DACITIC MAGMAS AS THE PRODUCT OF UPPER CRUSTAL MELTING AT LONG-LIVED VOLCANIC COMPLEXES: VOLCÁN AUCANQUILCHA, CENTRAL ANDES OF NORTHERN CHILE

Volcán Aucanquilcha is a large dacitic composite volcano in the Central Volcanic Zone of the Andes. It has erupted ~45 km³ of dacite over the past 1 Ma as effusive lava flows, forming an 8-km east-west ridge composed of at least 7 vents. Aucanquilcha is the youngest product of the largely nonexplosive Aucanquilcha Volcanic Complex (AVC) that has been active for >11 m.y.. Aucanquilcha lavas are remarkably homogenous, ranging ~5 wt% SiO₂ (62.3 to 67.5 wt%), which sets it apart from coeval Volcán Ollagüe (31 km to the SE) whose lavas span ~15 wt% SiO₂. Ollagüe also lacks a history of protracted volcanism around it. Other Central Andean dacitic centers exhibit similar compositional ranges to Aucanquilcha (e.g., Chao Dacite: ~3 wt%) and are thought to be the product of waning stages of a pluton or new melting of upper crust (deSilva et al., 1994). Textural and mineral information at Aucanquilcha indicate a complex but persistent and repeated magmatic history leading to effusive eruptions.

Abundant reaction textures in amphibole and biotite indicate mixing of two (or more) magmas at different P-T conditions just prior to eruption. Ubiquitous pargasite and hornblende in dacite lavas and andesite inclusions suggest magmas are produced in 2 stages: a deeper, hotter source (5-8 km) and a shallower (1-3 km), cooler source. Mgt-Ilm thermometry indicates T of 800-950°C with no difference between lavas and inclusions. Compositions vary in successive volcanic stages (LILEs, Zr, εSr, εNd) during Aucanquilcha’s evolution. These data can be reconciled by a model of a “baseline” source (proto-batholith?) supplying heat and material to the upper crust to create smaller shallow, ephemeral chambers or cupolas that are relatively more sensitive to contamination by assimilation of upper crust. “Homogenous” dacite at Aucanquilcha is likely the product of new melting of upper crustal material in crust thermally “softened” by repeated volcanism in the AVC. Aucanquilcha could be tapping a long-lived batholith, as opposed to a younger source such as that at Ollagüe.