鄂尔多斯盆地东缘柳林地区煤系泥页岩稀土元素地球化学特征

摘要

Black shale is widely developed in eastern Ordos Basin.Core samples of black shale from the Taiyuan Formation and Shanxi Formation were systematically collected in the Liulin region and analyzed by inductively coupled plasma mass spectrometry (ICP-MS) to discern the characteristics of rare earth elements (REEs) in the study area.The results show that PAAS-normalized REE in shale are enriched of LREE with a deficit of HREE,and the REE patterns are mutual parallel,inclining to the right with slightly negative Ce anomalies and significantly negative Eu anomalies,which suggests a stable terrigenous input from upper crust.TheδCe anomalies show weak correlations between δE,ΣREE and w (La) N/w (Sm) N,and the values of w (U)/w (Th) were all less than 1,indicating reliable information provided by REE indexes for tracing the source and palaeoenvironment.The Ce anomalies incorporated with trace elemental redox indices (w (V)/w (Cr),w (Ni) / w (Co),w (V)/w (V + Ni),w (U) / w (Th) and δU) indicate that the coal-bearing shales were deposited under a week reduction to weak oxidizing environment,and the oxidation degree of Shanxi Period was higher than the Taiyuan Period.Also,the vertical variations of δCe,ΣREE and selected indicators (w (Ba),w (Sr)/w (Ba),w (Ba)/w (Ga)) coincide with sequence stratigraphic levels,which indicatively suggests that this region has experienced from a marine to continental depositional environment during the period from SQ2 to SQ3.The felsic provenance is offered by the old Yinshan in the north,which should be relevant to continental-margin environment tectonically.%鄂尔多斯盆地东缘晚古生代含煤地层中暗色泥页岩广泛发育,通过系统采集柳林地区上古生界太原组和山西组暗色泥页岩岩芯样品,借助电感耦合等离子质谱方法开展了稀土元素与微量元素测试.研究表明,柳林地区暗色泥页岩中稀土元素含量较高,轻稀土元素富集,重稀土元素亏损且平稳,配分曲线相互平行且右倾,可见微弱的Ce负异常和中等Eu负异常,指示稳定的上地壳物源注入.结合δCe与δE、ΣREE和w(La) N/w(Sm)N较差的相关性,以及w(U)/w(Th)＜1可排除泥页岩中REE的非源岩影响因素,REE指标的源岩及环境指示意义可靠.综合δCe,Ceanom以及氧化还原敏感元素指标(w(V)/w(Cr),w(Ni)/w(Co),w(V)/w(V+Ni),w(U)/w(Th)和δU)表明山西组与太原组泥页岩沉积环境整体为弱还原-弱氧化环境,且山西组沉积期氧化程度更强.δCe、∑REE和部分微量元素指标(w(Ba),w(Sr)/w(Ba),w(Ba)/w(Ga))的差异特征和变化规律在层序地层格架内指相意义明显,显示从SQ2到SQ3期,研究区具有从海相、过渡相到陆相的变化趋势.研究区物源来自盆地北部的阴山古陆,以长英质源岩为主,源区构造背景与大陆边缘环境相关.