Вестник МГТУ. 2020, Т. 23, № 1.

Their emplacement time complies with the cratonization of the system and generation of tectonic relief setting for the domain mainly at the expense of the reducing geothermal gradient. This resulted in an overall and essential cooling of the system triggered numerous faults in the domain’s peripheral areas and intrusion of abundant and imminently fluid-saturated magmas for alkaline granites, which represented the remobilized lower- crustal matter. It is worth emphasizing in this regard that the geodynamic setting of the median massif implies relative confinement of denudation of adjacent orogens. This resulted in removing all clastics to its surface, physical and chemical transformations of sediments and accumulation of thick sedimentary cover sequences. In our opinion, this favoured generation of giant aluminum deposits and wide fields of potassium metasomatism after terrigenous sediments within the Keivy median massif. In order to trace the process of chemical transformations within the Keivy sedimentary cover, a series of main reactions should be examined. Thus, at the denudation of orogenic units in the adjacent domains of the continental crust, orthoclase (K(AlSi 3 O8)), albite (Na(AlSi 3 O8)), anorthite (Ca(AlSi 2 O8)), and microcline (orthoclase polymorph) are among the major rock-forming minerals in trondhjemites, tonalities and granodiorites, which decompose in the presence of water by the following reactions: K(AlSi 3 O8) + nH2O + CO 2 ^ Al 4 [Si 4 O 10 ](OH )8 + K 2 CO 3 + SiO 2 x nH 2 O, orthoclase kaolinite potash opal or: 2 Ca(AlSi 2 O 8 ) + 6 H 2 O ^ Ab^O ioKOHh + 2 Ca(OH )2 anorthite kaolinite calcium hydroxide and Na(AlSi 3 O 8 ) + 2 H 2 O ^ AL,[Si 4 Oio](OH )8 + 4NaOH + 8 H 4 SiO 4 . albite kaolinite hydrated silica As well, under the hypergenesis conditions and in warm humid climate, orthoclase may decompose into hydromicaceous minerals, which further form kaolinite. For example: 6^AlSi3O8) + 2 CO 2 + 2 H 2 O ^ 2KAl2[AlSi3Oio](OH)2 + 2 K 2 CO 3 + 12 SiO 2 . Muscovite, in turn, may form kaolinite and potassium carbonate in the presence of carbon dioxide and water by the following reaction: 4KAl 2 [AlSi3Olo](OH )2 + 2CO 2 + 8 H2O ^ Al 4 [Si 4 Olo](OH )8 + 2K 2 CO3. Further hydrolysis processes occurred at the Archean and Proterozoic boundary in the Keivy domain could result in generation of laterite (red clay mineral) and silicon oxide: Al 4 [Si 4 Olo](OH )8 ^ H 2 Al 2 O 4 + SiO 2 x nH2O or hydrargillite and opal: Al 4 [Si 4 Olo](OH )8 ^ Al(OH )3 + SiO 2 x nH 2 O. The hydrargillite contains over 65 % of silica (Al 2 O3). Further, when the evolved sedimentary cover of the Keivy domain was overlain by younger sediments and tectonic thrusted plates of framing orogenic units in the Kola region, the sedimentary cover was dehydrated, heated up and yielded famous kyanite schists. This process is described by a simple endothermic reaction: Al4[Si4Oio](OH)8 ^ Al2OSiO4 + 2 H 2 O. kaolinite kyanite The same reaction may be presented in a simpler and understandable way: Al 2 O 3 x 2SiO 2 x 2H2O ^ Al 2 O 3 x 2SiO 2 + 2H 2 O. In our opinion, the unique kyanite deposits could occur in the Neoarchean. In addition to the above processes, it should be noted that the decomposition and redeposition of the tonalite-trondhjemite and granodiorite series rocks were accompanied by the generation of abundant potassium salts (K 2 CO3). These are highly water-soluble and most likely resulted in salinization of the Keivy water-saturated sedimentary sequences of that time. Later, during their metamorphic transformations potassium served as a building material for metasomatic microcline (alkaline) granites, which are abundant in the periphery of the domain. The established affiliation of the Keivy domain to the units similar to Phanerozoic median massifs, e. g. to the Tarim massif in terms of geodynamic features allowed Ye. N. Fomina and coauthors (Fomina et al., 2019) to draw, based on carbon isotopic data, a conclusion of possible Precambrian oil-and-gas-being basin existed in