小笠原群島父島に産する高Ｃａおよび低Ｃａボニナイト

Abstract

Chichijima, Bonin archipelago, is a Tertiary volcanic island approximately 1000 km south-south east of Tokyo, and has been known as the type locality of boninite. Two types of boninite are found in the Bonin archipelago: one is high-Ca boninite from the Mikazukiyama Formation (Mk-HCB) and the Marubewan Formation (Mr-HCB) in Chichijima, and another is low-Ca boninite from the Marubewan Formation (Mr-LCB), Anijima (Ani-LCB), and Mukojima (Muko-LCB). High-Ca boninite is distinguished by CaO content from dominant low-Ca boninite, and contains augite phenocrysts. The bulk MgO, Ni and Cr contents and the magnesian compositions of olivines and pyroxenes in Mk-HCB and Mr-LCB indicate their primitive nature. Bronzite phenocrysts in Mk-HCB and Mr-HCB are highly enriched in Wo molecule, implying existence of high-Ca primitive boninite magmas. Mr-HCB is a mixing product between a high-Ca boninite magma and a differentiate from Mr-LCB. Sr isotopic ratios and incompatible element ratios of Mr-HCB, Mr-LCB, Ani-LCB and Muko-LCB are nearly the same and therefore they could have been produced from the common source. In contrast, Mk-HCB has different Sr isotopic and incompatible element ratios from the others and then it may have been produced from different parental materials. Both REE model calculations and Cr-spinel compositions imply that the parental mantle of the Mr-LCB is more depleted than that of the Mk-HCB. The large enrichment of LILE and LREE in Mr-LCB compared to Mk-HCB suggests that Mr-LCB has been generated under more hydrous condition than Mk-HCB.