Petrochemical features and factors of variation of rock compositionin the Sredne-Tatarsky Massif

The Sredne-Tatarsky (Zaangarsky) alkaline massif located in the south-western part of the Yenisey Ridge, theformation of which occurred as the result of repeated intrusion of mantel feldspathoid magmas in rifting conditions, is characterized by avariety of rocks composing it and presence of rare-earth − rare-metal mineralization. Foyaites of the early phase and later ijolites oftenpresented by feldspar variations participate mainly in the structure of the massif. The main rock-forming minerals of foyaites and ijolitesare presented by two generations and correspond to their standard composition in whole. Alkaline rocks of the massif have similar isotopiccomposition of primary relations of Sr and Nd isotopes in foyalites and ijolites, characterizing close relation of foid magmasources. By their chemical composition the rocks of the massif in whole are poor in silicic acid (SiO2 = 46-62%), rich in alumina(Al2O3 = 17-28%) and alkalis (K2O + Na2O = 6-17%) with Na2O prevailing. The petrochemical certification of rocks of the massifallowed defining silicate and carbonate associations based on the calcium, silica, alumina and alkali ratio. The silicate association unitesplagioijolites, foyalites, muscovite syenites and their weathering products. The assessment of the petrochemical specialization allowsconsidering them as products of potassium-sodium alkaline series and reveals the presence of various evolution trends of imaging rockpoints in each massif phase, indicating various mechanisms of crystallization during their development. The observed accumulation ofsalic components in the residual melt in ijolites and their reduction or permanent composition in foyalites (as in case with Al2O3) testifyto the fact that the main factor during ijolite crystallization is fractionating of dark-coloured minerals (pyroxenes), and peritectic feldsparand feldspathoid crystallization is noted in foyalites. The component analysis of petrochemical information in the silicate associationshowed that the variations of petrochemical rock parameters within the selection are defined by three main components. The first factorallows considering fraction crystallization of femic minerals as the leading process during the formation of ijolites. The second componentevidently characterizes the orientation of exogenous processes defined by decomposition of dark-coloured minerals and feldspathoidswith muscovite formation. The third component characterized by high positive correlation of the sum of alkalis, potassium andthe potassium-to-sodium ratio at the background of stable negative relations with silica probably defines the distribution of feldspars inthe rocks under study and reflects their peritectic crystallization. The material heterogeneity of main rocks composing the Sredne-Tatarmassif, based on the performed study of their silicate analyses, is preconditioned by evolution of the foidite melt, associated with variousmagma fractionating and crystallization mechanisms.

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