The Kostomuksha greenstone belt,NW Baltic Shield: remnant of a late Archaean oceanic plateau? |
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| Authors: | I. S. Puchtel A. W. Hofmann K. P. Jochum K. Mezger A. A. Shchipansky A. V. Samsonov |
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| Affiliation: | Max-Planck Institut für Chemie, Postfach 3060, 55020 Mainz, Germany;GIN Russian Academy of Sciences, Pyzhevsky 7, 109017 Moscow, Russia;IGEM Russian Academy of Sciences, Staromonetny 35, 109017 Moscow, Russia |
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| Abstract: | The Kostomuksha greenstone belt consists of two lithotectonic terranes, one mafic igneous and the other sedimentary, separated by a major shear zone. The former contains submarine 2.8 Gyr old komatiite-basalt lavas and volcaniclastic lithologies with trace element and isotopic compositions resembling those of recent oceanic flood basalts [?Nd(T) =+ 2,8, μ.1= 8.73 (Nb/Th)N= 1.5–2.1 (Nb/La)N= 1.0–1.5]. We suggest that the mafic terrane is a remnant of the upper crustal part of an Archaean oceanic plateau derived from partial melting of a mantle plume head. When the plateau reached the continental margin, it 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 piece of continental crust. The deeper zones were subducted and disappeared from the geological record. |
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