SHRIMP U-Pb and Sm-Nd data for the Araxá Group and associated magmatic rocks: Constraints for the age of sedimentation and geodynamic context of the southern Brasília Belt, central Brazil

SHRIMP U-Pb and Sm-Nd isotopic data for Araxá Group micaschist, associated amphibolite and intrusive tonalite were used to estimate the age and tectonic setting of deposition of the original detrital sediments, in the southern part of the Neoproterozoic Brasília Fold Belt. One amphibolite sample from a layer interleaved tectonically within the metasedimentary rocks has the U-Pb zircon age of 838 ± 20 Ma. Its tholeiitic composition and depleted mantle isotopic signature (ε_Nd = +5.1) suggests that it represents a tectonic slice of the former oceanic lithosphere separating the São Francisco and Amazon continents. U-Pb ages of detrital zircon grains and T_DM model ages of the Araxá Group micaschists show a bimodal pattern, interpreted as the result of provenance from two contrasting sources: (i) young juvenile sources, probably represented by igneous rocks of the Goiás Magmatic Arc in the west and (ii) Paleoproterozoic sialic sources, most probably belonging to the São Francisco Craton, in the east. The detrital population is largely dominated by ca. 666-682 Ma old zircon grains derived from both mafic and felsic igneous source rocks. Detrital metamorphic grains and rims dated at ca. 715 and 643 Ma indicate that the original basin received sediments from Neoproterozoic sources with a previous metamorphic overprint, implying that young metamorphic rocks have been uplifted, exposed and eroded, providing sediment for at least part of what is presently mapped as the Araxá Group. The youngest detrital zircon age of ca. 643 Ma represents the upper limit for deposition of the original detrital sediments. Their minimum age is constrained by the crystallisation of an intrusive tonalite at 638 ± 11 Ma. The data suggest, therefore, that the Araxá Group metasediments were deposited and metamorphosed within a very short time interval, most probably during the tectonic events related with the accretion of the Goiás Magmatic Arc to the eastern part of the Brasília Belt and final closure of the Goiás Ocean.