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
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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).
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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
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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 ACCEPTED a 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 ordering of the carbonaceous matter (Baysal et al., 2016; Beyssac et al., 2002a, b, 2003a, b; Cuesta et al., 1998; González et al., 2002, 2003, 2004; Han et al., 2017; Jawhari et al., 1995; Kelemen and Fang, 2001; Kwiecińska et al., 2010; Marques et al., 2009; Pasteris and Wopenka, 1991; Sadezky et al., 2005; Suárez-Ruiz and García, 2007; Tuinstra and Koenig, 1970; Wopenka and Pasteris, 1993; Xueqiu et al., 2017). It can reveal the microstructural imperfections induced by the tectonic deformation (Han et al., 2017) as well as the metamorphic temperature of the carbonaceous materials from the relative area ratios of the defect band 1 (D1) and graphitic band (G) (Beyssac et al., 2002a). Therefore, the detailed documentation of the optical and the microstructural transformations of the anthracite samples in response to the Himalayan tectonic activities through the aid of petrographic analysis (compositional and RIS characterization), and Raman spectroscopy defines the novelty of this study in front of the world science repute.
2. Geological setting Sikkim State is located in the southern mountain ranges of Eastern Himalayas between 27° 05′ N to 28° 08′ N and 88°10′ E to 88° 55′ E with an elevation from 240 to 8484 m above mean sea level encompassing the total area of 7096 sq km (GSI, 2012). Sikkim includes parts of the Lesser Himalaya, the Higher Himalaya and the Trans Himalayan ranges (GSI, 2012). In Sikkim-Darjeeling Himalaya, the Lesser Himalaya ranges from Kalijhora in Darjeeling (south/west) to SinghikACCEPTED in Sikkim (north/east) MANUSCRIPT with an elevation between 300 m and 3050 m (Mt. Mainak) and contains low grade metamorphic rocks. The Higher Himalayan rocks in Sikkim are exposed at elevations ranging from 650 m to 8586 m (Mt. Kangchenjunga). It contains outcrops of gneisses with intrusions of granite forming a barrier between the Trans- Himalayan zone and the Lesser Himalayas with Kanchenjunga as Western limit and Lama Angdang as the eastern limit (GSI, 2012). The snow-capped ranges of Trans-Himalaya vary from 5300 m to 7000 m elevation above sea level. The Himalayan orogeny is considered to take place at the northern margin of East Gondwanaland (Bhattacharyya and Mitra, 2009; Goscombe et al., 2006; Valdiya, 1997, Yoshida and Upreti, 2006).
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The Himalayas were formed as the result of convergence and collision between the Indian and Eurasian lithospheric plates and the uplift of the Himalayas continues in present day due to the ongoing collision. This orogenic system gave rise to the Himalayan fold-thrust belts, which are accommodated by south-vergent thrusts, where early faults are very often folded by compressive stress (Bhattacharyya and Mitra, 2009; Gansser, 1964; Heim and Gansser, 1939; Medlicott, 1864; Pearson and DeCelles, 2005; Ray, 1995; Srivastava and Mitra, 1994; Valdiya, 1980). These thrust systems extend from north to south in a sequence of the Main Central Thrust (MCT 1 and 2), the Ramgarh thrust (RT), the Lesser Himalayan duplex (LHD), the Main Boundary thrust (MBT) and the Main Frontal thrust (MFT) (Fig. 1a). Within these thrust systems, MBT and MCT have been usually considered to mark the boundaries between the Greater- and the Lesser Himalayan lithologies, and the Lesser- and the sub- Himalayan rocks, respectively (Bhattacharyya and Mitra, 2009). Two north-south trending river systems, namely these of Teesta and Rangit, acted as the main agent of erosion and formed the Teesta half window (Schwan, 1980), which is bounded by MCT 2 (Bhattacharyya and Mitra, 2009). The amphibolite grade Paro-Lingste gneiss had been carried by MCT 2 on the top of the greenschist grade lithologies of Daling Formation in the lower Lesser Himalayan Sequence. The MCT 1, on the other hand, brings the granulite grade Kanchenjunga gneiss on its hanging wall (Bhattacharyya and Mitra, 2009). The Teesta half window (Schwan,1980; Bhattacharyya and Mitra, 2009; Mitra et al., 2010) exposes the metapelitic greenschist-facies of the Lesser Himalayan Sequence with the relatively high grade Greater Himalayan crystalline lithologies (Bhattacharyya and Mitra, 2011); within this half-window, there lies an inner window known as Rangit window (Gangopadhyay and Ray, 1980; Raina, 1976), which is bounded entirely by RT (Fig. 1b) and exposes the Rangit duplex (Bhattacharyya and Mitra, 2009; Bhattacharyya et al., 2006). This Rangit duplex consists of ten horses of the Lesser Himalayan Sequence and repeats the Gondwana, Buxa and upper Daling litho-units (Bhattacharyya and Mitra, 2009). The RT carries Daling quartzites over the sandstones of Gondwana time and in the south of the frontal RT, the MBT places the Gondwana lithologies on the top of younger Cenozoic Siwalik Formation (Acharyya, 1994; Bhattachryya and Mitra, 2009). Moreover, the youngest MFT places the Siwalik rocks over the Quaternary sediments deposited in the mountain front of Himalayas (Bhattacharyya and Mitra, 2009). The Sikkim domal structure, as well as Rangit window zone, are important tectonic features in Lesser Himalayans where Late Palaeozoic Gondwana-equivalent rocks are exposed in the Rangit window at the core of Sikkim domal structure. This includes coal bearing lithology of lower Gondwana exposed as tectonic strips in the Himalayan thrust belts (Ghosh, 1997). AACCEPTED cross-section depicting theMANUSCRIPT formations and the complex thrust systems is shown in Figure 2.
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Fig.1a. Location of Sikkim State in India with the regional map of Darjeeling-Sikkim Himalaya (after Bhattacharyya and Mitra, 2009)
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Fig.1b. Enlarged map of Rangit window (the box surrounding ‘RW’ in Fig. 1a) showing the exposure of Rangit duplex (after Bhattacharyya and Mitra, 2009)
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Fig. 2. Geological cross-section of Lesser Himalayan duplex system along A-B (following Bhattacharyya and Mitra, 2009) Explanations (for Fig. 1a, b and 2): RW= Rangit Window; MCT=Main Central Thrust; MBT=Main Boundary Thrust; RT= Ramgarh Thrust; MFT= Main Frontal Thrust; MHT= Main Himalayan Sole Thrust; Tatapani Th.= Tatapani thrust; Sippip Th.= Sikkip thrust; Dong Th.= Dong thrust; Jorethang Th.= Jorethang thrust; Sorok Th.= Sorok thrust; Kitam Th.= Kitam thrust; J — Jorethang; K—Kamling, N—Namchi; R—Reshi.
The generalised stratigraphic sequence of Sikkim Himalaya is given in Table 1 following GSI (2012).
