Geochronology of very low-grade Mesozoic Andean metabasites;: an approach through the K-Ar, 40Ar/39Ar and U-Pb LA-MC-ICP-MS methods

Multiple geochronological methods using different metamorphic minerals were combined to date the regional, very low-grade metamorphism affecting Upper Jurassic-Lower Cretaceous volcano-sedimentary successions in the Andes of central Chile. Early Late Cretaceous metamorphic ages (between 82 and 108 Ma) were obtained by the K-Ar and U-Pb methods for celadonite and titanite. A much younger thermal event is responsible for actinolite formation at 8 Ma, most probably related to the intrusion of proximal Miocene granitoids. Previous models for the metamorphism should be reinterpreted taking into account the absence of a greenschist-facies event. The combination of different metamorphic minerals and chronometers is regarded as a powerful analytical tool to date the very low-grade metamorphism associated with the Mesozoic extensional regime developed within the Andes.