A U-Pb geochronological study of migmatitic gneiss in the Busan gneiss complex, Gyeonggi massif, Korea

Zircon U-Pb geochronology was applied to a migmatitic gneiss in the Busan gneiss complex, Gyeonggi massif, Korea. Pb-207/Pb-206 ages of concordant zircon grains reveal that overgrowth rims with Th/U ratios lower than 0.1 formed at 1937 +/- 6 Ma during the migmatization and mantled the inherited cores of ca. 3500-2200 Ma. The (207)Pn/Pb-206 age of the rims is consistent with the monazite Th-total Ph age (ca. 1938 Ma), suggesting that the rims are the product of Paleoproterozoic regional metamorphism. Our zircon age data are not sufficient for discerning the tectonic affinity of the Busan gneiss complex belonging to either Gyeonggi or Yeongnam massif. However, consistency of these ages with the ca. 1930 Ma Pb-Pb age reported from the Bagdalrycong gneiss complex, Gyeonggi massif, supports the tectonic model that the Busan gneiss complex is a part of the Gyconggi massif. Discordant U-Pb data obtained from the zircon rims (293 +/- 31 Ma) and the monazite Th-total Ph age (ca. 289 Ma) suggest that the Busan gneiss complex was affected by the Early Permian regional metamorphism during the Ogeheon (Okcheon) Orogeny. Therefore, we conclude that the Busan gneiss complex is a part of the Gyconggi massif occurring as either a basement rock of the Ogcheon (Okcheon) metamorphic rocks, or a tectonically emplaced, exotic piece affected by the Early Permian Ogcheon (Okcheon) metamorphism. In addition, our U-Pb data suggest that the Busan complex is possibly linked with the Hida-Oki terrane in Japanese Islands.