Petrology and deformation of metamorphosed volcanic-exhalative sediments in the Gairloch Schist Belt,N.W. Scotland

The Gairloch Schist Belt in the Archaean to Early Proterozoic Lewisian Complex of north-west Scotland is largely composed of amphibolite facies metabasites and metagreywackes. These are associated with a distinctive suite of metamorphosed volcanic-exhalative sediments including quartz-magnetite rocks, garnet-grunerite rocks and compositionally variable, siliceous calcite- and dolomite-bearing lithologies. The carbonate horizons are locally rich in sulphide and carry Cu-Zn-Au mineralization. Meta-exhalites occur within parts of metavolcanic units characterized by metamorphosed tuffs and tuffs mixed with exhalative material. Quartz-magnetite and carbonate horizons were dismembered and underwent mylonitic recrystallization during regional compression. The associated metabasic rocks in the shear zones have suffered extensive phyllonitization. This style and degree of deformation are not developed elsewhere in the immediate area which suggests that ductile shear zones in the Gairloch Schist Belt were preferentially initiated near and localized around the meta-exhalative horizons.