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

Abstract

The Bonar Range consists predominantly of variably mylonitised granitoidal gneiss, informally named the Bonar orthogneiss. Amphibolite facies metasedimen‐tary 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