PLAGIOCLASE ZONING AS AN INDICATOR OF MAGMA PROCESSES AT SAN PEDRO VOLCANO, CENTRAL VOLCANIC ZONE, CHILE

The textures and compositions of plagioclase and other minerals can be used to identify the physical and chemical processes operating in sub-volcanic magma plumbing systems. These mineral phases record the interaction between different magma compositions, disequilibrium melting of country and changes in volatile content during magma ascent. Given that plagioclase is an abundant phenocryst phase in intermediate composition magmas in the Central Andes understanding the link between texture and process is critical to understanding the volcanic system. This study presents major and trace-element chemistry of plagioclase from San Pedro volcano in the Chilean Central Andes. Back-scatter electron (BSE)-derived zoning profiles of plagioclase phenocrysts from four lavas are used to identify magma process at San Pedro volcano. The erupted magmas are two pyroxene andesites which are suggested to have been stored at upper crustal pressures and ~900°C to 1000°C. Textural and compositional zoning of individual crystals includes a core-to-rim sequence of repeated oscillatory zoning, with dissolution surfaces and truncated zones. Some crystals with these zoning patterns contain sieving in the mantled rims. This zoning pattern is interpreted to be the result of frequent replenishment of the magma chamber which caused both a thermal and chemical interaction between plagioclase and melt. Spongy and coarse sieved cores are next most common texture. These crystals are interpreted to be dissolution of decompression melting in a H₂O-undersaturated magma because of repeated eruptions during crystal residence and crystal inheritance during recharge. Despite the considerable variation in textures and composition of plagioclase phenocrysts, distinct segments have been cross correlated between crystals. Variation in Fe, Mg, Sr and Ba in the melt and in the plagioclase record crystal residence in deeper melts and record the ascent through the crust, potentially recording mixing with a partially molten large volume magma body, the Altiplano-Puna Magma Body.