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  

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

作　　者：于鑫 孙作玉[1] 孟庆强[2] 季承[3] 周敏[1] YU Xin;SUN Zuoyu;MENG Qingqiang;JI Cheng;ZHOU Min(School of Earth and Space Sciences,Geological Museum,Peking University,Beijing 100871;Sinopec Petroleum Exploration and Production Research Institute,Beijing 100083;State Key Laboratory of Palaeobiology and Stratigraphy,Nanjing Institute of Geology and Palaeontology and Center for Excellence in Life and Paleoenvironment,Chinese Academy of Sciences,Nanjing 210008)

机构地区：[1]北京大学地球与空间科学学院,北京大学地质博物馆,北京100871 [2]中国石油化工股份有限公司石油勘探开发研究院,北京100083 [3]中国科学院南京地质古生物研究所和生物演化与环境卓越创新中心,现代古生物学和地层学国家重点实验室,南京210008

出　　处：《北京大学学报（自然科学版）》2021年第2期225-240,共16页Acta Scientiarum Naturalium Universitatis Pekinensis

基　　金：国家自然科学基金(41872164,41972003);国家重点研发计划(2016YFC053301)资助。

摘　　要：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、δ^(13)C值增高、δ^(18)O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(δ^(13)C=−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。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,δ^(13)C values increase,δ^(18)O 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 ofδ^(13)C(−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.

关 键 词：碳酸盐岩结核 同心生长 硫化 早三叠世 巢湖 

分 类 号：P534.51[天文地球—古生物学与地层学] P593[天文地球—地质学] Q915.2[生物学—古生物学]