Vestnik MGTU (Mar 2017)
Graphite as an indicator of contact influence of Western Keivy alkaline granite intrusion, the Kola Peninsula
Abstract
The results of complex petro-mineragraphic, Raman and isotope- geochemical study of three types of graphite- bearing rocks circulated at different distances from the alkaline granites: (1) kyanite schists of Bolshiye Keivy, sampled at a considerable distance from a contact with alkaline gran ites; (2) sillimanite schists, sampled close to the contact, and (3) silexites, located in the inner part of th e alkaline granite massif Western Keivy have been presented. Five morphogenetic types of graphite have been revealed in the rocks under consideration: fine- grained Gr-1, intergranular Gr-2, nest-shaped Gr-3, vein Gr- 4 and spherulitic Gr-5. Current study demonstrates that these five types of graphite distinctly vary not only i n morphology, but also in temperature of crystallization, as determined by RSCM-Raman geothermometer, and in carbon isotop e composition. The most likely source for the anomalous "light" graphite Gr-1 and Gr-2 [δ 13 C(PDB) = −43...−45 ‰] from kyanite schists is a water- methane fluid originating from sedimentary rocks with org anic compounds. The carbon of graphite Gr-5 of the silexites selected at the inner part of alkaline granite massif West ern Keivy, on the contrary, proved to be most "heavy" [δ 13 C(PDB) = −8 ‰], which indicates its origin from the lower crustal or mantle carbon dioxide fluid. Thus, carbon extracted into the rocks of Keivy structure from at least two contrasting isotope sources. Graphite Gr-3, that makes up the bulk of graphite of exocontact sil limanite schists, is also isotopically light, but not anomalously [δ 13 C(PDB) = −17...−28 ‰]. The crystallization temperature of the gi ven graphite (435−520 ºC), and its structural relationships with other minerals of th e rock evidence of its synmetamorphic origin. The presence of veinlets of isotopically heavy [δ 13 C(PDB) = −10 ‰...−11 ‰] high-temperature (570−670 ºC) graphite intersecting minerals of the metamorphic paragenesis (i ncluding Gr-3) indicates that the introduction of the granites occurred into the previously metamorphosed strat a and was accompanied by thermal and fluid influence on these strata.
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