Introduction

In the Spiti Basin, Batal Formation (Neoproterozoic) lies over the crystallines of Vaikrita Group in the Spiti Basin, and is primarily characterized by the rocks such as shale, siltstone, and sandstone. The clastic rocks of the Batal Formation are well exposed along the Batal-Chandratal road in this basin (Fig. 1). The geochemistry of clastic sediments has been used widely by various authors to infer the provenance, type of source rocks, and intensity of weathering (McLennan and Taylor 1991; Cullers 1994; Nagarajan et al. 2007a, b; Madhavaraju and Lee 2010; Armstrong-Altrin et al. 2013, 2015a, b, 2017, 2018; Madhavaraju 2015; Madhavaraju et al. 2016; Pandey and Parcha 2017). Siliciclastic sediment composition is mainly controlled by the source rock composition due to this major and trace element geochemistry of sediments that provide information about its origin as well as the weathering conditions in the source area (Ahmad et al. 2014; Armstrong-Altrin et al. 2015a). The main objectives of the present study are (1) to infer the provenance and tectonic setting and (2) to infer the paleoweathering and depositional condition. To achieve the objectives, major, trace, and rare earth element geochemistry of the Batal Formation was carried out.

Fig. 1
figure 1

Geological map and lithostratigraphic column of the Batal section exposed in the Batal-Chandratal section of the Spiti Basin

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Study area

Spiti Basin lies in the Lahaul-Spiti district of Himachal Pradesh, northern India. The Spiti Basin forms a part of the Spiti-Zanskar Basin that belongs to the geotectonic zone of the Tethyan Himalaya, and together, they make the largest basin in the Indian Tethys Himalaya. The basin is bounded by Indus-Tsangpo Suture Zone (ITSZ) in the north and metamorphic basement of the Central Crystalline zone in the south. The Paleozoic sequence in Spiti Basin is well exposed in the Parahio, Pin, Kunzum La-Takche, and Chandratal sections. The present studied section is exposed along the track route at 32° N and 77° E in between Batal and Chandratal (Fig. 1).

Stratigraphy and geological setting

Haimanta Group rests over the crystalline rocks of Vaikrita Group (Fig. 1). The term “Haimanta System” has been given by Griesbach (1891) to the sedimentary rocks above the metamorphic rocks of the Higher Himalaya (Vaikrita System). Hayden (1904) modified this term and suggested that whole of Haimanta System lies below an angular unconformity, which is no younger than late Ordovician. Srikantia (1981) adopted the term Haimanta Group instead of Haimanta System. Subsequently, Kumar et al. (1984) and Srikantia and Bhargava (1998) divided this group into Batal and Kunzum La formations. The contact between the Haimanta Group with underlying crystalline rocks of Vaikrita Group is still debatable. Thakur (1980) stated that the contact between these two units is gradational; on the contrary, Srikantia (1981) considers it to be as conformable. Dragnits (2000) and Myrow et al. (2006) considered it as gradational, whereas Bhargava (2008b) considered it as faulted. A gradual change in the lithology from the Vaikrita Group to Haimanta Group was observed during present studies; hence, we consider it to be of a gradational contact.

From the top of the Batal Formation except a record of cryptarch by Kumar et al. (1984) from the top of the Batal Formation, there is no record of fossils in this section so far. The age of Batal Formation remains a point of discussion among the workers due to lack of age diagnostic fossils. Srikantia (1981) and Srikantia and Bhargava (1998) assigned a terminal Proterozoic age to this formation on the basis of its stratigraphic position, while some workers consider it to be of a late Proterozoic age (e.g., Frank et al. 1995; Dragnits 2000; DiPietro and Pogue 2004). Bhargava (2008a) assigned Ediacaran-Early Cambrian age on the basis of overlying stratigraphy. Myrow et al. (2010) assigned Early Cambrian age based on the detrital zircon U-Pb age spectra of one sample from the Spiti Basin. The Batal Formation can be assigned Late Neoproterozoic age based on its stratigraphic position.

