SHRIMP U-Pb zircon ages of Jigokri migmatitic gneisses at Garorim bay, Southwestern Gyeonggi massif

We report zircon SHRIMP U-Pb ages for Jigokri migmatitic gneisses at Garorim Bay, southwestern Gyeonggi massif. The age data can be subdivided into four groups based on the internal structures and Th/U ratios of the zircons. The Group 1 and Group 2 are inherited cores of the zircons. They have high Th/U ratios (0.11-1.62) with well-developed zoning (such as, oscillatory, concentric, and banded zoning), indicating its igneous origin. The upper intercept ages on the Tera-Wasserburg concordia diagrams for the Group 1 zircons are 2502 +/- 92 Ma and 2528 +/- 100 Ma, whereas that of the Group 2 zircons is 2051 +/- 25 Ma. The Groups 1 and 2 ages are considered to represent the timing of magmatism in this area. The Group 3 zircons are characterized by unzoned or faintly zoned internal structure, and have relatively low Th/U ratios (0.01-0.16). The Group 3 zircons yield an upper intercept age of 1833 +/- 10 Ma, which is interpreted as the age of metamorphic recrystallization. Lastly, the Group 4 zircons have a relatively high Th/U ratio (0.44-1.43), and yield an upper intercept age of 1695.5 +/- 5.4 Ma, representing possibly the timing of high-temperature metamorphism accompanied by the breakdown of monazite. The Groups 1 and 3 ages are consistent with the timing of the regional igneous and metamorphic activities (ca. 2.5 and 2.0-1.8 Ga) in the Gyeonggi massif. Especially, the Group 1 age coincides with the oldest Neoarchean formation age (ca. 2.5 Ga) of tonalitic migmatite at Daeijak Island, western Gyeonggi massif. However, ca. 2.1 and 1.7 Ga ages of the Group 2 and Group 4 zircons have not been reported in previous studies. The newly reported ages indicate that studies on locally generated Precambrian thermal activity in the Gyeonggi massif are still lacking. To better understand Precambrian crustal evolution of the Korean Peninsula, it means that additional precise geochronological and petrological data must be collected in the future.