Regional Trends of Chemical Variation and Thermal Erosion in the Upper Critical Zone, Western Bushveld Complex

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

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 complex • Critical Zone • geochemistry • South Africa

Mineralogical Magazine; March 1988 v. 52; no. 364; p. 63-79; DOI: 10.1180/minmag.1988.052.364.06
© 1988, The Mineralogical Society
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