Little Barrier Volcano: geology and geochemistry

Abstract

Little Barrier Island is a large (10km3) dacite volcano situated in the Hauraki Rift, 80km NE of Auckland. U-Pb and K-Ar age determinations reveal two episodes of volcanism at 1.5 and 3Ma. These produced two petrographically and geochemically distinct lavas: the Haowhenua Dacite (1.5Ma) and the Waimaomao Rhyodacite (3Ma). These lavas are overlain by a series of predominantly debris flow-derived breccia units, collectively termed the Haowhenua Breccia. Haowhenua Dacites are porphyritic, with phenocrysts of plagioclase (An20-60); orthopyroxene (En51-72Fs25-23Wo1-8); resorbed hornblende; clinopyroxene (En36-44Fs14-27Wo37-43); titanomagnetite ± quartz, and accessory ilmenite, zircon and apatite. They contain evidence of disequilibrium in the form of sieve-textured plagioclase megacrysts. The Haowhenua Dacites belong to the medium-K calc-alkaline suite, and their geochemistry is characteristic of convergent margin volcanism. Modelling by Perfect Fractional Crystallisation shows that the observed variation in major and trace elements can be explained by 59% crystallisation of the least evolved Haowhenua Dacite, with an extract assemblage consisting of 71% plagioclase; 12% quartz; 9% orthopyroxene; 3% titanomagnetite; 3% clinopyroxene; 1% apatite and 1% ilmenite, although quartz is not a major phenocryst phase. Waimaomao Rhyodacites are petrographically similar to the Haowhenua Dacites, except ferromagnesian minerals are generally present as opaque pseudomorphs. Waimaomao lavas are medium-K calc-alkaline rhyodacites, and have higher LILE and lower Zr concentrations than the Haowhenua Dacites. Little Barrier lavas have petrographic and geochemical characteristics that are not consistent with derivation by magma mixing, or partial melting of either the Waipapa Terrane basement or the subducted slab. They could be crustal melts, in which case their geochemistry requires a source rock high in HFSEs and low in LILEs, which appears to be inconsistent with the absence of inherited zircon. Little Barrier Volcano does not have the characteristics of continental rift zone magmatism: like other volcanic material in the Hauraki Rift (Kiwitahi and Waitoa Volcanics, Anchorite Rock) it is subduction-related, which suggests that the rift is amagmatic. Little Barrier lavas are depleted in LILEs and enriched in HFSEs relative to other North Island volcanic centres, except for three young dacite domes in Northland. These centres (the Hikurangi, Parakiore and Opuawhanga domes) and Little Barrier Island form a volcanic association that is geochemically and temporally distinct from the Coromandel and Northland arc-type associations, and that may be related to volcanism on the Colville Ridge.