Geochemistry characteristics of rare earth elements in the late Paleozoic black shale from eastern Ordos Basin

Black shale is widely developed in eastern Ordos Basin. Core samples of black shale from the Taiyuan For- mation 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 pat- terns are mutual parallel,inclining to the right with slightly negative Ce anomalies and significantly negative Eu anoma- lies,which suggests a stable terrigenous input from upper crust. The δCe 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 ele- mental 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.