The microstructural analysis was conducted by Electron backscatter direction analysis (EBSD), and it was coupled with elemental mapping (QEMSCAN) and mineral chemistry (electron probe micro-analyser - EMPA). All the analyses were performed at the University of Cambridge. Twenty samples of oxide layers and their surrounding rocks have been analysed by EBSD. The EBSD data produced crystal orientation maps, crystallographic pole figure data, and maps showing intra-crystalline deformation. Additional grain size analaysis have been done on the chromite grains from the chromitite layers. Data was used to interprete the depositional enviroments for the two oxide layers and their surrounding rocks. Samples that have been studied from the magnetitite layers of the Upper Zone of the Bushveld Complex are coming from the Bierkaal borehole drilled in the Western Limb.
The Magnetitite Layers and the surrounding anorthosites
The EBSD data showed that the amount of deformation intragrain microstructures (e.g. microstructure recorded within the single crystal) in the plagioclase crystals is the same in anorthosites that are located below and above the magnentite layers. Anorthosites are also characterised by two populations of plagioclase grains, large >1000 µm in diameter in small grains < 1000 µm (Fig 2). The small population of the plagioclase grain surrounds the large plagioclase grains (Fig. 2). These small grains are not a result of recrystallisation of the large grains but represent a generation of plagioclase that crystallised from the interstitial melt.
Plagioclase crystals within the magnetitite layers show three types of appearance: (1) isolated round grains; (2) single elongate, grains that record evidence of minor plastic deformation with only minor marginal recrystallisation; and (3) grains that preserve their original shape but with more extensive recrystallization along grain boundaries with other plagioclase grains. I proposed that these isolated plagioclase grains moved within this liquid-rich slurry, but in case of plagioclase aggregates, strain localised along the contacts of aggregated grains. The numerous small, rounded plagioclase grains may have originally formed part of recrystallised laths that subsequently disintegrated during the flow of the liquid-rich mush.
The Chromitite Layer and the surrounding anorthosites
The grain size analysis of the chromite crystals from the UG2 chromitite layer showed that there is no significant grain size variation within the main chromitite stream (Fig. 3a). Due to the uniformity of the crystal size, gravitational sorting is unlikely to have a strong effect in the deposition of chromite crystals. Crystallographic data show that chromite grains show no crystallographic preferred orientation and that chromite cluster shows no evidence of crystallographic control of individual crystal within a cluster (Fig. 3d).
The remnant of olivine crystals and pegmatoidal orthopyroxene in underlying harzburgitic rock is likely a product of the metasomatic reaction between olivine-rich rock and late Si-rich melt. Chromite texture is not affected by this metasomatic reaction. The overlying pyroxenite rock on the other side shows strong very magmatic fabric formed by deposition of pyroxene grains from a magmatic current. The pyroxene grains are mantled by fine chromite crystals. Like in the case of magnetitite layers, there is no strong microstructural evidence, that compaction had a significant role in the formation of these rocks.
