Geochemistry of Neoproterozoic calcareous rocks from the Barrandian (Czech Republic)


Authors: Drábek M, Pouba Z

Published in: Bulletin of Geosciences, volume 75, issue 3; pages: 265 - 270;

Keywords: Neoproterozoic, Barrandian, calcareous rocks, limestone, oolitic limestone, geochemistry, trace elements, REE,

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Neoproterozoic calcareous rocks of the Barrandian are associated with mafic volcanic rocks. Current geochemical investigations show that carbonate chemistry does not correspond to the Proterozoic ocean chemistry but reflects rather the chemistry of shallow-water lagoons. The lagoon conditions were hypersaline, additionally strongly affected by ubiquitous volcanic activity and volcanic exhalations. Therefore, a negative Ce anomaly, well established in seawater, has not been confirmed in the set of carbonates studied. On the basis the REE distribution patterns and trace elements distribution, four distinct groups of calcareous rocks were selected for study. Group I and particularly group II are closely connected with mafic volcanic rocks. The REE distribution between carbonates of Group I and II are similar to those of basalt andesite or tholeiite. Carbonates of these two groups are enriched in Mn, Cr, Eu and Y relative to the average Phanerozoic marine limestones (PML). Conversely, Group III includes carbonates less affected by volcanic activity. Carbonates of this group are depleted in REE, Y, Fe, Ni, Mn and Zr. The low Mn/Sr ratio, mostly <1, indicates their low degree of post-depositional alterations. Group IV includes carbonates that exhibit a negative Eu anomaly connected with a later high-temperature polymetallic hydrothermal activity.