Accepted Manuscript Petrographic and Raman spectroscopic characterization of coal from Himalayan fold-thrust belts of Sikkim, India Santanu Ghosh, Sandra Rodrigues, Atul Kumar Varma, Joan Esterle, Sutapa Patra, Sitindra Sundar Dirghangi PII: S0166-5162(18)30489-0 DOI: doi:10.1016/j.coal.2018.07.014 Reference: COGEL 3062 To appear in: International Journal of Coal Geology Received date: 20 May 2018 Revised date: 26 July 2018 Accepted date: 26 July 2018 Please cite this article as: Santanu Ghosh, Sandra Rodrigues, Atul Kumar Varma, Joan Esterle, Sutapa Patra, Sitindra Sundar Dirghangi , Petrographic and Raman spectroscopic characterization of coal from Himalayan fold-thrust belts of Sikkim, India. Cogel (2018), doi:10.1016/j.coal.2018.07.014 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. ACCEPTED MANUSCRIPT Petrographic and Raman Spectroscopic Characterization of Coal from Himalayan Fold- Thrust Belts of Sikkim, India Santanu Ghosh1, Sandra Rodrigues2 Atul Kumar Varma1, Joan Esterle2, Sutapa Patra3, Sitindra Sundar Dirghangi3 1Coal Geology and Organic Petrology Laboratory, Department of Applied Geology, Indian Institute of Technology (Indian School of Mines) Dhanbad-826004, India 2School of Earth and Environmental Sciences, The University of Queensland, QLD 4072, Australia 3Department of Earth Sciences, Indian Institute of Science Education and Research Kolkata, Mohanpur, India. * Corresponding author. E-mail address: [email protected] (Atul K Varma). ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT Abstract: The present study focuses on the investigation of the optical anisotropy, optical sign, textural heterogeneity and deformational features of the maceral grains along with the Raman spectral characteristics of the seven coal samples collected from the Himalayan fold-thrust belts of Sikkim State, India. The coal samples were extremely fragile and pulverized due to the intense tectonic deformation. The maceral composition revealed the dominance of semifusinite over collotelinite grains. The calculated maximum vitrinite reflectance (5.94 – 8.66 %) and mean random reflectance (4.11 – 5.36 %) suggest anthracite A rank of the coal samples following ISO 11760:2005. The proximity of the intermediate reflectance axis value (RINT) to maximum reflectance axis value (RMAX) as well as the range of Reflectance Indicating Surface (RIS) style (Rst) values (-9.98 to -19.37) indicates the biaxial negative optical texture of the vitrinite grains. The augmented bireflectance values due to enhancement of the RMAX associated with strong decline in RMIN may suggest the commencement of pregraphitization. In addition, the strong linear correlation (r = 0.94) of the RIS-anisotropy (Ram) parameter with the bireflectance values may imply the role of tectonic stress on the optical transformations of the samples. The range of the peak temperature (334.94 – 369.01 ℃) calculated from mean random vitrinite reflectance may suggest the effect of thermo-stress coupling on the metamorphism of these coal samples. Microlithotype study combined with deformational aspects of macerals shows the presence of “deformed”, “sheared” and “smashed” grains within each sample, which may, additionally, document the tectonic stress influence on the coal particles. Moreover, relatively, larger area of ‘defect band 1 (D1)’ than that of ‘graphitic band (G)’ along with the broad G band in the first order Raman spectra may indicate the considerable presence of structural dislocations and aromatic compounds with disordered bond angle within the microstructure induced by the tectonic deformation. The lowest intensity of the ‘defect band 4 (D4)’ may suggest the preferential removal of aliphatic compounds from the samples in response to the tectonic stress degradation. In addition, the relative area ratio calculated from the D1 and the G bands (AD1/(AD1+AG)) may indicate that the studied anthracite samples would have attained the metamorphic temperature ranging from 325.12 – 387.89 ℃. Keywords: Himalayan fold-thrust belts; reflectance indicating surface; anthracite A; onset of pregraphitization; Raman spectral characteristics ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT 1. Introduction The tectonic framework coupled with structural characteristics of lithologies from the Himalayan fold-thrust belts (FTBs) is well documented (Bhattacharyya et al., 2015; Bhattacharyya and Mitra, 2009, 2011, 2014). Earlier studies (Ghosh, 1997; GSI, 2012) had investigated the physico-chemical properties along with the petrography and metamorphism of Rangit valley coal