GEOCHRONOLOGY AND FORMATION SUBDIVISIONS OF THE LOWER PRECAMBRIAN IN THE NORTHWESTERN LADOGA REGION

A brief geological review of the northwestern Ladoga region and system atics of the Lower Precambrian subdivisions in this territory are pres ented. The respective stratigraphic scale and supplementary geologic s cheme include the following stratigraphic and metamorphic units applic able in cartography: (I)the 2700-Ma-old North Ladoga granite-gneiss as sociation of the Archean; (2) the Sortavala Group with lower metaclast ic and upper metatholeiitic subgroups of the Lower Karelian (Jatulian, Ludicovian), which are 2120-1980 Ma old; and (3) the Ladoga Group of the Upper Karelian (Kalevian) metaturbidites, which are 1910-1880 Ma o ld. Eight metamorphic suites of the Lahdenpohja Group are referred to the undivided Ludicovian-Kalevian. The Late Karelian plutonic rocks ar e subdivided into the following units: the Kurkijoki and Lauvatsaari-I mpiniemi complexes of the early orogenic stage (1880-1860 Ma) represen ted by metanorite-enderbite and metadiorite-tonalite associations resp ectively; the Tervu metadiorite-granite complex of the late orogenic s tage (1830-1810 Ma); and the Ojajarvi monzonite-granite association (1 800-1760 Ma) of postorogenic origin. Geological relations between thes e subdivisions in metamorphic zones of the Ladoga region and pertinent geochronological problems are discussed.