Abstract

The lava consists mainly of clinoenstatite phenocrysts (29%), sometimes enclosing euhedral chromite, a lesser amount of microphenocryst bronzite (20%), and a glassy mesostasis. Clinoenstatite phenocrysts are compositionally homogeneous (En 89 Fs (sub 10.5) Wo (sub 0.5) ) with polysynthetic twins on (100), a feature which is believed to indicate inversion from protoenstatite. Bronzite is variable in composition with En (sub 74-84) Fs (sub 14-22) Wo (sub 1.5-4) . Hypersthene (En (sub 43-61) Fs (sub 36-52) Wo (sub 3-4) ), calcic and subcalcic augite and iron-rich hornblende occur as overgrowths on phenocrysts and as quench microlites. The New Caledonian clinoenstatite boninite is similar chemically, mineralogically, texturally and in terms of age (Eocene-Oligocene) and tectonic setting (with ophiolite complexes) to the three other known clinoenstatite boninite localities (Cape Vogel, Papua New Guinea, Bonin Islands and Mariana Trench). Clinoenstatite boninite magmas are believed to result from extensive partial melting of depleted mantle peridotite under hydrous conditions and an abnormally high geothermal gradient.--Modified journal abstract.

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