Comparison between Raman spectra of carbonaceous material and carbon isotope thermometries in low-medium grade meta-carbonates: Implications for estimation of metamorphic temperature condition

Abstract

The changes in the physicochemical conditions of the carbonaceous material during progressive metamorphism in metapelitic rocks is widely used as a geothermometer with the aid of Raman spectroscopy. However, the application of this technique in carbonate rocks has not been established yet. Here, we compare Raman spectroscopy of carbonaceous material thermometry and carbon isotope thermometry in low- to medium-grade metacarbonate rocks from the Archean Chitradurga Schist Belt in the Dharwar Craton, India. The carbonates in the lowermost Bababudan Group have metamorphosed under lower amphibolite facies metamorphic conditions giving consistent estimates for both Raman spectra of carbonaceous material thermometry (460–592 °C) and carbon isotope thermometry (450–560 °C). Contrastingly, in the Vanivilas Formation, the carbonaceous material with very fine flaky morphology in the grain boundary has slightly lower crystallinity, when compared to the coarse-grained ones near the vein boundary. Nevertheless, the carbon isotope thermometry estimated a lower temperature around 400 °C. The inconsistencies between the temperature estimates are ascribed to the pervasive post-metamorphic aqueous hydrothermal fluid infiltration indicated by the presence of numerous criss crossing quartz veins. This is also corroborated by the lowering of the oxygen isotope ratio of the carbonates near the vein boundary by 2.3‰, but without much difference in the carbon isotope ratios. The coarsening of the carbonaceous material near the vein boundary signifies fluid assisted recrystallisation, that enhanced the crystallinity as evidenced in the Raman spectra. Moreover, the negative δ13C shift in the carbonaceous material (c. –8.5 to –13‰) was possibly due to recrystallization following partial CO2 degassing. In the Chitradurga Schist Belt, the Bababudan Group and the Vanivilas Formation have regionally metamorphosed under lower amphibolite facies condition, possibly related to the widespread granitic intrusion at c. 2.61 Ga and later affected by hydrothermal event at c. 2.5 Ga.