The distribution of pressure and temperature during Dalradian metamorphism

Abstract

The formation of sillimanite in association with the development of gneisses and migmatites in the Grampian Highlands and north-eastern Scotland is interpreted as a later superimposition upon a slightly earlier metamorphic pattern characterized in its higher grades by andalusite and kyanite. This earlier pattern is subdivided by three significant isograds: (i) the andalusite isograd, along which andalusite developed from hydrous mineral assemblages; (ii) a kyanite isograd, along which kyanite developed from hydrous mineral assemblages; and (iii) an andalusite–kyanite isograd, dividing the andalusite and kyanite zones, (i) and (ii) are interpreted as dehydration isograds, virtually isothermal; (iii) represents the polymorphic inversion of kyanite to andalusite, with a P/T slope of approximately 12.8 bars/degC.