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

The Lower Zone–Critical Zone boundary of the Bushveld Complex is an intrusion-wide, major stratigraphic transition from ultramafic harzburgite and pyroxenite in the Lower Zone to increasingly plagioclase-rich pyroxenites and norites in the Critical Zone. Quantitative textural and compositional data for 29 samples through this transition show the following: 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. The larger grain size of the Lower Zone results in crystal size distribution plots that are rotated to lower slopes and intercepts relative to those in the Critical Zone. Although all rocks show differing amounts of foliation, mineral lineations are weak to absent. These data are consistent with significant compaction-driven recrystallization in the study section. Numerical modeling of concurrent compaction and crystallization provides 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. However, as the crystal pile grows, plagioclase saturation is reached in the interstitial liquid before compaction is complete in more evolved pyroxenites, producing more abundant but still modest amounts of plagioclase characteristic of the Lower Critical Zone. It is concluded that both the textures and the modal mineralogy are largely controlled by compaction and compaction-driven recrystallization; primary magmatic textures are not preserved.