Extract

Eclogite exposed in the Glenelg region of north-west Scotland is of interest because of its antiquity, it is the oldest crustal eclogite known, and also because it is associated with apparently cofacial metagranitoids, marbles, pelites and meta-ironstones. This paper briefly describes the eclogite-facies assemblages and their geological setting, and attempts to establish peak metamorphic P-T conditions. Cal-Dol and Grt-Cpx thermometry, and Grt-Omp-Qtz and Grt-Pl-Ky-Qtz barometry suggest equilibration at 16.5 ± 1 kbar and 730 ± 25°C. Also, the Ca correction in the Grt-Cpx thermometer is found to be inappropriate where X_Ca,Grt < 0.29.

The geology of north-west Scotland is dominated by the Moine thrust, a northnorthwest-southsoutheast-trending, east-dipping reverse fault which separates cratonized Archaean and early Proterozoic gneisses (the Lewisian gneisses) in the west from their reworked counterparts in the east. The eclogite at Glenelg is contained within the reworked gneisses (Fig. 1), and was probably formed before 1082 Ma (eclogite Sm-Nd internal isochron) as the margin of the Laurentian craton became tectonically depressed during a Grenville collision event (Sanders et al. 1984).

Post-eclogite uplift and erosion was probably rapid since the eclogite-bearing gneisses are overlain unconformably by a low-grade psammitic cover sequence, the Moines, which was probably deposited before about 1000 Ma (cf. Brewer et al. 1979). Cover and basement were subsequently deformed and metamorphosed together on more than one occasion, culminating in the formation of the Moine thrust (Craig 1983, chapters 2 & 3). As a result of this reworking, the eclogite and cofacial gneisses in the basement were