Metamorphic, structural, igneous and sedimentary geology of Mount Brewster, Haast Pass, New Zealand.


Daly, Adrian Bernard.


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This thesis documents the structural, metamorphic, igneous and sedimentary geology of Mount Brewster, Haast Pass, which is located within the Haast schist terrane.
Quartzofeldspathic schist, the metamorphosed and deformed derivative of greywacke and argillite eugeosynclinal sediment is subdivided into textural zones 1 to 4, in the map area. Subordinate lithologies include metabasite, metachert and conglomerate horizons.
Three phases of folding are identified. Phase I folds are dominant throughout the map area, except in textural zone 4 in the northwest of the area, where a second phase of folding is recognised. Phases I and II folding pre-date the metamorphic maximum. Phase III kink folds are recognised throughout the map area. Phase III folds post-date the metamorphic maximum. Three phases of faulting are recognised.
The dominant structure in the map area is a macroscopic F 1 fold, which is given the name Brewster Synclinoriurn. The fold axes of this structure plunges steeply (70-75°) to the south-southwest. Textural zone 1-2 rocks are restricted to the core of this fold, with textural grade increasing to textural zone 4 in the west, and textural zone 3 in the east.
Mineral zonation trends are studied, with particular attention to the isograd marking the disappearance of pumpellyite, defining the boundary between pumpellyite-actinolite facies and greenschist facies.
A strain analysis of deformed conglomerates is documented for two locales which differ in textural grade. Interpretation of data, suggests that at high textural grades (textural zone 3 and 4) plain strain is very important, whereas at lower textural grades (textural zone 1 and 2) there is a higher component of flattening. Percentage flattening is 56% and 80% respectively for textural zone 2 and textural zone 4 locales.
Lamprophyric dykes are mapped. All dykes are consistent in mineral composition, with camptonitic compositions in the centre of the dyke 1 and monchiquitic compositions at the chilled contact to country quartzofeldspathic schist. Spheroidal and ellipsoidal leucocratic patches called ocelli, are recognised, forming stratified zones parallel to the trend of the dykes. Two phases of dyke intrusion are recognised. Phase I dykes are related to phase I faulting. Both phases of dykes have a general east to west trend, which cuts across the trend of the foliation. (Foliation strikes -north-north-east.)
Sedimentary structure is preserved in the map area. Bedding is recognised best in textural zone 1-2 and 2, and its recognition persists to textural zone 3. A textural zone 4 conglomerate is documented. Three sediment facies are recognised. Facies 'A' sediment is characteristic of channel deposits where debris 1 grain 1 fluid and turbidity flow processes predominate. Facies 'B' sediment is intimately related to Facies 'A' 1 representing hemipelagic fallout. Facies 'C' sediment is characteristic of interchannel deposits, in which turbidity flow - with a minor component of fluid flow - is active. The association of Facies 'A', 'B' and 'C' would suggest an 'inner' to 'midfan' submarine environment.
An unsuccessful attempt to place an age on the map area, by analysing calcareous sandstone concretions for conodonts, was made. Biozoning of rocks of the Torlesse terrane, would suggest a possible Triassic to Permian age.

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145 leaves : illus., maps (2 fold. col. in pocket) ; 30 cm.


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Daly, Adrian Bernard., “Metamorphic, structural, igneous and sedimentary geology of Mount Brewster, Haast Pass, New Zealand.,” Otago Geology Theses, accessed May 22, 2024,

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