Oxygen isotopic investigation of silicic magmatism in the Stillwater caldera complex, Nevada Oxygen isotopic investigation of silicic magmatism in the Stillwater caldera complex, Nevada: Generation of large- volume, low-δ18O rhyolitic tuffs and assessment of their regional context in the Great Basin of the western United States Kathryn E. Watts1,†, David A. John1, Joseph P. Colgan2, Christopher D. Henry3, Ilya N. Bindeman4, and John W. Valley5 1U.S. Geological Survey, Menlo Park, California 94025, USA 2U.S. Geological Survey, Denver, Colorado 80225, USA 3Nevada Bureau of Mines and Geology, Reno, Nevada 89557, USA 4Department of Earth Sciences, University of Oregon, Eugene, Oregon 97403, USA 5Department of Geoscience, University of Wisconsin–Madison, Madison, Wisconsin 53706, USA ABSTRACT occurrence of these important magma types nimbrite flare-up may have been related to that fingerprint recycling of shallow crust steepening, or roll-back, of the subducting Successive caldera-forming eruptions altered by low-δ18O meteoric waters. The Farallon plate beneath the western edge of from ca. 30 to 25 Ma generated a large nested appearance of low-δ18O rhyolites in the North America (Coney and Reynolds, 1977; caldera complex in western Nevada that was Stillwater caldera complex is overprinted Coney, 1978; Humphreys, 1995). Although subsequently dissected by Basin and Range on a Great Basin–wide trend of miogeoclinal most calderas occur within intact crustal extension, providing extraordinary cross- sediment contribution to silicic magmas that blocks, some were dissected and significantly sectional views through diverse volcanic and elevates δ18O compositions, making identifi- tilted by Basin and Range extension during the plutonic rocks. A high- resolution oxygen cation of 18O depletions difficult. Though not late Cenozoic, providing valuable windows isotopic study was conducted on units that a nominally low-δ18O rhyolite, the tuff of into the volcanic and plutonic processes that represent all major parts of the Job Canyon, Eleven mile Canyon possesses both low-δ18O underpinned them, and perhaps by analogue, Louderback Mountains, Poco Canyon, and and high-δ18O zircon cores that are over- the life spans and eruptive potential of mod- Elevenmile Canyon caldera cycles (29.2– grown by homogenized zircon rims that ap- ern-day caldera volcanoes. 25.1 Ma), and several Cretaceous plutons proximate the bulk zircon average, pointing Caldera exposures are exemplified by the that flank the Stillwater caldera complex. to batch assembly of isotopically diverse up- Stillwater–Clan Alpine complex in western We provide new oxygen and strontium iso- per crustal melts to generate one of the most Nevada (Figs. 1–2; abbreviated here and else- tope data for 12 additional caldera centers voluminous (2500–5000 km3) tuff eruptions where as the Stillwater caldera complex). The in the Great Basin, which are synthesized in the Great Basin. Despite overlapping in western margin of the complex in the Stillwa- with >150 published oxygen and strontium space and time, each caldera-forming cycle ter Range is steeply tilted, exposing an ~10 km isotope analyses for regional Mesozoic base- of the Stillwater complex has a unique oxy- depth section through the upper crust, includ- ment rocks. Stillwater zircons span a large gen isotope record as retained in single zir- ing several overlapping calderas and plutons 18 isotopic range (δ Ozircon of 3.6‰–8.2‰), and cons. Most plutons that were spatially and that provide a unique opportunity to elucidate all caldera cycles possess low-δ18O zircons. temporally coincident with calderas have the petro genesis of large-volume silicic mag- In some cases, they are a small propor- isotopic compositions that diverge from the mas in the Great Basin ignimbrite flare-up. tion of the total populations, and in oth- caldera-forming tuffs and cannot be their Here, we present the results of a comprehensive ers, they dominate, such as in the low-δ18O cogenetic remnants. oxygen isotopic investigation, with particular rhyolitic tuffs of Job Canyon and Poco emphasis on refractory zircon crystals, which 18 18 Canyon (δ Ozircon = 4.0‰–4.3‰; δ Omagma = INTRODUCTION demonstrate significant diversity in Stillwater 5.5‰–6‰). These are the first low-δ18O rhy- magmas, including two low-δ18O rhyolitic tuffs 18 olites documented in middle Cenozoic calde- During the middle Cenozoic, tens of thou- (Job Canyon and Poco Canyon; δ Omagma = ras of the Great Basin, adding to the global sands of cubic kilometers of silicic magma 5.5‰–6‰). To our knowledge, these are the erupted from dozens of calderas that span first low-δ18O rhyolites described in the Great across Nevada in a broad northwest to south- Basin ignimbrite flare-up. The low-δ18O Am- † 18 [email protected] east belt (Fig. 1; Best et al., 2013a). This ig- monia Tanks Tuff (δ Omagma = 5.4‰–6‰) in GSA Bulletin; July/August 2019; v. 131; no. 7/8; p. 1133–1156; https://doi.org/10.1130/B35021.1; 12 figures; 4 tables; Data Repository item 2019060; published online 14 February 2019. © 2019 The Authors. Gold Open Access: Geological Society of America Bulletin, v. 131, no. 7/8 1133 This paper is published under the terms of the CC-BY license. