Multistage dolomitization and its interaction with ELIP in Southwestern Sichuan Basin, China: Insights from U-Pb dating and geochemical analysis

The Emei Mountain large igneous province (ELIP) considerably impacts the formation of Middle Permian dolomite in the Sichuan Basin, but the specific mechanisms of this influence remain unclear. This paper combines data from petrology, geochemistry, laser U-Pb isotope dating, and fluid inclusion microthermometry to elucidate the properties of the diagenetic fluids and the diagenetic model of dolomitization. The relationship between the dolomitization process and the ELIP is also discussed. The results show the following: (1) Four types of dolomite are identified: Micritic to finely crystalline matrix dolomite with preserved original structure (Md1, <50 μ m), fine-to-medium crystal euhedral–subhedral matrix dolomite (Md2, 50–300 μ m), medium-to-coarse crystalline allotriomorphic matrix dolomite (Md3, 250–800 μ m), and coarse-to-macrocrystalline saddle-shaped cemented dolomite (CD, >500 μ m). (2) Md1 exhibits a dark orange-red fluorescence under cathodoluminescence (CL). The REE + Y distribution pattern, Mn content, δ 13 C, δ 18 O, and 87 Sr/ 86 Sr ratios of Md1 are similar to those of marine carbonates from the same period. This suggests that the diagenetic fluid was seawater that did not undergo significant evaporation during the penecontemporaneous period. In contrast, the homogenization temperature (T h ) values and salinity of inclusions in Md2 are higher than both the temperature of shallow-buried strata and the salinity of seawater from the same period. The U-Pb isotope age of Md2 (262.1 ± 6.5 Ma) is consistent with the stratigraphic age. The diagenetic fluid is high-salinity seawater that has been concentrated owing to rising residual temperatures in the formation pores. Md3 and CD appear bright red under cathodoluminescence (CL) and exhibit a positive Eu anomaly in the REE + Y distribution pattern. Additionally, the Mn content, 87 Sr/ 86 Sr ratio, T h value, and salinity of inclusions are significantly higher. The U-Pb isotopic age of Md3 ( 254.2 ± 6.4 Ma ) is closer to the ELIP active period ( 259-258 Ma ), while the U-Pb isotopic age of CD ( 244.5 ± 3.2 Ma ) represents that the hydrothermal activity of the strata has been weakened, and its formation is more likely to be changed by burial diagenesis. The diagenetic fluid associated with Md3 and CD is primarily hydrothermal fluid that migrated along faults. (3) Diagenetic model: Md1 results from seepage reflux dolomitization, Md2 from thermal convection dolomitization, Md3 from the superposition of thermal convection and tectonic-hydrothermal transformation, and CD from the precipitation of hydrothermal–seawater mixed dolomitization fluid. Among these, Md3 is the most promising reservoir rock and should be the focus of future exploration. This study offers valuable insights for research on high-temperature hydrothermal systems.