Geochemical Characteristics of REE and Trace Elements of Middle to Late Triassic Sedimentary Rocks and Its Tectonic Setting in Zoige Basin

Situated in Northeast of Songpan-Ganzi terrain,the Zoige Basin is a significant sedimentary area of this terrain from Middle to Late Triassic.37 greywacke samples of Middle and Late Triassic collected from Hongcan well in Zoige were analyzed by ICP-MS for REE and trace elements.The result shows that these samples were originated continental crust because they enriched in Cs,Rb,Th and high REE contents and depleted in Sr,Nb and Ta.Negative Eu anomalies and strong differentiation between heavy and light REE are significant.Both the contents and ratios of the inactive trace elements and the results based on discriminant diagrams indicated that the sedimentary tectonic setting of Zoige Basin was dominated by continental island arc and the sediment in this basin was strongly affected by continental island arc between Middle and Late Triassic.The change of REE composition between the upper and lower samples from Hongcan well implied the tectonic setting and property of the sediment provenance in Zoige Basin were altered obviously between late-period of Middle Triassic and mid-period or late-period of Late Triassic.This change was probably due to elevation and erosion of the matured continental crust on the east of Zoige Basin,and lots of clastic materials transported into the basin in late-period of Late Triassic.