Geology and geochemistry of Gannet (Karewa) Island, Tasman Sea: A rift‐related nephelinitic tuff ring

Abstract

Gannet (Karewa) Island is a small (0.06 km2) island situated in the Tasman Sea, northwest of Kawhia Harbour, western North Island, New Zealand. It consists of well indurated, palagonitic tuff and lapilli tuff with subordinate scoriaceous basalt bombs and blocks (Karewa Volcanic Formation) which are considered to represent the eroded remnants of a tuff ring. Evidence for this includes such features as cross‐bedded coarse ash units, lensoidal beds, U‐channels, and impact sag structures. Juvenile bombs and blocks consist of fine‐grained olivine nephelinite, comprising microphenocrysts of olivine and acicular clinopyroxene, set in a devitrified glassy groundmass. Accidental lithic fragments are abundant and include ultramafic xenoliths (up to 5 cm), sedimentary rock fragments (15 cm), and hornblende andesite crystal tuff (20 cm). The island has previously been mapped as part of the Alexandra Volcanics, but the major and trace element abundances and isotopic composition indicate that it has lower SiO2 (40–42%) and higher TiO2 (3.2–3.5%), MgO (14–16%),Nb(110–125 ppm), Zr(307–343 ppm)andLREE abundances, and lower 87Sr/86Sr (0.7030) and relatively higher SNd (+6.8) compared with the alkaline intraplate basaltic fields of Okete, Ngatutura, or South Auckland, or the convergent margin basalts of the Alexandra Volcanics and Egmont. Juvenile olivine nephelinite bombs have been dated by K‐Ar determination at 0.51 ± 0.05 Ma, much younger in age than the nearby Alexandra Volcanics (2.74–1.60 Ma) or offshore middle‐late Miocene volcanics. Gannet Island is located approximately above the Taranaki Fault, which is shown on offshore seismic profiles as extending northwards, parallel to the coast off western North Island. Movement on the Taranaki Fault, which forms the eastern boundary of the North Taranaki Graben, ceased in the late Miocene. Since the Pliocene, this region of western North Island has been in extension, and hence Gannet Island is interpreted on geochemical constraints and tectonic position to be an isolated rift‐related volcano. This supports the interpretation that the North Taranaki Graben formed as a back‐arc rift rather than an actively spreading back‐arc basin. The nephelinites of Gannet Island, and also those in the South Auckland volcanic field, which have similar ages, lie just to the northwest of the limit of seismic activity on the subducting Pacific plate, and are probably close to the most distal expression of the contemporary plate boundary.