Metamorphic evolution of the southwest Okcheon metamorphic belt in South Korea and its regional tectonic implications

The Hwasan area of the southwest Okcheon metamorphic belt in South Korea underwent intermediate-P/T MI metamorphism in the Late Carboniferous to Early Permian. Mica K-Ar and 40Ar/39Ar ages of the pelitic and psammitic rocks are concentrated at similar to 160 Ma. Granites in the northern and western parts of the Hwasan area have K-Ar mica ages of similar to 156 Ma. Carbonaceous material throughout the Hwasan metasediments exhibits a narrow range in d(002) of 3.353 to 3.359 angstrom, corresponding to that of fully ordered graphite, consistent with amphibolite-facies conditions of similar to 500 degrees C. Graphitic carbon is also distributed throughout lower-grade greenschist-facies rocks formed during the main MI metamorphic episode. These results, along with the occurrence of a thermal metamorphic aureole (> 500 degrees C) in the metasediments close to the mid-Jurassic granites, suggest that the study area was overprinted by low-P/T M2 metamorphism due to the intrusions of the plutons. This latter event, however, failed to re-equilibrate mineral assemblages of the earlier metamorphism, except within a narrow aureole of 1-2 km around the Jurassic granites.