Proterozoic alkaline-ultramafic magmatism in the eastern part of the Baltic Shield

Data obtained by the Rb-Sr and Sm-Nd isochron techniques and K-Ar method in dicate that K-rich ultramafic Proterozoic magmatism occurred in the Baltic Shield in two stages: in the Early Proterozoic (Svecofennian, at 1720 +/- 8 Ma) and the Middle Riphean (1230 +/- 5 Ma). These stages resulted, respect ively, in the Por' ya Bay dike complex of lamproites of the transitional ty pe and the Kostomuksha dike complex of olivine lamproites. The two complexe s were produced in different geodynamic environments. Some geochemical feat ures (such as a negative Ti-Ta-Nb anomaly and the Nb-Zr and La-Ta relations ) suggest that the lamproites of the transitional type were produced in con nection with postcollisional processes, a concert that is consistent with t he setting of the Por' ya Bay Complex at the boundary between the Kola crat on and the Belomorian-Lapland belt, where collisional processes terminated at 1.75 Cia. The olivine lamproites of the Kostomuksha Complex are typical intraplate rocks. The rocks of both complexes display isotopic features tha t are atypical of Precambrian lamproites: their epsilon(Nd) varies from -9. 2 to -9.3 (enriched mantle), and the epsilon(Sr) equals -20 (depleted mantl e). The similarity between the epsilon(Nd) values of the two complexes sugg ests that the Early Riphean and Middle Riphean lamproites could not have a common protolith. The protolith of the Kostomuksha lamproites was more depl eted, but the evolution of the magmas resulted in their significant enrichm ent in incompatible elements. The occurrence of an enriched component in th e lamproites was most probably caused by the addition of enriched mantle ma terial(which was compositionally close to EMI) to a depleted component (lik e MORE). The fact that the CO2 and H2O concentrations in the lamproites of both complexes are two to three orders of magnitude higher than the average contents of these components in the wall rocks indicates that the probabil ity of melt contamination with crustal fluids is low. The N/C values of the lamproites (<0.003) suggest a source resembling MORE, but the WC ratios of the rocks are notably higher (>3-4) than that in the source of MORE. The C /Ar-36 and N/Ar-36 ratios of the phlogopite (which are higher than the aver age crustal values) indicate that the fluids contained a mantle component.