On reasons of specific composition of the Keivy domain rocks (the Kola peninsula)

The Keivy domain differs from the other structures of the Kola region by the specific composition of the metamorphic complexes, the presence of the oldest for the region alkaline granites and gabbroanorthosites associated with them as well as by the presence of unique mineral deposits. The previous studies of the Keivy structure showed that within this structure there are metasedimentary formations that differ from both Early Precambrian metasedimentary rock associations of the Kola region and Phanerozoic sedimentary complexes in the content of titanium and the ratio Al2O3/SiO2. The present study confirms the correctness of this conclusion. The authors have attempted the search of the generalized indicator (trend) describing the nature of changes in the chemical composition of rocks in the transition from Low-Ti to High-Ti varieties of rocks (with simultaneous increase in the ratio Al2O3/SiO2). To solve this problem the method of search of differences in the specified number of sets has been used with the partial order relation introduced by the researcher. The formations of the same type and age, namely metapelites of the Chervurtskaya suite of the Keivy series, have been chosen as the informational basis. Such indicator has been found. It describes the process which besides a significant titanium input was accompanied by a simultaneous increase in the content of CaO and decrease in the content of Al2O3, ΣFe, and Na2O. It must be emphasized that the obtained trend is not related to the supergene processes characterized by the similar behavior of titanium with aluminum and iron which is especially clearly exhibited in the formation of crusts of weathering. The location on this generalized indicator of the points of rock compositions of all strata of the geological section of the Keivy domain taken far or close to alkaline granites corresponds to the position of less or more titaniferous rocks, thus confirming the assumption of a metasomatic alteration of the latter. The fact that all the metasedimentary Keivy formations are subjected to metasomatic alteration most probably related to the Archean complexes of alkaline granites, which is confirmed by direct geological observations of the relationships of the Lebyazhinskiye and Chervurtskiye rock complexes with alkaline granites, allows us to speak about their Archean age.