Mineral facies in pelitic rocks, with particular reference to the Buchan type metamorphism of north-eastern Scotland
The Dairadian rocks of Aberdeenshire and Banffshire have been affected by a regional metamorphism giving rise to andalusite, cordierite, staurolite and garnet in pelitic rocks (Bu.cha.n. Type Metamorphism). Four zones are recognised in which the following assemblages (with muscovite and quartz) first appear: Biotite Zone (chlorite—biotite, B.1; Cordierite Zone (eordierite.-chloritebiotite, CA); Andalusite Zone (andalusite.-côrdierite -biotite A.1); Stauroli.tc Zone (staurolite.andalusite-"biotite, The biotite zone separates two N.E.—S.W. trending sets of higher grade zones, The staurolite zone is absent from the eastern set, Both sequences pass into sillimanite bearing rocks at higher grade.. Theabove distribution of assemblages along wit.' , he trend. of upgrade Mg enrichment indicated by analyses of coexisting cordIorite-biotite (in A.1) and staurolite-'biotite (in s.) suggest the :foj.lowing isograd forming reactions: Cordierite Isograd: chlorite + muscovite cordierite +biotite Andalusite Isograd: cordierite + muscovite and.alusite + biotite 2 Staurolite Isograd: andalusite -V biotite staurolite + muscovite 3 These reactions are dior multi— variant in nature and isograds in the field mark lines where the reactions occur for the most Fe rich bulk compositions present. Mineral analyses suggest that this is close to lOOMgO/MgO+FeO = 40, whilst bulk rock analyses suggest N/FM of 38.