The geochemistry and petrology of the upper critical zone in the boshoek section, of the Western Bushveld complex

Abstract

The Boshoek section is located in the mafic part of the southwestern Bushveld Complex. The tectonic setting of the section is characterized by two deformational stages prior and during emplacement of the lower and critical zones. The warped floor of the southwestern Bushveld Complex resulted in lithological differences of the lower zone sequences in the Rustenburg and Marikana sections, and the subsequent transgression northwards towards the Boshoek section in response to the addition of fresh magma into the magma chamber. The upper critical zone in the Boshoek section is stratigraphically subdivided into several units on the basis of lithological features and geochemical trends. The Intermediate and Merensky Footwall units are further subdivided into sub-units on the basis of modal-mineral proportions and whole rock geochemical trends. The mechanism introduced for the formation of the upper critical zone is based on mineralogical and geochemical trends, together with the characteristics of the potential parental magmas to the lower and critical zones. The feldspathic pyroxenite layers are produced by the slumping of Bl magma from an elevated position down to the floor, while the sub-units of the Intermediate and Merensky Footwall units are initiated by periodic, gentle influxes of B2/B3 magma. Such an influx of magma can cause an increase in temperature and thus the return to plagioclase as the only mineral phase on the liquidus, in order to produce a mottled anorthosite. On cooling, orthopyroxene will join plagioclase on the liquidus to produce the noritic rocks. The formation of the Merensky and Bastard units is initiated by the emplacement of the main zone magma, B4, together with the slumping of Bl magma from an elevated position. The Bl magma is responsible for the feldspathic pyroxenite layers, while the hybrid magma consisting of Bl and B4, together with the B4 magma produced the more felsic layers by crystal settling.