Genesis and developmental pattern of Middle Permian dolomite in Northwestern Sichuan, China

The formation of large-scale dolomite reservoirs in the Middle Permian of Northwest Sichuan shares a good temporal correlation with the occurrence of major events in the Emeishan Large Igneous Province (ELIP). However, because the dolomitized fluids, the timing of dolomitization and the genesis mechanism of the dolomites are not comprehensively understood, its developmental pattern requires further analysis. This paper identified the diagenetic fluids of different types of dolomites via petrological characteristics of dolomites, carbon and oxygen isotopes, strontium isotopes, and other geochemical data, as well as fluid inclusions and laser U-Pb isotope dating. The dolomite genesis model related to the dynamical formation mechanism of the ELIP was also reconstructed. The Middle Permian dolomites in Northwest Sichuan are divided into matrix dolomite (Md) and cement dolomites (Cd). Importantly, the matrix dolomites include very finely to finely crystalline matrix dolomite (Md1), finely to medium crystalline matrix dolomite (Md2), and medium to coarsely crystalline matrix dolomite (Md3). The dolomites were primarily modified by three phases of dolomitizing fluids. The Md1 phase yields a U-Pb age of 262 ± 9.2 Ma, consistent with the host stratigraphic age, and exhibits ⁸⁷Sr/⁸⁶Sr ratios matching Permian seawater values. These features indicate Md1 formed dominantly through penecontemporaneous seawater reflux dolomitization. In contrast, the Md2 phase shows a U-Pb age of 261.3 ± 7.4 Ma—also stratigraphically consistent—but contains later-stage subsurface cements with a distinct U-Pb age of 245.7 ± 1.9 Ma. This significant age offset demonstrates that Md2 resulted from overprinting by multiple dolomitizing fluid phases. These fluids were mainly formed owing to the temperature difference between the warming stratigraphic water and the cold seawater in the ELIP under the geothermal warming effect. The cold seawater on the slope creates a fluid potential energy difference, generating lateral open circulation and causing thermal convection. Furthermore, Md3 and Cd have negatively skewed δ¹⁸O values, along with higher Th and Eu/Eu∗ values, suggesting that the diagenetic fluids were substantially shallow buried strata water, which resembled Permian seawater. They were mostly produced under the higher geothermal temperatures during the ELIP active period, which generated a substantial fluid potential energy difference, and were driven by a more rapid thermal convection cycle. This study quantitatively constrains the timing of dolomitization using laser U-Pb isotope dating, aiming to give a reliable and valuable reference for the impact of tectonic activities on dolomite genesis caused by the Large Igneous Province.