K-Ar dating of illites for time constraint on tectonic burial metamorphism of the Jurassic Nampo Group (West Korea)

Abstract

The timing of tectonic burial metamorphism of the early Mesozoic Nampo Group (western Korea) related to the Jurassic Daebo orogeny was first constrained by K-Ar dating of illite in sedimentary rocks. Two shale samples show a clear decreasing trend of both the amount of detrital component and K-Ar age of illites with decreasing grain size from >4 μm to 4–2 μm, and to <2 μm fractions. This observation most likely reflects a binary mixture of old detrital 2M1 and young authigenic 1Md illites, usually seen in argillaceous sedimentary rocks. In this sense, the latest illitization attributable to the crustal loading can be estimated to be 157–140 Ma, the extrapolated illitic age inferred from a linear regression between the amount of detrital component and apparent K-Ar age of different size fractions. This result suggests that the maximum tectonic burial metamorphism of the Nampo Group has occurred in the last stage of the Daebo orogeny.