COMPOSITION AND STRUCTURE OF THE LOWER CRUST OF THE BELOMORIAN MOBILE BELT, BALTIC SHIELD

Abstract

The lower crust of the Belomorian Mobile Belt consists predominantly of garnet peridotites with subordinate amounts of pyroxenites and spinel peridotites, which occur as xenoliths in Devonian diatremes and dikes in the southern part of the Kola Peninsula. When transported to the surface by ultrabasic melts, the xenoliths were affected by fluids from the host ultrabasic lamprophyres with the introduction of Ca, Mg, and such trace elements as Ba, Nb, Sr, and P. The concentrations of trace elements (Sm, Nd, Y, Ti, Zr, Ni, Cr, and others) and the Sm-Nd isotopic composition were not significantly modified, which makes it possible to use them to compare the xenoliths with the near-surface complexes and to reproduce the composition of the protoliths. The Paleoproterozoic lower crust was produced during the emplacement of mantle magmas into metabasites in the Neoarchean lower crust, a process that was accompanied by the contamination of the melts and the origin of rocks showing characteristics of mantle and crust material. The emplacement of significant melt volumes into the Neoarchean lower crust caused its heating and enabled its viscous-plastic flow. This flow could likely also affect the material of the upper mantle, as follows from the occurrence of spinel peridotite nodules among the garnet granulites with an increase in the amount of mantle xenoliths from the roof to bottom of the lower crust. The overall amount of ultrabasic rocks in the lower crust was evaluated at 8–10%.