Petrology and geochemical characteristics of dolomite in the Middle Permian Maoukou Formation, central Sichuan

Abstract

Dolomites of the Maokou Formation in the central Sichuan were mainly developed in the middle-upper part of Member 2 and the lower part of Member 3 of Maokou Formation, it could be divided into fine-to medium-grained calcareous dolomite, fine-to medium-grained dolomite and breccia dolomite according to petrology characteristic, and the latter two were dominant. Through contrast of geochemical characteristic between micritic limestone and different types of dolomite, the dolomite was characterized by low Fe content, high Mn content and low Sr content relative to the micritc limestone. The micritic limestone and different types of dolomite had similar rare earth element (REE) distribution patterns which were characterized by depletion of light REE, weak positive anomaly of La and negative anomaly of Ce, and the dolomite also had weak positive anomaly of Eu. Compared with the micritic limestone, the fine-to medium-grained dolomite and breccia dolomite had low values of δ18O and obvious high ratios of 87Sr/86Sr. REE distribution patterns of different types of dolomite indicated that some geochemical characteristics of primitive limestone were preserved during the dolomitization process in the Maokou Formation, while weak Eu positive anomaly of dolomite and isotopic difference in dolomite and limestone might be caused by high-temperature thermal fluids during the dolomitization process. The dolomitization of the Maokou Formation was controlled by some factors: (1) strata residual seawater and hydrothermal fluids derived from magmatism were major sources of Mg during the dolomitization process; (2) heating effect of Emeishan large igneous province provided abnormal high paleo-geotemperature for dolomitization; (3) fault systems offered rapid migration channels for abnormal geothermal convection and dolomitization fluid; (4) limestone of grain-shoal facies with good porosity and permeability was more prone to dolomitization than micritic limestone. Abnormal geotemperature and abundant Mg supply derived from Emeishan Large Igneous Province were key factors for dolomitization in the Maokou Formation, and different types of dolomites were formed in different periods of the dolomitization process.