THE DISCOVERY OF IGNEOUS XENOLITHS IN THE UPPER ZONE OF THE BUSHVELD IGNEOUS COMPLEX AND ITS IMPLICATIONS ON THE MODEL OF EMPLACEMENT OF UPPER ZONE MAGMAS

The new discovery of a layer of xenoliths just below the magnetite layer known as Layer 21 in the Upper Zone of the Bushveld Complex directly challenges the idea that the ca. 2km thick Upper Zone crystallised from a single pulse of magma. The xenoliths are of mafic origin, and contain ilmenite mineral growths indicating that the original rock contained magnetite prior to incorporation into the host magma. Contact metasomatism and metamorphism are also observed in these samples, evidenced by the presence of low grade metamorphic mineral assemblages that are not present in the host rock. In this study we provide data obtained through extensive field work as well as petrographical and geochemical investigations in order to prove that these samples are xenoliths of igneous origin and most likely also from lower stratigraphic units of the Upper Zone. This discovery could be the physical evidence to prove that at least one other magma pulse contributed to the formation of the Upper Zone. This suggests that an open system model should be examined rather than the previously accepted closed system fractionation theory.