THE ORIGIN OF IGNEOUS LAYERING IN THE NUNARSSUIT SYENITE, SOUTH GREENLAND

The rhythmic modal layering in the Nunarssuit syenite has a vertical e xtent of >150 m and a lateral extent of >15 km. Individual layers aver age 20 cm thick and grade from a relatively melanocratic base into mor e leucocratic syenite over a distance of up to 5 cm. The major cumulus phases are alkali feldspar, ferro-salite/hedenbergite and fayalite. T wo basic stratigraphic cycles have been identified in which faint moda l layering becomes more pronounced up section, each cycle terminating in a thick melanocratic zone. Slumps, slump breccias, troughs, micro-r hythmic layering and one occurrence of crossed layers were observed. Q ualitative grain size analysis indicates no size sorting in the layers . Preliminary application of crystal size distribution theory to ferro -salite/hedenbergite and fayalite from the bases of individual layers gives results which may be interpreted as indicating a relative lack o f coarse grains. If the layers were deposited from density currents it would be expected that the coarsest grains would be deposited close t o the source of the currents. There is no evidence in the majority of the syenite that the cumulus pile underwent compaction during crystall ization. There was little, or no, primary chemical variation across in dividual layers. Whole-rock compositions and the ferro-salite/hedenber gite, fayalite, biotite and amphibole present in the syenites show a s light, but statistically significant, increase in the ratio Mg/(Mg+Fe- total), from the base up to the top of the layered succession. A model is suggested in which successive magma layers become more ferroan tow ards the top of the chamber. Cooling is concentrated at the top of the chamber and layers of magma crystallize sequentially, the uppermost, ferroan layers first. As layers of magma cool and crystallize they sin k, as crystal-melt plumes, to the bottom of the chamber where they sou rce density currents from which layers are deposited.