Ophiolites record a series processes of oceanic decomposition, subduction and continental accretion. Ophiolites of the Bangongco- Nujiang Suture in northern Tibet record the history of terrain assemblage and uplift of the Qinghai- Tibet Plateau. Ophiolites of the middle section of the Bangongco- Nujiang Suture is well developed and exposed, mainly composed of mantle peridotite, cumulate complex, gabbro diabase and basalt. Basalt occurs in several outcrops, each of which ranges from more than ten square meters to tens of square meters. Diabase occurs in veins (or dikes) in basalt. This study carried out detailed field geological investigation and laboratory lithofacies, chronology and geochemistry analysis of basalts and diabase outcropped in the west of Dongqiao to understand the origin and formation background of the ophiolite. Research shows that diabase and basalt belong to low- K tholeiite series and calc- alkaline series. The content of LREE in diabase is slightly higher than that in N- MORB, which has the characteristics of IAT and N- MORB. Basalt is enriched in HREE relative to LREE, and has the characteristics of E- MORB. Nb/U values of the samples (30~48) are higher than the average values of the crust (10) and (Th/Nb)N values are less than 1, indicating that the rocks were not contaminated by the crust. Zircon U- Pb ages of three diabase and basalt samples show large variation ranges. Combined with previous chronological study of ophiolites in the Bangongco- Nujiang Suture, it can be divided into three groups. The first group （156~239 Ma） represents the age of ophiolite itself. The second group （37~141 Ma） is later than the age of ophiolites formation, indicating that the formation of the second group （37~141 Ma）resulted from late magmatic events. The third group （277~2454 Ma）is earlier than the age of ophiolites formation, suggesting that the zircons are from the primitive mantle which carried by the early subduction plates from the crust. Comparative lithogeochemical study shows that diabase and basalt are considered to be back- arc basin basalt (BABB). Based on the previous studies of mantle peridotite, this study can conclude that basaltic rocks of Dongqiao formed during the spreading period of back- arc basin and was influenced by subduction zone fluids to varying degrees (SSZ).