Geochemical distinctions between the Lewisian of Cassley, Durcha and Loch Shin, Sutherland, and the surrounding Moinian

In Strath Oykell and Lower Loch Shin, Sutherland, several outcrops of Lewisian-Gneiss-like rocks occur, previously described at the Airde of Shin and Durcha as being part of the Moinian. They form a discontinuous sill-like body up to 300 m. thick, extending in a 180° arc round a centre 8 km. north of Cassley Falls, dipping 15–50° outward throughout. They show little evidence of folding of other than isoclinal style. Contrary to previous published statements, hornblende-bearing gneisses are confined to a distinct, mappable unit, largely composed of biotite-oligoclase-quartz gneisses which do not grade laterally into Moine psammites. Diopside marbles with later tremolite in the Shin Group may indicate retrogressive metamorphism of earlier high-grade rocks. Chemical analysis shows that the Shin Group is low in TiO2, has high FeO total/TiO2, low K2O and low Rb/Sr, all Lewisian characteristics. The calcsilicates are MgO- and K2O-rich compared with normal Moinian calc-silicates. It is concluded that the Shin Group is another Lewisian inlier.