TEXTURAL AND CHEMICAL VARIATION IN PLAGIOCLASE PHENOCRYSTS FROM THE POLÁN STAGE AND AUCANQUILCHA STAGE OF THE AUCANQUILCHA VOLCANIC COMPLEX, CHILE

Aucanquilcha Volcanic Complex (AVC) is an 11 m.y. old volcanic system that is constructed over the western geophysical boundary of the partially molten Altiplano-Puna Magma Body (APMB) in the central Andes; however, the architecture of the magma plumbing system and magma storage times are poorly understood. The AVC provides a long-lived system providing a time series analysis of potential interaction between the APMB and arc-front stratovolcanoes. This study will test the hypothesis that the Polán (~4-2 Ma) and Aucanquilcha (<1 Ma) stage lavas have long magma residence times from slower rates of magma intrusion which hybridized the crust and created more homogeneous plagioclase and pyroxene mineral compositions. To test this hypothesis we use backscatter electron images and major element mineral chemistry of plagioclase analyzed by an electron microprobe (EMPA), and a petrographic analysis of mineral textures. Seven polished thin sections of andesitic to dacitic composition lavas, have well-zoned, euhedral to subhedral plagioclase phenocrysts. Crystal diameters on the c-axis range from ~1 mm to ~300 um while the a-axis and b-axis diameter is ~250 um to ~100 um. Textures suggest the presence of multiple populations of plagioclase crystals with different patterns of defined core-to-rim chemical zoning. Four distinct plagioclase textures are observed in cores: clear and rounded, clear and euhedral, patchy/spongy, and sieved. Two textures are observed in the rims: sieved and resorbed. Oscillatory zoning profiles separated and interrupted by these textures suggest occasional replenishment of the magmatic system with a chemically similar magma. For each crystal, a complex evolutionary path was reconstructed, and several growth and resorption episodes were identified. Despite the considerable variation in textures and composition of plagioclase phenocrysts, distinct segments have been cross correlated between crystals suggesting that crystallization of plagioclase upon magma ascent caused a shift in melt composition, but sufficient time after incorporation occurred to re-equilibrate the crystals into the melt. Diffusion geospeedometry will provide timescales of magma storage for plagioclase grains that will ultimately elucidate magma storage times as they relate to magma evolution at the AVC.