A Lewisian basement sheet within the Moine at Ribigill, north Sutherland

Synopsis A Lewisian basement sheet, consisting of hornblende and hornblende-biotite gneiss and amphibolite with a central zone of feldspathic migmatite, tectonically emplaced within the Moine is described using 98 major and trace element analyses of Lewisian and Moine rocks. The Lewisian rocks are quite distinct from the unmigmatized local Moine metasediments and can be distinguished from a Moine pre-metamorphic basic intrusion (the ‘Ben Hope Sill’) which forms part of the boundary of the Lewisian sheet. It is inferred, by comparison with published data, that the Ribigill Lewisian originally had a ‘Scourian-type’ geochemistry depleted in K, Rb, Pb, etc., during an early Lewisian high grade metamorphism. The rocks were retrogressed and migmatized during a later Lewisian event, with the introduction of Ce, Th, K, Rb, and possibly Na. The Ribigill migmatites differ from published accounts of Laxfordian migmatites in several respects most notably in having high K/Rb ratios.