Mineralogy of some igneous rocks from the Kermadec Islands and Taupo volcanic zone, New Zealand

Abstract

The Kermadec volcanic arc and Taupo Volcanic Zone (TVZ) are related to a convergent section of the plate boundary between the Indian-Australian and Pacific plates. The Kermadec segment has developed on oceanic crust while the TVZ has developed on continental crust. The Kermadec volcanics are dominated by basalts and basaltic andesites which have tholeiitic affinities, with minor andesites, dacites and rhyolites. The mineral assemblages within all lavas are very similar and relatively simple. Calcic plagioclase (An97-49) is the dominant phenocryst in all but one lava. Olivine (Fo84-50) occurs in lavas with <53% SiO2 and is replaced by orthopyroxene (En75-46) in the more silicic lavas. Clinopyroxene (augite-ferroaugite) is typically the dominant pyroxene and pigeonitic and subcalcic pyroxenes have crystallised in some basaltic andesites. Magnetite appears as a phenocryst phase in the andesites and more silicic lavas. Hydrous phases are absent. In general, Fe/Fe+Mg ratios of olivines and pyroxenes and Usp contents in magnetite increase, while An contents of plagioclase decrease with increasing SiO2 of the host lava. The mineralogy indicates high liquidus temperatures (1350-1100°C) for the lavas with plagioclase ± olivine ± clinopyroxene crystallising through an interval of 50-200°C before being joined by orthopyroxene or pigeonite ± magnetite. Phenocrysts of all lavas equilibrated with their host liquids at low pressures (<6kb) under anhydrous or strongly water undersaturated conditions. Mixing calculations, together with mineralogy and chemistry indicate that both basaltic and basaltic andesite magmas were parental to the Kermadec lavas and that fractionation of phenocryst phases from these magmas could have produced most of the more silicic lavas. In contrast, the basalts, basaltic andesites, andesites and dacites in the TVZ are distinctly subordinate with respect to the vast outpourings of rhyolitic eruptives, and the mineral compositions and assemblages within the various lava types are more diverse than those in the Kermadec lavas. Four primary magma associations have been recognised; a high alumina basalt association, a 'normal' basaltic andesite-andesite association in which modal (plagioclase/Σferromagnesians) ratios are >1, a high-Mg basaltic andesite-andesite association in which magnesian olivines and pyroxenes are the dominant phases with plagioclase a subordinate phase, and a rhyolitic association. The three dacites studied are hybrids derived by mixing of basaltic andesite or andesite magma with rhyolitic magma. Magma mixing has also been operative in the derivation of some basaltic andesites. The mineral assemblages within the TVZ lavas indicate a greater influence of water during crystallisation, crystallisation of some magmas at higher pressures, and that processes of crystal accumulation and magma mixing have been operative within the arc. Mixing calculations demonstrated that the four magma associations are unrelated, but that members within each of the basaltic andesite-andesite associations could be related by crystal fractionation from a basaltic andesite parent. It is concluded that interaction of andesitic melt, derived by water-poor melting of subducted oceanic crust, with overlying mantle peridotite, followed by varying degrees of partial melting of this modified peridotite and subsequent fractionation of these magmas at crustal level could have produced the range of magma within the Kermadec arc. To account for the different magma associations in the TVZ the following processes of magma genesis are proposed; partial melting of mantle peridotite, possibly related to MORB-type spreading, for the basalts; melting of hydrous refractory peridotite, enriched in some incompatible elements by slab derived IRS fluids, for the high-Mg lavas; mixing of slab derived melt with mantle peridotite, followed by partial melting of the resultant hybrid and subsequent re-equilibration of these magmas with upper mantle or lower crust for the 'normal' basaltic andesites and andesites; and crustal fusion for the rhyolitic eruptives.