Age and geotectonic setting of Late Neoproterozoic juvenile mafic gneisses and associated paragneisses from the Ribeira belt (SE Brazil) based on geochemistry and Sm-Nd data - Implications on Gondwana assembly

摘要

Mafic gneisses and associated paragneisses from the Cabo Frio Tectonic Domain in the southeastern part of the Ribeira Belt, along the coast of Rio de Janeiro State in southeast Brazil, were subjected to a geochemical and Sm-Nd isotope study. Four lithotypes are distinguished: aluminous paragneisses (mainly sillimanite-kyanite-garnet-biotite gneiss), calesilicate lenses, quartzo-feldspathic metasedimentary gneisses and mafic-ultramafic lenses. The whole-rock major and trace, including rare earth element distributions in the mafic-ultramafic intercalations indicate that derivation from subalkaline basalt/gabbro of tholeiitic affinity with E-MORB signature from a non-subduction environment. These mafic rocks have positive epsilon(Nd)(t) and T-DM of 1.1 Ga. The metasedimentary rocks have negative epsilon(Nd)(t) and T-DM of 1.7 Ga. A Sm-Nd whole rock isochron of mafic rocks yielded an age of 604 +/- 38 Ma for the crystallization. This matches with the age of some detrital zircon grains from the paragneisses. The depositional basin, named Buzios-Palmital, was active at least until 620 Ma (age of the youngest detrital zircon) and was subsequently deformed and metamorphosed at ca. 525 Ma (age of metamorphic zircons) during the Buzios Orogeny. It is interpreted as a back arc basin with relation to the 630 Ma magmatic arc of the Oriental Terrane in the Ribeira Belt to the NW. However, after 600 Ma, the Buzios-Palmital basin changed to an active margin setting because the arc collided with the continental margin and the subduction shifted to the back are environment. By 610 Ma, most of the Brasiliano belts registered collisional events related to multiple convergent blocks. The stress fields and paleocontinent shapes would have allowed the occurrence of extensional areas with not only sedimentary deposition but also ocean floor spreading. Its remnants are preserved in this Brazilian coastal region as an ancient suture, reworked intensively during the Mesozoic rifting events. The reconnaissance of Late Neoproterozoic basins in the Brasiliano-Pan-African belts is of major importance to partially unravel the final amalgamation events of SW Gondwana. Considering that the Buzios-Palmital basin rock units are mostly covered by the marginal Atlantic basins, it is possible that other evidence could be preserved in the coastal regions of SW-Africa and SE-South America. (C) 2007 International Association for Gondwana Research. Published by Elsevier B.V All rights reserved.