The Lewisian chronology of the southern region of the Scottish mainland

Abstract

Synopsis K-Ar measurements on gneisses and basic intrusive bodies from the Torridon, Gairloch and Gruinard Bay areas demonstrate (i) the existence of a few relict Badcallian (≡ Early Scourian of Sutton and Watson 1969) dates of c. 2600 m.y. in the Gruinard Bay area, (ii) the emplacement of Scourie dykes at Torridon, as well as of the Aundrary basite sheet near Gairloch, at least 2200 m.y. ago, and in the case of the dykes probably during the waning stages of the Inverian metamorphism, (iii) the occurrence of the first Laxfordian metamorphism at c. 1700 m.y. ago, of the second Laxfordian metamorphism at c. 1500 m.y. ago, and of the third (retrogressive) Laxfordian metamorphism in the Gairloch area at c. 1400 m.y. ago, (iv) post-Laxfordian cataclasis and reheating at Torridon c. 1150 m.y. ago. The significance of these results is discussed in detail and compared and contrasted with published age data. A statistical re-examination of the lead isotope results of Moorbath et al. (1969), from the areas covered in this paper only, confirms that the parent rocks of the typical basement gneisses were already in existence c. 2900 m.y. ago and were probably subjected to their highest-grade metamorphism at that time.

Keywords

Affiliated Institutions

• University of Oxford GB
• Keele University GB

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