Nd isotope variation across the Archaean-Proterozoic boundary in the North

In order to investigate the boundary between Archaean crust of the Karelian Craton and Paleo- proterozoic 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 e 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 e 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 e Nd values and occurs within a 10-20 km-wide zone north of the inferred suture. The rocks of the Svecofennian Orogen south of the suture are characterized by relatively uniform initial Nd isotopic composition (average e Nd value at 1880-1860 Ma +0.7, n = 17). In terms of Nd isotopic composition, this domain, the Lahdenpohja Domain, is comparable to the Svecofennian terranes to the west: the Central Finland Granitoid Complex and the Accretionary Arc Complex of Southern Finland. The Primitive Arc Complex of Central Finland has, in contrast, significantly more juvenile Nd isotopic composition.