Ediacaran cap dolostones from south China: Petrography, dolomite stoichiometry, and crystal architecture

Abstract

Using the global Ediacaran cap dolostones to understand the post-Marinoan Earth is based on the premise that the original environmental features have not been diagenetically altered. On the southwest side of the Yangtze Block in southeast China, the Doushantuo cap dolostones that are part of the shelf-to-slope transition are characterized by distinctive petrographic, stoichiometric, and crystal architectural features that recorded their genesis and evolution. Standard petrographic and X-ray diffraction analyses show that these cap dolostones are formed of populations of near-stoichiometric (average molar %Ca = 50.8 %; n = 73), highly ordered (average cation ordering of 0.83; n = 73) dolomite. Six microfacies recognized in the cap dolostones indicate that the rocks have experienced early cementation, dolomitization, fracture filling, recrystallization, and late-stage dolomite precipitation. All of the matrix dolomite crystals from the Liujing, Duodingguan, and Chuanyandong sections, and >95 % of those from the Wuhe section, are formed of unzoned dolomite crystals that are each formed of numerous subcrystals (200–700 nm wide, <2 μm long). In contrast, the zoned dolomite crystals from the Wuhe section are characterized of different degrees of etching between their cores and cortices, and/or between different growth zones. The zoned dolomite crystals demonstrate that the Doushantuo cap dolostones were initially formed via replacive dolomitization followed by multiple phases of dolomite precipitation in a manner similar to many Phanerozoic marine dolostone counterparts. The homogeneity in the dolomite populations and crystal architecture and the increases in dolomite stoichiometry and cation ordering in the Doushantuo cap dolostones suggests that they have been pervasively recrystallized.