Stable isotope and fluid inclusion study of carbonate and siliceous sediments of the Barrandian Neoproterozoic


Authors: Žák K, Dobeš P, Hladíková J, Pudilová M

Published in: Bulletin of Geosciences, volume 75, issue 3; pages: 271 - 284;

Keywords: Stable isotopes, Neoproterozoic, Teplá-Barrandian Unit, oolitic limestone, chert, fluid inclusions,

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The Neoproterozoic volcanosedimentary sequences of the Tepl.-Barrandian Unit contain several types of carbonate rocks. Most carbonate rocks are characterized by low delta;13C and low delta;18O values and were formed in connection with submarine volcanism and related hydrothermal activity. Significant amount of carbonate carbon of these rocks was derived from oxidation and/or pyrolysis of organic matter. In the Příbram area rare oolitic carbonate lenses were found with high delta;13C values up to +6 ‰ corresponding to the typical isotopic composition of Neoproterozoic limestones of the world. Oxygen isotope data of oolitic limestones of the Příbram area and of adjacent rocks, chert with intercalated dolomitic limestone, indicate that the formation of the studied rocks was also related to submarine hydrothermal activity. Abundant H2S-rich fluid inclusions characteristic of diagenetic environment support the idea that the isotope data reflect original depositional conditions with only minor alterations during later weak regional metamorphism. In the Mítov-Kokšín area the fluid inclusion study indicates a much higher pressure characteristic of subsea-floor metamorphism. Oxygen isotope data of several chert types are in the range between +16.6 and +21.1 ‰(SMOW). Within this range the delta;18O data for stromatolite-like cherts are the highest, indicating lower temperature of formation while dark-coloured bedded cherts (lydites) and light-coloured cherts were formed at a higher temperature. Individual growth zones of siliceous stromatolitic structures showed almost identical delta;18O data.