Post-collisional evolution of Korea: slab break-off, magmatism, metamorphism, fast cooling and gravity spreading shown by 243-220 Ma 40Ar/39Ar laser-probe and U-Pb SHRIMP ages
Abstract
We present a detailed calendar for the Late Triassic tectonic evolution of Korea founded on field-based research in the northern Gyeonggi Massif, the Hongseong belt, and adjacent Taean Formation, combined with isotopic dating using 40Ar/39Ar single grain laser-probe and SHRIMP technologies. Metamorphic titanite in a calc-silicate rock (Taean Formation, Anmyeondo) gave a 232.5±3.0 Ma 206Pb/238U lower intercept age that is concordant with a 230.7±1.0 Ma (1σ) 40Ar/39Ar age for a muscovite cross-cutting the main tectono-metamorphic fabric in metapelites. Magmatic zircon from a syenite that intruded the meta-sediments following two folding phases has a 229.6±3.5 Ma 206Pb/238U age. These dates These dates show that the timing of magmatism and greenschist-facies metamorphism that affected the Taean Formation is indistinguishable, suggesting both have a common tectonic cause. Some age spectra are saddle-shaped with step ages that are older (237-243 Ma) for initial and final degassing than the main central parts with ca. 230 Ma ages. These age spectra imply recrystallization at about 230 Ma of muscovite that had already recorded an earlier tectono-metamorphic phase around 240 Ma.