gener- similar to the most mafi c part of intracaldera of emplacement of the top of the Freeman Creek ally contain 30–45 vol% phenocrysts mostly tuff of Job Canyon (Fig. 25). Weighted mean pluton is indeterminate due to structural uncer- composed of quartz, sanidine, and plagioclase SHRIMP zircon U-Pb ages for intracaldera tuff tainties, but probably it was somewhat deeper (Figs. 25 and 26, Supplemental Fig. 26; Table 2; of Job Canyon and the IXL pluton are 29.43 ± (>6–7 km from the paleosurface) than the top John, 1995; John et al., 2008). The Caetano Tuff 0.29 and 29.20 ± 0.29 Ma, respectively. Whole- of the IXL pluton (John, 1995). Eruption of the has relatively uniform crystal content, is compo- rock geochemical data, U-Pb zircon ages, and thick younger dacite and andesite unit after col- sitionally zoned upward from high-silica rhyo- fi eld relations suggest a genetic relationship lapse of the Job Canyon caldera and prior to lite to low-silica rhyolite, and except for sparse between caldera-fi lling rocks and the IXL emplacement of the IXL pluton suggests that crystal-poor pumice clasts, lacks a crystal-poor pluton. pressure caused by magma resurgence was eruptive phase. The earliest eruptive part of The composite Freeman Creek pluton under- released by magma eruption rather than by the Caetano magma is geochemically the most lies the Poco Canyon caldera and northern part structural doming of caldera-related deposits; highly evolved, but these rocks still contain of the Elevenmile Canyon caldera and is inferred subsequent emplacement of the IXL pluton may ~25% phenocrysts. The upper tuff of Poco Can- to represent the plutonic roots of both caldera have been at depths too great to uplift the cal- yon is less well characterized than the Caetano systems (John, 1995). The Freeman Creek dera and the overlying lava fl ows. Tuff, but available data suggest compositional pluton consists of two main phases, an earlier medium-grained hornblende biotite granodio- Volumes of the Caetano and Stillwater Tuffs 6Supplemental Figure 2. Plots showing whole- rite (66–67 wt% SiO2) and later coarse-grained The volume of erupted magma that formed rock composition and phenocryst abundance in 3 porphyritic biotite granite (71–74 wt% SiO2) the Caetano Tuff was estimated as ~1100 km crystal-rich rhyolites of (A) the Caetano Tuff and that locally contains K-feldspar megacrysts as by John et al. (2008). This estimate presumes a (B) upper unit of the tuff of Poco Canyon. Data from much as 1.5 cm long. On the basis of whole- restored caldera area of ~280 km2 after remov- John (1995), John et al. (2008), and this study. If you are viewing the PDF of this paper or reading it offl ine, rock geochemical data and distribution of the ing 100% post-caldera extension (Colgan et al., please visit http://dx.doi.org/10.1130/GES00867.S6 two phases, the granodiorite is inferred to be 2008), an average thickness of 3 km for intracal- or the full-text article on www.gsapubs.org to view related to the Elevenmile Canyon caldera sys- dera Caetano Tuff, and caldera collapse ~1 km Supplemental Figure 2.

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ABCaetano Caldera Poco Canyon Caldera

4500 2500

4000

Major cooling break 2000 3500

3000 1500 2500 Intracaldera tuff 2000 Intrusive rocks 1000

1500

1000

Height above tuff base (m) Height above 500

500

0 Stratigraphic position (m above caldera floor) 0 010203040506070 0 102030405060 Phenocrysts (volume percent) Phenocrysts (volume percent) 4500 2500

4000

Major cooling break 2000 3500

3000

1500 2500

2000 1000 1500

1000 tuff base (m) Height above 500

500

Stratigraphic position (m above caldera floor) 0 0 0.00 0.10 0.20 0.30 0.40 0.50 0 0.1 0.2 0.3 0.4 0.5 TiO (weight percent) 2 TiO2 (weight percent)

