In Situ Crystallization Under Contrasting Compaction Regimes Across the Boundary between the Lower and Upper Critical Zones of the Rustenburg Layered Suite, Bushveld Complex | In situ-kristallisation under kontrasterande kompaktionsregimer över gränsen mellan  den Nedre och Övre Kritiska Zonen,  Rustenburg Layered Suite, Bushveldkomplexet

The boundary between the Lower Critical Zone (LCZ) and Upper Critical Zone (UCZ) in the Rustenburg Layered Suite (RLS) of the Bushveld Complex in South Africa represents a key lithostratigraphic transition. It is marked by the first appearance of cumulus plagioclase and a shift in mineralogy and texture. This study focuses on textural analysis to investigate magmatic processes across the transition from the LCZ to the UCZ, with supporting insights from mineral chemistry. A ~302 m interval from borehole 5067_2_D on the Eastern Limb is investigated using petrographic observations, μX-ray fluorescence (µXRF) spectrometry, Electron Probe Microanalysis (EPMA), and Crystal Size Distribution (CSD) analyses. These techniques were used to explore crystallization mechanisms for orthopyroxene, plagioclase, and chromite, with emphasis on the MG3F anorthosite layer and surrounding lithologies.  CSD analyses for both orthopyroxene and plagioclase reveal largely log-linear trends with concavedown curvature at smaller grain-size fractions, suggesting crystal aging and coarsening due to compaction. Grain alignment factor analyses show high degrees of shape preferred orientation, particularly in the MG3F anorthosite layer and orthopyroxenites in the LCZ, consistent with compaction driven recrystallisation. This compaction likely led to upward expulsion of interstitial melt, reflected by increasing concentrations of interstitial plagioclase upsection in the LCZ. Additionally, orthopyroxene grains in the LCZ are coarser and more euhedral than in the UCZ, implying different magmatic and post-crystallization conditions between the two. Also, a compositional break is observed across the LCZ–UCZ transition, marked by an increase in FeO content in orthopyroxene and a decrease in plagioclase An%. While such trends may reflect changing cumulus assemblages, they are consistent with the input of compositionally distinct magma. The absence of crystal size grading, the log-linear trends of CSD slopes, and the lack of curved or kinked CSD trends at larger grain sizes suggest that gravitational settling was not the dominant accumulation process. Instead, the data support in situ crystallization at the mush-magma interface, potentially influenced by a sill-like injection of plagioclase-saturated melt forming the MG3F anorthosite at the LCZ-UCZ transition. If a sill intrusion did occur, the data indicate that it likely also crystallized in situ through continued differentiation. Finally, BSE images and EPMA analyses of orthopyroxene grains embayed by clinopyroxene reveal Ca-rich rims depleted in FeO and MgO, consistent with late-stage crystallisation of clinopyroxene.

Bushveld Drilling Project (BVDP)