Detrital zircons from the Upper Três Marias Formation, São Francisco basin, SE Brazil: Record of foreland deposition during the Cambrian?

Abstract

The Bambuí Group, a succession of mixed carbonate and siliciclastic rocks, accumulated in the intracratonic and poly-historic São Francisco successor basin by the end of the Neoproterozoic. The São Francisco basin, which occupies a substantial portion of the eponymous craton in southeastern Brazil, was converted into a major foreland depocenter in response to orogenic loads generated along the craton margins during the assembly of West Gondwana in the Ediacaran Period. Various studies carried out in recent years have led to a better understanding of many aspects of the evolution of the São Francisco basin during the Neoproterozoic. However, the age of the Bambuí Group, critical for a better understanding of the basin history and establishment of regional and global correlations, remains poorly constrained and matter of a long-standing debate in the literature. In order to contribute to this debate, we performed U-Pb LA-ICPMS age determinations on detrital zircons extracted from the Três Marias Formation, the youngest unit of the group. Our study, based on a stratigraphic survey, was conducted in an occurrence of the formation located near to the eastern border of the basin, i.e., in a proximal position in relation to its potential source, the Brasiliano Araçuaí orogen. The detrital zircon age spectra (involving 432 concordant ages) of the lower alluvial and upper marine units of the Três Marias Formation are characterized by a unimodal distribution with c.83% of the grains dated between 735 and 522 Ma. These results indicated that lithostratigraphic assemblages of the Araçuaí orogen, namely the 630-580 Ma granitoids of Rio Doce magmatic arc and 585-540 Ma syn-collisional granites were the most important sources of the Três Marias sediments. Moreover, post-collisional granite suites younger than 530 Ma also contribute with sediments to the formation. Using four different metrics, we obtained 527 ± 4 Ma as the most accurate and reliable estimation for maximum depositional age of the Upper Três Marias Formation, an age that has major implications for the developmental history of both the São Francisco basin and Araçuaí orogen.