Geochemistry of black shale at the bottom of the Lower Cambrian in Tarim Basin and its significance for lithosphere evolution

The systematic analyses of trace elements, REEs and PGEs of black shale at the bot-tom of the Lower Cambrian in Tarim Basin have been made for the first time in this work. The basic geochemical features are that some trace elements having something to do with the deep-level fluids are highly enriched, including V, Cr, Ni, Cu, Zn, Ge, As, Sr, Y, Mo, Ag, Cd, Sb, Ba, Pb and U, and the Th/U and Th/Sc ratios decreased compared with those in the same kind of rocks in the crust, that the enriched degrees of LREEs are reduced, Eu and Ce depleted distinctly in the chon-drite-normalized REE patterns, and that the contents of PGEs and Au are increased. These char-acteristics indicate that there were more mafic interior sources in the basin when the black shale was deposited. Generally, the interior sources show an extensional tectonic setting of lithosphere. It can be seen from the change of the characteristics of trace elements, REEs and PGEs along the black shale section that the quantity of interior source from inner earth is gradually increased upwards in the lower part of the section, reaches the peak value as shown by sample No. 4 (XCM7-1), and then decreased, which might indicate the episodic extension of lithosphere in the beginning of the Lower Cambrian.