A geochronological study of the Lewisian from Loch Laxford to Durness, Sutherland, N. W. Scotland

Potassium-argon biotite and hornblende ages from near Laxford Bridge, Sutherland, range up to 1770 m.y.: higher apparent ages of 2060 and 1840 m.y. occur in the biotite pods along the southern margin of the Laxford assemblage. A rubidium-strontium isochron from whole-rock specimens from the gneisses of the Laxford assemblage gives 1850 ± 50 m.y., initial 87 Sr/ 86 Sr = 0.7061 ± 0.0005 (T 1 2 = 4.85. 10 10 y). It is suggested that 1850 ± 50 m.y. is a minimum age for the climax of the Laxfordian metamorphic episode, and that this metamorphism had effectively terminated by 1750 m.y. the highest K-Ar age of a mineral from the type area. Younger apparent ages of minerals from and close to the type area spread down to 1575 ± 50 m.y., associated with late effects of the metamorphism, continued pegmatite-formation, and a general slow-cooling. Closure of radiogenic systems occurred later at Durness than at Laxford Bridge. Comparison with the sparse data from nearby shield areas suggests a closer correlation of the Lewisian with comparable rocks in Greenland than with Fennoscandia.