XXV.—The Mineralogy and Metamorphism of the Moine Schists of the Morar and Knoydart Districts of Inverness-shire

Abstract

This study of the mineralogy and metamorphism of the Moine schists approaches the problem through consideration of the chemical composition of the minerals. The principal minerals of the Moine schists are studied individually, thirty-three new analyses of minerals being given, with four of schists. For all but six analyses the trace elements have been determined and are tabulated. Muscovites, biotites and garnets all show variation of composition with increasing metamorphic grade, respectively with decreasing Fe2O3 and SiO2, increasing MgO and Al2O3, and decreasing CaO plus increasing MgO, these being only the principal variations. The crystal chemistry of the muscovites, biotites and garnets is briefly discussed. Some new data are presented for the relationship of plagioclase and epidote, a relationship which can only be used with extreme caution to indicate metamorphic grade. A dependence of the incidence of a sutured quartz aggregate in psammitic schists upon the microcline content is noted, and it is suggested that the incidence of the breakdown of quartz under the combined effect of temperature (the dominant cause), stress and potassio solutions, all active in the core, is responsible for the production of the core-envelope boundary. This boundary transgresses the strike; within it retrogressive changes have converted Garnet-zone schists to Biotite-zone schists, but outside it the only retrogressive change is garnet to chlorite. The retrogressive metamorphism formed a distinct episode separated from the initial regional metamorphism by an unknown time-interval; during the second metamorphism isotherms were concentric with the dome, declining outwards. The evidence of an epidiorite from the west coast of Morar suggests that during the first metamorphism the zonal state there was that of the Dalradian Biotite-zone; “almandine” garnets in the adjacent schists are extremely rich in CaO and are in equilibrium with microcline.