Abstract: Diabase dikes are extensively distributed in the Tangbai area on the southern margin of the Lhasa terrane. In order to discuss their formation age,genesis and tectonic implications,the authors investigated their petrography,geochemistry,zircon U-Pb geochronology and Hf isotope. Zircon U-Pb dating yielded an age of (54±1) Ma for the Tangbai diabase dikes,indicating that they were formed in the Early Eocene. The trace elements are characterized by enrichment of LILEs (such as Rb,Sr and Ba) and depletion of HFSEs (such as Nb,Ta and Ti). Compared with typical arc magmas,Sangri Group basalts and Yeba Formation basalts in this area,the Tangbai diabase dikes have higher values of Nb,TiO₂ and Zr. Trace element tectonic discrimination diagrams show that Tangbai diabase dikes fall in intraplate basalts field,and show geochemical affinities with intraplate magmatism. The race element geochemical characteristics and zircon Hf isotopic data suggest that the diabase dikes were likely derived from enriched lithospheric mantle which had been metasomatized by slab-derived fluids during previous subductions,and mixed with upwelling ashospheric mantle. The intrusion age of Tangbai diabase dikes was close to the peak period (52 Ma) of Linzizong volcanic activity. Combined with their genesis and tectonic setting,the authors hold that the formation of the Tangbai diabase dikes was related to slab break-off of the northward subduction of the Neo-Tethyan slab ca. 54~52 Ma in age. In addition,according to the latest 3-D numerical models of continental collision and slab break-off,it is shown that the onset of India-Eurasia continental collision should have occurred at 65 Ma or earlier.