The Gangdese batholith, located in south margin of Lhasa terrane, is mainly composed of granitoid rocks, which were related to the subduction of the Neo Tethys and collision of Indian—Asian plates. The Xaitongmoin—Quxu shear zone, lied in the southern margin of Lhasa terrane, is a detachment shear zone, with a width of 1~2 km, extending from the west to east and dipping to the north. Mylonites from the shear zone demonstrates obvious foliation, and stretching lineation observed in the foliation reveals a typical S—L structural tectonites. Achievements and studies were mainly on geochemistry, geochronology and petrogenesis. However, geological structural research is not well understood, especially about the Xaitongmoin—Quxu shear zone. Based on this issue, we focus on the geochronology of the shear zone to provide new and accurate constraints on formation of the Xaitongmoin—Quxu shear zone.Methods: Biotite and sericite grains from 2 crushed samples were handpicked under a binocular microscope, then selected by a magnetic separator and cleaned in an ultrasonic ethanol bath. The analyzed grains should be over 99% for the purity. The two samples were irradiated for over 24 hours in the nuclear reactor at the Chinese Institute of Atomic Energy in Beijing and then cooled for about 3 months. Mica 40Ar 39Ar step heating analysis was conducted by mean of an MM 1200B mass spectrometer at the key laboratory of isotopic geology of CAGS. The measured isotopic ratios were adjusted or corrected for mass discrimination atmospheric argon, blanks and irradiation induced mass interference. The correction factors of interfering isotopes produced during irradiation were determined by analyzing the irradiated pure K2SO4 and CaF2. The values were listed below: (36Ar/37Ar)Ca=0.000240, (40Ar/39Ar)K=0.004782, and (39Ar/37Ar)Ca=0.000806. The decay constant was λ=5.543×10-10/a. The detailed procedures and calculations are referred by Chen et al.(2006, 2011) and Zhang et al.(2006). Crystallographic preferred orientation (CPO) measurements of quartz were analyzed using the EBSD (electron back scattered diffraction) technique at the key laboratory of continental and dynamics of Institute of Geology, CAGS. The EBSD technique is a scanning electron microscope JEOL JSM 5610LV(acceleration voltage for 20 kv; working distance for 20 mm). Thin sections for the EBSD measurements were cut parallel to the structural XZ plane (X parallel to the lineation, Y parallel to foliation and normal to lineation and Z normal to foliation). Detailed analytical procedures were given by Xu et al. (2009).Results: The study results suggest that basal slip system and prismatic slip system are dominant with little rhombohedral slip, indicating a low—middle temperatures (300~550℃), namely from greenschist facies to low amphibolite facies. In addition, by means of dating of 40Ar 39Ar of sericite and biotite from mylonites of the Xaitongmoin—Quxu ductile shear zone, we obtained good 40Ar 39Ar data (21.81 Ma and 23.74 Ma). Conclusions: The data has the similar geologic significance in essence, and shows that formational age of the shear zone might be early period of the Miocene (21.81~23.74 Ma). Combined with tectonic setting in southern Tibet, it belongs to the post collision of Indian—Asian plates, as having a close relationship with tectonic transfer from compression to extension in southern Tibet. Moreover, identification of the Xaitongmoin—Quxu ductile shear zone provides accurate timing constraints on Cenozoic tectonic evolution process, and further advances geologic studies of the middle—south Tibetan plateau.