Multi-stage dolomitization and its interaction with the Emei Mountain large igneous province in southwestern Sichuan Basin, China: insights from U–Pb dating and geochemical analysis

The Emei Mountain large igneous province (ELIP) affected the formation of Middle Permian dolomite in the Sichuan Basin, but the specific mechanisms of this influence remain unclear. We combined data from petrology, geochemistry, laser U–Pb isotope dating and fluid inclusion microthermometry to determine the properties of the diagenetic fluids and the diagenetic model of dolomitization. We also considered the relationship between the dolomitization process and the ELIP. We identified four types of dolomite: micritic to finely crystalline matrix dolomite with a preserved original structure (Md1, <50 μ m); fine- to medium-grained crystalline euhedral–subhedral matrix dolomite (Md2, 50–300 μ m); medium- to coarse-grained crystalline allotriomorphic matrix dolomite (Md3, 250–800 μ m); and coarse-grained to macrocrystalline saddle-shaped cemented dolomite (>500 μ m). The Md1 dolomite shows a dark orange–red fluorescence under cathodoluminescence. The rare earth element (REE) + Y distribution pattern, Mn content, δ 13 C, δ 18 O and 87 Sr/ 86 Sr ratios of the Md1 dolomites are similar to those of marine carbonates from the same time period. This suggests that the diagenetic fluid was seawater that did not undergo significant evaporation during the penecontemporaneous period. By contrast, the homogenization temperature and salinity of the inclusions in the Md2 dolomites are higher than both the temperature of shallowly buried strata and the salinity of seawater from the same time period. The U–Pb isotope age of the Md2 dolomites (262.1 ± 6.5 Ma) is consistent with the stratigraphic age. The diagenetic fluid was high-salinity seawater that had been concentrated as a result of an increase in the residual temperatures in the formation pores. The Md3 and cemented dolomites appear bright red under cathodoluminescence and show a positive Eu anomaly in the REE + Y distribution pattern. The Mn content, 87 Sr/ 86 Sr ratio, homogenization temperature and salinity of the inclusions are significantly higher than those of the other types of dolomites. The U–Pb isotopic age of the Md3 dolomites (254.2 ± 6.4 Ma) is closer to the active period of the ELIP (259–258 Ma), whereas the U–Pb isotopic age of the cemented dolomites (244.5 ± 3.2 Ma) suggests that the hydrothermal activity of the strata had weakened and its formation is more likely to be a result of burial diagenesis. The diagenetic fluids associated with the Md3 and cemented dolomites are primarily hydrothermal fluids that had migrated along faults. In our diagenetic model, the Md1 dolomites resulted from seepage reflux dolomitization, the Md2 dolomites from thermal convection dolomitization, the Md3 dolomites from the superposition of thermal convection and tectonic–hydrothermal transformation, and the cemented dolomites from the precipitation of a mixed hydrothermal–seawater dolomitization fluid. Among these, the Md3 dolomites form most promising reservoir rocks and should be the focus of future exploration. This study offers valuable insights for research on high-temperature hydrothermal systems.