Implication of vertical and lateral compositional variations across the pyroxene marker and its associated rocks in the upper part of the main zone in the eastern Bushveld Complex

Abstract

On the basis of a detailed mapping program in the area between Laersdrif and Roossenekal in the eastern compartment of the Bushveld Complex, three composite profiles nearly perpendicular to the stratigraphic sequence from the upper parts of the subzone B of the main zone up to the main magnetite layer were constructed. Samples taken along these profiles were investigated by means of geochemical, petrographical, and mineralogical methods. These profiles, together with the data of the comparable profiles from Von Gruenewaldt (1973a and b) and Molyneux (1974), allowed construction of a north-south-trending cross section through this part of the layered sequence showing the vertical and lateral compositional variations.The anorthite (An) and ferrosilite (Fs) distribution along each profile within this cross section displays a reversal of the normal differentiation trend which commences a significant distance below the stratigraphic position of the pyroxenite marker. This zone of reversed differentiation has its maximum thickness near Roossenekal and wedges out toward the south.The isopleths of both the anorthite and ferrosilite values within the cross section wedge out in a similar fashion. Comparable isopleth patterns are displayed by the distribution of the modified differentiation index. This pattern is interpreted as reflecting lateral changes in the chemical composition of an evolved residual magma, modified by the influx of a new magma some time prior to the formation of the pyroxenite marker. The decrease in thickness of this abnormal zone toward the south parallels the degree of influence caused by this influx.The formation of the reversed zone is discussed in terms of recent experimental models of magma mixing within a density-stratified crystallizing magma chamber. The abnormal layer of magnetite gabbro and the directly overlying pyroxenite marker are regarded as products of an increase in the oxygen fugacity at the liquid solid boundary of the magma chamber, possibly caused by a short-lived increase in pressure thereby resulting in monocrystallization of pyroxene.