⁴⁰Ar/³⁹Ar laser dating of the Zongwulong ductile shear zone in northeastern Tibetan Plateau: Constraints on the time of Indosinian orogeny

The Zongwulong tectonic belt (ZTB) is located between the northern Qaidam tectonic belt and the south Qilian orogenic belt and contains Late Paleozoic and Early-Middle Triassic strata. Structural features and geochronology of the Zongwulong ductile shear zone have key implications for the tectonic property of the ZTB. This study integrated field structure, microscopic structure and ⁴⁰Ar/³⁹Ar laser probe analysis. The shear zone strikes ~NEE-SWW and dips at a high angle, with a NWW-SEE trending and W-E stretching lineation, indicating the shear zone as a thrust-slip shear ductile shear. The asymmetric folds, rotating porphyroclast, structural lens and crenulation cleavage can be seen in the field. Mica fish, S-C fabrics, σ type quartz porphy roblasts and quartz wire drawing structure can also be observed under the microscope, indicating that the strike-slip related ductile deformation and mylonitization occurred under low-grade greenschist facies conditions at temperatures of 300℃ -400℃. The highly deformed mylonite schist yielded ⁴⁰Ar/³⁹Ar ages (245.8±1.7)Ma and (238.5±2.6)Ma for muscovite and biotite, respectively, indicating that the shear deformation occurred during the Early-Mid Triassic. Combined with a comprehensive analysis of regional geology and petrology, the authors hold that the age of ductile shear deformation represents the time of Indosinian orogeny in the ZTB. The orogenic activity was probably related to the oblique collision between the South Qilian block and the Oulongbuluke block after the closure of the ocean basin.