Pb−Pb age and uplift history of the Busan gneiss complex in the Okchon Belt, Korea: a comparison with the Bagdalryeong gneiss complex in the Kyongki Massif

Abstract

The Busan gneiss complex is a unique body in the Okchon Belt in that it consists of high-grade gneisses surrounded by low-grade schists. In order to elucidate the origin of the Busan gneiss complex, we compared Pb isotope data from the complex with those from the nearby Bagdalryeong gneiss complex of the Kyongki Massif. The two complexes share common lithology, i.e., migmatitic gneiss and augen gneiss. The Pb data of whole rock and feldspar from the two complexes appear to form a single linear array in the²⁰⁷Pb/²⁰⁴Pb–²⁰⁶Pb/²⁰⁴Pb diagram, corresponding to an age of 2932±65 (1σ) Ma. It represents either the formation age or the age of high-grade metamorphism, i.e., migmatization. The Th/U ratios calculated from the²⁰⁸Pb/²⁰⁴Pb and²⁰⁶Pb/²⁰⁴Pb data of both complexes show a close relationship in lithology. The Pb isotope data suggest that the two complexes were originally one continuous body and experienced a similar geologic history since the early Proterozoic. Thus, we conclude that the Busan gneiss complex is either an extension of the basement rocks of the Kyongki Massif beneath the Okchon Belt or a tectonically emplaced piece of the Kyongki Massif. We reinterpret reported Rb−Sr muscovite and biotite ages for the Busan and Bagdalryeong gneiss complexes as the cooling age. The Rb−Sr muscovite ages were originally interpreted as the reset (or partially reset) ages during the Triassic metamorphism of the Okchon Belt (Cliff et al., 1985). Our alternative interpretation suggests that the Bagdalryeong gneiss complex was uplifted rapidly during the Jurassic, whereas the Busan gneiss complex was uplifted slowly since the late Permian. The fast uplift of the Bagdalryeong gneiss complex might have been related to the normal shear movement between the Kyongki Massif and the Okchon Belt (Ree et al., 1995).