THE LILLOISE INTRUSION, EAST GREENLAND - FRACTIONATION OF A HYDROUS ALKALI PICRITIC MAGMA

The Lilloise is an 8 kmx4 km layered mafic intrusion which cuts the plateau basalts of the East Greenland Tertiary province. Lilloise was intruded at similar to 50 Ma, 4-5 Ma after cessation of the voluminous tholeiitic magmatism which accompanied rifting of the East Greenland continental margin. Lilloise is unusual among layered intrusions in the province because it had a hydrous alkali picrite parent magma and generated a late-stage efflux of magmatic water from the intrusion into the aureole rocks. The three major subdivisions of the layered rocks are: olivine-clinopyroxene, olivine-clinopyroxene-plagioclase and plagioclase-amphibole cumulates Massive subsidence of the intrusion before complete solidification resulted in deformation of the internal layering and downturn of the bedding in the surrounding basalts. A striking feature of the intrusion is the injection of the layered rocks by a plexus of magmatic sheets which formed at the time of subsidence. The composition of these sheets is representative of the fractionation trend of the intrusion and ranges from hawaiite to mildly saturated quartz trachyte. The fractionation trend is successfully explained by extraction of cumulus minerals of the layered rocks from a parent magma represented by alkali picrite dykes of a contemporaneous regional dyke swarm. Saturated to mildly over-saturated syenites are a major component of the East Greenland province and the Lilloise intrusion is illustrative of an important magmatic trend towards such compositions at this stage in the opening of the North Atlantic.