Intracaldera Caetano Tuff Sections Poco Canyon Caldera Toiyabe Range Upper cooling unit, tuff of Poco Canyon Tub Spring Freeman Creek pluton, granite phase Rocky Pass North Carico Valley Mt Caetano North wall Toiyabe Range Intrusive rocks

Figure 26. Plots showing whole-rock composition and phenocryst abundance in crystal-rich rhyolites of (A) the Caetano Tuff and (B) the upper unit of the tuff of Poco Canyon. Data from John (1995), John et al. (2008), and this study.

zoning is more irregular than in the Caetano rial, the crystal-rich, high-silica rhyolite (77%– locally overlain by as much as 200 m of coarse

Tuff. These data indicate that both the Caetano 78% SiO2) lower unit of the tuff of Poco Canyon collapse breccia, sedimentary rocks, and minor and Poco Canyon magma systems erupted large has distinctly highly evolved compositions. This tuff (sandstone and sedimentary breccia unit, volumes (>200–1100 km3) of crystal-rich, mostly small(?)-volume tuff has not been identiﬁ ed out- Fig. 22), collapsed in trapdoor fashion. Major high-silica rhyolite. side of the caldera, and its eruption may have caldera subsidence accompanied eruption of The eruptive history of the Poco Canyon cal- caused crustal sagging rather than caldera col- the overlying megabreccia of Government Trail dera is more complicated than that of the Caetano lapse (John, 1995). Alternatively, perhaps only Canyon, which is a highly evolved, crystal-poor caldera (Fig. 22), but the earliest erupted mate- the southern part of the caldera, where the tuff is (5%–10% pheno crysts) high-silica (77%–78%

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SiO2) rhyolite that contains numerous large exposed as the tilted and extended Caetano and Caldera Hydrothermal Systems wall-rock blocks including some composed Stillwater calderas. Consequently, a signifi cant Large hydrothermal systems were active in of the lower tuff of Poco Canyon. The mega- group of western Nevada volcanic fi eld calderas Caetano and Job Canyon calderas shortly after breccia is overlain by the volumetrically domi- appear to have retained large volumes of resid- their formation; a wide variety of hydrothermal nant, crystal-rich (25%–45%) upper unit of the ual magma after ash-fl ow eruption (see also features are exposed in caldera-fi lling rocks tuff of Poco Canyon, which also is mostly high- Lipman, 2007; Zimmerer and McIntosh, 2013), and in underlying plutons (Fig. 27; John and

silica rhyolite (74%–78% SiO2) and which, which contrasts with interpretations that some Pickthorn, 1996; John et al., 2011). However, based on 40Ar/39Ar ages, is correlative with the calderas have small magma chambers that are the types of hydrothermal systems were sig- regionally widespread New Pass Tuff and tuff of completely evacuated during ash-fl ow eruption nifi cantly different in the two calderas, and no Chimney Spring. (Glazner et al., 2008; Tappa et al., 2011; Zim- signifi cant mineral deposits have been identifi ed The Caetano Tuff and tuff of Poco Canyon are merer and McIntosh, 2012). in either caldera. similar to several other large-volume ignimbrites In the Caetano caldera, hydrothermally in the western Nevada volcanic fi eld, includ- Overlap of the Poco Canyon and Elevenmile altered rocks are exposed over >100 km2 and ing the tuffs of Arc Dome and Toiyabe (John, Canyon Calderas represent depths from the paleosurface to 1992a), the tuff of Round Mountain (Shawe, Overlap of the Elevenmile Canyon and Poco >1–1.5 km throughout much of the west half of 1995), the tuff of Moores Creek (Boden, 1994), Canyon calderas both in time and space sug- the caldera and farther east along the caldera’s and the Fish Creek Mountains Tuff (McKee, gests that two nearby, shallow-level, silicic south wall (Figs. 27A and B; John et al., 2011). 1970). All are phenocryst rich (>20%–50%), magma chambers were erupting nearly simulta- The hydrothermal system affected intracaldera

