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The geochemical characteristics of the Upper Jurassic to Cretaceous sedimentary rocks in north margin of the mid Yangtze and tectonic setting discrimination

YAO Ji ming , YU Bing song, CHEN Jian qiang and LI Shan ying(State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083)  
The chemical composition of sedimentary rocks contains important records for understanding how the continental crust evolves through geological time.Several previous studies have shown that the characteristics of trace element and rare earth element(REE) in sediments are very important for tracing the environment of deposition, the source province and tectonic settings.In this paper, ten argillaceous rock samples collected from the Upper Jurassic to Cretaceous sediments in the north margin of the mid Yangtze region, China, were analyzed for trace element and rare earth concentrations.Trace elements such as Li, V, Cr, Co, Ni, Cu, Ga, Rb, Sr, Y, Ba and U are depleted, which are lower than the mean value of crustal argillaceous rocks, whereas Zr and Hf are highly enriched, indicating major contributions from massive detrital material of terrigenous origin.The REE patterns also change dramatically with time from the Upper to the Lower Cretaceous, with reducing Eu negative anomaly and light rare earth element enrichment, and increasing Ce negative anomaly, which reflect decreased sedimentary differentiation.The tectonic setting of sedimentary provinces of the north margin of the mid-Yangtze is classified as mainly continental island arc, with some similarities to passive continental margin environment, based on La-Th-Sc, Th-Co-Zr/10, Th- Sc-Zr/10, La/Th-Hf, Th/Sc-Sc, Th/U discrimination plots.
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