Abstract: To confirm the emplacement age and tectonic setting of the NE-striking diabase dykes developed in the Tangwangzhai-Yangtianwo syncline of northern Longmen Mountain as well as to reveal whether or not the diabases were associated with the Emeishan mantle plume activity, the authors investigated U-Pb baddeleyite age, major element content and trace element abundances of the diabase samples collected from the Maliucun area within the syncline. The results showed that the diabases yielded a baddeleyite concordant U-Pb age of 261.1±1.8Ma; they belong to the high-Ti (Ti/Y>500) tholeiitic series, and are characterized by high Al (13.39%~14.02%), high Fe (13.88%~14.02%), and low Mg (4.43%~4.56%) values as will as enrichment of the large iron lithophile ele-ments and light rare earth elements; coupled with the data Nb/La (0.84~0.85), Th/Ta (2.40~2.45), and Ta/Hf>0.25, the features of the diabases are similar to the geochemical characteristics of ocean island basalt; the slightly negative anomaly of Nb-Ta indicates that they were probably derived from the enriched mantle with minor contamination by crust materials. A comparison with the Emeishan basaltic rocks shows that they are consistent highly with each other in ages and geochemical characteristics. It is considered that the diabases were formed by Emeishan mantle plume activity and the activity of Emeishan basaltic magma was spread to the northern section of Longmen Mountain and northwestern Sichuan, which had an impact on the biotic environment and hydrocarbon accumulation of this area.