Abstract: The Yidun arc, located between the Songpan-Garze Terrane and Qiangtang Block of southwestern China, was formed in response to the Late Triassic large-scale subduction orogenic process. The Daocheng batholith consists of granite, granodiorite and K-feldspar granite. Abundant massive mafic microgranular enclaves (MMEs) developed mainly within the granodiorite and K-feldspar granite, forming clear contacts with the host granites. The MMEs are characterized by the quartz eye structure, quenched apatite, and plagioclases phenocrysts with obvious oscillatory zones. Based on the systematic petrographical, mineralogical and geochemical characteristics of plagioclase, hornblende and biotite from the host rock and MMEs of the Daocheng batholith of western Sichuan, we provide here not only the constraints on the physicochemical conditions for mineral and host rock formation, but also important insights into understanding the magma mixing process and structural setting. Our study shows that the plagioclases in the granodiorite and MMEs are all dominated by andesine with rounded or embayed cores. In the former, the An contents in the cores vary between 21 and 50, significantly higher than the An contents in the mantles (2134), and in the latter, the An contents (2944) are waved and concentrated in the mutation annulus. The overlap of An value between MMEs and host granite diorite indicates that magma mixing occurred during their formation. The An contents increase with the increase of Al2O3, FeO, MgO and CaO, but decrease with the increase of SiO2, Na2O and K2O. The hornblendes from the host rock and MMEs are rich in Mg and Fe, with CaB and Ti values of 1.561.75 and 0.080.13, respectively. They both belong to the calc-amphiboles, crystallized at 697725 ℃ and 680705 ℃, respectively. The biotites of granodiorite are enriched in iron, depleted in calcium and magnesium, which belongs to typical magmatic biotite. These biotites have Mg/(Mg+Fe2+) ratios of 0.370.45, TiO2 contents of 3.54%4.62%, oxidation coefficient of 0.080.11, Mg# values of 0.390.46, MF values of 0.360.44, AlⅥ value (molecular number of cations in one unit) of 0.030.11. The biotites from granodiorite display low CaO, MgO and AlⅥ values and high FeO and TiO2 contents, and were most likely crystallized between 584 ℃ and 624 ℃ as calculated by the cation numbers of Ti and Al. According to the tectonic setting discrimination diagrams and the geochemical features of biotites, we conclude that the Daocheng granodiorite belongs to the I-type granitederived from the partial melting of the Late Triassic arc lower crust with minor mantle-derived materials, and the MMEs were generated by the mixing of mafic and felsic magma.

Key words: zoned plagioclase, mineral chemistry, magma mixing, Daocheng batholith, western Sichuan