Geoquímica dos ortognaisses do Complexo Região dos Lagos, Araruama-Cabo Frio, Rio de Janeiro, Brasil

Abstract

The study area is situated on the mid-east part of the Rio de Janeiro state, Brazil, in the Cabo Frio Terrane. This terrane is located on the southeastern extremity of the Ribeira belt, which corresponds to a NE-SW structure composed of thrusts and folds generated during the Brasiliano Orogeny. In this research, the units that constitute the basement of the Cabo Frio terrane, orthogneisses and orthoamphibolites of Regiao dos Lagos Complex, of Palaeoproterozoic age, were studied based on petrography and geochemistry of the orthogneisses. These are hornblende-biotite gneisses of granodioritic to monzogranitic composition, leucogneisses of granitic composition and biotite gneisses of tonalitic and monzogranitic composition. Chemical data reveal that the orthogneisses have a subalkaline, calc-alkaline character and can be divided into two groups: a) gneisses of intermediate composition and lower K 2 O values, covering hornblende-biotite gneisses and biotite gneisses of diverse compositions; b) gneisses of acid compositions and relatively higher K 2 O values, including, predominantly, the granitic leucogneiss. Rare earth element data indicate that not all the samples of the Regiao dos Lagos Complex are cogenetic, but four different suites can be considered. The four identified groups have geochemical signatures similar to the suites generated in modern compressive tectonic environments, particularly those from the volcanic arcs in Chile. The orthogneisses of the Regiao dos Lagos Complex are interpreted as the result of mature magmatic arc construction, generated during the Palaeoproterozoic.