Geology of the Western Hohonu Range, North Westland


Hamill, L. J.


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Alkaline dike rocks, of early Cretaceous age, intruding the Tuhua Granite complex of the Hohonu Range, North Westland, form an almost continuous series from camptonitic lamprophyre through trachyandesites to trachytes and soda rhyolites. The series shows extreme fractionation of a relatively differentiated alkali basaltic magma, saturated with silica, to yield extremely oversaturated peralkaline rhyolites. A fractionation trend arose as a consequence of crystallization of olivine, pyroxene, kaersutitic amphibole and feldspar phases.
The Hohonu Range dike rocks are part of a regional swarm bounded to the south by the Alpine Fault and extending from Mt. Bonar to the Buller River some 200 km further north. (See fig. 16).
A comparison is drawn between the Hohonu swarm and similar intrusives, possibly cogenetic, south east of the Alpine Fault in the Haast River area. Dissimilarities between the two areas, thought to have arisen from different tectonic histories, resulting from Cainozoic movement on the Alpine Fault, are included in a possible model.
An account of the Greenland series, Tuhua Granite and Tertiary rocks, encountered in the area studied, is included.
In 1908 J. P. Smith described a series of lamprophyric rocks, found in pleistocene gravels, interpreted as having been derived from the Hohonu Range. Comparison of these dike rocks to similar varieties found in situ along the West Coast has resulted in a number of significant structural and genetic implications. Since 1908 little has been done to substantiate Smith's study. During the summer of 1972 the writer spent some 27 days mapping the dike rocks of the Hohonu Range. The aim of the study being to examine closely evolutionary trends in the intrusive lamprophyric swarm.
All available analyses of lamprophyres and allied rocks, both published and unpublished were compiled and a preliminary geochemical comparison between the Haast River and Hohonu swarm was attempted.
Other than noting their presence little attention was given to other rock types encountered during the mapping of the dike rocks.
The writer is indebted to both Academic and Technical Staff for assistance and advice provided during the year. Special thanks are due to Dr. A. F. Cooper who supervised the project and provided the writer with seven unpublished whole rock chemical analysE;;s. Financial assistance from the Benson Memorial fund is gratefully acknowledged.

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91 leaves : maps (1 in pocket), plates ; 29 cm.


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POLYGON ((171.33593565194036 -42.749586471617498,171.308096464941286 -42.733095754148735,171.35909210388192 -42.668315140509833,171.393726344653999 -42.671188807775025,171.430302259308462 -42.725305687397054,171.373115493679791 -42.749525133307905,171.33593565194036 -42.749586471617498))




Hamill, L. J., “Geology of the Western Hohonu Range, North Westland,” Otago Geology Theses, accessed May 22, 2024,

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