Baltic Shield - Northeastern Baltic Shield

Northeastern Baltic Shield

The Keivy complex in the NE Baltic shield consists mainly of sheet-like peralkaline granite bodies, granosyenite dykes and some nepheline syenite fault-type intrusions in the total exposed ~2500 km. square area.

According to a study by Bayanova and Zozulya (1999), the emplacement ages for peralkaline granite magmatic vary from 2610 Ma for the White Tundra massif to 2670 Ma for the Western Keivy massif and are spatially confined to voluminious gabbro-anorthosite magmatism of 2.66–2.68 Ga. The predominantly "juvenile" Sm-Nd isotopic signatures from most suites of Keivy complex suggest that they must be of mantle derivation or else have has short-lived crustal precursors. "The granites are petrologically and geochemically similar to Phanerozoic A-type granitoids, presumably emplaced into noncompressive or extensional environments. The distinct tectonic regime of such type of granites indicates that the Keivy peralkaline granite magmatism can be regarded as a consequence of post-collisional events. Collision in the region has possibly taken place earlier than 2.74 Ga. The granites studied were formed after the Late Archaean Keivy-Voronja greenstone belt evolution." (Bayanova, 1999). The above model suggests that the NE Archaean portion of the Baltic shield was dominated by plume tectonics.

The Laplandian Granulite Belt is in the central northeast section of the Baltic shield. Garnet plagiogranitoids occur in the northeastern part, crystallised from melting of host rock acid granulites. The absence of stratification in the north part of the Lapland Granulite Belt are related to the E-W extension at the final period of thrusting. This deformation stage was characterised by persistently high temperatures and increasing water activity. (Kozlov and Kozlova, 1999).