A geochemical study of Lewisian metasedimentary granulites and gneisses in the Scourie-Laxford area of the north-west Scotland

Abstract

AbstractMajor element, trace element, and REE data for metasedimentary granulites and their retrogressed derivatives formed from Archaean parent-rocks at two localities in the Lewisian complex of north-west Scotland are presented.The metasedimentary rocks are enclosed in and intruded by metatonalites and related calc-alkaline rocks and have been highly deformed and metamorphosed along with these rocks. They are geochemically distinct from this meta-igneous suite and range from highly aluminous to highly siliceous types. Comparisons of major and trace element data with those for unmetamorphosed Archaean and post-Archaean sediments suggest that they are derived from a detrital shale-greywacke assemblage, which may have included a volcaniclastic component. High ΣREE and ΣLREE suggest that the source-rocks included fractionated felsic igneous rocks.Evidence that depletion in the large ion lithophile elements K, Rb, Th accompanied high-pressure granulite metamorphism supports the view, based on data derived from the associated meta-igneous rocks, that depletion was effected by active fluids rather than by abstraction of a partial melt and suggests that removal of K and Th began only when a considerable reduction of Rb had taken place.