Incompatible trace element behavior in the Bushveld Complex

Borehole core from the uppermost 500 m of the Rustenburg Layered Suite of the Bushveld Complex has been analyzed for Rb, Sr, Zr, Y, and Nb. Plots of absolute trace element abundance and trace element ratios against stratigraphic height are presented. These are used to test for multiple injection of magma and postcumulus migration of residual liquid.These and previously published data are used to support the hypotheses that there was a new injection of magma above the Merensky Reef and that large-scale infiltration metasomatism did not occur. No evidence from incompatible element data exists for an addition of magma near the pyroxenite marker where a distinct reversal in cumulus mineral composition occurs. However, this may be the result of relatively small degrees of fractionation of the host magma prior to the addition of new magma. The ability to recognize the entry of new magma may be restricted by the analytical precision. The uppermost 500 m of the layered sequence appears to behave as a single, broadly homogeneous magma in showing a systematic, predictable upward increase in incompatible element abundances, consistent with a model assuming 10 to 30 percent trapped liquid in the cumulate. Deviations from the predicted trends for Zr, Y, and Nb are attributed to these elements being compatible in magnetite (for Zr) and apatite (for Y and Nb) which are present in the uppermost rocks.