Abstract: The Chuabu composite granitic pluton, with an outcrop area of about 400 km2, is located in the middle segment of the Gangdese belt. This paper presents comprehensive zircon U-Pb age, elemental geochemistry and Sr-Nd-Hf isotopic compositions for the hypabyssal biotite monzogranite porphyry and subvolcanic dacite porphyry within the pluton, aiming to elucidate their origin and to discuss their geological significances. Zircon LA-ICP-MS U-Pb dating yields crystallization ages of about 13-16 Ma for these rocks, indicating that they were generated during Miocene. Geochemically, these rocks are high-K calc-alkaline, metaluminous-weakly peraluminous, enriched in light rare earth elements and large ion lithophile elements (LILEs, e.g., Rb, Th and U), and depleted in high field strength elements (HFSEs, e. g., Nb, Ta and Ti). They also have low Cr, Ni contents, and show indistinct Eu negative anomalies. These rocks have high Sr, low Y and Yb concentrations, with high Sr/Y and (La/Yb)N ratios, suggesting that they can be grouped into adakitic rocks. Isotopically, these rocks have higher (87Sr/86Sr)i ratios (=0.706347-0.707584), negative εNd(t) values (=-4.15- -7.33), and with wide range of zircon εHf(t) values varying from negative to positive (=-5.99-7.78). The integrated petrology, and elemental and isotopic compositions suggest that during Miocene, the Indian ancient continental crust had been subducted underneath the Lhasa terrane. The Chabu adakitic rocks were formed under a post-collisional extensional setting, and were likely produced by mixing between magmas from partial melting of the thickened mafic juvenile crust and components from the subducted ancient Indian continental crust.

Key words: adakitic rocks, petrogenesis, Miocene, Indian continental crust, Chabu, Xietongmen County