INSIGHTS INTO THE ORIGIN OF SHALLOW MARINE DOLOSTONES, LONGWANGMIAO FORMATION (LOWER CAMBRIAN), NORTHEAST SICHUAN BASIN, CHINA: PETROGRAPHIC AND GEOCHEMICAL CONSTRAINTS

For a better understanding of the origin of Early Cambrian dolomites in the Sichuan Basin, which serve as valuable oil and gas reservoirs, we present and interpret complex multiparameter petrographic and geochemical analyses of dolomites in the Cambrian Longwangmiao Formation at the Yangsiqiao section. Our data show that these dolomites mainly occur in the middle and upper parts of the Longwangmiao Formation, and we identify two types of dolomites: micrite and fine-medium crystalline dolomites. The micritic dolomites, occasionally coupled with quartz and mica minerals, show dirty crystal surfaces, nonautogenous crystal shapes, and dark cathodoluminescence and mainly occur in pure micrite and granular dolostones, which retain the original sedimentary texture. Contrarily the fine-medium crystalline dolomites, mainly developed in crystalline and grain dolostones and generally coupled with residual calcites and altered sedimentary textures, have a dark heart-bright boundary texture under a polarized microscope and occasional zonal texture under cathode luminescence. Furthermore, the micritic dolomites have CaO and MgO contents approximately similar to those of the ideal stoichiometric dolomite, relatively high Sr concentration, Σ rare-earth element (REE) +Y concentrations, and δ13C and δ18O values located in the corresponding range of dolomite deposited from Early Cambrian marine water. However, light-REE enrichment (LREE), heavy-REE (HREE) depletion patterns, and high Al2O3 and SiO2 contents distinguish these micritic dolomites from typical marine carbonate deposits. Notably, the fine-medium crystalline dolomites have higher CaO content and lower MgO, Al2O3 and SiO2 contents, lower Sr and higher Fe and Mn concentrations than those of micritic dolomites. The ΣREE+Y concentrations and REE patterns of these fine-medium crystalline dolomites belong to the typical marine carbonate deposits. These features indicate considerable differences in origins between micrite and fine-medium crystalline dolomites. The micritic dolomites show a significant genetic response to the precipitation of seawater in shallow marine with the injection of terrestrial water during the syngenetic period, while the fine-medium crystalline dolomites have a typical response to the secondary metasomatism products of original calcites at the shallow burial stage.