Diorite in the Bieruozecuo area of Tibetoccurs in the Early Cretaceous Qushenla Group or intermediate- felsic rocks as irregular or elliptic stocks, with minor mafic enclaves. LA- ICP- MS zircon U- Pb dating of the diorite yields a crystallization age of 81.4±0.9 Ma (MSWD=0.55), suggesting late Cretaceous age for magmatic activity. Geochemical analysis shows that the diorite is rich in K2O and poor in CaO, and should belong to quasi aluminum, high K and Ca, alkaline series rock (KCG). The total amount of rare earth elements ranges from 113.80×10-6 to 197.28×10-6, with (La/Yb)N of 11.98～20.26 and the δEu of 0.83 to 0.98. Chondrite normalized REE patterns show relative enrichment of LREE, right- dipping distribution trend, and no obvious Eu depletion. Primitive mantle normalization spider diagram reveals enrichment of K and Rb and depletion of Nb and P, and these features are similar to that of arc volcanic rocks. Geochemical compositions of the diorite show double features of mantle and newly- formed crust, indicating that the diorite likely resulted from linear thermal uplift and thinning decompression of the crust under the effect of mantle flow diaperism in the early stage of subduction and collision. The mantle thermal energy and materials, together with partial melting of the crust, resulted in the formation the diorite in the post collision- extension environment during the ocean- continent transition of the Bangonghu- Nujiang suture zone.