Andesites of the Tongariro volcanic centre, North Island, New Zealand

Abstract

The Tongariro volcanic centre comprises four major andesite massifs, Kakaramea, Pihanga, Tongariro and Ruapehu, and four smaller cones and flows, Maungakatote, Pukeonake, Hauhungatahi and Ohakune. Older lavas from Kakaramea, Pihanga and Tongariro were erupted from a series of vents aligned NW-SE and more recent lavas from vents aligned NNE-SSW. The most voluminous lava types in the centre are labradorite and labradorite-pyroxene andesite containing phenocrysts of plagioclase (An 68—An 48), orthopyroxene (mainly bronzite) and clinopyroxene, in a fine to medium-grained groundmass. Smaller amounts of pyroxene andesite (from Pihanga), olivine andesite (from NNE-trending vents), and hornblende andesite also occur. Pigeonite surrounds orthopyroxene in some of the pyroxene andesites. Average major and trace element compositions of each petrographic type indicate that all lavas are “normal” calc-alkaline andesites and low-Si andesites. Low-Si andesites, represented mainly by pyroxene and olivine andesites, are typified by higher FeO, MnO, MgO, Sr, Cu, Ni, V, Sc and Cr and lower Al 2O 3, Na 2O, K 2O, Rb, Ba, Zr and Pb than “normal” andesites. Some low-Si andesites are regarded as phenocryst-rich cumulates. The location of the Tongariro andesites and alignments of eruptive vents suggest a source from a subduction zone underlying the area. However, the lavas differ chemically from island arc andesites such as those of Tonga by having higher Na 2O, K 2O, Rb, Ba and Zr. This suggests some crustal contamination, but the overall chemistry of the andesites is inconsistent with an origin by mixing, en route to the surface, of high-alumina basalt with either Mesozoic greywacke-argillite or with a partial melt of the greywacke-argillite equivalent in composition to Taupo rhyolite. A model is presented in which oceanic crust assimilates Mesozoic and Cenozoic sediments at the eastern side of the North Island, and is subducted to produce amphibolite. This amphibolite would subsequently break down, hydrating the overlying upper mantle and lower crust, and, in steeply dipping subduction zones such as that under Tongariro, would produce phlogopite eclogite below 90 km. The latter will partially melt at 150–200 km and the resultant magma will fractionate in the upper mantle or lower crust to produce andesite.