TEMPORAL EVOLUTION AND ELEVATED POST-LAST GLACIAL MAXIMUM MAGMA FLUX AT CALBUCO VOLCANO (41.3°S), SOUTHERN ANDES, CHILE

Calbuco (41.3°S) is a highly active composite frontal arc volcano in the Andean Central Southern Volcanic Zone (CSVZ). It is currently ranked by Chile’s Geological Survey (SERNAGEOMIN) as among the most dangerous volcanic centers of the Andean arc, with sub-Plinian explosive activity occurring as recently as 2015. Data from a suite of geochemically and geochronologically characterized samples (n = 50) constrain the Pleistocene-Holocene eruptive chronology for Calbuco. New ⁴⁰Ar/³⁹Ar ages and previously published ¹⁴C dates for Holocene tephra units reveal that much of the current edifice is < 20 ka in age. This finding necessitates a revised geological map. Additionally, these data indicate that Calbuco has experienced a greater Holocene eruptive flux, 3.4 km³/ka averaged over the last 10 ka and 4.8 km³/ka over the last 4 ka, than other arc volcanoes both in the CSVZ and globally. Morphological and stratigraphic observations along with new measurements of lava compositions suggest that this remarkable magma flux is expressed as repeated eruptions of intermediate lavas and tephra with a narrow SiO₂ range (54-58 wt.%). This compositional monotony, the presence of resorbed amphibole, and elevated fluid-mobile trace element ratios differ from the post-glacial compositional differentiation inferred at other CSVZ arc front volcanoes and indicates that mid-crustal processing at or just below the amphibole stability zone plays a key role in the genesis of explosive andesitic eruptions at Calbuco.