巢湖地区早三叠世晚斯密斯亚期含鱼化石碳酸盐岩结核的地球化学特征及其地质意义

以安徽省巢湖地区下三叠统和龙山组顶部的黑色泥页岩段及其产出的含鱼化石碳酸盐岩结核为研究对象, 综合岩相学、地球化学(主微量元素和碳、氧同位素)和环境矿物学等分析手段, 探讨富有机质页岩中含鱼化石碳酸盐岩结核的形成机制及可能的地质环境指示意义。结核中心三维保存的鱼化石和较高的碳酸盐含量(80%)表明结核形成于早期成岩阶段, 结核内部呈同心环状结构, 从中心至边缘, 碳酸盐矿物含量降低, 硅质碎屑矿物增多并逐渐出现纹层, Fe和S含量升高、δ¹³C值增高、δ¹⁸O值降低和TOC含量逐渐增多等特征均表明结核为同心生长模式。对围岩中草莓状黄铁矿的粒径分析结果表明, 结核形成于硫化还原环境。结核出现在斯密斯亚阶与斯帕斯亚阶界线(SSB)之下1 m处, 与界线层有机碳埋藏增加、碳同位素组成快速正漂移(δ¹³C = -1.3‰~1.9‰)和水体硫化还原事件吻合, 有潜力作为华南地区SSB的辅助识别标志。

Late Smithian (Early Triassic) Fish-bearing Concretions from Chaohu Area: Geochemistry and Its Significance

Fish-bearing carbonate concretions are found in the organic-rich shale at the top of Helongshan Formation, Lower Triassic in Chaohu area, Anhui Province. In order to study the formation mechanism and environmental significance of the carbonate concretions, petrography, major and trace elements, carbon and oxygen isotopes and environmental minerals were analyzed for the concretions and surrounding rocks. The 3D-preserved fish fossils and high carbonate content (80%) in the center of concretions indicate that concretions were formed in the early diagenetic stage. Concretions have a concentric zoning structure inside. From the center to the edge, the content of carbonate decreases, the content of siliceous clastic increases and laminae gradually appears, the content of Fe and S elements increases, δ¹³C values increase, δ¹⁸O values decrease, and TOC contents gradually increases, all indicating a concentric growth of concretions. The analysis of the size of framboidal pyrites in the host rock indicates a euxinic environment where concretions were formed. The level of concretions appears 1 m below the Smithian-Spathian boundary (SSB), consistent with the increased burial of organic carbon, rapid positive excursion of δ¹³C (-1.3‰ to 1.9‰) and seawater euxinic event. The concretions have the potential to be regarded as an auxiliary identification marker of SSB in South China.