The paper presents the results of detailed geological, petrographic and geochemical studies of Archean acidic subvolcanic rocks from the Koikary domain located in the central part of the Vedlozero-Segozero greenstone belt. Three petrologically contrasting groups are clearly distinguished in the investigated dacite-rhyolite association: 1) dacite with high content of Cr, Ni, Ti, Nb, and heavy REE, which are characterized by εNd (2935 Ma) from +0.72 to +2.93 and contain porphyritic phenocrysts of several generations; 2) rhyolite with a strongly fractionated REE distribution, negative anomalies in Nb, Sr and Ti, and with εNd (2935 Ma) from +1.15 to +3.37; 3) dacite and rhyolite with a moderately fractionated REE distribution, negative anomalies in Nb, Sr and Ti, and with εNd (2935 Ma) = –6.47. The above mentioned features of the rocks reflect the contrasting conditions of generation of acidic subvolcanic rocks in the Koikary domain and mark different time stages of the greenstone belt development.

A 1:750 000 scale geological map, covering an area of over 320 000 sq. km occupied by the Republic of Karelia and adjacent areas of Northwest Russia and Eastern Finland, has been compiled for the first time. A new legend, based on the International Stratigraphic Scale, was constructed by the authors. It differs from the common legends of Russian state geological maps. The rank of Archean and Proterozoic strata, comparable in the duration of formation to Phanerozoic systems, was determined. They are described briefly with regard for modern geochronological data. New colours and numerical indexing for all the strata shown in the map were used. The multi-level geoinformation system “1:500 000 scale Southeastern Fennoscandia”, based on new geological, geophysical, geochronological and petrological data collected by the Institute of Geology and other organizations, was developed. Geodynamic models for the Archean to Paleozoic crustal evolution of SE Fennoscandia have been proposed.