Thermobaric evolution of the peraluminous orthogneiss, of north-western Velasco range, La Rioja

Abstract

The Antinaco Orthogneiss forms a mountain block located in the north-western sector of the Velasco range. It consists of coarse-grained biotite-muscovite-bearing porphyritic orthogneisses, with accessory garnet and kyanite/sillimanite. The protolith is a porphyroid peraluminous granite that was deformed into protomylonitic zones with NNW- striking, steeply ENE - dipping foliation which contain down-dip stretching of mica minerals. Rotated feldspar porphyroclasts indicate an east-over-west movement sense. Chemical data from garnet, biotite, plagioclase, muscovite, and alumino-silicates as well as their textures, indicate high temperature and middle pressure conditions of deformation. The thermobaric evolution of this important block records a pressure peak of 6 kb and three episodes of isothermal decompression with temperatures ranges between 591 and 588 ºC and decreasing pressures of 4.3, 3.4, and 2.7 kb, respectively. The unloading could have allowed the emplacement of the early Carboniferous San Blas porphyritic granite to the east. Despite the lack of a crystallization age for the Antinaco Orthogneiss, the geological relationships with San Blas Granite and their correlation with the mylonites of the La Puntilla de Copacabana, suggest it formed as the result of a Devonian intrusion and deformation.