The Archaean Greenstone Belts of Karelia (Eastern Finland) and their Komatiitic and Tholeiitic Series

Abstract

Abstract The geological and geochemical features of three greenstone belts of Eastern Finland (Suomussalmi, Kuhmo and Tipasjarvi) have been studied. We have analysed about 70 metavolcanic rocks from the low greenstone sequence for their major and trace element compositions. The field relationships between various volcanic rocks are rather obscure; but the chemical data allow us to distinguish two general magmatic series, namely, the komatiitic and the tholeiitic series. We have concluded from this preliminary geochemical study that most volcanic rocks in each may have been derived from fractional crystallization of some parental magma. The REE data, mainly presented for the rocks from the Tipasjarvi belt, provide a strong evidence for a “depleted” mantle source, a feature very similar to that of Abitibi, Canada. The REE data also suggest that not all rocks are formed by fractional crystallization; a mechanism of different degree of partial melting is called to account for some rock types. Although the Baltic Shield is one of the first regions in which the Archaean rocks were described (Sederholm, 1897, 1932), it is only recently that the existence of greenstone belts has been clearly demonstrated (Blais et al., 1976, 1977 and in print a and b; Bowes, 1976; Gaal et al., 1976 and in print; Lobach-Zhuckenko et al., 1976; Mutanen, 1976; Suslova, 1976). In this contribution, the principal characteristics of the belts, which we have studied in Finland, are briefly described and we expose the present state of our petrological and geochemical research in this context.