The Geology of Dun Mountain, Nelson, New Zealand

Abstract

The sedimentary and volcanic rocks of the Dun Mountain area belong to the Te Anau and Maitai Groups of probably Permian age. The Te Anau Group includes the Little Twin Formation (new), composed of tufTaceous argillite and greywacke with fragments of Atomodesma (Little Twin Argillite Member), and fine-grained spilite (Little Twin Spilite Member), above and interdigitating with the argillite; and the Pelorus Greywacke (new).The lower formations of the Maitai Group (those examined in this paper) are the Rangitoto Formation and the Greville Argillite Formation. The Rangitoto Formation is subdivided into the Rangitoto Volcanic Breccia Member at the base, the Rangitoto Marble Member, and at the top the Rangitoto Greywacke Member. Calcite from the marble has been disseminated by heat from ultrabasic rocks giving in some places calc-silicate rocks and in others calcite replacement of feldspar.The Te Anau and Maitai Groups form an anticline with its axis along the Dun Mountain – Little Twin ridge. The ultrabasic rocks of Dun Mountain cut the rocks of the Te Anau and Maitai Groups.At all contacts dikes and veins of serpentinite invade the contact rocks, which show considerable metamorphism and metasomatism.This is particularly prominent at the western contact, where the disintegration of the Rangitoto Marble Member has produced copious volatiles.