Preliminary radiometric ages for the Arroyo Pescado Formation

LINARES E; HALLER MJ; OSTERA HA; PAGE SM

Trelew

Simposio; II Simposio Argentino Paleozoico Superior; 2001

The Arroyo Pescado Formation metasediments conform the oldest rocks in the Sierra de Tecka (43-44ºS). In their type - locality, these rocks are present in isolated and sporadic outcrops on the left riverbank of the Arroyo Pescado valley, near Estancia Ap Iwan.  The rocks of the Arroyo Pescado Formation are products of low-grade metamorphism on protoliths constituted by sands and sandy pelites.  They correspond to dark gray color metasandstones and low-grade quartz-schists.  The rocks underlain conglomerates interpreted as the base of Tepuel Group of Carboniferous age. The age of Arroyo Pescado Formation is discussed. The original description assigned a pre-carboniferous age to the metasediments.  Recently a carboniferous age has been proposed for the unit. Preliminary Rb/Sr data obtained for the analyzed samples suggest that the homogenization of the isotopic system has been incomplete during the metamorphism of these rocks. Therefore, a solid state diffusion process is postulated during the low-grade metamorphism that affected the Arroyo Pescado rocks.  With this assumption, the persistence of inherited ages attributable to the protholith could be detected.  Two isochrons, of 559 ± 29 and 349 ± 29 Ma respectively have been defined. The older age (559 ± 29 Ma) reflects a minimum provenance age for the original sedimentary package.  The younger age (349 ± 29 Ma) suggests the existence of a metamorphic event which is coincident with that detected in the near Río Chico - Cushamen district (North Patagonian Massif), whose effects extend here closer to the Cordilleran axis. The 349 Ma age marks as well a lower limit for the deformative episode previous to the sedimentation of the Tepuel Group.