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

Abstract : 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.
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Submitted on : Wednesday, February 18, 2015 - 10:45:20 AM
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Koenraad de Jong, Gilles Ruffet. 243-220 Ma 40Ar/39Ar laser probe muscovite single grain ages in the northernmost Gyeonggi Massif (Juksung area) and their tectonic implications. Journal of the Geological Society of Korea, Geological Society of Korea, 2014, 50 (6), pp.771-782. ⟨10.14770/jgsk.2014.50.6.771⟩. ⟨insu-01117910⟩

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