The Bushveld Igneous Complex gives

The Bushveld Igneous Complex gives an overall impression of differentiation (with Fe-enrichment) as suggested for Skaergaard. The initial cumulates are ultramafic in nature (magnesian orthopyroxenites and harzburgites, followed by a noritic sequence, a gabbronoritic sequence and eventually magnetite-bearing gabbros and ferro-diorites. However, Early Sr-isotopic data were instrumental in demonstrating the multiple intrusive nature of the Bushveld Complex, and the close association of intrusion with mineralisation, particularly at the level of the Merensky Reef (Kruger and Marsh, 1982; Kruger 1992). Numerous magma influxes dominate the lower part of the stratigraphy, whilst differentiation dominated the upper parts of the stratigraphy. According to Kruger, 1994, the Bushveld Complex as a whole can be viewed as having three
main magmatic lineages– the Lower and Critical Zone harzburgite to noritic lineage (with low Sr ratio from 0.705 – 0.7064), the Main Zone gabbronorite lineage (with high Sr ratio c, 0.7082) and the Upper Zone Ferich gabbronorite lineage (with Sr ratio c. 0.7075). The boundaries between these major magmatic episodes are major unconformities within the magma chamber coincident with the base of the Merensky Reef and the Pyroxenite Marker (Figure 23). However, isotopic evidence also suggests that during the formation of the Lower and Critical Zones there were repeated influxes of new magma, which expanded the chamber both upwards and outwards (Kinnaird et al, 2002).
During the accumulation of the Lower and Critical Zones, the chamber was continually fed in the
eastern and western lobes by olivine- and orthopyroxene- crystallizing magmas that formed the Lower and Critical Zones. Progressive mixing of new and residual fractionated magma resulted in the slow evolution from a harzburgite/orthopyroxenite dominated Lower Zone, through a feldspathic orthopyroxenite dominated lower Critical Zone, to a norite/anorthosite dominated upper Critical Zone. More than one magma type was intruded during this time and may have varied over time from more ultramafic magmas in the initial stages to more noritic magmas in the upper Critical Zone. Interaction of the influxes of new melt with a roof melt, now represented by the granophyric rocks, resulted in variations in isotopic ratio and the production of major chromitite layers. This process is schematically illustrated in Figure 25.