Petrology and Sm-Nd and Pb-Pb Systematics of the Early Archean Highly Magnesian Metavolcanics of the Vodla Block,Baltic Shield |
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| Abstract: | The mafic-ultramafic metavolcanics of the Volotsk sequence are the earliest formations of the Karelian gneiss-greenstone region. They are fragments of Early Archean komatiite-basalt crust, constituting the base of the section of the Vodla block, on the periphery of which greenstone belts formed in the Late Archean. The komatiites of the Volotsk sequence are of the Al-nondepleted geochemical type (Al2O3/TiO2)N, (Gd/Yb)N = 1.0], which may be related to relatively low temperatures existing in the Early Archean mantle of the Karelian gneiss-greenstone region. The parent magmas of the komatiites were generated in a depth range of 450-250 km under conditions of high (>50%) degrees of partial melting in equilibrium with an olivine-orthopyroxene restite. Differentiation of the komatiite melts after eruption on the surface was controlled by fractionation of olivine (Fo93) and a small amount of liquidus chromite. Sm-Nd isotopic dating of the komatiites and metabasalts to 3391±76 Ma reflects the time the Volotsk sequence accumulated. The positive value of εNd(T) = +1.2±0.21 suggests the existence in the Early Archean mantle of a region of reservoirs depleted in highly incongruent elements (La/Zr)N = 0.78] produced during early stages of melting and extraction of basaltic magmas during a 100-200-m.y. interval before formation of the Volotsk sequence. The 2764±150 Ma Pb-Pb isotopic age of the metavolcanics corresponds to the time of metamorphism and granite formation, extensively manifested in the region. |
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