Conditions of Dalradian metamorphism in the Buchan area, NE Scotland

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Abstract
P–T conditions in the Buchan Dalradian have been estimated using published equilibria. Garnet-biotite (corrected according to Hodges & Spear (1982) ) and garnethornblende temperatures are in good agreement, recording 430 ± 15°C, 490 ± 15°C and 510 ± 20°C at the cordierite, andalusite and staurolite isograds and 550 ± 40°C in the upper staurolite zone. Garnet-clinopyroxene-plagioclase-quartz and garnet-plagioclase-epidote-quartz-fluid (with aH 2 O = 1) record a consistent minimum pressure of 1.7kbar for the andalusite isograd though the uncertainties in the equilibria indicate this could be 2 kbar higher. In the staurolite zone the latter geobarometer is in agreement with pressures of 2–3.5 kbar derived from garnet-plagioclase-andalusite-quartz. A calcite-dolomite temperature of 537°C from marbles at the Portsoy kyanite/andalusite isograd, implies pressures near 4.5 kbar. The P–T array defined by these data lies at lower pressures and temperatures than that deduced from pelite petrogenetic grids. The possibility of reduced aH 2 O is considered unlikely, as the calc-silicate assemblages are indicative of H 2 O-rich fluids, and it is suggested that the discrepancy may result from inadequate garnet activity formulations. Inferred overburden estimates are consistent with recent overburden estimates for the Newer Gabbro aureole implying that little or no erosion had occurred by the time of emplacement of the basic intrusions. The overburden estimates are combined with the stratigraphic thickness data of Sutton & Watson (1955) to estimate differential erosion/uplift rates in the area.