POLYMETAMORPHISM IN THE BUCHAN TYPE AREA, NE SCOTLAND

Re-investigation of the metamorphism of the Buchan type-area in Scotland has revealed that the famous NW Buchan coastal sequence is polymetamorphic. Whereas the eastern and southern Buchan domain shows the classic Buchan-type prograde sequence cordierite – andalusite –sillimanite – migmatite/gneiss, and the west Buchan domain shows a higher-pressure prograde sequence of staurolite+andalusite – sillimanite – gneiss/migmatite, the NW Buchan coastal sequence uniquely contains a domain in which staurolite+cordierite+andalusite assemblages occur in muscovite-bearing rocks. Global assessment of metapelitic mineral assemblage sequences suggests that muscovite-bearing staurolite+cordierite+andalusite assemblages should not be a stable association. Another unique aspect of the NW Buchan coastal sequence is that, in contrast to other parts of the Buchan domain, the order of porphyroblast development inferred from textures in individual rocks (staurolite-cordierite-andalusite) does not accord with the sequence of porphyroblast isograds on the ground (cordierite-andalusite-staurolite). We ascribe these features to polymetamorphism, involving an early generation of staurolite±andalusite-bearing schists partially overprinted by a second, more static, lower pressure metamorphism characterized by cordierite+andalusite mineral assemblages. Two broad scenarios present themselves to account for the local domain of polymetamorphism in the NW Buchan: (1) an early domain of staurolite+andalusite metamorphism restricted to the west Buchan, overprinted by a Buchan-wide episode of lower-pressure cordierite+andalusite metamorphism that only impinged on the west Buchan domain in its NW (coastal) portion; or (2) an early Buchan-wide, depth-dependent metamorphism characterized by higher-pressure staurolite+andalusite metamorphism in the west and lower-pressure cordierite+andalusite metamorphism in the south and east, locally overprinted by cordierite+andalusite metamorphism in the NW Buchan. The second scenario is preferred, although a cause for a local overprint remains elusive. Monazite U-Pb geochronology of the schists and gneisses of the NW Buchan domain, whether polymetamorphic or not, yield dates in the range 465-470 Ma, suggesting that the time gap between the overprinting metamorphic episodes falls within the uncertainties of the dates.