Cation ordering and stoichiometry of the Upper Ediacaran Dengying dolomites, South China: an extensive diagenesis-controlled pattern

Cation ordering in dolomites and their stoichiometry are key proxies for the conditions of formation, yet high-resolution data from Precambrian dolomites remain scarce, hindering our understanding of ancient dolomite formation and transformation. The Upper Ediacaran Dengying Formation in the Sichuan Basin, South China provides an exceptional archive for addressing this gap due to its continuous deposition, significant thickness and diverse lithologies. We analysed 442 dolomite samples from three stratigraphic sections (Hujiaba, Yangba and Wuxi), representing microbial dolomite, sand-bearing dolomite, crystalline dolomite and silicified dolomite. Powder X-ray diffraction quantified the cation ordering ( S ) and stoichiometry, revealing the pattern of a single peak with two shoulders linked to microbial ( S = 0.65), silicified ( S = 0.75) and sand-bearing ( S = 0.53) dolomites. The stoichiometry ranged from 46 to 54% MgCO 3 , with lower values (43–45%) in the sand-bearing dolomites. Strong correlations between cation ordering, O isotopes, Sr concentrations and the Mn/Sr ratios indicate early to late diagenesis as a dominant control. Elevated 87 Sr/ 86 Sr values exceeding those of contemporaneous seawater and lighter O isotopes suggest the involvement of continental fluids. These findings, the first high-resolution dataset for Upper Ediacaran dolomites, highlight cation ordering as a more reliable proxy for microbial activity than the stoichiometry and emphasize the crucial role of diagenesis in shaping dolomite attributes over geological time.