Geochemistry of the massive dolomites in Eastern Black Sea region: REE implications for dolomite petrogenesis

Abstract

Dolomitization is an important diagenetic process observed in carbonate rocks ranging in age from Precambrian to Holocene. The formation of massive dolostone bodies has long been a challenge due to complex sedimentary and diagenetic conditions. The presence of massive dolostone successions which pervasively occur in the Late Jurrasic-Early Cretaceous carbonates in Eastern Pontides (NE Turkey) can provide an excellent opportunity to gain a better understanding of the dolomitization process. Previous studies of these carbonates interpreted dolomite as a replacement phase after calcite formed at shallow burial depths. The nature of fluids for dolomitization has been attributed to the Late Jurassic–Early Cretaceous seawater. Here, we report new geochemical data, including rare earth elements (REEs) on the formation of dolomites of the Berdiga Formation and its relationship to the Late Jurassic magmatic event. These dolomites are grouped into two categories: (1) microcrystalline replacive dolomites (D1 and D2) corresponding to the shallow subsurface realm formed at relatively low-temperature conditions from seawater parentage fluids, and (2) coarse-crystalline replacive dolomites (D3) and cement dolomite (Cd) formed at shallow to intermediate burial depth under relatively high-temperature conditions from seawater affected by the hydrothermal fluid flux in Late Jurassic-Early Cretaceous. High-temperature input can be inferred from high fluid inclusion homogenization temperatures (170–210°C), low δ18O values, relatively high Eu/Eu*, Eu/Sm and Sm/Yb ratios, low Y/Ho ratios, and enrichment of LREE over HREE in these dolomites compared to the seawater signatures. The Late Jurassic magmatic event may have provided a heat supply for the generation of high-temperature input to the ambient seawater. This probably led to the rapid convection and circulation of seawater in the carbonate strata resulting in a water-rock alteration process and massive dolomitization. Therefore, we suggest that the dolomites in the Eastern Pontides are mainly formed at shallow burial associated with the Late Jurassic Magma generation. This model provides new insights into the mechanism of dolomite formation associated with a contemporaneous magmatic activity.