METAMORPHISM OF THE BUCHAN TYPE-AREA, NE SCOTLAND, AND ITS RELATION TO THE ADJACENT BARROVIAN DOMAIN

The metamorphism of the Buchan type-area in northeast Scotland, and its relation to the Barrovian domain to its northwest, has been reassessed from consideration of mineral assemblages, microstructures, phase equilibria modelling and U-Pb monazite geochronology of some of the metamorphic rocks. The metamorphism of the eastern and southern parts of the Buchan domain shows the classic Buchan-type prograde sequence cordierite-andalusite-sillimanite- migmatite/gneiss, with andalusite relics in the gneisses, representing pressures of 3.0-3.5 kbar. The western part of the Buchan block is more complicated, in places showing a prograde sequence involving staurolite+andalusite, indicating somewhat higher pressures (~3.5-4.0 kbar) than the cordierite+andalusite sequence. In other places (e.g., west Buchan coastal section), staurolite±andalusite schists are variably overprinted by cordierite+andalusite assemblages. Upgrade of the polymetamorphic porphyroblastic schists in the west Buchan coastal section is a sillimanite zone followed by the Cowhythe gneisses, the latter interpreted to be the highest grade part of the prograde sequence. West of the Cowhythe gneisses is a high strain zone (Portsoy shear zone) that juxtaposes the Buchan-type metamorphic rocks against higher-pressure, lower-temperature Barrovian (kyanite+staurolite+garnet-bearing) schists. The westward transition from lower pressure metamorphic rocks of the Buchan block to higher pressure Barrovian rocks is not a continuous metamorphic transition, and the kyanite-andalusite “inversion isograd” of the literature is a tectonic artifact. U-Pb monazite geochronology of porphyroblastic schists and gneisses from the Buchan block fall in the range 475-465 Ma, including the Ellon gneisses (southeastern part of Buchan block) and Cowhythe gneisses (western part of Buchan block), supporting geological and petrological evidence that these gneisses are part of the prograde Buchan-type metamorphism of Dalradian strata, rather than representing older, tectonically interleaved, basement gneisses. There is a suggestion in the data that the metamorphism in the eastern part of the Buchan block may be a little older (ca. 475 Ma) than in the western part of the Buchan block (465-470 Ma). Monazite geochronology of rocks in the Barrovian domain to the immediate west of the Portsoy shear zone is more difficult to interpret, with indications of monazite growth episodes at approximately 500 Ma and 450 Ma.