Fractionation and Assimilation Processes in the Alkaline Augite Syenite Unit of the Ilímaussaq Intrusion, South Greenland, as Deduced from Phase Equilibria

Abstract

The early augite syenite unit in the 1·13-Ga-old Ilímaussaq intrusive complex, South Greenland, consists of a magmatic assemblage of ternary alkali feldspar + fayalitic olivine + augite + titanomagnetite + apatite + baddeleyite ± nepheline ± quartz ± ilmenite ± zircon. Feldspar, nepheline and QUILF thermometry yield T = 1000–700°C, at P = 1 kbar, which is derived from fluid inclusion data from other parts of the complex. Ternary feldspar was the first major liquidus phase. It crystallized at temperatures between 950 and 1000°C from a homogeneous magma with aSiO2 = 0·8 and fO2 about 1·5–2 log units below the fayalite–magnetite–quartz (FMQ) buffer. Later, closed system fractionation produced nepheline-bearing assemblages with aSiO2 = 0·4 and log fO2 = FMQ – 3 to FMQ – 5. Assimilation of wall rocks produced local variations of melt composition. Four traverses through the unit were sampled parallel to the assumed direction of crystallization. They exhibit significant differences in their mineral assemblages and compositions. The chemical zoning and calculated intensive parameters of four sample suites reflect both closed system fractional crystallization and local assimilation of wall rocks.