The Neoproterozoic volcano-sedimentary unit, Gapyeong Formation (GF), is found in the Gapyeong area which locates in the Hongseong-Yangpyeong-Odesan collision belt, the Permo- Triassic collision boundary in the Gyeonggi Massif, South Korea. The Gapyeong Formation mainly consists of the amphibolite which is interlayered with meta-calcsilicate, marble, biotite gneiss and meta-trachyandesite and has serpentinized ultramafic boudin. The GF contacts with the Yongmunsan Group (YG) by shear zone and the YG unconformably overlies the Jangrag Gruop (JG). The YG mainly consists of the migmatitc gneiss and hornblende-biotite gneiss and the JG mainly composes of the Jangrag quartzite and migmatitic biotite gneiss. The meta-trachyandesite in the GF gives igneous age of the 854 Ma and the whole rock geochemistries indicate that the interlayered meta-trachyandesite and amphibolite were originally formed in within plate tectonic setting. These data suggest that the meta-trachyandesite and amphibolite in the GF were trachyandesite and basalt which formed by bimodal volcanism in the continental rift tectonic setting at ca. 854 Ma which is matched with the timing of the break up of the Rodinia supercontinent. The interlayered meta-calcsilicate, marble, biotite gneiss in the GF were originally the calc-silicate sediment, limestone and shale which deposited during the Neoproterozoic bimodal volcanism. Whereas the serpentinite is found to be formed in the supra-subduction zone indicating that they were incorporated into the GF by tectonic process.
The amphibolite and biotite gneiss in the GF underwent upper granulite facies metamorphism (918-855℃, 12.8-10.9kbar) at ca. 250 Ma during the collision between the North and South China Cratons. Whereas the hornblende-biotite gneiss in the YG was metamorphosed under lower granulite facies (767-725℃, 8.4-7 kbar) at ca. 414 Ma and the migmatitic biotite gneiss in the JG was metamorphosed at ca. 380 Ma. Considering that the GF shows strong mylonitic texture without the middle Paleozoic metamorphic age, it can be interpreted that the GF formed in the rifting tectonic setting during the Neoproterozoic and metamorphosed during the Permo-Triassic collision between the North and South China Cratons and then tectonically transported to the present area contacting with the YG by shear zone.