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

Abstract

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, d 13C fluctuations indicate three stratigraphic levels of carbonate sedimentation. Marbles at the older carbonate horizon display d 13C between +8.3 and +11.8%0. Marbles at the intermediate carbonate horizon exhibit d 13C values between +6.7 and +8.7%0, and the youngest carbonate horizon in the Jucurutu Formation marbles, yielded d 13C 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 d 13C values between +8.9 and +10.7%0, and negative d 13C values mark the upper marble carbonate horizon of the Seridó Belt that exhibit a narrow range of d 13C 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.