EVIDENCE OF NEOPROTEROZOIC BACKARC BASIN DEVELOPMENT IN THE CENTRAL RIBEIRA BELT, SOUTHEASTERN BRAZIL: NEW GEOCHRONOLOGICAL AND GEOCHEMICAL CONSTRAINTS FROM THE SÃO ROQUE - AÇUNGUI GROUPS

Abstract

The Ribeira Belt (RB) of southeastern Brazil represents an important manifestation of the Brasiliano Orogeny formed during the assembly of West Gondwana. Contemporaneous sedimentation and volcanism within the RB provide a basis for helping understand its tectonic evolution and paleogeography. U-Pb monazite data from the basal metavolcanic rocks of the Sao Roque Group indicate a crystallization age of 628 Ma and the upper sequence is cut by a 605 Ma (U-Pb zircon) rhyolite intrusion. Zircon and monazite analyses of metavolcanic (mafic) rocks and from metagabbros of the lower Acungui Supergroup yield crystallization ages of 614 and 617 Ma, respectively. This supergroup is intruded by a 607 Ma granite. Geochemical signatures of basal mafic units in both sequences are characteristic of E-MORB subalkaline tholeitic basaltic rocks. Nd isotopic signatures of the metamafic rocks indicate that they were derived in part from the asthenospheric mantle (consistent with emplacement in an extensional setting), whereas the felsic bodies appear to have come from the melting of Paleoproterozoic lithosphere. The paleogeographic reconstruction of part of the RB suggests that the Sao Roque/Acungui groups represent extensional sequences, with features of backarc basins, which evolved during the syn-collisional phase of the Brasiliano Orogeny. These data support the hypothesis that we have a rapid evolution (10-20 Ma) between extensional and compressional tectonics during the geological history of the Sao Roque/Acungui Backarc.