samples. However, the detailed optical analysis thorough Reflectance Indicating Surface characterization as well as the Raman spectroscopic characteristics of these coal samples promoted by the Himalayan orogeny have been under cover hitherto. The anthracite and the graphite from other basins in the world have been, widely, studied from detailed petrographic, and microstructural points of view (Han et al., 2017; Kwiecińska et al., 2010; Marques et al., 2009; Okolo et al., 2015; Rodrigues et al., 2011a, b, 2013; Xueqiu et al., 2017, among others) and these parameters have also been correlated and used in the investigation for the experimental production of graphite like particles under laboratory environment (González et al., 2004; Pusz et al., 2003; Rodrigues et al., 2011a, b). But these coal samples from the Rangit duplex of Sikkim Himalayan fold-thrust belts (FTBs) have not been explored until now from these aspects. The Gondwana anthracite samples from the complex structures of Himalayan FTBs have drawn the attention of the authors to carry out this study because coal can record the peak temperature and its structures and textures can be used as indicators for tectonic or metamorphic events (Bruns and Littke, 2015; Cao et al., 2000; Daiyong et al., 2009; Duber et al., 2000; Levine and Davis, 1989; Wu et al., 2012). Anthracite is characterized by having a typical structure and micro-texture related to the spatial arrangement of nano-meter sized turbostratic polyaromatic layers called basic structural units (BSUs) (Bonijoly et al., 1982; Marques et al., 2009; Oberlin, 1984). The preferred orientation of these aromatic lamellae is usually governed by tectonic stress (Hower, 1997; Hower et al., 1993; Levine and Davis, 1989; Marques et al., 2009; Ross and Bustin, 1997). The growth and preferred orientation of these BSUs parallel to the minimum compressive stress can be considered as the foremost driving mechanism in developing the anisotropy of vitrinite grains (Levine and Davis, 1984; Ross and Bustin, 1997). The anisotropic tectonic stress could promote the reorientation of BSUs, exhibited by strong bireflectance (Bw) of the organic material. The role of both stress and temperature on the anisotropy of vitrinite has already been proved from high temperature-pressure experiments on bituminous coal and anthracite (Bustin et al., 1995; Komorek and Morga, 2002; Ross et al., 1991; Wu et al., 2012). The vitrinite reflectance anisotropy in anthracite is usually represented by ACCEPTEDa three-dimensional ellipsoid MANUSCRIPT called Reflectance Indicating Surface (RIS) (Duber et al., 2000; Kilby, 1988; Levine and Davis 1989). The principal axes of the RIS, i.e., maximum reflectance value (RMAX), intermediate reflectance value (RINT) and minimum reflectance value (RMIN) can determine the evolution of the stress ellipsoid under the differential stress conditions as well as the optical sign of the coals. The magnitude of the vitrinite reflectance anisotropy along with the RIS style can be determined more extensively using transformation parameters of the RIS (Ram, Rev, and Rst) introduced by Kilby (1988, 1991) and later on modified by Duber et al. (2000). These parameters can also be applied to estimate the degree of heterogeneity in micro-textures of these coals. Furthermore, ACCEPTED MANUSCRIPT deformational features of the coal particles may also indicate the intensity of deformation associated with the coals. In this study, the deformation aspects of the macerals were studied through microlithotype analysis to provide additional information on the distribution of the coal particles with different deformational characteristics in the samples collected from the foreland and hinterland-dipping horses of the Himalayan FTBs of Sikkim. In addition, the peak palaeotemperature (Tpeak) parameter, which had been applied in some earlier investigations to determine the thermal maturation of organic matter (Barker and Pawlewicz, 1986), to reconstruct the burial history (Barker and Pawlewicz, 1994), to illuminate the effect of igneous intrusions, geothermal fluids on the sediments (Sweeney and Burnham, 1990) and to document the paleothermometry of the sedimentary basins (Barker and Goldstein, 1990; Bostick et al., 1979; Disnar, 1986; Piedad-Sánchez et al., 2004; Sweeney and Burnham, 1990) is used in this study. In complementary, this study includes the microstructural properties of the anthracite samples through Raman spectroscopy, which were also not explored, earlier. Raman spectroscopy is one of the most widely used tools to determine the microstructural