Some petrological aspects of mafic rocks from four borehole sections between the Merensky Reef and the Main-Zone Gabbro in the Western and Eastern Bushveld complex

Abstract

Some petrological aspects of rocks from four borehole sections in the Main Zone of the Western and Eastern Bushveld Complex were investigated. Of these boreholes, two were drilled near Rustenburg, one near Northam, and the fourth one about 55 miles to the east of Potgietersrus. Rock types with similar volumetric mineral composition and texture occur in the same sequential positions in these boreholes and this makes it possible to distinguish ten zones. In some of these zones layering is very well developed. Some well developed changes in the volumetric mineralogical composition and texture, and particularly pyroxenite inclusions, seem to show beyond doubt that several heaves of magma were emplaced at intervals so that each heave normally came to rest upon the previous emplacement. These heaves of magma probably changed more or less systematically in chemical composition. The different associated rock and textural types which were distinguished, in order of decreasing abundance, are: hypidio­morphic hyperite, norite, and gabbro; synophitic hyperite and norite; ophitic to nesophi⁺ic hypidiomorphic hyperite; subophi⁺ic hypidiomorphic hyperite; hypersthene- and bronzite-beering mottled anorthosite and mottled leucohyperite; spotted anorthosite and spotted leuconorite; anorthosite; and leucohyperite. Plagioclase, orthopyroxene, end clinopyroxene are the main constituents of these rocks and seem to be mostly cumulus crystals. The hypersthene in the synophitic hyperite and norite and in the hypersthene-bearing mottled anorthosite and mottled leucohyperite originally crystallized as pigeonite which on slow cooling inverted to hypersthene. The inversion was in the majority of cases of such a nature that none of the original crystallographic axes were retained as is evident from the random orientation of sets of ( 001)- augite exsolution lamellae and blebs in the hypersthene. In the synophitic hyperite large clusters of hypersthene "grains" with very nearly the same optical orientation were formed when diversely orientated pigeonite grains inverted o hypersthene. A number of features in the hypersthene such as the spacing of inversion nuclei, the size and spacing of augite blebs, and the development of vermicular exsolution seem to be temperature controlled.