BISAC SCI019000 Earth Sciences / General
The geochemical (REE & traces elements) characteristics of Permo-Carboniferous sandstone has been taken into account to decipher tectonic setting and provenance of the Rangit Pebble Slate Formation of Sikkim Lesser Himalaya. The chondrite normalized REE pattern with Eu negative anomaly and a bivariate plot (Th/Co-La/Sc) clearly indicates that studied sediments were likely derived from upper crust felsic source. The average elemental ratio of traces elements La/Sc (∼ 3.85), Th/Sc (∼ 2.57), Cr/Th (∼ 6.64), Th/Co (∼ 2.52), La/Co (∼ 3.74), and Eu/Eu* (∼ 0.32) also shows close affinities with Upper Continental Crust. The trivariate plot (La-Th-Sc plot, Th-Sc-Zr/10 plot, & Th-Co-Zr/10 plot) and a bivariate plot (Ti/Zr-La/Sc) plotted on the field of passive tectonic region for the Rangit Pebble Slate Formation sandstone. A binary plot between the ratio of Th/Sc-Zr/Sc and Th/U-Th reflects the enrichment of zircon and weathering trend during sedimentary recycling.
Geochemistry, provenance, tectonic setting, Sikkim Lesser Himalaya, India
The geochemical (REE & traces elements) characteristics of Permo-Carboniferous sandstone has been taken into account to decipher tectonic setting and provenance of the Rangit Pebble Slate Formation of Sikkim Lesser Himalaya. The chondrite normalized REE pattern with Eu negative anomaly and a bivariate plot (Th/Co-La/Sc) clearly indicates that studied sediments were likely derived from upper crust felsic source. The average elemental ratio of traces elements La/Sc (∼ 3.85), Th/Sc (∼ 2.57), Cr/Th (∼ 6.64), Th/Co (∼ 2.52), La/Co (∼ 3.74), and Eu/Eu* (∼ 0.32) also shows close affinities with Upper Continental Crust. The trivariate plot (La-Th-Sc plot, Th-Sc-Zr/10 plot, & Th-Co-Zr/10 plot) and a bivariate plot (Ti/Zr-La/Sc) plotted on the field of passive tectonic region for the Rangit Pebble Slate Formation sandstone. A binary plot between the ratio of Th/Sc-Zr/Sc and Th/U-Th reflects the enrichment of zircon and weathering trend during sedimentary recycling.
Introduction The Pre-Pleistocene glacial record of Earth as a whole was reviewed by Hambrey and Harland [2011]. In this classical review they have cov- ered almost all the glacial deposits of North and South America, Africa, Antarctica, Asia and Aus- tralia in East and West Gondwana land [Tewari and Sial, 2007]. Pre-Mesozoic global record of cli- mate change of the earth was reviewed by Histon et al. [2013]. The Neoproterozoic global glaciations (Snow Ball Earth) has been recorded from west- ern Lesser Himalaya of India [Tewari, 2001, 2010, 2012]. These are the main cryospheric events in the earth’s history and well preserved in Indian subcon- tinent. The evidence of Late Paleozoic Gondwana glaciation is one of the important cryospheric event which have been documented from southern hemi- sphere. In, India it has been witnessed in both peninsular and extra-peninsular parts of Indian sub-continent. Rangit Gondwana Basin (RGB) of Sikkim Lesser Himalaya is one of the Himalayan Gondwana basin of eastern Himalaya in the Ran- git window (Figure 1). Diamictites and boulder beds at the base of the Rangit Pebble Slate Forma- tion shows the glacial environment based on field and petrography. The alternate bands of sandstone and shale overlying the glacial beds are influenced by the glacio-marine environment (Figure 2a, Fig- ure 2b). The Rangit Pebble Slate Formation of the Sikkim Lesser Himalaya belongs to Lower Gond- wana sequence and correlated with the Talchir For- mation of Gondwana master basin in Peninsular India [Priya et al., 2019]. The main objective of the present study is to understand the source area composition, tectonic setting and paleo-weathering conditions during deposition of the Rangit Pebble-Slate (RPS) Forma- Geological Setting of the Sikkim Lesser tion. We have analyzed the REE and traces elements of the