In this paper, we report petrography, geochronology, lithogeochemistry and Sr- Nd- Hf isotopic data for the Early Jurassic- Eocene composite pluton in Siborongqu, Gyaca County, the eastern segment of the Gangdese belt, aiming to understand the petrogenesis and its tectonic implications. The investigation results indicate that the Siborongqu composite pluton is composed of Early Jurassic gabbro- granite complex (188~185Ma) and Eocene granitoids (~47Ma). Mafic microgranular enclaves (MMEs) with various plastic shapes are generally dispersed in both stages of granitoids. The Early Jurassic complex consists predominantly of hornblende gabbros and tonalites, with mafic minerals in the gabbroic rocks are mainly of amphiboles. They can geochemically be grouped into a set of calc- alkaline magmatic rocks with arc signatures, and show depleted Sr- Nd- Hf isotopic compositions. The Eocene granitoids constitute dominantly of monzogranites and granodiorites. They are more alkaline and show somewhat enriched Sr- Nd- Hf isotopic compositions with respect to the Early Jurassic tonalites, and can be grouped into calc- alkaline or high- K calc- alkaline I- type granites. Combined with our data and previous studies, we concluded that the Early Jurassic gabbroic rocks were generated by partial melting of the overlying depleted mantle wedge which was metasomatized by aqueous fluids released from the subducted slab. The coexisting tonalites were produced by mixing of felsic magmas derived from partial melting of juvenile lower crust and mafic magmas related to the gabbroic rocks. The Eocene granitoids were derived from partial melting of juvenile lower crust triggered by underplating of a contemporaneous mafic magma, and with somewhat involvement of enriched components from the subducted Indian continental crust. From the perspective of tectonic evolution, the petrogenesis of Early Jurassic complex is controlled by the northward subduction of the Neo- Tethyan oceanic slab, while the Eocene granitoids correspond to the upwelling of asthenosphere after break- off of the Neo- Tethyan oceanic slab.