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 (Dn) and extensional (Dn+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-Dn) + staurolite (syn-Dn and/or inter Dn and Dn+1, and syn-Dn+1) + andalusite (syn-Dn+1)-bearing and (2) garnet (pre- and/or syn-Dn) + epidote-bearing assemblages, depending on bulk-rock compositions. Various compositions (An10–40 in the former assemblage and An45–70 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-Dn garnet and plagioclase (An30–40 & An45–55) 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-Dn+1 staurolite with growth of syn-Dn+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.
Bibliographical noteFunding Information:
This study is supported by the Basic Science Research Program through the National Research Foundation (NRF) funded by the Ministry of Science and ICT 2017R1D1A1B04028521 to H. S. Kim. We thank Prof. M Cho and an anonymous reviewer for helpful and constructive suggestions that improved the manuscript significantly. Appendix A
This study is supported by the Basic Science Research Program through the National Research Foundation (NRF) funded by the Ministry of Science and ICT 2017R1D1A1B04028521to H. S. Kim. We thank Prof. M Cho and an anonymous reviewer for helpful and constructive suggestions that improved the manuscript significantly.
© 2019 Elsevier Ltd
- Contractional and extensional deformation
- Deformed plagioclase
- Gyeonggi Massif
- Okcheon Metamorphic Belt
- Shear zone
ASJC Scopus subject areas
- Earth-Surface Processes