Metamorphism of the Buchan type-area, NE Scotland, and its relation to the adjacent Barrovian domain

The metamorphism of the Buchan block in northeast Scotland, and its relation to the Barrovian domain to its west, have been reassessed from consideration of mineral assemblages, microstructures, phase equilibrium modelling and monazite U-Pb geochronology. Zones of increasing metamorphic grade surround a central low grade domain (biotite zone) and define a northward-opening, U-shaped metamorphic map pattern ascribed to post-metamorphic folding. The eastern and southern Buchan domain show the classic Buchan-type prograde sequence cordierite - andalusite - sillimanite - migmatite/gneiss, representing a metamorphic field gradient of gently increasing pressure between 2.5-3.0 kbar, approximately 550 degrees C and 3.5-4.0 kbar, approximately 750 degrees C. A lower pressure metamorphic field gradient (by less than approximately 0.5 kbar) is interpreted for the northeastern Buchan domain. The west Buchan domain shows a prograde sequence of staurolite+andalusite - sillimanite - gneiss/migmatite, representing a metamorphic field gradient higher by approximately 1 kbar or less than the classic sequence. Uniquely in the northwestern Buchan domain is a staurolite-cordierite-andalusite domain in which staurolite-bearing schists are interpreted to have been overprinted by cordierite+andalusite assemblages. Monazite U-Pb geochronology of schists and gneisses from the Buchan block, including the Cowhythe and Ellon gneisses, yields ages in the range 470+ or -5 Ma, supporting geological evidence that the gneisses are metamorphosed Dalradian strata rather than older basement gneisses. The metamorphic ages are similar to the ages of mafic igneous intrusions in the Buchan block, even though many of the exposed intrusions post-date the regional metamorphic zones. The Buchan metamorphic zones are truncated to the west by the Portsoy-Duchray Hill lineament (PDHL), a ductile shear zone that juxtaposes the Buchan rocks against higher-pressure, lower-temperature (kyanite-bearing) Barrovian schists to the west. A 2-15 km wide corridor of andalusite pseudomorphed by kyanite occurs between the PDHL and the Keith shear zone to its west. Monazite geochronology of the Barrovian rocks west of Portsoy shows little evidence of the c. 470 Ma signature of the Buchan block, instead yielding a dominant cluster of ages at c. 450 Ma and a more poorly defined grouping at c. 490 Ma. Supplementary materials: Analytical methods, thermodynamic modelling; tables S1-S13; figures S1-S5 are available at https://doi.org/10.6084/m9.figshare.c.5536745