243-220 Ma 40Ar/39Ar laser probe muscovite single grain ages in the northernmost Gyeonggi Massif (Juksung area) and their tectonic implications

International audienceLaser step-heating of muscovites from strongly retrogressed and ductilely deformed rocks inthe top of the northern Gyeonggi massif (Juksung area) yielded 1σ 40Ar/39Ar (pseudo)plateauages of: 242.8 ± 1.0 Ma and 240.3 ± 1.0 Ma (mica schists) and 219.7 ± 0.9 Ma (myloniticquartzite). A biotite single grain yielded a hump-shaped age spectrum with ∼245-250 Mastep ages pointing to 39Ar recoil, an irradiation artifact. It is possible that the ∼243-240 Mamuscovite ages record an early phase of exhumation following or during collision, and thatthe much younger muscovite age from the mylonitic quartzite implies extended or renewedrecrystallization. These ages are ∼10 million years older and younger, respectively, than40Ar/39Ar ages from amphibolites in anatectic gneisses (Hongseong area) and low-grademetasediments on Anmyeondo, linked to post-collisional tectonic and magmatic processes.Our study thus shows that cooling, exhumation and recrystallization in the Triassic occurredin distinct phases that were not coeval in all areas. This underscores that the younger age setsin metamorphic terranes cannot always be simply interpreted as due to passive post-tectoniccooling, but rather reflects distinct tectonic phases