Metamorphic Evolution of the Southwest Okcheon Metamorphic Belt in South Korea and Its Regional Tectonic Implications

摘要

The Hwasan area of the southwest Okcheon metamorphic belt in South Korea underwent inter-mediate-P/T M1 metamorphism in the Late Carboniferous to Early Permian. Mica K-Ar and ~(40)Ar/ ~(39)Ar ages of the pelitic and psammitic rocks are concentrated at ~160 Ma. Granites in the northern and western parts of the Hwasan area have K-Ar mica ages of approx 156 Ma. Carbonaceous material throughout the Hwasan metasediments exhibits a narrow range in d_(002) of 3.353 to 3.359 A, corresponding to that of fully ordered graphite, consistent with amphibolite-facies conditions of ~500 deg C. Graphitic carbon is also distributed throughout lower-grade greenschist-facies rocks formed during the main Ml metamorphic episode. These results, along with the occurrence of a thermal metamorphic aureole (>500 deg 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.