The Biotite Isograd and the Lower Greenschist Facies in the Dalradian Rocks of Scotland

Analyses of a series of coexisting minerals and their host rocks, from the Aberfoyle area, indicate that previously proposed biotite-forming reactions are inadequate to account for the appearance of biotite in the Dalradian greywackes and pelites. Initially biotite is formed as a product of the breakdown of the assemblage chlorite+microcline+phengitic muscovite in greywackes of suitable bulk composition. With increase in grade the stability field of the biotite-bearing assemblage is enlarged through the reduction in the amount of celadonite substitution that can be tolerated by the white micas. The stability field increases so that it includes progressively more aluminous rocks. The sequence of biotile-forming reactions is used to define three zones of increasing metamorphic grade in the Dalradian greywackes and pelites. The interrelationships of the assemblages developed in these zones are considered in detail. The assemblages developed in the Green Beds are explained on the assumption that the fugacity of CO2 is a third intensive variable.