U-Pb and Lu-Hf isotope systematics on detrital zircon from the southern Sao Francisco Craton's Neoproterozoic passive margin: Tectonic implications

摘要

Isotopic Lu-Hf and U-Pb systematics can provide important insights into the tectono-metamorphic processes that occurred in the source regions of sedimentary/metasedimentary units, helping to further clarify regional crustal evolution. The studied metasedimentary sequence is located in the southern Brasilia Orogen, SE Brazil, a foldand-thrust belt composed of a thick nappe stack transported towards the southern Sao Francisco Craton (SFC) during the Neoproterozoic. Adjacent to the Cratonic border is a reworked Neoproterozoic metasedimentary passive margin sequence, that includes the Carrancas succession, for which we present new U-Pb and Lu-Hf detrital zircon analyses (ca. 700 grains in total). Results yielded a maximum depositional age of 0.988 +/- 0.013 Ga and an extensional tectonic setting for the precursor basin. Major source areas, constrained by comparing our data to isotopic analyses compiled from the existing literature, are probably located in the adjacent southern SFC. Lu-Hf analyses of Paleoproterozoic and Archean zircon grains, presented in this study, yielded similar results to those obtained for the basement of the SFC. However, important U-Pb age peaks found in this study do not have strong correlatives in the southern SFC (e.g. 1.84,1.87 and 1.88 Ga), with the exception of a few localized examples (e.g. Espinhaco I magmatism). These ages are abundant in the southwestern Congo Craton (CC), which was potentially relatively close to the studied area during sedimentation, since the SFC and CC probably constituted a single continental mass after the Paleoproterozoic until the opening of the south Atlantic in the Mesozoic.