Evolution of the Azufre volcano (northern Chile): Implications for the Cerro Pabellón Geothermal Field as inferred from long lasting eruptive activity

Azufre volcano (21°47′S, 68°15′W) is part of the Pleistocene NW-SE trending Azufre-Inacaliri Volcanic Chain, located in the Chilean Central Volcanic Zone of the Andes, near the currently exploited Cerro Pabellón Geothermal Field. Geochronological data and geomorphological features indicate that Azufre was constructed between ~1300 and ~ 120 ka through four magmatic Stages (namely Stage I, II, III and IV), with eruptions from different vents that generated the Northern (Stages II and IV) and Southern (Stages I and III) edifices. Whole rock geochemical data indicates that lavas erupted in Stage II (700–500 ka) and Stage IV (300–120 ka) mainly show lower Al2O3, Na2O and Sr concentrations, and higher FeO, MgO and Ti2O concentrations at similar SiO2 contents than those of Stage I (1,300–700 ka) and Stage III (500–300 ka). These differences suggest dominant plagioclase and Fe–Mg rich mineral phases (i.e. olivine and orthopyroxene) fractionations in the Northern edifice with respect to the Southern one. This is in agreement with petrographic characters that reveal more olivine and orthopyroxene contents in less evolved (<60 wt% SiO2) lavas of the Northern edifice whilst those of the Southern edifice shows higher amphibole content at similar SiO2 compositions. Finally, mineral chemistry of samples of the Northern edifice was used to establish a preliminary estimation of the physical conditions of the magmas below Azufre volcano. Two-pyroxene thermobarometry gave temperature values spanning from 1000 °C up to 1060 °C (±39 °C), and fractionation pressures between 350 and 470 (±370) MPa, whereas amphibole thermobarometry indicated pressures of magma fractionation between 140 and 220 (±11.5%) MPa, with temperature ranging from 810 °C to 890 °C (±23.5 °C). These thermobarometric conditions are similar to those previously published of the nearby Apacheta-Aguilucho Volcanic Complex (~1050–600 ka) and the young (< 150 ka) dacitic domes of Chanka, Chac-Inca and Pabellón, therefore suggesting common petrogenetic systems. Evolution of Azufre volcano fill the time gap of the volcanic activity between the growth of the adjacent Apacheta-Aguilucho Volcanic Complex and the emplacement of the young dacitic domes surrounding the Cerro Pabellón Geothermal Field. This possibly unravels a long lasting (more than 1 Ma) active magmatic system in the northeastern-most sectors of the Azufre-Inacaliri Volcanic Chain and new constraints on the longevity of the heat source regions of the high-enthalpy Cerro Pabellón Geothermal Field, possibly opening new perspectives in the geothermal exploration of this area.