Origin of grained dolomite from the Buqu Formation of Zaring area in southern Qiangtang depression

The genetic mechanism of dolomite of the Middle Jurassic Buqu Formation in the Qiangtang depression was studied by using microscopy, fluid Inclusion data, and carbon and oxygen isotope analysis. The results show that the dolomites in the study area can be divided into fine-powdery dolomite, medium dolomite and coarse dolomite according to grain size. Dolomite generally has a fogged center and clear margin structure, in the cracks near the extensive development of grained saddle dolomite. Fluid inclusions in the dolomite have homogenization temperature ranging between 150.2 and 216.0℃ and average salinity of 24.5%NaCl, much higher than the homogenization temperature and salinity of fluid inclusions in calcite, indicating a high temperature and high salinity diagenetic environment. The isotopic analysis shows that δ¹³C_PDB values vary from-0.01‰ to 3.43‰, and δ¹⁸O_PDB values vary from-11.17‰ to -7.68‰; according to the dolomite-fluid oxygen isotope fractionation equation, the δ¹⁸O_SMOW values of dolomitization fluid are in the range of 4.82‰-12.85‰, It is believed that the matrix dolomite and the saddle dolomite that are filled in the pores in the study area are the products of host limestone altered by the high salinity fluid heated by the magma activity in a close environment, while the high salinity fluid due to the consumption of magnesium ions led to over-saturation of fluid calcite and then to the precipitation if high-temperature calcite. Therefore, hydrothermal activity was very significant for the development of dolomite of the Middle Jurassic Buqu Formation in the study area.