Bindeman, IN; Schmitt, AK; Valley, JW
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 Ar-40/Ar-39 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 sheetlike 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 Ar-40/Ar-39 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.
paintbrush tuff; Timber Mountain tuff; oxygen isotopes; geochronology; isotope zoning; zircon; Yucca Mountain; ash-flow sheets; plateau volcanic field; western north-america; southwestern nevada; oxygen-isotope; residence times; long-valley; southern nevada; rhyolitic magma; low-delta-o-18 rhyolites