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

摘要

Sedimentary provenance analysis based on lithogeochemistry and isotope data has been extensively applied in the investigation of ancient sedimentary successions. This approach contributes to understand the tectonic setting in which these strata were deposited and to recognize major provenance patterns in time and space. The Ediacaran to Cambrian Bambui Group is exposed in the Sa tilde o Francisco craton (SE Brazil) and represents the sedimentary record of a foreland basin system developed during the assembly of the West Gondwana. Different lines of evidence indicate that this basin system evolved into a confined setting, due to the overloads imposed on the western and eastern margins of the Sa tilde o Francisco paleoplate by the diachronic Brasilia and Aracuai orogenic systems, respectively. New whole-rock lithogeochemistry and Sm-Nd isotopic data were applied to evaluate the overall sedimentary provenance patterns of the Bambui strata exposed in the southern portion of the Sa tilde o Francisco craton (i.e., within the Sete Lagoas basement high) and to identify its possible sources. Coupled (La/Yb)(n) and Th/Sc ratios with Sm-147/Nd-144 and epsilon Nd isotopes allowed the recognition of three contrasting source patterns associated with different 2nd order sequences. The lower sequence comprises carbonate ramp deposits and shows highly heterogeneous and more negative epsilon Nd 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 TDM model ages between 1.5 and 2.0 Ga. The overlying 2nd-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 epsilon Nd-(t) values between-6.8 and-5.2, with low Th/Sc (0.6-0.8) and (La/Yb)(n) (0.60-0.84) ratios. The uppermost Bambui 2nd-order sequence, marked by greenish siltstones and arkoses grading upward into storm-bedded arkoses and lithic sandstones, shows homogeneous epsilon Nd-(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 1.5-1.7 Ga, characteristics compatible with Ediacaran arc-related rocks and post-tectonic granites exposed in the Aracuai orogen. Our data indicate that the Proterozoic cratonic covers, their correlatives in the surrounding orogens, and magmatic rocks from the Aracuai orogen acted as major sedimentary sources during the first evolutionary stages of the Bambui basin-cycle in the Sete Lagoas basement high, which behaved as a forebulge depocenter. During the deposition of the overlying sequences, the Aracuai orogen 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.