Distribution, petrography, and chemistry of Kiwitahi and Maungatautari volcanics, North Island, New Zealand

Abstract

The ‘Kiwitahi Volcanics’ form a chain on the west side of the Hauraki Depression. Individual cones and flows occur at Waiheke Island, Miranda, Pukekarnaka, Tahuna, Maungatapu, Kiwitahi, Maungakawa, and Te Tapui. Maungatautari is a volcanic complex 8 km SE of Cambridge which is surrounded by a lahar ‘ring-plain’. The lahars overlie ignimbrites of the Pakaumanu Group, but because they probably represent slope wash from Maungatautari formed soon after deposition of the ignimbrite, they cannot be used to date activity within the Complex. Petrographically, all lavas are porphyritic, the Kiwitahi Volcanics comprising olivine low-Si andesite, pyroxene low-Si andesite, labradorite-pyroxene andesite and low-Si andesite, and hornblende andesite. Maungatautari lavas comprise olivine basalt, labradorite andesite, and hornblende andesite. Major and trace element analyses of the Kiwitahi Volcanics and Maungatautari Volcanic Complex differ, Maungatautari lavas having higher K,O, Zr, V, and Sc than Kiwitahi Volcanics. The Koo value of Kiwitahi lavas is 1'10 whereas that of Maungatautari is 2.05. It is suggested that Maungatautari lavas represent a fractionation series, while some of the Kiwitahi Volcanics are probably cumulates. On the basis of distribution and chemistry the Kiwitahi Volcanics are considered part of the Hauraki Volcanic Region and to have originated by partial melting of oceanic crust which has assimilated small amounts of sediment and been subducted to a depth of 150–200 km. Maungatautari lavas are thought more likely to have resulted from partial melting of the subduction zone at a depth of 200 km. This would account for their higher K,O content, and, because of the greater thickness of mantle and crust through which the magma has risen, may account for the greater fractionation which has taken place.