Petrography, mineral chemistry and geochemistry of phyllossilicates as indicators of the provenance of the neoproterozoic sediments of the Serra de Santa Helena and Serra da Saudade formations (Bambuí Group)

Abstract

The mineral and geochemical characterization of pelites of the Serra de Santa Helena (FSSH) and Serra da Saudade (FSS) formations may contribute to the interpretation of the evolution of the Bambuí Group deposition basin, which has been considered as a foreland type related to Brazilian Orogenenesis. This work aims to compare the composition of the pelites along the stratigraphic column - FSSH and FSS - and the geographical distribution between the regions of Vila Boa - Bezerra to the west and the Serra de São Domingos to the east, looking for indications of conditions sedimentation. For this, petrographic analyzes were carried out by X-ray diffraction, total rock geochemistry and mineral chemistry of phyllossilicates. It is observed that in the FSSH to the west there is a greater variation of sedimentary structures, mineral composition and chemical composition than in the east, while in the FSS the pelites tend to have greater homogeneity besides containing greater contribution of ferromagnesian minerals, represented by chlorite and biotite. In both formations the granulometric selection is confirmed, in which the finest pelites are constituted essentially by phyllosilicates while the coarser ones contain a greater percentage of non-clayey silicates, mainly albite. The values of the Index of Compositional Variability (ICV) in the FSSH and the FSS indicate immature sediments in the two units both west and east, but distinct values reflect variation of the source rock. In the FSSH to the west there are also more mature pellets, with contribution of recycled sediments from the Paranoá Group (glauconite and zircon). These immature sediments, with ICV around 1, tend to be found in tectonically active areas, probably associated to the evolution of the Brasilia Folding Range.