Tuomoerrite ophiolitic mélange is located in the eastern segment of the northern margin of Qaidam terrane. This paper presents a systematic geochronological and petrogeochemical study of the meta-basaltic rocks in the ophiolite mélange. The metabasalts have whole-rock SiO₂ contents of 47.78%~50.01%,MgO contents of 4.52%~9.36%, ^TFeO contents of 8.00%~14.94%,K₂O contents of 0.04%~0.80%,and Na₂O contents of 1.02%~4.20%, resembling those features of tholeiitic magma. They show slightly depletion in LREE, enrichment in LILE and LREE,and depletion in HFSE,similar to the patterns of arc-related volcanic rocks. These features collectively suggest a back-arc setting for the meta-basalts. The zircon U-Pb age of these metabasalts determined by LA-ICP-MS is 480.1±1.8 Ma,suggesting that the expansion time of the back-arc basin in this area should be around 480 Ma,and the oceanic slab subduction occurred in Early Ordovician.