RAMAN Analysis of Carbonaceous Material and Deduced Peak Metamorphic Temperatures of Metasediments From Western Himalaya, NW Pakistan

Abstract

Raman spectroscopy of carbonaceous materials (RSCM) was applied to 10 carbonaceous material (CM) -rich low- to medium-grade metasedimentary rock samples of Western Himalaya, Pakistan to assess their optimum thermal evolution. The RSCM thermometry is based on the degree of graphitisation of CM as a function of peak metamorphic temperature. Petrographic observations of the studied samples revealed two dominant CM morphologies. Type-I CM, represented by fine-grained discontinuous dust-like and scattered to thin sporadic layers, was more prevalent in low-grade metamorphic samples. Type-II CM, corresponding to continuous and extended thick fibres and elongated grain constellations, was dominant in medium- to high-grade metamorphic samples. The degree of crystallinity of CM and subsequent metamorphic temperatures were quantified by considering the intensity-based R1 and the area-based R2 ratio parameters. Low-grade samples from the Lesser Himalayan Sequence (LHS), having Type-I CM, resulted in RSCM peak temperatures between 306°C and 403°C. Medium- to high-grade samples from Higher Himalayan Crystalline (HHC), containing Type-II CM, yielded peak temperatures in the 482°C–560°C range. The extremely deformed sample from a fault zone in the LHS produced relatively higher temperature of 403°C, thereby suggesting heat associated with shearing due to fault activities resulting in CM recrystallisation and elevated temperature values. The study revealed that the peak metamorphic temperatures increase from south towards north following a progressive path of metamorphism in the region.