TEXTURAL DEVELOPMENT OF MIXED, FINELY PORPHYRITIC SILICIC VOLCANIC-ROCKS, INYO DOMES, EASTERN CALIFORNIA

Magma mixing in silicic volcanic rocks of the Inyo chain is manifest by bimodal mineral chemistry and distinctive banding, defined by either variations in (1) glass color and chemistry or (2) microlite abundance within colorless glass. Bands composed of brown or microlite-rich, colorless glass are characterized by more Mg-rich mafic silicates and more calcic plagioclase than microlite-poor, colorless glass domains. A rhyolitic and a dacitic mixing end-member can be defined on the basis of this mineral distribution. Thermobarometry using the mineral assemblages unique to each end-member indicates that prior to mixing, the dacite was more oxidized and water-rich than the rhyolitic magma. Determined mineral crystallization conditions compare favorably with experimentally derived phase equilibria for melts of broadly similar composition. The spatial association of contrasting mineral assemblages with the flow banding in these rocks implies that the banding is a direct product of magma mixing. It formed as either (1) volumes of the comingled magmas were immediately vented to the surface and quenched, preserving domains of two distinct glass compositions, or (2) bands of dacitic magma precipitated microlites when comingled with the rhyolitic magma in an attempt for the two magmas to approach equilibrium.