INVESTIGATING MAGMA MIXING AT AGUAS CALIENTES VOLCANO, CHILE

Magma mixing is widely accepted to be a fundamental petrogenetic process in the formation of intermediate magma compositions at continental arc volcanoes. In this study, we look at mixing processes in a dacite from Aguas Calientes volcano in the Central Andes of Chile. The sample shows evidence of magma mingling, with light and dark bands on a mm-cm scale. We used a petrographic microscope and SEM X-ray element maps to determine the textural relations and modal abundances of the mineral phases in each band. Both bands contain the same mineral phases (plagioclase (plag), clinopyroxene (cpx), orthopyroxene (opx), magnetite, ilmenite, apatite), but the dark bands contain more cpx and the light bands have more plag and apatite. Groundmass contains fine microlites in the dark band, but is glassy in the light bands. Both bands contain different textural populations of plag: clear phenocrysts and sieve-textured phenocrysts with clear rims. Electron microprobe analyses show that plag in the dark band is richer in Mg and Fe. Sieve-textured cores have higher %An (77.2-82.6) than their clear rims (57.8-66.4). Core to rim profiles for clear plag have higher %An in the cores and rims compared to the middles. Cpx in the dark band tends to have higher Mg# (0.67-0.86) compared to the light band, (0.69-0.81). Opx in the dark band tends to have much higher Mg# than in the light band. Based on the different textural populations of plag and their high variability in %An, it seems as though each band in this mingled magma is the product of previous mixing events. The compositional data and modal abundances suggest that the dark band is a more primitive magma; however similarities in the phases present, textures, and compositional ranges also suggest that these magmas formed from similar processes. LA-ICP-MS trace element data for plag, cpx, and groundmass will also be presented in order to better constrain the origins of the crystals and melt of both bands.