Table 1: Stratigraphic sequence of Sikkim Himalaya (GSI, 2012)
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3. Materials and methods
3.1. Sampling The coal samples were collected from road cut sections that traverse through Jorethang,-Namchi- Nyabazar- Legship- Sikkip- Reshi areas of south and west Sikkim State. Samples CG3300, CG3301, and CG3302 were collected from and around Jorethang area and sample CG3303 near Namchi at South Sikkim (Fig. 1b). Sample CG3304 was picked up from the north of Kamling village (Fig. 1b). Furthermore, CG3305 and CG3306 had been collected near the Reshi khola in Reshi town (Fig. 1b).
3.2. Petrographic study The samples were ground and processed to obtain 1mm sized particles; epoxy impregnated polished pellets were prepared according to the standard procedure. The petrographic study was conducted at the School of Earth and Environmental Sciences, The University of Queensland, Australia. The macerals were identified with following the ICCP System 1994 (ICCP, 1998, 2001) using a Leica DM 6000 microscope and the Diskus Fossil software (Technisches Büro Carl H. Hilgers) was used to measure the reflectance. Furthermore, a modified microlithotype study (microlithotypes were identified following ISO, 2017) with deformational aspects was carried out under 20x oil objective.
For RIS characterization the motorized stage of the Leica DM 6000 was substituted by a rotating stage in order to adopt the procedures described by Ting and Lo (1978) where three reflectance values are collected at every 45° interval during rotating the stage for a single vitrinite grain and where possible, 100 particles were measured. From this obtained data, maximum and minimum apparent reflectance (Rmax and R′min) of each grain were calculated using the equations formulated by Ting and Lo (1978). Thereafter, the three principal axes (RMAX, RINT and RMIN) of RIS as well as its parameters (Rev-reflectance equivalent of the RIS, Ram—RIS anisotropy, Rst—RIS style) were calculated following Kilby’s method (1988,1991) modified by Duber et al. (2000) for complete characterisation of RIS. Bireflectance values (Bw) were obtained by calculating the difference between RMAX and RMIN values (González et al., 2004; Khorasani et al., 1990; Marques et al., 2009; Wu et al., 2012). Coefficient of relative variabilities of the RIS parameters (Hst, Hev Ham) along with the total heterogeneity (Ht) values were also calculated using the equations suggested by Duber et al. (2000). The textural classes hadACCEPTED been formed based on theMANUSCRIPT vitrinite reflectance data following Duber et al. (2000). The reflectance measurements in these textural classes may vary in different classes. For instance, some classes comprise more than 10% of total measurements and some other even less and this situation influences the Ht values (Duber et al., 2000). Taking this concern under consideration, H10 values had been calculated from the relative variability of RIS parameters considering the textural classes having >10% of total measurements following Duber et al. (2000) to document the degree of heterogeneity commonly found in anthracite. Moreover, random vitrinite reflectance (Rr) values were computed following the equation formulated by Kilby (1988). From these values mean random vitrinite reflectance (R̅ r) parameter was estimated for all the studied samples. The peak palaeotemperature (Tpeak) was ACCEPTED MANUSCRIPT
calculated from the mean random vitrinite reflectance (R̅ r) values from the linear equation (i) between R̅ r and Tpeak following Barker and Pawlewicz (1986).
푇푃푒푎푘 = {ln (R̅푟 ) + 1.20}/0.0078………………..….(i)
3.3. Raman spectroscopy Raman spectroscopy was carried out using a Horiba Jobin Yvon LabRam spectrometer on the powdered coal samples at Indian Institute of Science Education and Research, Kolkata, West Bengal, India. The Rayleigh radiation was blocked using edge filters and the scattered light was dispersed by a grating with 1800 groves/mm. The 633 nm line of the Nd- YAG laser was used for excitation with the laser power of 18mW. The samples were scanned for 10 seconds between 1000 and 2000 cm-1 spectral region, commonly known as the ‘first order Raman spectra’. Three spectra per sample were collected and the average values of Raman shifts and intensities were calculated from those three spectra for each sample. Then, the spectrum obtained after averaging those parameters (Raman shifts and intensities) for each sample was considered for deconvolution. The raw spectra were treated with curve-fitting procedure (combination of Gaussian-Lorentzian area fit with mean coefficient of regression (R2) values of 0.98) using PeakFit version 4.12.
4. Results
4.1. Macroscopic description Coal deposits in the Himalayan FTBs belong to the Damuda Group of Gondwana Supergroup and are affected by folds and thrusts and show pinching and swelling structures (GSI, 2012). The coal occurring in these regions are mainly crushed and fragile due to brittle failure owing to tectonic deformation (GSI, 2012) compounded by the weathering effects. A significant amount of outer weathered portion of the exposures was removed to get the fresh samples from inside and to avoid the contamination from the recent sediments.
The first three coal samples (CG3300, CG3301, and CG3302) are exposed by the foreland dipping Jorethang horse of Rangit duplex in the southern part of the Rangit window (Fig. 1b) whereasACCEPTED CG3303 is exposed byMANUSCRIPT Namchi horse, the uppermost horse of the antiformal stack (Bhattacharyya and Mitra, 2009). The Namchi horse is underlain by the unexposed Namchi thrust and exposes the medium-grained sandstones and coal bearing shales of Damuda Group of Gondwana Supergroup (Table 1) (Bhattacharyya and Mitra, 2009). The eastern edge of the Rangit window is demarcated by a steep fault, which may be considered as the surface expression of an N-S trending lateral ramp (Bhattacharyya and Mitra, 2009). The Jorethang horse is underlain by the Jorethang thrust on its north side (Matin and Mazumdar, 2009) and is exposed along Jorethang ridge (Bhattacharyya and Mitra, 2009). The Gondwana coaly sandstone of thickness about 430 m is exposed by this ACCEPTED MANUSCRIPT
Jorethang horse along with the green slates of Daling Formation and calcareous shales of Buxa Formation (Bhattacharyya and Mitra, 2009).
The last three samples (CG3304, CG3305, and CG3306) are exposed at the hinterland dipping horses of the Rangit duplex in the northern part of the Rangit window (Fig. 1b). In the west side of Rangit river and to the north of Kamling village, the southern limb of an anticlinal fold exposes the Gondwana sandstone (Bhattacharyya and Mitra, 2009) along a 2 m thick coal outcrop. The southern limb of this anticline is exposed by the Sikkip horse, which is, further, underlain by the Sikkip thrust (Bhattacharyya and Mitra, 2009). Moreover, near Reshi, the northern limb of that anticlinal fold is exposed (Bhattacharyya and Mitra, 2009). The Gondwana Supergroup of this northern limb starts with medium-grained sandstone in the south near Reshi, which grades to coal bearing slate towards north near Tatapani thrust with a thickness of about 900 m (Bhattacharyya and Mitra, 2009).
