Geochronology of the pre-Andean metamorphism and magmatism in the Andean Cordillera between latitudes 30° and 36°S

In the 30°-36°S section of the Andean Cordillera the pre-Andean basement is composed by metamorphic, igneous, and sedimentary rocks of Precambrian, Paleozoic and Lower Triassic ages. They crop out in the morphostructural units named from east to west: Sierras Pampeanas, Precordillera, Cordillera Frontal and Bloque de San Rafael in Argentina, and Cordillera de la Costa in Chile. Isotopic age determinations of the metamorphic rocks indicate a migration of the metamorphic events from east to west, such as: Precambrian to Lower Paleozoic ages (900 m.y. and 650-400 m.y.) in the Sierras Pampeanas; Lower Paleozoic ages (about 500 m.y.) in the Precordillera and Cordillera Frontal; and Upper Paleozoic to Lower Triassic (?) ages (340-210 m.y.) in the Cordillera de la Costa. These metamorphic belts contain a large variety of mafic and ultramafic rocks. The distribution of the pre-Andean granitoids show no apparent chronological zonation. However, the plutonic activity persisted until the Lower Carboniferous in the Sierras Pampeanas; to the Lower Triassic in the Cordillera Frontal and Precordillera, associated with an important vulcanism which also develops in the Bloque de San Rafael; and until the Jurassic in the Cordillera de la Costa.