Abstract

The Azufre volcano (21 degrees 47'S; 68 degrees 14'W) is emplaced above the western boundary of the Altiplano Puna Magma Body. The youngest lava of the Azufre volcano (50-331 ka) was selected for detailed studies because of the relevant petrological information that could emerge to understand its reservoir as a potential heat source of the neighboring Cerro Pabellon geothermal system. The studied lava corresponds to an andesite-dacite (61-63 SiO2 wt%) with phenocrysts of plagioclase, amphibole (Group 1), biotite, pyroxenes, quartz, and olivine. The lava also contains aphanitic enclaves (58-60 SiO2 wt%), whose groundmass have the same mineralogy of lava samples groundmass consisting of amphibole microphenocryts (Group 2) and microlites of plagioclase, pyroxenes, and Fe-Ti oxides. Disequilibrium textures are commonly observed in the studied samples such as partially resorbed plagioclase phenocrysts, amphibole breakdown, and reverse zoning in pyroxene and plagioclase phenocrysts. Thermobarometry calculations indicated pressures of similar to 2 kbar for Group 1 amphiboles, temperatures of 905- 1097 degrees C for Mg-rich pyroxene phenocrysts and Ca-rich plagioclase (An >= 66), and near-solidus temperatures of 712-788 degrees C for Group 1 amphibole-plagioclase pairs. Group 1 amphiboles also indicate crystallization from an evolved liquid (63-79 wt% of SiO2). Oxidation conditions of QFM + 0.9-2.5 log units were recorded in amphibole and Fe-Ti oxides. Rhyolite-MELTS models reproduce the composition of the high-temperature phases from a melt composition (andesitic enclave) at similar P-T-fO(2). The arrival of new hot andesite magma into a crystal-rich shallow reservoir, thermally and compositionally zoned, would have triggered the studied eruption. Diffusion models in Fe-Ti oxides microlites indicated cooling temperatures of 742-866 degrees C during the sub-aerial emplacement of the lava.