Extensive crustal extension existent in northeast Asian continent since Late Mesozoic has resulted in the formation of one of the world's largest extensional provinces. Metamorphic core complexes (MCCs) from Lake Baikal to the southern margin of North China constitute a typical extensional phenomenon in this province. The duration and mechanism of such large-scale crustal extension, especially the mechanism of the extension as to whether the stretching took place simultaneously in this region or it was gradually expanded, deserve further research. Obviously the precise chronological limit of the beginning of each metamorphic core complex is an effective means to solve this problem. Syn-tectonic mylonitized granite (vein) is an ideal symbol body for studying the kinematics, dynamics and chronology of the metamorphic core complexes, and is also an important object for determining the structural features and deformation time. Southern Liaoning was a rather intense and typical lithospheric thinning and destruction area in North China Craton during Late Mesozoic, with the formation of the Liaonan metamorphic core complex serving as one of the marks. The authors tentatively studied the deformation features and zircon SHRIMP U-Pb geochronology of the mylonitized granite vein exposed in the Jinzhou detachment fault of the Liaonan MCC, with the purpose of providing new evidence for constraint on the ductile detachment time. The vein intruded into Archean mylonitic biotite plagioclase gneiss along the mylonitic foliation of the wall rock and underwent a strong extensional ductile shear deformation together with the wall rocks. Field observations and micro-structural analysis show that the vein is syn-kinematic. Fry's finite strain measurements of feldspars show that the sample's Flinn K values is 0.83, Lodυvalues is 0.09, and strain intensity (Es) is 0.71, indicating that the mylonite is dominated by plane strain. The kinematic vorticity number (Wk) is 0.89, implying that the deformation is dominated by simple shear. SHRIMP U-Pb dating of the magmatic zircon from the mylonitized granite vein yields an age of 129±2 Ma (n=10, MSWD=1.6), suggesting that the vein was emplaced in Early Cretaceous. Recent researches have shown that the Liaonan MCC resulted from regional and lithosphere thinning in the North China Craton in Late Mesozoic. The formation of the Jinzhou detachment fault and the uplift on its eastern side are the major indicator of the turning point of the tectonic regime from contraction to extension. In combination with the results obtained by the authors, it is suggested that the Jinzhou detachment fault started its ductile shearing at least at 129±2 Ma and the tectonic regime changed from contraction to extension at this time.