243-220 Ma(40)Ar/Ar-39 Laser Probe Muscovite Single Grain Ages In The Northernmost Gyonggi Massif (Juksung Area) And Their Tectonic Implication

Laser step-heating of muscovites from strongly retrogressed and ductilely deformed rocks in the top of the northern Gyeonggi massif (Juksung area) yielded ls Ar-40/Ar-39 (pseudo)plateau ages of: 242.8 +/- 1.0 Ma and 240.3 +/- 1.0 Ma (mica schists) and 219.7 +/- 0.9 Ma (mylonitic quartzite). A biotite single grain yielded a hump-shaped age spectrum with similar to 245-250 Ma step ages pointing to Ar-39 recoil, an irradiation artifact. It is possible that the similar to 243-240 Ma muscovite ages record an early phase of exhumation following or during collision, and that the much younger muscovite age from the mylonitic quartzite implies extended or renewed recrystallization. These ages are similar to 10 million years older and younger, respectively, than Ar-40/Ar-39 ages from amphibolites in anatectic gneisses (Hongseong area) and low-grade metasediments on Anmyeondo, linked to post-collisional tectonic and magmatic processes. Our study thus shows that cooling, exhumation and recrystallization in the Triassic occurred in distinct phases that were not coeval in all areas. This underscores that the younger age sets in metamorphic terranes cannot always be simply interpreted as due to passive post-tectonic cooling, but rather reflects distinct tectonic phases.