OCEANIC PLATEAU MODEL FOR CONTINENTAL CRUSTAL GROWTH IN THE ARCHEAN, A CASE-STUDY FROM THE KOSTOMUKSHA GREENSTONE-BELT, NW BALTIC SHIELD

Field studies combined with chemical and isotope data indicate that th e Kostomuksha greenstone belt in the NW Baltic Shield consists of two lithotectonic terranes, one mafic igneous and the other sedimentary, s eparated by a major shear zone. The former contains submarine komatiit e-basalt lavas and volcaniclastic lithologies, and the latter is compo sed 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, resp ectively, Their trace-element compositions resemble those of recent Pa cific 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 ar e and most continental magmas, which are characterized by Nb/(Th,La)(N ) much less than 1. Calculated initial Nd-isotope compositions (epsilo n Nd(T) = +2.8 to +3.4) plot close to an evolution line previously inf erred for major orogens (''MOMO''), which is also consistent with the compositions of recent oceanic plateaux. The high liquidus temperature s of the komatiite magmas (1550 degrees C) and their Al-depleted natur e require an unusually hot (1770 degrees C) mantle source for the lava s (> 200 degrees 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 similar to 30 km underlain by a permanently buoyant refractory lithospheric mantle keel. Nb/U rat ios in the komatiites and basalts calculated on the basis of Th-U-Pb r elationships range from 35 to 47 and are thus similar to those observe d in modem MORE and GIB. This implies that some magma source regions o f the Kostomuksha lavas have undergone a degree of continental materia l extraction comparable with those found in the modem mantle. The mafi c terrane is interpreted as a remnant of the upper crustal part of an Archaean oceanic plateau. When the newly formed plateau reached the ac tive continental margin, its upper part collided with the sedimentary terrane but was too buoyant to subduct. As a result, the volcanic sect ion of the plateau was imbricated and obducted thus becoming a new seg ment of continental crust. The deeper zones were delaminated and tecto nically underplated or subducted. (C) 1998 Elsevier Science B.V.