Litogeoquímica e isótopos de Sm-Nd-Sr e sua relação com a proveniência sedimentar de rochas do Grupo Bambuí na porção Sul do Craton São Francisco

Abstract

The Bambuí Group is an association of siliciclastic and carbonatic rocks deposited in a foreland basin developed after West-Gondwana amalgamation during the Neoproterozoic. In the southern portion of the São Francisco craton, this foreland basin is bounded by two diachronic orogenic fronts: the South Brasilia orogen, to the west, and the Araçuaí-Ribeira Orogenic System, to the east. In this context, the sedimentary provenance allows the evaluation of geochemical changes in the sedimentary rocks together with the evolution of the source rocks in the marginal orogens. In order to address this evaluation, we used lithogeochemistry and Sm-Nd-Sr isotopic methods in the sedimentary rocks of the Bambuí Group. The coupled Sm-Nd with Th/Sc and La/Yb elemental ratios showed three major changes in the sedimentary provenance possibly correlated to second order shallowing upward cycles. The εNdt parameter becames less negative toward the upper successions of the Bambuí Group. The mean εNdt values in the basal Carrancas Formation are between -18 and -23; continuously increasing in the Sete Lagoas Formation (εNdt = -7.9) and in the Serra de Santa Helena and Lagoa do Jacaré formations (εNdt = -6.1). In the upper Serra da Saudade and Três Marias formations, the εNdt becames, again, more negative (εNdt = -7.4), while it is followed by increase in the Th/Sc ratio to values between 0.7 and 1.3. These geochemical changes are relatable to the increasing influence of sources from the Araçuaí orogen on the sedimentary provenance of Bambuí Group, especially in Serra de Santa Helena formation and the upper successions. In the other hand, the Nd-Sr isotopic pair was used as a resource to the investigation of the Bambuí Group isotopic homogeneity and the influence of source mixing against compositional change of the crustal reservoirs during Gondwana amalgation in the Criogenian-Ediacaran periods. The proposed models can add to the knowledge of geodynamic models Bambuí basin evolution, and to the investigation of crustal evolution during the Neoproterozoic collisional events