Pressures and temperatures of metamorphism in the eastern Dalradian

Abstract

Pressures and temperatures have been determined from locations in the eastern Dalradian using a number of different geothermometers and geobarometers. Temperatures range from 550°C in the low kyanite zone to about 800°C in the sillimanite–K feldspar zone. Pressures range from about 5.5 kbar in Glen Esk to 9–10 kbar in the Central Highlands. The barometric calibrations employed are consistent with the kyanite–sillimanite equilibrium. Evidence from mineral zoning suggests that rocks outside the Buchan area (in the Duchray Hill Gneiss, Glens Clova and Avon and in the Schichallion area) underwent a pressure decrease synchronous with the metamorphic peak. Regional temperature variations define a thermal high centred on the Cromar–Glen Muick–Duchray Hill Gneiss area. To the W of this high temperatures decrease rapidly. A sharp thermal break occurs between high grade sillimanite–K feldspar gneisses in Glen Muick and kyanite–staurolite schists to the W. This thermal contrast is attributed to a structural break which may be analogous to a number of other thermal features in the Dalradian.

The nature and origin of the Barrovian metamorphism, Scotland: diffusion length scales in garnet and inferred thermal time scales

“…Metamorphic T and P within the sillimanite zone of the Barrovian metamorphic series attained values of the order of 670 8C and 6 kbar, respectively (Harte & Hudson 1979;Baker 1985). With knowledge of the starting Mn concentration geometry and the diffusion parameters for Mn in almandine, one could use forward models to determine the heating durations required to produce the modified Mn profiles observed for the sillimanitegrade garnets.…”

The Dalradian rocks of the north-east Grampian Highlands of Scotland

“…But, using the garnet-clinopyroxene (Ganguly, 1979;Ellis and Green, 1979) and garnet-biotite (Ferry and Spear, 1978) Fe-Mg exchange thermometers, Baker and Droop (1983) calculated peak pressure in the area to have been close to 8 kbar at 820 o C. Baker (1985) subsequently suggested that pressures might even have exceeded 9 kbar, and these are undoubtedly the most extreme metamorphic conditions so far recognized in the Grampian Terrane.…”

“…In a well-defined zone up to 10 km wide, immediately to the west of the lineament, original andalusite is overprinted by later kyanite as a result of an increase in pressure due to westward overthrusting during D3 (Chinner and 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 Heseltine, 1979;Baker, 1985;Beddoe-Stephens, 1990). Pseudomorphs of kyanite after andalusite are well seen in the Auchindoun Castle GCR site and immediately west of Portsoy in the Cullen to Troup Head GCR site.…”

“…Consequently the highest grade rocks are found in close contact with these igneous bodies, with local pressures and temperatures of over 8 kbar and c. 820°C, characteristic of granulite-facies conditions (Baker and Droop, 1983;Baker, 1985). Granulite-facies hornfelses and migmatites, characterized by garnet-orthopyroxenecordierite assemblages, are found in the roof-zones, inner aureoles and in screens within the mafic and ultramafic intrusions, notably the Huntly-Knock Pluton (Fletcher and Rice, 1989).…”

“…Cairn Leuchan was the focus of detailed studies of metamorphic conditions by Baker and Droop (1983) and Baker (1985). The area was resurveyed by staff of Aberdeen University and Queen's University, Belfast under contract to the British Geological Survey, resulting in a new edition of the 1:50 000 Sheet 65E (Ballater, 1995) and an accompanying memoir .…”