Proterozoic eclogites from the Lofoten islands, northern Norway

Abstract

Massif lenses and bands of variably retrogressed eclogites occur within the granulite facies Proterozoic basement of the Lofoten islands in northern Norway. Eclogites are often related to shear and mylonite zones suggesting that deformation and fluid access was important for their formation. The eclogitic rocks are restricted to gabbroic or troctolitic rock compositions and show, in the least retrogressed samples, the typical assemblage garnet + omphacite + rutile. The widespread occurrence of eclogite shows that the whole group of the Lofoten islands was affected by high pressure metamorphism. Eclogites were found on three of the five major islands so far (Flakstadøy, Vestvågøy, Austvågøy). Thermobarometry reveals minimum conditions during eclogite formation of 680 °C/15 kbar on Flakstadøy and 540 °C/14 kbar on Austvågøy. Geological, petrological and geochemical data suggest the following succession of events: (a) Intrusion of mafic to mangeritic magmas (at about 1.8 Ga; Griffin et al., 1978). (b) Incomplete recrystallization under high-pressure granulite facies conditions. Associated with granulitization was the formation of Cl-rich hydrosilicates. Granulite formation was incomplete except for zones of intensive deformation. (c) Eclogites formed in response to a significant pressure increase in the same high strain zones. However, scattered eclogite lenses and boudins also occur in gneisses. (d) Post-eclogite retrogression occurred under middle to high pressure amphibolite facies conditions (probably the Leknes event at 1.1 Ga of Griffin et al., 1978). Eclogite formation was related to crustal thickening during the final stages of early Proterozoic magmatism and granulite facies metamorphism.