The Grampian Group, Scotland

Abstract

Rocks now classified as part of the Grampian Group (Harris et al, 1978) crop out extensively in the Central Highlands of Scotland (Fig. 13.1). Formerly referred to collectively as the ‘Central Highland Granulites’ before 1978, these rocks were assigned to the Moine Assemblage, which they closely resemble both in lithology and geochemistry. However, recent mapping and isotopic work has revealed that a gneissose assemblage of rocks occurring near Laggan, Kincraig, and north of Carrbridge (Fig. 13.1) appears to be older than, and has been reported to be unconformably overlain by dominantly psammitic rocks currently assigned the Grampian Group (Piasecki and van Breemen, 1979a, 1979b; Piasecki, 1980). If so, the areas in which these gneissose rocks occur may be interpreted as inliers. In some recent references the term `Grampian Division’ has been applied to the Grampian Group, while yet other papers have used the term ‘Younger Moine’. The earlier assemblage, termed by Piasecki (1980) ‘Highland Division’, has yielded isotopic ages similar to those obtained from Moine Assemblage rocks which crop out west of the Great Glen Fault (Brewer et al, 1976; Brook et al, 1977; Piasecki and van Breemen, 1979a). In addition the Central Highland Division contains paragneiss lithologies and minor amphibolite pods, which texturally, mineralogically and geochemically mirror those of the Glenfinnan Division of the Moine Assemblage. Hence it is the Central Highland Division migmatitic gneisses which appear to represent the Moine Assemblage east of the Great Glen and the Grampian Group rocks thus appear to form a cover sequence to the Moine Assemblage in the Grampian Highlands.