Compositional Zoning of the Bishop Tuff

Compositional data for4400 pumice clasts, organized according to eruptive sequence, crystal content, and texture, provide new perspec-tives on eruption and pre-eruptive evolution of the4600 km3 of zoned rhyolitic magma ejected as the BishopTuff during formation of Long Valley caldera. Proportions and compositions of different pumice types are given for each ignimbrite package and for the intercalated plinian pumice-fall layers that erupted synchronously. Although withdrawal of the zoned magma was less systematic than previously realized, the overall sequence displays trends toward greater propor-tions of less evolved pumice, more crystals (05²4 wt %), and higher FeTi-oxide temperatures (714⁸188C). No significant hiatus took place during the 6 day eruption of the BishopTuff, nearly all of which issued from an integrated, zoned, unitary reservoir. Shortly before eruption, however, the zoned melt-dominant portion of the chamber was invaded by batches of disparate lower-silica rhyolite