An archaean granulite-grade tonalite-trondhjemite-granite suite from scourie,NW Scotland: Geochemistry and origin

The Lewisian complex of the Scourie-Badcall area is composed predominantly of banded tonalitic gneiss which intrudes layered gabbro-ultramafic complexes. Intrusive into both gabbro and tonalitic gneiss are homogeneous acid sheets which are trondhjemitic to granitic in composition. All rocks were subjected to granulite facies metamorphism. Smooth continuous trends on chemical variation diagrams suggest that the evolution of these rocks was dominated by fractional crystallisation. A scheme is proposed whereby a tonalitic melt was parental to trondhjemite and granite. Variation within tonalites was a function of the fractional crystallisation of hornblende and plagioclase, and trondhjemite was derived from tonalite by the fractional crystallisation of hornblende and/or plagioclase. Granite and granodiorite represent residual liquids which evolved along the quartz-feldspar cotectic surface; they were derived by the fractional crystallisation of plagioclase from a trondhjemite liquid. Some trondhjemitic sheets are quartz-plagioclase residues from which a granitic melt was removed. The associated gabbros and ultramafic rocks are not directly related to the proposed fractional crystallisation scheme and are not crystal residues removed from the tonalitic melt. Tonalites were probably derived from a basaltic source by partial melting or fractional crystallisation with either hornblende and/or garnet as residual phases.