A chemical study of some rocks of the Moine Series of Scotland

Abstract

The Moine Series in Ardnamurchan consists of three types of lithological unit: psammites, striped schists, and garnetiferous mica-schists. Chemical analyses and variation-diagrams show that the mica-schists that occur as thin bands in the psammitic and striped schist groups of the Moine Series are of pelitic character, but the garnetiferous mica-schists are comparable with greywackes, especially in their low K₂O and high CaO and Na₂O contents. The garnetiferous mica-schists are similar to some rocks from other areas in the Moine Series that have been described as injected, permeated, pegmatitized, or migmatized, and thus have been assumed to have different chemical compositions from those in which they were formed as sediments. All but one of the analysed Ardnamurchan rocks come from west of the belt of regional injection of the Moine Series, and textural, mineralogical, and chemical evidence shows that there are no grounds for supposing that the injection process was effective west of the injection-belt. It is concluded that the garnetiferous mica-schists represent immature poorly sorted sediments of greywacke or silt composition that have not changed appreciably in composition as a result of mechanical or chemical processes of regional injection. The occurrence of this rock-type as an important constituent of the Moine sediments suggests that some of the Moine rocks described as injected may be rocks of greywacke composition that have acquired an ‘injected’ appearance as a result of small-scale differential mobilization during isochemical metamorphism, rather than originally pelitic rocks into which new material has been introduced during the regional metamorphism.