Geochemistry and petrogenesis of S-type granites in the East Greenland Caledonides

Abstract

In the Kong Oscar Fjord region, East Greenland Caledonides, the formation of peraluminous leucogranite by low-temperature (<800°C) anatexis of metasedimentary rocks, and collection of the resulting melts into granite sheets and plutons can be studied in detail. Field observations suggest that a significant proportion of the biotite in the granites represents remnants of biotite from the metasedimentary source. This complicates geochemical modelling of granite formation, because chemical analyses do not necessarily represent melt compositions. Rb, Sr, Ba, and Eu concentrations in the granites are variable and suggest variations in the proportions of feldspar and mica retained in the residue. Despite uncertainties in modelling, Rb–Sr relationships suggest that granite formation was aided by the presence of externally derived H 2O-rich fluids, which could have been introduced along fluid pathways such as shear zones. The distribution of granite plutons indicates that they did not move over great distances, consistent with relatively wet magmas. Some biotite-rich granitoid rocks were formed by partial melting of the same metasediments, but at higher temperatures, during an earlier metamorphic event, while other mafic granites may have a totally different petrogenesis.