40Ar/39Ar isochron ages of lawsonite blueschists from Jiuquan in the northern Qilian Mountain,NW China,and their tectonic implications

Using laser ⁴⁰Ar/³⁹Ar dating method, we have gotten the metamorphic ages of lawsonite blueschist and epidote blueschist from Jiuquan, northern Qilian Mountain, NW China. The high quality laser ⁴⁰Ar/³⁹Ar dating of glaucophane from lawsonite blueschist gives an isochron age of 413±5 Ma. The isochron age obtained from phengite in epidote blueschist is 415±7 Ma. These data, combining with peak metamorphic P-T conditions and regional geological setting, allow us to infer that the lower limit of the ages of the prograde subduction metamorphism from lawsonite blueschist facies to epidote blueschist facies occurred at ca. 413–415 Ma, which also suggests that the formation of lawsonite blueschist in the northern Qilian Mountain maybe resulted from the corner flow in the cold subduction zone. This study shows that the final closing time of the northern Qilian remnant oceanic basin is about 413–415 Ma, which also represents the convergent age between the North China Craton and the Qaidam block.