Abstract
The Merensky Unit, Bushveld Complex, is commonly described using genetic terms such as “cyclic unit”, typically without careful consideration of the connotations. We suggest that this contributes to the debate on processes forming the unit. This study integrates an extensive field study with detailed petrographic and textural analyses of the Merensky Unit to determine whether it is a “cyclic unit” sensu stricto. The study indicates that the bulk of the platinum-bearing chromitite-feldspathic orthopyroxenite developed through heterogeneous nucleation and in situ growth during multiple replenishment events. The overlying leuconorite developed above a gradational boundary, reflecting mixing following replenishment by a relatively more evolved magma. The bulk of this unit also formed in situ. The uppermost poikilitic anorthosite formed above a distinct boundary through a subsequent injection of a plagioclase-saturated magma, which crystallised in situ. Processes of gravitational settling and local remobilisation of crystals cannot be discounted from contributing to the development of the unit. The final textures throughout the unit developed through pervasive textural equilibration, with extensive fluid-mediated textural equilibration forming the megacrystic feldspathic orthopyroxenite. The evidence for at least five replenishment events indicates that the Merensky Unit is not a cyclic unit; therefore, the genetic term, “Merensky Cyclic Unit”, is misleading and its use should be carefully considered in future work.
Highlights
Igneous layering is a spectacular feature of many complexes around the world, with some of the best-known occurrences described in the Bushveld Complex, South Africa (Figure 1)
The first stage in the formation of the Merensky Unit was a replenishment event by a superheated magma
This rapidly spread over the magma chamber floor and resulted in widespread erosion, developing the undulating basal contact of the Merensky Unit and potholes in spatially restricted regions
Summary
Igneous layering is a spectacular feature of many complexes around the world, with some of the best-known occurrences described in the Bushveld Complex, South Africa (Figure 1). This displays laterally persistent layering, which can be traced for up to 100 km across outcrops [1]. Despite much geological interest in the development of igneous layering, there is surprisingly little consensus on the processes involved ([2] and references therein) Even fundamental concepts, such as the existence of magma chambers, have recently been questioned [3]. In combination with detailed petrographic and textural
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