CHIME geochronology of granitic gneiss from Baekdong in the Hongseong area of the Gyeonggi Massif, South Korea

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CHIME dating was carried out on zircon and allanite in a sample of granitic gneiss from the Deokjeongri Granitic Gneiss, exposed at Baekdong (36°33.521'N, 126°44.131'E) in the Hongseong area, which lies in the southwestern Palt of the Gyeonggi Massif. Geochemically, the sample belongs to the high-K(calc-alkaline series or shoshonite series of A-type granites, and is typical of within-plate granitoids. The CHIME ages for concentrically-zoned zircon cores and the core of an exceptionally large allanite grain are 824±29 Ma and 825±110 Ma, respectively. Outer growth zones in zircon with low Y2O3 and margins with high Y2O3 yield poorly defined ages of ca. 500 Ma and 230 Ma, respectively. Allanite rims give ages which cluster along two isochrons, 438±99 Ma for low-Y2O3 domains in equilibrium with Ca-rich garnet, and 254±30 Ma for high Y2O3 domains that formed during the breakdown of garnet. The present sample differs from granitic gneisses in the same lithological unit from other sites, in terms of both ages and geochemistry. Such diversity suggests that the Deokjeongri Granitic Gneiss is a composite unit, comprised of a variety of protoliths from differing tectonic environments and with differing tectonometamorphic histories.

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