Provenance of fine-grained sedimentary rocks derived from Rb-Sr and Sm-Nd analyses: the example of the Santa Maria Formation (Triassic, Parana Basin, Southern Brazil)

In this work, the Rb-Sr and Sm-Nd methods were applied to fine-grained sedimentary rock samples of the Santa Maria Formation (Triassic, Paraná Basin) in order to have provenance information. The Sm-Nd, although being considered as a new technique when applied to sedimentary rocks is an important parameter to the determination of potential source rocks. The acquired information with the Sm-Nd method and the integration with Rb-Sr data allowed to propose the reconstruction of the source areas concerning Santa Maria Formation. The results are fundamental to improve these methodologies when applied in sedimentary rocks

O método radiométrico RB-Sr aplicado em rochas sedimentares o exemplo da bacia do Paraná, Brasil

The Rb-Sr method has been successfully applied to argillaceous sedimentary rocks in order to supply parameters for the definition of their depositional ages. In many studied cases, despite the geologically significant results, there are doubts concerning the interpretation of the obtained data and their limitations. This happens because sedimentary rocks are considered to be the mixture of detrital fragments of diverse sources, and therefore is the possibility of contain an isotopic record of their source rocks. The Rb-Sr method, when applied to mudrocks, allows constraining absolute depositional ages, owing to the resetting of the isotopic Rb/Sr system during deposition. This process is called isotopic homogenization in the depositional environment. Some special care must be taken with respect to sampling, sample grain size, depositional setting, and mineralogical composition. Nevertheless, questions still remain when concerning this methodology, particularly on the real occurrence and extent of the isotopic homogenization in the sedimentary environments. In the present work, the Rb-Sr method has been applied to samples of sedimentary rocks collected from different units representing several depositional settings from the Paraná Basin. The parameters responsible by the isotopic homogenization of Sr have been evaluated. The obtained results allowed the improvement of the application of the Rb-Sr method to sedimentary rocks, and significant results concerning depositional ages were obtained