The origin of the lowermost succession of the Rustenburg Layered Suite (RLS) of the Bushveld Complex remains poorly understood, mainly because of limited exposure and lack of drilling. This study describes the lithostratigraphy and whole-rock and mineral major element geochemistry of three drill cores from the Clapham area in the eastern Bushveld Complex. The cores provide a continuous 2100 m section from the ultramafic Lower Zone downwards through the noritic Marginal Zone into a previously

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... nown section (750 m thick) of highly magnesian pyroxenites, harzburgites and dunites that is here called the Basal Ultramafic Sequence (BUS). The Clapham BUS consists of hundreds of distinct layers, with a great variety of compositional changes. For comparison with the BUS new data are presented for a section of drill core from the northern limb of the Complex and the extensive ultramafic body in the Burgersfort area. This work provides greater understanding of the nature of the lowest succession in the eastern Bushveld and how the major rock units of the Lower Zone, Marginal Zone and the BUS were formed and related to each other. The BUS represents the first stage of filling of a number of precursor magma compartments in the eastern Bushveld Complex, which are separated from each other by basement highs, and which amalgamated at higher stratigraphic levels as the chambers expanded. The compositions of olivine and orthopyroxene through much of the BUS are the most magnesian (molecular magnesium number or Mg# > 0Á91) observed for the Bushveld Complex and, therefore, require a parental magma that was more ultramafic than the widespread B1 magma, generally regarded as the parent for the Lower and Critical Zones. The new field observations and the mineral geochemical data also reveal that the Marginal Zone norites were formed by fractionation of the BUS magmas at the roof of the early magma chamber and do not represent the oldest rocks of the Bushveld Complex, as has been previously suggested. This is also supported by the occurrence of abundant countryrock xenoliths within them. Prior to its complete solidification, the Marginal Zone was intruded at its upper levels by primitive magmas of the Lower Zone, causing a steep but smooth reversal in orthopyroxene compositions from Mg# 0Á71 to 0Á86 over a vertical interval of less than 50 m. The interface between these magmas is seen in field exposures as finger-like crystal mush interactions between norite and pyroxenite. Apart from the chill sequence at the base of the BUS (lowermost 150 m), the Mg# of olivine and pyroxene in harzburgites and pyroxenites remains relatively constant throughout the section, despite important lithological variations. Mineral composition trends commonly transcend boundaries of modal layers with no evidence of compositional reversals. This suggests processes of physical separation of olivine and pyroxene, possibly as crystal slurries. The compositions and forms of the layering indicate a process of continuous small-volume magma replenishment, rather than episodic large influxes, creating a balance between fractionation and magma input. Discontinuities in geochemical trends indicate that parental magmas of different compositions may have been emplaced synchronously during the formation of the Lower Zone and the BUS.