Neoarchean Volcanites of the Hedozero-Bolshozero Green-Stone Structure of Central Karelia: Composition, Age and Tectonic Setting
The composition was studied and the age was determined (the U–Th–Pb method for zircon, SIMS) of the Neoarchean volcanites of the Hedozero-Bolshozero greenstone structure of Central Karelia. The volcanogenic sequence of this structure is composed primarily of dacites and andesidacites, to less andesites and andesibasalts. The ultra-acidic volcanics, that was previously included in the sequence, make up intruding dikes and are represented by rhyolite-porphyries. According to geochemical characteristics, the volcanic rocks of the sequence are close to the rocks of the basalt-andesite-dacite-rhyolite series of modern island arcs, and their chemical composition is comparable with the high-silica and low-silica adakites. The age of andesibasalts and dacites of the Hedozero-Bolshozero structure is Neoarchean and is 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, and they are result by the mantle–crust interaction. The ancient Sm–Nd model ages of dacites and low ε<sub>Nd</sub> values (0.4 and 0.3) indicate a significant part of the crustal source in the initial melt. The younger model age of metaandesibasalts and the high value of ε<sub>Nd</sub> (2.8) indicate the predominantly mantle nature of lavas with a small part of 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 the more ancient crustal protolith (3.17 Ga). The volcanites of the Hedozero-Bolshozero structure cannot be compared with the rocks of the Kontoksky series of the Kostomuksha greenstone belt, since they differ from the latter in composition and younger age and occupy a higher position in the stratigraphic scale. There are alternative geodynamic settings for the formation of the studied rocks: an active continental margin, turning into collision, or the result of a combined action of the mantle plume and plate tectonics.