Retrieval of P–T–t–d paths in a syn-metamorphic shear zone: Implications for P-T calculations and the Permian–Triassic orogeny on the Korean Peninsula

Crustal-scale shear zones developed on the boundaries of the Precambrian Gyeonggi Massif (GM) can play key role on understanding collisional orogeny during the Late Paleozoic to Early Mesozoic in the Korean Peninsula. Garnet schist formed on the boundary between the GM and Okcheon Metamorphic Belt (OMB) experienced contractional (D_n) and extensional (D_n+1) shearing combined with a clockwise pressure–temperature–deformation (P–T–d) path. Two distinct mineral assemblages were found in the schists, (1) garnet (pre- and/or syn-D_n) + staurolite (syn-D_n and/or inter D_n and D_n+1, and syn-D_n+1) + andalusite (syn-D_n+1)-bearing and (2) garnet (pre- and/or syn-D_n) + epidote-bearing assemblages, depending on bulk-rock compositions. Various compositions (An_10–40 in the former assemblage and An_45–70 in the latter one) and texture of plagioclase resulted from bulk-rock compositions, heterogeneous strain and dissolution and precipitation during the multiple shearing. The P–T conditions estimated for the early contractional shearing (top-to-the-north reverse shear) are 560 °C–600 °C and 5.5–7.0 kbar based on the intersections between compositional isopleths for pre- and/or syn-D_n garnet and plagioclase (An_30–40 & An_45–55) calculated in the MnO–Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O and MnO–Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–Fe₂O₃ systems. Subsequent extensional shearing (top-to-the-south normal shear) was characterized by recrystallization of fine-grained syn-D_n+1 staurolite with growth of syn-D_n+1 andalusite porphyroblasts during cooling and decompression to 560 °C and 3.5 kbar. The contractional and extensional shearing probably occurred at the Permian and the Late Triassic to Early Jurassic, respectively, based on the previous geochronological data. Consequently, inferred P–T–t–d paths on the GM–OMB boundary probably suggest not only crustal thickening followed by thinning, but also multiple amalgamations of several terranes.