Petrology,age, and polychronous sources of the initial magmatism of the Imandra-Varzuga paleorift,Fennoscandian shield

This paper reports new geochemical and isotope data on the volcanogenic complexes of the Arvarench sequence of the Imandra-Varzuga paleorift structure of the Fennoscandian shield. It was established that these complexes are made up of komatiites, basalts, high-Mg andesites, and dacites and occupy a Sumian chronostratigraphic position with U-Pb (SHRIMP) age of 2429 ± 6.6 Ma in the regional Early Precambrian stratigraphic scale of the Kola-Norwegian province of the Fennoscandian shield, thus constraining the Sumian Subhorizon of the Lower Karelian Complex of the Northeastern Fennoscandian shield within 2450–2430 Ma. The high negative ε_Nd, LREE enrichment, and the presence of different-age Archean zircons with REE patterns indicative of disequilibrium crystallization suggest that the parental dacitic melts were derived by anatectic melting of polychronous (3.2, 2.9, 2.8, 2.7 Ga) lithological complexes of the Archean continental crust of the Kola-Norwegian province of the Fennoscandian shield. Numerical petrological-geochemical modeling of generation and evolution of primary melts of the metavolcanic rocks made it possible to establish that the isotope-geochemical peculiarities of the komatiites, basalts, and basaltic andesites can be best described by fractional crystallization of primary komatiite melt contaminated by ∼ 2% of the Archean crustal material of tonalitic composition. The mantle protolith of primary melt in terms of its isotope-geochemical parameters was similar to the “enriched” mantle source of the Paleoproterozoic (2430–2450 Ma) mafic-ultramafic layered intrusions of the Kola-Norwegian province and Sumian metavolcanic rocks of the Fennoscandian shield. The high-Mg andesites of the Arvarench sequence were derived by fractionation of crustally contaminated (∼ 2%) magnesian basalts with elevated Al content (Al₂O₃ ∼ 15.6 wt %) in equilibrium with 40–50% Cpx ₄₀-Ol ₂₀-Opx ₁₀-Pl ₁₀-Mag ₂₀ assemblage at P < 1 GPa. Obtained isotope-geochemical data and modeling results could be interpreted by off-subduction geodynamic model of the evolution of mantle plume and its interaction with the Archean continental lithosphere at the early stage of intracratonic rifting.