Baltic Shield - Southeastern Baltic Shield

Southeastern Baltic Shield

The Sumozero-Kenozero greenstone belt in the southeastern section of the Baltic shield is ~400 km long and up to 50 km wide. It comprises a 5-km thick oceanic plateau sequence of submarine komatiite-basalt lava and volcanic sediments. The belt is intruded and overlain by an island arc-like sequence of intermediate-felsic volcanic rocks including andesitic basalts, andesites, dacites and rhyolites. According to a study by Puchtel et al. (1999): "The komatiites were derived from a liquid containing ~30% MgO. This liquid was initiated at depths of 300–400 km in a mantle plume that was some 250°C hotter than the ambient mantle. Both komatiites and basalts of the lower sequence are strongly depleted in LREE, have high Nd(T) of +2.7±0.3, relatively unradiogenic Pb isotope compositions (µ m1 = 8.7±0.2) and show Nb-maxima (Nb/Nb* = 1.2±0.2, Nb/U = 43±6)." These parameters are found in a number of other early Precambrian greenstones and in recent Pacific OFB. "They are regarded as plume source characteristics and provide further evidence for the existence of certain Nb-excess in the Archaean mantle due to the early extraction of large volumes of continental crust with low Nb/U ratios. The intermediate-felsic volcanic and subvolcanic rocks of the upper unit are enriched in LREE, depleted in HFSE, but have positive Nd(T) values of +2.5±1.2. They represent both mantle wedge-derived basalt-andesite-dacite-rhyolite (BADR), and slab-derived (adakite) melts, erupted in the inner and frontal parts of an intraoceanic island arc." U-Pb zircon ages for the felsic volcanic rocks are 2875±2 Ma, and Pb-Pb and Sm-Nd ages of 2892±130 and 2916±117 Ma for the komatiites-basalts.