The mode of occurrence and petrogenesis of the sapphirine-bearing and associated rocks of West Greenland

Abstract

Seven occurrences of sapphirine-bearing rocks in the Fiskenæsset region are described in detail. They occur within a chromite-layered anorthosite complex that was metamorphosed by a hornblende-granulite facies metamorphism and then by a cordierite-amphibolite facies metamorphism. They were derived from spinel-layered ultramafic rocks that mostly occur as layers and lenses along the contacts between major metaanorthosite and pyribolite/amphibolite horizons within the complex. There are four types of sapphirine-bearing rocks - enstatite, pargasite, gedrite and phlogopite types, which represent a petrogenetic sequence involving increasing degrees of Si, Ca, K and H₂O metasomatism related to shearing and deformation along meta-anorthosite-pyribolite junctions. Their relationship to associated non-sapphirine-bearing rocks is described. Brief mineralogical and petrological data are given for the principal minerals and rocks. In addition, two occurrences in the Sukkertoppen region are described in detail; these are also localised in meta-norite-ultramafic lenses in high grade gneisses. It is concluded that sapphirine is stable within a wide range af pressure and temperature conditions, but within a limited range of chemical environments.