Massive magnetitite layers of the bushveld complex, South Africa: geology, geochemistry, and genesis

Abstract

The petrogenesis of monomineralic rocks in layered intrusions is a petrological enigma containing important clues regarding the inner workings of magma chambers. Massive magnetitite layers of the Bushveld Complex, South Africa, are particularly useful to unravel magma chamber processes due to the exceptionally high crystal/liquid partition coefficients of Cr in magnetite. By studying the field relationships and distribution of Cr within and across the lowermost massive magnetitite layers of the Complex, new insights are derived regarding some of the most fundamental questions pertaining to our understanding of magmatic crystallization and differentiation. These include where and how do crystals accumulate, what is the thickness of crystal mushes in magma chambers, and how does crystal/liquid fractionation occur to drive magmatic evolution. Two-dimensional geochemical maps of magnetitite layers reveal fossilized solidification fronts that strongly argue for the in situ crystallization of this rock type as indicated by the presence of Cr-rich, dome-shaped growth nodes, the accumulation of magnetite on sub-vertical footwalls, and the outward growth of magnetitite on anorthositic inclusions. Extremely steep Cr gradients, coupled with a near-constant V concentration, show that the solidification front propagates as a near-solid surface, suggesting that crystal mushes may not be of significant thicknesses in layered intrusions. The latter requires effective exchange of liquid at the solidification front by the convective removal of thin compositional boundary layers surrounding magnetite crystals. Depressions in the anorthositic footwall underneath massive magnetitite layers together with the presence of an interconnected network of anorthosite inclusions suggest that thermochemical erosion (or magmatic karstification) of the basal cumulates occurred prior to massive magnetitite crystallization. This observation, along with large Cr reversals recorded in massive magnetitite layers, suggest magmatic recharge preceded the formation of this rock type. A pressure reduction associated with magmatic ascent from a deeper seated staging chamber, possibly coupled with contamination of the melt by crustal material, ensured the production of a relatively Cr-rich, superheated melt capable of crystallizing only magnetite upon entry and cooling in the Bushveld magma chamber. Mixing between the incoming and resident melt is responsible for the formation of anorthositic footwalls commonly associated with the massive magnetitite