EXPLOSIVE SILICIC VOLCANISM PRECEDING THE PEACH SPRING TUFF SUPERERUPTION, PART 2: MAGMATIC PROCESSES RECORDED BY PETROCHEMISTRY
Zircon saturation thermometry (Boehnke et al 2013; SEM-EDS & LA-ICPMS glass analyses) indicates melt T of ~680-700°C for Fort Rock tuff on the Plateau, 740° for the upper, possibly correlated fall deposit at Kingman, and 690° for fallout tuffs in the SBM. In contrast, the tuffs enveloping the CCT yield 860°, matching values for CCT (Perry et al, 2015). Tuff glasses from Fort Rock, SBM, and upper Kingman deposit also have low REE (e.g. Lan ~100 ppm) suggesting accessory fractionation, but CCT and its enveloping tuff has high REE (Lan ~400). Likewise, whole-rock tuffs from Fort Rock, SBM, and upper Kingman have relatively low Zr/Sr (<1 @ 70 wt% SiO2), in contrast to CCT and enveloping deposits (>2 @ 70 wt% SiO2), suggesting that the former evolved under cooler, wetter conditions (Miller et al 2014). Like CCT, PST has a hot, dry signature, with high zircon saturation T and very high Zr/Sr. These distinctions hint at different modes of silicic magma generation and evolution in the region before and including PST. We are expanding the dataset to further evaluate conditions and evolution of the tuffs, with amphibole barometry (Schmidt 1992), Rhyolite-MELTS (Gualda et al. 2012), and trace element modeling.