Abstract: The Yangfanggou biotite quartz diorite mass in the Muli region,southwestern Sichuan lies in the area east of the southern part of the Garze-Litang ophiolitic mélange zone,and belongs to high-K calc-alkaline granitic rock series. The diorite mass gives a SHRIMP zircon U-Pb age of 210±1 Ma,and may be traced back to the Late Triassic in age. Geochemically,the REE distribution curves for the diorite mass display the gently right-leaning patterns. The enrichment of the light rare earth elements and negative Eu anomaly show that part of plagioclase once experienced crystallization-differentiation processes during the magmatic evolution. The discrimination diagrams for tectonic interpretation show that the Yangfanggou biotite quartz diorite mass in the Muli region was formed in the volcanic arc environment associated with the westward subduction of the Garze-Litang ocean. The arc-continent and continent-continent collision of the Garze-Litang ocean gave rise to the asthenospheric upwelling and slab roll-back, and thus led to the formation of the Yangfanggou diorite mass.