Located in Mengyang area of the Tengchong block in western Yunnan, the Mengyang intrusion consists of biotite monzogranite (128 Ma) and granodiorite (113 Ma) with abundant gabbroic enclaves (123 Ma) and dioritic enclaves (120 Ma). The dark enclaves, which are mainly gabbroic enclaves and dioritic enclaves, are mainly distributed in the granodiorite. The dark enclaves have igneous texture, and some of them are characterized by plastic flow deformation, quenching boundary, reverse pulse and acicular apatite, suggesting petrographic characteristics of magma mixing. Geochemical data show that the host granitoids and mafic enclaves are all metaluminous to peraluminous and high-K calc-alkaline enclaves enriched in LILE ( such as Rb,Th,U, Nd and Pb) and depleted in HFSE ( such as Nb,Ta,Sr,Zr) . The host rock and dark enclaves are metaluminous, calc-alkaline and high-K calc-alkaline series rocks. Geochemical data show that the host granitoids and mafic enclaves are all metaluminous to peraluminous, calc-alkaline and high-K calc-alkaline series rocks enriched in LILE ( such as Rb,Th,U, Nd and Pb) and depleted in HFSE ( such as Nb,Ta,Sr,Zr) . The REE patterns and primitive mantle normalized trace element spider diagrams of the host rock and dark enclaves are nearly of the same shape,which indicates that they have a close relationship in magma mixing. The Early Cretaceous granitoids are considered to be of the volcanic island arc tectonic setting. The gabbroic enclaves and dioritic enclaves are characterized by low SiO₂, high MgO, high K₂O and high Mg^#(Mg^# = 48.1-68.5). Zircon in the dioritic enclaves has ε_Hf(122 Ma) values of 3.6-6.2, T_DM1Hf ratios of 535-665 Ma, and Nb/Ta ratios of 12.1-15.4, suggesting that the dioritic enclaves had the nature of mantle derived magmas, and that mafic magma was directly derived from depleted mantle source region and was subjected to magma mixing in late reformation. The Mg^# value of biotite monzogranite is 33.5, and Nb/Ta ratio is 10.9. The Mg^# value of granodiorite is 44 and Nb/Ta ratio is 12.5. It is thus held that the biotite monzogranite and the dark enclaves were derived from partial melting of the Mid-Proterozoic metamorphosed basement (Gaoligong Group), and granodiorite from the mixing of mantlederived magma and partial melting crust. The early Cretaceous intrusive rocks in the Tengchong block were intimately related to the oceanic crust of Zuogong -Baoshan plate subducted to the bottom of the Bomi-Tengchong block in the southwest; as a result, the regional crustal was continuously thickening and led to continuous regional gravity isostatic uplift; deep crustal partial melting occurred after heating uplift during decompression and formed lots of granite. Because of plate subduction and collision, the mantle magma and melting substances produced magma mixing, and formed granodiorite and dark enclaves.