The Oligocene biotite monzogranites in Neymo of Tibet contain abundant melanocratic mafic microgranular enclaves (MMEs) with different shapes. This paper presents geochronology, elemental geochemistry and Sr- Nd- Hf isotopic compositions for the host granites and mafic enclaves, with the aims to elucidate the origin of magma mixing and discuss its implications for petrogenesis and continental crust accretion. Zircon LA- ICP- MS U- Pb dating yields the crystallization ages of about 30 Ma for both the host granites and the M MES, which are within analytical errors, suggesting that the host rocks and MMEs were coeval. Geochemically, the host biotite monzogranites are high- K calc- alkaline, metaluminous, and enriched in large ion lithophile elements (LILEs, e. g. , Rb, Th and U), and depleted in high field strength elements (HFSEs, e. g. , Nb, Ta, Ti and P). High Sr, low Y and Yb concentrations, high Sr/Y ratios and indistinct Eu anomalies show geochemical signatures of adakitic rocks. The MMEs are relatively depleted in Si and enriched in Fe and Mg, and have similar distribution pattern of trace elements and rare earth elements (REE) to the host granitic rocks. The host biotite monzogranites and MMEs also have close whole- rock Sr- Nd and zircon Hf isotopic compositions (host rocks: (87Sr/86Sr)i =0.7057~0.7064，εNd(t)=-1.45~0.35，εHf(t)=1.21~7.34; MMEs: (87Sr/86Sr)i=0.7058~0.7064，εNd(t)=-2.23~-1.57，εHf(t)=2.40~7.04). The integrated petrology and elemental and isotopic compositions suggest that the host biotite monzogranite originated mainly from partial melting of thickened juvenile mafic lower crust, and the MMEs were produced by inhomogeneous magma mixing between mantle- derived basaltic magmas and the adakitic parental magmas of the host granitic rocks. In combination with the comprehensive analyses of the available data for the magmatic rocks along the Gangdese belt, it can concluded that underplating of mantle- derived basaltic magmas and their induced magma mixing is not only the main mechanism for the formation of Gangdese batholith, but also an important way resulting in continental accretion of the Tibetan Plateau.