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

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.