Material and method

From the Batal Formation, 14 representative samples of sandstone, shale, siltstone, and phyllite were collected for the petrography and geochemical studies. Seven samples of sandstone, four samples of siltstone, two samples of shale, and one sample for phyllite were taken for the analyis. Petrographical and geochemical analyses of the samples were carried out at Wadia Institute of Himalayan Geology, Dehradun. Thin sections were studied under transmitted and reflected light using a Nikon Eclipse E600 POL microscope; they were prepared by cutting the rocks into small pieces using BUEHLER Delta Petrocut GEOLOGICAL CUTTER. One side of the rock wafers was grinded to make it smooth. The smooth side of the rock chips was mounted on glass slide using araldite/canada balsam. Wavelength dispersive XRF system (Siemens SRS 3000) was used to analyze major and trace elements. Ten grams of each sample was powdered in an agate mortar which was further analyzed using pressed powder pellet. For accuracy, analytical values were compared with the GSR-1 values by Govindaraju (1994). Major oxide and trace element data for each sample were recalculated to 100% after deduction of loss on ignition. The accuracy and precision for major oxides and trace elements was ± 2–5%. The REE data was analyzed by using inductively coupled plasma mass spectrometer (ICP-MS; PerkinElmer). Analysis was done by acid digestion which involves frequent treatment with HF-HClO4 in 1:2 for 24 h in open system. REE data was normalized to chondrite values of Taylor and McLennan (1985). Eu/Eu* ratios were calculated based on formula given by Bau and Dulski (1996).

Result

Petrography

Sandstones are mostly fine- to medium-grained, poorly sorted, and contain angular to sub-angular grains of low to moderate sphericity. In sandstone, 80% of the total framework grains comprise of quartz (Fig. 2a), feldspar is generally less abundant than quartz and is ~ 10–15%, and remaining is lithic fragment. Monocrystalline quartz is present, and the edges of quartz grains are broken (Fig. 2a). Lithic fragments and heavy minerals like zircon are present (Fig. 2b). Matrix is commonly between 10 and 15%, which comprises of clay minerals, lithic and feldspathic fragments, fine-grained quartz, and mica. In siltstone, quartz is the most abundant mineral with almost 50–60%; the other minerals like feldspar, mica, and lithic fragments occur in less abundance. The unidirectional alignment of mica flakes indicates that the rocks were subjected to low-grade metamorphism (Fig. 2c–e). The siltstone consists ~ 20–30% of matrix, comprising of clay minerals, lithic and feldspathic fragments, fine-grained quartz, and micas. Fine-grained shale contain more than 50% of matrix supported by lithic fragments, mica, and poorly sorted quartz grains (Fig. 2f). Single sample of phyllite consists of feldspar, clay, and quartz with few ferruginous matter. At places, preferred orientation of micaceous minerals was also observed.

Fig. 2
figure 2

a, b Photomicrographs of sandstone with presence of Zircon. c–e Siltstone with quartz, feldspar, and mica. Mica grains show alignment in one direction. f Photomicrographs showing shale with quartz and lithic fragments. Qz quartz, M mica, F feldspar, Lt lith

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Major oxide

Major oxide data of clastic sediments collected from Batal Formation is listed in Table 1. In the studied samples, a large variation in chemical composition was observed. Sandstone contains moderate to high concentration of SiO2 (64 to 84; average 74%), whereas siltstone and shale reflects low concentration of SiO2 range from 54 to 62% with an average of 59 (Table 1); this composition indicates enrichment relative to the upper continental crust (UCC, 65.89 wt%) (Taylor and McLennan 1985). The Al2O3 content in sandstone shows a large range from 7 to 14%, whereas in the shale and siltstone, it ranges from 13 to 22% (Table 1). Elements such as Al2O3, Fe2O3, K2O, and TiO2 show a negative correlation with SiO2 (r = 0.91 for Al2O3, 0.85 for Fe2O3, 0.79 for K2O, 0.89 for TiO2, respectively), which suggests that the rocks of studied section mostly contain clay sediments (Fig. 3). Concentration of high Al2O3 in sediments suggests their association either with clay or phyllosilicates in relationship with other elements (Nagarajan et al. 2017). Whereas, TiO2 (r = 0.88) and K2O (r = 0.86) exhibit positive correlation with Al2O3 (Fig. 4) which indicates that their association with clay mineral indicates chemical weathering in the source area (Nagarajan et al. 2017). It also suggests that TiO2 is likely associated with phyllosilicates especially with illite and the enrichment of K2O may be related to granitic/gneisses source (Dabard 1990; Armstrong-Altrin 2009, 2015). In the clastic sediments, if the value of SiO2/Al2O3 ratio is > 5, it indicates sediment maturity (Armstrong-Altrin et al. 2012, 2013; Hernández-Hinojosa et al. 2018). The SiO2/Al2O3 ratios vary from 1.7 to 10.9 in the rocks of Batal Formation, hence indicating a moderate textural maturity. In the Al2O3-Fe2O3-K2O ternary plot, samples fall towards Al2O3 (Fig. 5) which indicate abundance of clay minerals in rocks (Wronkiewicz and Condie 1987).

