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Title: 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
ABSTRACT 18 Successive caldera-forming eruptions from ~30-25 Ma generated a large nested 19 caldera complex in western Nevada that was subsequently dissected by Basin and Range 20 extension, providing extraordinary cross-sectional views through a diverse range of 21 eruptive and intrusive products. A high-resolution oxygen isotopic study was conducted 22 on units that represent all major parts of the Job Canyon, Poco Canyon, Elevenmile 23 Canyon, and Louderback Mountains caldera cycles (29.2-25.1 Ma), and several 24 Cretaceous basement plutons that flank the Stillwater caldera complex. We also provide 2 25 new oxygen and strontium isotope data for regional caldera centers in the Great Basin, 26 which are synthesized with published oxygen and strontium isotope data for regional 27 Mesozoic basement rocks. Stillwater zircons span a large isotopic range (d18Ozircon of 3.6 28 to 8.2‰), and all caldera cycles possess low-d18O zircons. In some cases, they are a small 29 proportion of the total populations, and in others they dominate, such as in the low-d18O 30 rhyolitic tuffs of Job Canyon and Poco Canyon (d18Ozircon=4.0-4.3‰ and calculated d18O 31 magma=5.5-6‰). These are the first low-d18O rhyolites documented in middle Cenozoic 32 calderas of the Great Basin, adding to the global occurrence of these important magma 33 types that fingerprint recycling of shallow crust altered by low-d18O meteoric waters. more » The 34 appearance of low-d18O rhyolites in the Stillwater caldera complex is overprinted on a 35 Great Basin-wide trend of miogeoclinal sediment contribution to silicic magmas that 36 elevates d18O compositions, making identification of 18O depletions difficult. Oxygen and 37 strontium isotopic data and U-Pb zircon age inheritance points to derivation of the low38 d18O tuff of Job Canyon from melting and assimilation of hydrothermally altered 39 Cretaceous granitic basement. In contrast, oxygen and strontium isotopic modeling points 40 to derivation of the low-d18O tuff of Poco Canyon from melting and assimilation of 41 hydrothermally altered intracaldera Job Canyon rocks. Though not a nominally low-d18O 42 rhyolite, the tuff of Elevenmile Canyon possesses both low-d18O and high-d18O zircon 43 cores that are overgrown by homogenized zircon rims that approximate the bulk zircon 44 average, pointing to batch assembly of isotopically diverse upper crustal melts to 45 generate one of the most voluminous (2,500-5,000 km3) tuff eruptions in the Great Basin. 46 Low-d18O zircons in the tuff of Elevenmile Canyon are inherited from the Poco Canyon 47 cycle based on their unique trace and rare earth element chemistry. The tuffs of Poco and 3 48 Elevenmile Canyon both support a caldera cannibalization model in which 49 hydrothermally altered extrusive and intrusive rocks from previous caldera cycles are 50 recycled into later magmas. Though overlapping in space and time, each caldera-forming 51 cycle of the Stillwater complex had a unique oxygen isotope record as retained in single 52 zircons. Even plutons that were spatially and temporally coincident with calderas diverge 53 from the caldera-forming tuffs and cannot be their cogenetic remnants in most cases. « less
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