The authors studied geochronology and geochemical data of the Songjianghe biotite monzogranite in the southern Zhangguangcai Ｒange in order to determine its formation age， magma source，and tectonic environ- ment． The results indicate that the Songjianghe biotite monzogranite was formed in the Middle Jurassic with an age of 168． 2 ±2． 0 Ma (MSWD =0． 93)． The monzogranite was characterized by high alkali and low CaO and MgO，belonging to high- potassium calc- alkaline，metaluminous I- type granite． The rock is enriched in large ion lithophile elements such as Ｒb，Ba，and K and strongly depleted high field strength elements such as P，Ti， Nb，and Ta． It is concluded that the Songjianghe biotite monzogranite was derived from partial melting of am- phibolite facies metamorphism mafic lower- crust and its formation was controlled by the Pacific Plate subduc- tion．