黔南地区早石炭世黑色岩系稀土元素地球化学特征及沉积-构造环境分析

摘要

In order to explore the sedimentary-tectonic setting of the Early Carboniferous black rock series in southern Guizhou,the emphasis of the present paper is placed upon the REE geochemistry of the Early Carboniferous black shale samples from the CY1-well through the Dawuba Formation in the Changshun region, southern Guizhou. The results of research show that the ΣREE values for the Early Carboniferous black shales vary from 213. 08 í 10-6 and 308. 1 í 10-6 ,with an average of 251. 43 í 10-6 greater than that(173. 21 í 10-6 )for the North American Shale Composites(NASC). The geochemical parameters such as w(ΣLREE)/ w(ΣHREE)and w (Ce)N / w(Yb)N ratios indicate the highly differentiation of ΣLREE and ΣHREE. The smooth REE distribution patterns display the enrichment of LREE and the depletion of HREE. The δEu values range from 0. 53 to 0. 67( <1),indicating the moderately negative Eu anomalies and slightly Ce anomalies. The changes in δCe and Ce anomalies suggest a redox environment of the study area during the Early Carboniferous. It is inferred from the REE distribution patterns and relevant parameters such as δEu values and w( La)N / w( Yb)N vs. ΣREE diagram in combination with regional geological data that the principal source of the black shales from the Dawuba Formation in southern Guizhou lay to the Central Guizhou uplift and Xuefengshan uplift,and the source rocks are interpreted as the mixtures of granites and sedimentary rocks. The tectonic setting of the study area should be attributed to the passive continental margin.%为了深入探讨黔南地区早石炭世黑色岩系沉积-构造环境，笔者对页岩气钻孔 CY1井打屋坝组黑色岩系进行了详细采样测试，并重点分析了页岩稀土元素地球化学特征。分析结果表明，该区早石炭世页岩稀土元素总量（ΣREE）为（213.08~308.1）×10-6，平均值为251.43×10-6，大于北美页岩的平均值（173.21×10-6）；w（ΣLREE）/ w （ΣHREE）及 w（Ce）N / w（Yb）N等地球化学参数表明，轻、重稀土元素分异明显；REE 分布模式呈平坦状，表现出轻稀土富集、重稀土元素亏损；δEu 值为0.53~0.67约1，显示中等程度的负 Eu 异常，Ce 异常表现不明显。根据δCe 和 Ce异常值的变化，反映研究区早石炭世为缺氧还原环境。根据稀土元素组合特征、δEu 值的变化及 w（La）N / w（Yb）N-ΣREE 的图解特征，结合区域地质背景，认为黔南地区打屋坝组黑色页岩的物源来自黔中隆起和雪峰山隆起，母岩为沉积岩和花岗岩的混合，沉积区构造环境为被动大陆边缘构造背景。