Sr- and Nd-Isotope Geochemical Effects of Assimilation and Fractional Crystallization (AFC) Processes in the Heterogeneous Crust: The Example of Ferrobasites from the Ladoga Graben (Karelia, Russia)

Petrographic, geochemical, and isotope–geochemical studies of the Mesoproterozoic volcanic association (lava flows and the Valaam sill) in the Ladoga graben in the suture zone of the Karelian craton and Svekofennian orogenic complex are conducted. Ferrobasites of close composition and geological position in lava flows and the sill have different Nd isotopic compositions, differing by 5 units of ε_Nd(t): = –4…–5 in ferrobasalts of lava flows and a very low-radiogenic Nd isotopic composition in mafic rocks of the sill (ε_Nd(t) to –11 for ferrogabbros), and identical to the isotopic composition of its acidic rocks (ε_Nd(t) to –11 for granophyres). It is shown that in the crust, which is heterogeneous in age, the process of fractional crystallization along the tholeiitic trend combined with assimilation of melts from wall rocks of magma chambers could have been an effective mechanism for forming such “paradoxical” features of the Nd isotopic composition. The isotopic composition of sill rocks indicates the likely occurrence of Mesoarchean blocks in the lower crust of the Ladoga graben region.