ND AND SR ISOTOPIC GEOCHEMISTRY OF MAFIC LAYERED INTRUSIONS IN THE EASTERN BALTIC SHIELD - IMPLICATIONS FOR THE EVOLUTION OF PALEOPROTEROZOIC CONTINENTAL MAFIC MAGMAS

Nd and Sr isotopic data are presented for the 2449-2441 Ma Olanga and Burakovka layered mafic complexes in the eastern Baltic Shield. These complexes have similar tectonic position, but differ in two aspects: t he age of the enclosing crust and the post-crystallization metamorphic history. The Sm-Nd isotopic results for the Kivakka and Lukkulaisvaar a intrusions, Olanga Complex, are consistent with the model of closed- system crystallization of a single magma without significant wallrock assimilation. The Rb Sr systems of minerals were disturbed by late Rb addition during 1.75-1.50 Ga metamorphism. The Nd and Sr isotopic syst eins in the Burakovka complex show no metamorphic disturbance and indi cate mixing of at least four isotopically distinct components. Isotopi c variations in the Burakovka Complex can be explained by a 4-20 per c ent contamination of a primary komatiitic or picritic magma with a Mes oarchean crust. similar to that exposed in the region. A similar model , applied to the Olanga Complex using a Neoarchean crustal isotopic co mposition, cannot reproduce the observed isotopic signature. The nearl y uniform initial EN, values between -1 and -2.3, observed in the Kiva kka and Lukkulaisvaara intrusions of the Olanga Complex, as well as in the other 2.50-2.44 Ga layered mafic intrusions throughout the easter n Baltic Shield, are better explained by a mantle plume model with sma ll amounts of crustal contamination and minor involvement of asthenosp heric material. This model is also consistent with the geological obse rvations and the temporal distribution of the Paleoproterozoic mafic m agmatism in the eastern Baltic Shield. As an alternative, the enriched isotopic characteristics may be explained by melting of a metasomatic ally modified lithospheric mantle source.