Shear stress in the graphitization of carbonaceous matter during the low-grade metamorphism from the northern Parang Mountains (South Carpathians) — Implications to graphite geothermometry

• Applicability of graphite geothermometers is tested in low-grade metamorphic rocks.
• Graphite geothermometers show inconsistency in different shear stress conditions.
• Graphite geothermometers require recalibration that considers the effect of strain.
• A new correction parameter for graphite geothermometers is proposed.

Low-grade metamorphic rocks from the northern Parang Mountains (South Carpathians) were examined to infer the temperature of metamorphism and to obtain new data to study the applicability of the geothermometers relying on the graphite crystallinity to rocks that underwent different shear stress conditions. There were analyzed samples of graphitic carbonaceous matter-bearing rocks that exposed dissimilar degrees of strain — for which the temperature of metamorphism was calculated using the graphite geothermometers based on the lattice parameter c0 determined through X-ray diffractometry and based on the peak area ratio of the first-order Raman spectrum. The temperature was also inferred from the chemical composition of the chlorite and from the Fe–Mg partitioning between chlorite and chloritoid, the mineral chemistry being determined through electron microprobe analyses. The results obtained have shown (i) differences of up to more than 150 °C between vicinal strained and less-strained zones when applying the geothermometers based on the graphite crystallinity and (ii) considerable discrepancies between the temperatures calculated based on the graphite crystallinity and those based on the chlorite and chloritoid chemical compositions. These inconsistencies are most likely caused by the fact that the calibration of the graphite geothermometers considers temperature as being exclusively responsible for enhancing the degree of graphite crystallinity and, therefore, these graphite geothermometers should be adjusted accordingly by introducing a correction factor that quantifies the degree of strain of the samples analyzed in order to obtain more accurate results.