Fast cooling following a Late Triassic metamorphic and magmatic pulse: implications for the tectonic evolution of the Korean collision belt

• Large parts of Korea have been affected by a thermal pulse in the late Triassic.
• This is manifest by magmatism, and metamorphism, and isotopic ages generally between 235 and 228 Ma.
• Ar/Ar ages 231–228 Ma on a regional scale, in low- and high-grade rocks, and sediments are due to this event.
• Ar/Ar ages point to very fast cooling 100–150 °C/Ma, probably during core complex like exhumation.
• This evolution maybe due to slab detachment/delamination.

We discuss the evolution of Korea in the context of a relatively short-lived, tectonically induced, magmatic and metamorphic pulse that affected large portions of the crust of the peninsula's southern part during the Late Triassic. Recent 40Ar/39Ar single grain laser step-heating dates imply a prolonged metamorphic recrystallization between 243 and 220 Ma, which occurred in distinct phases that were not coeval throughout the peninsula. We obtained identical plateau ages between 231.4 ± 0.8 and 228.9 ± 0.8 Ma (1σ; 85–95% 39Ar release) on single grains of detrital muscovite from Jurassic sandstones (Gimpo Group). A literature review shows that the ages of detrital muscovites are identical to: (1) concordant 40Ar/39Ar ages of biotite (228 Ma) and amphibole (230 Ma) in amphibolites of the Deokjeongri Gneiss Formation and the Weolhyeonri Complex, pointing to very rapid cooling of 100–150 °C/Ma, and (2) 231–229 Ma muscovite from the low-grade metamorphic mid-Paleozoic turbidites of the Taean Formation. The efficiency of cooling is further underlined by the near-coincidence of these 40Ar/39Ar ages with 243–229 Ma (average: 234.6 Ma) zircon U–Pb ages in the Gyeonggi Massif and the Hongseong belt, in the literature. It is argued that the Late Triassic magmatic and metamorphic pulse is superimposed on an earlier tectono-metamorphic event, possibly related to collision, indicated by: (1) ~ 243–237 Ma muscovite ages, or age components in age spectra, and (2) two generations of folds and associated tectonic foliations truncated by ~ 229.5-Ma-old syenites and earlier mafic dykes. The Late Triassic thermal pulse could have been the result of post-collisional delamination of the lower crust and uppermost mantle, and/or oceanic slab break-off, which is also suggested by almost coeval, widespread mantle-sourced Mg-rich potassic magmatism. Continuing ductile deformation is shown by mylonitization of Late Triassic magmatic rocks; an ~ 220 Ma muscovite age may be related to this.

Fig. 13. Cartoon section of a possible post-collisional configuration in the Korean tectonic system in the Late Triassic, showing a mantle upwarping, and resulting collapse of the isotherms over the extended and thinned region that coincides with the Gyeonggi Massif. Pre-stretching thickness of the brittle upper crust is about 18 km. Isothermal surfaces in °C. The particle path (double line) shows early near-isothermal decompression followed by fast cooling that takes over the thermal regime close to the detachment fault; polygon and square: approximate positions of biotite and hornblende closure, respectively. The dotted line represents the solidus, implying that magmatic rocks may be ductilely deformed during advanced exhumation and thinning, as is demonstrated by a number of Late Triassic intrusions in Anmyeon Island (Fig. 11d–f). Modified after Rey et al. (2009) and Whitney et al. (2013).Figure optionsDownload as PowerPoint slide