Massive magnetitite layers of the bushveld complex, South Africa: geology, geochemistry, and genesis
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Date
2021
Authors
Kruger, Willem Abram Jacobus
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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
Description
A thesis submitted in fulfilment of the requirements for the degree of Doctor of Philosophy to the Faculty of Science, School of Geosciences, University of the Witwatersrand, Johannesburg, 2021