Subsurface andesite lavas and plutonic rocks in the Rotokawa and Ngatamariki geothermal systems, Taupo Volcanic Zone, New Zealand

Abstract

Deep (> 2 km) drillholes into the Rotokawa and Ngatamariki geothermal systems, Taupo Volcanic Zone (TVZ), New Zealand, penetrate a sequence of silicic volcanic rocks and andesite lavas, the latter being locally more than 1100 m thick. One drillhole at Ngatamariki bottoms in diorite (2200 m), the first plutonic rocks reached by any well drilled into the Taupo Volcanic Zone. The Rotokawa Andesite lavas, which occur below 1400 m depth, are older than 330 ka, and locally rest upon Mesozoic metasedimentary basement rocks of the Torlesse terrane. They are typically dense, jointed, and, where fresh, contain phenocrysts of andesine-labradorite, augite, hypersthene, titanomagnetite and, in places, hornblende. Geochemically, these rocks are medium-K, calc-alkaline, orogenic andesites. Their major- and trace-element contents and Sr isotopic ratios show them to be chemically coherent, with little lateral or vertical variation. Hydrothermal alteration has typically added volatiles but has been largely isochemical on a hand specimen scale. Mineralogical, geochemical and stratigraphic evidence indicates that the Rotokawa Andesites are not genetically or temporally related to andesite lavas exposed at Rolles Peak, 5 km east of Rotokawa, nor to andesite or diorite penetrated at Ngatamariki, but they have closer chemical affinities with the plagioclase-pyroxene phyric lavas of the Tongariro Volcanic Centre at the south end of the TVZ. The Ngatamariki Andesites have distinctive trace-element compositions (particularly Ti and Zr) and are chemically and petrographically different from most other TVZ lavas. The occurrence of thick, subsurface andesite sequences in the Rotokawa-Ngatamariki area suggests that early eruptives here were of intermediate composition rather than the more silicic volcanics now so widespread both at the surface and in the shallow levels of several nearby geothermal systems.