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 pl ateau basalts of the East Greenland Tertiary province. Lilloise was in truded at similar to 50 Ma, 4-5 Ma after cessation of the voluminous t holeiitic magmatism which accompanied rifting of the East Greenland co ntinental margin. Lilloise is unusual among layered intrusions in the province because it had a hydrous alkali picrite parent magma and gene rated a late-stage efflux of magmatic water from the intrusion into th e aureole rocks. The three major subdivisions of the layered rocks are : olivine-clinopyroxene, olivine-clinopyroxene-plagioclase and plagioc lase-amphibole cumulates Massive subsidence of the intrusion before co mplete solidification resulted in deformation of the internal layering and downturn of the bedding in the surrounding basalts. A striking fe ature of the intrusion is the injection of the layered rocks by a plex us of magmatic sheets which formed at the time of subsidence. The comp osition of these sheets is representative of the fractionation trend o f the intrusion and ranges from hawaiite to mildly saturated quartz tr achyte. The fractionation trend is successfully explained by extractio n of cumulus minerals of the layered rocks from a parent magma represe nted 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 stag e in the opening of the North Atlantic.