~(40)Ar/~(39)Ar Age Constraints on the Ductile Deformation of the Detachment System of the Louzidian Core Complex, Southern Chifeng, China

The kinematics of ductile shearing is not compatible with that of detachment faulting in the detachment systems of the Louzidian metamorphic core complex, which is different from the Cordillera metamorphic core complex in North America. Structural analysis of the detachment systems on the two sides of the core complex shows consistent top-to-the-northeast ductile shearing. Three biotites separated from mylonitic rocks in the detachment systems on the two sides of the core complex yielded 40Ar/39Ar plateau ages between 126-128 Ma and one hornblende separated from mylonitic rocks on the west side yielded a 40Ar/39Ar plateau age of 134 Ma. Four 40Ar/39Ar plateau ages, which are concordant with corresponding isochronal ages, represent a range of ductile shearing and suggest ductile extension. Studies show that the ductile shear zones on the two sides of the Louzidian core complex have the same formation and kinematics and ductile shearing is an important stage of the formation and evolution of the core complex, which provides chronic evidence for the constraints of ductile extension.