New Zealand Journal of Geology and Geophysics abstracts
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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