The role of mafic input in highly evolved volcanism within the Altiplano-Puna Volcanic complex: A geochemical study of Cerro Chao

Abstract

Cerro Chao is a crystal-rich dacitic coulée-type lava flow located within the Altiplano-Puna Magmatic Complex (APVC) in Northern Chile, being the largest lava body in the area (14 km long) and one of the largest silicic lava flows in the world. Together with a group of dacitic to rhyolitic lava domes <1 Ma in age (i.e., Cerro La Torta, Chillahuita, Chanka and Chac-Inca), Cerro Chao represents the last steady-state stage of the APVC eruptive cycle. Through a petrological and geochemical study of melt inclusions hosted in plagioclase, amphibole, biotite, and quartz, we aim to present novel data on the composition of Cerro Chao's parental magma and highlight the role that the input of deeper, mafic magma had in the evolution and generation of shallow (∼ 4–8.5 km) parental magma bodies feeding the silica-rich domes. Chao's parental magma (68.8–80.1 wt% SiO₂) originated from a dacitic mush intruded by an andesitic to basaltic andesitic magma(s) (53–59 wt% SiO₂) from the Altiplano-Puna Magma Body (APMB) ascending between ∼20 to 8 km depth. This intrusion(s) caused mixing and later evolution of the magma through assimilation and fractional crystallization (AFC). As a result, the magma consisting of newly formed crystals, older mush crystal aggregates, and melt ascend diapirically through the heating mush. Additionally, unmixed mafic intrusion(s) contributed to the presence of these different xenocrystic glomerophyres. An injection of magma first triggered an explosive eruption, forming Phase 1 of Chao Dacite (de Silva et al., 1994). Then, during its ascent, degassing occurred, allowing the crystal-rich magma to rise through the upper crust via processes of stopping and dyking, eventually erupting effusively.