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/131/7-8/1133/4729506/1133.pdf by guest on 26 September 2021 -120°-119° -118° Watts -1et17 al.° -116° -115° -114° 42° 49 48 47 35 Wells 41° Winnemucca 34 b 17 33 Battle Elko a 26 36 32 Mountain Sr =0.708 44 Sr =0.706 i Accreted oceanic i c CM 22 18 terranes 28 RC 27 b 16 FCM CT Stillwater 25 23 40° caldera complex NYC Rifted 20 transitional a (29.2–25.1 Ma) Precambrian crust HC 1 AC 24 Reno 45 JC PD basement 14 DC a Eureka PC NH? Austin LC Fallon LB CC 46 46 SS b Central Nevada Ely UD Volcanic Field 42 EC c 15 (~36–18.4 Ma) 39° 43 FP c 41 AD 37 29 a b Western Nevada MJL Volcanic Field MJU study area of a 40 b Larson &Taylor Indian Peak (34.4–19.5 Ma) 1986 (~32–27 Ma) MH 30 PD2 Tonopah 31 Caliente (~24–14 Ma) Regional granites (King et al., 2004) mountain range sampled Ammonia Tanks Tuff; 19 study area of Bindeman 1 sample locality number & Valley 2003 Cenozoic Cretaceous Jurassic LOCATION MAP Precambrian Regional granites (Wooden et al., 1999) Cenozoic Las Vegas Cretaceous Jurassic Regional granites (this study, Cretaceous) sample locality AC Alameda Canyon (84 Ma) LC La Plata Canyon (87 Ma) SS Sand Springs pluton (89 Ma) 100 km Humboldt mafic complex (Kistler and Speed, 2000) gabbro, basalt sample locality (172 Ma) Regional calderas and their associated tuffs (this study, Eocene-Miocene) AD: tuff of Arc Dome, 25.2 Ma FCM: Fish Creek Mountains Tuff, 24.9 Ma MJL: lower tuff of Mount Jefferson, 27.3 Ma CC: tuff of Campbell Creek, 28.9 Ma FP: tuff of Fairview Peak, 19.5 Ma MJU: upper tuff of Mount Jefferson, 27.0 Ma CM: tuff of Cove Mine, 34.4 Ma HC: tuff of Hall Creek, 34.0 Ma NH: Nine Hill Tuff, 25.4 Ma CT: Caetano Tuff, 34.0 Ma JC: tuff of Job Canyon, 29.2 Ma PC: tuff of Poco Canyon, 25.2 Ma DC: tuff of Deep Canyon, 30.4 Ma LB: tuff of Louderback Mountains, 25.2 Ma UD: Underdown Tuff, 25.0 Ma EC: tuff of Elevenmile Canyon, 25.1 Ma MH: Manhattan, Round Rock Formation, 24.8 Ma Figure 1. Map showing the location of the Stillwater caldera complex and other middle Cenozoic calderas in the Great Basin, Nevada. Cal- deras are subdivided into the western Nevada, central Nevada, and Indian Peak–Caliente volcanic fields (Best et al., 2013a). Dashed pale- 87 86 87 86 gray lines show the boundaries between volcanic fields, black solid lines show the locations of the Sr/ Sri = 0.706 and Sr/ Sri = 0.708 isopleths (Farmer and DePaolo, 1983), and dotted black lines show the paleodivide boundaries of Henry and John (2013) (“PD1”) and Best et al. (2013a) (“PD2”). The mountain ranges sampled by King et al. (2004) are shaded in gray and numbered according to their numbering scheme. Labels and ages for the calderas and granites included in this study are shown in the figure legend. Labels are included for two Cretaceous granite localities to the north of the Stillwater caldera complex, the New York Canyon (NYC) stock and the Rocky Canyon (RC) pluton, which may be pertinent to the basement crustal architecture (see text for detail). Stars show town localities in Nevada. 1134 Geological Society of America Bulletin, v. 131, no. 7/8 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/131/7-8/1133/4729506/1133.pdf by guest on 26 September 2021 Oxygen isotopic investigation of silicic magmatism in the Stillwater caldera complex, Nevada −118º00′ −118º15′ ? 08-IXL2 13-DJ11 A 08-IXL4 10-DJ2 Job Canyon caldera −117º45′ 10-DJ3 (29.2 Ma) Deep ? Canyon 10-DJ4 10-DJ5 caldera 10-DJ6 E (30.4 Ma) Poco Canyon NG A caldera (25.2 Ma) R 12-DJ35 08-IXL8 Nine Hill S N ER ? 14-DJ75A Elevenmile T caldera? MT A JC13-9 Canyon caldera (25.4 Ma) W 12-DJ36 EY (25.1 Ma) L L L L TI S K C VA 39º30 A ′ ? 12-DJ37 B MTNS R E 12-DJ38 D E U N O 14-DJ79 L PI L D E A S A T O E Y I Mesozoic sedimentary rocks 12-DJ33 A N H00-104A A Cretaceous granite DIX L tuff of Job Canyon 13-DJ7 C IXL pluton 12-DJ34 tuff of Louderback Mountains MT tuff of Poco Canyon N N Louderback JC11-32 T S breccia-rich tuff of Poco Canyon M JC11-26 Mountains Freeman Creek pluton K AL caldera tuff of Elevenmile Canyon H C (25.2 Ma) rhyolite lavas/intrusions N andesite-dacite lavas/tuff sample location tuff of Deep Canyon Campbell Creek 10 km tuff of Campbell Creek caldera (28.9 Ma) Nine Hill Tuff? 39º15′ Approximate longitude: −118º15′ B Poco Job S Elevenmile Canyon Canyon Canyon N 0 0 tuff of Elevenmile tuff of 2 2 Canyon Poco Canyon Depth (km tuff of Job 4 Canyon 4 level o f modern exp 6 osure 6 ) IXL 8 pluton 8 Freeman 10 Creek pluton 10 Figure 2.