Cyclicity in the Main and Upper Zones of the Bushveld complex, South Africa: crystallisation from a zoned magma sheet

The major element composition of plagioclase, pyroxene, olivine, and magnetite, and whole-rock 87Sr/86Sr data are presented for the uppermost 2.1 km of the layered mafic rocks (upper Main Zone and Upper Zone) at Bierkraal in the western Bushveld Complex. Initial 87Sr/86Sr ratios are near-constant (0.7073 ± 0.0001) for 24 samples and imply crystallization from a homogeneous magma sheet without major magma recharge or assimilation. The 2125 m thick section investigated in drill core comprises 26 magnetitite and six nelsonite (magnetite–ilmenite–apatite) layers and changes up-section from gabbronorite (An72 plagioclase; Mg # 74 clinopyroxene) to magnetite–ilmenite–apatite–fayalite ferrodiorite (An43; Mg # 5 clinopyroxene; Fo1 olivine). The overall fractionation trend is, however, interrupted by reversals characterized by higher An % of plagioclase, higher Mg # of pyroxene and olivine, and higher V2O5 of magnetite. In the upper half of the succession there is also th