Constraining the magmatic evolution of the Andean arc at 21⁰S using the volcanic and petrologic history of Volćán Aucanquilcha, Central Volcanic Zone, northern Chile

Abstract

Volcan Aucanquilcha, a Pleistocene to Recent composite volcano in northern Chile, is the youngest and center-most member of the 11 m.y.-old Aucanquilcha Volcanic Cluster (AVC). The AVC has produced [approximately]350 km³ of lava over its history. Magmatism started slowly and was punctuated by high rates of eruption between 4.5 and 2.5 Ma. The AVC records a history of progressive fractionation and homogenization as the upper crustal becomes thermally and compositionally buffer through repeated intrusion. The pattern of volcanism at the AVC, from diffuse to central, mimics that seen in exposed intrusive bodies. The AVC is likely the volcanic record of the incremental emplacement of an intermediate batholith over a span of at least 11 million years. Volcan Aucanquilcha has produced [approximately] 37 km³ of compositionally and mineralogically monotonous dacite over the past 1 million years. The volcano has erupted dominantly effusive dacite lava flows with one debris avalanche deposit and one dome collapse pyroclastic flow. It also contains a ubiquitous bimodal amphibole population of hornblende and pargasite phenocrysts. P-T data from amphibole and Fe-Ti oxides indicate a multilevel geometry for the magmatic system within the top 10-km of the upper crust. The generation of magma at Aucanquilcha begins with intrusion of andesite that fractionates to mafic dacite. Magmas will then either mix with low [delta]¹⁸0 upper crustal and creating a high-K dacite or will continue to fractionate to silicic dacite. In both cases, two similar dacite magmas, stored at different P-T condition mix rapidly during eruption. The repeated intrusion of magma into the upper crust during the history of the AVC has allowed for thermal and compositional buffering of the crust, creating the conditions for Volcan Aucanquilcha to erupt monotonous dacite lava. The location of eruptive vents appears to be controlled by crustal lineaments that not only focus hydrothermal fluids but may allow for the eruption of the relatively degassed dacite. Otherwise, the lavas of Aucanquilcha may have stalled underground to form a porphyry-style stock.