Mafic-Ultramafic Subduction Complexes of the Belomorian Eclogite Province: Composition and Formation Conditions

Research subject. The Salma metamorphic rock association (Belomorian Eclogite Province (BEP), Fennoscandian Shield, East European Craton) represents a manifestation of a single Meso-Neoarchean subduction-accretion complex of the active continental margin of the Kola paleocontinent. Aim. To investigate genetic relationships between eclogites, metaperidotites, products of eclogite melting in the subduction zone, as well as the tonalite-trondhjemite-granodiorite (TTG) gneisses that host these rocks. Key points. Information on the rocks of the Meso-Neoarchean oceanic paleosubduction zone beneath the active continental margin of the Kola paleocontinent is reviewed and generalized. The work compiles both published and new petrological and geochemical data. Eclogites. Salma subduction eclogites of Salma in the BEP are localized within TTG gneisses and have variable protolith compositions of oceanic crust ranging from N-MORB to E-MORB. The eclogites preserve a record of prograde evolution of the oceanic crust, from formation in a slow-spreading center and low-temperature seafloor alteration to subduction into the mantle to depths >70 km. Adakites and andesites. Adakitic leucosomes in eclogites and adakitic gneisses indicate partial melting of relatively young eclogitized oceanic crust in the subduction zone at depths > 70 km and temperatures > 750 °C. Large volumes of andesitic magmas probably originated from the melting of the mantle wedge metasomatized by supra-subduction fluids. Both adakitic and andesitic supra-subduction magmatism contributed to the growth of the continental margin. Metaperidotites. The eclogites contain magnesian ultramafic rocks, including serpentinites, talc schists, carbonate-bearing chlorite harzburgites with layers and veins of garnet pyroxenites. Metaperidotites formed by low-temperature hydration of abyssal mantle peridotites in a slow-spreading oceanic ridge or in the plate-bending zone near a deep-sea trench, which transformed them into serpentinites. Large serpentinite fragments were tectonically incorporated into the subducting plate. Chlorite harzburgites resulted from partial dehydration of serpentinites at depths >30 km and temperatures of ≈ 650 °C. At depths > 70 km and temperatures > 750°C, mantle rocks interacted with slab-derived melts, forming layers and veins of garnet pyroxenites of websteritic composition. Conclusions. The rocks studied in the Belomorian Eclogite Province record a complete cycle of oceanic lithosphere evolution (crust and mantle) in an Archean subduction zone and the synchronous formation of an active margin of the Kola paleocontinent. The rocks described in the paper reflect key stages of this process.