THE RIO-CHICO PALEOZOIC CRYSTALLINE COMPLEX AND THE EVOLUTION OF NORTHERN PATAGONIA

The Rio Chico Complex, which includes the Cushamen Metamorphics and th e Mamil Choique Granitoids (Ordovician, 439+/-10 Ma), constitutes the crystalline basement of the Rio Chico region (southwestern North Patag onian massif). The metamorphics and migmatites resulted from a medium to high grade tectono-thermal event which caused local anatexis of a p rotolith of graywackes, pelites and some quartz-rich sandstones. These rocks show a main deformational episode, trending mainly NNW, which c ontrolled the post-Paleozoic structures. A Devonian age granite (387+/ -17 Ma) intruded this complex. The Ordovician and Devonian granitoids, which are calc-alkaline, and per to metaluminous, may be related to a natexis during the Taconian and Acadian Laurentia-Gondwana collisional events. During the Permian (260+/-5 Ma) leucogranites, some bearing g arnet, were intruded into the ancient orogenic belt following an impor tant phase uplift. They are related to further melting of continental crust during extension in an intraplate environment, and are included in the Choiyoi acidic Province.