EVIDENCE FOR MIXING AND SELF-MIXING AT VOLCÁN AUCANQUILCHA, CENTRAL VOLCANIC ZONE OF THE ANDES, NORTHERN CHILE

Volcán Aucanquilcha is a Pleistocene to Recent composite volcano in the Andes of northern Chile. It consists of at least 5 eruptive centers forming an east-west ridge that rises to 6150 m. The volcano covers over 200 km² and has an eruptive volume of at least 120 km³. Volcán Aucanquilcha is the youngest volcano in the Aucanquilcha Volcanic Cluster whose activity dates to 11 Ma, and its eruptive material could be considered the climax composition erupted at the cluster. ⁴⁰Ar/³⁹Ar age dates from biotite in the stratigraphically oldest lava flows reveal isochron ages of 0.6 ± 0.22 Ma and 1.0 ± 0.08 Ma. Aucanquilcha is high-K dacite lavas, with SiO₂ of 63.7 to 67.2 wt%, K₂O of 2.5 to 3.3 wt%, and MgO of 1.35 to 2.32 wt%. Quenched magmatic inclusions found at Aucanquilcha are not considerably more mafic than the host lavas, but have SiO₂ of 60.2 and 62.3 wt% and MgO of 3.63 and 2.63 wt%. SiO₂ vs. K₂O or Rb show a strong bifurcation of trends at higher SiO₂ for lavas. Lava flows have 10-20% phenocrysts with at least 4 textural populations of plagioclase including textural elements of spongy cores, fritted rims, euhedral overgrowths, and compositional zoning for large phenocrysts. Micro-phenocrysts and microlites of plagioclase rarely show textural diversity. Hornblende and biotite exhibit as many textural populations as plagioclase; most of phenocrysts display reaction rims along with oxidation and dissolution of the cores. Rare clino- and ortho-pyroxene is found as glomerocrysts with plagioclase and hornblende or as rims of other phases. Trace, rounded xenocrysts (< 0.5 vol%) of quartz are found in some flows and are rimmed by pyroxene. Magmatic inclusions are finer grained and have a higher vol% of hydrous phases, particularly amphibole microlites. Evidence for open and closed system processes is found in the lavas at Volcán Aucanquilcha. A high percent of flows with quenched magmatic inclusions (> 70%) indicate that an influx of more mafic magma played an important role. The textures of phenocrysts along with remarkable homogeneity of bulk composition are consistent with a convective self-mixing (Couch, et al., Nature, 2001) magma chamber where thermal disequilibrium controls reaction textures. The climax composition at Aucanquilcha is the product of balanced thermal and compositional mixing during the evolution of the magma system.