Geochemical Studies on the Scourian Complex, N. W. Scotland

The gabbro-tonalite-trondhjemite-granodiorite-granite suite of the Scourian complex of northwest Scotland was studied at Scourie, Gruinard Bay and Torridon and represents highly deformed and metamorphosed plutonic igneous rocks, whose formation ca. 2900 Ma. ago represents the generation of new continental crust. Amphibolites, found predominantly in amphibolite facies gneisses at Gruinard Bay may be metavolcanic and are chemically similar to present-day island arc tholeiites; they are complementary to the layered cumulitic gabbro-anorthosite complexes in granulite facies gneisses and together they represent the remnants of an early basic volcanic-plutonic suite. Tonalite was generated by the partial melting of amphibolite, similar in chemistry to that found at Gruinard Bay. Trondhjemite, granodiorite and granite evolved from tonalite by the fractional crystallisation of plagioclase and hornblende at depths of less than 60 km. Granulite facies metamorphism at Scourie depleted originally wet tonalitic and trondhjemitic magmas in LIL elements but not potassium. Relatively dry granitic magmas were not depleted during granulite facies metamorphism. There is a change in the principal magmatype from Scourie in the north (tonalite) through Gruinard Bay (trondhjemite) to Torridon in the south (granodiorite) reflecting a southerly increase in K20 across the complex. Granulite facies grieisses at Scourie show a continuum of rock types between ultramafic and granitic, but amphibolite facies gneisses show a distinct bimodality. Igneous temperatures of 1000°C to 1150°C were obtained from feldspar and ilmenite-magnetite thermometry. The peak of the granulite facies metamorphism was at 820°C and 10 kb. and was determined using plagioclase-scapolite and garnet-pyroxene equilibria. Many mineral pairs equilibrated at lower temperatures and reflect the cooling history of the complex. Water was introduced into granite sheets during cooling at about 600°C. The Archaean crust at Scourie was originally 60 km thick and the thermal history of the area suggests that the crust was magmatically thickened. The plutonic igneous and metamorphic history of these rocks invite comparison with modern Cordilleran batholiths and suggest that the Scourian complex formed in an analogous tectonic setting.