贵州瓮安陡山沱组磷块岩稀土元素地球化学特征与沉积期后变化

贵州瓮安陡山沱组磷块岩保存了可能是全球最早的后生动物化石 (瓮安动物群 ),对于瓮安动物群出现过程中的古海洋环境重建具有重要科学意义。但必须对成岩后生作用对磷块岩中的稀土元素改造进行评估。通过对贵州瓮安陡山沱组磷块岩的磷质碎屑、磷质和白云质胶结物、磷条带和泥条带等的稀土元素地球化学特征研究,确定沉积期后变化对稀土元素的改造影响不大。上矿层磷块岩沉积期形成的磷质碎屑、成岩期形成的白云质和磷质胶结物具相同的稀土元素配分模式,暗示了沉积期后的改造作用对瓮安陡山沱组磷块岩保存的原生沉积信息影响不大。瓮安陡山沱组磷块岩具有显著的重稀土亏损特征。磷块岩的磷质和白云质胶结物、伴生磷质碎屑、强风化磷块岩相近的ErN/LuN 比值,表明沉积期后的改造作用不是重稀土元素亏损的主要原因。磷块岩的ErN/LuN、LaN/NdN 与Ce/Ce 间的相关性,表明越氧化的沉积环境中,轻和重稀土元素亏损越强。

REEs Geochemistry of Doushantuo Phosphorites and Modification during Post Sedimentary Stages in Weng'an Area, South China

The Neoproterozoic Doushantou phodphorites in Weng'an, South China, preserved a unique assemblage of a most likely the earliest metazoan in the world. The reconstruction of paleoenvironmental change, when the Weng'an metazoan fauna emerged, is most important. But REE diagenetic modification of the Doushantou phosphorites must be evaluated. This paper is to discuss the REE modification in the Weng'an phosphorites during post sedimentary stages by REE geochemistry of phosphatic and dolomitic cements, phosphatic clasts in the upper phosphorites, phosphorite and shale bands in the lower phosphorites. The later diagenetic mineral phases: dolomitic and phosphatic cements display similar North American Shale normalized REE patterns with associated authigenic phosphatic phases and suggest that the REE geochemical characteristics in the Doushantuo phosphorites could be taken as primary signatures of paleoseawater conditions and may imply that the REE in the authigenic phosphatic phases were not modified during their burial, digenesis and surface weathering. The different REE characteristics in the phosphatic bands and associated shale bands in couplet samples suggested that the REE digenetic reaction might be only taken place within-strata but not between strata. Negative correlation of Er N/Lu N and positive correlation of La N/Nd N with Ce/Ce * suggests that the HREE and LREE depletions may derive from the change of paleoseawater redox condition, and HREE, LREE appear to be more depletion in the more oxidizing condition caused more Ce depletion.