Table 1 Major oxide concentration data of the Neoproterozoic metasedimentary rocks of Batal Formation, Spiti Basin, Tethys Himalaya
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Fig. 3
figure 3

Major and trace element vs. Si2O graph showing the distribution of samples from the Neoproterozoic succession of Batal Formation in the Spiti Basin. Sandstone (circles); siltstone (plus signs); shale (filled triangles); phyllite (empty triangles)

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Fig. 4
figure 4

Major and trace elements vs. Al2O3 graph showing the distribution of samples from the Neoproterozoic succession of Batal Formation. Symbols are as in Fig. 3

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Fig. 5
figure 5

Major element distribution of Batal Formation in Fe2O3-Al2O3-K2O compositional space by Wronkiewicz and Condie (1987). Symbols as per Fig. 3

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Trace element

Trace element data of silisiclastic sediments collected from Batal Formation is listed in Table 2. Trace elements such as Co, Sc, Gb, and Y show a negative correlation with SiO2 (for Co, r = − 0.86; Sc, r = − 0.85; Ga, r = − 0.94; for Y, r = − 0.53) (Fig. 3). Positive correlation between Co, Rb, Sc, Nb with Al2O3 (for Co, r = 0.69; Rb, r = 0.74; Sc, r = 0.91; Nb, r = 0.79) (Fig. 4). Trace element such as Rb, Ga, Sc, and Nb shows positive correlation with K2O (e.g., r = 0.99 for Rb; 0.88 for Ga; 0.81 for Sc; 0.79 for Nb, sample n = 14) suggesting that elements are primarily controlled by clay minerals (Fig. 6). The concentration of Zr/Rb ratio of the rocks shows wide variation from ~ 0.4 to 10.4 which reflects the grain size variations of the sediment, with higher values in the relatively coarse-grained units and lower values in fine-grained units.

Table 2 Trace element concentration data of the Neoproterozoic metasedimentary rocks of Batal Formation, Spiti Basin, Tethys Himalaya
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Fig. 6
figure 6

Trace elements vs. K2O graph showing the distribution of samples from the Neoproterozoic succession of Batal Formation. Symbols are as in Fig. 3

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REE

Rare earth element data of the samples collected from Batal Formation is listed in Table 3. The REEs are plotted against the chondrite-normalized REE pattern given by Taylor and McLennan (1985). The source rock signature in sedimentary rock is revealed by the pattern of REE, LREE/HREE ratio, and Eu anomaly (Cullers and Graf 1983; Fu et al. 2010; Madhavaraju et al. 2010; Armstrong-Altrin et al. 2013). It was observed that in the present study, all the samples show similar REE patterns with enriched LREE, depleted HREE, and a negative Eu anomaly (Fig. 7). The depletion of HREE, relative with LREE, was due to the lower concentration of heavy minerals (Nyakairu and Koeerl 2001). The siliciclastic rocks usually contain higher LREE/HREE ratios and negative Eu anomaly, whereas the basic rocks contain low LREE/HREE ratios and no Eu anomaly (Cullers and Graf 1983). The pattern of chondrite-normalized REE (including a negative Eu anomaly) and high LREE/HREE ratio (2.32–5.56) in the studied samples reveals that the source rock is of felsic type (Cullers et al. 1997). A statistically significant positive correlation was observed between ΣREE and Al2O3 and K2O for the Batal Formation (r = 0.93 and r = 0.52, n = 14) suggesting that REE is mainly housed in detrital or heavy minerals (Armstrong-Altrin et al. 2017). Correlation between Zr versus Yb and ΣHREE was observed statistically significant as this reflects that the sediments are influenced by sediment recycling (Armstrong-Altrin et al. 2017).

