1 Introduction

Bentonite deposits throughout the world occur either by diagenetic or hydrothermal alteration of pyroclastics of intermediate to acidic character (Christidis et al. 1995; Christidis and Dunham 1997; Christidis and Scott 1997; Christidis 1998; Yalcin and Gumuser 2000; Caballero et al. 2005; Ddani et al. 2005; Arslan et al. 2006; Christidis and Huff 2009). Bentonite has been successfully applied in stratigraphy as perfect time markers horizon for correlation. During the last three decades, numerous works have been carried out worldwide on bentonite geochemistry (MacDonald 1987; McLennan 1989, 2001; Barr and Cooper 2013; Batchelor 2014; Kiipli et al. 2015; Pozo et al. 2016; Göncüoğlu et al. 2016; Elliott et al. 2018; Ge et al. 2019; Siritongkham et al. 2020).

The bentonite occurring in the Sub-Himalayan rocks represents an invaluable time marker horizon in the Siwalik. There is extensive literature on the volcanic ash beds of Himalayas and their tectonic settings (Middlemiss 1930; Bhola 1948; Johnson et al. 1982; Arif et al. 2005; Bhat et al. 2008; Ullah et al. 2020). From time to time volcanic ash beds/bentonite beds have been reported and studied from many places of the Siwalik Group (Visser and Johnson 1978; Opdyke et al. 1979; Johnson et al. 1982; Tandon and Kumar 1984; Burbank and Tahirkheli 1985; Bhat et al. 2008).

The bentonite/bentonitized tuff band exposed in the Upper Siwalik Subgroup of Jammu was reported about 60 yrs back and their geochemistry, geochronology and depositional origin are well studied (Tripathi 1986; Yokoyama et al. 1987; Ranga Rao et al. 1988; Mehta et al. 1993; Gupta 1996a, b, 2000, 2001; Gupta et al. 1999; Kundal et al. 1999, 2011; Gupta and Kochhar 2002; Bhat et al. 2008). However, these authors have not described the exact level (bottom, middle, top) of bentonite sampled in their studies as the bentonite bed is 3.6 m thick at Badakhetar. The bentonite bands occurring in Siwalik of Jammu area are exposed at 21 sites. Among these 21 sites, we carried out sampling only from Badakhetar site (figure 1) as the Badakhetar exposure is thickest of all. The aim of the present work is to know the nature and source of bentonitized tuff band of Siwalik of Jammu area.

Figure 1
figure 1

(A) Major tectonic units of Himalaya (after Sorkhabi and Macfarlane 1999) and (B) Bentonitized tuff band exposure in the Pinjor Formation/Nagrota Formation, Upper Siwalik of Jammu province (modified after Gupta and Verma 1988).

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1.1 Stratigraphy of the study area

Stratigraphically, the Siwalik Subgroup was divided into seven formations, viz., Kamlial and Chinji (Lower Siwalik Subgroup), Nagri and Dhokpathan (Middle Siwalik Subgroup) and Tatrot, Pinjor and Boulder Conglomerate (Upper Siwalik Subgroup) by Pilgrim (1934). In Jammu, all seven formations of the Siwalik are well exposed in the study area. Ranga Rao et al. (1988) classified the rocks of Upper Siwalik Subgroup of Jammu into Purmandal Sandstone (=Tatrot Formation), Nagrota Formation (=Pinjor Formation) and Tawi Conglomerate (=Boulder Conglomerate). Agarwal et al. (1993) accepted the classification of Ranga Rao and classified the Nagrota Formation into three members, viz., Nagrota Member A(NA), Nagrota Member B(NB) and Nagrota Member C(NC). Gupta and Verma (1988) classified the Siwalik rocks of Jammu into five formations, viz., Mansar Formation (Lower Siwalik Subgroup), Dewal Formation and Mohargarh Formation (Middle Siwalik Subgroup) and Uttarbaini and Dughor formations (Upper Siwalik Subgroup). Gupta (2000) refined his classification and divided the Mansar Formation into Dodenal Member and Ramnagar Member and Uttarbaini Formation into Labli Member and Marikhui Member. Recently, Eliyas et al. (2017) classified the Siwalik Group of Jammu province into six formations, viz., Mansar Formation (Lower Siwalik Subgroup), Dewal and Mohargarh formations (Middle Siwalik Subgroup) and Labli Formation, Marikhui Formation and Dughor Formation (Upper Siwalik Subgroup). The comparative up-to-date lithostratigraphic classification of Jammu Siwalik is given in table 1. The bentonite samples in the present study have been collected from the Nagrota Member-B of Nagrota Formation (Agarwal et al. 1993; Ranga Rao et al. 1988)/Pinjor Formation (Pilgrim 1934)/Labli Member of Uttarbaini Formation (Gupta and Verma 1988)/Marikhui Formation (Eliyas et al. 2017) for geochemical analysis. The field photograph of bentonitized tuff band of Badakhetar section is given in figure 2.

