Structural and Geochronological Study of High-Pressure Metamorphic Rocks in the Kekesu Section (Northwestern China): Implications for the Late Paleozoic Tectonics of the Southern Tianshan

Abstract : Blueschist- and eclogite-facies high- to ultrahigh-pressure (HP/UHP) metamorphic rocks occur in the southern Tianshan Belt. Their deformation and metamorphic history is important for understanding the Paleozoic tectonics of the Central Asian Orogenic Belt. Our study focuses on the structural analysis and geochronology of the HP metamorphic rocks and the surrounding rocks in the Kekesu Section in the southern Chinese Tianshan. Geometric and kinematic analyses indicate three ductile deformation events: a top-to-the-north thrusting, a top-to-the-south shearing, and a dextral wrenching. New 40Ar/39Ar laser probe plateau ages were obtained on white mica from retrograde blueschist ( 316+/-2and331+/-1Ma; 1j) and greenschist-facies metasediments (323+/-1 Ma; 1j). These ages are interpreted as the time of retrograde recrystallization during exhumation of the HP metamorphic rocks. New structural and isotopic data, in conjunction with previous results, suggest that (1) the collision event occurred during the latest Devonian to earliest Carboniferous, resulting in HP/UHP metamorphism and the top-to-the-north thrusting; (2) the postcollisional exhumation of the HP/UHP metamorphic rocks and extensive retrograde metamorphism under greenschistfacies conditions took place in the Mid-Late Carboniferous and are correlated with south-dipping normal faulting; and (3) Permian ductile dextral shearing and associated granitic intrusion and fluid activity severely overprinted the earlier fabrics.