The mechanism of dolomitization has long been debated due to complex sedimentary and diagenetic conditions. Large-scale dolomite bodies were recently discovered in the Permian Cisuralian Qixia Formation (284–273 Ma) in the northwestern Sichuan Basin, providing an opportunity to understand the dolomitization associated with the Emeishan Large Igneous Province (ELIP). Traditionally, the dolomites in the Sichuan Basin have been interpreted as hydrothermal dolomitization because of the hydrothermal impacts from the ELIP. Here, we report new petrographic and geochemical data on the Permian dolomites, which we grouped into two categories (matrix dolomites or Md1–Md3 and cement dolomites or Cd), that support thermal convection dolomitization. The petrographic and geochemical features (δ13C, δ18O, REE, Mn/Sr ratios, 87Sr/86Sr ratios) suggest that Md1 were formed in penecontemporaneous seawater, whereas Md2, Md3, and Cd were formed in fluids resembling Permian seawater during shallow burial stage, of which both dolomitizing fluids are non-hydrothermal. High formation temperature inferred from fluid inclusion is consistent with the low δ18O values of the Md2, Md3 and Cd, suggesting that the ELIP event may have provided the heat for the dolomitization in the Qixia carbonate platform. The formation of the widespread dolomite bodies 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. Therefore, we demonstrate that the dolomites in the northwestern Sichuan Basin are mainly due to thermal convection dolomitization in the shallow burial stage. This study provides important implications for the mechanism of dolomite formation linked to abnormal geothermal activity.
- Geochemical analysis
- Thermal convection