Table 1 Comparative lithostratigraphic classification of Siwalik Province of Jammu.
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Figure 2
figure 2

Field photograph showing bentonitized tuff band of Badakhetar section.

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1.2 Field observations and age of bentonitized tuff band

The bentonitized tuff bands (BTB) are persistent laterally occurring in detached outcrops for about 70 km and are embedded in mudstone–siltstone horizons. These detached outcrops of bentonitized tuff bands were reported at 21 sites in the Upper Siwalik Subgroup of Jammu. The Badakhetar bentonitized tuff band is the thickest (3.6 m) among the reported sites and is exposed 375 m northwest of village Badakhetar. The associated mudstones yielding a diverse type of micro-fauna and flora include rodents, lizards, fishes, ostracodes, gastropods and charophytes (Prasad et al. 2005; Bhandari and Kundal 2008; Kundal 2013, 2015, 2018; Kundal et al. 2017). The bentonitized tuff band is mainly composed of three types of sediments: (1) Dark gray tuffaceous mudstone, (2) Cream-coloured bentonite having high swelling property, and (3) White coloured, powdery and sandy bentonite clay having little swelling nature. Dip of the bentonitized tuff beds and the associated strata varies between 15° to 23° SW in the study section. The bentonitized tuff band generally shows sharp basal contact with the underlying mudstones and gradational contact with the overlying siltstone. The bentonitized tuff band units are internally composed of many subunits and exhibit diverse type of sedimentary structures. These structures include graded bedding, planar- and wavy lamination, crossed bedding, load and flame structures, convolute bedding, oscillation ripples, bioturbation, pinch and swell structure and escape burrows (vertical and inclined). The cream colour bentonite shows maximum plasticity, high jel value index and high swelling properties as compared to other two varieties. The bed exhibits diffused internal stratification, picked out by subtle, variation in grain size and sorting, which is typical of many air fall deposits (Fisher and Schminke 1984). Both normal and inverse grading are common, but normal grading dominates. In a few beds, a rhythmic pattern exists, indicating that stratification is the product of a succession of discrete layers of tuff separated by discrete bedding planes. Moreover, the beds and laminae are naturally grouped into bedsets (e.g., predominantly fine-grained tuff and successive graded packages) suggesting that the tuff was emplaced by multiple episodic eruptions. The change in colour from bottom to top in different laminae also indicates episodic eruption. The dark gray beds of tuffaceous bentonitized tuff band contain heavy minerals such as biotite, zircon, apatite, epidote, etc., and white/pinkish colour beds contain light-coloured minerals such as glass shards, quartz, feldspars, etc. Bhat et al. (2008) inferred four palaeolakes in the study area based on the facies studies of the bentonitized tuff band and associated mudstones. The recovery of fauna and flora underlying and overlying mudstone horizons indicates a interfluvial–lacustrine depositional environment of the study area (Bhat et al. 2008; Bhandari and Kundal 2008).

Previous works have dated the bentonitized tuff bands of the Siwalik of Jammu area (Tripathi 1986; Yokoyama et al. 1987; Ranga Rao et al. 1988; Mehta et al. 1993). Tripathi (1986) reported a fission-track age (1.6 ± 0.2 m.y.) of zircon grains extracted from bentonite band exposed at Uttarbehani section of Nagrota Formation. Subsequently, Yokoyama et al. (1987) reported fission-track age of 1.6 ± 0.56 m.y. from zircons recovered from the bentonite band of Barakhetar section. Further, Ranga Rao et al. (1988) gave fission-track ages of 2.8 ± 0.56 m.y. and 2.31 ± 0.54 m.y. for bentonite bands of Badakhetar and Nagrota sections, respectively. In conjunction with magnetostratigraphy, Ranga Rao et al. (1988) arrived at a conclusion that these bentonite bands straddle across Gauss–Matuyama dated 2.48 m.y. Following this, Mehta et al. (1993) reported the fission-track age (1.59 ± 0.32 m.y.) for the bentonite layer of Badakhetar.

