Geology of an area about N.W. Arm, Middle Fiord, Lake Te Anau
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Abstract:
A layered gabbro overlying granodiorite and a sequence of hornblende-bearing and quartzofeldspathic gneisses have been described from an area of 25km2 about Lake Hankinson and the N.W. Arm, Middle Fiord, Lake Te Anau.
The gneissose rocks represent a westward younging sequence of basic and semibasic (basic-intermediate) lavas and tuffs overlain by quartzofeldspathic and possibly calcareous sediments. This sequence has been completely recrystallised by amphibolite grade metamorphism.
South of a roughly E-W trending line through the centre of the area (the Howitt Peaks Fault) gneissose rocks are uncommon. Instead, a flat-lying sequence of differentiated ultrabasic rocks, approximately 550 feet thick, overlie felsic igneous rocks of a dominantly granodioritic composition.
This sequence is in various stages of recrystallisation under the influence of amphibolite grade metamorphism~ The degree of recrystallisation is largely dependent on the composition of the original rock. Study of a range of secondary phases by optical and X-ray methods, suggest that mineral composition is also largely dependent on the composition of the original rock.
Two phases of faulting accompanied by retrogressive metamorphism and crush zones disrupt all lithologies and give rise to intermixing of adjacent rock types.
On the basis of tectonic position and associated rock types the N.W. Arm gabbro is thought to be genetically related to the Mt Luxmore Ultrabasics of the South Fiord, Lake Te Anau. Comparisons with Central Fiordland gabbroic bodies are also considered.
The gneissose rocks represent a westward younging sequence of basic and semibasic (basic-intermediate) lavas and tuffs overlain by quartzofeldspathic and possibly calcareous sediments. This sequence has been completely recrystallised by amphibolite grade metamorphism.
South of a roughly E-W trending line through the centre of the area (the Howitt Peaks Fault) gneissose rocks are uncommon. Instead, a flat-lying sequence of differentiated ultrabasic rocks, approximately 550 feet thick, overlie felsic igneous rocks of a dominantly granodioritic composition.
This sequence is in various stages of recrystallisation under the influence of amphibolite grade metamorphism~ The degree of recrystallisation is largely dependent on the composition of the original rock. Study of a range of secondary phases by optical and X-ray methods, suggest that mineral composition is also largely dependent on the composition of the original rock.
Two phases of faulting accompanied by retrogressive metamorphism and crush zones disrupt all lithologies and give rise to intermixing of adjacent rock types.
On the basis of tectonic position and associated rock types the N.W. Arm gabbro is thought to be genetically related to the Mt Luxmore Ultrabasics of the South Fiord, Lake Te Anau. Comparisons with Central Fiordland gabbroic bodies are also considered.
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88 leaves : illus. maps (fold) ; 29 cm.
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1973Morrison
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POLYGON ((167.589182872000038 -45.129208670999958,167.543631172000119 -45.130419058999962,167.524545568000121 -45.056757791999985,167.521113179000054 -45.043490254999938,167.64234400700002 -45.039251233999948,167.658252799000024 -45.127121700999965,167.658291904000066 -45.127337365999949,167.658233059000054 -45.127338977999955,167.589182872000038 -45.129208670999958))
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Morrison, Gregg W., “Geology of an area about N.W. Arm, Middle Fiord, Lake Te Anau ,” Otago Geology Theses, accessed December 7, 2024, https://theses.otagogeology.org.nz/items/show/72.