P-T modeling of margarite + anorthite-bearing Al-rich metapelites in the Taebaeksan basin, South Korea: Implications for accretion-related metamorphism during the Late Permian-Triassic orogeny

Greenschist-facies Al-rich metapelites of the Pyeongan Supergroup in the eastern part of the Taebaeksan Basin, South Korea, contain unusual mineral assemblages, including the associations Mrg + Cld + Ms ± Chl ± Cal, An + Mrg + Ms, and An + Ms ± Grt ± St (all rocks contain Qtz). The X _Mrg [Ca/(Ca + Na + K)] and X _Pa [Na/(Ca + Na + K)] values of margarite range from 0. 12 to 0. 92 and from 0. 10 to 0. 28, respectively. High-Ca plagioclase porphyroblasts are slightly zoned, from An ₈₅ in the core to An98 in the rim. The Mrg + Cld-bearing assemblage yields P-T conditions of ca. 450-480 °C, 4. 0-5. 0 kbar based on the intersection of isopleths of Fe/(Fe + Mg) of chloritoid and X _Pa of margarite in the MnO-Na ₂O-CaO-K ₂O-FeO-MgO-Al ₂O ₃-SiO ₂-H ₂O (MnNCKFMASH) system. The anorthite-bearing mineral assemblages yield P-T conditions of ca. 480-520 °C, 3. 8-4. 4 kbar, and the An contents of plagioclase increased during decompression rather than during heating, based on isopleths of An content and the modal proportion of plagioclase. Phase relations in T-X _CO2 pseudosections at 4 kbar in the CASH-CO ₂ system indicate that anorthite in the Al-rich metapelites grew during the consumption of margarite at T-X _CO2 values of ca. 0. 2-0. 5 during sub-isothermal decompression at temperatures of 480-500 °C. Consequently, the overall P-T path for the formation of chloritoid, margarite and anorthite is characterized by relatively rapid compressional heating up to 480 °C at 5. 0 kbar (geothermal gradient of 27-30 °C/km), followed by sub-isothermal retrograde decompression. This clockwise P-T path in the eastern part of the Taebaeksan Basin reflects the effects of the Late Permian-Triassic Songrim orogeny, which probably occurred at an accretionary forearc-related active continental margin.