Investigação estrutural do Gnaisse Neto Rodrigues e sua relação com Complexo Granítico Caçapava do Sul

Abstract

The Neto Rodrigues Gneiss (GNR) is located at southeast of the city of Caçapava do Sul, in the state of Rio Grande do Sul, Brazil. The GNR lithology is composed for monzogranites, sienogranites and granodiorites disposed in layers. Two hypotheses about the origin of the GNR were proposed in previous works: (1) the GNR is a very deformed portion of the Caçapava do Sul Granite Complex (CSGC); or (2) the GNR is a protolite that had an active participation in the generation of the magmas that give rise to the CGCS. Through the geological and structural mapping and the the analysis of the structural data obtained in the field, it was possible to observe that the GNR rocks were less deformed to the southwest in the study area and more deformed to the northeast in the area. During the microstructural analysis it was observed that the quartz was recrystallized dynamically, while K-feldspar and plagioclase were deformed in a dull manner. As a consequence of our studies was possible to compare the GNR and the CGCS and to delimit conditions about the deformation for the GNR rocks. In this way, the conclusion is that the GNR is a portion of the CGCS that was deformed in a simple ductile shear zone, under thermal conditions between 400° C and 500° C, and that your layered structure was generated for the parallelization of the structures of compositional banding in a zone of high deformation. In view of these conclusions, it was suggested that the term gneiss for the rocks of the study area is no longer used and the term banded granitoids is adopted as one of the CGCS facies.