Mineralogy, Chemistry, and Genesis of the Boninite Series Volcanics, Chichijima, Bonin Islands, Japan

Abstract

The Bonin archipelago represents an uplifted fore-arc terrain which exposes the products of Eocene supra-subduction zone magmatism. Chichijima, at the centre of the chain, represents the type locality for the high-Mg andesitic lava termed boninite. The range of extrusives which constitute the boninite series volcanics are present on Chichijima, and are disposed in the sequence boninite-andesite-dacite with increasing height in the volcano-stratigraphy. Progression to evolved compositions within the Chichijima boninite series is controlled by crystal fractionation from a boninite parental magma containing ? 15% MgO. Olivine and clinoenstatite are the initial liquidus phases, but extraction of enstatitic orthopyroxene, followed by clinopyroxene and plagioclase, is responsible for the general evolution from boninite, through andesite, to dacite. Some andesites within the overlying Mikazukiyama Formation are petrographically distinct from the main boninite series in containing magnetite phenocrysts and a high proportion of plagioclase. As such, these andesites have affinities with the calc-alkaline series. Major and trace element data for 74 boninitic series rocks from Chichijima are presented. Although major element variation is dominantly controlled by high-level crystal fractionation, the large variations in incompatiable trace element concentrations at high MgO compositions cannot be explained by this mechanism. Nd, Pb, and Sr isotopic data indicate the following: (1) a strong overprint on 87Sr/86Sr by seawater alteration; (2) Pb isotopes lie above the northern hemisphere reference line (NHRL) and are thus similar to the <30-Ma are and basin lavas of the Izu—Bonin system, and (3) ?Nd(40 Ma) ranges between 2.8 and 6.8 within the boninite series volcanics. Differences in rare-earth elements (REE), Zr, Ti, and 143Nd/144Nd at similar degrees of fractionation can be explained by the addition of a component of fixed composition from the down-going oceanic crustal slab to a variably depleted source region within the overlying wedge. Data presented for Sm/Zr and Ti/Zr indicate that boninite series volcanics are characterized by low values for both of these ratios. In particular, boninites appear to have uniquely low Sm/Zr ratios. These characteristics may be the result of slab melting in the presence of residual amphibole; the resultant melt could combine with typical slab dehydration fluids and infiltrate the overlying mantle wedge. Such a fluid—melt component could mix either with shallow mantle or directly with primitive melts from depleted mantle. Trace elements, REE, and isotope data thus point to a model for boninite genesis which requires tightly constrained pressure—temperature conditions in the slab combined with melting of a variably depleted source in the overlying wedge. Such constraints are rarely met except during the subduction of juvenile oceanic crust beneath a young, hot overriding plate.