Fil: Serra Varela, Samanta. Universidad Nacional de Río Negro. Instituto de Investigación en Paleobiología y Geología. Río Negro, Argentina.Fil: González, Pablo Diego. Universidad Nacional de Río Negro. Instituto de Investigación en Paleobiología y Geología. Río Negro, Argentina.Fil: Giacosa, Raúl. Servicio Geológico Minero Argentino (SEGEMAR), Delegación Comahue; Argentina.Fil: Heredia, Nemesio. Instituto Geológico y Minero de España; España.Fil: Sato, Ana María. Centro de Investigaciones Geológica (CIG), UNLP- CONICET; Argentina.Metamorphic basement rocks of San Martín de los Andes area comprise schists, gneisses and migmatites. They are intruded by Devonian granitoids that cut the metamorphic internal structure of these rocks. This basement belongs to a pre-Gondwanan cycle and its regional extension in the Nordpatagonian Andes is unknown. Petrographic analysis and pseudosections allowed us to determine physical parameters in order to characterize the metamorphic conditions. P-T conditions for migmatites range between 710° and 740°C below 0.44GPa. These conditions correspond to a low pressure ? high temperature (LP-HT) metamorphism between amphibolite and granulite facies. Considering its metamorphic characteristics and age (pre-Devonian), no equivalent rocks have yet been found in nearby areas with outcrops of pre-Andine basement.