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Rapid generation of both high- and low-¹⁸O, large-volume silicic magmas at the Timber Mountain/Oasis Valley caldera complex, Nevada

Ilya N. Bindeman^,1 and John W. Valley^,1

¹ Department of Geology and Geophysics, University of Wisconsin, 1215 West Dayton Street, Madison, Wisconsin 53706, USA

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 ¹⁸O values of quartz, sanidine, sphene, magnetite, and zircons in rhyolites and latites of each tuff were measured and used to estimate ¹⁸O(melt) at 700–900 °C. Temperatures were determined by ¹⁸O(quartz-magnetite) and Fe-Ti thermometers. Each tuff is characterized by a distinct range of ¹⁸O(melt): 8.0–9.0 (TS), 7.1–7.8 (TC), 7.4–8.6 (RM), and 5.4–6.0 (AT), with higher ¹⁸O values for rhyolites in each unit. The distinct ¹⁸O 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. Low-¹⁸O AT and normal-¹⁸O TC tuffs were erupted from the same nested caldera complex only 100–150 k.y. after eruption of voluminous high-¹⁸O TS and RM magmas, respectively. These short time intervals, distinct ¹⁸O, ⁸⁷Sr/⁸⁶Sr_i, and _Nd of each tuff, the same loci of their eruption, and energy-constrained 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 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 pre- and post-caldera lavas are 2.5 lower in ¹⁸O than in the RM tuff and represent the largest known low ¹⁸O magma. We find that all units of the AT cycle contain isotopically zoned zir cons that have up to 2 core-to-rim zoning and correspondingly smaller, out-of-equilibrium quartz-zircon and melt-zircon fractionations. Air-abraded cores of quartz and sphene do not preserve any ¹⁸O zoning. The higher-¹⁸O zircon cores in low-¹⁸O magmas of SWNVF are similar to zoned zircons in low-¹⁸O lavas at Yellowstone. In both places, normal-¹⁸O zircons have been inherited from precursor volcanic rocks in that the matrix suffered depletion in ¹⁸O (down to +4% to +5% according to AFC modeling), but zircons and quartz survived hydrothermal alteration. These precursor rocks were later rapidly remelted to form low-¹⁸O melt and caused progressive exchange of oxygen with normal-¹⁸O 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-¹⁸O magmas worldwide and is a signature of nearly total remelting of hydrothermally altered roof rocks, in hot-spot (Yellowstone) and in extensional (SWNVF) environments.

Key Words: Paintbrush tuff • Timber Mountain tuff • oxygen isotopes • isotope zoning • zircon • zone refinement melting

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