4.2. Maceral composition The vitrinite content (Vmmf) varies from 15.41 vol.% in CG 3304 to 41.71 vol.% in CG3302 whereas, the inertinite content (Immf) varies between 58.29 vol.% and 84.59 vol.% (Table 2). Vitrinite grains, mostly, comprise the collotellinite (Fig. 3 a-b), while, mostly, the semifusinite (Fig. 3 c-d) and often, few fusinite grains characterize the inertinite group. The semifusinite grains are found to be the most dominantly occurring maceral in every sample. Moreover, the samples contain a large amount of mineral matter ranging from 15.2 vol.% in CG3306 to 53.8 vol.% in CG3303 (Table 2).
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Fig. 3. (a-b) Optical photomicrographs of collotelinite grain (structureless, homogeneous appearance) with homogeneous anisotropy; (c-d) semifusinite grain (with partially visible cell cavities of varying size and shapes) in polarised light (polarise filter rotated to 90° from each image) in sample CG3305
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Table 2. Maceral composition of the studied anthracite samples Sample V (vol.%) I (vol.%) MM (vol.%) Vmmf (vol.%) Immf (vol.%) no. CG3300 18.6 48.2 33.2 27.8 72.1 CG3301 23.2 41.0 35.8 36.1 63.9 CG3302 35.2 49.2 15.6 41.7 58.2 CG3303 11.8 34.4 53.8 25.5 74.4 CG3304 8.6 47.2 44.2 15.4 84.5 CG3305 12.4 60.0 27.6 17.1 82.8
CG3306 18.0 66.8 15.2 21.2 78.7 Explanations: V=vitrinite; I=inertinite; MM=mineral matter; mmf=mineral matter free basis
4.3. Vitrinite reflectance, bireflectance and maximum palaeotemperature
Table 3 presents the RIS main axes, its parameters, together with the bireflectance (Bw),
mean random vitrinite reflectance (R̅ r %), optical sign of the vitrinite grains as well as peak palaeotemperature (Tpeak) that the coal samples would have experienced. The RMAX values have been found to range from 5.94 - 8.66% (Table 3) and the peak of this parameter is assigned to sample CG3300. The RINT values vary from 4.94-6.38% whereas the RMIN values are placed between 1.32-3.03%. Regarding the RIS parameters, Ram values exhibit variation from 0.128-0.260 while Rev values are placed between 3.23 and 4.74 and the Rst values range from -9.98 to -19.37. The bireflectance (Bw) values shows variation between 3.55 and 7.27 %
Moreover, the R̅ r parameter exhibits a range from 4.11 – 5.36 %. Finally, the Tpeak values vary from 334.94 – 364.01 ℃.
Table 3. RIS axes, weighted means of RIS parameters, optical property, bireflectance, mean random reflectance and peak palaeotemperature of the studied anthracite samples
Optical
Samples RIS axes RIS parameters property Bw R̅ r (%) Tpeak (℃) R R R MAX INT MIN R R R (% ) (% ) (% ) am ev st CG3300 8.66 ACCEPTED6.30 1.39 0.260 4.01 MANUSCRIPT -9.98 B(-) 7.27 5.12 363.28 CG3301 6.57 5.55 3.03 0.128 4.74 -10.77 B(-) 3.55 4.94 358.71 CG3302 7.10 6.04 2.28 0.187 4.56 -16.06 B(-) 4.82 5.15 364.01 CG3303 6.95 6.21 2.37 0.180 4.62 -19.37 B(-) 4.58 5.36 369.01 CG3304 5.94 4.94 1.84 0.192 3.49 -14.55 B(-) 4.10 4.11 334.94 CG3305 7.37 5.61 1.32 0.249 3.23 -12.06 B(-) 6.06 4.15 336.39 CG3306 8.13 6.38 1.42 0.250 4.11 -13.98 B(-) 6.71 4.73 353.05
Explanations: RMAX = Maximum reflectance axis value; RINT = Intermediate reflectance axis value; RMIN = Minimum reflectance value axis; Ram = RIS anisotropy magnitude; Rev = Reflectance of equivalent volume isotropic RIS; Rst = RIS style; B(-) = ACCEPTED MANUSCRIPT
Biaxial negative when, RMAX>RINT>>RMIN; Bw = Bireflectance; R̅ r = Mean random vitrinite reflectance; Tpeak = Peak palaeotemperature
4.4. Textural heterogeneity parameters
The Ham values range from 0.859 – 1.797, while the Hev parameter varies between
0.04 – 0.8, and the Hst parameter exhibits its range from 0.124 – 3.353 (Table 4). The range of the Ht values lies between 0.016 and 32.429, whereas the H10 values are placed between 0.011 – 8.844 (Table 4).
Table 4. Textural heterogeneity of the studied anthracite
Sample Ham Hev Hst Ht H10 Ham= CG3300 0.922 0.39 1.936 3.481 1.393 Coeff icient CG3301 1.797 0.04 3.353 32.429 8.844 of CG3302 0.859 0.052 0.124 0.016 0.011 relati CG3303 0.918 0.218 0.487 0.293 0.195 ve CG3304 1.394 0.81 2.26 25.481 7.644 varia bility CG3305 1.043 0.55 1.59 7.339 2.752 Ram; CG3306 1.055 0.22 1.89 1.794 0.896 Hev= Coefficient of relative variability Rev; Hst= Coefficient of relative variability Rst; Ht= Heterogeneity coefficient of a sample; H10= Heterogeneity coefficient of a sample (for textural classes having >10% of total reflectance measurements)
4.5. Deformational features associated with the macerals: Three main microlithotypes have been recognised; vitrite (> 95% vitrinite), inertite (>95% inertinite), and bimacerite (combination of vitrinite and inertinite >95%). Given the fact that the samples presented a high degree of deformation, three deformation categories, i.e., “deformed”, “sheared” and “smashed”, (Fig. 4 a-c) are recognised and quantified together with the microlithotype composition as presented in Table 5. Furthermore, the given the amount of mineral matter determined during the maceral analysis (Table 2), carbominerite (mixture of maceral and minerals matter, generally with values higher than 20% of mineral matter except pyrite, Taylor et al., 1998) and minerite (>95% mineral matter) contents are also determined together with their deformational characteristics (Table 5).
