Carbon, oxygen, and strontium isotope study on the genesis of paleoreservoir dolomites in Longeni-Angdaercuo area, southern central uplift belt, Qiangtang Basin

This study investigates the origin of Buqu Formation palaeoreservoir dolomites in the Longeni-Angdaercuo area, southern flank of the central uplift belt, Qiangtang Basin. Based on dolomite and associated limestone samples collected from field sections and route geological surveys, we analyzed rock fabric characteristics using thin-section identification and cathode luminescence analysis. The fluid properties and diagenetic environment of dolomitization were examined, and the genetic mechanisms were revealed by combining major and trace element data with carbon, oxygen, and strontium isotopic analyses. The results show that: (1) Dolomites in this area mainly develop in three kinds of sedimentary facies belt‌s—upper subtidal high-energy zone, tidal flats, and platform-margin algal mound reef, which are near ancient land. Rock fabrics include primary and diagenetic fabrics, among which the primary fabric comprises residual granular and algal laminae structure, and the diagenetic fabric comprises uneven granular structure, fog-core bright-edge structure, rim edge cementation, and generation growth structure. (2) Carbon and oxygen isotope Z values of the dolomites show a near-shore environment subjected to the interaction of sea-land and fresh water influx. δ¹⁸O_PDB and δ¹³C_PDB values of dolomites are generally higher than those of associated limestones, consistent with typical mixed-water metasomatism dolomites. Dolomites may form in the relatively high point in paleogeography with oxidation environment susceptible to fresh water input (atmospheric precipitation and surface water) . The fluid salinity was lower than that of normal seawater, and the temperature was higher than that of the surface (25℃), indicating a mixed-water metasomatism origin. (3) The ⁸⁷Sr/⁸⁶Sr values in both dolomite and associated limestone are affected by supergene alteration. After excluding this factor, the overall value is still higher than that of contemporary strata, and the ⁸⁷Sr/⁸⁶Sr values of dolomite are slightly lower than that of limestone, suggesting formation in relative high point of palaeogeography and the nearshore fresh water environments where clay minerals are not easy to precipitate and the loss of strontium occurred during dolomitization. (4) Carbon, oxygen, and strontium isotopes of dolomites and associated limestones in this area are slightly affected by late-stage paleokarst, tectonism, and epigenesis, but the specific effects need to be further studied.