Abstract

In this study, we conducted metamorphic P-T estimation and zircon U-Pb age dating for the Chiaksan gneiss in the Wonju-Yeongwol-Hoengseong area in the southeastern Gyeonggi Massif to examine Permian-Triassic metamorphic events occurred in the region. The results of metamorphic P-T estimation using conventional geothermobarometer and pseudosection analysis show that the Chiaksan gneiss underwent intermediate-P/T (656-696°C/8.3-8.7 Kbar) and low-P/T (756-759°C/6.3-6.9 Kbar) metamorphism during prograde and peak metamorphic stages, respectively. After the peak metamorphism, the Chiaksan gneiss experienced retrograde metamorphism (567-612°C/2.2-2.8 Kbar). By combining the results of metamorphic P-T estimation with mineral paragenesis and texture, it was confirmed that successive metamorphic events, from prograde to retrograde metamorphism, occurred along a clockwise P-T path. Additionally, it was identified that partial melting of Chiaksan gneiss occurred due to the dehydration reaction of muscovite during prograde to peak metamorphism. From zircon U-Pb age analysis, metamorphic ages corresponding to the Paleoproterozoic era (1923 Ma) and Permian-Triassic (278 Ma, 229.9 Ma) are recognized in the Chiaksan gneiss, and the peak and retrograde metamorphism of the Chiaksan gneiss were confirmed to have occurred at 278 Ma and 229 Ma, respectively. Considering the metamorphic geothermal gradient identified in this study and the results of previous studies, the regional metamorphism related to the orogenic event occurred in the southeastern Gyeonggi Massif during the Permian-Triassic period. This metamorphic event occurred in an arc-related tectonic setting that was developed in the southeastern part of the Korean Peninsula at that time.

Keywords Gyeonggi Massif, orogeny, Permian-Triassic, metamorphism, zircon U–Pb age analysis