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Fig. 4. Optical photomicrographs of the different deformational categories: (a) Deformed, (b) sheared and (c) smashed grains of the studied samples ACCEPTED MANUSCRIPT
Vt Vt Vt It It It Bm Bm Bm CM CM CM Mt Mt Mt Sample (sh) (sm) (de) (sh) (sm) (de) (sh) (sm) (de) (sh) (sm) (de) (sh) (sm) (de)
CG 3300 9.6 2.8 3.2 37.2 7.2 6.4 0 0 0.8 4.4 28.4 0 0 0 0
CG3301 6 7.2 1.6 18.4 8.8 0 0 0 0 18.8 22 0 4.4 12.8 0
CG3302 12 6.8 6.8 20.8 13.2 2 0.4 0 0 24.4 12.4 0 0 1.2 0
CG3303 1.6 2.8 0.4 5.6 12.8 0 0 0 0 17.6 38.4 0 7.6 8.8 4.4
CG3304 5.2 2.4 0 48.4 12 0.4 1.6 4 0 6.8 12 0 2.8 0.8 3.6
CG3305 12 3.2 0 47.2 2.4 1.2 16 7.2 0 3.6 5.6 0 1.2 0 0.4
CG3306 14.4 3.6 4.4 58 2.4 1.6 4 2 0 5.2 4 0 0 0 0.4 Table 5: Microlithotype composition of the studied anthracite samples Explanations: All the components are in vol.%. Vt = vitrite; It = inertite; Bm = bimacerite; CM = carbominerite; Mt = minerite sh = sheared; sm = smashed; de = deformed
4.6. Deconvoluted Raman spectra Four bands are obtained after the deconvolution of the first order Raman spectra in each -1 -1 sample. These bands are the D1 band at ~1360 cm , D3 band at ~1500 cm , D4 band at ~1250 cm-1 and finally G band at ~1600 cm-1. As all the samples exhibit similar type of deconvoluted spectra, the spectral deconvolution of the sample CG3301 is presented in Figure 5 as the representative. The position of D1, D3, D4 and G bands, Full Width at Half Maxima (FWHM) of D1 and G bands, peak height ratio of D1 to G band (ID1/IG) as well as peak area ratio of D1 band to G band (AD1/AG) are presented in Table 6.
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Fig. 5. Curved-fitted Raman Spectra of sample CG3301 showing D1, D3, D4 and G bands
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Table 6: Raman spectral parameters of the studied anthracite samples
Position of D1 Position of D3 Position of D4 Position of G FWHM of D1 FWHM of G Sample ID1/IG AD1/AG band (cm-1) band (cm-1) band (cm-1) band (cm-1) band (cm-1) band (cm-1)
CG3300 1357.93 1523.49 1257.44 1604.79 64.79 45.70 1.53 2.45
CG3301 1356.99 1490.79 1274.09 1608.55 92.46 51.49 1.29 2.27
CG3302 1349.88 1482.17 1253.07 1597.12 91.75 72.04 1.23 1.42
CG3303 1360.27 1487.41 1259.56 1604.91 79.71 64.21 1.98 2.21
CG3304 1359.75 1488.26 1260.53 1608.98 104.39 54.62 1.69 2.39
CG3305 1363.42 1492.33 1293.04 1603.41 76.95 66.99 1.20 1.31
CG3306 1359.83 1486.91 1265.39 1608.36 85.47 55.18 1.58 2.02
Explanations: FWHM = Full Width of Half Maxima; ID1/IG = Intensity ratio of the D1 and G bands; ID1/AG = Peak height ratio of D1 to G band; AD1/AG= Peak area ratio of D1 to G band
5. Discussion 5.1. Micropetrography
The authors were aware that for the coal samples with R̅ r > 4%, the petrographic study could be difficult to carry out. The reflectance of the vitrinite and the inertinite grains merges at this high rank (Taylor et al., 1998) and hence, only the relict botanical structures, if present and prominent enough to be observed under the microscope can differentiate the macerals of these two groups. The collotelinite grains appeared as structureless and homogeneous organic components (Fig. 3 a, b). Comparatively, the semifusinite grains of inertinite group depicted partially visible and not well-developed cell cavities of varying shape and size (Fig. 3 c, d). The samples consisted of very few fusinite grains exhibiting preserved cellular structures. These observations are in agreement with the ICCP System 1994 for ‘vitrinite’ and ‘inertinite’ classifications (ICCP 1998, 2001).
The macerals mostly had exhibited a sheared texture, and often they were just smashed. The sheared and smashed grains indicate that the intense tectonic shearing had affected the coals. A large amount of mineral matter in the samples was observed to cement the smashed macerals and oftenACCEPTED the bigger grains as well. MANUSCRIPT Furthermore, within the cemented association of maceral and minerals, the maceral grains were found to be randomly oriented, that is, not all the particles exhibited similar optical anisotropy during the rotation of the polarise filter (Fig. 6). Hence, it can be considered that the epigenetic minerals acted as the cementing agent in binding the randomly oriented maceral grains in the samples. These mineral cements, mostly, comprised the clay minerals and often, quartz. Syngenetic minerals like quartz, feldspar, mica etc. were also present but the concentration of the epigenetic cementing minerals was, perhaps, higher than that of the former one. The occurrence of both syngenetic and epigenetic minerals in the Rangit valley coal samples was also reported by Ghosh (1997). ACCEPTED MANUSCRIPT
Fig. 6. Variable optical anisotropy of the collotelinite grains cemented by the epigenetic minerals in CG3305 under polarised light (polarise filter rotated to 90° from each image)
5.2. RIS characterization and rank of the coal samples
The studied RMAX parameter (Table 3) along with the range of the R̅ r values (Table 3) place the samples in the anthracite A rank (ISO, 2005). Further, according to ISO, 2005, the
RMAX value of 8 % may imply the stage that is beyond the rank where the R̅ v, min parameter declines rapidly from its peak value of 3.5 % at R̅ v, max of 6 % and indicates the commencement of the graphitization. The very similar observation is obtained from this study, where the acceleration of RMAX is in the strong accordance with the sharp decline of RMIN parameter (Fig. 7).
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Fig. 7. Enhancement of Bireflectance (Bw) in the anthracite samples with acceleration of RMAX and rapid decline of RMIN parameters
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The RINT values were close to RMAX values and RMAX > RINT >> RMIN (Table 3), which indicates that the studied anthracite samples have biaxial negative optical character (Duber et al., 2000; Levine and Davis, 1989). Such optical texture of the coals can be developed when the coal bearing stratum is subjected to intense tectonic activity (such as oriented pressures and shears) and deformation (Cook et al., 1972; Duber et al., 2000; Grieve, 1991; Kilby, 1988; Langenberg and Kalkreuth, 1991; Levine and Davis, 1989; Ting, 1981).
The Rangit duplex structure as proposed by Bhattacharyya and Mitra (2009) had evolved in a combination with footwall imbrication of thrusts and reactivation of those imbricates. The Rangit duplex started to evolve as a foreland-dipping duplex consisting of three horses (the Kitam, the Sorok, and the Jorethang horses) in the northernmost part of the Gondwana Basin after the emplacement of the Daling lithologies over the northern edge of this Basin (Bhattacharyya and Mitra, 2009). After that, the hinterland dipping Tatapani horse was emplaced at the next stage of footwall imbrication with concurrent reactivation of Ramgarh thrust and translation of the foreland-dipping horses farther southward (Bhattacharyya and Mitra, 2009). This Tatapani horse along with the Sikkip and the Dong horses formed the hinterland dipping component of the Rangit duplex and during the evolution of these hinterland-dipping horses, the Ramgarh Thrust was reactivated continuously to generate the deformed state geometry of the Rangit duplex (Bhattacharyya and Mitra, 2009). Finally, the Namchi horse along with other two underlying horses was emplaced to form the antiformal stack, the final stage of the proposed kinematic model of the Rangit duplex exposed in the Rangit window (Bhattacharyya and Mitra, 2009). Hence, these complex Himalayan thrust kinetics had, intensely, affected the studied anthracite samples.
