Geochronology of the Banks Peninsula Volcanoes, New Zealand

Abstract

Banks Peninsula, on the central east coast of South Island, New Zealand, consists of two shield volcanoes, Lyttelton and Akaroa. Both are built mainly of mildly alkaline basaltic and andesitic lavas. Akaroa is the younger volcano and its lavas have buried much of the south-eastern part of Lyttelton. Potassium-argon dating on whole rock samples shows that Lyttelton volcano was active between about 10 m.y. and 12 m.y. ago, and that the main Akaroa activity occurred between about 8.0 m.y. and 9.0 m.y. ago. The central regions of both volcanoes have been very deeply eroded. The Diamond Harbour Group of Liggett and Gregg (1965) consists of basaltic lavas that lie within the confines of Lyttelton volcano, and to a large extent within the erosion caldera. These lavas range from 5.8 m.y. to 8.2 m.y, in age. The oldest formation of the Diamond Harbour Group, the Church Formation, gives ages that overlap with those of the Akaroa lavas of the Mt Herbert region and are similar petrographically. This points to a possible equivalence of these lavas. The younger formations of the Diamond Harbour Group (Kaioruru and Stoddart) have considerably younger ages than the Akaroa lavas and probably were erupted from local sources. The isotopic dates, however, generally are in excellent agreement with field interpretations and indicate that volcanism in Banks Peninsula was confined to the late Miocene and the Pliocene.