GEOCHEMISTRY OF ALUMINOUS PLAGIOGNEISSES: THE ORIGIN OF KYANITE-BEARING ROCKS OF THE BELOMORIAN BELT

Abstract

Geochemical features (including REE contents) are studied for many samples of fine-grained massive ("dry") aluminous plagiogneisses, which occur as relics among the migmatized and granitized varieties in different parts of the Belomorian mobile belt. Superimposed kyanite is found in various rocks of the region. It is demonstrated that the kyanite-garnet-biotite plagiogneisses represent their garnet-biotite varieties with superimposed kyanite. Kyanite originated under high and medium pressures. High Al content in the rocks is not a leading factor of kyanite crystallization. Kyanite was formed in several stages from the earlier granitization in the Late Archean (2.8-2.6 Ga) to the Svecofennian epoch (1.9-1.75 Ga).