Abstract: The Huayangchuan ductile shear zone in the Xiaoqinling Mountains, Shaanxi, formed in the Neoarchean Taihua Group complex. Field survey and microscopic observation have revealed this ductile shear zone is a deep-level zone, consisting of tectonic schist and augen gneiss, which have features of thrust and sinistral strike-slip oblique subduction. 40Ar/39Ar dating of biotite from tectonic schist in the ductile shear zone yielded a plateau age of 419±0.6 Ma and an inverse isochron age of 417±0.8 Ma. Therefore the authors think that the Huayangchuan ductile shear zone and its related regional principal tectonic deformation of the Xiaoqinling Mountains formed in the Caledonian event at 419± Ma.