Mineralogical evolution of fayalite-bearing rapakivi granites from the Prins Christians Sund

The Prins Christians Sund rapakivi granite pluton in South Greenland is a member of the early Proterozoic 'Rapakivi Suite ' and is emplaced into early Proterozoic Ketilidian migmatites. The pluton is composed predominantly of black or dark brown monzonites and quartz monzonites (collectively, rapakivi granites), although a loealised white facies is developed adjacent to metasedimentary xeno-liths. The back rapakivi granites are extremely fresh and have an anhydrous primary mafic mineralogy of olivine and orthopyroxene, with rare inverted pigeonite and clinopyroxene; minor amounts of biotite and amphibole occur in most fayalite-bearing rapakivi granites. Feldspars in these rocks are black and non-turbid. The white rapakivi granites have a wholly hydrous mafic silicate assemblage and turbid, white or cream-coloured feldspars. Electron microprobe analyses of the mafic silicates in the black rapakivi granites how that they are Fe-rich, comprising fayalite (Fa93_96 5), orthopyroxene (Fs77_51), ferro-pargasitic and ferro-edenitic hornblende (Fe/(Fe + Mg) = 0.72-0.93), and biotite (Fe/ (Fe + Mg) = 0.77-0.88). Both biotite and amphibole crystallised subsolidus, and often adopt symplectic morphologies. Biotite has formed in response to a fayalite-consuming reaction at temperatures below 650-700 ~ and fo, of 10-16.5 to 10-17.5 bars, and continued to grow under reducing conditions belo