2 Methodology

Twelve fresh bentonitized tuff samples (BKB1–BKB12) have been collected for major oxides, trace and rare earth elements. While collecting the samples, weathered surfaces were avoided and each sample collected after one foot from bottom to top of the bentonitized tuff section. The collected samples were dried in the sunlight and converted into powdered form for geochemical analysis at Wadia Institute of Himalayan Geology, Dehradun. Oxides and trace elemental analyses were carried out by using Standard XRF (Wavelength Despersive XRF Sequential Spectrometer -WD-XRF) technique, Bruker S8 Tiger. The particle size of the powdered sample is less than 74 micrometer and the quantity of sample was 5–6 gm. The standards used for sediment calibration were USGS (U.S.A.): GXR-2, GXR-5 and the overall accuracy (%RSD) for major oxides was <5% and for trace elements was <12%. The average precision was always better than 1.5%.

For rare earth elements (REE), samples were analyzed by Inductively Couple Plasma-Mass Spectrometer (ICP-MS; PerkinElmer SCIEX ELAN DRC-e) using the open system rock digestion method (Khanna et al. 2009). In this method, 100 mg of homogenous powder of each sample (200 mesh) with 10 ml of HF:HNO3 mixture (2:1) is used. The dried mass of sample in 10 ml of 20% HNO3 and the standard used for sediment calibration and validation during the analysis was SCo-1, MAG-1, SRG-1 (USGS, USA) the accuracy and precision of Rare earth elements.

2.1 Results

The result obtained after geochemical analysis by XRF and ICPMS of bentonitized tuff samples from Badakhetar bentonitized tuff section is given in table 2.

Table 2 Major oxides, trace and rare earth elements data of bentonitized tuff samples (BKB1 to BKB12) from the Badakhetar bentonitized tuff section of Samba district, Jammu province of J&K, India.
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3 Discussion and interpretation

The original chemical composition of volcanic ash layers is altered by digenesis (Glanzman and Rytuba 1979; Namayandeh et al. 2020). Therefore, the types of material/source from which these ash layers were derived cannot be determined from major element data alone as with unaltered glass. For this purpose, whole-rock major, trace and rare earth element geochemistry of bentonitized tuff band was carried out on powdered samples. The obtained geochemical data is used to infer magmatic composition of the parent magma and tectonic setting of the source volcanoes. Many previous studies based on immobile trace and rare earth elements have been carried out in this direction (e.g., Teale and Spears 1986; Merriman and Roberts 1990; Huff et al. 1997a, b, 2000; Pattan et al. 2001; Nambiar and Sukumaran 2002; Pattan 2002). Some of these studies have generally relied on the use of empirically-based discriminate plots derived from studies of igneous rocks of known origin (e.g., Teale and Spears 1986; Merriman and Roberts 1990; Huff et al. 1997a, b, 2000). Although these plots do not provide absolute proof of magma origin or affinity, these plots serve as a useful source of identification of the tectonic settings and general magma chemistry, particularly where other geological evidences are ambiguous (Huff et al. 1998). The immobile trace element plots (Zr/TiO2 vs. Nb/Y) for the bulk ash layers are useful for the classification and inferred the nature of magma. During early alteration, most major elements and many trace elements can be mobile (Floyd and Winchester 1975; Winchester and Floyd 1977; Baragar et al. 1979; Gélinas et al. 1982), although some plotting using the major elements seem to be useful provided that the analyses are carefully screened and selected before plotting (Pearce 1976; Pearce et al. 1977).

3.1 Major oxides

The rocks having high silica content are depleted in MgO and FeO and enriched in Na2O and K2O. On the other hand, the rocks having low silica content are enriched in MgO and FeO and depleted in Na2O and K2O. The major oxides and trace elements data of Badakherar bentonitized tuff are listed in table 2. The major oxide (wt%) concentration of 12 samples of Badakhetar bentonitized tuff band is presented in figure 3.

Figure 3
figure 3

Showing concentration of chemical oxides in bentonitized tuff samples at Badakhetar, Jammu province.