The temperature has been considered as the triggering factor of coalification, which influences the coalification stage gradually whereas, the frictional heat from shearing events in tectonic regime may enhance the coalification rate (Daiyong et al., 2009; Stach et al., 1982; Teichmüler and Teichmüler, 1966) and also affect the optical and micro-chemical structures of the coals. Suchy et al. (1997) as well as Bruns and Littke (2015) had suggested that the RMAX parameter is very sensitive to both temperature and stress at relatively high maturity level. The increasing temperature would control the degree of aromatization with increasing thermal maturity, which in turn may influence the reflectance parameters through controlling the refraction and absorption of light (Bruns and Littke, 2015). The tectonic stress, on the other hand, modifies the structure and texture of the coals through affecting the arrangements of ACCEPTED BSUs, which in turn influences MANUSCRIPT the optical property of the vitrinite grains (Bruns and Littke, 2015). In the present study, the immense tectonic stress and shearing heat produced due to complex structural evolution of the Himalayas would have significantly affected the reflectance parameters as evidenced from the enhanced RMAX values. The maceral grains had suffered intense tectonic differential stress that had been upgraded with the enhancement of tectonic agents (Wu et al., 2012) and hence, the vitrinite grains had exhibited biaxial negative optical texture, typically found in the orogenic regime.
Among the RIS parameters, the range of Rst values (Table 3) would again point toward the biaxial negative optical texture of the vitrinite grains. Further, this parameter shows ACCEPTED MANUSCRIPT
strong positive linear correlation (r = 0.98; Fig. 8) with an index computed (equation (ii)) from the principal axes of the RIS and is termed as optical sign index (OSI). The OSI declines with the shift of the RINT values towards the RMAX, i.e., with increase in the biaxial negativity of the vitrinite grains. However, the value of OSI becomes zero in the case of uniaxial negative (RMAX=RINT>RMIN) vitrinite, while becomes infinite for uniaxial positive (RMAX>RINT=RMIN) grains. This index, thus, calculates finite values for the biaxial positive or negative vitrinite grains. As this index has no definite range or sharp boundaries of values for determining the biaxial optical sign (whether it is positive or negative), it requires correlation with additional parameters to confirm that. Hence, in the present study, the strong agreement of the OSI with the Rst parameter, further, confirms the biaxial negative optical character of the vitrinite grains in the studied anthracite samples.
푂푆퐼 = (푅푀퐴푋 − 푅퐼푁푇 )/(푅퐼푁푇 − 푅푀퐼푁 )…………………..(ii)
Fig. 8. Relation between Optical Sign Index (OSI) and RIS-style (Rst) parameter of the anthracite samples Explanation: OSI = (RMAX − RINT )/(RINT − RMIN )
Moreover, the Ram (characterizes the RIS anisotropy) values varying from 0.128 - 0.260 imply high magnitudeACCEPTED of anisotropy in the MANUSCRIPT samples. This parameter is correlated with the study of Wu et al. (2012) regarding RIS characterization in tectonically deformed terrain at three different regions of Huaibei coalfield in the south-east area of North China craton. The study area was reported to be deformed by strike-slip faulting (Zhu et al., 2005; Yin and Nie, 1993) with an arcuate thrust system (Guiliang et al., 1998). A synclinal fold associated with the thrust system, had been considered to affect the burial depth and rank of the coal (Liu et al., 2009). According to Wu et al. (2012), among the three regions, the samples from third one (Region ‘C’) were strongly influenced by the tectonic deformation and thermal shearing. They had reported that the magnitude of anisotropy (Ram) was higher (0.04 ~ 0.08) in the ACCEPTED MANUSCRIPT
Region C (third region) than that of the Region A (first region), which may substantiate the geo-tectonic setting of that particular region. Although having different geo-tectonic origin, the anthracite samples in the present study exhibits higher range of Ram values than reported in the work of Wu et al. (2012), which may, further, substantiate the influence of the
Himalayan tectonism on the optical characters of the studied anthracite samples. Rev results from the ordering of chemical structure due to heating (Duber et al., 2000). Indeed, it did not show good correlation with RMAX, RINT, and RMIN, which may imply that this parameter does not depend on the optical properties.
5.3. Optical anisotropy of the macerals
The collotelinite grains exhibited homogeneous anisotropy under the microscope during the rotation of the polarizer (Fig. 3 a-b). The semifusinite had also shown this anisotropy but these grains were not at all homogeneously anisotropic rather they exhibited wavy or net-like anisotropy. The optical anisotropy of the vitrinite grains (Fig. 9) is usually represented by bireflectance values (Bw), which are presented in Table 3. It was observed in Figure 6 that the enhanced RMAX with strong decline in RMIN values had boosted up the Bw parameter. Hence, the strong inverse correlation between Bw and the RMIN parameters may be considered as the beginning phase of pre-graphitization (Stach et al., 1982). This stage also corresponds to the influence of stress (Bustin et al., 1995; Han et al., 2017; Wilks et al., 1993) on the studied anthracite samples. This observation is, further, supported by the study carried out by Reinhardt (1991) who suggested that the high amplitude of tectonic shear stress coupled with heating phase would enhance the parallel alignment of the aromatic lamellae inducing high bireflectance of the vitrinite grains. Taylor et al. (1998) opined that the shearing plays effectively in promoting the maximum reflectance and anisotropy of the vitrinite grains, which, additionally, substantiate the enhanced bireflectance values observed in this work.
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Fig. 9: Optical photomicrographs in polarised light (polarise filter rotated to 90° from each image) showing reflectance anisotropy of vitrinite (collotelinite) in sample CG3305
In addition, the Bw parameter correlates linearly (r = 0.94) with the Ram values (Fig. 10), which may indicate that the magnitude of anisotropy was enhanced with the acceleration of the bireflectance. As the tectonic stress had been considered to have a major influence on the ACCEPTED MANUSCRIPT
degree of bireflectance (Reinhardt, 1991), the strong correlation between the Bw and Ram parameters put an additional evidence on the optical transformations of the maceral grains in response to the deformation events of the Himalaya.
