Detrital zircon (U-Pb) and Sm-Nd isotope studies of the provenance and tectonic setting of basins related to collisional orogens: The case of the Rio Preto fold belt on the northwest Sao Francisco Craton margin, NE Brazil

摘要

The Rio Preto fold belt (RPFB), in northeastern Brazil, is represented by Precambrian metasedimentary sequences (Canabravinha and Formosa formations) that were accreted to the northwestern Sao Francisco Craton margin, during the collisional Brasiliano Orogeny (similar to 630-500 Ma). The analysis of the detrital zircon age spectra of samples from both formations revealed a broadly distinct sedimentary provenance for each one of them. While the Canabravinha Formation yielded late-Archean to Tonian zircons, the Formosa Formation yielded a relatively simple, bimodal detrital zircon age pattern at 1900-2200 Ma and 2500-2650 Ma. The Nd isotope compositions of each unit are also broadly distinct. The Canabravinha Formation shows variable T-DM model ages ranging from 1.6 to 2.7 Ga, with associated epsilon Nd-(600) values of -8.7 to -19.5, thus reflecting a wide age range in the source areas. The Formosa Formation T-DM distribution reflects the bimodal detrital zircon pattern, with two broad T-DM peaks of 1.9-2.0 and 2.4-2.6 Ga, and epsilon Nd-(600) values from -12.9 to -26.5. Based on the detrital zircon and Sm-Nd isotope data, we suggest that the Canabravinha and Formosa formations represent two distinct sedimentary sequences in time and space. A sill-like amphibolite body interleaved within the Formosa Formation mica-schists yielded a U-Pb zircon age of 1962.6 +/- 9.9 Ma that supports this interpretation. Thus, we suggest that the Formosa Formation represents part of the basement of the Neoproterozoic fold belt, presenting a very simple sedimentary provenance derived uniquely from the Cristalandia do Piaui Complex (Archean/Paleoproterozoic) to the north. The Canabravinha Formation, on the other hand, represents the infilling of a Neoproterozoic extensional (rift) basin (similar to 900-600 Ma), whose debris came mainly from the Sao Francisco Craton to the south and from secondary source areas further northeast in the Borborema Province, such as the Cariris Velhos belt (920-1000 Ma). During the Brasiliano Orogeny, both the Proterozoic supracrustal sequences and the Archean/Paleoproterozoic basement were tectonically juxtaposed, deformed and metamorphosed under greenschist fades, thus complicating the original stratigraphic relationships between them. (C) 2013 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.