Large-scale liquid immiscibility at the top of the Bushveld Complex, South Africa

Detailed study of the top 625 m of the Bushveld Complex (South Africa) suggests that the magma developed a siliceous cap generated by liquid immiscibility. The lower 300 m of this section, which consists of magnetite-bearing ferrodiorite with several magnetite-rich layers, contains cumulus apatite with relatively low rare earth element (REE) concentrations and no chondrite-normalized Eu anomaly. In contrast, apatite in the overlying 325 m thick portion of the section has three times higher REE concentrations and a negative Eu anomaly, and the cumulate rocks contain cumulus orthoclase and quartz. The contact between the two parts of the section occurs over a limited stratigraphic range (<50 m). Large-scale silicate liquid immiscibility resulting in the physical separation of a dense, Fe-rich magma to form the lower part of the sequence and a buoyant Si-rich magma to form the upper part of the sequence is best able to explain the observations. In addition, the phases in the upper siliceous cap of the Bushveld Complex are in major and trace element equilibrium with portions of the overlying Rooiberg rhyolites and/or Rashoop granophyres, consistent with these rocks having originated as residual liquids after extensive fractional crystallization of the Bushveld magmas.