Metamorphic history of mafic granulite and garnet-bearing amphibolite from the Oki-Dogo Island, Japan

Oki metamorphic rocks have long been considered as a constituent of the Hida Belt based on their geographic proximity, lithology, and Permo-Triassic metamorphism. However, recent geochronological studies have demonstrated that both para- and ortho-gneisses in the Oki-Dogo Island display Paleoproterozoic protolith formation and two separate phases of metamorphism at 1.85 Ga and 250–230 Ma. Consequently, the Oki metamorphic rocks are closely connected to the Paleoproterozoic massifs in the Korean Peninsula, although little is known about their pressure (P)–temperature (T) history. Here, we provide petrological data on mafic metamorphic rocks in the Oki-Dogo Island. The mafic lithologies are classified into mafic granulite, amphibolitized granulite and amphibolite. In addition, we first discover a garnet-bearing variety of amphibolite from the Oki-Dogo Island. The texture and composition of Ca amphibole suggest these rock types share a common P–T history but the dominant mineral assemblage in each rock type records different stages of metamorphism. The inferred P–T history includes two distinct events. The first event includes a low-P granulite facies stage (~900°C, 0.7–0.8 GPa) and subsequent amphibolite facies retrogression. This event is linked to the continuous compositional change of Ca amphibole from Ti-rich pargasite to hornblende/actinolite. The second event is prograde amphibolite facies metamorphism, which is associated with the formation of tschermakitic hornblende and calcic plagioclase. In high Fe/(Mg + Fe) rocks, garnet was formed at ~550–580°C, 0.45–0.50 GPa in this stage. Depending on the age of the first event, the low-P granulite facies metamorphism is likely to have occurred in a similar tectonic setting as the Paleoproterozoic crustal metamorphism in the Yeongnam Massif or the Permo-Triassic ultrahigh-T metamorphism in the northern Gyeonggi Massif.