Multistage magma mingling and the origin of flow banding in the Aliso lava dome, Tumacacori Mountains, southern Arizona

Flow banded rocks of the middle Tertiary Aliso lava dome contain light-colored dacitic bands, dark-colored rhyolitic bands, and dark-colored ellipsoidal enclaves of rhyolite. Groundmass glass of the light- and dark-colored flow bands is trachyte and high-silica rhyolite, respectively. A two-stage 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 band magma composite mingled with alkalic trachyte magma, now represented by the light bands. Calculations of viscosities of the three magmas across ranges of equilibration temperature, strain rate, water concentration, and crystallinity indicate that magma viscosities are more strongly affected by water concentration and crystal abundance than by temperature or strain rate. -from Authors