Thermal metamorphism in the lesser Himalaya of Nepal determined from Raman spectroscopy of carbonaceous material

The determination of metamorphic conditions is critical to the understanding of the formation of mountain belts. However, all collisional mountain belts contain large volumes of accreted sediments generally lacking metamorphic index minerals and are therefore not amenable to conventional petrologic investigations. By contrast, these units are often rich in carbonaceous material, making it possible to determine thermal metamorphism through Raman spectroscopy of carbonaceous material (RSCM method), a technique that has been recently calibrated [Beyssac et al., J. Metamorph. Geol. 20 (2002) 859-871]. The Lesser Himalaya (LH) is one of these problematic cases with a very poor mineralogy, but a key structural position within the Himalayan system that makes LH considered as diagnostic of the overall thermal behaviour of the orogen. This work demonstrates the performance of the RSCM technique and shows that this technique might thus be used to detect inter-sample variations as small as ∼10-15 °C, but absolute temperatures can only be determined to ±50 °C due to the uncertainty on the calibration. This study reveals that the LH has undergone a large-scale thermal metamorphism, with temperature decreasing progressively from about 540 °C at the top to less than 330 °C within the deepest exhumed structural levels.