Phonolitic Diatremes within the Dunedin Volcano, South Island, New Zealand

Abstract

The Port Chalmers Breccia is a vent-filling, clastic volcanic unit exposed within the Miocene Dunedin Volcano of South Island, New Zealand. Clasts (up to in excess of 1 m but generally <20 cm) are supported in ash and fine lapilli of phonolitic (ne-benmoreite or tephro-phonolite) composition and the dominant clast type (55 to almost 100%) is also phonolitic. Less abundant lithologies include ne-normative basalt (basanite), hawaiite, mugearite and trachyandesite, syenites and microsyenites, coarse-grained mafic (gabbros) and ultramafic rocks (pyroxenites, hornblendites), schists and sediments. The breccias were emplaced as diatremes associated with localized, but highly explosive, eruptive events in which mantle-derived CO2 was an important component. The syenitic and ultramafic clasts could represent intrusive suites produced by crystal fractionation acting on parental ne-benmoreite magmas that may themselves have been derived by crystal fractionation from basanitic precursors. An alternative variation on this model is that the parental ne-benmoreites were generated through partial melting of an alkalic igneous underplate. Sr, Nd and Pb isotopic compositions are strikingly similar to those of intraplate igneous rocks, ranging in age from 100 to less than 10 Ma, from elsewhere in the South Island, and New Zealand's sub-Antarctic islands, the south Tasman Sea and the Ross Sea region. This regional, HIMU-influenced, isotopic signature is believed to be derived from within the lithospheric mantle.