The Dongbo peridotite massif in the western part of the Yarlung Zangbo Suture Zone in western Tibet represents a mantle thrust sheet of more than 400 km2. In order to find out the origin of the massif, a borehole (DSD- 1) has been drilled in the massif. The upper- most part of the drilling core is composed of Quaternary sediments with a thickness of 23m. The rest part of the core is dominated by peridotite which could be divided into two main rock units (further divided into 44 sub- rock units). The upper unit of the core (from 23. 1m to 340. 7m) consists of strongly serpentinized clinopyroxene (Cpx)- bearing harzburgites with orthopyroxene (Opx) contents ranging from 15% to 20% and Cpx contents less than 5%; while the lower unit (from 310. 17m to 1002. 06m) mainly consists of less serpentinized harzburgites (with Opx ranging from 10% to 15%, and no or little Cpx). Thin layers of dunite and pyroxenite, as well as diabase dikes, occur in the harzburgites. Detailed studies on the mineralogy and whole- rock geochemistry of these rocks have revealed that: ① the Dongbo peridotite massif is dominated by harzburgites, accompanied by subordinate Cpx- bearing harzburgites; they have depleted whole- rock major- element and mineral compositions, suggesting that they are residues of mantle rocks after middle- high degrees of basaltic extraction; the occurrence of amphibole replacing Opx indicates that these rocks have experienced melt (fluid)- rock interactions; ② the dunites and pyroxenites occurring as thin layers are composed of minerals of magmatic origin, and amphibole of metasomatic origin is seen locally, which indicates the matasomatic origin for the dunites and pyroxenites; ③ the diabases exhibit both N- MORB- like and arc- like geochemical affinities; they have low SiO2 contents, and high contents of MgO and Al2O3, and their REE patterns are similar to those of the basalts in Western Lau Basin; these features suggest that they are formed in a mature back- arc basin. The formation of the Dongbo peridotites (including the dunite, pyroxene and diabase veins) are closely related with the subduction process.