Stratigraphy and isotopic age of the Lopian complex in the Lekhta structure of northern Karelia

New data on the stratigraphy and isotopic age of supracrustal rocks from the lower part of the section constituting the northeastern limb of the Lekhta structure (northern Karelia) and their relationships with the basement are considered. Geological-petrographic, lithological-geochemical, and isotopic data are used to define three formations united into the Okhta Group. Immediate relationships between volcanics of the greenstone belt and granitoids of the basement represented by the oldest (for the Baltic Shield: 2.8 Ga) continental weathering crust after granites, are discussed. Isotopic age of volcanics and granite gneisses of the basement indicates that the Lopian supracrustal complex of the Lekhta structure was deposited in a period lasting 16 myr with duration of periods corresponding to formation of the Okhta and Pebozero groups being as long as 8 and 11 myr, respectively. In the regional stratigraphic scale, the entire Archean part of the supracrustal section in the Lekhta structure should be attributed to the Middle Lopian.