Origin and ancient history of gneisses older than 2,800 Myr in Lewisian Complex

Abstract

THIS work records new field observations (to be discussed in detail elsewhere) which suggest that the pre-2,800 Myr history of the Lewisian Complex in the type localities of Scourie and Laxford, north-western Sutherland, was closely comparable with that of Godthab, western Greenland. In western Greenland Precambrian rocks have been grouped into older (Amitsoq) and younger (Nuk) gneisses1, separated in time by the emplacement of metamorphosed mafic–ultramafic Ameralik dykes. Supracrustal rocks and layered cumulitic intrusions, probably younger than the Amitsoq gneisses but older than the Nuk gneisses are also present and have all been varyingly affected by high-grade amphibolite or granulite facies metamorphism. Although the Lewisian Complex contains gneisses which, together with supracrustal and layered cumulitic igneous rocks, have suffered the widespread granulite facies metamorphism dated at about 2,800 Myr BP and can be compared with those of western Greenland1,8, equivalents of the Amitsoq and Nuk gneisses have previously not been distinguished.