Regional Geochronology of Chile South of 50° Latitude

Abstract

In southern Chile the oldest known rocks are metamorphosed. Gneiss from the basement rocks of the Magellan Basin at the Atlantic entrance to the Strait of Magellan has been dated by Rb-Sr total-rock analyses at 306 ± 156 m.y. (λβ = 1.47 × 10 −11 yr −1 ), with an initial Sr 87 Sr 86 ratio of 0.7112 ± 0.0033. Biotite from a sample of gneiss has been dated as Permian age by the Rb-Sr and K-Ar methods; this implies that the basement rocks of the Magellan Basin have been involved in one or more Paleozoic geologic events. Paraschist from the basement complex along Chile9s Pacific margin indicated Paleozoic to Mesozoic total-rock Rb-Sr ages, based on an assumed initial Sr 87 Sr 86 ratio of 0.710; minerals separated from the schists gave late Mesozoic Rb-Sr and K-Ar dates. Volcanic rocks that overlie the basement rocks and are generally accepted as being of Late Jurassic or Early Cretaceous age based on stratigraphic position gave total-rock Rb-Sr and K-Ar dates of Late Cretaceous to Tertiary age; these dates are considered to represent the time of final closure of their isotopic systems, perhaps associated with deformation in the region. The plutonic igneous rocks that constitute the southern Andes Patagonian batholithic complex range in age from Jurassic to Tertiary. Late Jurassic to Early Cretaceous (155 to 120 m.y.), Late Cretaceous (100 to 75 m.y.), and middle to late Tertiary (50 to 10 m.y.) phases of magmatic activity have been recognized. In the Cordillera Darwin region of the Beagle Canal, minerals separated from plutonic rocks of the Cordillera Darwin suite and from the metamorphic rocks it intrudes gave Rb-Sr and K-Ar dates of Late Cretaceous to early Tertiary age.