The magmatic evolution of the Scourian complex at Gruinard Bay

Summary Scourian (Archaean) gneisses at Gruinard Bay form a bimodal suite of trondhjemites and amphibolites in which the trondhjemites form about 75% of the complex. The gneisses form a large-scale agmatite complex in which amphibolite, ultramafic rocks and tonalitic gneiss are enclosed in trondhjemitic and associated granitic gneiss. All lithologies have been metamorphosed to hornblende-granulite facies and subsequently retrogressed. Amphibolites are the earliest recognizable component of the agmatite complex and have a varied geochemistry, which in part reflects the heterogeneous nature of their source. It is postulated that they are fragments of Archaean ocean floor. As such they are potential source rocks for the quartzofeldspathic gneisses, and calculations show that the tonalite and trondhjemite can be derived by 30% and 20–30% partial melting respectively, of a basaltic source, similar in chemistry to the amphibolites, leaving a residue of clinopyroxene, garnet and hornblende. Granitic gneisses associated with the trondhjemite require a slightly lower degree of partial melting ( c. 10%) and an eclogitic residue. Comparisons with experimentally determined phase equilibria indicate that the partial melting took place in the pressure range 10–20 kb, significantly shallower than the depth of partial melting in modern subduction zones.