The oldest (ca. 2.51 Ga) rock in South Korea: U-Pb zircon age of a tonalitic migmatite, Daeijak Island, western Gyeonggi massif

Abstract: We discovered the oldest rock in South Korea, and dated this migmatite using a Sensitive High-Resolution Ion Micro-probe (SHRIMP). Zircon grains from a tonalitic migmatite, Daeijak Island, western Gyeonggi massif, are distinctly zoned and commonly characterized by oscillatory-zoned cores and structureless rims. The U-Pb zircon ages estimated from six spot analyses of the rims in five grains define a concordant age group with the weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 2508 ± 18 Ma, interpreted as the age of migmatization. On the other hand, the cores yield an older ²⁰⁷Pb/²⁰⁶Pb age of ca. 2.58 Ga, interpreted as the crystallization age of tonalitic protolith. These results demonstrate not only the presence of Neoarchean granitoids but also high-grade migmatitic metamorphism at ca. 2.51 Ga in the Gyeonggi massif, which are correlative with those reported from the North China craton.