The cooling history of the North Patagonian Massif: first results for the granitoids of the Valcheta area, Río Negro, Argentina

LÓPEZ DE LUCHI, M. G.; WEMMER,K.; RAPALINI, A.E.

San Carlos de Bariloche

Simposio; 6ºSimposio de Geocronología y Geología Isotópica; 2008

At the NE sector of the NPM, in the area west of Valcheta the Late Paleozoic Complejo Plutónico Navarrete (CPN) and the Early Jurassic Complejo Plutonico-Volcánico Treneta (CPVT) are emplaced in pre-Ordovician Nahuel Niyeu Formation as well as in the Carboniferous (?) meta-magmatic Yaminué Complex. Southeast of Valcheta, a medium grade metamorphic basement hosts the Tembrao Granodiorite and the intruding Permian Pailemán Granite both part of the Complejo Plutónico Pailemán. The aim of this contribution is to present 5 new K-Ar mica ages on pegmatites and granitoids west of Valcheta plus the Paileman Granite which are combined with the available cooling ages in order to set some time constraints on the cooling path of the NE sector of the NPM. The late Ordovician muscovite age for a mostly undeformed leucogranite that intrudes the Nahuel Niyeu Fm. is an evidence for the regional extension of the Ordovician magmatism in the NE corner of the NPM. The Permian biotite cooling age for the undeformed.Plutón San Martin, a part of the CPN would indicate that this granite was cooled down to ~300ºC by the Late Permian. As the only available crystallization age for the CPN belongs also to an undeformed granodiorite the more penetrative Permian deformation that affected this segment of the NPM must be older than 281 Ma. The early Jurassic ages for muscovite booklets of the pegmatoids of the Flores Granite support the Rb-Sr isochron of 188 Ma because their closure temperature is around 500ºC.  The Permian 264 Ma muscovite age for the dykes at the area of Tembrao-Paileman support the Permian age of the Paileman granite.