Silica indices of volcanoes in and around New Zealand with reference to volcanic zones in the North Island

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Abstract
Silica indices have been calculated for lavas from volcanoes in and around New Zealand. Comparison of geographic distribution of the indices with the ages of the volcanoes indicates there are two different Quaternary volcanic zones within the North Island. One is the Rotorua-Taupo zone with the “volcanic front” lying along White Island, Whale Island, Mt Edgecumbe, Tauhara, and Ngauruhoe. Silica indices of those volcanoes on the “volcanic front” are in range 41–43. There is a steep Benioff zone with an inclination 70° to WNW underneath the volcanic zone and a correspondingly sharp lateral rock suite transition. This is shown by a low silica index of 36 for Ongaroto Basalt and 34 for Kakuki Basalt at Orakeikorako. Both lie on the 200-km-depth contour line of the Benioff zone. A second zone lies along the Northland Peninsula with a “volcanic front” corresponding to volcanoes of high silica indices: Little Barrier Island, the Kiwitahi andesite volcanoes near Cambridge, and Titiraupenga and Kakaramea, NW and SW of Lake Taupo. This trend also includes Kerikeri basalts, Auckland Basalts, Franklin Basalts, Alexandra Volcanics, and the Mt Egmont group of volcanoes. Volcanism in this zone was a consequence of island arc activity in late Oligocene to early Pleistocene times. Nevertheless, volcanoes in the outer (western) part of the zone, with low silica indices, have continued active into late Quaternary, and represent relict volcanism of an old island arc. Judging from the recent speed of spreading motion of the Pacific plate, descent of the plate along the Tonga—Kermadec—North Island arc commenced 12–13 m.y. ago, or 19–21 m.y. ago if the change speed at 5 m.y. ago is taken into account. Prior to this, during the period from 40 to 20 m.y. ago, it is assumed the plate descended along the Northland trend.