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

Abstract

Metasedimentary rocks of the Pyeongan Supergroup in the northeastern flank of the Taebaeksan Basin, South Korea, commonly contain two Al 2SiO 5 polymorphs (And + Ky and And + Sil), or more rarely, three coexisting Al 2SiO 5 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 Al 2SiO 5-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–K 2O–FeO–MgO–Al 2O 3–SiO 2–H 2O (MnKFMASH), MnKFMASH–T(TiO 2)–O(Fe 2O 3), and Mn–N(Na 2O)–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. 5.0–6.0 kbar at 550–580 °C, and those for HT/LP metamorphism are ca. 3.0–4.8 kbar 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.