Combined use of Sm-Nd isotopes and lithogeochemistry in the sedimentary provenance of the southern Ediacaran-Cambrian Bambuí foreland basin system, Brazil

The Ediacaran to Cambrian Bambuí Group represents the sedimentary record of a foreland basin system that follows the West Gondwana orogeny. Different lines of evidence indicate that this basin system evolved in a confined setting, due to the overloads imposed by diachronic orogenic events in the continental margins of the São Francisco paleoplate. Coupled (La/Yb)_n and Th/Sc ratios with ¹⁴⁷Sm/¹⁴⁴Nd and ¹⁴³Nd/¹⁴⁴Nd isotopes allowed the recognition of three contrasting source patterns associated with different 2^nd order stratigraphic sequences. The lower sequence comprises carbonate ramp deposits and shows highly heterogeneous and more negative εNd ranging from -10.5 to -4.5, as well as Th/Sc and (La/Yb)_n ratios between 0.6 and 0.7 and 0.54 and 0.76, respectively. The provenance patterns point toward intermediate-dominated and isotopically evolved crustal sources, with mean T_DM model ages between 1.5 and 2.0 Ga. The overlying 2^nd-order sequence is composed of fine-to medium-grained siliciclastics grading upward into platformal oolitic calcarenites and calcilutites. This succession exhibits homogeneous and less negative εNd_(t) values between -6.8 and -5.2, with low Th/Sc (0.6 to 0.8) and (La/Yb)_n (0.60 to 0.84) ratios. The uppermost Bambuí 2^nd-order sequence, marked by greenish siltstones and arkoses grading upward into storm-bedded arkoses and lithic sandstones, shows homogeneous εNd_(t) ranging from -8.5 to -6.1, slightly more negative than the underlying successions, and relatively higher Th/Sc and (La/Yb)_n ratios ranging from 0.7 to 1.3 and 0.85 to 1.28, respectively. The Sm-Nd isotopic signature of the upper deposits indicates felsic-dominated and more isotopically juvenile crustal sources with T_DM model ages of 1.5 to 1.7 Ga_, characteristics compatible with Ediacaran arc-related rocks and post-tectonic granites. Our data indicate that the Proterozoic cratonic covers, and their correlatives in the surrounding orogens, and magmatic arc rocks acted as major sedimentary sources during the first evolutionary stages of the Bambuí basin-cycle in the southern São Francisco Craton. During the deposition of the overlying sequences, the magmatic arc rocks became increasingly important in sourcing the Ediacaran- Cambrian foreland system in this depocenter. Besides revealing the major provenance patterns through the evolution of an ancient and partially confined foreland basin system, our data also demonstrates that even minor changes in the Sm-Nd system might be useful as reliable elements on the provenance analysis of sedimentary systems elsewhere.