Evidence for high‐Ca boninite magmatism from Paleogene primitive low‐K tholeiite, Mukoojima, Hahajima Island group, southern Bonin (Ogasawara) forearc, Japan

Abstract

Abstract High‐Ca boninitic inclusions are found in primitive low‐K tholeiite from Mukoojima (Mukoo‐Jima), an islet in the Hahajima Island group, Bonin (Ogasawara) forearc, Japan. While Chichijima Island group, 50 km north of Hahajima Island group, is well known as a type locality of boninite, there has been no report of boninitic rocks from the Hahajima Island group. The high‐Ca boninitic inclusions are aphanitic and contain olivine, Ca‐rich clinopyroxene, plagioclase, chromian spinel, opaque minerals and dark brown glass. The mode of occurrence of the inclusions and host tholeiite under the microscope indicates mingling of these two magmas, suggesting intimate association in space and time of the boninite and primitive tholeiite magmas around the Hahajima Island group in Paleogene time. Primitive compositions and slightly different Sr and Nd isotopic ratios suggest that these two magmas are derived from two distinct mantle sources. These two mantle sources were present at the same time around the Hahajima Island group, southern Bonin forearc. The source of the high‐Ca boninite was higher in water content and/or shallower in depth compared to that of the primitive tholeiite.