LITHOGEOCHEMISTRY OF PALEOPROTEROZOIC ORTHOGRANULITES FROM THE RIO PRETO (MG)-VASSOURAS (RJ) REGION, CENTRAL RIBEIRA BELT, SE BRAZIL

Abstract

The granulites of the Juiz de Fora Complex (JFC) integrate the pre-1.8 Ga basement of the Intermediate Thrust Sheet of the Ribeira Belt's central segment. This tectonic domain is characterized by intense tectonic imbrication of the JFC granulites with Meso- to Neoproterozoic metasedimentary units. In the Rio Preto (MG)-Vassouras (RJ) region, the JFC comprises three lithological associations: orthogranulites, orthogneisses and paragranulites which crop out as several thrust sheets formed by intense shortening during the Brasiliano Orogeny. The main contacts are characterized by the transformation of the granulites into amphibolite facies mylonites. The geochemistry of the orthogranulites leads to the identification of two tectonic associations. One comprises two calc-alkaline series compositionally similar to rocks of modern magmatic arcs, while the other contains two groups of basic rocks, respectively the tholeiitic series and alkaline to transitional basalts, both of intraplate extensional tectonic setting. The relationships between these distinct tectonic environments need further geochronological background, as the Brasiliano shortening obliterated the original paleogeography of this important Paleoproterozoic segment of the Ribeira belt.