First age constraints on the timing of metamorphism of the Taean Formation, Anmyeondo: concordant 233 Ma U-Pb titanite and 231-229 Ma 40Ar/39Ar muscovite ages
Résumé
Isotopic dating has established that the Middle Paleozoic turbidites of the Taean Formation on Anmyeondo in the West Sea were affected by metamorphism during the Late Triassic. We obtained a 206Pb/238U lower intercept age of 232.5 ± 3.0 Ma (95% confidence, MSWD = 1.2) of metamorphic titanite from a calc-silicate rock by Multi Collector Sensitive High-Resolution Secondary Ion Mass Spectrometry and 40Ar/39Ar laser probe pseudo-plateau ages of 230.7 ± 1.0 Ma and 228.8 ± 1.0 Ma (1σ) for two single grains of metamorphic muscovite. We consider that the metamorphic temperature was below the closure temperatures of titanite and muscovite. Consequently, the concordant U-Pb and 40Ar/39Ar mineral ages are not cooling ages but demonstrate that the metamorphism of the Taean Formation on the island occurred in the earliest Late Triassic (Carnian). The dated muscovite occurs as undeformed grains that cross-cut the main tectono-metamorphic fabric in greenschist facies metapelites, or form undeformed grains in rocks with a well-developed secondary crenulation cleavage. This suggests that the two phases of ductile deformation that affected these Paleozoic sediments occurred earlier. The muscovite age spectra show evidence of an earlier isotopic system of about 237 Ma, which could relate to the observed pre-magmatic folding of the Taean Formation.