The Dongdegou peridotites are located at the northern of orogenic belt between the Tarim Plate and the Yili—Central Tianshan Plate, southwestern Tianshan Mountains, China, have a lot of amphibole, phlogopite and other hydrous minerals, which shows various degrees of metamorphism. Major elements of the peridotites change in a wider range and toward Al rich or Ca rich tendency during the late alteration, their Nb/Ta and Zr/Hf ratios display that they have the characteristics of refractory mantle residuals. The chondrite normalized REE patterns of the peridotites are types of LREE enriched patterns, based on the obvious fractionation between LREE and HREE, and also enriched in LILE of Rb, Ba, Sr and the radioactive element Th. However, the major elements are refractory, when the rare earth elements or incompatible trace elements are enriched in the same samples. Thus, the decoupling phenomenon indicates the major elements and the trace elements have their own evolutionary characteristics, respectively. Therefore, the Dongdegou peridotites are the depleted wedge mantle residuals after partial melting, and the characteristics of trace elements indicate they might have been metasomatized by the melts or fluids from a subducted slab. Geochemical studies of the peridotites illustrate also that they most likely come from a SSZ Mantle wedge, just as the result from the research of their mineralogy and petrology.