Magma addition and possible decoupling of major- and trace-element behaviour in the Bushveld Complex, South Africa

Abstract

Incompatible trace-element abundances and ratios in cumulate rocks from the Bushveld Complex may be used to predict the levels at which new magmas may be injected. Sudden and sustained changes in absolute abundances suggest new magma addition while gradual increases are consistent with fractionation of a large body of magma. The present data suggest input above the Merensky Reef at the top of the critical zone. Near the top of the lower and main zones there may be other additions. However, in the latter case, the chemical effect is spread through a large vertical interval and suggests very slow diffusive mixing between new and residual liquids. The different behavioral patterns may be related to whether new magma intrudes above or below the residual magma, the former situation resulting in delays between timing of injection and significant influence in the chemistry of the cumulates.