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All the samples of bentonitized tuff considerably have high SiO2 content (59.92–72.05 wt%) with variable Na2O (0.09–1.88 wt%), MgO (0.76–4.67%), Al2O3 (11.73–23.88%), P2O5 (0.00–0.07%), K2O (0.07–4.29%), CaO (1.27–2.03%), TiO2 (0.06–0.10%) and Fe2O3 (0.84–3.39%). Samples BKB1, BKB2 and BKB4 have silica content SiO2 < 66% indicating intermediate nature of bentonitized tuff samples and the samples BKB3, BKB5 to BKB12 have high silica content (SiO2 > 66%) indicating a felsic nature of the bentonite samples.

Similarly, the range of Na2O (0.09–1.04 wt%) and K2O (0.07–2.40 wt%) of BKB1–BKB2, and BKB4 is less as compared to Na2O (1.16–1.91 wt%) and K2O (2.75–4.29 wt%) of BKB3, BKB5 to BKB12. This indicates that BKB3, BKB5 to BKB12 are enriched in Na2O and K2O, whereas BKB1–BKB2, BKB4 are depleted in Na2O and K2O.

Al2O3 in BKB1–BKB2, BKB4 ranges from 14.70 to 23.88 wt% which is much higher than BKB3, BKB5–BKB12 (11.73–14.52 wt%). Similarly, CaO in BKB1–BKB2, BKB4 ranges from 1.83 to 2.03 wt% which is higher than that of BKB3, BKB5 to BKB12 (1.27–1.80 wt%). This indicates that with increase in silica content, the CaO content decreases. The BKB1–BKB2, BKB4 have less silica content and high CaO values as compared to BKB3, BKB5–BKB12 which have higher silica content and low CaO values.

The alkali content of the BKB1, BKB2 and BKB4 was much smaller than BKB3, BKB5–BKB12 samples, indicating a major loss of alkalis during alteration (e.g., Senkayi et al. 1984; Shiraki et al. 1987; Altaner and Grim 1990; Christidis and Scott 1997). The relatively higher K2O and Na2O contents of BKB3, BKB5–BKB12 of the bentonitized tuff samples reflect the presence of authigenic K-feldspar and mordenite, respectively.

3.2 Trace elements

The high field strength elements like Zr, Nb, Hf, Ta and rare earth elements and TiO2 are commonly considered to be immobile under most conditions of diagenesis and low-grade metamorphism and thus are useful indicators of petrogenetic processes (Huff et al. 1998). The Nb/Y ratio is widely used as a measure of alkalinity and Zr/TiO2 ratio as an index of differentiation. Izett (1981) concluded that many explosively erupted volcanic ashes tend to have moderate to high Nb and Zr content reflective of their silicic and high volatile (H2O) character. The Zr/TiO2 vs. SiO2 (wt%) and Zr/TiO2 vs. Nb/Y chemical discrimination diagrams (Winchester and Flyod 1977) are used for the classification of bentonites.

Samples BKB1–BKB12 plotted on Zr/TiO2 vs. SiO2 (wt%) classification diagram (figure 4) resulted in a range from rhyodacite to trachyandesite with SiO2 between 59.92 and 72.05 wt%. In bentonitized tuffs/clay, Si tetrahedral shows least variation of major elements and has a mean value relative to continental crust of nearly one.

Figure 4
figure 4

Zr/TiO2 vs. SiO2 (wt%) classifying studied samples as rhyodacite to trachyandesite with an SiO2 range of 59.92–72.05 wt% (Winchester and Floyd 1977).

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Similarly, BKB1 to BKB12 are plotted on Zr/TiO2 vs. Nb/Y diagram (figure 5). The Zr/TiO2 vs. Nb/Y suggests derivation from felsic magma having trachyte, trachyte–andesite and andesitic affinity. The high Zr/TiO2 ratio (0.01–0.08) also suggests trachyte–andesite composition for the bentonitized tuff samples. The low Nb, Th and Zr values are suggestive of subalkaline origin for these samples.

Figure 5
figure 5

Bentonitized tuff band range from andesite to trachyandesite to trachyte with range Zr/TiO2 between 0.01 and 0.08 and Nb/Y between 0.57 and 10.

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The trace elemental data has also been plotted on several widely referred discrimination plots to reveal the tectomagmatic origin of altered volcanic ashes. Immobile elements Y (ppm) and Nb (ppm) values of bentonitized samples (BKB1–BKB12) are plotted on a discrimination diagram (after Pearce et al. 1984) (figure 6). All the samples of bentonitized tuff band plot in the fields of Volcanic Arc Granites Plus Syn-Collision tectomagmatic setting (VAG + Syn-COLG). This indicates that the source of Badakhetar bentonitized tuff band was Volcanic Arc Granite + Syn Collision tectomagmatic setting.

