The Gangdese magmatic arc in the south of the Tibet Plateau was formed during the Mesozoic subduction of the Neo- Tethyan oceanic lithosphere. Middle- to high- grade metamorphic rocks exposed in the eastern Gangdese magmatic arc represent the middle- lower crust components of the arc, which provide a rare opportunity to study the formation and evolution of continental crust. The Lilong batholith, forming as main part of the middle- high metamorphic rocks, is composed of meta- gabbro, meta- diorite and meta- granite. In this paper, we conducted a petrological and geochronological study for the meta- diorite and meta- granite (orthogneisses) from the upper part of the Lilong batholith in order to reveal their protolith, metamorphic ages and P- T conditions, and tectonic significance. The orthogneisses are mainly composed of plagioclase, K- feldspar, quartz, biotite, epidote, and with or without amphibole. Their SiO2 contents and alumina saturation indexes (A/CNK) range from 61. 94% to 74. 39%, and from 0. 89 to 1. 03, respectively. They show enrichment of LREE and LILE and depletion of HREE and HFSE. The zircon from the orthogneisses consists of magmatic core and metamorphic rim. The zircon core yielded the crystallization age of 92~86 Ma, while the zircon rim yielded the metamorphic age of 81~72 Ma. The  ε Hf( t ) values of zircon magmatic cores range from +10. 2 to +12. 1. These gneisses underwent metamorphism and partial melting under P- T conditions of 740~750℃ and 0. 5~0. 6 GPa. By combining available results, we conclude that the Lilong batholith is composed of arc magmatic rocks with geochemical characteristics of the depleted mantle, and the granite studied were the crystallization products of fractionated magma. We suggest that the accretion of voluminous mantle- derived magma during the early Late Cretaceous subduction of the Neo- Tethyan mid- oceanic ridge led to significant juvenile crustal growth of the Gangdese arc. In the latest Late Cretaceous, the flat subduction of the young Neo- Tethyan lithosphere resulted in intense shorting and thickening of the arc crust, and the Lilong batholith was transported to the middle to lower crust, and underwent middle- to high- grade metamorphism and anatexis. Our study indicates that the juvenile crust of the Gangdese magmatic arc experienced significant reworking during the late subduction of oceanic lithosphere.