New isotopic ages and the timing of orogenic events in the Cordillera Darwin, southernmost Chilean Andes

The Cordillera Darwin, a structural culmination in the Andes of Tierra del Fuego, exposes an orogenic core zone that has undergone polyphase deformation and metamorphism. Some of the classic problems of orogenic zones have remained unanswered in the Cordillera Darwin: the age of deformed plutonic rocks, the distinction of structurally reactivated basement and metamorphosed cover rocks, and the timing of orogenic events. This study addresses and partially answers these questions. A well-constrained Rb-Sr isochron age of 157±8 m.y. and an initial ⁸⁷Sr ⁸⁶Sr ratio of 0.7087 obtained from a pre-tectonic granitic suite suggest a genetic relation between this suite and Upper Jurassic silicic volcanic rocks in the cover sequence (Tobifera Formation), and also suggest involvement of continental crust in formation of these magmas. A poorly constrained Rb-Sr isochron age of 240±40 m.y. obtained from supposed basement schists is consistent with field relations in the area which suggest a late Paleozoic/early Mesozoic metamorphism for these pre-Late Jurassic rocks. However, because of scatter in the data and the uncertainties involved in dating metasedimentary rocks, the significance of the isotopic age is dubious. Compilation of previously published ages in the area [9] with new mineral ages reported here indicate that "early Andean" orogenic events occurred between 100 and 84 m.y. ago, and that subduction-related magmatism has contributed, probably discontinuously, to the crustal evolution of the region throughout the Mesozoic.