Typification of banded iron formations of the Kostomuksha Greenstone Belt (Fennoscandian Shield): Petromagnetic Evidence

As a result of petro-paleomagnetic studies, banded iron formations (BIF) have been typified by their magnetic properties. The main magnetization carrier minerals in BIF 2 and BIF-4 samples are single-domain magnetite and monoclinic pyrrhotite. The latter is identified by a characteristic rise in the cooling curves at 350 °C. It is shown that magnetite in the rocks is single-domain and ‘encapsulated’ in quartz grains. Investigation of the Anisotropy of magnetic susceptibility (AMS) permitted differentiating between highly and weakly magnetic samples: highly magnetic samples are dominated by a high degree of anisotropy associated with the transformation of the magnetic fraction and remagnetization of the rocks. The high AMS in the BIF-2 samples and Neoarchean conglomerates (2710 Ma) is associated with the predominance of layered-structure monoclinic pyrrhotite in the rocks. Weakly magnetic samples are predominantly anisotropic. Definitive stepwise demagnetization allowed us to distinguish at least two magnetization components in the studied samples. The average direction of the low-temperature component identified in all sites is near-aligned with the Svecofennian remagnetization for the Karelian Craton. The average direction of the high-temperature component in BIF-2 samples is close to that in Neoarchean conglomerates and coincides with the direction of the Lapland-Kola remagnetization (ca. 1.96 Ga). The identified metachronous magnetization components testify to uneven remagnetization of rocks within the same stratum. The Mesoarchean (2.8 Ga) rhyolite and Paleoproterozoic (2.42 Ga) gabbro samples may contain primary magnetization components dated to 2.72 and 2.42 Ga, respectively. Selective remagnetization of rocks differing in composition is observed.