Rangit Pebble Slate Formation of Himalaya Sikkim Lesser Himalaya and interpreted the data to achieve this objective. The distributions of REE pattern, elemental ratio, bivariate plot (Th/Co vs La/Sc, Ti/Zr vs La/Sc, La/Th vs Th/Yb, and Th/Sc vs Zr/Sc) and a trivariate plot (La-Th-Sc, Th-Co-Zr/10 and Th-Sc-Zr/10) of traces elements have been significantly used to decipher the geo- chemical nature of provenance and discriminate tectonic setting of the studied sediments. A bi- nary plot (Th/Sc vs Zr/Sc and Th/U vs Th) also have been used to measure the intensity of chemical weathering and sedimentary recycling. Material and Methods The systematic samples were collected from the Namchi-Sikkip area, Sikkim Lesser Himalaya stud- ied area and powdered for the geochemical analysis. All the standard procedures have been followed for 2 of 9 ES5002 priya et al.: geochemical characteristics ES5002 Figure 2. Field photograph depicting the Rangit Pebble Slate sequence in Sikkim Lesser Himalaya (a) alternate band of pebble-slate and coarser sandstone and (b) alternate band of coarser and fine sandstone. wana mainly consists of phyllite, quartzite, varie- gated slates and dolomites [GSI, 2012]. However, the dolomite is integral part of the underlying Buxa Group [Tewari, 2011]. Gondwana Group/Damuda Group of rocks (Carboniferous-Permian) is uncon- formably overlain by the stromatolitic and micro- fossiliferous cherty carbonate rocks of the Buxa Group [Priya et al., 2019; Schopf et al., 2008; Tewari, 2011]. The Gondwana group of the sed- iments is the youngest stratigraphic unit in the Sikkim Lesser Himalaya which consists of two ma- jor formations i.e. Rangit Pebble Slate (RPS) For- mation and Damuda Formation. The lower litho units mainly consists of pebble-slate, diamictites, coarser sandstone and olive-green shale. The upper Gondwana sediments (Namchi Sandstone) consti- tute the alternate bands of sandstone, fossiliferous shale and coal beds [Priya et al., 2019; Raichud- hari, 2002]. The Rangit Pebble Slate Formation is exposed along Namchi-Sikkip road in South Sikkim. The Rangit Pebble Slate Formation belongs to the lower litho units of Gondwana Group. The Ran- git Pebble Slate Formation is chiefly consists of glacial diamictite, boulder beds, pebble-slate, alter- nate band of sandstone and shale (Figure 2a, Fig- ure 2b). The Rangit Pebble Slate sequence of the Sikkim Lesser Himalaya is tectonically disturbed, highly complex folding and over thrusting can be seen along the Namchi-Jorethang, Namchi-Sikkip and Namchi-Damthang road sections [Priya et al., 2021a; 2021b]. Provenance and Tectonic Setting Trace and Rare earth elements (REE) is one of the most important geochemical tools which has been widely used to deciphering the sediment provenance, tectonic setting and depositional envi- ronment of any sedimentary geological Formation [Bhatia, 1983; Taylor and McLennan, 1985]. Due to less mobility and highly resistant to the chemi- cal weathering, some elements like Sc, Zr, Cr, Co, Th, V, and La are preserved as a geochemical sig- nature to indicate the source of parents’ materials [McLennan et al., 1983]. The dominance of Sc, V and Co traces elements is higher in mafic source rock than felsic rock [Bhatia and Crook, 1986; Tay- lor and McLennan, 1985]. The lower abundance of V, Sc and Co in all samples of RPS Fm. sandstones strongly suggests and confirms that these sand- stones were derived from felsic source rock. The elemental ratio of traces elements such as La/Sc, Th/ Sc, Cr/Th, Th/Co, and La/Co (Table 2) helps to distinguish and constrain the geochemical na- ture of the provenance [Culler, 2000; Taylor and McLennan, 1985]. The value (elemental ratios) of studied samples of RPS Fm. sandstones shown more affinities to- wards felsic rocks derivation (Table 2). The high value of Th/Co and La/Th ratio in all studied sam- ples have shown the enrichment