Neoarchean Granitoids of the Hautavaara Structure, Karelia:the Melting of the Heterogeneous Lithosphere of the Accretional Orogen
New results the isotope-geochemical study and U-Pb isotope zircon dating (SHRIMP-II) of four post-tectonic granitoid massifs in the south-east of the Karelian Granite-Greenstone province (GGP), Fennoscandian Shield, are discussed. The massifs are located near the Hautavaara Structure, in the south-eastern part of the Mesoarchean (3.05–2.85 Ga) Vedlozerian-Segozerian Greenstone Belt, which is confined to the western margin of the Vodlozerian crustal block with Paleoarchean (TNd(DM) > 3.2 Ga) prehistory. All four massifs: Hautavaara, Chalka, Shuya and Nyalmozerian, were shown to occupy a similar structural-tectonic position, to have intruded at about the same time (2745–2740 Ma ago) and display a variety of compositions associated with differences in the composition and conditions of melting of magma sources. The Hautavaara Massif in the central part of the structure and the Chalka Massif on its western margin make up moderately alkaline high-Mg granitoids (sanukitoids), the initial dioritic melts of which formed upon the melting of the lithospheric mantle metasomatized in an active margin setting 3.00–2.90 Ga ago. Shuya granodiorites and Nyalmozerian leucogranites, confined to the eastern flank of the structure, yield highly fractionated имеют heavy REE ((Dy/Yb)n 3.5 to 5.14), negative εNd(Т) values of –0.9 to –2.8, and were produced by the variably deep melting of a Mesoarchean crustal source similar to 3.05–2.90 Ga felsic volcanics from the Hautavaara Structure. Shuya granodiorites contain elevated Cr and Ni concentrations, suggesting their formation in the crust contributed to by basic magma, which seems to be the same age as primitive sanukitoids. The melting of the continental lithosphere at mantle and crustal levels in the Karelian GGP in the late Neoarchean are assumed to be associated with an extension setting upon the disintegration of the collisional orogen in accordance with the model presented in (Laurent et al., 2014).