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Abstract: The Gongsuwon thrust together with the Deokpori (Gakdong) thrust in northeastern Okcheon belt is considered to be an important structure in understanding the tectonic evolution of the Korean peninsula following the Permian-Triassic collision between North and South China continents. We obtained Rb-Sr ages for Paleozoic sedimentary rocks around the Gongsuwon thrust to determine the age of petrographically observed thermal metamorphism and possibly constrain the movement age of the thrust. Andalusite, codierite and biotite porphyroblasts overgrow cataclastic foliation of siliceous fault rocks in the Gongsuwon thrust, indicating a post-faulting thermal metamorphism. A marble of the early Paleozoic Makgol Formation in the hanging wall (∼200 m northwest from the fault zone center) and a siliceous cataclasite in the fault zone center gave Cretaceous Rb-Sr whole rock fragment ages: 113±21 (2σ) Ma and 68±17 (2σ) Ma, respectively. Sandstones of the late Paleozoic Hambaeksan Formation in the footwall (about 680 m southeast from the fault zone center) yielded an early Tertiary Rb-Sr biotite age of 56.3±6.0 (2σ) Ma, but a Paleoproterozoic Rb-Sr muscovite age of 1774±30 (2σ) Ma, suggesting that the early Tertiary thermal effect for the sandstones was strong enough to reset the Rb-Sr biotite age but weak enough not to affect the Rb-Sr muscovite age, i.e., between ∼300°C and ∼500°C, and that detrital muscovites in the sandstones were derived from the Precambrian basement rocks. These isotopic ages suggest that there was Cretaceous to early Tertiary thermal event around the Gongsuwon thrust, which is probably associated with well-known plutonism at that time (so-called Bulguksa granite). Available age data, albeit limited, suggest that Cretaceous to early Tertiary thermal metamorphism in the Taebaeksan basin might have been extensive. The movement age of the Gongsuwon thrust is constrained to be older than ∼60 Ma.

We report a ring dike of Jurassic granite for the first time in Korea. There is a prominent circular structure in Seoul granitic batholith near Uijongbu, north of Seoul. The circular structure consists of large eastern half-circle (ca. 13 km in diameter) and small western half-circle (ca. 6 km in diameter) divided by a north-south-striking fault. A comparison with recently available geologic maps indicates that the geologic structure corresponds to a ring dike (or cone sheet) of garnet biotite granite, which negates an impact origin. We suggest that the Uijongbu structure was formed by differential weathering between resistant garnet biotite granite and easily weathered biotite granite. A ring dike origin for the Uijongbu structure probably associated with a volcanic cauldron subsidence is supported by rather shallow emplacement depth for the Jurassic granite and fast cooling rate (ca. 100°C/Ma) estimated from mineral ages, although no associated volcanics are present in the surrounding region.

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).