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Metasomatism of cumulus magnesian olivine by iron-rich postcumulus liquids in the upper Critical Zone of the Bushveld Complex

Published online by Cambridge University Press:  05 July 2018

Roger N. Scoon
Roger N. Scoon*
Affiliation:
Department of Geology, Rhodes University, P.O. Box 94, Grahamstown 6140, South Africa
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Abstract

Hybrid olivine grains from locally concordant contact zones between bodies of iron-rich ultramafic pegmatite (postcumulate) and layered harzburgite (cumulate) are distinguished from cumulus olivine grains by petrographic features and compositional differences. The hybrid olivines, which in comparison with the cumulus crystals are Fe-Mn-rich and Mg-Ni-poor, represent an arrested stage of replacement and exhibit unusually high NiO/MgO ratios. These features are explained by a disequilibrium process of cation-for-cation exchange between crystals and silicate liquid i.e. magmatic metasomatism. Plots of cations across a contact zone give straight line relationships, a function of the extent to which the metasomatizing liquid infiltrated the cumulate layer. A plot of NiO against MgO enables a distinction to be made between metasomatic olivine and olivine that has fractionally crystallized from a tholeiitic magma. These metasomatic olivines all formed by the replacement of pre-existing cumulus olivine and no chemical evidence has been found to support the formation of olivine in iron-rich ultramafic pegmatite bodies by metasomatism of other phases.

Keywords

Bushveld Complexiron-rich ultramafic pegmatitemetasomatismnickelolivinepostcumulusSouth Africa
Type
Geochemistry
Information
Mineralogical Magazine , Volume 51 , Issue 361 , September 1987 , pp. 389 - 396
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1987

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