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Boninite, first described by KIKUCHI (1888, 1889) and named by PETERSEN (1891) in Chichi-jima of the Bonin Islands, recently gained a profound petrogenetic significance after about eighty years missing. Its re-appraisal was accompanied by the discovery of some boninite localities in the western Pacific island arcs and ophiolites. Among them of particular interest in relation to ophiolites is clinoenstatite-bearing boninite discovered by SAMESHIMA et al.(1983) near Nepoui on the west coast of New Caledonia, because numerous, huge masses of ophiolites are exposed on New Caledonia.During a short trip to New Caledonia, we also found a new occurrence of boninite pillow lavas near Col dAmieu in the central part of the island. While the Népoui boninite is possibly a member of the “Formation des Basaltes” of late Cretaceous to Eocene age, the Col dAmieu boninite occurs in the weakly metamorphosed pre-Senonian rocks. Close inspection of the literature reveals that there are several other boninite localities associated with island arc tholeiites and/or mid-ocean ridge basalts in the New Caledonia ophiolites. In Népoui, an MgO- and SiO2-rich, hypabyssal rock crystallizing quartz and feldspar and a cumulate olivine orthopyroxenite were found together with the glassy clinoenstatite boninite breccias, accompanying a mylonitized serpentinite sheet. Both the hypabyssal rock and the clinoenstatite boninite are characterized by very low CaO content and Ca/Al ratio, suggesting that they have been derived from the same boninite magma. Proto-and ortho-pyroxenes from the Nepoui boninite are lower in Ca at a given ‘Mg’ value than those from the Bonin Island boninites, and the Népoui pigeonites and augites are less magnesian, indicating their later crystallization. In contrast, the Col dAmieu boninite has early formed augites with high ‘Mg’ and Cr, owing to its high bulk rock CaO content. The crystallization stage of pigeonite like augite appears to depend on CaO content in the host rock. Mg-rich augites from boninites are generally poor in Al, in agreement with the low Al/Si ratio of boninites, but the Al in augite increases rapidly with decreasing‘Mg’, which reflects the effective quenching and no crystallization of plagioclase in boninites.Chromite compositions of the two New Caledonia boninites are similar; both have very high Cr content and Cr/Al ratio, a feature characteristic of boninite chromites. The boninite primary magmas show a considerably wide range of CaO contents. The Col dAmieu boninite, as well as boninites with augite phenocrysts from the Bonins, represents the most CaO-rich end. The Ca/Al ratio in the boninite primary magmas decreases in order of Col dAmieu, Bonin, Papua, Sanukitoid and Népoui. In New Caledonia, various types of boninite, in terms of chemistry, mineralogy, texture, age and tectonic setting, may occur in close association with island arc tholeiites and midocean ridge basalts. New Caledonia would be the most favorable place for boninite studies.

ニューカレドニアの無人岩

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