Nd isotopic study of the Neoproterozoic Mara Rosa Arc, central Brazil: implications for the evolution of the Brası́lia Belt

Abstract

The Goiás Magmatic Arc is a 0.9–0.64 Ga island arc underlying large areas of the western part of the Brası́lia Belt, in central Brazil. It is exposed in two separate sections, known as the Arenópolis Arc in the south and the Mara Rosa Arc in the north. It is made of island arc volcanic–sedimentary sequences forming narrow NNE to NNW belts, separated from each other by tonalitic–granodioritic–granitic orthogneisses. All rocks are metamorphosed under upper greenschist to amphibolite facies conditions, but have kept major geochemical and isotopic features indicating the juvenile nature of the igneous protoliths. The meta-igneous and metasedimentary rocks are intruded by a number of late- to post-orogenic granites (<600 Ma), associated with gabbro–dioritic bodies, constituting a typically post-orogenic bimodal association. Tonalitic and minor granitic orthogneisses dated at ≈850 Ma are the oldest rocks in the Mara Rosa Arc and have ε Nd(850) ranging from +1.5 to +4.6, with T DM values between 1.0 and 1.1 Ga. Metasedimentary rocks in the Mara Rosa supracrustal belts display geochemical features suggesting that they represent first cycle, immature sediments which preserve chemical characteristics of their igneous sources. Their Nd isotopic characteristics support this interpretation. T DM model ages for detrital metasediments are mostly in the interval between 0.9 and 1.2 Ga, indicating that they are a product of erosion of the arc rocks. The absence of older contributions to these sediments seems to indicate an intraoceanic setting. This is also suggested by the Nd isotopic composition of the ≈0.8 Ga old tholeiitic amphibolites, which display positive initial ε Nd (+4.9), indicating derivation from a depleted mantle source. Syn–orogenic deformed granites forming small intrusions within the supracrustal belts have T DM ages ranging from 0.77 to 2.54 Ga. The older model age values indicate the presence of hidden fragments of ancient crust, which were probably tectonically interleaved with younger rocks during accretion of the arc. The post-orogenic granites are moderately peraluminous and the Nd model ages between 1.0 and 1.3 Ga indicate that their parental magmas were predominantly re-melts of the arc rocks. Their peraluminous nature reflects the incorporation of substantial amounts of arc sediments in the original magma. Garnet-whole rock Sm–Nd ages for the metasediments may be interpreted to refer to two metamorphic events: an older event at ≈0.76–0.73 Ga and a younger event at ≈604–610 Ma. Mineral chemistry indicate that the older episode was characterized by higher P/T and the younger by lower P/T. The older metamorphic event is interpreted preliminarily as the result of the accretion of island arc terranes to the western edge of the orogen and the younger event, as a result of final ocean closure and continental collision between the Amazon and São Francisco continents. The Goiás Arc formation and accretion is partially coeval with Rodinia break up, suggesting that the western margin of the São Francisco–Congo was peripheral in that supercontinent and faced a wide oceanic basin during most of the Neoproterozoic.