Geochronology and geochemistry of the Dunedin Volcanic Group, eastern Otago, New Zealand

Fifty-six previously unpublished K-Ar ages for the Dunedin Volcanic Group and previously published K-Ar and 40Ar/39Ar ages demonstrate that activity in the centrally situated Dunedin Volcano (here given formal lithostratigraphic status) lasted from 16.0 ± 0.4 to c. 10.1 Ma, and that of the surrounding Waipiata Volcanics lasted from 24.8 ± 0.6 to 8.9 ± 0.9 Ma. Apart from a gap at c. 20 Ma, recorded Waipiata activity climaxed at c. 16-14 Ma when activity of the Dunedin Volcano was beginning; it outlasted that of the Dunedin Volcano by c. 1 m.y. The total volume erupted by the Dunedin Volcano may have exceeded that of the largely monogenetic Waipiata Volcanics by an order of magnitude. New major- and trace-element analyses are given for 87 whole-rock samples and kaersutite. The whole-rock data demonstrate the exclusively alkalic nature of the group, the Waipiata Volcanics being more strongly alkalic than most of the mafic members of the central volcano. This fractionated to give a much greater volume of phonolitic differentiates than the Waipiata Volcanics. As for other intraplate Cenozoic volcanism in the New Zealand region, ranging overall from tholeiitic to highly alkalic, major- and trace-element patterns support an origin from a garnet-bearing ocean island basalt source region with high U/Pb mantle characteristics. (author). 77 refs., 13 figs., 4 tabs