Polymetamorphism of Ogcheon Supergroup in the Miwon area, central Ogcheon metamorphic belt, South Korea

Metasedimentary rocks in the Miwon area of the central Ogcheon metamorphic belt (OMB) are composed of schist, phyllite, slate, quartzite, and marble, together with minor mafic and felsic layers, and are divided into Pibanryeong and Poeun units. Mineral assemblages of the Poeun unit are represented by low-grade assemblages including biotite and chlorite, whereas those of the Pibanryeong unit by kyanite, staurolite, garnet, and biotite. Retrograde minerals include chlorite and white mica. Subsequent to regional metamorphism, thermal flux during the Mesozoic magmatism produced sillimanite, andalusite, cordierite, staurolite, garnet, biotite, and white mica in the contact aureole. Contact metamorphism, however, did not broadly reset mineral compositions of the regional metamorphism. P-T conditions for regional metamorphism of the Pibanryeong unit are in the range of 4.3–8.0 kbar and 510–620°C, which belong to the kyanite stability field. Mineral assemblages and P-T conditions of the Pibanryeong unit in the Miwon area are identical to those reported in the Jeungpyeoung—Deokpyeong area. However, mineral assemblages of the Poeun unit in both areas are not compatible, and suggest a decrease in metamorphic grade towards the southwest along strike of the OMB.