U-Pb zircon geochronology of silicic tuffs from the Timber Mountain/Oasis Valley caldera complex, Nevada: rapid generation of large volume magmas by shallow-level remelting

Large volumes of silicic magma were produced on a very short timescale in the nested caldera complex of the SW Nevada volcanic field (SWNVF). Voluminous ash flows erupted in two paired events: Topopah Spring (TS, >1,200 km(3), 12.8 Ma)-Tiva Canyon (TC, 1,000 km(3), 12.7 Ma) and Rainier Mesa (RM, 1,200 km(3), 11.6 Ma)-Ammonia Tanks (AT, 900 km(3), 11.45 Ma; all cited ages are previously published 40Ar/39Ar sanidine ages). Within each pair, eruptions are separated by only 0.1-0.15 My and produced tuffs with contrasting isotopic values. These events represent nearly complete evacuation of sheet-like magma chambers formed in the extensional Basin and Range environment. We present ion microprobe ages from zircons in the zoned ash-flow sheets of TS, TC, RM, and AT in conjunction with delta O-18 values of zircons and other phenocrysts, which differ dramatically among subsequently erupted units. Bulk zircons in the low-delta O-18 AT cycle were earlier determined to exhibit similar to 1.5 parts per thousand core-to-rim oxygen isotope zoning; and high-spatial resolution zircon analyses by ion microprobe reveal the presence of older grains that are zoned by 0.5-2.5 parts per thousand. The following U-Pb isochron ages were calculated after correcting for the initial U-Pb disequilibria: AT (zircon rims: 11.7 +/- 0.2 Ma; cores: 12.0 +/- 0.1 Ma); pre-AT rhyolite lava: (12.0 +/- 0.3 Ma); RM: 12.4 +/- 0.3); TC: (13.2 +/- 0.15 Ma); TS: (13.5 +/- 0.2). Average zircon crystallization ages calculated from weighted regression or cumulative averaging are older than the Ar-Ar stratigraphy, but preserve the comparably short time gaps within each of two major eruption cycles (TS/TC, RM/AT). Notably, every sample yields average zircon ages that are 0.70-0.35 Ma older than the respective Ar-Ar eruption ages. The Th/U ratio of SWNVF zircons are 0.4-4.7, higher than typically found in igneous zircons, which correlates with elevated Th/U of the whole rocks (5-16). High Th/U could be explained if uranium was preferentially removed by hydrothermal solutions or is retained in the protolith during partial melting. For low-delta O-18 AT-cycle magmas, rim ages from unpolished zircons overlap within analytical uncertainties with the 40Ar/39Ar eruption age compared to core ages that are on average similar to 0.2-0.3 My older than even the age of the preceding caldera forming eruption of RM tuff. This age difference, the core-to-rim oxygen isotope zoning in AT zircons, and disequilibrium quartz-zircon and melt-zircon isotopic fractionations suggest that AT magma recycled older zircons derived from the RM and older eruptive cycles. These results suggest that the low-delta O-18 AT magmas were generated by melting a hydrothermally-altered protolith from the same nested complex that erupted high-delta O-18 magmas of the RM cycle only 0.15 My prior to the eruption of the AT, the largest volume low-delta O-18 magma presently known.