El Sótano Granodiorite: a deformed Jurassic pluton in the basement of Las Grutas, Atlantic Northpatagonian Massif

Abstract

Igneous and metamorphic rocks belonging to a crystalline basement crop out to the south of Las Grutas beach, Río Negro Province. Within these rocks, a small granodioritic exposure was identified and named El Sótano Granodiorite. The granodiorite shows a magmatic foliation developed by NW-trending feldspar phenocrysts, and an overprinting brittle to ductile deformation at low metamorphic grade coeval with a hydrothermal activity. The conventional U-Pb zircon age of 186 ± 9 Ma is interpreted as crystallization at an early magmatic stage, whereas the Rb-Sr whole rock and mineral isochron of 174 ± 22 Ma might indicate the timing when the cooling system passed around 300°C and the deformation fabric was almost completed. The crystallization age and the location of the granodiorite are consistent with the initial stages of the extensive Jurassic volcanism developed throughout Patagonia. The Nd and Sr isotopic characterization (eNd -5.7, and initial ⁸⁷Sr/⁸⁶Sr 0.7071) as well as the granodioritic composition are all comparable with those of the initial andesitic to dacitic layers of the volcanism. However, the volcanic rocks do not show a pervasive brittle-ductile deformation, and therefore, the deformation found in the El Sótano Granodiorite must be the result of a local stress field, despite the NW orientation of the folitation that is also shared by both the compressive Permian structures and the extensional Triassic to Jurassic structures of the region.