Abstract：There occurs a suite of argillite in the lower plate of the Daqingshan thrust nappe structure. It formed by dynamic metamorphism caused by thrust deformation of Permian mudstone, silty mudstone, siltstone and fine sandstone. A set of penetrative plate cleavages are developed in the rock. The identification of rock thin sections shows that syntectonic neogenic minerals such as sericite, muscovite and chlorite are formed along the cleavage planes. Dating of the syntectonic neogenic muscovite by the laser microprobe 40Ar/39Ar method yielded a 40Ar/39Ar isochron age of 121.6±1.6 Ma. Field geological evidence suggests that two phases of thrust deformation occurred in the Indosinian and late Yanshanian periods respectively in the Daqingshang region. The 40Ar/39Ar isochron age of 121.6±1.6 Ma represents the age of the late Yanshanian thrust deformation event.