Mineralogical Evolution of Fayalite-Bearing Rapakivi Granites from the Prins Christians Sund Pluton, South Greenland

T. N. Harrison1, I. Parsons2 and P. E. Brown1, *
1 Department of Geology and Mineralogy, Aberdeen University, Aberdeen AB9 1AS, Scotland
2 Grant Institute of Geology, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, Scotland
*Now at: Dept. of Geology, The University, St. Andrews, Fife KY16 9ST, Scotland.

Abstract: 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 localised white facies is developed adjacent to metasedimentary xenoliths. 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 show that they are Fe-rich, comprising fayalite (Fa93−96.5), orthopyroxene (Fs77−81), 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°C and fO2, of 10−16.5 to 10−17.5 bars, and continued to grow under reducing conditions below the QFM buffer to temperatures below 450–500°C. Orthopyroxene formed in response to a low-pressure fayalite-consuming reaction in the melt. The correlation of black, pristine feldspar with anhydrous mafic silicates, and of turbid feldspar with hydrous phases suggests either that the feldspars reflect the anhydrous nature of the parent magma, or more likely that the mafic mineralogy of the white rapakivi granites is secondary.

Keywords: Rapakivi granite • South Greenland • fayalite • subsolidus biotite

Mineralogical Magazine; March 1990 v. 54; no. 374; p. 57-66; DOI: 10.1180/minmag.1990.054.374.05
© 1990, The Mineralogical Society
Mineralogical Society (www.minersoc.org)