Table 3 Rare earth element concentration data of the Neoproterozoic metasedimentary rocks of Batal Formation, Spiti Basin, Tethys Himalaya
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Fig. 7
figure 7

Chondrite-normalized REE distribution spectra of Neoproterozoic succession of Batal Formation in Spiti Basin

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Discussion

Major, trace, and REE concentrations in sediments have been used widely to know the provenance and environment conditions during deposition and tectonic setting of the source area and were defined by various discriminant diagrams (Bhatia 1983; Bhatia and Cook 1986; Roser and Korsch 1986; Sari and Koca 2012; Hernández-Hinojosa et al. 2018; Anaya-Gregorio et al. 2018). The geochemical composition of siliciclastic rocks is useful to define the provenance, weathering, and depositional condition, because these rocks are mainly controlled by the plate tectonic settings of their provenances, so the rocks from different tectonic settings have specific geochemical signatures (Bhatia 1983; Bhatia and Cook 1986; Roser and Korsch 1986; McLennan et al. 1993). In the present work, the geochemical studies of the Batal Formation rocks were undertaken aiming to provide a comprehensive and detailed analysis of provenance and depositional environment.

Verma and Armstrong-Altrin (2013) defined tectonic setting by using major element–based discrimination diagrams. They proposed discriminant function diagram based on major element to define the tectonic discrimination of siliciclastic sediments for high-silica [(SiO2) adj = 63–95%] and low-silica [(SiO2) adj = 35–63%] types, from three main tectonic settings: island or continental arc, continental rift, and collision. The continental arc and collision field represent active margin, whereas continental rift represents passive margin. Armstrong-Altrin and Verma (2005) evaluated this discrimination diagram and showed a low percentage success rate (0%–23%) for the Bhatia (1983) diagram and 31.5%–52.3% for the Roser and Korsch (1986) diagram. Further, this multidimensional diagram can be considered as a tool for successfully discriminating the tectonic setting of older sedimentary basins (Armstrong-Altrin 2015a). Recently, Hernández-Hinojosa et al. (2018), Armstrong-Altrin et al. (2017), Tobia and Aswad (2015), and Zaid (2015b, c) used the multidimensional discrimination diagram to study the tectonic setting of source region of clastic sediments based on geochemistry. In the present study, authors used major element–based discrimination diagrams proposed by Verma and Armstrong-Altrin (2013). It is observed that the rocks which contain high silica percentage plotted in the collision field (Fig. 8a) whereas at the same time, the rocks containing low silica percentage are also plotted in the collision field (Fig. 8b). On the basis of these high silica and low silica multidimensional diagrams (Fig. 8a, b), the present studied samples are plotted mostly in the collision field suggesting that the deposition of sediments took place in active continental margin environment.

