LAUSR

Abstract A study of the composition and the age (U–Th–Pb method for zircon, SIMS) of the Neoarchean volcanic rocks of the Khedozero-Bolshozero greenstone structure of Central Karelia was carried out. The volcanogenic sequence of this structure is composed primarily of dacites and andesite-dacites, and to a lesser extent of andesites and andesite basalts. The ultra-felsic volcanic rocks, previously attributed to the sequence, compose intersecting dikes and are represented by rhyolite porphyry. According to geochemical characteristics, the volcanic rocks of the sequence are similar to the basalt-andesite-dacite-rhyolite rock series of modern island arcs, their chemical composition being comparable with the high-silica and low-silica adakites. The age of andesibasalts and dacites of the Khedozero-Bolshozero structure is Neoarchean: 2705 ± 9 Ma for andesibasalts and from 2712 ± 6 to 2703 ± 7 Ma for dacites. The Nd isotopic composition shows that the sources of melts for volcanic rocks were mixed owing to the mantle–crust interaction. The ancient Sm–Nd model ages and low ε Nd values (0.4 and 0.3) of dacites indicate a significant part of the crustal source in the initial melt. The younger model age of meta-andesibasalts and the high value of ε Nd (2.8) indicate the predominantly mantle nature of lavas with a minor crustal contamination. According to the age of crystallization, dikes of rhyolite porphyry (2704 ± 12 Ma) are close to the volcanic rocks of the sequence, but they have geochemical differences and a more ancient crustal protolith (3.17 Ga). As compared to the rocks of the Kontok Group of the Kostomuksha greenstone belt, the Khedozero-Bolshozero volcanic rocks have a different composition, a younger age, and a higher position in the stratigraphic scale. It is possible that alternative geodynamic settings, such as an active continental margin, turning into collision, or the combined action of the mantle plume and plate tectonics, led to the formation of the rocks studied.