Waipata volcanic formation

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Donnelly, Kathleen E., 1972-

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Abstract:

The W aipiata Volcanic Formation (WVF) are in trap late, sodic alkaline volcanics erupted during the Miocene in Otago, South Island. The WVF precedes the Dunedin Volcano, a major shield volcano centered over Port Chalmers. The WVF is peripheral to the Dunedin Volcano, lying up to 95 km from the Volcano center. Volcanism is characterized by short-lived, mongenetic eruptions of intrusives, flows, and tephra. The WVF has trace element chemistry characteristic of OIB. Volcanics are LREEenriched. with La/Lu ratios of 71-436, La abundances 70-500 times chondrite values, and Lu abundances 8-15 times chondrite values. Incompatible element contents increase with increasing silica undersaturation in samples. Thus, nephelinites are expected to have highest incompatible element contents; however, relative depletions in HREE, HFSE, K, Rb, and Ba occur in nephelinites and to a lesser extent in basanites. Depletions in these trace elements is attributed to residual garnet and amphibole in the source. The proposed mantle source is a garnet lherzolite with minor amphibole (5%). It is LREE-enriched with La abundance 15 times chondrite values and Yb abundance 4.5 times chondrite values. Three lineages, alkali basalt, basanite, and nephelinites, can be derived by varying degrees of melting. Silica undersaturation increases with decreasing partial melting. Alkali basalts result from 16% partial melting, while basanites result from 7.5% partial melting. Nephelinites are difficult to model because no primary melt nephelinites are observed it the WVF. It is suggested that nephelinites result from <5.5% partial melting. Fifteen samples represent primary melts from the mantle. All fifteen are basanites except for one alkali basalt. The remaining WVF samples have undergone fractionation. Early fractionation in the alkali basalt lineage to form more evolved alkali basalts involves olivine, c.linopyroxene, and spinel. Hawaiite formation requires the fractionation of olivine, clinopyroxene, plagioclase, and titanomagnetite. Fractionation in the basanite lineage is dominated by olivine, clinopyroxene, kaersutite, and titanomagnetite. The nephelinite series could not be modeled because a suitable parent is not known. Fractionation likely involves olivine, clinopyroxene, kaersutite, and oxides. Reverse zoning in clinopyroxene and plagioclase megacrysts and the presence of green-cored pyroxenes suggest that magma stagnation is an important process producing chemical and mineralogical variations in the WVF. Two different types of green cores occur: fassaites and salites. Trace element data show that fassaites have similar patterns to mantle, rim, and groundmass pyroxenes, but salites do not (Duda and Schmincke, 1985). Plagioclase megacrysts are sodium-rich. They are more sodic than most groundmass plagioclase compositions. An evolved, cognate magma is the source of the fassaites and plagioclase megacrysts. The source of the salite is enigmatic. At depth a common plumbing channels all magmas. Duda and Schmincke ( 1985) provide evidence that magmas stagnate near the crust/mantle boundary where magmas spread out into smaller, discrete magma chambers. Here magmas begin fractionate, gradually becoming more evolved. Fassaites and plagioclase megacrysts are among the phases precipitated. Occasionally, fresh, primitive pulses of magma rise through the same conduits. Pulses may be energetic, scouring the sides of the conduits and entraining fractionated phases such as the green pyroxenes and plagioclase megacrysts. The volume of evolved magmas is small. Thus, mixing may go undetected. Isotope studies show that the WVF is influenced by two mantle components: HIMU and DMM. The Dupal anomaly is not present. HIMU originates from the deep mantle as a plume. This plume upon impingement of the lithosphere flattens out and becomes fossilized. HIMU signatures are prevalent in South Pacific volcanism following the break-up of Australia, Antarctica, and New Zealand, suggesting that the HIMU plume rose prior to the break-up of Australia, Antarctica, and New Zealand. The HIMU plume may have played a role in the break-up.

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121 p. : ill. (some col.), maps (some col.) ; 30 cm.

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1996Donnelly

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http://download.otagogeology.org.nz/temp/Abstracts/1996Donnelly.pdf

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Donnelly, Kathleen E., 1972-, “Waipata volcanic formation ,” Otago Geology Theses, accessed May 22, 2024, https://theses.otagogeology.org.nz/items/show/320.

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