The Archean Tonalite-Trondhjemite-Granodiorite Association of the Karelian Province: Geology, Geochemistry, Formation Stages and Conditions

On the basis of the available geochronological data, the main stages of formation of Archean rocks belonging to the tonalite-trondhjemite-granodiorite (TTG) association, with ages of about 3240, 3150, 2900, 2850, 2800, and 2750 Ma, are identified and characterized within the Karelian Province of the Fennoscandian Shield. The distinctive features of geological positions and chemical compositions of TTG rocks of different stages are shown. For example, early TTG melts could have been melted not from basalts of the greenstone belts of the Karelian Province, but from lower crustal basites or from early amphibolites of the Vodlozero domain. Features of the geological position, chemical composition, and ε Nd (t) isotopic evolution do not support the assumption that most of the TTGs formed from tholeiites of the greenstone belts, thus excluding their genetic connection with subduction of the ancient oceanic crust. The most acceptable model seems to be the formation of TTGs in the Karelian Province as a result of plume-affected melting of lower crustal mafites. Archean TTGs differ markedly from Phanerozoic plagiogranitoids not only in the shape and size of the massifs but also in a number of geochemical features, in particular, lower HREE, Sc, Y, and Ti contents.