Evolution of high-magnesian primary magma in the eastern part of the baltic shield

In distribution pattern of rare-earth and other trace elements, the primary magmas of high-magnesian volcanics in the eastern part of the Baltic shield can be clearly divided into three types: Archean komatiite, Proterozoic high-magnesian and Proterozoic picritic ferrobasalts. This means that there were marked changes in composition of the parent mantle substrate at the Archean-Proterozoic boundary due not only to deepening of the zone of magma generation as a result of the growth in crustal thickness, but also to involvement in the melting process of upper mantle zones with a relatively full complement of original elements and rich in titanium, iron, nickel, volatiles and other incongruent elements. Such changes seem to reflect the overall cratonization of the Earth's crust in the Proterozoic and the onset of continental rifting at that time. The data do not support the view that products of a komatiite magma continued to crystallize in the Proterozoic.