Structure and origin of the Dun Mountain ultramafic massif, New Zealand
Dun Mountain, the type locality of dunite, forms part of a major belt of predominantly mafic and ultramafic rocks in South Island, New Zealand. The core of the mountain consists of unserpentinized dunite and harzburgite tectonite. Foliations in the ultramafic rocks strike approximately northeast-southwest, parallel to the elongation of the crestal plateau. Lineations are horizontal and northeast-southwest. Texture is usually porphyroclastic, with olivine elongated parallel to lineations. Petrofabric analyses show strong preferred orientations of olivine a crystallographic axes subparallel to foliations and lineations. Shear senses within the ultramafic are dextral, indicating that the northern portion of Dun Mountain has been displaced to the east relative to the southern portion. Undulatory extinction of olivine, resulting from kinking and associated slip on (010) [100] and {Ok1} [100] slip systems, is interpreted as originating within the upper mantle beneath oceanic crust that formed at a spreading ridge or in a marginal basin. Seismic anisotropy of the Dun Mountain ultramafic is similar to observed anisotropy in the upper oceanic mantle.