Fig. 10. Relation between bireflectance (Bw) and magnitude of anisotropy (Ram) parameters of the anthracite samples
5.4. Textural heterogeneity The RIS parameters exhibited variance among the samples mainly because of the variation in the number of textural classes in each sample and these variations in textural classes are usually controlled by the heterogeneity of the anthracite texture. Therefore, the higher the degree of heterogeneity, the higher is the variation of RIS parameters. These heterogeneity parameters had been calculated following Duber et al. (2000) as discussed in the methodology section. In the present study, the Hst, the Hev and the Ham parameters have values greater than zero (Table 4), rather they exhibited a wide variation in all the samples, which implies the presence of large textural heterogeneity. Additionally, the Ht values had been found to be always greater than corresponding H10 values (Table 4), which may imply a higher degree of heterogeneity in the anthracite micro-texture (BSUs) caused by the tectonic impacts of the Himalayan orogeny. ACCEPTED MANUSCRIPT 5.5. Vitrinite reflectance palaeothermometry The vitrinite reflectance palaeothermometry can be, efficiently, used to determine the peak palaeotemperature in orogenic tectonic terrains with complex burial history (Barker and Pawlewicz, 1994). As, the studied samples were collected from the Himalayan orogenic regime, the peak temperature calculated from the mean random vitrinite reflectance values would indicate about the maximum temperature that the coal particles experienced during the course of coalification. From that aspect, the anthracite samples, in the present study, have been interpreted to reach the peak temperature (Tpeak) range of 336.39 – 369.01 ℃ (Table 3), which is a bit higher than postulated (<300 ℃) by Ghosh (1997) for most of the Rangit valley ACCEPTED MANUSCRIPT
coal samples. Also, from the mosaic and ribbon structures within the coal samples, he suggested that the peak temperature might have raised about 500 ℃. However, the Tpeak values calculated for the studied samples may substantiate his work that the coal of this area was thermally metamorphosed under tectonic stress generated from thrust kinetics of the Himalayan FTBs.
5.6. Deformational characteristics of the macerals
The “deformed” grains (Fig. 4 a) were very rarely observed in the samples. However, the “sheared” vitrites and inertites were the most dominant species, observed under the microscope (Table 5). These may indicate a high amount of shearing stress had influenced the samples significantly, which is in good agreement with the observations from the RIS characteristics. Within the anthracite, the parallel alignment of the aromatic lamellae to the bedding plane is disrupted under shear stress through rotation of the bonds of the chemical compounds (Han et al., 2017), perhaps, by Stone-Wales defect or through the generation of edge and screw dislocations (explained in ‘5.7. Raman spectroscopy’). The chemical bonds, often get chances to adapt themselves to the tectonic shear stress and the organic components exhibit plastic deformation without any structural failure (Han et al., 2017). However, if the amplitude of shear stress is, extremely, high and the chemical bonds do not get the enough chance for adapting themselves to that excessive stress level, bonds are broken and the organic components suffer the structural failure or breakage. The Himalayan tectonic movements would have exerted a high amplitude of shear stress and, consequently, induced large amount of structural defects within the microstructure of the anthracite. In extreme cases, the chemical structure of the maceral grains has been assumed not to get enough chance for adaptation to that high stress level and suffered the structural failure. The “smashed” grains within the studied anthracite samples may be the consequence of this structural failure.
An increasing trend of the concentration of the “sheared” inertites, and vitrites had been observed in the samples collected from the Jorethang and Namchi horses towards the Sikkip horse (Fig. 11 a, b). Here, the proximity of the samples to the respective thrusts might have played a role. The anthracite samples collected from the Sikkip horse, would have proximal exposures to the Sikkip thrust in comparison to these of Jorethang and Namchi horses from their respective thrusts (Jorethang thrust, unexposed Namchi thrust) and this had resulted in more deformationACCEPTED of the organic components MANUSCRIPT in the samples collected from Sikkip horse. Hence, the samples of the hinterland dipping horse exhibited more “sheared” grains than that of foreland-dipping complexes and antiformal stack.
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Fig. 11: Distribution of (a) inertites, and (b) vitrites in the studied anthracite across the samples
5.7. Raman spectral characteristics
-1 The peak at 1360 cm (Fig. 5), known as the ‘D1’ band (‘defect band’) was interpreted to appear due to the in-plane defects like heteroatoms (O, H, N etc.) or structural defects (Beny-Bassez and Rouzaud, 1985; Beyssac et al., 2002a, b) and had been attributed to A1g vibration mode related to those structural defects (Ferrari and Robertson, 2000). The G band at -1 4 1600 cm (Fig. 5) corresponds to E2g2 vibration mode of graphite with D6h crystal symmetry (Beyssac et al., 2002a, b) and arises from the aromatic ring quadrant breathing (in-phase stretching vibrations, where all the bond lengths change by similar amount in-phase and the atoms move radially (Colthup, 1990)) of sp2 hybridized carbon atoms (Sadezky et al., 2005; -1 Xueqiu et al., 2017). The D3 band at 1500 cm (Fig. 5) represents the out-of-plane defects released early in the graphitization process (Bény-Bassez and Rouzaud, 1985; Beyssac et al., 2002a, b) and also may arise due to the small crystallite size (Beyssac et al., 2002a; -1 3 Nemanich and Solin, 1979). The D4 band at 1250 cm (Fig. 5) corresponds to sp or admixed sp3-sp2 hybridized carbon atoms (Chabalala et al., 2017; Han et al., 2017). The sp3 hybridized carbons correspond to aliphatic fraction whereas the polyaromatic rings contain sp2 hybridized carbon atoms (Han et al., 2017).
The intensity ratio of D1 and G bands (ID1/IG, Table 6) corresponds to in-plane crystallite size (Tunistra and Koenig, 1970) and has been used extensively to detect the degree of structural defects in coals (Bustin et al., 1995; Han et al., 2017; Schito et al., 2017). The area ratio ofACCEPTED the D1 and G bands (AD MANUSCRIPT1/AG, Table 6) is used in the present study for determining the degree of structural imperfections in place of ID1/IG ratio as the AD1/AG ratio is in good correlation with the intensity ratio. Also, Han et al. (2017) and Xueqiu et al. (2017) amongst many others had used the AD1/AG ratio to determine the degree of crystallinity of the coal samples. The in-plane structural defects within the aromatic lamellae in the studied anthracite might have played a significant role for the relatively high AD1/AG ratio (Table 6). The anthracite had been observed to exhibit four types of structural dislocations (Han et al., 2017; Sun et al., 2011). Among them edge and screw dislocations had been found to be the main structural defects (Sun et al., 2011). The edge dislocation is characterised by distortion of planes caused by the introduction of an extra half-plane of ACCEPTED MANUSCRIPT
atoms at the midway through the crystal whereas in the screw dislocations, one continuous spiralling plane is formed after rendering the all the planes and this spiralling plane cannot be separated without breaking the covalent bonds between the sp2 hybridized carbon atoms (Sun et al., 2011). Additionally, the Stone -Wales (SW) defect induced by 90° in-plane rotation of the C–C bonds, leads to the plastic deformation of the graphene and nanotube (Huang et al., 2006; Nardelli et al., 1998; Tang et al., 2008). As the structural defects in the anthracite are quite similar to those of graphite, the SW dislocations would also play significant role in deformation of the anthracite (Han et al., 2017). These structural dislocations would have broadened the D1 band (Fig. 12) and influenced the AD1/AG values of the studied anthracite samples. Han et al. (2017) had also studied the microstructural properties of anthracite samples from two collieries and found the range of AD1/AG values from 1.43 – 1.81 in Sihe colliery and 2.00 – 2.18 in Wangtaipu colliery Those anthracite samples were reported to be affected by the normal and thrust faults as well as anticlinal folds, which would have enhanced the AD1/AG ratio (Han et al., 2017). The AD1/AG parameter (Table 6) of the anthracite samples from the Himalayan FTBs have been observed to possess almost similar values as reported by Han et al. (2017) for their samples from Sihe and Wangtaipu collieries. Hence, the microstructural characteristics of the studied anthracite samples can be interpreted to be affected, significantly, by the Himalayan fold-thrust kinetics. Furthermore, the introduction of structural defects would have weakened the stability of the chemical bonds from adapting to the external high stress level induced by the Himalayan tectonism. This might have resulted in ‘deformed’ and ‘sheared’ grains observed in the samples and when the chemical structures are assumed to fail to adapt, completely, the grains suffered the breakage as depicted by the ‘smashed’ grains.
