The Evolution of the Peach Spring Giant Magma Body: Evidence from Accessory Mineral Textures and Compositions, Bulk Pumice and Glass Geochemistry, and Rhyolite-MELTS Modeling

Abstract

The Miocene Peach Spring Tuff is a giant (� 640 km 3 dense rock equivalent) pyroclastic deposit that is extensively exposed in the southwestern USA. Evidence from geochemical and textural analyses of bulk-rocks, glasses, and accessory minerals (zircon, titanite, allanite, chevkinite, magnetite) from outflow and intra-caldera pumice clasts and fiamme, in combination with field observations and rhyolite-MELTS modeling, suggests that the Peach Spring magma body was compositionally and thermally zoned with a basal cumulate, and that it crystallized over millennial timescales before being remobilized by mafic input prior to erupting. Crystal contents, bulk compositions, spatial distributions, and temperatures (recorded by Ti in zircon and Zr in titanite) of pumice clasts and fiamme vary systematically: distal outflow high-silica rhyolites are crystal-poor and document lower temperatures; intra-caldera trachytes are crystal-rich and record higher temperatures. These variations indicate that the Peach Spring magma body was zoned. We interpret the outflow high-silica rhyolites to represent the first portion of the magma body to erupt. Zircon and titanite display core-to-edge reductions in rare earth element (REE) concentration and temperature, suggestive of relatively uninterrupted crystallization as the magma body cooled; crystallization temperature intervals from rhyoliteMELTS are consistent with those recorded by zircon and titanite. Exponential size distributions for accessory minerals and phenocryst textures are consistent with geochemical evidence for a simple cooling and crystallization history. Intra-caldera trachytes and outflow low-silica rhyolites represent the later portion of the magma body to erupt.This magma experienced a late-stage heating event potentially associated with the onset of the eruption.The edges of titanite crystals are enriched in REE and Zr, and zircon edges are enriched in Ti, suggesting higher temperatures during edge crystallization (at least 9008C). Concave-down crystal size distributions and resorption features on phenocrysts are additional signs of heating. Rare trachyandesite enclaves and the presence of mafic to intermediate lavas immediately underlying the Peach Spring Tuff suggest that a mafic magma input may have been the cause of the heating. Evidence further suggests that the intra-caldera trachytes may represent a remobilized cumulate at the base of the magma body, which retained some melt prior to rejuvenation. Bulk pumice and fiamme compositions are very rich in feldspar and accessory mineral phenocrysts, indicative of accumulation of these minerals; high crystal contents (� 35%) and evidence of heating and resorption imply that this magma was even more crystal-rich prior to the heating event. Rhyolite-MELTS simulations suggest that the trachyte magma had roughly 1wt % water, which cannot be totally accounted for by hydrous phases, thus requiring some amount of melt within the cumulate. Kinked magnetite size distributions are interpreted to represent a change from growth-dominated crystallization (larger crystals, shallow slopes) to nucleation-dominated (small crystals, steep slopes) owing to the onset of eruptive decompression. Timescales of magnetite crystallization calculated from these slopes indicate that the Peach Spring magma body crystallized over millennial timescales, and that eruptive decompression began 10 � 1 ^10 2 years prior to eruption.