Aspects of the geology of Moeraki Peninsula and the geochemistry of the Moeraki dolerite

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Brown, Justin.

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The Moeraki Peninsula is located onĀ· the east coast of the South Island of New Zealand, approximately 70 kilometres north of Dunedin.
The regional schist basement, and the overlying fault derived breccias of the Shag Point Group both outcrop to the west (Mutch, 1963), and are not exposed on t~e Peninsula. These units are overlain by the marine sediments of the Moeraki, Kurinui, Hampden, and Iviokihi Formations, which collectively represent a marine transgressive sequence formed during a period of regional subsidence in the Eocene. The latter formations comprise fossiliferous mudstones and greensands, and are overlain by late Eocene or younger hyaloclastic volcanic agglomerates, breccias and tuffs, erupted in a submarine environment.
Voluminous basaltic magmas, similar in age to the volcanics, have intruded the Tertiary sedimentary sequence to form numerous dykes and sills. The largest of these structtires is the Moeraki Dolerite, an elongated sill-like body which outcrops along the southeast coast of the peninsula, and has an estimated minimum volume of 0.06 cubic kilometres.
The mineralogy of the Moeraki Dolerite is plagioclase feldspar (Anoo-An2o), diopside, augite, ilmenite, magnetite, and an unidentified amphibole. The minerals occur as both phenocrysts displaying a doleritic texture, and groundmass, where the feldspars display prominent chill textures, indicating quenching of the body after emplacement.
The main body of the sill is basaltic in composition, ranging from ~48-53% silica, and has been intruded subsequent to emplacement and solidification, by a more differentiated liquid (Si02 ~52-62%) which has formed thin, laterally continuous veins, occurring throughout the main sill, spaced approximately lm apart, and running parallel to the upper and lower contacts of the latter.
The major and trace element chemistry of these two rock-types suggest that the se~ond, more differentiated liquid was derived from the same source as the more basic main sill rocks, chemically differentiated by the fractional crystallization of olivine, clinopyroxene and plagioclase. Some crusta! assimilation of the regional schist basement is indicated by the dramatic silica enrichment seen in the differentiated veins, which can not be accounted for by fractional crystallization alone.
The textures along the contact between the main sill rocks and the differentiated veins indicate that the latter were injected after the solidification of the former, but while it was still hot enough to prevent quenching of the second liquid. This short time frame suggests that the source magma chamber was probably already differentiated and zoned prior to the emplacement of the Moeraki Dolerite.
Spatial chemical variation within the main sill rocks indicate that the outer margins of the sill are more differentiated than the central portions.
Two petrogenetic models have been proposed, both involving multiple pulses of injection from a parent magma chamber at depth. One proposes that in-situ fractionation is responsible for the variation within the main sill rocks, while the second s~ggests that the variation is entirely the result of the tapping of a zoned magma chamber at depth.
Schist xenoliths occur in the main sill rocks, and within a vein-like structure which is parallel to the upper contact of the Moeraki Dolerite, consisting of -90% xenoliths in a fine grained igneous groundmass. The layer may represent the injection of a liquid tapped from near the top of the parent magma chamber, where quartz xenoliths had been concentrated through floatation.
Recent lithologies in the study area include extensive loess cover, raised beach deposits which represent sea-level fluctuations, and a mafic heavy mineral beach sand which occurs on the south coast of the peninsula, and is derived predominantly from the local dolerites through erosional processes.

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149 p. : col. ill., maps ; 30 cm.

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1993Brown

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POLYGON ((170.87805011965915 -45.352666839861563,170.877434875500171 -45.40296724934916,170.850795918331045 -45.403361996964378,170.824853421067445 -45.38560393584121,170.827818367106602 -45.35231756677905,170.87805011965915 -45.352666839861563))

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http://download.otagogeology.org.nz/temp/Abstracts/1993Brown.pdf

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Brown, Justin., “Aspects of the geology of Moeraki Peninsula and the geochemistry of the Moeraki dolerite ,” Otago Geology Theses, accessed May 22, 2024, https://theses.otagogeology.org.nz/items/show/274.

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