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Vol. 1 (1998 year), No. 3

Vetrin V.P.
The Belomorian Megablock Lower Crust: Its Age, Structure and Conditions of Formation (by Results of Deep Seated Xenolith Studying)

The fragments of the lower crust of the Belomorian megablock located in a boundary zone between the Kola and Karelian subcratons are submitted by deep seated xenoliths in explosion pipes and explosive dykes of the Kandalaksha and Tersky coasts of the White Sea. The most part of xenoliths are formed by garnet granulites, there are pyroxenites, websterites, amphibole-pyroxene rocks and some others at smaller amount as well. The age stages of the lower crust rocks are determined by results of Pb-Pb and U-Pb researches. The conditions of metamorphic paragenesis formation in garnet granulites are described. This process is characteristic for many granulite belts of the world and is supposed to be connected to processes of basalt underplating.

(in Russian, стр.12, fig. 5, tables. 3, ref 37, MS Word 95, MS Word 95 453 Kb)

Vol. 1 (1998 year), No. 3

Vetrin V.P., Ovchinnikova G.V., Neimark L.A., Gorokhovsky B.M., Smirnov Yu.P.
Muscovite-Microcline Metasomatites in Rocks of the Archaean Complex of the Kola Superdeep Borehole: Stages of Formation and Sources of Substance

Muscovite-microcline metasomatites on gneisses of the Archaean complex of the Kola superdeep borehole are found out in the bottom part of the section on the depth 12172-12235 m and by the geological position, structure and age are correlated with garnet-like rocks of the metasomatite formation occupying the area not less than 300 km² in the Archaean rocks of the Pechenga riftogenic structure's northern frame. The metasomatite formation occured in conditions of increased potential of CO₂, P, F, B and was accompanied by increase in gneisses of the content substratum as elements of siderophile (Fe, Ti, Cu, Ni, Co, Cr, V), and litophile (K, Rb, Ba, Nb, Zr, Y, TR, Pb, Th) group as well. These features of the structure are peculiar for subalkaline rocks of the early-orogenic trachybasaltic formation, which rocks have mantle origin and, as it is supposed, were a source of fluids rich of carbonic acid. On the Pb-isotope data, lead in the metasomatites was formed by mixing of mantle lead introduced by fluids with lead from gneiss substratum with age 2.8 Ga. The more radiogenic isotope structure of Pb of some plagioclases adequate to lead of upper crust formations, does not contradict the assumption of reworking of Pb-Pb systems of feldspars from muscovite-microcline metasomatites during intrusion of anatectic bodies of the Svecofennian granites.

(in Russian, стр.8, fig. 4, tables. 2, ref 13, MS Word 95, MS Word 95 204 Kb)