大巴山早寒武世黑色岩层稀土元素特征及沉积环境分析

Analysis of REE Geochemical Characteristics and Sedimentary Environment of the Early Cambrian Black Rock Strata in Daba Mountain

为了探讨大巴山早寒武世黑色岩层的稀土元素特征及古沉积环境,采用电感耦合等离子体质谱法（ICP-MS）对大巴山城巴断块内的水井沱组黑色岩层的稀土元素及V、Ni、Cr、U、Th、Zn、Co等微量元素质量分数进行了测定,并与前人研究的鲁家坪组黑色岩层进行了对比分析。研究结果表明,城巴断块内的黑色岩层轻稀土元素相对重稀土元素富集,Ce异常值δCeN为0.47~0.94（平均值0.69）,δCeS为0.46~0.91（平均值0.67）呈负异常,Eu异常值δEuN=0.27~1.03（平均值0.57）,δEuS=0.42~1.57（绝大多数小于0.87）,δEu基本呈现负异常;U/Th比值为2.52~67.51,Sr/Cu比值为5.83~27.96,V/（V＋Ni）比值为0.65~0.98,V/Cr比值为1.35~17.79及δU值为1.77~1.99,结合REE的北美页岩标准化曲线、La-Ce图解、Zn-Ni-Co三角图解及La/Yb-REE判别图解的分析表明,大巴山城巴断块内的早寒武世黑色岩层沉积时所处的古气候为干热型气候,黑色岩层形成于被动大陆边缘的缺氧浅海陆棚-斜坡过渡带的沉积环境,且明显受到热液作用的影响。

In this research, ICP-MS method was used to determine the abundance of REE and some trace elements,e, g. , V, Ni, Cr, U, Th, Zn, and Co, of black rock strata of Shuijintuo Formation from Early Cambrian in Chengba rift belt, Daba Mountain. By analyzing these elements, we researched the REE geo- chemical characteristics of black rock strata and their sedimentary environment. The results show that the black rock strata displayed LREE-enriched type in study area, were characterized by obviously negative Ce anomalies （δCeN=0. 47-- 0. 94, δCeS= 0.46- 0. 91） and negative Eu anomalies （δEuN= 0.27- 1.03, δEuS=0.42--1.57）, and that the ratios of U/Th range from 2.52 to 67.51, the ratios of V/（V-＋-Ni）= 0. 65--0. 98,Sr/Cu=5. 83--27. 96, V/Cr=I. 35--17. 79, and δU=1. 77--1. 99 were in relatively high level. The analysis of the NASC-normalized REE-patterns, La-Ce diagram, Zn-Ni-Co triangular diagram and La/Yb-REE diagram reveals that the black rock strata were formed in a dry, anoxic and shallow ma- rine shelf and slope marginal environment belonging to passive continental margin affected by hydrothermal fluids.