243-220 Ma 40Ar/39Ar laser probe muscovite single grain ages in the northernmost Gyeonggi Massif (Juksung area) and their tectonic implications
Résumé
Laser step-heating of muscovites from strongly retrogressed and ductilely deformed rocks in
the top of the northern Gyeonggi massif (Juksung area) yielded 1σ 40Ar/39Ar (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 ∼245-250 Ma
step ages pointing to 39Ar recoil, an irradiation artifact. It is possible that the ∼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 ∼10 million years older and younger, respectively, than
40Ar/39Ar 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.