The petrogenesis of the Kangerdlugssuaq alkaline intrusion,east Greenland

Evidence of layering and physical conditions of emplacement are discussed for this saucer-shaped, differentiated syenite mass. The intrusion is thought to derive from a magma of quartz trachytic composition,rather than from partial melting of the basement gneiss. The derivation of an undersaturated residual liquid, necessary to produce the pulaskite and the foyaite, is discussed, together with the mechanism whereby the ‘thermal barrier’ is crossed. Possible explanations considered are the depression of the thermal barrier through increased vapour pressure; the formation of iron-bearing feldspar; escape of silica and some potassium with volatile constituents; and crystal ? liquid equilibrium control.