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

Abstract

The Upper Zone is the uppermost portion of the Rustenburg Layered Suite of the Bushveld Igneous Complex and has long been interpreted to have formed from the differentiation of a single magma pulse through closed-system fractionation. This is in contrast to theories proposed for the formation of the lower portions of the Bushveld, which state that multiple cycles of new magma injections occurred. In this study, the discovery of rocks believed to be igneous xenoliths in Subzone C of the Upper Zone in the Eastern Limb of the Bushveld Complex is investigated. The study area has not been extensively investigated in the past and the outcrops referred to in this study will be properly described for the first time. The xenoliths feature remnant trellis ilmenite exsolution textures that have not been observed elsewhere before. Extensive field work was conducted, as well as petrographical and geochemical analyses, that led to the conclusion that these foreign blocks were xenoliths of igneous origin. Furthermore, when geochemical and SEM data are analysed, the xenoliths show strong evidence of hydrothermal alteration and contain highly altered minerals, which the host rock does not, further solidifying the idea that these samples are, in fact, xenoliths. The presence of remnant ilmenite suggest that the xenoliths originated from magnetite bearing units, and since the only magnetite-bearing units in the Bushveld Complex are found in the Upper Zone itself, this implies that the xenoliths originated from elsewhere in the Upper Zone. Therefore, it was concluded that they could be further classified as being autoliths. This directly challenges the single magma theory, since the presence of autoliths from the Upper Zone in subzone C suggests that at least one other magma pulse contributed to the formation of the Upper Zone, and thus the Upper Zone did not form in a closed system.