Abstract
The Korean Collision Belt originally constituted a single coherent southwards-thinning tectonic wedge formed by accretion of the Qinling-type Barrovian metamorphic Jingok and Samgot units (Yeoncheon Complex) and Taean Formation onto the Sino–Korean Craton’s southern extension (Precambrian Gyeonggi Massif) while being underthrust by the South China Block. Detailed structural geological study and 40Ar/39Ar laser-probe dating of 47 mineral single-grain and 2 whole-rock samples with both syn-collisional peak metamorphic and retrograde mineral assemblages reveal a prolonged tectonic evolution from the Devono–Carboniferous pre-collisional stage (375–370 Ma; ~315 Ma) to the Midde–Late Jurassic reactivation of the Permian–Triassic wedge during the second major exhumation phase of the Gyeonggi Massif (194–165 Ma). The latest Permian to Late Triassic main orogeny comprises three distinct correlated tectono-metamorphic phases. Essentially concordant (pseudo-)plateau ages (255.2–249.9 ± 0.4–0.9 Ma, 1σ) for main-phase-fabric-forming muscovite and biotite in the garnet, staurolite and kyanite zones and discordant late-stage andalusite-quartz veins (Jingok Unit) show fast cooling during exhumation from ~30 km to ≤12.5 km. Hornblendes (248.8–247.0 ± 0.6–1.6 Ma, 1σ) in the underlying higher-grade metamorphic, deeper underthrust Samgot Unit show later cooling. Fabric asymmetry implies cooling and age differences stem from (S)SE-ward exhumation along low-angle ductile normal faults. Incipient exhumation of mid-Triassic eclogites (Hongseong Belt) induced strong tectono-metamorphic reworking of the overriding plate (242–237 Ma; Yeoncheon Complex, Taean Formation, Gyeonggi Massif). Structurally downwards increasing resetting of mica (225–220 Ma), retrogression and overprinting by top-to the-north post-main-phase-shear constrains the Gyeonggi Massif’s further exhumation and cooling.
Key Points:
Korean Collision Belt: a southwards-tapering tectonic wedge with discrete tectono-metamorphic phases at 255–247, 245–237, 225–220 Ma
255–247 Ma: fast cooling, rapid exhumation, top-to-SE normal shear; 225–220 Ma: downwards increase retrogression, top-to-N normal shear
245–237 Ma: strong reworking; exhumation mid-Triassic eclogite; 194–165 Ma: final metamorphic reactivation and top-to-N normal shear
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