Rapid generation of both high- and low-d18O, large-volume silicic magmas at the Timber Mountain/Oasis Valley caldera complex, Nevada

We present an oxygen isotope and petrologic study of four voluminous, zoned ash-flow sheets of the Southwestern Nevada Volcanic Field (SWNVF): Topopah Spring (TS, .1200 km3, 12.8 Ma), Tiva Canyon (TC, 1000 km3, 12.7 Ma), Rainier Mesa (RM, 1200 km3, 11.6 Ma), and Ammonia Tanks (AT, 900 km3, 11.45 Ma). The d18O values of quartz, sanidine, sphene, magnetite, and zircons in rhyolites and latites of each tuff were measured and used to estimate d18O(melt) at 700–900 8C. Temperatures were determined by D18O(quartzmagnetite) and Fe-Ti thermometers. Each tuff is characterized by a distinct range of d18O(melt): 8.0–9.0‰ (TS), 7.1–7.8‰ (TC), 7.4–8.6‰ (RM), and 5.4–6.0‰ (AT), with higher d18O values for rhyolites in each unit. The distinct d18O of rhyolitic versus latitic portions of each tuff suggests that they can not be related by in situ fractionation and assimilation in a single zoned magma chamber. It is more likely that latite and rhyolite represent two magmas that were juxtaposed prior to eruption. Lowd18O AT and normal-d18O TC tuffs were erupted from the same nested caldera complex only 100–150 k.y. after eruption of voluminous high-d18O TS and RM magmas, respectively. These short time intervals, distinct d18O, Sr/Sri, and «Nd of each tuff, the same loci of their eruption, and energyconstrained assimilation modeling suggest that TS, TC, RM, and AT represent independent magma batches that were rapidly generated, fractionated, and erupted from shallow, sheet-like magma chambers. Such geometry is a result of extensional tectonics †E-mail: [email protected]. in the Basin and Range province, and it favors nearly total evacuation of the magma chamber during a single eruption. Each silicic magma unit was generated by a shallow influx of new mafic magma that melted 18O/16O-depleted (as in the case of AT) or 18O/16O-enriched (RM, TS) rocks. The AT tuff and associated preand post-caldera lavas are 2.5‰ lower in d18O than in the RM tuff and represent the largest known low d18O magma. We find that all units of the AT cycle contain isotopically zoned zircons that have up to 2‰ core-to-rim zoning and correspondingly smaller, out-ofequilibrium quartz-zircon and melt-zircon fractionations. Air-abraded cores of quartz and sphene do not preserve any d18O zoning. The higher-d18O zircon cores in lowd18O magmas of SWNVF are similar to zoned zircons in low-d18O lavas at Yellowstone. In both places, normal-d18O zircons have been inherited from precursor volcanic rocks in that the matrix suffered depletion in d18O (down to 14% to 15% according to AFC modeling), but zircons and quartz survived hydrothermal alteration. These precursor rocks were later rapidly remelted to form low-d18O melt and caused progressive exchange of oxygen with normal-d18O zircon and quartz xenocrysts. Based on modeling of oxygen diffusion in zircon and quartz, the time between xenocryst entrapment and eruption is estimated to be 104 yr in SWNVF versus 103 yr for Yellowstone. We suggest that zircon recycling is a common feature of low-d18O magmas worldwide and is a signature of nearly total remelting of hydrothermally altered roof rocks, in hot-spot (Yellowstone) and in extensional (SWNVF) environments.