U-Pb and Lu-Hf zircon analysis in orogenic basins - A case study in a hot orogen, SE Brazil

During the convergence of the São Francisco and the Congo-Angola paleocontinents, between 600 and 500 Ma, sedimentary basins were generated and metamorphosed to paragneisses within the Ribeira Orogen, SE Brazil. The Oriental Terrane, part of this orogen, and the focus of this study, is constituted of paragneisses intruded by voluminous pre-, syn- and post-orogenic batholiths. Here, we investigate the origin of the paragneisses that crop out in the city of Rio de Janeiro. The São Fidélis Group consists mostly of cordierite-sillimanite-garnet-biotite paragneisses with minor quartzite layers at granulite facies. Peak metamorphic conditions are estimated at 850 °C and 7 kbar. Our goal is to study the provenance of the protoliths and the basin's tectonic evolution, using a combination of U-Pb and Lu-Hf methods in zircons, even though these rocks underwent metamorphism above the zircon closure temperature for the U-Pb system. Fieldwork was conducted at Niemeyer Avenue and Morro Cara de Cão. Three samples from distinct lithologies were analyzed: a cordierite-sillimanite-garnet-biotite paragneiss (kinzigite), a fine-grained biotite paragneiss, and a quartzite. All samples show similar detrital zircon populations, with predominant Neoproterozoic sources and minor Mesoproterozoic and Paleoproterozoic sources. The older populations might come from African sources, such as the Kunene Anorthosite Complex and the Eburnean magmatic units of the Angola Craton and the Cabo Frio Tectonic Domain. The main Neoproterozoic populations are 980-930 Ma, 856-838 Ma, 800-740 Ma, and 690-580 Ma. These peaks coincide with the ages of the magmatic arcs described in the Oriental Terrane, suggesting them as sources. The Neoproterozoic εHf values for the dated detrital zircons range from +5 to −18, which corroborates with magmatic arc nature of these sources. The youngest detrital zircon population (590-570 Ma) could be either interpreted as metamorphic or magmatic due to Pb-loss caused uncertainties. Therefore, we propose that the maximum age of sedimentation is a time interval of 590-570 Ma and the minimum age is about 560 Ma (age of the metamorphic rims and of the Facoidal Orthogneiss syntectonic intrusion). The short period between initiation of sedimentation and metamorphism, 590-560 Ma, suggests a syn-orogenic basin, related to an arc terrane (Oriental Terrane), either a back-arc or an intra-arc basin. This period coincides with the regional metamorphism in this hot orogen.