Abstract
ABSTRACTRaman spectroscopy of carbonaceous materials (RSCM) was applied to 10 carbonaceous material (CM) ârich lowâ to mediumâgrade 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. TypeâI CM, represented by fineâgrained discontinuous dustâlike and scattered to thin sporadic layers, was more prevalent in lowâgrade metamorphic samples. TypeâII CM, corresponding to continuous and extended thick fibres and elongated grain constellations, was dominant in mediumâ to highâgrade metamorphic samples. The degree of crystallinity of CM and subsequent metamorphic temperatures were quantified by considering the intensityâbased R1 and the areaâbased R2 ratio parameters. Lowâgrade samples from the Lesser Himalayan Sequence (LHS), having TypeâI CM, resulted in RSCM peak temperatures between 306°C and 403°C. Mediumâ to highâgrade samples from Higher Himalayan Crystalline (HHC), containing TypeâII CM, yielded peak temperatures in the 482°Câ560°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.
Published Version
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