XRD-based crystallinity degrees and formation temperature of graphite in some regional-metamorphic rocks from the Central and Eastern Rhodopes, Bulgaria: a test of the new graphitization degree GD(0–30) scale

Abstract

Graphitization degrees and temperatures of the regional metamorphism in the Central and Eastern Rhododpes have been determined by the values of the structural parameter d002 (Å) of graphite in graphite-bearing marbles and shists, from Vacha and Madan-Erma Reka areas (Madan lithotectonic unit), Ardino–Nedelino area (Startsevo lithotectonic unit), and Chernichevo–Boturche area (Byala Reka lithotectonic unit). Equations of different authors, formulated between 1951 and 2021, were used. They are integrated in one system, allowing a conversion of the results and deriving new quantitative correlations between the temperature of metamorphism, the structural parameter d002 (Å) and the degree of crystallinity order of semi-graphite and graphite. The comparison of the data allows the creation of a new scheme, GD(0–30), based on the values of the parameter d002 (Å) of the natural carbon matter. The validity range of this geothermometer is large, from 84 °C to 804 °C (from zeolite to granulite facies of the regional metamorphism). The temperature peak of metamorphism in the studied areas is 660 °C (i.e., the upper limit of the amphibolite facies), and the lowest temperature is 468 °C, which is characteristic for the lower part of the greenschist facies P-T field. The temperature range and graphitization degrees of the regressive metamorphism in the Central and Eastern Rhodopes are: 660 °C (GD(0–30) = 24, well-crystallized graphite); 588 °C (GD(0–30) = 21, graphite); 564 °C (GD(0–30) = 20, graphite); 516 °C (GD(0–30) = 18, graphite); 492 °C (GD(0–30) = 17, graphite); and 468 °C (GD(0–30) = 16, graphite). The calculated temperatures of metamorphism in the Central and Eastern Rhodopes, by equations of all authors, correspond to the conditions of metamorphism established by other methods without using of the structural parameter d002 (Å) of the graphite. The GD(0–30) system for determination of graphitization degree of the natural carbon is simple, informative and functional. It is more convenient than earlier schemes showing correlation: metamorphic temperature − d002 (Å) − graphitization degree. In addition to the influence of the temperature, the values of the degrees of graphitization also include the effect of all secondary factors on the graphitization processes.