Finding of Neoarchean (2.71 Ga) Conglomerates in the Kostomuksha Iron Ore Province: On the Youngest Archean Association of the Karelian Craton

A lens of previously unknown polymictic conglomerates has been found in the axial part of the Kostomuksha greenstone belt (KGB) of the Karelian Craton. Their clastic portion consists of poorly graded, largely angular pebbles. In composition, they are comparable with rocks of the country greenstone complex: (1) amphibolites similar to the Mesoarchean KGB gabbroids and basalts, (2) magnetite–amphibole quartzites similar to the Mesoarchean magnetite–biotite–amphibole quartzites associated with banded iron formations, and (3) Neoarchean (2.75 Ga) quartz metagraywackes. The conglomerates are intensely deformed (γ >10) under conditions of a sinistral shear. The conglomerates were deposited about 2.71 Ga ago, as indicated by analysis of zircons from the matrix. Thus, a new Neoarchean stratotectonic association, the youngest in the greenstone complex, was found in KGB. It probably formed in a pull-apart basin at the final stage of the formation of the KGB from an important role of shear movements. Volcanic and sedimentary associations similar in age are known in the Khedozero–Bolshozero, Kuhmo, and Takanen greenstone belts of the Karelian Craton.