The Archaean Evolution of the Lewisian Complex of Gruinard Bay

Abstract

South-east of Gruinard Bay, the Lewisian gneiss complex is cut by metadolerites derived from Scourie dykes which occupy two directional sets and record low Laxfordian finite strains. The gneisses record effects of pre-dyke deformation of an early complex of plutonic granodioritic and tonalitic gneisses which in places retain magmatic features and show intrusive relationships with remnants of supracrustal rocks and layered ultramafic-leucogabbro sheets occurring as agmatite complexes within the gneisses. The regional structure outlined by these agmatites is dominated by large-scale mushroom-type interference patterns produced from the refolding of major tight to isoclinal folds with NW dipping axial surfaces by close to tight major folds with NE dipping axial surfaces. Minor shear belts occur on the limbs of the larger late folds. The pre-dyke history of the Gruinard Bay complex is similar to that of the type-Laxfordian complex, north of Scourie.