Permian Arc Cumulates and Adakitic Magmatism in the Oura Plutonic Complex, Southwest Japan: Implications for Ridge Subduction Along the Proto-East Asian Continental Margin

Two Paleozoic ophiolites, the Ordovician Oeyama and Permian Yakuno ophiolites, juxtaposed in the inner zone of southwestern Japan, record the tectonic evolution of the proto-East Asian continental margin during the Paleozoic. The Oura plutonic complex, located in the northern part of Maizuru City, Kyoto Prefecture, consists mainly of mafic to ultramafic rocks with minor amounts of intermediate intrusive rocks. Despite its key position for understanding the geotectonic framework of the Inner Zone of southwestern Japan, the geological attribution of the Oura plutonic complex has remained controversial. Zircon U–Pb dating of metagabbro and metadiorite from the Oura plutonic complex yields Permian ages of 273 and 292–254 Ma, respectively, indicating an affinity with the Permian Yakuno ophiolite of the Maizuru belt. Whole-rock and mineral geochemical data from metagabbro and ultramafic rocks indicate formation in an oceanic island arc setting, suggesting that the Oura plutonic complex represents lower crustal cumulates beneath a Permian island arc. The metadiorite is characterized by high Sr/Y and La/Yb ratios, typical of adakitic magmas derived from the melting of subducted oceanic crust. The Oura metadiorite is therefore interpreted to have originated from adakitic magma, most likely generated during backarc spreading. Late Paleozoic adakitic rocks are distributed not only in the Maizuru belt but also in northeast Japan and the Khanka–Jiamusi massif, and plausibly provide crucial clues to deciphering the tectonic evolution along the Permian proto-East Asian continental margin.