Estructura y metamorfismo de la Formación la Modesta en la estancia El Tranquilo (Santa Cruz), Patagonia

Abstract

Structure and metamorphism of the La Modesta Formation in the El Tranquilo ranch (Santa Cruz), Patagonia. In this paper are exposed the results of a geological survey about the lower Paleozoic metamorphic rocks of La Modesta Formation in the El Tranquilo farm in Santa Cruz province. Structural, petrographic and geochemical studies carried out in this rock unit indicated that it includes pelitic and psammitic quartz - muscovite - chorite schists, with blastesis of biotite. These rocks show evidences of a regional metamorphism associated with two deformational events. The protolith has been recognized as a pelitic and psamo-pelitic marine sedimentary succession that was affected by metamorphism with a temperature up to 550°C and a pressure between 2 and 4 Kb, reaching green schists metamorphic facies (biotite-garnet grade). It was established a regional metamorphic foliation, S1, subparallel to the sedimentary stratification S0. The schistosity S1 was deformed by a second deformational event, generating a non penetrative S2/L2. This rock unit is differentiated from the La Modesta Formation in the type locality because of its protoliths and the second deformational event is most evident in this area. Nevertheless, the Formation has been correlated with metasedimentites located to the west and its sedimentary protoliths could have been related with a forearc Pacific basin that afterwards formed part of the gondwanic accretionary prism.