Fluid Inclusion and Geochemistry Studies of Calcite Veins in Shizhu Synclinorium, Central China: Record of Origin of Fluids and Diagenetic Conditions

Calcite veins in carbonate fracture have been investigated by petrographic, fluid inclusion, geochemical analyses and coupled with basin modeling techniques to provide useful insights into fluid activity and deformation conditions of the Cambrian to Triassic Shizhu synclinorium from the western region of Mid-Yangtze, central China. The results of the fluid inclusion microthermometry show a wide range of homogenization temperatures (78.6-215.5 ℃) and salinities (0.18-23.11 wt.% NaCl equivalent), indicating the formation under diverse fluid conditions. All the calcite veins have negative Ce anomalies, which are the typical characteristic of marine carbonate sediments; it is therefore plausible that calcite veins were precipitated from the marine basin fluid. The stable carbon isotopic compositions of calcites (δ¹³C _V-PDB=-2.5‰-4.26‰) and host limestones (δ¹³C _V-PDB=-3.56‰-5.80 ‰) are very similar with a correlation coefficient of 0.86, however, four calcites from the Lower Permian and Lower Triassic show lower δ¹³C values relative to the host limestones, and they are depleted in total REE concentrations (∑REE ratio varies from 0.74 to 2.06), suggesting the derivation of dissolved carbon from marine carbonates hosting the calcite veins and, less commonly, from the degradation of organic matter. Calculated δ¹⁸O of the fluids-precipitating calcites (δ¹⁸O _V-SMOW=-0.41‰-14.42‰), ⁸⁷Sr/ ⁸⁶Sr ratios varying in the range of coeval seawater and the distinct REE pattern simultaneously suggest calcite-forming fluids in each stratigraphic unit could have formed from the involvement of fluids that originated from coeval seawater and evolved through different degrees of water rock interaction. However, the presence of more radiogenic ⁸⁷Sr/ ⁸⁶Sr ratios than coeval seawater and pronounced positive Eu anomalies in calcites of Lower to Middle Ordovician rocks indicate that terrestrial input from upper strata mudstone and siliciclastic rocks could be involved in the precipitation of the Ordovician calcite. Fluid-inclusion data combined with burial and thermal history modeling indicate there was large-scale flow of evolved basinal fluids through the carbonate formation fractures spanning a time frame from 135 to 50 Ma (Early Cretaceous-Eocene). Therefore, the geochemical characteristics of calcite veins can provide the basis for deformation events in Late Yanshanian and Early Himalayan orogeny.