composed of high-silica (73%–78% SiO2) rhyo- neously. However, these calderas have discrete Caetano Tuff—especially thin ash ﬂ ows and lites, and have volumes of hundreds to more than collapse features and different distributions of interbedded lacustrine sediments in the upper one thousand cubic kilometers. None of these intracaldera rocks. In addition, lithic fragments unit of the tuff—the Redrock Canyon intrusion, tuffs are known to have appreciable volumes in caldera-related pyroclastic rocks are notably and locally, Paleozoic wall rocks. The Carico of crystal-poor (<20 vol%) tuff. Crystal-rich different; pre-Tertiary lithic clasts are scarce Lake pluton postdates and truncates the hydro- rhyolite tuffs also constitute a substantial part to absent in rocks related to the Poco Canyon thermal system, which indicates (1) very rapid of the central Nevada volcanic ﬁ eld (Best et al., caldera but are nearly ubiquitous in the tuff of development of the hydrothermal system fol- 2013c). These tuffs are strikingly different Elevenmile Canyon, suggesting that these rocks lowing caldera collapse and (2) that the hydro- from the two end members usually envisioned were erupted through different parts of the crust. thermal system had a short lifespan (<<100 for large zoned caldera-forming ignimbrites: Thicknesses of intracaldera rocks, locations of k.y.). The hydrothermal system formed in two crystal-poor, high-silica rhyolite, which grades collapse features, and the location of the Free- stages. An early stage resulted in widespread, upward into lower-silica rhyolite, and crystal- man Creek pluton suggest that the magma relatively homogeneous, advanced argillic alter- rich dacite (monotonous intermediates of chamber of the Elevenmile Canyon caldera was ation dominated by a quartz-kaolinite-pyrite Hildreth [1981]) (Bachman and Bergantz, 2008; located somewhat south, west, or east of the assemblage that also includes local occurrences Huber et al., 2012), suggesting that some cur- magma chamber that formed the Poco Canyon of dickite, alunite, and residual quartz altera- rent models for large-volume ignimbrites are caldera. Compositions of intracaldera rocks tion, mostly along the south caldera wall (Fig.

incomplete. of the two calderas overlap at ~74 wt% SiO2, 27A). Alteration intensity generally decreases The compositional continuum from ash-fl ow but their trace element and REE contents have to the north and locally to the west, where it tuff to post-collapse intrusions in the Caetano distinct trends and differences at similar silica grades into an intermediate argillic assemblage and Stillwater calderas and the overlap in their contents (Fig. 25), which suggests that post- (montmorillonite, illite, and/or kaolinite after ages demonstrate that large volumes of residual, collapse caldera-fi lling rocks of both calderas plagioclase phenocrysts), in which sanidine unerupted magma remained in their source did not tap different parts of a single, compo- phenocrysts are unaltered, and eventually into magma chambers following ash-flow tuff sitionally zoned magma chamber. Although unaltered tuff. The alteration assemblage appar- eruption. Several other calderas in the western the post-collapse sedimentary tuff unit in the ently varies little vertically, which suggests rela- Nevada volcanic fi eld, such as the lower and southern and central parts of Elevenmile Can- tively uniform temperatures ≤270 °C based on upper Mount Jefferson calderas of the Toquima yon was inferred to be related to the Elevenmile the absence of pyrophyllite. Early hydrothermal caldera complex and the Fairview Peak caldera, Canyon caldera (John, 1995), its REE pattern fl uids are inferred to represent upfl ow of hot erupted ring-fracture rhyolites that are indis- more closely resembles REE patterns of units in magmatic volatiles released from crystallizing tinguishable in age from or slightly younger the Poco Canyon caldera; consequently this unit residual Caetano magma at depth and conden- than their associated intracaldera tuffs (this may be a late, small-volume eruptive product of sation into the lake covering the caldera fl oor study). The Campbell Creek caldera includes the Poco Canyon caldera. These data suggest and the underlying water-saturated upper unit of post-collapse rhyolite and dacite intrusions that the existence of two separate magma chambers Caetano Tuff. Magmatic volatile and heat fl uxes are indistinguishable in age from and compo- beneath the Poco Canyon and Elevenmile Can- likely were greatest along the south caldera wall sitionally similar to intracaldera tuff (Henry yon calderas. Characteristics of their eruptive where alunite, dickite, and residual quartz alter- et al., 2012a). Moreover, the dacite intrusions products preclude physical interaction of these ation was most strongly developed. The hydro- are petrographically and compositionally simi- magma chambers during caldera-forming activ- thermal fl uids fl owed laterally, mostly to the lar to sparse dacite pumice in the intracaldera ity, although eruption of one magma chamber north and east along the caldera wall, resulting tuff. Whether or not other calderas in the west- may have triggered eruption of the other (e.g., in the pervasive advanced argillic alteration. The ern Nevada volcanic fi eld also retained sig- Christiansen, 1979). However, multiple, compo- later stage of the hydrothermal system is charac- nifi cant volumes of unerupted residual magma sitionally varied intrusions within the Freeman terized by oxidation of early-formed pyrite and is not known, because (1) they have not been Creek pluton suggest magma interaction during cross-cutting hematite ± barite alteration (Fig. studied suffi ciently or (2) they are not as well pluton emplacement. 27B). This stage probably represents decreased

