Mineralogy and Geochemistry of the Younger Volcanic Islands of Tonga, S.W. Pacific

Abstract

The Tongan Islands comprise a double island chain, and lie at the northern end of an active island arc extending NNE from northern New Zealand. The eastern chain are limestone covered, with an underlying pre-Upper Eocene gabbro-basalt-andesite series exposed on one island. The active volcanoes, the subject of this paper, form the western island chain, and lie some 100 km above a westward dipping seismic zone. Basaltic andesite dominates the Islands of Hunga Ha'apai and Late (and also Tofua and Kao, which are not described in this paper). Dacite is the main lava of Fonualei and the 1967-68 Metis Shoal eruption. Acid andesites occur only sporadically.Chemically, the lavas are moderately iron-enriched, with relatively low concentrations of K, Rb, Ba, Zr, REE, Pb, and U. K/Rb ratios are 500-700. The lavas are not typically 'calcalkaline'. Phenocryst minerals (normally poorly zoned) are dominated by bytownite with subordinate orthopyroxene, augite, and sometimes pigeonite in the basaltic andesites. Titanomagnetite occurs only in the more silicic andesites and dacites. The pyroxenes show a limited iron enrichment through the series. Coexisting pyroxenes exhibit equilibrium distributions of Mg, Mn, V, Sc, and Ni. Pyroxene partition coefficients for V, Sc. Mn, and Co increase with increasing iron enrichment. Groundmass minerals include intermediate plagioclase, pigeonite, hypersthene, magnetite, plus potash feldspar and quartz in the dacites. The Metis Shoal lava consists of rhyolitic glass with xenocrysts of forsteritic olivine and enstatite and phenocrysts of bytownite, hypersthene, augite, and titanomagnetite.The basaltic andesites are interpreted to be the product of direct peridotitic fusion within the mantle, presumably in the region overlying the seismic zone, and modified by olivine fractionation during ascent. Least squares numerical calculations, using major and trace element data, support the derivation of the more siliceous lava compositions by low pressure crystal fractionation of basaltic andesite Sr/Sr ratios exhibit a small range of variation (0.7036-0.7043), possibly indicative (except for the Metis Shoal lava) of minor limestone (?) contamination within the more silicic lavas during fractionation.