THERMOCHRONOLOGIC CONSTRAINTS ON THE PERMO-TRIASSIC PALEO-TETHYS TECTONICS IN EASTERN TIBETAN PLATEAU

Structural and thermochronologic studies of the Batang-Deqin area in eastern Tibetan Plateau provide constraints on the Paleo-Tethys orogeny. The NS-trending Paleo-Tethys orogen at Batang-Deqin includes ophiolitic mélange zone, magmatic belt, and fold-thrust belt. Two Paleo-Tethys deformation stages, i.e., the dextral oblique top-to-the-west thrusting (D₁) and a top-down-to-the-southeast shearing (D₂), and associated metamorphism, are identified in this area. The P-T estimates of the garnet schist of the Barrovian metamorphism suggest the formation condition of ~500-600 °C and ~5-6 kbars. The ⁴⁰Ar/³⁹Ar analyses on synkinematic and metamorphic micas of the garnet schist yield Late Permian to Early Triassic and Late Triassic ages throughout the Batang-Deqin area. The biotite ⁴⁰Ar/³⁹Ar and zircon (U-Th)/He analyses on the migmatite yield ages of ~190 Ma and Cenozoic ages, respectively. Integrated with previous ⁴⁰Ar/³⁹Ar and U-Pb ages, the reconstruction of cooling path for the garnet schist, plutons, and migmatites reveals a rapid cooling from crystallization of igneous zircons through closure temperature of mica (~700 ºC to ~300 ºC), between ~230 Ma to ~210 Ma. Therefore, we propose that the post-orogenic extension and ductile thinning (D₂) cooled and exhumed the thickened Paleo-Tethys orogenic crust through the biotite ⁴⁰Ar/³⁹Ar temperature during the late Late Triassic.

Keywords: Paleo-Tethys tectonics; eastern Tibetan Plateau; Jinsha suture zone; thermochronology; structural analysis