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fl uxes of heat and magmatic volatiles, cooling, SUMMARY AND COMPARISON WITH analyses range from 66.6% to 69.6% SiO2. This and downward collapse of the hydrothermal THE CENTRAL NEVADA AND INDIAN tuff is a composite unit consisting of as many as system, which resulted in narrow cross-cutting PEAK–CALIENTE CALDERA FIELDS four cooling units possibly emplaced over sev- veins and hydrothermal breccias. eral hundred thousand years, much longer than In the north block of the Job Canyon caldera, The western Nevada volcanic fi eld, the west- typical of caldera-forming ignimbrites. How- most caldera-fi lling rocks, including the pre-cal- ernmost part of the Great Basin ignimbrite ever, the Monotony Tuff, a defi nite monotonous dera lava fl ow sequence, and the upper parts of province, covers ~40,000 km2 in western and intermediate of the central Nevada fi eld, has a the underlying IXL pluton are pervasively hydro- central Nevada. At least 23 calderas and their wider compositional range and also consists of thermally altered (Fig. 27C; John and Pickthorn, caldera-forming tuffs are reasonably well iden- multiple cooling units (Best et al., 2013c). Isom- 1996). Alteration is dominated by near-neutral tifi ed in the western Nevada volcanic fi eld, and type trachydacites are restricted to a few, small- pH, propylitic alteration assemblages of quartz– the presence of another 14 areally extensive, volume, probably non–caldera forming tuffs. In Na-plagioclase–sericite–chlorite and quartz–Na- apparently voluminous ash-fl ow tuffs whose contrast, monotonous intermediates and Isom- plagioclase–sericite–chlorite–epidote (both ± sources are unknown suggests a similar number type trachydacites are common and particularly calcite, K-feldspar, and pyrite). Epidote-bear- of additional calderas. Most tuffs are rhyolites, voluminous in the other two fi elds (Best et al., ing rocks are present at paleodepths >3–4 km with approximately equal numbers of sparsely 2013b, 2013c). Volumes of individual tuffs are except along the south margin of the caldera, porphyritic (≤15% phenocrysts) and abundantly diffi cult to estimate in the western Nevada fi eld, where epidote at much shallower paleodepths porphyritic (~20–50% phenocrysts) tuffs. Both because many tuffs primarily fi lled their source (<1 km) likely indicates an upfl ow zone for hot sparsely and abundantly porphyritic rhyolites calderas and/or fl owed and were deposited in hydrothermal fl uids. Hydrothermal fl uids were commonly show chemical or petrographic paleovalleys, so that they are irregularly distrib- low salinity (mostly ≤1 wt% NaCl equivalent), evidence of normal compositional zoning uted. Nevertheless, volumes of individual tuffs 3 neutral pH, and supercritical, and had low CO2 from silicic bases to more mafi c compositions could range as large as ~3000 km ; at least four contents. Fluid inclusions in quartz-calcite upward. Several well-exposed, thick (2–4 km) are probably greater than 1000 km3. veins at 3–4 km paleodepth indicate fl uid tem- intracaldera tuffs are almost entirely crystal-rich The tuffs and calderas range in age from 34.4 peratures of ~290–315 °C. Oxygen and hydro- (30%–45% phenocrysts) rhyolite or high-silica to 23.3 Ma, and one caldera formed at 19.5 Ma. gen isotope data indicate that the fl uids were rhyolite, an end member generally not included Most caldera-forming eruptions clustered into dominantly derived from heated 18O-enriched in current models for large zoned caldera-form- fi ve ~0.5–2.7-Ma-long episodes separated by meteoric groundwater and that large volumes ing ignimbrites. The only possible “monotonous quiescent periods of ~1.4 Ma. Caldera-related of meteoric water reacted with caldera fi ll and intermediate” tuff is the plagioclase-biotite– as well as other magmatism migrated to the with fractured upper parts of the IXL pluton to dominated, probably