Corroded Plagioclase Feldspar Inclusions in Orthopyroxene and Olivine of the Lower and Critical Zones, Western Bushveld Complex

H. V. Eales, W. D. Maier and B. Teigler
Department of Geology, Rhodes University, Grahamstown, South Africa

Abstract: The occurrence of spheroidal, ovoid or deeply embayed plagioclase inclusions within cumulus bronzite and olivine grains of both pyroxenites and norites here implies the existence of plagioclase feldspar in melts prior to the separation of ultramafic cumulates from these melts. Variations of Sr-isotope initial ratio are shown to exist within the feldspar populations of such rocks, even on a grain-to-grain scale. Five separate stratigraphic intervals within the Lower and Critical Zones exhibit this texture, and in each case these are intervals wherein normal fractionation trends (decline in both whole-rock and orthopyroxene Mg/(Mg+Fe2+); decline in Cr contents of orthopyroxene) became reversed. The data support a model for evolution of the texture during major episodes of influx of hot, primitive magmatic liquid into the chamber. Progressive mixing of the new liquid with resident liquids (bearing feldspar on the liquidus) effected progressive changes in the composition of the hybrid liquids produced, and resorption and imposition of reversed zoning upon the inclusions.

Keywords: Bushveld Complex • magma mixing • resorption • plagioclase

Mineralogical Magazine; September 1991 v. 55; no. 380; p. 479-486; DOI: 10.1180/minmag.1991.055.380.17
© 1991, The Mineralogical Society
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