The Lower Zone-Critical Zone Transition of the Bushveld Complex: a Quantitative Textural Study

The Lower Zone–Critical Zone boundary of the BushveldComplex is an intrusion-wide, major stratigraphic transitionfrom ultramafic harzburgite and pyroxenite in the Lower Zoneto increasingly plagioclase-rich pyroxenites and norites inthe Critical Zone. Quantitative textural and compositional datafor 29 samples through this transition show the following: LowerZone orthopyroxene grains are larger, have higher aspect ratios,are better foliated and have a lower trapped liquid componentthan those of the Critical Zone. The larger grain size of theLower Zone results in crystal size distribution plots that arerotated to lower slopes and intercepts relative to those inthe Critical Zone. Although all rocks show differing amountsof foliation, mineral lineations are weak to absent. These dataare consistent with significant compaction-driven recrystallizationin the study section. Numerical modeling of concurrent compactionand crystallization provides a quantitative model of how theLower Zone–Critical Zone transition may have formed: plagioclaseis rare in the Lower Zone because compaction removes interstitialliquid before it reaches plagioclase saturation. However, asthe crystal pile grows, plagioclase saturation is reached inthe interstitial liquid before compaction is complete in moreevolved pyroxenites, producing more abundant but still modestamounts of plagioclase characteristic of the Lower CriticalZone. It is concluded that both the textures and the modal mineralogyare largely controlled by compaction and compaction-driven recrystallization;primary magmatic textures are not preserved. KEY WORDS: Bushveld Complex; compaction; crystal size distributions; crystal aging; igneous textures