large-volume tuff of Dog- southwest with time, probably resulting from paleodepths greater than 7 km (John and Pick- skin Mountain–McCoy Mine, for which fi ve rollback of the shallow-dipping Farallon slab. thorn, 1996). The fl uid chemistry of hydrothermal systems in the two calderas was fundamentally different with near-neutral pH fl uids (K-feldspar stable) Figure 27 (on following page). Schematic north-south cross sections outlining the evolution dominant in the Job Canyon caldera system of hydrothermal systems in the Caetano and Job Canyon calderas (modifi ed from John et al. and moderately acidic fl uids (kaolinite stable) [2011] and John and Pickthorn [1996], respectively). (A) Early stage of Caetano hydro- dominant in the Caetano caldera system. The thermal system. Hot magmatic gases released from crystallizing residual Caetano magma Job Canyon caldera hydrothermal system was rise up through the Redrock Canyon intrusion and along the south caldera wall and mix similar to alkali-chloride geothermal systems with cool lake water and with groundwater in interbedded poorly welded ash-fl ow tuffs, that form where magmatic sources of heat and clastic and lacustrine sedimentary rocks, and breccias in the upper unit of Caetano Tuff. volatiles are relatively deep, such as in the The resulting moderately acidic (pH < ~4), moderate-temperature (<270 °C) fl uid fl ows Taupo volcanic zone, New Zealand (Fig. 27C; laterally mostly through the upper unit of Caetano Tuff, hydrothermally altering the tuff Henley and Ellis, 1983; Simmons and Browne, and the upper parts of the Redrock Canyon intrusion mostly forming pervasive quartz- 2000). In contrast, in the Caetano caldera, much kaolinite-pyrite alteration that grades northward and westward to quartz-kaolinite or shallower magmatic sources of heat and vola- illite/montmorillonite alteration with relict sanidine. CCHS—Caetano caldera hydrother- tiles (represented by the Redrock Canyon intru- mal system. (B) Late stage of the Caetano hydrothermal system. Flux of hot magmatic gas sion and other residual magma) resulted in more has diminished or ended and hydrothermal fl uids cool and oxidize as they continue to mix intense and sustained fl ux of acid-forming mag- with cold oxygenated meteoric water. The hydrothermal system collapses with downfl ow of matic volatiles into the Caetano hydrothermal cooler denser fl uids into previously altered and fractured upper Caeteno Tuff and Redrock system where it formed the acidic fl uids typical Canyon intrusion. These fl uids oxidize pyrite formed during early alteration and precipitate of magmatic-hydrothermal systems (Figs. 27A hematite and barite in widespread fractures and breccias. (C) Job Canyon hydrothermal and 27B; Henley and Ellis, 1983; Giggenbach, system showing fl uid circulation paths in the Job Canyon caldera and IXL pluton. Recharge 1997; Hedenquist and Lowenstern, 1994). The areas are in the north part of the caldera and outside the caldera in Tertiary volcanic rocks relatively homogeneous alteration and lower to the south and Triassic metasedimentary rocks to the north. Meteoric fl uids circulate to alteration temperature in the Caetano caldera depths of 7 km or more before rising along the sides and fl owing into the uppermost part probably refl ect a caldera-fi lling lake and under- of IXL pluton and into the overlying caldera. An upwelling zone of high-temperature fl uids lying water-saturated, interbedded sedimentary was present along the south margin of the caldera that offsets the epidote isograd to higher rocks and poorly welded upper Caetano Tuff, structural levels, and a zone of mixing was present in the central part of the caldera between which allowed widespread dispersion, dilution, fl uids fl owing from the south that were rising buoyantly and cooler, more dense fl uids with and cooling of the acidic fl uids. higher δ18O that were fl owing laterally from the north.

