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-formingreactions are inadequate to account for the appearance of biotitein the Dalradian greywackes and pelites. Initially biotite isformed as a product of the breakdown of the assemblage chlorite+microcline+phengiticmuscovite in greywackes of suitable bulk composition. With increasein grade the stability field of the biotite-bearing assemblageis enlarged through the reduction in the amount of celadonitesubstitution that can be tolerated by the white micas. The stabilityfield increases so that it includes progressively more aluminousrocks. The sequence of biotile-forming reactions is used to definethree zones of increasing metamorphic grade in the Dalradiangreywackes and pelites. The interrelationships of the assemblagesdeveloped in these zones are considered in detail. The assemblagesdeveloped in the Green Beds are explained on the assumptionthat the fugacity of CO₂ is a third intensive variable.