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Paired Zircon and Allanite Dating of late Pleistocene-Holocene Rhyolites in the Mono Basin, Eastern California, USA

Chiaro, G.R ., Vazquez, J., Calvert, A., Hildreth, W., Fierstein, J., Hurwitz, S.

Abstract

The ~30 high-silica rhyolite domes and flows of the 15 km-long Mono Craters volcanic chain are some of the youngest rhyolitic volcanoes in the western United States. Their ages range from ~40 ka to 0.7 ka with a total erupted volume of ~8.5 km3 and distinctions based on mineralogy (biotite, orthopyroxene, or fayalite-bearing) and crystallinity. Ion microprobe 238U-230Th dating of allanite and zircon crystal surfaces was previously used to date the final crystallization period of the ~40--20 ka biotite-bearing domes and younger ~12--9 ka orthopyroxene-bearing domes. We present new 238U-230Th dates for the fayalite-bearing and sparsely porphyritic Mono domes. The fayalite-bearing domes are compositionally indistinguishable despite having the widest spatial distribution of all the Mono domes, with ~12 km separating the northernmost from the southernmost dome. Fayalite-bearing whole-rock compositions are tightly constrained from 76-77 wt.% SiO2 and represent extremely fractionated (Sr <8, Ba <20 ppm) melts. Glass compositions from the fayalite-bearing domes return rhyolite-MELTS quartz + 2 feldspar storage pressures of ~7 km. Results suggest that all nine fayalite-bearing domes are essentially the same age and erupted ~6 ka. At least one dome from the sparsely porphyritic group appears older with a U-Th zircon-allanite date of ~20 ka. This dome, as well as the older biotite and orthopyroxene-bearing domes, all contain distinctly different whole-rock trace-element compositions than the younger fayalite-bearing, sparsely porphyritic, and aphyric domes. By further constraining the eruptive history of the Mono Basin, we improve our understanding of volcanic hazards in eastern California.