Chemostratigraphy of Medium-grade Marbles of the Late Neoproterozoic Seridَ Group, Seridَ Fold Belt, Northeastern Brazil

The age of metasedimentary rocks of the Seridó Belt, Borborema Province, a large Neoproterozoic orogen in northeastern Brazil, has been discussed for over three decades. Previous field and structural studies pointed to a Paleoproterozoic age, but recent isotopic data raised the possibility that the protoliths were deposited and metamorphosed during the Neoproterozoic. We carried out detailed C and Sr isotope studies in marbles of the Seridó Group to help clarifying this issue. In the Jucurutu Formation, at the base of the Seridó Group, d13C fluctuations indicate three stratigraphic levels of carbonate sedimentation. Marbles at the older carbonate horizon display d13C between +8.3 and +11.8%0. Marbles at the intermediate carbonate horizon exhibit d13C values between +6.7 and +8.7%0, and the youngest carbonate horizon in the Jucurutu Formation marbles, yielded d13C values between –8.9 and +3.8%0. In the Seridó Formation, at the top of the Seridó Group, two stratigraphic levels of carbonate sedimentation are observed. Marbles at the older carbonate horizon exhibit d13C values between +8.9 and +10.7%0, and negative d13C values mark the upper marble carbonate horizon of the Seridó Belt that exhibit a narrow range of d13C ratios –4.6 to –4.0%0. The C-isotope fluctuations observed in marbles of the Seridó Belt compared to C-isotope secular variation curves, suggest depositional age intervals of 640–570 and 590–570 Ma. The former age estimate is corroborated by 87Sr/86Sr values which varies from 0.7074 to 0.7077, typical of pre-Varangerian carbonates and is also in agreement with 650 Ma old zircon crystals within pelitic rocks of the Seridó Formation. Mica and amphibole 40Ar/39Ar ages indicate metamorphic events of 544±3 Ma and 500±5 Ma.