Evolution of the Yuzhno-Vygozero Greenstone Belt, Karelia

Comparative analysis of magmatism in the Shilos zone (southern part of the Yuzhno-Vygozero greenstone belt) and the Vodlozero block, which borders the zone to the east and is a fragment of the ancient (>3 Ga) sialic crust of the Baltic Shield, was conducted with the aim of developing a geodynamic mo del for greenstone belts of Karelia that frame the Vodlozero block. Preexis ting and newly obtained isotopic Sm-Nd and U-Pb (on zircon) data suggest th at the main Archean magmatic events that produced the Shilos greenstone bel t acted synchronously throughout the Vodlozero block. The metabasalts of th e Shilos greenstone belt differ from analogous rocks of greenstone belts: o f central Karelia by their elevated contents of Cr (350-400 ppm) and Ni (14 0-150 ppm) at MgO contents of 7.5-8.5 wt %, features that testify to the or igin of the rocks from higher temperature melts. The differences in the REE contents reflect compositional variations in the mantle source. The tonali tes and dacites of the Shilos greenstone belt are more mafic (this paramete r of the rocks is comparable with analogous features of Phanerozoic tonalit es), have lower contents of Rb, Ba, Nb, Th, and LREE, and are characterized : by relatively weak REE fractionation. The rocks show positive epsilon(Nd) (T), ranging from +1.5 to +3.3, and a low initial Sr isotopic ratio (0.701) , These geochemical features are explained by the origin of the tonalites b y the partial melting of amphibolites under dry conditions at moderate dept hs. The sources of the melts were basalts that had not undergone a crustal evolution and whose composition differed from the composition of the Shilos basalts, The rhyodacites that are coeval with these rocks and intrude the Vodlozero tonalites have a negative epsilon(Nd)(T) and a high primary Sr-87 /Sr-86 ratio (0.708), features that suggest that the rocks were derived fro m older sialic crustal material.