Geochemical signatures of chert concretions and bedded cherts from the Kraków-Częstochowa Upland (Poland), the southeastern Franconian Alb, and the Bayerwald (Germany)

The Upper Jurassic bedded limestones developed within the microbial-sponge megafacies along the northernmargin of the Tethys Ocean host, among other things, chert concretions and bedded cherts. This megafacies hasbeen identified in a number of exposures located in the Kraków-Częstochowa Upland (Poland) as well as in thesoutheastern Franconian Alb and the Bayerwald (Germany), where it revealed mineralogical, petrographic andgeochemical similarities. Hence, siliceous rocks formed by the replacement of primary calcium carbonate withsilica also showed significant resemblance. The major chemical component of the analyzed siliceous rocks isSiO2, accompanied by minor amounts of Al2O3, TiO2, Fe2O3, and trace elements. The field observations combinedwith the results of geochemical analyses have indicated that the silica forming the chert concretions andthe bedded cherts was primarily supplied by hydrothermal solutions rather than being supplied by the skeletonsof siliceous sponges and radiolarians. The results of geochemical analyses document the formation of siliceousrocks during multistage diagenetic processes strongly influenced by hydrothermal solutions. The negativeCe anomalies observed in samples from all studied locations suggest oxic conditions during the silicificationprocesses whereas the positive Eu anomalies indicate the local contribution of hydrothermal solutions underpartially anoxic conditions. The differences in Eu anomalies found between particular locations are the result ofthe varying temperatures and intensity of hydrothermal solutions influx during the replacement processes, thusrevealing the spatial variability of hydrothermal activity in the studied regions of the northern Tethys margin.