Fig. 8
figure 8

a Discriminant function multidimensional diagram for high-silica clastic sediments (Verma and Armstrong-Altrin 2013). The subscript m1 in DF1 and DF2 represents the high-silica diagram based on log ratios of major elements. The discriminant function equations are DF1(Arc-Rift-Col)m1 = (− 0.263 × In(TiO2/SiO2)adj) + (0.604 × In (Al2O3/SiO2)adj) + (− 1.725 × In(Fe2O3t/SiO2)adj) + (0.660 × In(MnO/SiO2)adj) + (2.191 × In(MgO/SiO2)adj) + (0.144 × In(CaO/SiO2)adj) + (− 1.304 × In(Na2O/SiO2)adj) + (0.054 × In(K2O/SiO2)adj) + (− 0.330 × In(P2O5/SiO2)adj) + 1.588; DF2(Arc-Rift-Col)m1 = (− 1.196 × In(TiO2/SiO2)adj) + (1.604 × In(Al2O3/SiO2)adj) + (0.303 × In(Fe2O3t/SiO2)adj) + (0.436 × In(MnO/SiO2)adj) + (0.838 × In(MgO/SiO2)adj) + (− 0.407 × In(CaO/SiO2)adj) + (1.021 × In(Na2O/SiO2)adj) + (− 1.706 × In(K2O/SiO2)adj) + (− 0.126 × In(P2O5/SiO2)adj) − 1.068. b Discriminant function multidimensional diagram for low-silica clastic sediments (Verma and Armstrong-Altrin 2013). The subscript m2 in DF1 and DF2 represents the low-silica diagram based on log ratio of major elements. Discriminant function equations are DF1(Arc-Rift-Col)m2 = (0.608 × In(TiO2/SiO2)adj) + (− 1.854 × In(Al2O3/SiO2)adj) + (0.299 × In(Fe2O3t/SiO2)adj) + (− 0.550 × In(MnO/SiO2)adj) + (0.120 × In(MgO/SiO2)adj) + (0.194 × In(CaO/SiO2)adj) + (− 1.510 × In(Na2O/SiO2)adj) + (1.941 × In(K2O/SiO2)adj) + (0.003 × In(P2O5/SiO2)adj) − 0.294; DF2(Arc-Rift-Col)m2 = (− 0.554 × In(TiO2/SiO2)adj) + (− 0.995 × In (Al2O3/SiO2)adj) + (1.765 × In(Fe2O3t/SiO2)adj) + (− 1.391 × In(MnO/SiO2)adj) + (− 1.034 × In(MgO/SiO2)adj) + (0.225 × In(CaO/SiO2)adj) + (0.713 × In(Na2O/SiO2)adj) + (0.330 × In(K2O/SiO2)adj) + (0.637 × In(P2O5/SiO2)adj) − 3.631

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To describe the provenance of sediments, Roser and Korsch (1986) define four different major group of rocks; they are mafic igneous provenance (P1), intermediate igneous provenance (P2), felsic igneous provenance (P3), and quartzose sedimentary provenances (P4). In the studied discriminate diagram, samples mostly represent the quartzose sedimentary provenance and to some extent the felsic igneous provenance, suggesting that sediments may derived mainly from the silica-rich sedimentary provenance (Fig. 9) with subordinate felsic igneous source. According to Armstrong-Altrin et al. (2017), the REE pattern and the range of Eu anomalies are highly useful to discriminate mafic and felsic rocks and to interpret the provenance of sediments. In the present study, it was observed that the REE pattern and negative Eu anomaly suggests felsic-type rock as the source rock for Batal Formation. Whereas, LREE enrichment and low HREE pattern reflects that the studied sediments were derived from the silica-recycled sediments or felsic igneous source and deposited on active continental margin.

Fig. 9
figure 9

Discriminant function 1 against discriminant function 2 variation diagram. Fields after Roser and Korsch (1988), wherein Fl = − 1.733TiO2 + 0.607Al2O3 + 0.76Fe2O3 − 1.5MgO + 0.616CaO + 0.509Na2O − 1.224K2O − 9.09 and F2 = 0.445TiO2 + 0.07Al2O3 − 0.25Fe2O3 − 1.142MgO + 0.438CaO + 1.475Na2O + 1.426K2O − 6.86. Provenance fields (P1) mafic igneous provenance, (P2) intermediate igneous provenance, (P3) felsic igneous provenance, and (P4) quartzose sedimentary provenance. Symbols as per Fig. 3

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Based on the molecular composition (Na2O, CaO, K2O, and Al2O3), few indices of weathering have been proposed to quantify the weathering effects. The most widely used chemical index to quantitatively measure the intensity of source area weathering is the chemical index of alteration (CIA) proposed by Nesbitt and Young (1982). The intensity of weathering in the source area of sediments can be estimated by using CIA (chemical index of alteration). Nesbitt and Young (1982) established the CIA, as CIA = 100 × Al2O3 / (Al2O3 + CaO* + Na2O + K2O). In the present study, the analyzed CIA values after Nesbitt and Young (1982) range between 63 and 93 with an average of ~ 72 (Table 1). This suggests that the source area suffered moderate to intensive weathering.

