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, in the Tumacacori Mountains of southern Arizona, contain light‐colored dacitic flow bands, dark‐colored rhyolitic flow 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. Enclaves are fine grained, holocrystalline, and phenocryst‐poor. 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. Both mingling events may have happened in the magma chamber, or partially in the magma chamber and partially in the volcanic conduit. Deformation of the magmas and the development of flow bands are likely to have occurred in the conduit. During flow, the highly crystalline enclave magma was slightly stretched and deformed, contaminating the dark band magma. The dark band magma deformed into thin bands, decoupling in some instances from the rhyolitic enclaves which it hosted. Enclaves and phenocrysts from the dark band magma were locally captured by the light band magma. Calculations of viscosities of the three magmas across fanges 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.