Neoproterozoic igneous activity and Permo-Triassic metamorphism inthe Gapyeong area within the Gyeonggi Massif, South Korea, and theirimplication for the tectonics of northeastern Asia

Although Neoproterozoic rocks were recently found in the western margin of the Gyeonggi Massif, no Neoproterozoic rocks were believed to be present within the interior of this massif. However, in this study, Neoproterozoic rocks are recognized for the first time within the interior of the Gyeonggi Massif through the discovery of the Neoproterozoic volcano-sedimentary rocks of the Gapyeong Formation (GF). The GF is located in the Gapyeong area within the northern Gyeonggi Massif to the north of the Permo-TriassicHongseong-Odesan continental collision boundary in South Korea. The GF mainly consists of mafic granulite interlayered with metacalcsilicate, marble, biotite gneiss, and metatrachyandesite and contains a serpentinized ultramafic block and hornblende biotite gneiss slab. The ages and whole-rock geochemistry indicate that the interlayered metatrachyandesite and mafic granulite in the GF were trachyandesite and basalt that formed through bimodal volcanism in a continental rift tectonic setting at ca. 854 Ma, which matches the timing of the Rodinia supercontinent breakup. The GF can be correlated to the 900–850 Ma Neoproterozoic rifting sequences in the southern Nangrim and northern Gyeonggi Massifs in the Korean Peninsula, which can be correlated to the 930–830 Ma Neoproterozoic rifting sequence along the southern margin of the North China Craton. The GF is difficult to correlate with the ca. 890–790 Ma subduction-related Neoproterozoic rocks in the Hongseong area in the southwestern Gyeonggi Massif, which are considered equivalent to rocks in the northern margin of the South China Craton. The GF underwent upper granulite-facies metamorphism (816–882 °C, 9.0–10.9 kbar) at ca. 250 Ma during the collision between the North and South China Cratons. The serpentinite block in the GF formed in a suprasubduction zone, and the hornblende biotite gneiss slab in the GF was metamorphosed under lower granulite-facies conditions (725–767 °C, 7.0–8.4 kbar) at ca. 414 Ma, indicating that the block and slab were tectonically incorporated into the GF. A strong mylonitic texture is observed in the GF but not in the basement Paleoproterozoic gneiss except at its boundary contact with the GF. These data indicate that the GF, together with the serpentinite block and hornblende biotite gneiss slab, was tectonically transported to the present area after the Triassic metamorphism.