of Th and La which significantly indicates that RPS Fm. sandstones were substantially derived from a felsic source. A bivariate plot between the ratio of Th/Co vs 3 of 9 ES5002 priya et al.: geochemical characteristics ES5002 Figure 3. Bivariate plot of Th/Co versus La/Sc showing the felsic provenance of the studied sedi- ments (after [Cullers, 2002]). La/Sc and La/Th vs Th/Yb (Figure 3, Figure 5) also indicates that all studied sediments of Rangit Pebble Slate Formation were derived from the felsic source. The total REE concentrations in the studied sample of Rangit Pebble Slate Formation sand- stone ranged from 44.1 ppm to 169.57 ppm (Ta- ble 3) and were being normalized with normaliz- ing factors of upper continental crust [Taylor and McLennan, 1985]. In contrast, the high value of LREE/HREE ratio with negative Eu anomalies ad- vocate felsic source rocks while mafic and/or ultra- mafic rocks have low LREE/HREE ratios with no or positive Eu anomalies [Taylor and McLennan, 1985]. The REE distribution pattern of studied samples compared with UCC and shown the en- richment of light REE with a negative Eu anomaly (Figure 4). The traces elements such as Sc, Co, La, Th, Ti and Zr play a significant role in deciphering the tectonic setting of clastic sedimentary rock. In the present study, triangular plots given by Bhatia and Crook [1986] have been used to discriminate the tectonic setting. The content of elements in the studied samples was converted into percentage and each axis of equilateral triangular plots equally di- vided into percentage (i.e. 0-100%). The four dis- tinctive tectonic settings i.e. Oceanic island arc (A), Continental island arc (B), Active continental margin (C) & Passive margin (D) are recognized Figure 4. Chondrite-normalized REE pattern for RPS Fm. sandstone and comprehensive compar- ison with Upper Continental Crust (UCC) value (after [Taylor and McLennan, 1985]). on the triangular plot of La-Th-Sc, Th-Co-Zr/10 and Th-Sc-Zr/10. The differentiated tectonic en- vironment (i.e. A, B, C, D) are demarcated on the basis of previous study done by Bhatia and Crook [1986] which indicate the relative dominance of traces elements in each of trivariate plots (Fig- ure 7a, Figure 7b, Figure 7c). The high value of Th/Sc ratio in the ternary plot of La-Th-Sc (Fig- ure 7a) depicted the passive margin setting. The similar passive setting of the tectonic environment is also evident from triangular plot of Th-Co-Zr/10 and Th-Sc-Zr/10 (Figure 7b, Figure 7c). A bivari- ate plot between the ratio of Ti/Zr and La/Sc also imply passive tectonic setting (Figure 6). All these plots reflect the enrichments of Th & Zr elements and depletion of Sc element in studied sediments of Rangit Pebble Slate Formation which indicate that these sediments were derived from the felsic prove- nance. The ratio of Th and U (Th/U) reflects the signature of weathering and sedimentary recycling histories due to loss of U4+ during sedimentation [Taylor and McLennan, 1985]. If the ratio value of Th/U is higher than 4.0 then, it is considered to be result of high intensity of weathering and/or sediment recycling in the provenance area [Rahman and Suzuki, 2007]. In the present study, a binary plot between the ratio of Th/U vs Th and Th/Sc vs Zr/Sc have been used to shown the weathering trend and sedimentary recycling for the RPS Fm. sandstone (Figure 8, Figure 9). The elemental ra- tio (Th/U) of RPS Fm. sandstones ranges 3.17 to 7.39 with an average of 4.8 which implies that these sediments were influenced by weathering and sedimentary recycling (Figure 8). Furthermore, a binary plot between the ratios of Th/Sc and Zr/Sc is another reliable indicator has been used to mea- sure the role of sorting and sedimentary recycling [McLennan et al., 1983]. The addition of zircon in studied sample significantly correlated with a bi- variate plot of Th/Sc vs Zr/Sc