A Petrological Study of the Limestones in the Moine Series of Ardgour, Argyllshire

Abstract

Summary and Comparison with Other Areas in Scotland. Limestones in Ardgour are found to vary considerably in chemical and mineral composition. They have been subjected to high-grade metamorphism either dynamic (regional) or thermal, or sometimes to both. The dioritic intrusion has been responsible for the thermal alteration but does not show any widespread or conspicuous changes which might be due to contamination with limestone, although a certain amount of chemical exchange is apparent at some points of contact between the two. Mineral and chemical changes in some of the limestones have resulted from the metasomatizing action of solutions, the source of which is as yet uncertain. The various limestone types are found to exhibit a number of analogies with the Deeside limestone investigated by Hutchison and their petrography has been worked out in some detail. Apart from certain hornblendic types in Central Sutherland, scapolite has not been recorded in the Moine Series. In Ardgour it has a wide distribution in the limestones associated with the diorite-gneiss. Wollastonite has not been recorded elsewhere in the Moine Series. Its distribution in Ardgour is like that of scapolite; and this applies also to prehnite. It must be remembered when thus comparing the Ardgour limestones with otters in the Moine Series firstly that these have not yet been studied in detail; secondly, that they are comparatively rare, and lastly, that there is doubt regarding the systematic position of some of the “Lewisian inliers” wherein limestones occur as a considerable proportion of the paragneisses. Scapolite, according to Flett (26) is a characteristic mineral in these rocks. Wollastonite occurs also as, for example, in the Glen Urquhart “inlier” (Cunningham-Craig, 27). In the Lewisian gneisses of Coll prehnite has a wide distribution and scapolite also is of frequent occurrence (Bailey & Eyles, 28). Pyroxene-amphibole rocks similar to those in Ardgour are found in Coll, Glen Urquhart, and as xenoliths in norite (Read, 29). The mineralogical character of the limestone is, therefore, in support of the conception of their forming part of an inlier in the Moine Series. The writer hopes to publish shortly a more detailed study of the geological problems of this area.