MULTISTAGE MAGMA MINGLING AND THE ORIGIN OF FLOW BANDING IN THE ALISOLAVA DOME, TUMACACORI MOUNTAINS, SOUTHERN ARIZONA

Flow banded rocks of the middle Tertiary Aliso lava dome, in the Tumac acori Mountains of southern Arizona, contain light-colored dacitic flo w bands, dark-colored rhyolitic flow bands, and dark-colored ellipsoid al enclaves of rhyolite. Groundmass glass of the light- and dark-color ed flow bands is trachyte and high-silica rhyolite, respectively. Encl aves are fine grained, holocrystalline, and phenccryst-poor. A two-sta ge sequence of mixing and mingling of discrete magmas can account for the compositional and textural characteristics of the Aliso rocks: (1) enclave magma mixed with dark band magma and (2) the enclave/dark ban d magma composite mingled with alkalic trachyte magma, now represented by the light bands. Both mingling events may have happened in the mag ma chamber, or partially in the magma chamber and partially in the vol canic conduit. Deformation of the magmas and the development of flow b ands are likely to have occurred in the conduit. During now, the highl y crystalline enclave magma was slightly stretched and deformed, conta minating the dark band magma. The dark band magma deformed into thin b ands, decoupling in some instances from the rhyolitic enclaves which i t hosted. Enclaves and phenocrysts from the dark band magma were local ly captured by the light band magma. Calculations of viscosities of th e three magmas across ranges of equilibration temperature, strain rate , water concentration, and crystallinity indicate that magma viscositi es are more strongly affected by water concentration and crystal abund ance than by temperature or strain rate.