Multi-stage metamorphic history of the Oki gneisses in Japan: Implications for Paleoproterozoic metamorphism and tectonic correlations in northeastern Asia

Metamorphic P-T conditions, monazite chemical ages and zircon U-Pb ages from the gneisses exposed in the Oki belt, Japan, were integrated to unravel the multi-stage metamorphic history of the belt. Microstructural observations combined with obtained P-T conditions and metamorphic ages reveal three distinct stages of metamorphism: M1, M2 and M3. The M1 stage occurred c. 1.85 Ga with high-T granulite-facies metamorphism (793-803 degrees C and 9.8-12.3 kbar and 738-755 degrees C and 9.1-12.0 kbar in the southwestern and southeastern Oki gneisses, respectively). The age of the M1 stage is well recorded in monazites included in large garnet porphyroblasts and low Th/U metamorphic rims in zircons from the Oki gneisses. The M1 metamorphism was overprinted by c. 230 Ma metamorphism (M2), which occurred at granulite-facies conditions (817-829 degrees C and 9.0-10.3 kbar) in the southwestern Oki gneisses and at upper amphibolite-facies conditions (693 degrees C and 5.3 kbar) in the southeastern Oki gneisses. Monazites in small garnets, euhedral zircons and outermost rims of zircons crystallized during this stage. The final metamorphism occurred as retrograde amphibolite-facies recrystallization (M3) at conditions of 558-638 degrees C and 3.7-4.8 kbar. The inherited cores in zircons yield ages from Paleoarchean to Paleoproterozoic but lack Palaeozoic ages. The detrital zircon distribution and the Paleoproterozoic metamorphic event in the Oki belt support the idea that the Oki gneisses are fragments of a Precambrian terrane rather than Palaeozoic sediments derived from the terrane. Combined with previous studies, we concluded that the c. 1.85 Ga M1 high-T granulite-facies metamorphism in the Oki belt could be related to that of the Jiao-Liao-Ji belt in the eastern North China block via the northern Gyeonggi and Nangrim Massifs on the Korean Peninsula, whereas the c. 250-230 Ma M2 stage could be associated with collision between the North and South China blocks. The Oki belt geologically corresponds to the northern Gyeonggi Massif in South Korea due to their similar Paleoproterozoic and Triassic tectonothermal events.