A. B. Vrevsky, A. B. Kuznetsov, A.V. Yurchenko
Institute of Precambrian geology and geochronology, Russian Academy of Sciences, St-Petersburg, Russia
Keywords: Fennoscandian shield, Paleoproterozoic, Inari Terrane, Kaskama Formation, komatiite-tholeiite association, Sm-Nd systematics
This paper deals with new geological, mineralogical, geochemical (major, trace, and rare earth elements) and Sm-Nd isotopic data for the unique Kalevian (1923-1926 Ma) komatiite-tholeiite association 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.
Relic igneous minerals (olivine, pyroxene) make it possible to estimate (COMAGMAT3.73 program) their liquidus temperatures in the range of 1500-1200 °С.
The komatiites of the Kaskama Formation belong to the Al-nondepleted type and are characterized by a low level of REEN content (1-2 relative to C1 chondrite), their total concentration (∑REE = 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 the plume source in depleted mantle (εNdT=+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 the upper crustal volcanic apparatus.
