Volcanic geology of Chichijima, the Bonin Islands ( Ogasawara Islands).

Abstract

Chichijima in the Bonin (Ogasawara) Islands consists mostly of submarine effusive rocks of Eocene age unconformably overlain by Oligocene to early Miocene foraminifer limestone (Minamizaki Formation). The volcanic rocks are divided into the Marubewan, Asahiyama and Mikazukiyama Formations in ascending order on the basis of both lithology and modes of occurrence. Radiolarian fossils found in the lower portion of Mikazukiayama Formation indicate early Middle Eocene, while a K-Ar age for the 2-pyroxene andesite is Middle Oligocene (29 Ma), and those for boninite and dacite are Middle Eocene (39-43 Ma). Two cases are considered. 1) Volcanism in Chichijima is not younger than the activities of island arc tholeiite and andesite in Hahajima. The fossil age suggests the existence of subduction before 43 Ma when the Pacific Plate moved towards NNW. 2) Volcanism of boninite was at 43 Ma and was contemporaneous with or slightly later than volcanism in Hahajima. These volcanisms were related to the subduction caused by the change in plate motion.-from Author