Geology of the Riverton Peninsula, Western Southland

Author:

Macfarlane, D. F., 1951-

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Abstract:

A sequence of spilitic pillow lavas, pillow breccias and tuffs, cut by numerous dikes, which crops out over the eastern part of the peninsula is defined as the Riverton Volcanics. An overlying, poorly exposed, group of lithologies is mapped as Undifferentiated Permian. These two lithologic groupings are infaulted against fossiliferous tuffaceous metasediments belonging to the Greenhills Group.
Two phases of dike intrusion are recognised. The earlier dikes are spilitic, in some cases feeders to pillow lavas. They are crosscut by porphyritic basaltic dikes, which also cut the Undifferentiated Permian lithologies. There are no dikes within the Greenhills Group rocks.
Field and petrographic evidence suggests that the Riverton Volcanics were emplaced in shallow water. Partial analyses reveal a very low K20 content (average 0.15%) which cannot be readily explained in terms of metasomatism. These rocks are tholeiites and may represent oceanic crust. The andesitic to basaltic tuffs of the Greenhills Group show calc-alkaline affinities. Double grading within these rocks is explained in terms of shallow submarine eruption.
The area has been burial metamorphosed to prehnite-pumpellyite and pumpellyite-actinolite facies. In the west, the Greenhills Group tuffs have been contact metamorphosed by the emplacement of the Longwood Intrusives. Fossil evidence indicates an upper Lower Permian (Mangapirian-Braxtonian) age for the Greenhills Group. The underlying rocks are placed in the lowest Permian but may be older. The association of calc-alka1ine tuffs and tholeiitic volcanics is explained in terms of a plate tectonic model in which the Undifferentiated Permian rocks, in part, represent continent-derived sediments.

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92 leaves : illus., maps (fold.)

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OU geology Identifier:

1973MacFarlane

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Location (WKT, WGS84):

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http://download.otagogeology.org.nz/temp/Abstracts/1973MacFarlane.pdf

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Citation

Macfarlane, D. F., 1951-, “Geology of the Riverton Peninsula, Western Southland,” Otago Geology Theses, accessed May 22, 2024, https://theses.otagogeology.org.nz/items/show/71.

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