The Tokunoshima Belt: A New Geotectonic Unit in Central Ryukyus, Japan

The latest geological and geotectonic aspects of Tokunoshima Island in Central Ryukyus are reviewed. The pre-Quaternary basement geology of the island is two-fold; i.e., the Cretaceous accretionary complex (Amagidake and Omo units) and the high-grade metamorphic unit newly named the Inokawadake metamorphic complex (IMC). The former is correlated with the northern subbelt of the Shimanto belt in SW Japan. The IMC is composed of high-grade metamorphic rocks composed of pelitic–psammitic schists, with minor lenses of serpentinite and “dioritic gneiss” with Paleoproterozoic (ca. 1.8 Ga) zircons. In the southern half of the island, the IMC structurally overlies the Cretaceous AC as a klippe, which is separated probably by a subhorizontal fault. The IMC suffered from high-grade metamorphism up to the amphibolite facies, which is extremely rare in the Shimanto belt. A new geotectonic unit, the Tokunoshima belt, is newly proposed for the area of the IMC, which is limited to a mountainous domain in southern Tokunoshima Island. Based on the youngest detrital zircon U–Pb ages (ca. 60 Ma) from psammitic schists, the metamorphism of the IMC probably occurred in the Paleocene or later. Possible sites for the Paleocene or later heat source may include the San-in (Paleogene batholith) belt in SW Japan on the north and/or an unknown collided island arc system from the south.