Cenozoic volcanism in north, east, and central Otago.

Three distinct periods of Cainozoic volcanism are recognized in eastern Otago. Volcanism commenced with submarine basaltic activity offshore of north Otago in Palaeocene time. Products of the second period (the Waiareka-Deborah volcanics) of late Eocene to early Oligocene age consist of extensive tephra and some pillow lavas from submarine eruptions, and shallow intrusives. Many of these volcanic products were tholeiitic in composition, but substantial volumes of mildly to highly alkaline mafic magma were also involved. The latter include the nephelinitic Kakanui mineral breccia which contains a suite of garnet pyroxenite blocks and other deep-seated xenoliths and xenocrysts. Activity in the third period of volcanism was spread over a large area of eastern and central Otago. It built up the Dunedin volcano in late middle Miocene time but the activity of some of the outlying vents extends back to early Miocene. The volcanic products are predominantly subaerial and alkalic, and range in composition from mildly alkalic basalts through basanites to nephelinites, and also intermediate types and relatively voluminous trachytes and phonolites. The glass chemistry, petrology, geochemistry and petrogenesis of these diverse suites of rocks are discussed, and their Sr and Nd isotope compositions are compared. The data imply derivation from a mantle source which had been depleted in light REE for some considerable time before recent enrichment in light REE and other incompatible elements.-R.M.B.