Abstract Igneous rocks of the Erebus Volcanic Province have been investigated for more than a century but many aspects of petrogenesis remain problematic. Current interpretations are assessed and summarized using a comprehensive dataset of previously published and new geochemical and geochronological data. Igneous rocks, ranging in age from 25 Ma to the present day, are mainly nepheline normative. Compositional variation is largely controlled by fractionation of olivine + clinopyroxene + magnetite/ilmenite + titanite ± kaersutite ± feldspar, with relatively undifferentiated melts being generated by <10% partial melting of a mixed spinel + garnet lherzolite source. Equilibration of radiogenic Sr, Nd, Pb and Hf is consistent with a high time-integrated HIMU sensu stricto source component and this is unlikely to be related to subduction of the palaeo-Pacific Plate around 0.5 Ga. Relatively undifferentiated whole-rock chemistry can be modelled to infer complex sources comprising depleted and enriched peridotite, HIMU, eclogite-like and carbonatite-like components. Spatial (west–east) variations in Sr, Nd and Pb isotopic compositions and Ba/Rb and Nb/Ta ratios can be interpreted to indicate increasing involvement of an eclogitic crustal component eastwards. Melting in the region is related to decompression, possibly from edge-driven mantle convection or a mantle plume.

The Westland dike swarm of South Island, New Zealand, consists of a variety of ultrabasic, alkaline rock types including camptonite lamprophyres, ouachitite peridotites, and carbonatites. At least part of the swarm is as young as Oligocene-Miocene age. The majority of Nd and Sr isotope analyses for 16 dikes ε Nd = +3.5 to +5.2, 87 Sr/ 86 Sr i = 0.7028 to 0.7035) fall to the left of the mantle array in Nd-Sr isotope correlation diagram. The data are similar to the ocean islands St. Helena and Tubuai and to some continental volcanics and mantle nodules (Menzies and Wass, 1983). The dikes contain radiogenic Pb ( 206 Pb/ 204 Pb = 19.19 to 20.59, 207 Pb/ 204 Pb = 15.64 to 15.71, 208 Pb/ 204 Pb = 39.0 to 40.2), which also is similar to the most radiogenic Pb in ocean islands. The trace element and isotope data are consistent with an origin involving a Paleozoic enrichment event, which added CO 2 , Sr, light rare-earth elements, and U to a depleted mantle. We propose that this first event was the manifestation of a deep-seated intrusion of carbonated hyperalkaline magma. A second event triggered the melting of this newly metasomatized mantle and the emplacement of the Westland dikes.