北秦岭庙湾组黑色岩系稀土元素地球化学特征及成因意义

笔者采用等离子质谱仪(ICP-MS)方法对北秦岭大荆区奥陶纪-志留纪庙湾组黑色岩系岩石的稀土元素及V、Ni、Cr、U、Th等微量元素含量进行了测定.结果显示岩石轻稀土元素相对重稀土元素富集,Eu异常值δEu为0.63~10.61(绝多数大于1.54)多呈正异常,Ce异常值δCe为0.65~1.02(平均为0.893)异常微弱且有正负差异;U/Th值为0.38~4.07,w(V)/w(V+Ni)值为0.38~0.93以及w(Ce)/w(La)值为1.27~2.17.将这.些特征值与前人研究成果对比,并利用La/Yb-Ce/La和La/Yb-REE判别图解判别后,认为北秦岭区庙湾组黑色岩系形成于活动陆缘一种干燥缺氧较深浅海环境,黑色岩系形成过程中有热液作用参与.

REE Geochemical Characteristics of the Miaowan Formation Black Rock Series in North Qinling Mountain and Their Genetic Significance

The abundances of rare earth elements and some trace elements such as V, Ni, Cr, U and Th of the black rock series from the Miaowan Formation in North Qinling Mountain were measured by using the ICP-MS method.The results show that the black rock series of the Miaowan Formation is usually of the LREE-enriched type, that the samples are characterized by slightly negative or positive Ce anomalies (δCe=0.65～1.02) and distinct positive Eu anomalies (with δEu mostly higher than 1.54), and that the ratios of U/Th display a wide variation range from 0.38 to 4.07, the ratios ofw (V)/w (V +Ni)(=0.38～0.93) are normal, and the ratios ofw (Ce)/w (La) (=1.27～2.17) are relatively high. A study of the parameters of REE and trace elements in comparison with the available data and an analysis of the La/Yb-Ce/La and La/Yb- REE diagrams reveal that the black rock series was formed in a dry, anoxic and shallow sea environment belonging to an active continental margin, with the participation of some hydrothermal fluids.