Pre-eruptive conditions of Mid-to-Late Holocene eruptions from Lonquimay Volcanic Complex, Southern Andes of Chile

Lonquimay Volcanic Complex (38°22′S, LVC) is one of the most active volcanoes in the Southern Volcanic Zone of the Andes. It comprises a main composite volcano and twelve monogenetic cones, and it has erupted since the Upper Pleistocene. Its latest eruption was in 1988–90 and had catastrophic consequences for the human health and environment. Here we estimated and compared key pre-eruptive conditions (pressure, temperature, dissolved H₂O content and crystallinity) from three of the most well-exposed and widespread fall deposits from the composite volcano, and three lavas of the volcanic complex. Magmas that fed the analysed eruptions were basaltic to trachytic in composition and contained a mineral assemblage of plagioclase±olivine±clinopyroxene and minor FeTi oxides and apatite. Evolved units contain unique fayalitic autocrysts (as low as Fo₂₁), which are proposed to have formed in a magma (1) under low pressure, (2) with an evolved composition, (3) a high Fe content, and (4) a reducing state. Based on geochemistry and thermobarometric calculations, we suggest two magma storage zones: (1) a deeper hot and mafic storage region (∼12 km) that host basaltic to basaltic-andesitic magmas at 1083–1151 °C, which undergo tholeiitic mafic injections and (2) a cooler and evolved eruption-feeding magmatic system with a major storage region at 5.6 km with magmas at 869–1046 °C, where fractional crystallisation, magma injections, mingling, and convection took place. Although the same magma injection and mingling triggering-mechanism is suggested for explosive and effusive eruptions, we suggest mingling timescale plays a key role in controlling eruptive style, along with volatile composition and microcryst content.