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
Successive caldera-forming eruptions from ca. 30 to 25 Ma generated a large nested caldera complex in western Nevada that was subsequently dissected by Basin and Range extension, providing extraordinary crosssectional views through diverse volcanic and plutonic rocks. A highresolution oxygen isotopic study was conducted on units that represent all major parts of the Job Canyon, Louderback Mountains, Poco Canyon, and Elevenmile Canyon caldera cycles (29.2– 25.1 Ma), and several Cretaceous plutons that flank the Stillwater caldera complex. We provide new oxygen and strontium isotope data for 12 additional caldera centers in the Great Basin, which are synthesized with >150 published oxygen and strontium isotope analyses for regional Mesozoic basement rocks. Stillwater zircons span a large isotopic range (δOzircon of 3.6‰–8.2‰), and all caldera cycles possess low-δ18O zircons. In some cases, they are a small proportion of the total populations, and in others, they dominate, such as in the low-δ18O rhyolitic tuffs of Job Canyon and Poco Canyon (δOzircon = 4.0‰–4.3‰; δOmagma = 5.5‰–6‰). These are the first low-δ18O rhyolites documented in middle Cenozoic calderas of the Great Basin, adding to the global