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. For some 50km southwards from the coastline along the Moray Firth on the western limb of the post-metamorphic Boyndie syncline, the andalusite and andalusite–kyanite isograds form virtually parallel traces; parallelism of isograds with such disparate P/T slopes suggests that isobars and isotherms in this region were parallel. In central Aberdeenshire the inversion isograd diverges from the andalusite isograd, and appears to run along Deeside to meet the kyanite isograd in the south. This is interpreted as indicating a divergence between isobars and isotherms; an antiformal monocline with approximately ne–sw axes is deduced for the thermal surface. To explain this disposition synmetamorphic monoclinal folding is suggested, in which pressure adjustment was instantaneous as sediments were depressed to deeper levels, but the heat content enabled pre-existing isothermal surfaces to be preserved.