Geology, petrology, and petrogenesis of Little Barrier Island, Hauraki Gulf, New Zealand

Little Barrier Island is the emergent part of a large, isolated, dacite-rhy odacite volcano in the active Hauraki Rift, 80 km northeast of Auckland. Tw o volcanic episodes are recognised: Waimaomao Formation was emplaced as a r hyodacite dome at 3 Ma, whereas the more extensive dacitic lavas of Haowhen ua Formation were erupted between 1.2 and 1.6 Ma. All Little Barrier lavas are strongly porphyritic and contain phenocrysts of plagioclase, orthopyrox ene, and hornblende. Geochemically, they are subduction related and distinc t from the older lavas of the Coromandel Volcanic Group, being Zr rich but Rb and Ba poor. Their Sr and Nd isotope ratios are similar to those of the Tonga-Kermadec are volcanoes. Modelling of the dacite supports petrographic evidence that recharge and mixing were important in the magmatic system. L ittle Barrier and two dacite domes of similar age and composition near Whan garei form a northwest-trending lineament subparallel to the Alexandra Volc anics and the Vening Meinesz Fracture Zone.