Graphite as an indicator of contact influence of Western Keivy alkaline granite intrusion, the Kola peninsula

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 granites; (2) sillimanite schists, sampled close to the contact, and (3) silexites, located in the inner part of the 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 in morphology, but also in temperature of crystallization, as determined by RSCM-Raman geothermometer, and in carbon isotope composition. The most likely source for the anomalous "light" graphite Gr-1 and Gr-2 [δ13C(PDB) = -43…-45 ‰] from kyanite schists is a water-methane fluid originating from sedimentary rocks with organic compounds. The carbon of graphite Gr-5 of the silexites selected at the inner part of alkaline granite massif Western Keivy, on the contrary, proved to be most "heavy" [δ13C(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 sillimanite schists, is also isotopically light, but not anomalously [δ13C(PDB) = -17…-28 ‰]. The crystallization temperature of the given graphite (435-520 ºC), and its structural relationships with other minerals of the rock evidence of its synmetamorphic origin. The presence of veinlets of isotopically heavy [δ13C(PDB) = -10 ‰…-11 ‰] high-temperature (570-670 ºC) graphite intersecting minerals of the metamorphic paragenesis (including Gr-3) indicates that the introduction of the granites occurred into the previously metamorphosed strata and was accompanied by thermal and fluid influence on these strata.