CONNECTING GEOCHEMISTRY AND TECTONICS: CRETACEOUS GONDWANA LAVAS OF NEW ZEALAND

In New Zealand, the Houhora Complex (~102 Ma) and the Mount Somers Volcanic Group (95 - 98 Ma) are both in situ eruptive piles of sub-alkaline Late Cretaceous basalt, andesite, dacite and rhyolite. During this time the tectonic regime of the New Zealand sector of the south-western Pacific Ocean changed from subduction to extension and great reliance is place on tectonic interpretation of igneous geochemistry to understand this change. The Houhora Complex and the Mount Somers Volcanic Group have never been compared and have been interpreted by different authors to be either intraplate or subduction related. Our recent comparison of the two sequences suggests they are geochemically similar and exhibit intraplate characteristics such as high Zr content compared with subduction suites when plotted against SiO2. Based on provisional rare earth (REE) and trace element data, both the Mount Somers Volcanic Group and the Houhora Complex are shown to span broadly similar compositions. Distinctly different trends within the data sets, involving Zr, Ba and V, have yet to be interpreted fully. Further REE and isotope work is to be undertaken on both the Mount Somers Volcanic Group and the Houhora Complex. This will inform on the contribution of crustal melting and/or assimilation in the petrogenesis of the lavas, and whether a slab was actively subducting beneath the Late Cretaceous volcanic centres.