A Tectonic Remnant of the Mesoarchean Oceanic Lithosphere in the Belomorian Province, Fennoscandian Shield

Abstract

The results of detailed geological mapping, coupled with the isotope-geochemical study of a metamorphosed mafic-ultramafic complex known as the Central Belomorian Belt located in the Belomorian province of the Fennoscandian Shield, are reported. The protholith of the complex is ~2.9‒3.1 Ga old. It has been subjected to two 2.87 and 1.87 Ga structural-metamorphic reworking. This complex is one of the oldest in the Belomorian Province. We present several lines of evidence showing that these lithologies constitute a tectonic remnant of the Mesoarchean oceanic lithosphere, rather than any other mafic-ulramafic complex from modern geodynamic settings. The Central-Belomorian high grade mafic-ultramafics reveal a clear geochemical coherency, which implies their genetic relationships. Their mafic protholiths stem from the partial melting of the mantle peridotite protholith. The petrologic modelling has shown that primary melts were formed in the garnet lherzolite field at pressure of 3.5–3.8 GPa at ambient mantle potential temperatures of 1520–1550°С which led to an emergence of ~25‒30 thick oceanic crust. The available geochemical data suggest that the complex was formed at the initial stage of subduction. It marks the start of early continental crust-forming processes in the Belomorian Province.