Abstract:
Field studies combined with chemical and isotope data indicate that the Kostomuksha greenstone belt in the NW Baltic Shield consists of two lithotectonic terranes, one mafic igneous and the other sedimentary, separated by a major shear zone. The former contains submarine komatiite–basalt lavas and volcaniclastic lithologies, and the latter is composed of shelf-type rocks and BIF. Komatiitic and basaltic samples yield Sm–Nd and Pb–Pb isochron ages of 2843±39 and 2813±78 Ma, respectively. Their trace-element compositions resemble those of recent Pacific oceanic flood basalts with primitive-mantle normalized Nb/Th of 1.5–2.1 and Nb/La of 1.0–1.5. This is in sharp contrast with island arc and most continental magmas, which are characterized by Nb/(Th,La)N≪1. Calculated initial Nd-isotope compositions (ϵNd(T)=+2.8 to +3.4) plot close to an evolution line previously inferred for major orogens (“MOMO”), which is also consistent with the compositions of recent oceanic plateaux. The high liquidus temperatures of the komatiite magmas (1550°C) and their Al-depleted nature require an unusually hot (1770°C) mantle source for the lavas (>200°C hotter than the ambient mantle at 2.8 Ga), and are consistent with their formation in a deep mantle plume in equilibrium with residual garnet. This plume had the thermal potential to produce oceanic crust with an average thickness of ∼30 km underlain by a permanently buoyant refractory lithospheric mantle keel. Nb/U ratios in the komatiites and basalts calculated on the basis of Th–U–Pb relationships range from 35 to 47 and are thus similar to those observed in modern MORB and OIB. This implies that some magma source regions of the Kostomuksha lavas have undergone a degree of continental material extraction comparable with those found in the modern mantle. The mafic terrane is interpreted as a remnant of the upper crustal part of an Archaean oceanic plateau. When the newly formed plateau reached the active continental margin, its upper part collided with the sedimentary terrane but was too buoyant to subduct. As a result, the volcanic section of the plateau was imbricated and obducted thus becoming a new segment of continental crust. The deeper zones were delaminated and tectonically underplated or subducted.
