MINERALOGY, CHEMISTRY, AND GENESIS OF THE BONINITE SERIES VOLCANICS, CHICHIJIMA, BONIN-ISLANDS, JAPAN

The Bonin archipelago represents an uplifted fore-arc terrain which ex poses the products of Eocene supra-subduction zone magmatism. Chichiji ma, at the centre of the chain, represents the type locality for the h igh-Mg andesitic lava termed boninite. The range of extrusives which c onstitute 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 compositio ns within the Chichijima boninite series is controlled by crystal frac tionation from a boninite parental magma containing approximately 15% MgO. Olivine and clinoenstatite are the initial liquidus phases, but e xtraction of enstatitic orthopyroxene, followed by clinopyroxene and p lagioclase, is responsible for the general evolution from boninite, th rough andesite. to dacite. Some andesites within the overlying Mikazuk iyama Formation are petrographically distinct from the main boninite s eries in containing magnetite phenocrysts and a high proportion of pla gioclase. As such, these andesites have affinities with the calc-alkal ine series. Major and trace element data for 74 boninitic series rocks from Chichijima are presented. Although major element variation is do minantly controlled by high-level crystal fractionation, the large var iations in incompatible trace element concentrations at high MgO compo sitions cannot be explained by this mechanism. Nd, Pb, and Sr isotopic data indicate the following: (1) a strong overprint on Sr-87/Sr-86 by seawater alteration; (2) Pb isotopes lie above the northern hemispher e reference line (NHRL) and are thus similar to the <30-Ma arc and bas in lavas of the Izu-Bonin system, and (3) epsilon(Nd)(40 Ma) ranges be tween 2.8 and 6.8 within the boninite series volcanics. Differences in rare-earth elements (REE), Zr, Ti, and Nd-143/Nd-144 at similar degre es of fractionation can be explained by the addition of a component of fixed composition from the down-going oceanic crustal slab to a varia bly depleted source region within the overlying wedge. Data presented for Sm/Zr and Ti/Zr indicate that boninite series volcanics are charac terized by low values for both of these ratios. In particular, boninit es 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 poin t to a model for boninite genesis which requires tightly constrained p ressure-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 bene ath a young, hot overriding plate.