Grampian metamorphic conditions deduced from mafic granulites and sillimanite–K-feldspar gneisses in the Dalradian of Glen Muick, Scotland

Metamorphic conditions of 8 ± 1.5 kbar and 820 ± 50°C are estimated for garnet-clinopyroxene-bearing mafic granulites and associated garnet-sillimanite migmatitic gneisses from the Dalradian sillimanite zone. Revised estimates for the kyanite zone in Glens Avon and Ey (W of the regional andalusite and sillimanite zones) are c . 7 kbar at 500-600°C. Results from rocks close to the kyanite-sillimanite isograd in Glen Clova testify to the accuracy of the barometric calibrations. Textural indications are that sillimanite preceded andalusite in the Glen Muick area: in this region the kyanite-andalusite isograd has no reality. In Glen Muick sillimanite grew synchronously with the development of the dominant steep foliation, and as part of a progressive evolution in metamorphic conditions, not as a separate late overprint. The western segment of the kyanite-andalusite isograd is thought to have developed before the attainment of the P-T conditions recorded in rocks from Glens Avon and Ey. Metamorphic and geochronological data are examined in the light of the Banff Nappe hypothesis. It is suggested that the eastern high-temperature zones were thrust over the present western kyanite zone and that rocks to the W of the kyanite-andalusite isograd were metamorphosed by thermal relaxation beneath this thrust sheet.