Abstract: The Eocene Nianbo Formation Adakitic volcanic rocks were discovered for the first time from the Yangyi basin in the Middle Gangdise Belt.This set of adakitic volcanic rocks consists of dacite, rhyolite, and volcanic lava.In order to determine their formation age, origin and geological tectonic significance, the authors conducted LA-ICP-MS zircon U-Pb geochronologic, petrographic and geochemical studies.The results show that the zircons used for dating are of magmatic origin, and the zircon ²⁰⁶Pb/²³⁸U dating yielded 55±0.8 Ma, which suggests that the volcano erupted in Eocene.Geochemical characteristics show that the volcanic rocks belong to a series of weakly peraluminous calc-alkaline rocks, and have rich Si(SiO₂ of 67.69%~71.93%), high Al(Al₂O₃ of 13.13% ~ 16.16%), low Mg(MgO of 0.52%~ 0.76%), higher Sr(Sr of 345×10^-6~875×10^-6), and lower Y(Y of 4.40×10^-6 ~11.30×10^-6)and Yb(Yb of 0.81×10^-6 ~1.49×10^-6).The total rare earth element values vary in the range of 116.86×10^-6~352×10^-6, exhibiting obvious fractionation between light and heavy REE((La/Yb)_N=23.31~43.66), with no obvious δEu anomaly(0.8~1.15).Enrichment of LILE(Rb, U and Th)and depletion of HFSE(Nb, P and Ti)are displayed in the primitive mantle normalized multi-element variation diagram, indicating geochemical characteristics of C-type adakitic volcanic rocks.Based on regional information, the authors hold that adakitic rocks of Nianbo Formation in Yangyi basin were formed by partial melting of thickened lower crust, indicating that some areas of the southern margin of Lhasa were thickened to about 50 km around 55±0.8 Ma.