A felsic end to Bushveld differentiation

The similarities in age (c. 2·06 Ga) and isotopic composition of the Bushveld Igneous Complex and the overlying Rooiberg Group felsic volcanic rocks suggest the possibility that the two are cogenetic. To investigate this possibility we derive a robust new estimate of the major and trace element bulk composition of the Bushveld Upper Zone and Upper Main Zone (UUMZ). Using MELTS thermodynamic modeling, we show that this composition fails to reproduce fundamental features of the cumulate sequence, such as the presence of primary orthopyroxene at the base of the magma column. We investigate the possibility that some amount of evolved magma escaped from the magma chamber and derive a new estimate of the UUMZ parent magma composition that reproduces generally the observed cumulate sequence. Our results constrain the amount and composition of the escaped magma and provide evidence of a genetic link between the UUMZ of the Bushveld layered series and the felsic volcanic rocks immediately overlying it. We conclude that between 15 and 25% of the original magma volume was expelled and is manifest either as part of the upper Rooiberg Group lava sequence or the Rashoop Granophyre (distinction is hindered by their almost identical major and trace element compositions). A parent magma in which 15-25% Rooiberg or Rashoop material is added back to the extant cumulate sequence is initially saturated in orthopyroxene and is in major and trace element equilibrium with the phases at the base of the UUMZ.