which might be the consequences of sedimentary recycling (Figure 9). Results and Discussion Figure 6. A bivariate tectonic setting discrimi- nation diagram for sandstone of RPS Fm. (after [Bhatia and Crook, 1986]). [McLennan et al., 1993]. The elemental ratios (Ta- ble 2) of trace elements have been positively used for the comparative study with felsic source, mafic source and UCC to interpret the geochemical na- ture of sediment. The dominance of Th and Zr el- ements in the studied sediments strongly indicate felsic provenance which has been depicted through a binary plot of Th/Co vs La/Sc and La/Th vs The distributions of rare earth element (REE) and traces elements of clastic sedimentary rock have been broadly applied for characterizing the provenance and tectonic setting discrimination Figure 7. A ternary plot (a, b & c) for tectonic setting showing the passive tectonic environment for Permo-Carboniferous sandstone of Rangit Peb- ble Slate Formation, Sikkim Lesser Himalaya (after [Bhatia and Crook, 1986]). 7 of 9 ES5002 priya et al.: geochemical characteristics ES5002 Figure 8. Binary plot of Th/U versus Th for the RPS Formation sandstone (after [McLennan et al., 1993]). Th/Yb (Figure 3, Figure 5). The REE distribution pattern of studied samples shows the enrichment of light REE with a negative Eu anomaly which im- plied the felsic rich source derivation (Figure 4). The triangular plot of La-Th-Sc, Th-Co-Zr/10 and Th-Sc-Zr/10 (Figure 7a, Figure 7b, Figure 7c) in- dicate the passive margin tectonic environment of deposition. A typical weathering trend depicted by the bivariate plot of Th/U vs Th/U and Th/Sc vs Zr/Sc strongly supports the role of weathering and sedimentary recycling during the sedimentation of RPS Formation (Figure 8, Figure 9). Conclusion The Permo-Carboniferous sandstone of the Ran- git Pebble Slate Formation of Sikkim Lesser Hi- malaya has been studied to decipher its geochem- ical characteristics through REE and trace ele- ments. The discrimination diagram for provenance and tectonic setting, elucidate that these sediments were derived from felsic rich provenance (continen- tal) and were deposited in a passive continental tec- tonic setting. The combined results of both REE and trace elements indicate the felsic source which may have been derived from high grade metamor- phic rocks and plutonic igneous rocks of Peninsu- lar Craton i.e. Chotanagpur Granite Gneiss Com- Figure 9. Binary plot of Th/Sc versus Zr/Sc for the RPS Fm. sandstones (after [McLennan et al., 1993]). Different geometry symbol indicate the average composition of basalt (BAS), andesite (AND), felsic (FEL), granite (GRA) and upper continental crust (UCC). plex (CGGC), Shillong-Meghalaya Gneissic Com- plex (SMGC) and Proterzoic Himalayan granites and gneisses rock . The higher LREE pattern with negative Eu anomaly and elemental ratios of trace elements of Rangit Pebble Slate Formation sand- stone strongly suggest that these sediments are en- riched with felsic source rock and closely associ- ated with upper continental crust (UCC). A typi- cal weathering trend depicted by Th/Sc vs Zr/Sc and Th/U vs Th plot suggested that these sedi- ments were strongly influenced by weathering and sedimentary recycling during sedimentation. Acknowledgment. The authors are thankful to the Head of Department, Sikkim University for his constant support and providing infrastructural facilities. Direc- tor, Birbal Sahni Institute of Palaeoscience, Lucknow, India is thanked for providing the facilities of the geo- chemical laboratory of the Institute. We are grateful to Dr. N. Wanjari for discussions and suggestions. Anony- mous reviewer is thanked for constructive comments and valuable suggestions to improve themanuscript. R.K.P. thanks Sikkim University for award of research fellow- ship to carry out this research.
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