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CAETANO CALDERA North Mixing zone of magmatic gases with meteoric water South and pervasive quartz-kaolinite-pyrite alteration Caldera wall Meteoric (lake/ground) water Megabreccia lenses Lake Caetano Pre-CCHS(?) hot springs and sinter Caetano Tuff, upper unit 0 0

1 1

2 2

3 Caetano Tuff, 3 Roberts Mountains lower unit 4 Allochthon 4 Hot magmatic gases 5 Redrock Canyon 5 Intrusion 6 6 North American 7 Miogeocline 7 Depth below surface (km) 8 8 Proterozoic 9 crystalline 9 basement Volatile-saturated Caetano magma 10 Residual Caetano Magma Chamber 10 (granite mush) A 11 11

Hematite veins Meteoric (lake/ground) water Barite veins 0 and breccias 0

1 1

2 2

3 3

4 4

Depth below surface (km) B 5 5

JOB CANYON CALDERA Job Canyon Meteoric water South Job Peak North

Younger 0 epidote-isograd dacite and 0 Older rhyolite Tuff of Job andesite 1 Canyon 1 quartz veins 2 2

3 3

4 Mesozoic 4 sedimentary Depth below surface (km) 5 rocks 5 skarn IXL district 6

IXL pluton 7 Older 1 km dacite and 8 andesite C Deep degassing of crystallizing IXL pluton

Figure 27.

