The geology of Dun Mountain, Nelson, New Zealand

Abstract

Dun Mountain is composed of dunite, orthopyroxene peridotite, and serpentlmte. The serpentinite crops out at the borders of the mass and appears to be formed from the ultrabasic rocks by metasomatism about faults at or near the contacts. The ultra basic rock cuts across bedding at all contacts, and at the contacts dikes and veins of serpentinite intrude the Te Anau and Maitai sedimentary and volcanic rocks. The unserpentinised ultrabasic rocks have alternate bands of orthopyroxene peridotite and dunite. The bands appear to be vertical and are probably concentric—like cylinders placed one inside the other. At one place the banding follows irregularities in the contact. The serpentinites and partly serpentinised ultrabasic rocks near the contacts are crossed by pyroxenite veins. At the eastern contact these veins are composed of orthpyroxene and at the western contact of monopyroxene (diallage). The origin of the latter may be related to the presence of Rangitoto Marble at the western contact. The texture of the orthopyroxene peridotite and dunite, the banding, and the ease with which olivine is crushed, seem to rule out diapiric intrusion or intrusion as a crystal mesh. It seems possible, however, that peridotites could be formed in situ by aggregation of phenocrysts in pipes and magma chambers of basaltic volcanoes. Preservation of complex textures in the ultrabasic rocks, and structure in the mass as a whole, indicate that these rocks have not been folded. It is probable, therefore, that they post date the Rangitata (Hokonui) Orogeny.