PETROLOGY OF KOMATIITES FROM THE BALTIC SHIELD AND ISOTOPE GEOCHEMICAL EVOLUTION OF THEIR MANTLE SOURCES

Abstract

This paper presents the results of investigations of the geochemical and isotopic compositions of komatiites from various Late Archean greenstone belts and structures of the Baltic shield. The analysis of major, trace, and rare earth elements demonstrated a considerable heterogeneity of their composition and lateral zoning in the distribution of their characteristics. It was found that variations in the contents of FeO, Al2O3, MgO, and REE and Al2O3/TiO2, Ti/Zr, Y/Zr, (Ce/Sm) N, and (Gd/Yb)N ratios in the complexes studied are related to various factors and have diverse origins. In order to decipher the latter, isotope geochemical data and methods of petrological and geochemical modeling of rock-forming processes were applied. The analysis of regularities in the spatial and temporal distribution of greenstone belts, the lithological characteristics of volcanosedimentary complexes, and the geochemical and isotopic heterogeneities of komatiites and their mantle sources demonstrated that they can be most adequately interpreted by the model of development of two mantle plumes of different ages.