Investigations into syn-crystallization processes in the lower and critical zones of the Bushveld Complex, Republic of South Africa

The extent to which mineral chemistry and textures represent primary crystal growth or post-crystallization modification in layered intrusions is a fundamental problem for igneous petrologists. Layered intrusions are used to understand the unobservable mechanics of magmatic differentiation, but interpretations based on what are considered to be primary features are invalid if they have undergone extensive recrystallization. The Bushveld Complex is used in this study because of its fresh exposures and extensive size and importance. Recrystallization may take place as the liquid-crystal mush at the base of the intrusion undergoes compaction, causing an upwards re-distribution of evolved interstitial liquids and the subsequent chemical evolution of the magma chamber.; In Chapter 2, a breccia pipe in the Bushveld Complex is investigated to ascertain the role of volatile fluid overpressure in its emplacement. Microprobe analyses of plagioclase in the breccia pipe blocks reveal that these grains have higher anorthosite content (∼An85) than host rock plagioclase (∼An75), indicating that the breccia pipe blocks were transported ∼1500 m upward. Whole rock geochemical data are enriched in fluid-soluble elements, supporting the role of volatile fluids in the formation of the pipe. Fluid overpressure can develop by compaction and during crystallization of silicate liquids as fluids exsolve from the solidifying interstitial liquid.; In Chapter 3 modeled fractional crystallization results indicate that observed Bushveld mineral chemistries are generated by parent magmas with significant H2O content.; Physical evidence for a compacting mush zone is presented in Chapter 4, where quantitative textural samples show that Lower Zone orthopyroxene grains are larger, have higher aspect ratios, are better foliated and have a lower trapped liquid component than those of the Critical Zone. These data are consistent with significant compaction-driven recrystallization. Numerical modeling of concurrent compaction and crystallization provide a quantitative model of how the Lower Zone-Critical Zone transition may have formed: plagioclase is rare in the Lower Zone because compaction removes interstitial liquid before it reaches plagioclase saturation. Both the textures and the modal mineralogy are largely controlled by compaction and compaction-driven recrystallization; primary magmatic textures are not preserved.