Genesis of Fibrous Calcite in the Chang 7 Member of the Yanchang Formation, Ordos Basin, China

To explore the genesis of the laminated calcite veins developed in the black shale of Chang 73 submember of the Ordos Basin, the petrology, microstructure and geochemistry of calcite veins are studied using thin sections, fluid inclusions, trace elements and isotopic geochemistry. The source, the time of formation, the mechanism of formation, and the dynamic background of the veins are discussed. The veins are mostly made up of calcite, mixed with lenticular or spindle-shape solid wall rock inclusions. Three structures are identified in the calcite minerals: fibrous, rhombic cleavage, and wedge-like structure. Trace elements and isotopes of carbon and oxygen confirm that the calcite veins were formed from a high density hydrothermal fluid. It is assumed that calcite veins formed prior to wall rock consolidation during the formation of the Qinling Orogenic Belt in the Middle and Late Triassic. The results show that the sedimentary sources of Chang 73 submember were influenced by hydrothermal materials besides terrigenous detritus. The fibrous calcite is different from the fibrous calcite reported in the morphology and mechanism of formation. In this way, the research complements and improves the morphology and the mechanism of formation of fibrous calcite. image

Automated summary

In this study, the petrology, microstructure, and geochemistry of the laminated calcite veins in the black shale of Chang 73 submember in the Ordos Basin were investigated. The results provide insights into the origin, formation mechanism, and dynamic background of the veins. The veins are predominantly composed of calcite and contain solid wall rock inclusions. The calcite minerals exhibit fibrous, rhombic cleavage, and wedge-like structures. Trace elements and isotopic geochemistry confirm that the calcite veins formed from a high-density hydrothermal fluid. The study suggests that the calcite veins formed prior to wall rock consolidation during the formation of the Qinling Orogenic Belt in the Middle and Late Triassic. The findings also indicate that the sedimentary sources of the Chang 73 submember were influenced by hydrothermal materials. Moreover, the fibrous calcite identified in this study differs from previously reported fibrous calcite in terms of morphology and formation mechanism.