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Fig. 12. Relation between the D1 band position and the FWHM of the D1 band
The G band shifts to a lower wavelength from 1600 – 1580 cm-1 with increase in pressure-temperature conditions or increase in metamorphic grade and becomes thinner (Beyssac et al., 2002a, b). However, in the present study the position of the G band shows consistency in its position (Table 6) with, relatively, large width in all the anthracite samples, ACCEPTED MANUSCRIPT
which may imply that all the samples lie in similar metamorphic grade. The width of the G band usually depends on the disorder of the bond-angle at sp2 hybridized sites in aromatic rings and it increases with the increase in the structures with disordered bond-angle (Ferrari and Robertson, 2000). The cross-plot of the position of the G band and its FWHM values (Fig. 13) shows broader G band in all the samples and hence, relatively large amount of structural disorder caused by the tectonic deformation events. The wide G band in the studied anthracite samples may imply the presence of aromatic structures of disordered bond angle owing to tectonic deformation and plausible high amount of turbostratic aromatic layers owing to the low degree of graphitization, which in turn may support the discussion regarding the onset of pregraphtization of the samples from the optical investigations.
Fig. 13. Relation between the G band position and the FWHM of the G band
The other two defect bands, i.e., D3 and D4 may also contribute in their own ways in depicting the effect of tectonic deformation. The small size of the crystallites exhibited by the presence of D3 bands may suggest the poor microstructural ordering, perhaps, as the consequences of the deformation events. Moreover, the peak intensity of the D4 band is the lowest among all the bands. As discussed above, this band corresponds both to the aliphatic and aromatic fractions in the coal. With increasing coalification stage, the aromatic fractions generally increaseACCEPTED and polycondensation takes MANUSCRIPT place resulting in larger aromatic layers along with the removal of aliphatic counterparts (Han et al., 2017). Stress degradation affects the large chemical molecules and hence, the chemical compounds of lower dissociation energy like side-chains, functional groups along with the aliphatic compounds (aliphatic C–C, C–H etc.) are degraded into molecules with lower molecular weight and would have escaped as hydrocarbon (Daiyong et al., 2009). The preferential removal of aliphatic compounds and functional groups would lead to reduction of the concentration of sp3 hybridized carbon atoms with respect to the sp2 hybridized carbon atoms in polyaromatic rings. In the present study, the lowest peak height of the D4 band may, therefore, suggest the effect of stress degradation owing to the Himalayan tectonism. ACCEPTED MANUSCRIPT
Moreover, all the samples, in the present study, exhibited higher intensity of the D1 band comparative to the G band (as evident from the representative sample CG 3301; Fig. 5), which is according to Lahfid et al. (2010), a spectral characteristic of the samples that had attained metamorphic temperature above 300 ℃ and may represent anchimetamorphism (Schito et al., 2017). Moreover, Beyssac et al. (2002a) proposed that the Raman spectra of the carbonaceous material can be used to document the maximum temperature reached during regional metamorphism. The R2 parameter (calculated by dividing the area of D1 band by the integrated area of the D1, D2 and G bands; AD1/(AD1+AD2+AG)), which quantifies the degree of organization within the carbonaceous materials can be used as a reliable indicator for providing information about the pressure or temperature of metamorphic zones (Beyssac et al., 2002a) and to track the graphitization pathway (Beyssac et al., 2002a, b). However, the Raman spectra of the studied anthracite samples lack D2 band and considering that the contributions from this band, if present, were low, the R2 parameter described by Beyssac et al. (2002a) is recalculated as Relative Area Ratio (RAR) from the equation (iii) (although the underlying ratio was calculated by Wopenka and Pasteris, 1993 but they had multiplied the ratio with 100) and the values are presented in Table 7.
푅퐴푅 = 퐴퐷1/(퐴퐷1 + 퐴퐺)…………………….(iii)
Then, following the linear relation given by Beyssac et al. (2002a) the metamorphic temperature (Tm) has been calculated from equation (iv).
푇푚 (℃) = −445 × 푅퐴푅 + 641……….(iv)
Table 7. Relative Area Ratio (RAR) and metamorphic temperature (Tm℃) of the studied anthracite samples
Sample RAR Tm (℃) CG3300 0.71 325.02 CG3301 0.69 331.97 CG3302 0.59 380.19 CG3303 0.69 334.61 CG3304 0.71 327.14 CG3305 0.57 387.89 CG3306 0.67 343.29 ACCEPTED MANUSCRIPT Although the values of the Tm (Table 7) calculated from the above equation (equation iv) are a bit different from the Tpeak values calculated from the R̅ r parameter (Table 3), both of them report that the peak metamorphic temperature attained by the anthracite samples is above 300 ℃. Moreover, the maximum value in the range of the Tm (Table 7) is quite near to 400 ℃, the temperature close to greenschist facies metamorphism (Fig. 16.6, pp. 420; Philpotts and Ague, 2009). As discussed in the geological setting of the study area, Bhattacharyya and Mitra (2011) reported that the Teesta half-window exposes the metapelitic greenschist-facies rocks of the Lesser Himalayan Sequence. This Sequence consisting of Daling, Buxa and Gondwana rocks is repeated a number of times along the horses constituting the Lesser ACCEPTED MANUSCRIPT
Himalayan Duplex (Bhattacharyya and Mitra, 2009) and the Rangit window exposes a part of this repeated Lesser Himalayan Sequence (Bhattacharyya and Mitra, 2009; Ghosh, 1956; Raina, 1976; Gangopadhyay and Ray, 1980) as Rangit duplex (Bhattacharyya et al., 2009, 2006). Thus, according to the geotectonic setting, the Gondwana lithology in the Rangit duplex is expected to have undergone greenschist facies metamorphism. Although the present study lacks the pressure data for substantiating the P-T conditions, the metamorphic temperature (Tm) would suggest that the coal samples had experienced metamorphism close to the greenschist facies. The ID1/IG (Table 6) or the R1ratio termed by Beyssac et al. (2002a, b) correlates well with the RAR and both of their values are consistent in all the studied samples. The lowering of these two ratios takes place with increase in metamorphic grade, although, this relation is debatable for the ID1/IG ratio. However, the consistent values of both of these parameters may indicate that all the studied anthracite samples belong to similar metamorphic grade.
