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 São Francisco Craton margin, NE Brazil

Abstract

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 São Francisco Craton margin, during the collisional Brasiliano Orogeny (~630–500Ma). 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 1900–2200Ma and 2500–2650Ma. The Nd isotope compositions of each unit are also broadly distinct. The Canabravinha Formation shows variable TDM model ages ranging from 1.6 to 2.7Ga, with associated εNd(600) values of −8.7 to −19.5, thus reflecting a wide age range in the source areas. The Formosa Formation TDM distribution reflects the bimodal detrital zircon pattern, with two broad TDM peaks of 1.9–2.0 and 2.4–2.6Ga, and εNd(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±9.9Ma 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 Cristalândia do Piauí Complex (Archean/Paleoproterozoic) to the north. The Canabravinha Formation, on the other hand, represents the infilling of a Neoproterozoic extensional (rift) basin (~900–600Ma), whose debris came mainly from the São Francisco Craton to the south and from secondary source areas further northeast in the Borborema Province, such as the Cariris Velhos belt (920–1000Ma). During the Brasiliano Orogeny, both the Proterozoic supracrustal sequences and the Archean/Paleoproterozoic basement were tectonically juxtaposed, deformed and metamorphosed under greenschist facies, thus complicating the original stratigraphic relationships between them.