Petrological and geochemical characteristics of the botryoidal dolomite of Dengying Formation in the Yangtze Craton, South China: Constraints on terminal Ediacaran 'dolomite seas'

摘要

Recent studies suggest that the massive Precambrian dolomite successions were likely the products of specific seawater conditions known as "dolomite seas". However, whether these so-called "dolomite seas" existed in the terminal Ediacaran ocean has not yet been constrained. The widespread botryoidal dolomite with well-preserved sedimentary fabrics from the Upper Ediacaran Dengying Formation (ca. 551 to 545 Ma), South China can be potentially an ideal target to constrain this issue. Results showed that within the botryoidal dolomite, fibrous dolomite cements are the dominant constituent part, and either grew upward from microbialite layers or occurred in sheet cracks within the microbialites, implying a syn-sedimentary diagenesis. Two different types of fibrous cements are newly redefined: fascicular and radial slow dolomite. Well-preserved chemical and cathodoluminescent zonations and length-slow optical characters are direct evidence for syn-sedimentary primary dolomite precipitation, different with the previous alternative interpretations of mimetic dolomitization of aragonite or high-Mg calcite precursors. In contrast, microbialite matrix consists of finely crystalline dolomicrite and preserved microbial fabrics, reveling an early mimetic dolomitization of aragonitic precursors. The dolomite precipitation and mimetic dolomitization likely occurred in coeval seawater or seawater-type fluids as indicated by comparable delta C-13 and (87)Sri/Sr-86 data with global changes. In particular, the presence of primary dolomite cement precipitates within botryoidal dolomite can reflect that the specific "dolomite seas", associated with anoxic or even euxinic shallow water conditions (inferred by redox-sensitive trace elements and REEs) and increased microbial activity, also prevailed in the terminal Ediacaran ocean. However, the occurrence of "dolomite seas" was not one occasional event, but common even if not persistent in the early to terminal Neoproterozoic oceans. This study helps to improve the understanding of "dolomite problem" and ubiquitous distribution of Precambrian dolomite worldwide. (C) 2020 Elsevier B.V. All rights reserved.