Discovery of a c.370 Ma granitic gneiss clast from the Hwanggangri pebble-bearing phyllite in the Okcheon metamorphic belt, Korea Kazuhiro Suzuki a, * , Daniel Dunkley a,1 , Mamoru Adachi b , Ueechan Chwae c a Center for Chronological Research, Nagoya University, Chikusa-ku, Nagoya 464-8602, Japan b Nagoya University Museum, Chikusa-ku, Nagoya 464-8601, Japan c Korea Institute of Geoscience and Mineral Resources, 30 Gajeong-dong, Yuseong-gu, Daejon, Korea Received 12 February 2005; accepted 15 June 2005 Available online 9 January 2006 Abstract The chemical Th – U total Pb isochron method (CHIME) of dating was carried out on accessory minerals in samples from the Okcheon metamorphic belt in Korea. Dated minerals include xenotime and monazite with overgrown mantles in a granitic gneiss clast from the Hwanggangri Formation, metamorphic allanite in garnet-bearing muscovite – chlorite schist of the Munjuri Formation, and polycrase and monazite in post-tectonic granite from the Hwanggangri area. Overgrowth of mantles took place at 369 T 10 Ma on c. 1750 Ma cores of xenotime and monazite in the granitic gneiss. Allanite, occurring in textural equilibrium with peak metamorphic minerals, yields a CHIME age of 246 T 15 Ma that is discriminably older than the polycrase (170 T 6 Ma) and monazite (170 T 3 Ma) ages of the post-tectonic granite. These chronological data suggest that some of the metasedimentary rocks in the belt formed through a single stage of metamorphism at c. 250 Ma from post-370 Ma sediments. Late Permian age signatures have also been reported from the Precambrian Gyeonggi and Yeongnam massifs that border the Okcheon metamorphic belt, and indicate that parts of the basement massifs and the metamorphic belt were affected by the same regional metamorphic event. D 2005 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. Keywords: Okcheon metamorphic belt; CHIME dating; Xenotime; Monazite; Hwanggangri Formation; Pebble-bearing phyllite 1. Introduction The Okcheon metamorphic belt ranges from Chungju to Jeonju within the NE – SW trending Okcheon mobile belt in the Korean peninsula (Fig. 1). The belt is composed mainly of schist and phyllite of pelitic composition with subordinate psammitic, calcareous and basic metamorphic rocks. The protoliths are inferred to have formed in an intracontinental rift-related environment (e.g. Cluzel, 1992), although the timing of sedimentation and metamorphism are controversial. Paleonto- logical evidence is limited owing to regional and contact metamorphism. Lee et al. (1972) reported the presence of an early Cambrian Archaeocyathid from a dolomitic limestone bed in the lower member of the sedimentary sequence. Chang and Park (1977) reported the marine trace fossil Chondrites from the black shale bed, and Lee et al. (1989) discovered Ordovician conodonts in limestone pebbles from a pebble-bearing phyllite. Further attempts to confirm the occurrence of early to middle Paleozoic fossils, however, have not been successful. Radiometric ages have yielded varied results. Cliff et al. (1985) found that Rb – Sr isotopic data from centimeter-scale samples of phyllite yield approximate isochrons of 200 Ma, and interpreted this as the time of metamorphism. Lee et al. (1998) analyzed four zircon fractions from a metatrachyte belonging to a suite of felsic volcanics that intrude sedimentary units, and identified a U–Pb upper intercept age of 755.8 T 1.3 Ma as the time of volcanism and a lower intercept age of 160 T 19 Ma as that of thermal metamorphism. Cheong et al. (2003), on the other hand, identified 207 Pb/ 206 Pb ages of c. 280 Ma as the time of peak metamorphism in the Okcheon belt. Published age data thus provide inconsistent estimates of both the timing of protolith formation (sedimentation) and the timing of medium-grade regional metamorphism. 1342-937X/$ - see front matter D 2005 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.gr.2005.06.004 * Corresponding author. E-mail address: suzuki@nendai.nagoya-u.ac.jp (K. Suzuki). 1 Present address: National Institute of Polar Research, Kaga, Itabashi-ku, Tokyo 173-8515, Japan. Gondwana Research 9 (2006) 85 – 94 www.elsevier.com/locate/gr