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

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

Jan 1978

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1978gecoa..42...21m&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 42, Issue 1, pp.21-26

Computer Science

11

Scientific paper

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, K 2 O and Na 2 O. During plagioclase fractionation, Ba and Rb enrich in successive solids and melt, while Sr is depleted. The K 2 O content of the solid phase (around 1%) is very different from that of the melt (greater than about 3%) while Na 2 O contents are similar (about 4-5%) so that variations in the amount of intercumulus melt result in wide variation in the Na 2 O/K 2 O 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 K 2 O and Na 2 O contents of melt and cumulate are now similar (around 5% K 2 O and 4% Na 2 O), so that little scatter in Na 2 O/K 2 O 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.

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