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Low-pressure corona textures between olivine and plagioclase in unmetamorphosed gabbros from Black Hill, South Australia

Published online by Cambridge University Press:  05 July 2018

Simon P. Turner  and
Kurt Stüwe
Simon P. Turner
Affiliation:
Department of Geology and Geophysics, University of Adelaide GPO Box 498, Adelaide, South Australia 5001
Kurt Stüwe
Affiliation:
Department of Geology and Geophysics, University of Adelaide GPO Box 498, Adelaide, South Australia 5001
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Abstract

Olivine-plagioclase corona textures occur in ophitic to sub-ophitic olivine gabbros at Black Hill, South Australia. Contrasting with many corona and symplectite textures previously described, these do not involve spinel or garnet as reaction products and did not form under high-pressure conditions. Rather, the coronas formed at no more than 1 kbar pressure and are composed of a shell of orthopyroxene around the olivine often succeeded by a shell of amphibole or occasionally biotite. Beyond this, a vermicular symplectite of anorthite containing orthopyroxene and rarer amphibole vermicules extends out to host plagioclase of labradorite composition. Textural relations are used to infer a subsolidus igneous origin for all but the orthopyroxene shell which may have formed in the presence of some magma. Compositional zonation is absent from all the constituent phases except the amphibole shell which is strongly zoned in Mg# and may have a late origin. An average maximum corona width of 150- 200 μm indicates a limiting distance for subsolidus chemical diffusion. The corona products involve the reactants olivine and plagioclase in the proportions 1:3 and symplectite formation may have been promoted by a Na potential gradient. The system must also have been open to minor components including H2O and TiO2, with H2O possibly being derived from a hydrothermal system. Such systems may have been set up in the country rocks on intrusion of the magma and subsequently collapsed inwards into the pluton during sub-solidus cooling.

Keywords

olivine-plagioclasecoronasymplectitelow pressure
Type
Research Article
Information
Mineralogical Magazine , Volume 56 , Issue 385 , December 1992 , pp. 503 - 509
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1992

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Footnotes

*

Present address: Department of Earth Sciences, Open University, Milton Keynes, MK7 6AA, England.

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