Petrogenesis and Tectonic Implication of the Newly Identified Rhyacian and Orosirian Orthogneisses in the Northwestern Part of the Gyeonggi Massif (Soyeonpyeongdo and Ganghwado Areas), South Korea: Constrained From Whole-Rock Geochemistry and Zircon U-Pb-Hf Isotopic Compositions

Abstract

The Gyeonggi Massif is one of the major Precambrian basements in the Korean Peninsula where Paleoproterozoic orthogneiss and paragneiss were widely exposed. In this study, newly identified Rhyacian to Orosirian meta-igneous rocks from the northwestern Gyeonggi Massif (Soyeonpyeongdo and Ganghwado areas) were investigated to elucidate their petrogenesis and tectonic implications based on zircon U–Pb–Hf isotopic data and whole-rock geochemistry. The garnet-bearing amphibole granite gneiss and biotite granite gneiss in the Soyeonpyeongdo area emplaced at ca. 2.06 Ga and 1.88 Ga, respectively, whereas the tonalite gneiss in the Ganghwado area formed at ca. 1.93 Ga. The 2.06 Ga garnet-amphibole granite gneiss shows both I-type and A₂-type features enriched in LILE and depleted in Nb–Ta, indicating generation in an arc-related extensional regime. The 1.93 Ga tonalitic gneiss exhibits adakitic characteristics, negative εHf(t) values (−8.2 to −4.9), and thickened crustal signatures (42–74 km), which are consistent with derivation from Archean mafic lower crust thickened during a regional syn-collisional stage. The 1.88 Ga biotite granite gneiss displays metaluminous to weakly peraluminous I- and A2-type features with slightly negative εHf(t) values (−1.7 to +0.6), implying formation in a post-collisional extensional setting. These successive magmatic events (2.06–1.85 Ga) in the Gyeonggi Massif are temporally and geochemically comparable to the Rhyacian–Orosirian magmatism in the Liao–Ji Belt of the eastern North China Craton. Therefore, the northwestern Gyeonggi Massif recorded a Paleoproterozoic tectono-magmatic evolution broadly comparable to that of the Liao–Ji Belt.