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

Abstract

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. The eastern and southern parts of the Buchan domain show the classic Buchan-type prograde sequence of cordierite – andalusite – sillimanite – migmatite/gneiss, representing a metamorphic field gradient of gently increasing pressure between (2.5–3.0 kbar, c. 550°C) and (3.5–4.0 kbar, c. 750°C). A slightly lower pressure metamorphic field gradient (by less than c. 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 of higher pressure by c. 1 kbar or less than the classic sequence over a similar temperature range. Unique to the northwestern Buchan domain is a staurolite+cordierite+andalusite domain in which staurolite-bearing schists 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 ± 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, a ductile shear zone that juxtaposes the Buchan rocks against higher pressure, lower temperature (kyanite-bearing) Barrovian schists to the west (c. 5.5–7.0 kbar, c. 580–650 °C). A 2–15 km wide corridor of andalusite pseudomorphed by kyanite occurs between the Portsoy–Duchray Hill Lineament 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. 500 Ma.Supplementary materials: Analytical procedures; Thermodynamic modelling; Tables S1 to S13 (mineral assemblages, whole rock compositions, mineral compositions, isotopic analyses); Figures S1 to S5 (supplementary mineral assemblage map, graphs of mineral composition vs. grade, supplementary thermodynamically calculated phase diagrams, back-scattered electron (BSE) images of monazite grains, concordia diagrams for monazite U–Pb results) are available at https://doi.org/10.6084/m9.figshare.c.5536745