Geochemical Characteristics and Tectonic Setting and of the Yueyashan-Xichangjing Ophiolite in the Beishan Area

The Yueyashan-Xichangjing ophiolite,located at the middle section of the Beishan area,is one of the well exposed ophiolitic belts in the Beishan area.The pillow basalt and cumulate gabbro are characterized by depletion of LREE,flat distribution pattern,and(La/Yb)N=0.47~1.62,similar to those of N-MORB;and by enrichment of LILEs,and depletion of HFSEs such as Nb and Ta,different from those of N-MORB,but similar to those by typical island-arc volcanic rocks,i.e.the mafic rocks have both the MORB-like and arc-like geochemical features.These features are similar to those of mafic rocks occurring in the Kuerti ophiolite which formed in the back-arc basin.Their geochemical features reflect a tectonic setting of back arc basin in which volcanic rock formed.The eNd(t) of other samples are positive and high(6.11~8.17),which implies that the source area is the depleted mantle.Combined with the geochemical and sedimentary characteristics,it can be concluded that the Yueyashan-Xichangjing ophiolite may form in a back-arc basin,which,along with Xieshan-Dongqiyi volcanic arc in the north of the basin,constituted an arc-basin system in the Tarim plate.