A GEOCHEMICAL AND EXPERIMENTAL STUDY OF THE ROLE OF K-FELDSPAR DURING WATER-UNDERSATURATED MELTING OF METAPELITES

Abstract

The production of S-type granites has been related to the extraction of melts during water-undersaturated melting of metasediments in the lower continental crust. If this mechanism is of widespread applicability, a chemically distinctive granulitic restite must be left behind with a positive Eu anomaly and depleted incompatible element concentrations. However, little geochemical evidence has been found in support of this hypothesis. The rare-earth element geochemistry of lower crustal granulites and associated melts will depend on the behaviour of the phases in which REE and trace elements are concentrated, i.e. zircon, monazite and K-rich feldspar. New experimental data on synthetic metapelites show that K-feldspar can be a product or a reactant during water-undersaturated melting of biotite, depending on the H2O/K2O of the melt relative to biotite. Past confusion over the role of K-feldspar during melting is due to the reaction being close to degeneracy and changing its stoichiometry in response to only small changes in bulk composition. Coherent interpretation of the role of K-feldspar in different experimental studies requires a better knowledge of phase water contents than has commonly been available. The leucosomes formed in some Antarctic granulite-facies migmatites are likely to have had low H2O/K2O ratios, which suggests that K-feldspar would have been a reactant during melting This could cause the positive Eu anomalies observed in the leucosomes, through the disequilibrium melting of K-feldspar during water-undersaturated melting. The undersaturation of trace elements and REE in the leucosomes strongly suggests that disequilibrium melting took place, caused by the incomplete dissolution of accessory phases, monazite and zircon. These phenomena give the melts a distinctive geochemical character which has also been seen in the other granulite-facies leucosomes analysed to date. This geochemical character contrasts with that of S-type granites and therefore the production of granulites in the lower crust may not necessarily be associated with this type of magmatism. If lower-crustal granulite-facies migmatites are the source region for peraluminous S-type granites then the melts generated must undergo considerable modification during ascent.