THE LAST ( Kalevian ) IMPULSЕ OF THE KOMATIITIC MAGMATISM IN THE PALEOPROTEROZOIC FORMATIONS OF THE FENNOSCANDIAN SHIELD

This paper deals with new geological, mineralogical, isotopic-geochemical (major, trace, and rare earth elements, Sm–Nd isotopic data) characteristics of the unique Kalevian (1926–1923 Ma) komatiite-tholeiitic magmatic manifestation for the first time identified in the supracrustal Kaskama Formation from the Inari terrane of the Kola-Norwegian region Fennoscandian Shield. In the massive and porphyritic komatiites, the primary (magmatic) mineral paragenesis represented by olivine 20–40%, orthopyroxene up to 5%, hornblende up to 10%, clinopyroxene 20–40% and plagioclase 20–30% was identified. Liquidus temperatures for olivine and pyroxenes calculated in the COMAGMAT3.73 software suite are in the range of ~ 1500–1200 °C. The komatiites of the Kaskama Formation belong to the Al-nondepleted type and are characterized by low REEN content (1–2 relative to C1 chondrite), a total concentration ∑REEavr = 0.15–0.36 ppm, and an unfractionated distribution of REEs, which is a consequence of the generation and evolution of their primary melts outside the field of thermodynamic conditions of garnet stability. The Zr–Y–Nb and Sm–Nd isotopic systematics of the rocks of the komatiite-tholeiitic association indicates the origin of their high-temperature primary melts from a plume source in depleted mantle (εNd(T) = +3.2 ± 0.3), which is different from the mantle sources of the Jatulian-Ludicovian picrite-komatiite association of the Central Lapland greenstone belt of Finland and the Pechenga intracratonic structure. Considering the amphibolite facies metamorphic conditions of the Kaskama Formation, it can be assumed that the komatiite-tholeiitic association is a deeply eroded (at least 10 km) section of a supracrustal volcan.