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

Abstract

A brief geological review of the northwestern Ladoga region and systematics of the Lower Precambrian subdivisions in this territory are presented. The respective stratigraphic scale and supplementary geologic scheme include the following stratigraphic and metamorphic units applicable in cartography: (1) the 2700-Ma-old North Ladoga granite-gneiss association of the Archean; (2) the Sortavala Group with lower metaclastic 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 old. Eight metamorphic suites of the Lahdenpohja Group are referred to the undivided Ludicovian-Kalevian. The Late Karelian plutonic rocks are subdivided into the following units: the Kurkijoki and Lauvatsaari-Impinierni complexes of the early orogenic stage (1880-1860 Ma) represented by metanorite-enderbite and metadioritetonalite associations respectively; the Tervu metadiorite-granite complex of the late orogenic stage (1830-1810Ma); and the Ojajärvi monzonite-granite association (1800-1760 Ma) of postorogenic origin. Geological relations between these subdivisions in metamorphic zones of the Ladoga region and pertinent geochronological problems are discussed.