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Unusual pegmatoid crystallisations in a nephelinite plug, near Round Lagoon, eastern Central Plateau, Tasmania

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Sutherland, FL, Barron, BJ, Pogson, RE, Everard, JL, Forsyth, SM and Hollis, JD 2008 , 'Unusual pegmatoid crystallisations in a nephelinite plug, near Round Lagoon, eastern Central Plateau, Tasmania' , Papers and Proceedings of the Royal Society of Tasmania, vol. 142, no. 2 , pp. 23-38 , doi: 10.26749/rstpp.142.2.23.

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Abstract

Pegmatoids in a Late Oligocene olivine nephelinite plug near Round Lagoon form a complex low-pressure fractionation suite. The host nephelinite contains meta-peridotite and meta-wehrlite mantle xenoliths and its composition (Mg# 0.63) may reflect both mantle and then limited fractionation. The pegmatoids range from ultramafic through mafic to feldspathic assemblages in a progressive, but discontinuous, fractionation sequence ~ olivine ~ sodalite ijolite, nepheline syenite ~ alkali syenite).
Within this sequence, olivine and clinopyroxene compositions decrease in content, while clinopyroxene becomes increasingly Na- and Fe-rich to produce late stage aegirine-augite and aegirine. Nepheline is prominent in the sequence and crystallised over a wide temperature range from 10000 to <500°C. The presence of sodalite suggests volatile Cl-rich fluxing. Mg-rich spinel crystallised in assemblages, distinct from Fe- and Ti-rich oxides of the magnetite-ulvospinel series in later assemblages. The Round Lagoon low-pressure pegmatoids developed by fractionation in a narrow, vertical feeder rather than in broad lava ponds such as those noted in nephelinite flows at Inverell, New South Wales, and at La Madera, Argentina.

Item Type: Article
Authors/Creators:Sutherland, FL and Barron, BJ and Pogson, RE and Everard, JL and Forsyth, SM and Hollis, JD
Keywords: Royal Society of Tasmania, RST, Van Diemens Land, natural history, science, ecology, taxonomy, botany, zoology, geology, geography, papers & proceedings, Australia, UTAS Library
Journal or Publication Title: Papers and Proceedings of the Royal Society of Tasmania
ISSN: 0080-4703
DOI / ID Number: 10.26749/rstpp.142.2.23
Collections: Royal Society Collection > Papers & Proceedings of the Royal Society of Tasmania
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