LAUSR

The eclogites exposed along northeastern boundary of the Belomorian orogen in the eastern Fennoscandian Shield were formed as a result of Mesoarchean–Neoarchean subduction and collision. As has been shown previously, the common protolith of the Salma-type subduction-related eclogite was oceanic layered gabbro. In this paper, we characterize eclogites formed from volcanic–sedimentary rocks of the upper oceanic crust, which comprised pillow lavas and associated aluminous sediments that filled the interpillow space intercalated with lava flows. As a result of eclogite-facies metamorphism, the aluminous sediments have been transformed into coarse-grained garnet–phengite–quartz rocks under pressure no lower than 21 kbar at a temperature of ~650°C. Alternatively, we cannot rule out the possibility that the garnet–phengite–quartz veins represent solidified felsic melts that were produced by melting of boron-bearing hydrothermally altered oceanic crust in the subduction zone. During transfer to the upper crust under high-pressure granulite facies conditions, phengite underwent incongruent melting with formation of complex polymineralic pseudomorphs consisting of feldspar, biotite, muscovite and kyanite with corundum and dumortierite. The peak of high-temperature metamorphism during exhumation of the eclogites is estimated at 850–900°C, i.e., at least 50–100°C higher than previous estimates. This article is protected by copyright. All rights reserved.