On the relationship between cumulus mineralogy and trace and alkali element chemistry in an Archean granite from the Barberton region,South Africa

A change in the liquidus mineralogy from plagioclase-quartz-biotite to plagioclase-quartz-K feldspar-biotite during the in situ fractional crystallization of a granitic magma has a marked effect on the abundance of and interrelationships between Ba, Rb, Sr, K₂O and Na₂O. During plagioclase fractionation, Ba and Rb enrich in successive solids and melt, while Sr is depleted. The K₂O content of the solid phase (around 1%) is very different from that of the melt (greater than about 3%) while Na₂O contents are similar (about 4–5%) so that variations in the amount of intercumulus melt result in wide variation in the Na₂O/K₂O ratio (from about 5 in cumulus-rich to about 1 in intercumulus-rich samples). The incoming of K feldspar as a cumulus phase causes Ba to be depleted along with Sr in successively formed solids, while Rb continues to be enriched. A pronounced compositional hiatus with respect to trace elements therefore results. The K₂O and Na₂O contents of melt and cumulate are now similar (around 5% K₂O and 4% Na₂O), so that little scatter in Na₂O/K₂O ratio (around 1) occurs as a result of variation in the amount of intercumulus melt. In general, trace element data from a natural example in the Barberton Mountain Land, South Africa, fit the models well, although the observed scatter of the data is somewhat greater than predicted by simple variation in cumulus-intercumulus proportions—possibly the result of the non-uniform distribution of biotite in the samples.