CRYSTALS THROUGH THE YEARS AT THE AUCANQUILCHA VOLCANIC CLUSTER: THE THERMAL EVOLUTION OF A LONG-LIVED ARC SYSTEM AS CONSTRAINED BY INTENSIVE PARAMETERS

Three different geothermometers are employed to evaluate the thermal history of the Aucanquilcha Volcanic Cluster (AVC), a long-lived (11 Ma – pr) magmatic center in the central Andes of northern Chile (21°S). Volcanoes of the AVC number ~20, are dacite dominant, and have a total eruptive volume of ~350 km³. The AVC experienced an eruptive pulse about half way through its lifetime, with ~60% of its volume erupted from ~5 – 2.5 Ma. Magmatic temperatures calculated from the composition of amphibole crystals and magnetite-ilmenite pairs (equations from Ridolfi et al., 2010 and Anderson & Lindsley, 1985, respectively) as well as previously published crystallization temperatures from Ti-in-zircon (Walker et al., 2010), allow us to track the thermal evolution of the AVC magma system as well as to compare the consistency of the thermometers.

Amphibole temperatures range from ~800 - 1000°C with the largest variation observed during the beginning and ending stages of AVC magmatism and more restricted temperature ranges (875 - 975°C) during the eruptive flare-up. Fe-Ti oxide temperatures also range from ~800 - 1000°C, and track amphibole Ts well. Ti-in-zircon temperatures are significantly lower (~690 - 920°C). This discrepancy is likely attributable to 1) the lower saturation temperature for zircon and 2) recycling of zircons grown in a low melt fraction crystal mush. Additionally, some of the difference may result from uncertainty in the Ti-in-zircon temperature calculations, in particular the activities chosen for SiO₂ and TiO₂.

Calculated pressures from amphiboles suggest AVC magmas last equilibrated between ~1 - 5 kbar. Relative homogeneity of pressures recorded during the flare-up period suggests a consolidation, of sorts, of the AVC underpinnings at ~2 – 4 kbar. In contrast, early and late stages of volcanism are marked by larger variations of P and T, suggesting multiple levels of magma storage. Through time, hotter (higher Al) amphiboles grew at progressively shallower levels, suggesting an upwelling geotherm for the system. Importantly, Volcán Aucanquilcha (the last pulse of volcanism, with the hottest recorded amphibole temps) erupted from the central AVC, suggesting that the heat associated with progressive magmatism served to buffer the temperature and sustain magma storage and passage in the central plexus.