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Regional trends of chemical variation and thermal erosion in the Upper Critical Zone, Western Bushveld Complex

Published online by Cambridge University Press:  05 July 2018

H. V. Eales ,
M. Field ,
W. J. de Klerk  and
R. N. Scoon
H. V. Eales
Affiliation:
Department of Geology, Rhodes University, Grahamstown, South Africa
M. Field
Affiliation:
Department of Geology, Rhodes University, Grahamstown, South Africa
W. J. de Klerk
Affiliation:
Department of Geology, Rhodes University, Grahamstown, South Africa
R. N. Scoon
Affiliation:
Department of Geology, Rhodes University, Grahamstown, South Africa
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Abstract

A comparison of R.P.M. Union and Amandelbult Sections reveals close geochemical and stratigraphic correlations, but the sequence at the latter is more complex. Mafic members at Union Section are consistently more magnesian than their equivalents at Amandelbult Section and (where available data allow comparisons) than at Rustenburg Section. This, taken together with regional patterns of progressive attenuation and elimination of leucocratic rocks beneath harzburgitic layers, identifies Union Section as proximally located with reference to an irruptive centre from which primitive liquids were injected along the floor/supernatant liquid interface. Successive injections led to thermal erosion of the floor, causing ‘dimpling’ and ‘potholing’ on a local scale, and elimination of noritic-anorthositic layers on a regional scale. Such erosion of the floors beneath new inputs of magmatic liquid is likely to be initiated by only partial remelting, mainly of lower-temperature phases (sodic rims to zoned plagioclase grains, and Fe-enriched pyroxenes) within intercumulus space. It is argued that resultant contamination of the primitive liquids may have led to direct superposition of anorthosites (accompanied by rare troctolite) upon harzburgite, without intervening pyroxenite members, as in the Pseudoreef Multicyclic Unit.

Keywords

Bushveld complexCritical ZonegeochemistrySouth Africa
Type
Geochemistry
Information
Mineralogical Magazine , Volume 52 , Issue 364 , March 1988 , pp. 63 - 79
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1988

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