Mamil Choique Granitoids, southwestern North Patagonian Massif, Argentina: magmatism and metamorphism associated with a polyphasic evolution

Abstract

Petrological and structural features of the metamorphic Cushamen Formation (CF), igneous units Tunnel Tonalites (TT) and Mamil Choique Granitoids (MCG) displaying polyphasic tectonic events in the Rı́o Chico-Mamil Choique area, southwestern corner of North Patagonian Massif (NPM), Argentina are studied. The metamorphic series of the CF displays a medium-pressure regional metamorphism, which is overprinted by the contact metamorphism related to the emplacement of the igneous units. The TT-MCG are calc-alkalic epidote-bearing plutons, with a metaluminous-peraluminous character. MCG include two magmatic series controlled by differences in oxygen fugacity, fluid interaction and crystallization dynamics. Four deformational phases have been recognized in the area. Regional relationships among metamorphism-deformation-magmatism agree with what is to be expected from a collisional setting, with peak metamorphic conditions being synchronous with the second deformation, and with magmatism postdating regional metamorphism. This would be consistent with the deep-seated magmatism interpreted from crystallization sequences in the TT and MCG. The TT and MCG could be ascribed to the same tectonothermal event since their primary magmatic foliations are overprinted by D 3 subsolidus deformation. Rb-Sr isochrons for the MCG are dubious, indicating either Ordovician/Silurian or Carboniferous ages. This datum would set a minimum age for D 3, and could bracket a crustal thickening event.