Multiple metamorphic episodes recorded in the Paleozoic Pyeongan Supergroup on the northeastern margin of the Yeongnam massif, South Korea: Implications for the Songrim (Indosinian) orogeny

Metasedimentary rocks of the Pyeongan Supergroup, which occur as roof pendants on the northeastern margin of the Yeongnam massif, South Korea, preserve a record of the impact of the Late Permian-Triassic Songrim (Indosinian) orogeny in the eastern Eurasia margin. Metamorphic results of this paper have revealed that the metasedimentary rocks were affected by three metamorphic events during the Middle Permian to Middle Mesozoic. Representative mineral assemblages from three metamorphic zones I, II and III are Cld+And�Ky�St, Grt+St+Ky+Bt�And, and And+Sill+Bt�Grt�St, respectively. Kyanite and sillimanite paramorphs after andalusite occur in zones I and III, respectively. Metamorphic P-T conditions and paths were determined from conventional geothermobarometers and compositional isopleths of chloritoid, staurolite, and garnet on P-T pseudosections in the MnO-Na₂O-CaO-K₂O-FeO-MgO-Al₂O₃-SiO₂-H₂O (MnNCKFMASH) and KFMASH-T (TiO₂)-O (Fe₂O₃) systems. In the study area, the Pyeongan Supergroup experienced medium pressure regional metamorphism (M1 and M2) belonging to greenschist to amphibolite facies followed by thermal metamorphism (M3). The M1 metamorphism occurred at 450-500�C and 2-4kbar and produced the kyanite paramorph after andalusite. The M2 metamorphism occurred at 520-600�C and 4-6kbar, and was associated with the growth of garnet, staurolite, and kyanite. The later thermal metamorphism occurred at ca. 580�C and ca. 4kbar, and was caused by intrusion of Jurassic granitoids. The medium pressure regional metamorphism (M1 and M2) have occurred around 270Ma during the Songrim orogeny based on the SHRIMP U-Pb zircon ages.