Meso-Neoarchean felsic volcanics of the Gimoly greenstone belt, Karelian Craton, Fennoscandian Shield

The Gimoly Greenstone Belt (GGB) is located in the central Karelian Craton on the Fennoscandian Shield. The volcanogenic and sedimentary rocks of the Gimoly Group are interpreted as a stratotype of an Upper Lopian (Neoarchean) volcano sedimentary rock sequence in Central Karelia. The GGB’s granite-greenstone complex is overlain with angular unconformity by Paleoproterozoic (Sumian, Jatulian) rocks. We propose a new model for the stratification of the GGB’s greenstone complex based on original geological and geochronological data. New stratigraphic units are given the older names that are already in use. The authors assume that the Gimoly Greenstone Belt consists of three stratotectonic associations (SТА): Gimolozero, Mezhozero, and Sukkozero. Isotopic dating of zircons (LA-ICP-MS) from felsic volcanics shows that the age of 1) andesites from the Mezhozero STA is 2824 ± 3 Ma, and 2) that of andesite-basalts interbedded with polymictic conglomerates is 2752 ± 2.4 Ma. Neoarchean felsic volcanics have been known as part of the GGB previously, but their Mesoarchean counterparts have been found for the first time. GGB’s western flank consists of Gimolozero STA’s basalts. These metabasalts are similar in chemical and isotopic composition to the mafic rocks occurring in the komatiitic-basaltic sequence of the Kontokki Group of the Kostomuksha Greenstone Belt. Thus, the new geochronological and geological data can now be used for differentiating three STAs in the GGB 1) the Gimolozero basaltic (most probably Mesoarchean) STA, 2) The Mezhozero STA made up of andesites and interrbedded with BIF and carbonaceous schists dated as Mesoarchean (2.82–2.81 Ga), and 3) the Sukkozero STA composed of andesite basalt-andesites with conglomerate lenses of Neoarchean (2.76–2.74 Ga) age.