Pressure-temperature-time-deformation (P-T-t-d) path for Devonian forearc deposits in the Imjingang Belt, South Korea: Implications for Permian-Triassic collisional orogenesis on the eastern margin of Eurasia

Abstract

The Taean Formation on the central western margin of the Korean Peninsula contains layered metapelite, quartzo-feldspathic schist, amphibolite, calcsilicate, and impure marble. The layering was formed by tectonic transposition of the original bedding. The formation has lithologic and isotopic age signatures similar to those of metasedimentary rocks of the Imjingang Belt, which is the eastern extension of the Qinling-Dabie-Sulu Belt, and the depositional age of the formation can be constrained to the Late Devonian. The metamorphic pressure-temperature (P-T) path for the formation is characterized by a high Delta P/Delta T prograde part and subsequent near-isothermal decompression with peak P-T conditions of 600-620 degrees C and 9.0-10.5 kbar, based on observations of mineral assemblages and phase equilibria modelling. Strong compressional deformation (D-n) produced a N-S-trending S-n foliation that transposed the earlier Sn-1 and isoclinal F-n folds and was coeval with an episode of medium- to high-pressure amphibolite-facies metamorphism. The NNE-SSW-trending left-lateral Ryeseonggang Fault played a key role in producing the N-S distribution of the metamorphosed forearc deposits along the western coast of the Korean Peninsula. The peak metamorphism and deformation occurred during the early Permian to Early Triassic (276-250 Ma), with retrogression taking place during the Late Triassic (c. 230 Ma), as inferred from new U-Pb zircon age data and previous titanite and muscovite ages for the formation. The clockwise P-T-t (time)-d (deformation) path for the Taean Formation indicates rapid burial during D-n followed by rapid exhumation during the Permian-Triassic collision between the Qinling microcontinent and the North China Craton.