The Lower Greenschist Facies in the Scottish Highlands

The characteristic mineral assemblage in the lowest grade Dalradian slates and phyllites in quartz + albite + phengite + chlorite + calcite + dolomite + anatase. New analyses of coexisting minerals include muscovite, phengite, chlorite, biotite, and chloritoid. Muscovite-paragonite relationships in chlorite zone rocks are examined. The claim of recent workers that phengites change towards a more muscovitic composition with increase in metamorphic grade is not sustained; the composition range of phengites is believed to increase with rise in pressure and temperature, and the ratio Fe+3 : Fe+2 also increases. Greenschist facies chlorites are usually ripidolites, and those from quartz-albite-phengite-chlorite schists are richer in aluminium than chlorites from actinolite schists, and poorer in aluminium than chlorites from chloritoid schists. Reactions of the type quartz + phengite + chlorite + carbonates = products + CO2 + H2O are suggested, from which biotite, epidote, and actinolite form in the course of dehydration and decarbonation; when dolomite rather than calcite participates in such reactions, the equilibrium temperature is markedly lower. Three new subfacies in chlorite zone rocks are defined: the lower-T muscovite-chlorite-dolomitequartz subfacies and the higher-T muscovite-chlorite-epidote-calcite subfacies are both represented in Scotland; the latter is equivalent to the muscovite-chlorite-epidote-actinolite subfacies in carbonate free rocks such as those in New Zealand. A probable P-T gradient for low grade Barrovian metamorphism is given.