DETERMINATION OF THE XIAMEN OPHIOLITE MELANGE IN THE EASTERN SECTION OF LAJI MOUNTAIN, QINGHAI PROVINCE: Tectonic Implication

An ophiolite melange belt with complex material compositions is newly identified in Xiamen Town in the eastern section of Laji Mountain, Qinghai Province. The tectonic blocks are mainly composed of ophiolites, exotic blocks, volcanic rocks and meta-sedimentary rocks. The basic volcanic lava and diabase are characterized by right-dipping LREE enrichment patterns, enrichment of LILEs (Ba, Th and Sr), and depletion of HFSEs (P, Ti and Nb), with the general petrogeochemistry similar to that of island-arc tholeiite. The comprehensive study of petrology, petrogeochemistry, isotope chronology and regional geology show that the Xiamen ophiolite is of supra-subduction zone (SSZ) type, which is probably a back-arc small ocean basin formed by Early Paleozoic Proto-Tethyan Ocean subduction, belonging to back-arc basin spreading ophiolite.