Abstract
The Precambrian rocks of the Keivy Terrane reveal five types of carbonaceous matter (CM): Fine-grained, flaky, nest, vein, and spherulitic. These types differ in their distribution character, carbon isotope composition, and graphitization temperatures calculated by the Raman spectra of carbonaceous material (RSCM) geothermometry. Supracrustal rocks of the Keivy Terrane contain extremely isotopically light (δ13CPDB = –43 ± 3‰) carbon. Presumably, its source was a methane–aqueous fluid. According to temperature calculations, this carbon matter and the host strata underwent at least two stages of metamorphism in the west of the Keivy Terrane and one stage in the east. The CM isotope signatures of several samples of kyanite schists (δ13CPDB = –33 ± 5‰) are close to those of oils and oil source rocks, and they indicate an additional carbon reservoir. Thus, in the Keivy territory, an oil-and-gas bearing basin has existed. Heavy carbon (δ13CPDB = −8 ± 3‰) precipitated from an aqueous CO2-rich fluid is derived from either the lower crust or the mantle. This fluid probably migrated from the Keivy alkaline granites into the surrounding rocks previously enriched with “methanogenic” carbon.
Highlights
The Keivy Terrane of the Archean Kola Province, located in the northeast of the Baltic Shield, is a unique object for studying carbonaceous matter (CM)
The morphology and structure of the CM-spherulites from quartzolites were inspected by using contrasting backscattered electron (BSE) images obtained with a Hitachi S-3400N scanning electron microscope
The graphitization temperature of CM was assessed by an Raman spectra of carbonaceous material (RSCM) geothermometer, calibrated
Summary
The Keivy Terrane of the Archean Kola Province, located in the northeast of the Baltic Shield, is a unique object for studying carbonaceous matter (CM). Different morphologies of CM are present in variable quantities in gneisses, amphibolites, quartzites, and schists of the Keivy and Tundra series [1]. The aluminous metapelites of the Keivy series (predominantly kyanite schists) are rich in carbon (0.1–2.3 wt % on average) [1,2,3]. These rocks are exposed over a wide area (Figure 1) of about 200 km in length and from several hundred meters to 10–14 km in width in the central part of the Keivy Terrane, as well as several smaller bands and lenses in its eastern and southern parts [3]. According to References [4,5,6], most of the stratum was metamorphosed under amphibolite facies conditions at Minerals 2019, 9, 94; doi:10.3390/min9020094 www.mdpi.com/journal/minerals
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