Abstract: This paper presents an analysis of LA-ICP-MS zircon U-Pb dating, major elements and trace elements of Tianba diabase in Sanchazi area of Mianlue tectonic belt,south Qinling. It discusses the tectonic evolution of the Mianlue tectonic belt in Neoproterozoic.The LA-ICP-MS zircon U-Pb dating is (805.9±2.6) Ma(MSWD=0.033,n=9), indicative of a middle Neoproterozoic formation age.The geochemical data show that the Tianba diabase belongs to the calc-alkaline series.The REE pattern shows enrichment of LREE; The REE patterns show rightward incline.Eu negative anomaly is not obvious(the average δEu is 0.91).The trace elements pattern shows enrichment in LILEs(Rb, Ba, Sr,etc), and LREEs, and depletion of HFSEs (Nb, Ta),relatively depletion of P and Ti.Comprehensive study indicates that Tianba diabase has the geochemical characteristics of continental marginal magmatic rocks,which was formed by partial melting of the subduction fluid metasomatised enriched lithospheric mantle in an extensional setting. Mianlue tectonic zone and its adjacent areas was from regional convergent environment to postcollision-cracking environment in the middle Neoproterozoic (about 800 Ma).