Some new chemical data on assemblages of the Assynt Alkali Suite

Abstract

Among the rock types described by Professor Shand in his extended paper “On borolanite and its associates in Assynt” (1910) were two referred to as “Younger Borolanite” and “Assyntite” (Shand, 1910, Nos. 26 and 28 respectively, pp. 400-401). Both were reported to contain fresh nepheline, a feature unusual in Borolan petrology and shared by the juvites outcropping N. of Cnoc na Sroine, S.E. of Loyne. Shand's study was accompanied by a report of Gemmell (1910) contributing eight chemical analyses, seven of them of rocks described by Shand in the paper noted. Unfortunately not all of Gemmell's analyses can be rated as superior in quality—for example the analysis of the assyntite despite the content of nepheline in the rock shows quartz in the norm and the analysis of the melanite syenite (Gemmell, 1910, p. 418) with 11 per cent. normative quartz is also under suspicion. Further analytical work on such rocks is clearly desirable. Shand presented to the Department of Mineralogy and Petrology at Cambridge a number of the types he described, including the analysed assyntite just referred to and the specimen of the Younger Borolanite of which no analytic data are on record. At my request Mr J. H. Scoon has subjected both these rocks to analysis together with a number of other alkali assemblages from the Assynt region. The results are now recorded in the accompanying Tables 1-3 and a brief note on the petrography of the rocks, supplementing earlier descriptions, has in some cases been appended. Younger Borolanite