Nd isotope variation across the Archaean-Proterozoic boundary in the North Ladoga Area, Russian Karelia

Abstract In order to investigate the boundary between Archaean crust of the Karelian Craton and Paleoproterozoic crust of the Svecofennian Orogen in the area north and west of the lake Ladoga (the North Ladoga Area) 24 samples of mostly granitoid rocks, collected along the 100 km long profile across the inferred suture, were analysed for their Nd isotopic composition. Ten previously published Nd data were incorporated in the dataset. It was established that gneisses from so-called “mantled domes” north of the suture have very low ∊Nd values and represent reworked Archaean basement. North of the Kirjavalahti Dome, the presence of Archaean basement under Kalevian sedimentary cover is registered by abundant Archaean cores in zircons, revealed by a SHRIMP study, and by low ∊Nd values in the rocks of the 1874±13 Ma-old Alattu dyke complex. The transition from Archaean to Proterozoic crust is registered by a shift from very low to Bulk-Earth-type ∊Nd values and occurs within a 10-20 km-wide zone north of the in...