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[SMP25-03] Characterization of diverse thermal trends in Western Himalayan Metamorphic rocks deduced from the Raman spectroscopy of carbonaceous material
Keywords:Raman Spectroscopy, Carbonaceous Material, RSCM, Western Himalaya, Peak Metamorphic Temperature
The Indian Plate in the Western Himalaya of northern Pakistan contains an array of rocks variably metamorphosed and deformed as a result of the Eocene Himalayan orogeny. This study extends the work of Ahmad et al. (2024) by applying Raman Spectroscopy of Carbonaceous Material (RSCM) to thirty low- to high-grade metapelites from the Lesser Himalayan Sequence (LHS) and Higher Himalayan Crystalline (HHC) units to explore their thermal conditions.
Petrographic observations revealed three distinct metamorphic zones: chlorite zone within LHS in the south that extends to the biotite-garnet-staurolite and kyanite-sillimanite-zones in the HHC in the north. Five types of CM morphologies were identified within the studied rocks. Type-I exhibited by fine and dusty grains was found in low-grade chlorite zone rocks. Type-II represented by continuous fibers was prevalent in the biotite-garnet-staurolite zone rocks. A transitional type morphology (type-III) showed the characteristics of both type-I and type-II, was specifically observed in a fault zone sample from the LHS. Type-IV was present as compact short layered and subrounded grains, as inclusions within garnet, epidote and tourmaline. Type-V, having textural features akin to type-IV, was primarily noticed in matrix of the kyanite-sillimanite zone samples of the HHC. Raman spectral processing of the acquired data and thermometry calculations were performed using the Phython code of Kaneki et al. (2024). Chlorite zone samples yielded temperatures between 302 and 409 C, whereas biotite-garnet-staurolite zone and kyanite-sillimanite zone samples resulted peak temperatures between 474 and 536 C, and 550 and 603 C, respectively. The focus of this study was to explore the newly identified type-IV CM found as inclusions in garnet and tourmaline. Two samples, one each from Swat and Besham areas of the HHC, with well-preserved type-IV CM were analysed in relatively clean garnet porphyroblasts. The former showed a temperature of around 470 C, noticeably lower than the matrix temperature of 530 C. Conversely, the latter showed a higher temperature > 600 C, surpassing the matrix temperature of 520 C. Additionally, two more samples having highly altered/dirty garnet porphyroblasts with type-II CM replicated the matrix temperatures of 515 C and 513 C, respectively. Three tourmaline-bearing samples, containing type-IV CM, were analysed too. A sample from Besham area showed temperature values comparable to the matrix CM, while a tourmaline-rich sample collected from a vicinity located near the contact of tourmaline granites from Swat area revealed a significantly high temperature of 610 C. We assume the thermal increase in this sample could be due to contact-metamorphism caused due to tourmaline granite, as also reported by earlier studies. The southern Swat area, having type-IV CM in tourmalines, produced temperatures exceeding 655 C, the upper limit of several Raman geothermometers. Conversely, the surrounding matrix with type-II CM yielded 450 C. Considering the unusually high temperatures, irregular alignment of tourmalines, lack of nearby tectonism, and distant location of the sample from collision zone, we propose a detrital origin for the tourmaline and enclosed CM. To summarise, the petrographically distinct metamorphic zones in the study area with five CM morphological types and the resulting RSCM data helped to elucidate the thermal structure of the Western Himalaya.
References
Ahmad, N., Faisal, S., Khan, A., Rehman, H.U., 2024. RAMAN Analysis of Carbonaceous Material and Deduced Peak Metamorphic Temperatures of Metasediments From Western Himalaya, NW Pakistan. Geological Journal.
Kaneki, S., Kouketsu, Y., Aoya, M., Nakamura, Y., Wallis, S.R., Shimura, Y., Yamaoka, K., 2024. An automatic peak deconvolution code for Raman spectra of carbonaceous material and a revised geothermometer for intermediate-to moderately high-grade metamorphism. Prog Earth Planet Sci 11, 35.
Petrographic observations revealed three distinct metamorphic zones: chlorite zone within LHS in the south that extends to the biotite-garnet-staurolite and kyanite-sillimanite-zones in the HHC in the north. Five types of CM morphologies were identified within the studied rocks. Type-I exhibited by fine and dusty grains was found in low-grade chlorite zone rocks. Type-II represented by continuous fibers was prevalent in the biotite-garnet-staurolite zone rocks. A transitional type morphology (type-III) showed the characteristics of both type-I and type-II, was specifically observed in a fault zone sample from the LHS. Type-IV was present as compact short layered and subrounded grains, as inclusions within garnet, epidote and tourmaline. Type-V, having textural features akin to type-IV, was primarily noticed in matrix of the kyanite-sillimanite zone samples of the HHC. Raman spectral processing of the acquired data and thermometry calculations were performed using the Phython code of Kaneki et al. (2024). Chlorite zone samples yielded temperatures between 302 and 409 C, whereas biotite-garnet-staurolite zone and kyanite-sillimanite zone samples resulted peak temperatures between 474 and 536 C, and 550 and 603 C, respectively. The focus of this study was to explore the newly identified type-IV CM found as inclusions in garnet and tourmaline. Two samples, one each from Swat and Besham areas of the HHC, with well-preserved type-IV CM were analysed in relatively clean garnet porphyroblasts. The former showed a temperature of around 470 C, noticeably lower than the matrix temperature of 530 C. Conversely, the latter showed a higher temperature > 600 C, surpassing the matrix temperature of 520 C. Additionally, two more samples having highly altered/dirty garnet porphyroblasts with type-II CM replicated the matrix temperatures of 515 C and 513 C, respectively. Three tourmaline-bearing samples, containing type-IV CM, were analysed too. A sample from Besham area showed temperature values comparable to the matrix CM, while a tourmaline-rich sample collected from a vicinity located near the contact of tourmaline granites from Swat area revealed a significantly high temperature of 610 C. We assume the thermal increase in this sample could be due to contact-metamorphism caused due to tourmaline granite, as also reported by earlier studies. The southern Swat area, having type-IV CM in tourmalines, produced temperatures exceeding 655 C, the upper limit of several Raman geothermometers. Conversely, the surrounding matrix with type-II CM yielded 450 C. Considering the unusually high temperatures, irregular alignment of tourmalines, lack of nearby tectonism, and distant location of the sample from collision zone, we propose a detrital origin for the tourmaline and enclosed CM. To summarise, the petrographically distinct metamorphic zones in the study area with five CM morphological types and the resulting RSCM data helped to elucidate the thermal structure of the Western Himalaya.
References
Ahmad, N., Faisal, S., Khan, A., Rehman, H.U., 2024. RAMAN Analysis of Carbonaceous Material and Deduced Peak Metamorphic Temperatures of Metasediments From Western Himalaya, NW Pakistan. Geological Journal.
Kaneki, S., Kouketsu, Y., Aoya, M., Nakamura, Y., Wallis, S.R., Shimura, Y., Yamaoka, K., 2024. An automatic peak deconvolution code for Raman spectra of carbonaceous material and a revised geothermometer for intermediate-to moderately high-grade metamorphism. Prog Earth Planet Sci 11, 35.