Abstract: To investigate the closure time of the Bangong -Nujiang Middle Tethys Ocean, the authors chose small acid rock masses in the ophiolitic zone near the Bangong Lake in Rutog County of Tibet as the study object to investigate their petrogenesis and structural environment. The rock type is plagioclase granite-porphyry. Chemically the rocks consist mainly of SiO2 and (K2O+Na2O), with their content being 65.78%~68.76% and 6.39%~6.86% respectively. In addition, the rocks have high LREE enrichment, high degree of fractionation and positive Eu anomalies They are rich in LREE such as Sr, Ba and Rb, but are depleted in high field strength elements (HFSE) like Ta, Ti and Nb, assuming a "valley" form. The depletion of Nb and Ta suggests that they have the typical characteristics of arc magmatite. LA-ICP-MS (inductively coupled plasma-mass spectrometer and a laser ablation system) dating shows that the zircon 206Pb/238U weighted mean ages of two plagioclase granite-porphyry samples are 96.08Ma±0.22Ma and 96.05Ma±0.30Ma respectively, implying that they both belong to the Late Cretaceous period of Mesozoic.