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The creation and evolution of crystal mush in Upper Zone of the Rustenburg Layered Suite, Bushveld Complex, South Africa

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Holness, Marian 
Stock, Micheal 
Roberts, James 


The Upper Zone of the Rustenburg Layered Suite of the Bushveld Complex contains the world’s largest Fe-Ti-V±P deposit and formed from the last major injection of magma into the chamber. Quantitative textural analysis of Upper Zone rocks was undertaken to constrain the processes operating during mush formation and solidification, focussing on horizons with the greatest density contrast to isolate the effects of gravitational loading. We examined three magnetitite layers, together with their underlying and overlying anorthosites. The similarity of microstructures in anorthosites above and below the dense magnetitite layers suggests that the rocks were not affected by viscous compaction driven by gravitational loading. The magnetitite cumulate layers formed by crystal accumulation from a mobile crystal slurry dominated by the Fe-rich conjugate of unmixed immiscible liquid. We suggest a new mechanism of crystal nucleation in deforming crystal-rich systems, driven by undercooling caused by cavitation as grains slide past each other during simple shear. We propose that the super-solidus deformation recorded in these rocks was caused by prolonged regional subsidence of the magma chamber at Upper Zone times.



anorthosite, magnetitite, Bushveld Complex, crystal mush, cavitation creep, subsolidus deformation, microstructure

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Journal of Petrology

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Oxford University Press
Natural Environment Research Council (NE/N009894/1)
European Commission Horizon 2020 (H2020) Marie Sk?odowska-Curie actions (708131)