New Zealand Journal of Geology and Geophysics abstracts

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Short communication 

Gneissic rocks of the Bonar Range, central Westland, New Zealand

Richard Jongens

Institute of Geological & Nuclear Sciences
Private Bag 1930
Dunedin, New Zealand
r.jongens@gns.cri.nz

Abstract  The Bonar Range consists predominantly of variably mylonitised granitoidal gneiss, informally named the Bonar orthogneiss. Amphibolite facies metasedimentary schist, deformed granitoids, and pegmatite/mafic dikes make up the remainder of basement rocks. Based on S-type granitoid characteristics and a Late Devonian to earliest Carboniferous granodiorite intrusion, the orthogneiss is probably associated with the Karamea Suite of Westland. The predominant foliation strikes east–west, dipping to the north. Late stage, pre-79 Ma mylonitisation, subparallel to the foliaton, occurred under greenschist facies conditions. Shear-sense indicators indicate a top to the northeast–east sense of shear. Hornfelsed low-grade Greenland Group metasediments and undeformed Devonian granites of the nearby Rangitoto Range demonstrate a structural and metamorphic change that is most easily explained by a NNW–SSE-trending fault along the intervening Waitaha valley. The mylonitisation and postulated fault are features consistent with Early Cretaceous extension recorded elsewhere in Westland.

Keywords  Bonar Range; Westland; Devonian; granite; gneiss; mylonite; structure; foliation; geochronology

G05055; Received 11 November 2005; accepted 24 January 2006; Online publication date 17 May 2006
New Zealand Journal of Geology & Geophysics, 2006, Vol. 49: 281–286
0028–8306/06/4902–0281  © The Royal Society of New Zealand 2006

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