Fractionation and assimilation processes in the alkaline augite syenite unit of the Ilimaussaq intrusion, South Greenland, as deduced from phase equilibria

The early augite syenite unit in the 1.13-Ga-old Ilimaussaq intrusive compl ex, South Greenland, consists of a magmatic assemblage of ternary alkali fe ldspar + fayalitic olivine + augite + titanomagnetite + apatite + baddelyit e +/- nepheline +/- quartz +/- ilmenite +/- zircon. Feldspar nepheline and QUILF thermometry yield T = 1000-700 degreesC, at P = 1 kbar which is deriv ed from fluid inclusion data from other parts of the complex. Ternary felds par was the first major liquidus phase. It crystallized at temperatures bet ween 950 and 1000 degreesC from a homogeneous magma with a(SiO2) = 0.8 and f(O2) about 1.5-2 log units below the fayalite-magnetite-quartz (FMQ) buffe r. Later, closed system fractionation produced nepheline-bearing assemblage s with a(SiO2) = 0.4 and log f(O2) = RMQ - 3 to FMQ - 5. Assimilation of wa ll rocks produced local variations of melt composition. Four traverses thro ugh the unit were sampled parallel to the assumed direction of crystallizat ion. They exhibit significant differences in their mineral assemblages and compositions. The chemical zoning and calculated intensive parameters of fo ur sample suites reflect both closed system fractional crystallization and local assimilation of wall rocks.