Geochemistry and geochronology of paleoproterozoic gneissic rocks of the Paraíba do Sul Complex (Quirino Unit), Barra Mansa region, Rio de Janeiro, Brazil

This paper presents and discusses new geochemical data from orthogneisses rocks of Paraiba do Sul Complex (PSC) from the NW of the Rio de Janeiro State, denominated Quirino Unit. U-Pb geochronological data (carried out on GEOTOP-UQAM, Canada), displayed in Valladares (1996) and Machado et al. (1996), are also discuss. This Unit, as well the  Metasedimentary Unit of PSC and intrusive brazilian granitoids, comprise the Paraiba do Sul Tectonic Domain (PSTD) or Upper Tectonic Domain, at the central segment of the Ribeira belt. The Quirino Unit occurs as extensive bodies of homogeneous gneisses of upper amphibolite facies metamorphism, with hornblende and or biotite comprising approximately 50% of the studied area and ca. 70% of the PSTD. This unit yields U-Pb zircon ages of 2185±3 Ma and 2169±3 Ma, temporally related to the Transamazonian event. Minimum ages of 2846 Ma and 2981 Ma (zircon U-Pb data) reveal the pre-existence of Archean crust as Pb source of part of the investigated gneisses. Geochemical studies of orthogneisses Quirino discriminate two calc-alkaline suites: one of medium to high-K and other of high-K, with enrichment in LILE. The chemical characteristics and the ages U-Pb allow to perform a tectonic correlation of the Quirino Unit with arc magmatic calc-alkaline granitoids, which were generated during the Transamazonian orogeny (2.2 - 1.9 Ga). Ages between 605-503 Ma (zircon and sphene U-Pb data) indicate reworking of Quirino Unit during Brazilian Orogeny (ca. 700-450 Ma).