Tectonometamorphic evolution of the Permo-Triassic Songrim (Indosinian) orogeny: evidence from the late Paleozoic Pyeongan Supergroup in the northeastern Taebaeksan Basin, South Korea

The deformed and metamorphosed middle Pennsylvanian to Early Triassic (?) rocks of the Pyeongan Supergroup, occurring in northeastern flank of the Paleozoic Taebaeksan Basin, east central South Korea, record the impact of the Late Permian-Triassic Songrim (Indosinian) collisional orogeny in the eastern Eurasia margin. Analysis of meso-and microstructures and their relationship with porphyroblast growth has revealed that these metasedimentary rocks were affected by four deformation events (D-1-D-4). During the polyphase deformations, successive foliations with alternately steep (S-1 and S-3) and flat-lying (S-2 and S-4) orientations were produced with growth of chloritoid, andalusite and kyanite porphyroblasts during D-1 and D-2, and growth of garnet, staurolite and andalusite porphyroblasts during D-3 and D-4. SHRIMP U-Pb detrital zircon ages (330-310 Ma) from metapelites in the oldest unit of the Pyeongan Supergroup in the study area, combined with the previous data on sandstone compositions, indicate magmatic activity that was contemporaneous with the initial deposition of the Pyeongan Supergroup. This also suggests that the Pyeongan Supergroup was deposited in an arc-related foreland basin at an active continental margin during the Late Carboniferous and Permian time. Two contractional deformation events (D-1 and D-3) and two events related to gravitational flattening (D-2 and D-4), along with Barrovian-type medium-pressure regional metamorphism, may have resulted from the Late Permian-Triassic Songrim orogeny, related to collision between the North and South China blocks and/or collision between the Eastern Eurasia margin and the proto-Japan terrane.