In the A-CN-K ternary plot (Fig. 10), molar proportion of Al2O3, Na2O, and CaO* (CaO in silicate fraction) is plotted to evaluate the mobility of elements during the process of chemical weathering. In the A-CN-K diagram, all the rocks exhibit a definite trend. In the present ternary plot, the rock sample of Batal Formation shows parallel trend to A-CN axis (CIA = 63–93, Fig. 10). The A-CN-K plot shows that the sample falls towards illite composition, which suggests moderate weathering (Fig. 10). The ternary plot source rock composition can be determined by backward projection and parallel to the A-CN line of the weathered samples to a point on the feldspar join (Fedo et al. 1997; Tang et al. 2012). This trend indicates that the sediments were emerging from granite as a potential ultimate source and reaching to the illite stability zone. This suggests that the effects of weathering had not proceeded to the stage of removal of alkali and alkaline earth elements from the clay minerals (Taylor and McLennan 1985). The plot further indicates that the samples were affected by moderate intensity of chemical weathering. In the bivariant plot (Fig. 11) of CIA % against Al2O3, 10 samples fall in the region of moderate weathering and four samples fall in the area which indicates the intensive weathering, which further support moderate to intense weathering in the source rock.

Fig. 10
figure 10

A-CN-K diagram showing the weathering trend of the siliciclastic rocks (after Nesbitt and Young 1982). a Al2O3; CN: CaO∗ + Na2O; K: K2O (molecular composition)

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Fig. 11
figure 11

Bivariant plot of CIA % against Al2O3 showing weathering conditions of source rock of the Batal Formation (after Nesbitt and Young 1982)

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Apart from this, some elemental ratio such as K2O/Al2O3, Ni/Co, and Cu/Zn has also been used to define the weathering and provenance. In the present study, the K2O/Al2O3 ratio ranges from 0.02 to 0.26 (Table 1), which shows distribution of samples close to illite line (Cox et al. 1995). This suggests decomposition of K-feldspar and muscovite during weathering under humid climate, and K remains fix in clay (Cox et al. 1995). Redox condition of sediments is indicated by Ni/Co ratio in sedimentary terrain (Dypvik 1984; Dill 1986). The Ni/Co below 5 reflects an aerobic environment, whereas above reflects an anaerobic environment (Jones and Manning 1994). The studied shale and siltstone sample shows Ni/Co ratio (1.2–2.55) which suggests that the sediments were deposited under low to moderate oxygenated environment (Table 2). This is further supported by Cu/Zn ratio as suggested by Hallberg (1976). According to Hallberg (1976), low ratio of Cu/Zn indicates well oxidizing and the high values suggest reducing depositional environment. The low to moderate values of Cu/Zn ratio (0.5–2.9) in the studied samples indicate that the sedimentary rocks of the Batal Formation were deposited under low to moderate oxidizing condition (Hallberg 1976; Ramos-Vázquez et al. 2017, 2018).

Conclusion

Based on the petrographical and geochemical data of Batal Formation, the following conclusions were derived:

  1. 1.

    The percentage of quartz in sandstone varies from 70 to 80% in siltstone, and in shale, it varies from 50 to 60%; at places, preferred orientation of micaceous minerals was observed.

  2. 2.

    The different major and trace elemental correlation suggests that these elements are primarily controlled by clay minerals.

  3. 3.

    The depletion of HREE, relative with LREE, was due to the lower concentration of heavy minerals, and the REE with negative Eu anomaly indicates felsic source. The significant statistical correlation between Zr versus Yb and ΣHREE represents that these sediments are influenced by sediment recycling.

  4. 4.

    The multidimensional discriminant function diagram indicates that the deposition of sediments took place in active continental marginal environment.

  5. 5.

    The CIA values suggest that the source area suffered moderate to intensive weathering with the presence of clay minerals derived during the cold and dry period.

  6. 6.

    The Ni/Co and Cu/Zn ratio indicates that the rocks of Batal Formation were deposited under low to moderate oxidizing condition.