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