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Intermediate to silicic lavas or shallow intru- The most obvious and probably most certain are Best, M.G., Christiansen, E.H., and Gromme, C.S., 2013a, sions commonly preceded caldera-forming that the western Nevada fi eld has a greater num- Introduction: The 36–18 Ma southern Great Basin, USA, ignimbrite province and flareup: Swarms of eruptions by 1–6 Ma in any specifi c area. Cal- ber of erupted ash-fl ow tuffs and resultant cal- subduction-related supervolcanoes: Geosphere, v. 9, deras are restricted to the area northeast of what deras, and mapped calderas are mostly smaller p. 260–274, doi:10.1130/GES00870.1. Best, M.G., Christiansen, E.H., Deino, A.L., Gromme, C.S., became the Walker Lane, although intermedi- and more dispersed through the fi eld. Only and Tingey, D.G., 2013b, The 36–18 Ma Indian Peak– ate and silicic, effusive magmatism continued the Stillwater and Toquima caldera complexes Caliente ignimbrite fi eld and calderas, southeastern to migrate to the southwest across the Walker cluster in a way somewhat similar to the Indian Great Basin, USA: Multicyclic super eruptions: Geo- sphere, doi:10.1130/GES00902.1 (in press). Lane, spatially and temporally transitioning into Peak–Caliente clusters. The differences between Best, M.G., Gromme, C.S., Deino, A.L., Christiansen, E.H., the ancestral Cascade arc (Cousens et al., 2008; the volcanic fi elds could mostly refl ect their and Tingey, D.G., 2013c, Central Nevada volcanic John et al., 2012). different crustal settings, with magmas rising fi eld: Geosphere. Bindeman, I.N., Schmitt, A.K., and Valley, J.W., 2006, U-Pb The caldera region of the western Nevada vol- through and interacting with the crust in different zircon geochronology of silicic tuffs from the Timber canic fi eld extends across the boundary between ways. This is somewhat analogous to the pres- Mountain/Oasis Valley caldera complex, Nevada: Rapid generation of large volume magmas by shallow-level accreted oceanic terrane on the west and rifted ence and absence of calderas across the Walker remelting: Contributions to Mineralogy and Petrology, North American craton as defi ned by radiogenic Lane. Continued study is necessary to test these v. 152, p. 649–665, doi:10.1007/s00410-006-0124-1. isotopic data. The central Nevada fi eld lies in the differences and their signifi cance. Bingler, E.C., 1977, New Empire geologic map: Nevada Bureau of Mines and Geology Map 59, scale 1:24,000. rifted craton, similar to the eastern part of the west- Bingler, E.C., 1978a, Abandonment of the name Hartford ern fi eld, whereas the Indian Peak–Caliente cal- ACKNOWLEDGMENTS Hill Rhyolite Tuff and adoption of new formation deras are in the relatively stable craton. names for middle Tertiary ash-fl ow tuffs in the Carson We gratefully acknowledge extensive discussions City–Silver City area, Nevada: U.S. Geological Survey Calderas are equant to slightly elongate, with many geologists, especially Joe Colgan, Jim Bulletin 1457-D, 19 p. at least 12 km in diameter, and as much as Faulds, Nick Hinz, Myron Best, Mark Hudson, Eric Bingler, E.C., 1978b, Geologic map of the Schurz quad- Christiansen, Brian Cousens, Al Deino, Sherman rangle: Nevada Bureau of Mines and Geology Map 60, 35 km in longest dimension. Exceptional three- scale 1:48,000. Gromme, Woody Brooks, Liz Cassel, Steve Castor, dimensional exposure of the Stillwater caldera Boden, D.R., 1986, Eruptive history and structural develop- John Dilles, Ted McKee, Jamie Conrad, Kathryn complex and Caetano caldera resulting from ment of the Toquima caldera complex, central Nevada: Watts, Dave Boden, Mike Ressel, Jerry Brem, Dick Geological Society of America Bulletin, v. 97, p. 61–74, post-collapse extensional tilting show they sub- Hardyman, and Dan Shawe. Precise 40Ar/39Ar dating doi:10.1130/0016-7606(1986)97<61:EHASDO>2.0 sided as much as 5 km as large piston-like blocks. done at the New Mexico Geochronology Research .CO;2. Caldera walls were vertical to steeply inward Laboratory under the patient guidance and with the Boden, D.R., 1987, Geology, structure, petrology, and mineralization of the Toquima caldera complex, central dipping to depths ≥4–5 km; topographic walls considerable help of Bill McIntosh, Matt Heizler, Lisa Peters, Rich Esser, Matt Zimmerer, and Jake Ross has Nevada [Ph.D. thesis]: Palo Alto, California, Stanford formed by slumping of wall rock into the caldera been essential to understanding the geology and evolu- University, 231 p. were only slightly outboard (≤1 km) of structural Boden, D.R., 1992, Geologic map of the Toquima caldera tion of the volcanic fi eld. Much of the geologic study complex, central Nevada: Nevada Bureau of Mines and margins. that contributed to this report was supported by the Geology Map 98, scale 1:48,000. Most calderas had abundant post-collapse National Science Foundation (grant EAR-0124869) Boden, D.R., 1994, Mid-Tertiary magmatism of the Toquima and the Mineral Resources and STATEMAP programs caldera complex and vicinity, Nevada: Development of magmatism expressed as resurgent intrusions, of the U.S. Geological Survey. Thorough reviews by explosive high-K, calc-alkaline magmas in the central ring-fracture intrusions, or intracaldera lavas Myron Best, Eric Christiansen, Ed du Bray, John Great Basin, USA: Contributions to Mineralogy and that are closely related temporally (~0–0.5 Ma Dilles, and Fred McDowell greatly improved the sci- Petrology, v. 116, p. 247–276, doi:10.1007/BF00306496. ence and presentation of this report. Bonham, H.F., 1970, Geologic map and sections of a part younger) to caldera formation. 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