INSIGHTS INTO MAGMA STORAGE AND EVOLUTION AT LICANCABUR VOLCANO, CENTRAL ANDES, CHILE

Lincancabur volcano is a young high-K andesite composite volcano along the border of Bolivia and Chile in the Central Andes. Volcanic rocks are dominantly crystal poor lavas and domes. Whole rock compositions and modal mineralogy are relatively homogeneous, but eruptive volumes inconsistent. Here we use plagioclase from three lava flows to determine major element contents by electron probe microanalyzer (EPMA) and combine these data with backscatter electron images and quantitative chemical maps to determine the magmatic processes leading to eruption. Plagioclase textures define multiple populations of phenocrysts. All plagioclases contain oscillatory zoning. Cores are commonly homogenous with a resorption surface separating cores from complex zoning profiles. Rims and mantles are mostly clean with sharp contacts with resorption only present close to the edge of the crystal. Anorthite contents define three populations of oscillatory crystals. FeO and MgO contents are consistent from core to rim with similar patterns. We suggest that plagioclase cores represent two end member magmas which were mixed in a shallow chamber and allowed to re-equilibrate. Eruption was triggered by a second mixing event defining the resorption surface in the plagioclase rims.