Fluid-driven carbonate diagenesis in the Central European Basin revealed by fluid inclusions and U–Pb carbonate geochronology

A detailed diagenetic reconstruction of the Anisian Schaumkalk (c. 245 Ma) and the Oxfordian Korallenoolith (c. 163 Ma) in the Central European Basin was conducted by combining petrographic techniques (including cathodoluminescence microscopy) with fluid inclusion analysis and in situ U–Pb dating of carbonates. Pack- and grainstone lithofacies were sampled from outcrops and cores, representing the present-day range from surface exposures in Central Germany to maximum burial depth in North Germany. U–Pb dating of carbonate grains and cementations yielded ages in the range of 244–63 Ma that cluster in seven distinct age groups. The earliest age group of 244–232 Ma (I) is related to eogenetic modifications of carbonate mud and cements of the Schaumkalk in shallow burial depth. Cementation groups II–VII are related to basin-scale fluid migration as evidenced by fluid inclusions trapped at 90–310°C with salinities of 13–24 wt% NaCl+CaCl₂. Trapping of highly saline basinal fluids in authigenic cementations at 239–237 Ma (II), 227–223 Ma (III), 202–189 Ma (IV), 170–156 Ma (V), 102–99 Ma (VI) and 64–63 Ma (VII) correspond to phases of structural reorganization and tectono-magmatic activity in the Central European Basin. Comparing the intense diagenetic overprint of the Schaumkalk (all age groups present) with the incipient diagenesis of the Korallenoolith (age groups V–VII present) highlights the importance of fluid-driven diagenetic alteration. This refers in particular to wide-spread neomorphism of carbonate matrix in the Schaumkalk that cannot be explained by burial diagenesis alone.