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XU Deming,HUANG Guicheng and LEI Yijun (Yichang Institute of Geology and Mineral Resources, Yichang, Hubei 443003, China)  
The Daba-Xiugugabu ophiolite massif is situated in the western portion of the Yalung Zangbo Suture Zone (YZSZ), composed mainly of mantle peridotites and of mafic dikes that intruded the mantle peridotites. The mafic dikes show similar geochemical features to back-arc basin basalts, characterized by LREE depletion with (La/Yb)N between 0.39 to 0.55, and clear negative anomalies of Ti and Nb and positive anomalies of Sr and Ba. Sr and Nd isotope systematics indicates an origin from a depleted upper mantle source modified by fluid released from the subducting oceanic slab. Sm-Nd isotopic data of four mafic dike samples yielded an isochron age of 126.2±9.1Ma (MSWD=0.44), representing the emplacement age of the mafic magma. The mantle peridotites have two types of REE patterns: spoon-shaped and U-shaped. The spoon-shaped peridotites display slightly LREE-enriched to roughly flattened patterns and have low and limited range of spinel Cr# values, features of mantle peridotites in back-arc extentional basins. Such trace element characteristics reflect lower degrees of partial melting (15%~20%) and moderate to low extent of metasomatism. By contrast, the peridotites having U-type REE patterns show significant LREE enrichment, and have higher and variable spinel Cr# values (0.40~0.77), implying that they experienced higher degrees of partial melting (25%~30%) and the following extensive incompatible element enrichment by melt/fluid metasomatism. Combined geochemical data of the mafic and ultramafic components of the Daba-Xiugugabu ophiolite massif suggests that the ophiolites were probably formed in an intraoceanic arc.
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