Accessory graphite in phyllites as indicator of metamorphic grade and stage (Szendrő Mts., NE Hungary)

The metamorphic temperature of rock bodies can be estimated from the degree of graphitization of carbonaceous material, best determined by Raman spectroscopy. Graphite-bearing rocks from intensely folded black phyllites (black schists) near Meszes village in the Szendrő Mts., NE-Hungary, were analyzed using ore microscopy, scanning electron microscopy (SEM-EDX), X-ray powder diffraction (XRD), Raman spectroscopy and X-ray fluorescence spectrometry (XRF). The exposed Szendrő Phyllite Formation is of Carboniferous age, represents a gradually deepening basin fill, metamorphosed under greenschist facies conditions. Due to its low quantity and the presence of significant muscovite (± illite) 2M₁, clinochlore and quartz content, graphite is not directly detectable by XRD. Graphite occurs as intensely deformed 50–150 μm sized flakes persistently with kink-band microstructure and undulose extinction. The absence of S and Cl (confirmed by EDX) indicates a total maturation of the carbonaceous material into pure, disordered graphite with higher crystal ordering supported also by the initial splitting of S2 Raman band. The average formation temperature is estimated at 425°C (±50°C) by the Beyssac-method and 417°C (±50°C) by the Aoya-method.