Nd-isotope effects through factional crystallisation-assimilation (AFC) processes in the continental crust, heterogeneous in age: the example of ferrobasalts from the Ladoga Graben (Karelia, Russia)

Petrography, geochemistry and isotope geochemistry of the Mesoproterozoic volcanic rocks occurred as lava flows and the Valaam sill in the Ladoga rift, have been studied. The Ladoga rift is located in the area of thrusting of the Palaeoproterozoic Svekofennian orogenic complexes onto the Archean Karelian craton. Ferrobasites of close composition and geological position in lava flows and sill have different Nd isotopic composition, differing by 5 units εNd(t): –4…-5 in ferrobasalts of lava flows and a very low-radiogenic Nd isotopic composition in mafic rocks of the sill (εNd(t) to –11 for ferrogabbro), and identical to the isotopic composition of its acidic rocks (εNd(t) to –11 for granophyres). It is shown that in the crust, heterogeneous in age, the process of fractional crystallisation along the tholeiitic trend combined with assimilation of melt from wall rocks can be an effective mechanism for forming such “paradoxical” features of the Nd isotopic composition. The isotopic composition of sill rocks indicates the probable presence of Mesoarchean blocks in the lower crust of the Ladoga graben region.