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 e..