Palaeoenvironment reconstruction, volcanic evolution and geochronology of the Cerro Blanco subcomplex, Nevados de Chillan volcanic complex, Central Chile

Nevados de Chillán Volcanic Complex, central Chile, has been active for at least 640 ka—a period spanning a number of glacial and interglacial periods. Geologic mapping, radiometric dating and geochemical analysis have identified six new volcanic units and produced four new 40Ar/39Ar ages for Cerro Blanco, the northern subcomplex of Nevados de Chillán volcano. Compositions range from dacite to basaltic-andesite and a new geologic map is presented. Examination of lava fracture structures on both newly mapped lavas and those mapped during previous studies has enabled interpretations of former eruptive environments. Palaeoenvironment reconstructions, combined with 40Ar/39Ar ages and comparison with the marine oxygen isotope record, show that at least three phases of volcanic activity have occurred during the evolution of Cerro Blanco: (1) a constructive, pre-caldera collapse period; (2) a period of caldera formation and collapse; and (3) a constructive period of dome growth forming the modern day volcanic centre. This style of volcanic evolution, whereby large-scale caldera collapse is followed by growth of a new stratocone is common at Andean volcanoes.