⁴⁰Ar-³⁹Ar ages of the ductile decollement shear zone in the Zhongba microterrane of Saga region, southwest Tibet, and their geological significance

The ductile decollement shear zones are in Zhongba microterrane which lies between southern subbelt and northern sub-belt in the western part of the Yarlung Zangbo suture zone.The determination of the activity time about the high strain tectonic belt can not only provide constraints for its formation evolution but also provide the important corroborative evidence for the India-Eur-asia collision.The samples of muscovite and biotite mylonitic rock were collected from the different parts of deformation in the shear zone to analyze the chronology with the mica ⁴⁰Ar-³⁹Ar dating.The tested ages including 53.39±0.57 Ma, 39.45±0.42 Ma and 29.41±0.32 Ma in three groups indicate that the shear zone has multi-period tectonic activities.Group one which represents the initial stage of the India-Eurasia collision is the age limit of the shear zone (minimum age).Thus the initial time of plate collision should be the end of the Late Cretaceous to the Early Eocene before 53.39±0.57 Ma because the shear zone was the product of the initial collision.Group two which is the cooling age of muscovite that formed under the SN-trending shearing compressive stress represents the peri-od of main collisions.Group three which is the biotite age that formed after the reslip of the shear zone slip at low temperature uplift-ing stage represents the period of post-collision.