Nd isotopes and the provenance of detrital sediments of the Neoproterozoic Brası́lia Belt, central Brazil

Abstract

The Neoproterozoic Brası́lia Belt, in central Brazil, includes in its eastern part a thick pile of sediments deposited and deformed along the western margin of the São Francisco–Congo Craton. Several lithostratigraphic units are identified (the Araı́, Paranoá, Serra da Mesa, Araxá, Ibiá, Vazante, Canastra and Bambuı́ groups) and have been traditionally interpreted as part of a passive margin association (<1.2Ga), with sediments being derived from Archaean or Paleoproterozoic continental sources to the north and east.Nd isotopic signatures of fine-grained detrital sediments of the several rock units of the belt were investigated in order to assess: (i) the nature and average crustal residence ages of the source areas, and (ii) the tectonic significance of the different sedimentary units in respect to the evolution of the Brası́lia Belt.TDM model ages of the ca. 1.2–0.9Ga old Paranoá and Canastra rhythmites, shales and phyllites vary within the interval between 1.9 and 2.3Ga, suggesting relatively uniform Paleoproterozoic continental sources within the São Francisco continent. The sediments of the detritic/carbonatic Vazante Group also display Paleoproterozoic model ages indicating, however, a distinct shift towards slightly younger TDM values (1.7–2.1Ga). These three sequences are interpreted as the typical representatives of the passive margin sequence, with dominance of Paleoproterozoic sources.The Ibiá and Araxá groups show a bimodal distribution of model age values, with a set of samples displaying TDM values between 1.8 and 2.1Ga (similar to the passive margin sequence), and another set with younger model ages, between ca. 1.0 and 1.3Ga. Younger sources such as those represented by the Neoproterozoic Goiás Magmatic Arc (0.93–0.64Ga) in the west, are required to explain the young model ages for these sediments. Immature sediments (feldspathic micaschists) within the magmatic arc, in fact, have TDM model ages mostly between 1.0 and 1.2Ga, indicating that the original sediments represent products of erosion of the arc itself. Therefore, part of the rock units that have been mapped as Araxá and Ibiá groups can be deeper water equivalents of a passive margin sequence (sediments with Paleoproterozoic model ages). However, another part has been clearly deposited under the influence of an island arc source, most probably in a back-arc basin.The Bambuı́ Group sediments (<0.8Ga?), at the top of the sequence, also indicate the presence of a young source. TDM model ages vary between ca. 1.4 and 1.9Ga. However, the distribution pattern of TDM values is more uniform, being intermediate between the Neoproterozoic juvenile source and the Paleoproterozoic continental source area. The Bambuı́ Group is interpreted as a post-inversion sequence, with detrital sediments being mostly derived from erosion of a mountain range in the west, after accretion of the Goiás Magmatic Arc.