The Metamorphism of the Dalradian rocks of Scotland.

Synopsis The metamorphism of the Dalradian rocks of Scotland is reviewed and commented on, attention being paid to the nature and spatial distribution of the zones and their significance in PT modelling. Reactions defining Barrovian and Buchan isograds are listed and figured and the importance of textural and chemical disequilibrium discussed. The effect of rock composition on the isograds is examined and a T-X diagram constructed to indicate the probable metamorphic mineral sequence for the average pelitic Dalradian composition. The history of ideas on the genesis of the migmatites is outlined and it is concluded that they may well be polygenetic, and those in the NE may even be of Pre-Caledonian age. The role of oxidation and reduction reactions is briefly mentioned and the late cooling reactions are described, from which it can be seen that the climatic assemblages have often been markedly modified. The pressures and temperatures of the metamorphism, determined from phase considerations are plotted on a PT diagram and compared to the isotope results. Finally, evidence is used to suggest that although the fundamental cause of metamorphism might be a normal heatflow across the mantle/crust boundary and within the tectonic pile, the evidence at the present level indicates some of the metamorphism is due to high level magma and perhaps heat focussing along major structures.