安徽巢湖三叠系东马鞍山组鸟眼构造成因研究

Genesis of bird’s-eye structure in the Triassic Dongmaanshan Formation in Chaohu,Anhui Province

鸟眼构造是碳酸盐岩地层中一种特殊的构造,指示潮上潮间带沉积环境。安徽巢湖地区三叠系东马鞍山组底部普遍发育一层鸟眼构造,但对其矿物学、地球化学特征及成因还缺乏基本的认识。通过光学显微镜、粉晶X-射线衍射(XRD)、拉曼光谱分析及LA-ICP-MS原位微区元素含量测定等方法,对东马鞍山组鸟眼构造的矿物学及元素地球化学特征进行探究。结果表明:鸟眼内部矿物组合以方解石为主,石英及黏土矿物含量较低,方解石晶粒较大且由核部到边部晶粒逐渐变小,富集Sr、REE等离子半径较大的元素;基质矿物组合以白云石为主,石英及黏土矿物含量多于鸟眼核部,有机质含量较高,主要富集Fe、Cu、Ti等离子半径较小或不易迁移的元素。这些综合信息反映了巢湖东鞍山组鸟眼构造是由成岩阶段基质白云岩化析出Ca,在沉积有机质分解聚集形成的气泡孔洞中重新沉淀形成方解石而成。

Bird’s-eye structure is a special structure in carbonate stratum indicative of the sedimentary environment of supratidal-intertidal zone.A layer of bird‘s-eye structure is widely developed in the bottom of the Triassic Dongmaanshan Formation in Chaohu,Anhui Province,but its mineralogy,geochemistry and genesis are still poorly understood.Here,we used optical microscope,X-ray diffraction(XRD),Raman spectroscopy and LA-ICP-MS to investigate the mineralogy and geochemical characteristics of bird’s-eye structure in the Dongmaanshan Formation.The results showed that the major mineral in bird’s-eye structure was calcite,enriched in Sr,REE,etc.,with decreasing crystalline size from the core to edge;while the major matrix mineral was dolomite,containing enriched Fe,Cu,Ti,etc.The contents of quartz,clay minerals and organic matters in matrix were higher than in bird’s-eye core.These results demonstrated that bird’s-eye structure in the Dongmaanshan Formation developed as a result of calcite formation,from matrix re-precipitation in cavities formed by accumulation and decomposition of sedimentary organic matters after Ca separation in the diagenetic stage due to dolomitization.