关键词: 埃迪卡拉纪, 纤维状白云石, 原生白云石, 白云石成因, 古环境

Abstract: The Ediacaran represented a key period of paleo ocean and paleoclimate evolution, and the extensively developed fibrous dolomites provided important evidences for reconstructing the oceanic chemical characteristics in this stage. The genetic types and discrimination criteria of fibrous dolomite have been discussed, focusing on the distribution, genetic mechanism, and paleo environmental significance of the Ediacaran fibrous dolomite in China. Multidimensional properties such as petrological characteristics, crystallographic properties, and geochemical composition are important criteria for identifying the dolomite genesis. The primary fibrous dolomites are typically characterized by fibrous structure, and length-slow optical properties, with well-developed growth girdles. While the secondary metasomatic fibrous dolomites commonly show acicular, botryoidal patterns or square terminations, with length-fast optical properties, but underdeveloped growth girdle. In addition, trace element content to some extent provides indicative significance in distinguishing genetic types of dolomites. Multiple types of fibrous dolomites have been identified in the Ediacaran base (first member of Doushantuo Formation) and top (second and fourth members of Dengying Formation) in Yangtze Plate, as well as in the upper Qigebulake Formation in Tarim Basin, including bladed dolomite, fascicular–length-fast dolomite, radial–length-fast dolomite, fascicular–length-slow dolomite and radial–length-slow dolomite. The former three types were mainly generated by secondary metasomatism of aragonite or high magnesium calcite, while fascicular–length-slow dolomite and radial–length-slow dolomite were more likely to be primary dolomites directly precipitated from paleo seawater or marine pore water. In the Ediacaran, seawater was featured by high Mg/Ca ratio, elevated alkalinity and low sulfate concentration, promoting the precipitation of fibrous aragonite and high magnesium calcite precursors. Evaporation further increased the Mg/Ca ratio, while metabolism of sulfate reducing bacteria in hypoxic environments released Mg2+ and increased pH and alkalinity, which effectively overcame precipitation barriers, thereby facilitating the nucleation of primary fibrous dolomites. In summary, the Ediacaran fibrous dolomites in China generally recorded critical geochemical signatures of contemporaneous seawater or marine pore water, and in some cases reflected the involvement of hydrothermal fluids. Their geochemical information effectively reveal the redox state, provenance, and temporal–spatial evolution of the Ediacaran ocean, offering critical geological evidence for reconstructing the Precambrian oceanic chemical system and assessing its environmental constraints on early life evolution.

Key words: Ediacaran, fibrous dolomite, primary dolomite, dolomite formation, paleo-environment