REGIONAL MAGMATIC SETTING OF CALLAQUI VOLCANO (S-ANDES, CHILE)

Callaqui, a Quaternary volcano in the frontal arc of the Andean Southern Volcanic Zone (SVZ; 37.92°S, 71.45°W) has had relatively frequent small-volume eruptions. Rock samples from lava flows, scoria cones, and dikes around the elongate Callaqui edifice were sampled and analyzed for major and trace elements, mineral chemistry, and radiogenic isotope compositions. These volcanics are calc-alkaline basaltic andesites and andesites, with MgO from 2.0 to 10 % and K₂O from 0.4 to 2.5 %. Trace element patterns show arc rock features: Nb-Ta-Ti depletions, and enrichment in LREE and Pb. Plagioclase is the dominant phenocryst, with smaller amounts of olivine (Fo 55-83%), two pyroxenes, and titanomagnetite. Microprobe analyses of glass inclusions (all with SiO₂>60%) in olivine, orthopyroxene and clinopyroxene show H₂O contents ranging from <1.0 % to 3.3 %, 86-560 ppm S, and 1800-3440 ppm Cl. ⁸⁷Sr/⁸⁶Sr ratios range from 0.703651 to 0.704187, and show no simple relationship with degree of magmatic evolution. ²⁰⁶Pb/²⁰⁴Pb ratios range from 18.483 to 18.626, with the most radiogenic value in the most mafic sample. An E-W cross-section of the Andean volcanoes Llaima-Tolhuaca-Callaqui-Copahue-Caviahue-back arc basalts shows the Callaqui samples occupying a compositional field between Copahue and Caviahue lavas with respect to most major and trace elements.

Geothermobarometry suggests crystallization temperatures of 1000-1200°C under highly variable pressure conditions. The glass inclusion analyses suggest volatile loss during the course of crystallization, which may have driven relatively rapid magma ascent, leading to the documented polybaric crystallization and, in some cases, the extrusion of very crystal-rich magmas. The latter may have been “squeezed out” by the buoyancy of underlying vesiculated magma. The isotope and trace element data suggest sediment source signatures in several Callaqui lavas, with variable overprinting from crustal interaction. Callaqui lavas fit into a S-N trend of incompatible element enrichment in the Andes segment between 36° and 39°S, which is most likely more controlled by partial melting processes than crustal interactions.