22The Lewisian Gneiss Complex of north-western Scotland consists of Archaean 23 gneisses, variably reworked during the Proterozoic. It can be divided into three 24 districts -a central granulite-facies district between districts of amphibolite-facies 25 gneiss to the north and south. Recent work has interpreted these districts in terms of 26 separate terranes, initiating a controversy that has implications for how Precambrian 27 rocks are understood worldwide. The northern district of the Lewisian Gneiss 28Complex (

S U M M A R Y: The early Proterozoic Loch Maree Group (LMG) is a supracrustal sequence occupying two belts, 5 km and 3.5 km wide respectively, at Gairloch and Loch Maree in the southern region of the Scottish Lewisian Complex. The LMG at Gairloch consists of about one-third metasediments, mainly metagreywackes, and two-thirds amphibolite. The LMG amphibolites are of igneous origin, possess a tholeiitic chemistry, and can be divided into two petrogenetic groups. Group A consists of the main amphibolite sheets, believed to represent lavas, which show little or no LREE enrichment and fiat incompatible element profiles, with some LILE depletion. Their chemical signatures are similar to those of oceanfloor (MORB) basalts. Group B consists of thin amphibolites and biotite-hornblende schists, believed to be sills, which show LREE and LILE enrichment trends quite different from group A and more similar to those of the well-documented Lewisian Scourie dykes.It is suggested that the voluminous group A amphibolites represent primitive basalts extruded rapidly, directly from the mantle, after a period of extensional rifting had caused significant crustal thinning. Subsequently, after the cessation of extension and the filling of the rift basin, later magmas, represented by the group B amphibolites, became subject to crustal contamination before being emplaced as sills within the supracrustal pile. An early, pre-rifting stage of extension is represented by the Scourie dykes, which possess normal continental basalt geochemistry.