Melting of eclogite facies sedimentary rocks in the Belomorian Eclogite Province, Russia

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 21kbar at a temperature of similar to 650 degrees 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-P 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-T metamorphism during exhumation of the eclogites is estimated at 850-900 degrees C, i.e. at least 50-100 degrees C higher than previous estimates.