P-T modeling of kyanite and sillimanite paramorphs growth after andalusite in late Paleozoic Pyeongan Supergroup, South Korea: Implication for metamorphism during the Mesozoic tectonic evolution

Metasedimentary rocks of the Pyeongan Supergroup in the northeastern flank of the Taebaeksan Basin, South Korea, commonly contain two Al₂SiO₅ polymorphs (And+Ky and And+Sil), or more rarely, three coexisting Al₂SiO₅ polymorphs. Lower-grade rocks preserve chloritoid+andalusite±kyanite in muscovite-chlorite phyllite, and higher-grade rocks contain andalusite+sillimanite (±kyanite)+staurolite±garnet in mica schists. Kyanite and sillimanite occur as partial paramorphs after andalusite. Textural relations and relative metamorphic P-T conditions of the Al₂SiO₅-bearing mineral assemblages suggest a crystallization sequence of andalusite→kyanite→sillimanite, with the growth of staurolite and garnet in the kyanite and sillimanite stability fields. Metamorphic P-T conditions determined from geothermobarometry, compositional isopleths of chloritoid and garnet, and petrogenetic grids in the MnO-K₂O-FeO-MgO-Al₂O₃-SiO₂-H₂O (MnKFMASH), MnKFMASH-T(TiO₂)-O(Fe₂O₃), and Mn-N(Na₂O)-C(CaO)-KFMASH systems could be divided into two regimes temporally: a clockwise path at lower temperature (LT)/medium pressure (MP) conditions, and higher temperature (HT)/low pressure (LP) conditions. The peak metamorphic P-T conditions for regional LT/MP metamorphism are ca. 5.0-6.0kbar at 550-580°C, and those for HT/LP metamorphism are ca. 3.0-4.8kbar and 590-610°C. Kyanite paramorphs after andalusite may result from a LT/MP clockwise P-T path associated with a rapid contractional deformation during the early to middle Mesozoic, and then sillimanite paramorphs after andalusite may have been formed by magmatic heating due to the later intrusion of leucocratic granite.