THERMAL CONVECTION DOLOMITIZATION INDUCED BY THE EMEISHAN LARGE IGNEOUS PROVINCE: GEOCHEMICAL AND PETROGRAPHICAL IMPLICATIONS

The Permian dolomites are widely distributed only in the paleotropical areas with arid and warm/hot climates, which are formed by dolomitization associated with high evaporation. However, large-scale dolomite bodies were recently discovered in the Middle Permian Qixia Formation in the northwestern Sichuan Basin where the paleoclimate was humid and hot with weak evaporation. Traditionally, the dolomites in the Sichuan Basin have been interpreted as hydrothermal dolomitization associated with the Emeishan Large Igneous Provinces (ELIPs). However, new petrographic and geochemical evidence on the Middle Permian dolomites (283.5 to 273.0 Ma) in the study area of the outer zone of ELIPs support thermal convection dolomitization. Petrographic analysis reveals four major types of dolomites, including matrix dolomites (Md1, Md2, and Md3) and cement dolomites (Cd). Md2, Md3 and Cd both have medium to coarse, fabric-destroyed crystals apart from the very finely crystalline, fabric-preserved penecontemporaneous Md1. The nearly constant Mn/Sr ratios, δ¹³C values, ⁸⁷Sr/⁸⁶Sr ratios, REE patterns, and weak negative Eu anomalies suggest that Md2, Md3, and Cd formed in fluids resembling Permian seawater, not hydrothermal fluids, during shallow burial stage. High formation temperature of the Md2, Md3 and Cd are suggested by the high fluid inclusion homogenization temperatures (T_h) and low δ¹⁸O values, which is consistent with the heat provided by the ELIPs events. Due to the unique geographic location of the northwestern Sichuan region in the carbonate platform margin, the extensive dolomitization was facilitated by the rapid convection exchange between warm pore seawater in the Qixia strata of the platform margin and cold water from outside the slope during the whole abnormal geothermal periods. Therefore, we conclude that the dolomites in the northwestern Sichuan Basin are mainly formed by thermal convection dolomitization in the shallow burial stage. Since the features of thermal convective dolomites tend to be overprinted by those of later hydrothermal fluids, this study provides useful implications for the possible widely existed mechanism of dolomite formation linked to geothermal activity.