湖北宜昌早寒武世岩家河组-水井沱组界线元素地球化学特征及其地质意义

Geochemical Characteristics of Elements and Their Geological Significance in the Boundary between Yanjiahe Formation and Shuijingtuo Formation of the Early Cambrian in Yichang area

为揭示宜昌地区早寒武世岩家河组-水井沱组转换时期的沉积环境、物源及构造背景,本文对宜昌暮阳溪剖面岩家河组-水井沱组界线上下采集的样品进行了元素地球化学分析。结果表明:排除成岩作用和陆缘碎屑影响后,对比稀土元素北美页岩标准化配分曲线、(La/Sm)N、(Gd/Yb)N特征,岩家河组-水井沱组轻、重稀土元素存在一定程度分异,且水井沱组轻、重稀土元素分异程度较岩家河组强;U、V、Ni、Co、Cr、Cu富集系数,MoEF-UEF协变图及V/(V+Ni)、δU表明岩家河组-水井沱组界线上下沉积时期总体处在缺氧还原环境中,但局部还原程度不一;La/Yb-ΣREE图解、Eu/ΣREE-ΣLREE/ΣHREE散点图和(K2O/Na2O)-SiO2图解特征显示岩家河组-水井沱组界线附近源岩主要为上地壳沉积岩类-玄武岩类,源区构造背景为被动大陆边缘。

In order to reveal the depositional environment,provenance and structural setting of the Early Cambrian Yanjiahe Formation-Shuijingtuo Formation transition period in Yichang area,this paper analyzed the element geochemistry of samples collected from above and below the Yanjiahe Formation-Shuijingtuo Formation boundary in Muyangxi Section,Yichang.The results show that:Excluding diagenetic and marginal clastic influences,the comparison of NASC standardized partition curve,(La/Sm)_Nand (Gd/Yb)_Ncharacteristics of rare earth elements shows that the light rare earth elements of Yanjiahe Formation and Shuijingtuo Formation have mild to moderate differentiation,while the heavy rare earth elements have weak differentiation,and the light and heavy rare earth elements of Shuijingtuo Formation have stronger differentiation than that of Yanjiahe Formation.The enrichment coefficients of U,V,Ni,Co,Cr,Cu,Mo_(EF)-U_(EF)covariant diagram and value of V/(V+Ni) and δU indicate that the sedimentary period above and below the boundary of Yanjiahe Formation and Shuijingtuo Formation was generally in anoxic reduction environment,but the local reduction degree was different.La/Yb-ΣREE、Eu/ΣREE-ΣLREE/ΣHREE and (K_2O/Na_2O)-SiO_2suggest that there may be a mixture source of sedimentary rocks and basalt from upper crust near the Yanjiahe Formation-Shuijingtuo Formation boundary.The tectonic setting of the source area is the passive continental margin.