Enigmatic evolution of rhyolitic magma, the Bishop, Calif. Tuff

For starters, the Bishop Tuff, although an exemplary, eutectoid high-silica rhyolite, is far from being a monolith. Wilson and Hildreth documented its stratigraphy in detail, establishing coeruption of a range of pumice types from at least two migrating vent systems with syndeposition of plinian pumice falls and pyroclastic flows. Some features of the Bishop Tuff remain enigmatic: 1) The earliest deposited pumice was plausibly the earliest erupted pumice and it probably derived from the uppermost and coolest part of the magma body. The enigma is that the earliest-deposited pumice is, in general, relatively poor in crystals, although both decompression and cooling promote crystallization. 2) Phenocrysts are reversely zoned: Sanidine phenocrysts have Ba-rich rims, Ba and CO2-rich melt inclusions are located in the outer zones of quartz phenocrysts, magnetite inclusions in some late-erupted quartz phenocrysts preserve compositions of magnetite in early-erupted pumice. The enigma is that reverse zoning of phenocrysts is commonly ascribed to a late influx of hotter, less differentiated magma, yet Bishop melt inclusion trace element compositions follow an exponential trend that is inconsistent with magma mixing. 3) Phenocrysts and melt inclusion compositions in a single pumice clast vary much less than for the whole deposit. The enigma is that a thick body of magma would tend to convect and thereby mix crystals and melts so as to form a uniform rather than stratified body. The solution? Crystal sinking, but would it not also cause mixing? Just what is the extent of crystal mixing within individual pumice clasts? New isotopic measurements of zircon and feldspar constrain the magmatic evolution at Long Valley Caldera J. I. SIMON, M. R. REID, AND E. D. YOUNG UCLA Los Angeles, CA 90095 (jisimon@ucla.edu) NAU Flagstaff, AZ 86011 (mary.reid@NAU.EDU) UCLA Los Angeles, CA 90095 (eyoung@ess.ucla.edu)