Abstract
Abstract Indurated, complexly deformed greywacke basement on northern Great Barrier Island is intruded by numerous Miocene porphyry dikes. The basement consists mainly of thin-bedded sands tonel mud stone and thick-bedded sandstone lithofacies with minor conglomerate, chert and sedimentary breccia. Conglomerate contains a variety of acidic igneous pebbles, and sandstones are dominated by volcanic detritus. Sparse belemnites indicate a Late Jurassic (Puaroan) age. Quartz porphyry and dacite dikes are more common and older than hornblende-pyroxene and two-pyroxene andesite porphyries. The dikes are hydrothermally altered, contain very minor copper mineralisation, and were mostly intruded along steep N and shallow NW trending joints. They are comparable with the Kai-iti Porphyrites of northern Coromandel Peninsula and probably have a similar age, i.e. about Middle Miocene. The relationship of dikes to the andesitic volcanics (Coromandel Group) of central and southern Great Barrier is uncertain.
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