6. Conclusion The optical and Raman spectroscopic investigations, hence, reveal the response of the coal microstructural characteristics to the complex thrust kinetics of Rangit duplex owing to the Himalayan tectonism along with the depiction of peak metamorphic temperature that the coal particles had attained. The semifusinite grains were identified by their relict botanical structure unlike the structureless collotelinite and all these grains were found to be present with characteristic anisotropy. The range of the RMAX and the R̅ r values suggests anthracite A rank of the studied coal samples following ISO, 2005. The interrelation between the Reflectance Indicating Surface (RIS) main axes, i.e., RMAX > RINT >> RMIN, indicates the biaxial negative optical property of the vitrinite grains. This is, further confirmed by the strong positive correlation between the Optical Sign Index (OSI) and RIS-style (Rst) parameters. This biaxial negative vitrinite maceral is the feature of the coals affected by folds and faults under high differential stress produced by intense tectonic activity. The acceleration of RMAX with sharp decrease in RMIN values had enhanced the bireflectance (Bw) values and may point towards the onset of pregraphitization phase. The magnitude of anisotropy of the studied anthracite samples is well expressed in the high values of Ram. This parameter is also positively correlated with the Bw suggesting, additionally, the strong influence of Himalayan tectonism on the optical properties of the anthracite samples. The higher values of heterogeneity coefficient (Ht) of the textural classes relative to heterogeneity coefficient calculated for texturalACCEPTED classes containing greater MANUSCRIPT than 10% of total reflectance values (H10) may be indicative of a higher magnitude of heterogeneity existing in the micro-texture of the samples. Furthermore, the microlithotypes identified taking the deformational aspects of the macerals within concern exhibit three different deformational characteristics, which may portrait the influence of tectonic shear stress on the morphology of the coal components. The higher concentration of the “sheared” inertite and vitrite grains in the samples collected from hinterland-dipping horses may be attributed to the proximity of those coal exposures to the respective thrust. Additionally, the high values of the area ratio of D1 to G band (AD1/AG) may imply the presence of structural imperfections induced by the intense tectonic ACCEPTED MANUSCRIPT
deformation events. The broad G band may point towards the aromatic structures with disordered bond angles due to large degree of deformation. The tectonic stress degraded the sp3 hybridized weak aliphatic compounds through preferential removal of those species resulting in extremely low intensity of D4 band in the first order Raman spectra. In addition, the Tpeak values calculated from R̅ r may suggest that these anthracite samples had attained the maximum temperature up to 369.01 ℃ suggesting anchimetamorphism, while the Tm determined from the RAR may indicate that the samples would have reached the metamorphic temperature up to 387.89 ℃ and that is close to the greenschist facies of metamorphism. This may support the greenschist facies metamorphism of the Lesser Himalayan Sequence exposed, partly, by the Rangit window. Moreover, the persistent values of the RAR, ID1/IG and the position of G band may imply similar metamorphic grade of all the anthracite samples.
Acknowledgements The authors would like to thank Ms Nikola Van de Wetering, School of Earth and Environmental Sciences, The University of Queensland, Australia for helping in preparing the well-polished pellets. The authors are thankful to Dr Kathakali Bhattacharyya, Associate Professor, Department of Earth Sciences, Indian Institute of Science Education and Research (IISER), Kolkata, West Bengal, India for her kind suggestions regarding this study. The authors are also indebted, especially, to the learned editor Dr. Cevat Özgen Karacan and the learned reviewers for their valuable suggestions to improve the quality of the manuscript.
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Mathematical formulas for treatment of vitrinite reflectance data: Following Ting and Lo. (1978)
1 ( ) 2 ( )2 2 푅 = 푅1+푅3 + {(푅1−푅2) + 푅2−푅3 } ………….1 푚푎푥 2 2 1 ( ) 2 ( )2 2 푅′푚푖푛 = 푅1+푅3 − {(푅1−푅2) + 푅2−푅3 } …………..2 2 2
Following Kilby (1988, 1991) and Duber et al. (2000)
max RMAX is calculated as the weighted means of the R’max values of each textural classes max RMIN is calculated as the weighted means of the R’min values of each textural classes
′푖 ′푚푎푥 Textural class: 푅 푚푎푥 ≥ 푅 푚푖푛
푖 푚푎푥 (푅′ +푅′ ) 푅 = 푚푎푥 푚푖푛 …………………………...... 3 퐼푁푇 2 1 2 2 푅푎푚 = (푎 + 푏 )2………………………………...4 1 푅푒푣 = (푅푀퐴푋 × 푅퐼푁푇 × 푅푀퐼푁 )3…………………...5 푅 = 30 − arctan (푎)…………………………….6 푠푡 푏 Where,
1 푅 [ − 푀퐼푁 ] 푎 = 3 푅푀퐴푋+푅퐼푁푇+푅푀퐼푁 − 푏푡푎푛30………………..7 푐표푠30 푏 = 푅푀퐴푋 − 1/3……………………..8 (푅푀퐴푋+푅퐼푁푇+푅푀퐼푁 )
푀푎푥 푀푖푛 푅푠푡 −푅푠푡 퐻푠푡 = …………………………………..9 푅푠푡 푀푎푥 푀푖푛 푅푒푣 −푅푒푣 퐻푒푣 = ………………………………….10 푅푒푣 푀푎푥 푀푖푛 푅푎푚 −푅푎푚 퐻푎푚 = ………………………………....11 푅푎푚 퐻푡 = 푛퐻푠푡 퐻푒푣퐻푎푚ACCEPTED………………………………..12 MANUSCRIPT 퐻10 = 푛10 퐻푠푡 퐻푒푣퐻푎푚…………………………….13 n = Number of textural class n10 = Number of textural classes consisting of ≥10% counts of the total reflectance counts. Following Kilby (1988) ′ ′ 푅푟 = √푅푚푎푥 × 푅푚푖푛 ……………………………14
Following Barker and Pawlewicz (1986) ( ) 푇 = {ln 푅푟 +1.20} …………………………….15 푝푒푎푘 0.00782
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Explanations: RMAX = Maximum reflectance axis value RINT = Intermediate reflectance axis value RMIN= Minimum reflectance axis value Rmax = Maximum reflectance value Rˊmin = Apparent minimum reflectance value Ram = Magnitude of RIS anisotropy Rev = Reflectance of equivalent volume isotropic RIS Rst = RIS style Ham = Coefficient of relative variability Ram Hev = Coefficient of relative variability Rev Hst = Coefficient of relative variability Rst Ht = Heterogeneity coefficient of a sample H10 = Heterogeneity coefficient of a sample Rr = Random vitrinite reflectance Tpeak = Peak palaeotemperature
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Graphical abstract
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Highlights Anthracite A samples from Rangit window of Sikkim Himalayan fold-thrust belts. Enhanced bireflectance and onset of pregraphitization. Biaxial negative optical texture and high magnitude of anisotropy of vitrinite. High AD1/AG ratio and wide G band implying presence of micro-structural disorders. Metamorphic temperature attained up to 387.89 ℃.
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