Some geochemical distinctions between Moinian and Lewisian rocks, and their use in establishing the identity of supposed inliers in the Moinian

Abstract

Synopsis Whole-rock analysis of Moinian rocks, and comparisons of their compositions with those of Foreland Lewisian rocks, shows that persistent chemical differences occur. Sedimentation processes explain adequately the lower FeO/ TiO 2 and K/Rb ratios, and the high concentration of zirconium characteristic of the Moinian. Whole-rock analysis of rocks from supposed Lewisian ‘inliers’ shows that they are compositionally similar to the gneisses of the Lewisian Foreland. It is suggested that post-Moinian metamorphism was not accompanied by wholesale chemical changes, and that original compositional differences between Lewisian and Moinian rocks are preserved. Knowledge of these compositional differences may therefore serve to identify or confirm the presence of Lewisian inliers in places where their field appearance is not readily distinguishable from the Moinian.