Metasedimentary rocks generally contain carbonaceous material (CM) deriving from the evolution of organic matter originally present in the host sedimentary rock. During metamorphic processes, this organic matter is progressively transformed into graphite s.s. and the degree of organisation of CM is known as a reliable indicator of metamorphic grade. In this study, the degree of organisation of CM was systematically characterised by Raman microspectroscopy across several Mesozoic and Cenozoic reference metamorphic belts. This degree of organisation, including within‐sample heterogeneity, was quantified by the relative area of the defect band (R2 ratio). The results from the Schistes Lustrés (Western Alps) and Sanbagawa (Japan) cross‐sections show that (1) even through simple visual inspection, changes in the CM Raman spectrum appear sensitive to variations of metamorphic grade, (2) there is an excellent agreement between the R2 values calculated for the two sections when considering samples with an equivalent metamorphic grade, and (3) the evolution of the R2 ratio with metamorphic grade is controlled by temperature (T). Along the Tinos cross‐section (Greece), which is characterised by a strong gradient of greenschist facies overprint on eclogite facies rocks, the R2 ratio is nearly constant. Consequently, the degree of organisation of CM is not affected by the retrogression and records peak metamorphic conditions. More generally, analysis of 54 samples representative of high‐temperature, low‐pressure to high‐pressure, low‐temperature metamorphic gradients shows that there is a linear correlation between the R2 ratio and the peak temperature [T(°C) = −445 R2 + 641], whatever the metamorphic gradient and, probably, the organic precursor. The Raman spectrum of CM can therefore be used as a geothermometer of the maximum temperature conditions reached during regional metamorphism. Temperature can be estimated to ± 50 °C in the range 330–650 °C. A few technical indications are given for optimal application.

The degree of recrystallization of carbonaceous material (CM), as monitored by Raman microspectroscopy, was examined as a function of metamorphic grade in two well‐studied contact aureoles containing carbonaceous pelites: the Nelson aureole, British Columbia and the Ballachulish aureole, Scotland. Here, we use (a) the R2 ratio extracted from the Raman spectrum of CM as a proxy for the degree of graphitization (0.0 in perfect graphite then increasing with structural defects) and (b) the second‐order S1 band (~2,700 cm−1) as a marker for the tridimensional ordering of CM. The Nelson aureole (garnet–staurolite–andalusite–sillimanite–K‐feldspar sequence, ~550–650°C, 3.5–4.0 kbar) was developed in rocks that were unmetamorphosed prior to contact metamorphism, whereas the Ballachulish aureole (cordierite–andalusite–K‐feldspar–sillimanite sequence, ~550–700°C, ~3.0 kbar) was developed in rocks that had been metamorphosed to garnet grade conditions (~7 kbar, ~500°C) c. 45 Ma before contact metamorphism. Thirty‐one samples were examined from Nelson and 29 samples from Ballachulish. At Nelson, the R2 ratio steadily decreases from ~0.25 to 0.0 as the igneous contact is approached, whereas at Ballachulish, the R2 ratio remains largely unchanged from regional values (~0.20–0.25) until less than 100 m from the igneous contact. The second‐order S1 band reveals that carbonaceous material (CM) was transformed to highly “ordered” locally tridimensional graphitic carbon at Ballachulish by regional metamorphism prior to contact metamorphism, whereas CM was still a disordered turbostratic (bidimensional) material before contact metamorphism in the case of Nelson. Pretexturation of CM likely induced sluggish recrystallization of CM and delayed graphitization in the Ballachulish aureole. Temperatures of recrystallization of the CM in the two aureoles were estimated using different published calibrations of the thermometry based on Raman Spectroscopy of Carbonaceous Material (RSCM), with differences among the calibrations being minor. In the Nelson aureole, temperatures are in reasonable agreement with those indicated by the metapelitic phase equilibria (all within 50°C, most within 25°C). In the Ballachulish aureole, the retarded crystallization noted above results in increasing underestimates of temperatures compared to the metapelitic phase equilibria (up to ~75°C too low within 200 m of the igneous contact). Our study calls for careful attention when using RSCM thermometry in complexly polymetamorphosed rocks to assess properly the meaning of the calculated temperature.