U−Pb zircon ages of a granitic gneiss boulder in metadiamictite from the Ogcheon metamorphic belt, Korea

Zircon grains separated from a granitic gneiss boulder of the Hwanggangri Formation, Ogcheon metamorphic belt, were analyzed using a SHRIMP II ion microprobe. This gravel-bearing formation consists primarily of greenschist to amphibolite facies metadiamictites. U−Pb zircon ages obtained from 31 grains provide not only diachronous inheritance ages ranging up to ca. 2.5 Ga but also define an apparent discordia with upper and lower intercept ages of ca. 1.87 Ga and ca. 290±40 Ma, respectively. The former age is consistent with the Paleoproterozoic age of granu-lite-facies metamorphism and granitic magmatism in the Gyeonggi and Yeongnam massifs. However, the latter age has not been reported from the basement gneisses and may be attributable to significant Pb loss during the Late Carboniferous Ogcheon orogeny recently dated at ca. 290–280 Ma. Alternatively, multiple Pb loss episodes including a ca. 400 Ma thermal disturbance may account for the array of discordant data. The tectonic implications for each hypothesis are enormous for deciphering the evolution of the Ogcheon belt, but precise interpretation awaits further geochronologic analyses.