Cretaceous and Paleogene Granitoids of the Kashevarov Bank and Origin of the Basement in the Northern Sea of Okhotsk

Cretaceous and Paleogene magmatic rocks consisting of calc-alkaline diorite–granite and andesite–rhyolite suites were dredged from the Kashevarov Bank. Among them, Cretaceous adakite granitoids and Late Cretaceous–Paleocene and Eocene adakite volcanics were identified by high Sr/Y ratios with low concentrations of Y and Yb. The igneous complexes of the Kashevarov Bank and other submarine rises in the northern Sea of Okhotsk are similar to coeval complexes of the Sikhote-Alin and northern Priokhotye or western Kamchatka, where granitoid emplacement is considered as having resulted from island arc collisions in the Albian–Cenomanian or the Early Eocene, respectively. It is suggested that the formation of adakitic granitoids is associated with destruction of the slab and upwelling of subslab asthenosphere. There is a boundary between the Albian–Cenomanian and Eocene orogenic belts and consequently different-aged blocks of continental crust in the area of the Kashevarov Bank. The Eocene collision zone can be traced from the Malkino Rise of western Kamchatka through the Kashevarov Bank and central Sakhalin to central Hokkaido. Island arc collisions—Early Eocene in Kamchatka and Middle Eocene on Sakhalin and Hokkaido—culminated in the formation of the continental crust that existed before the formation of the Sea of Okhotsk. We present a new tectonic zoning scheme of the Sea of Okhotsk basement and reconstructions of the continental margin before the opening of the Sea of Okhotsk.