Two-stage evolution of a bimodal reservoir: The case of Holocene lavas of the Lanín composite volcano, Southern Volcanic Zone, Chile

Abstract

Lanín is a composite volcano (∼39.38°S; 71.30°W) located in the Central Southern Volcanic Zone of the Andes. It is the southernmost member of the NW-SE volcanic alignment with the Villarrica and Quetrupillán composite volcanoes. During the Holocene, Lanín volcano erupted many lava flows, which we classify into three compositional groups: BAT (basalt, andesite, and basaltic trachyandesite), intermediate, and trachyte samples. The main recognised phenocrysts in the Lanín lavas are plagioclase, clinopyroxene, orthopyroxene, Fe–Ti oxides, and apatite. These mineral phases allow us to constrain the pre-eruptive (from the magma reservoir) conditions: temperature (985–1089 °C), pressure (1.8–4.1 kbar; equivalent to 6.5–15 km depth), and oxygen fugacity patch (∼QFM+0.4). In addition, textural features combined with pre-eruptive conditions suggest the existence of a crystal mush within the reservoir, which developed in two stages. The first stage considers the emplacement, crystal mush development, and eruption related to a melt of BAT composition. The residual melt evolved from BAT to trachytic primarily through fractional crystallisation. The second stage involves an evolved trachytic melt in the reservoir and the intrusion of a new, more mafic magma (presumably of BAT composition), which generates heating of the trachytic residual melt and crystal mush disaggregation. The fractional crystallisation also gives rise to scarce products of intermediate composition. This observation is consistent with the chemical constituents plots (TAS and Harker) and thermodynamic modelling.