GEOLOGY AND GEOCHEMISTRY OF GANNET (KAREWA) ISLAND, TASMAN SEA - A RIFT-RELATED NEPHELINITIC TUFF RING

Gannet (Karewa) Island is a small (0.06 km(2)) island situated in the Tasman Sea, northwest of Kawhia Harbour, western North Island, New Zea land. It consists of well indurated, palogonitic 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 c oarse ash units, lensoidal beds, U-channels, and impact sag structures . Juvenile bombs and blocks consist of fine-grained olivine nephelinit e, comprising microphenocrysts of olivine and acicular clinopyroxene, set in a devitrified glassy groundmass. Accidental lithic fragments ar e 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 Volcani cs, but the major and trace element abundances and isotopic compositio n 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) and LREE abundances, and lower Sr-87/Sr-86 (0.7030) and relatively higher Sigma(Nd) (+6.8) compared with the alkaline intraplate basaltic fields of Okete, Ngatut ura, or South Auckland, or the convergent margin basalts of the Alexan dra 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 t he Taranaki Fault, which is shown on offshore seismic profiles as exte nding northwards, parallel to the coast off western North Island. Move ment on the Taranaki Fault, which forms the eastern boundary of the No rth Taranaki Graben, ceased in the late Miocene. Since the Pliocene, t his region of western North Island has been in extension, and hence Ga nnet Island is interpreted on geochemical constraints and tectonic pos ition to be an isolated rift-related volcano. This supports the interp retation that the North Taranaki Graben formed as a back-are rift rath er than an actively spreading back-are basin. The nephelinites of Gann et Island, and also those in the South Auckland volcanic field, which have similar ages, lie just to the northwest of the limit of seismic a ctivity on the subducting Pacific plate, and are probably close to the most distal expression of the contemporary plate boundary.