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Rare-earth elements in the Archean iron formation and associated schists in Ukkolanvaara, Ilomantsi, SE Finland

1977; Geological Society of Finland; Volume: 49; Issue: 2 Linguagem: Inglês

10.17741/bgsf/49.2.005

1799-4632

K. Laajoki, S. Lavikainen,

Geomagnetism and Paleomagnetism Studies

This study describes the La, Ce, Nd, Sm, Eu, Tb, Yb and Lu concentrations in six iron-formation rocks (Fetot^> 15 °/o), four ferriferous schists (Fetofx.10-15°/o) and two schist interbeds of the Archean Ukkolanvaara iron formation, as well as in seven associated schists of the Ilomantsi Schist Belt.For the same elements, two samples were analysed from the Archean Siivikkovaara iron formation in Kuhmo, one sample from the Archean Otravaara pyrite ore and one sample from the Karhusaari pyrite ore, probably Middle Precambrian in age.All the iron-formation samples and the Otravaara pyrite ore show a marked positive Eu anomaly compared with the composite of North American shale (NAS) and are relatively poor in rare-earth elements (REE).The REE concentrations in the Ilomantsi schists are largely similar to those in the Sheba greywackes from the analogous ironformation-bearing association of the Archean Fig Tree Group, South Africa.The ferriferous schists show a marked positive Eu anomaly compared with the NAS, but contain the other REE in as high abundances as the Ilomantsi schists on average.The results confirm the general Eu excess in the Archean iron formations compared with younger iron-rich metasediments.The REE concentrations in the Archean schists characterized by an Eu excess in relation to younger pelitic schists are considered indicative of their general immaturity and a source area dominantly graniticgranodioritic in composition.