C-, Sr-isotope and Hg chemostratigraphy of Neoproterozoic cap carbonates of the Sergipano Belt, Northeastern Brazil

Abstract

Two cap carbonates overlying glaciogenic diamictites crop out extensively in the eastern Vaza Barris Domain of the Sergipano Belt, northeastern Brazil. They are represented by carbonates of the Jacoca Formation, resting on top of diamictite of the Ribeiropolis Formation, and by the Olhos D’Agua Forma- tion (carbonates, organic-rich towards the top), which overlies diamictite of the Palestina Formation. These two sequences were deformed and metamorphosed at sub-greenschist facies-conditions during the Brasiliano cycle (650–600 Ma). In the western Vaza Barris Domain, dolostone of the Acauã Formation rest, in sharp contact, on diamic- tites of the Juetê Formation and are much less deformed than the Olhos D’Agua Formation. Values of (13C for the Jacoca and Acauã Formations cluster around −4 to −5‰. In the Olhos D’Agua Formation, however, negative values (around −5‰) in the shallow-marine base of the sequence are replaced up section by values close to zero and, abruptly, by positive values between +8 and +10‰ at the top. Strontium-isotope ratios for carbonates of these three formations are within the range of late Neoproterozoic seawater (0.7060–0.7090). The Acauã Formation displays 87Sr/86Sr ratios from 0.7072 to 0.7073, while the Jacoca and Olhos D’Agua formations display values within the range from 0.7077 to 0.7081. Strontium-isotopes coupled with available C-isotopes, detrital zircon U–Pb ages and structural data suggest that the Jacoca Formation and Acauã are probably correlative and mid Cryogenian in age, and that the Olhos D’Agua Formation is lowermost Ediacaran (Marinoan) in age. Mercury concentrations in carbonates of the Jacoca, Olhos D’Agua and Acauã formations are usually much higher than those observed in carbonates deposited not concomitantly with volcanic activities (<3 ng g−1). They are, in fact, similar to those in carbonates deposited during volcanic activity (e.g. Tertiary Punta Rocallosa carbonates in Chile), with concentration values between 20 and 80 ng g−1, suggesting that CO2 in the basal portion of these carbonate formations is mostly mantle-derived, accumulated in the atmosphere during Cryogenian-Ediacaran glacial events.