Age of the Archean Strata with Banded Iron Formation in the Kostomuksha Greenstone Belt, Karelian Craton, Fennoscandian Shield: Constraints on the Geochemistry and Geochronology of Zircons

Abstract

The paper presents results of the first geochronologic study (LA-ICP-MS zircon method) of metasedimentary rocks that host the banded iron formation (BIF) and the metarhyolites (halleflinta) of the Gimoly Group in the Kostimuksha greenstone belt,Karelian Craton. Felsic magmatism that produced the rhyolite sills and dikes was dated at 2759 ± 19–2743 ± 15 Ma. The clearly predominant group of zircons in the schists (metagraywacke) has an isotope age of 2753 ±19 Ma, and scarce zircon grains of these rocks vary in age from 3.1 to 2.8 Ga. Geochemically, zircons from metarhyolites fall into three types (1) Eu-depleted and Ce-enriched, (2) Eu- and Ce-enriched, and (3) LREE- and MREE-enriched. The zircons of geochemical types (1) and (2) are also predominant in the sediments. The morphology and structure of zircon crystals in the rhyolites and sediments are also identical. The rhyolites were thus a major source of zircons for the metagraywacke, which alternates with BIF, while the Mesoarchean rocks played a subordinate role. The BIF of the Gimoly Group in the Kostomuksha Greenstone Belt was formed simultaneously with felsic volcanism at 2760–2740 Ma.