K-Ar age data and geochemistry of the Kiwitahi Volcanics, western Hauraki Rift, North Island, New Zealand

The Kiwitahi Volcanics of late Miocene age crop out in a number of localities situated along the western side of the Hauraki Rift in northern North Island, New Zealand. They extend from Waiheke Island in the north to Te Tapui in the south. From 19 new K-Ar ages presented here and geochemical data from volcanics at each locality, it is suggested that the Kiwitahi Volcanics should be informally subdivided into five groups: (1) an oldest group of agglomerates and volcanic breccias at northeastern Waiheke, containing pyroxene and hornblende andesites with an age range of 14.4-16.02 Ma; (2) a volcanic centre at Stony Batter (6.85-8.34 Ma) comprised of olivine basaltic andesites which should be assigned to the geochemically and temporally similar Ti Point Volcanics; (3) a group including the andesitic breccias at Ness Valley and the volcanic centres of Miranda (pyroxene basaltic andesite, pyroxene and hornblende andesite, hornblende dacite) and Pukekamaka (hornblende andesites), all within the age range 10.22-12.96 Ma; (4) a separate group at Tahuna (6.36-6.80 Ma) consisting of pyroxene basaltic andesites and pyroxene andesites; and (5) a southern group of Maungatapu, Ruru, Maungakawa, and Te Tapui (5.52-6.23 Ma), forming eroded cones of olivine basaltic andesites, pyroxene basaltic andesites, and pyroxene andesites. The distinct incompatible element ratios and age differences between these groups suggest that these magmas were derived from distinct magma source regions, although all lavas have geochemical characteristics (low Nb, Ti contents, high LIL/LREE and LIL/HFSE ratios) typical of convergent margin magmas. There is no convincing geochemical signature for any rift component. This suggests that the age of initiation of the Hauraki Rift postdates the youngest age of these volcanics at 5.5 Ma.