Figure 6
figure 6

Y (ppm) and Nb (ppm ) values of bentonitized tuff samples (BKB1–BKB12) plotted on the discriminate diagram (after Pearce et al. 1984) indicating volcanic arc granites plus syn-collision tectomagmatic setting (VAG + Syn-COLG) source of bentonitized tuff band.

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3.2.1 La/Th vs. Th/Yb

The La/Th vs. Th/Yb plots have been used to differentiate between felsic and mafic nature of source rocks (Bhatia and Crook 1986). The La/Th ranges between 0.168 and 1.765 and Th/Yb values between 5.042 and 40.0 of 12 samples of bentonitized tuff band from Badakhetar section has been plotted on La/Th vs. Th/Yb plots (figure 7). BKB3–BKB12 shows felsic character of source rock by its unusual high La/Th (felsic provenance) as compared with samples (BKB1 and BKB2) having low La/Th value and high Th/Yb value (intermediate provenance).

Figure 7
figure 7

La/Th vs. Th/Yb plot (Bhatia and Crook 1986) indicating BKB1−BKB2 intermediate provenance and BKB3–BKB12 indicating felsic provenance.

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3.3 Rare earth elements

3.3.1 Chondrite normalized plot

The chondrite normalized values (Taylor and McLennan 1985) of rare earth elements for 12 bentonitized tuff samples from Badakhetar section are plotted on the spider diagram (figure 8). All the samples show negative sloping curves with an overall increase in light rare earth elements of 4–17 times chondritic and heavy rare earth elements by a factor of 1–3. These observations are characteristic of a calc-alakline nature magmas of subduction related to volcanic arc environments (Taylor and McLennan 1988; McVey and Huff 1995).

Figure 8
figure 8

Chondrite-normalized REE patterns of 12 bentonitized tuff band samples of Badakhetar section showing slightly enrichment of LREE over HREE during the alteration processes (chondrite normalizing values are from Taylor and McLennan 1985).

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3.3.2 Degree of fractionation elements

Degree of fractionation/fractionation indices of rare earth elements can be expressed by Lachondrite normalized/Ybchondrite normalized. It is an index of the concentration of light rare earth elements (La or Ce) over heavy rare earth elements (Yb). A combination of Lachondrite normalized/Ybchondrite normalized ratio with Eu anomaly plays a measure role to describe the rare earth elements pattern and is very useful to determine the source rock. The Lachondrite normalized/Ybchondrite normalized values of 12 samples (BKB1–BKB12) are within the range of 4.58 and 8.03 with an average 5.95 suggests that the light rare earth elements are enriched with respect to heavy rare earth elements.

3.3.3 Europium (Eu) anomaly

The past events that existed in the reducing igneous environment (events occurred in upper continental crust) can be indicated by anomalous activity of Eu.

Europium anomaly (Eu anomaly) is generally represented by Eu/Eu* and can be expressed (Liao et al. 2016) as:

$$ {\text{Eu}}/{\text{Eu}}^{*} = \frac{{{\text{Eu}}_{{{\text{chondrite}}\;{\text{normalized}}}} }}{{0.5 \times {\text{Sm}}_{{{\text{chondrite}}\;{\text{normalized}}}} \times \, 0.5 \times {\text{Gd}}_{{{\text{chondrite}}\;{\text{normalized}}}} }}. $$

In felsic rocks, plagioclase majorly controls the Eu anomaly. The Eu/Eu* values >0.85 indicate a positive Eu anomaly while Eu/Eu* values <0.85 indicate a negative Eu anomaly and values =0.85 indicate no anomaly.

The Eu anomaly of the bentonitized tuff band samples ranges between 0.56 and 0.93 with an average of 0.81 suggesting a negative Eu anomaly. Individual Eu anomaly values of 0.73 (BKB1), 0.57 (BKB2), 0.79 (BKB4), 0.83 (BKB5), 0.78 (BKB6), 0.81 (BKB8) and 0.83 (BKB11) is <0.85 which also corresponds to negative Eu anomaly, sample BKB12 (0.85) and BKB3 (0.85) corresponding to no Eu anomaly and sample BKB7 (0.93), BKB9 (0.87) and BKB10 (0.88) >0.85 indicate slightly positive Eu anomaly. The negative Eu anomaly is usually in felsic rocks and sediments due to intracrustal fractionation/lithospheric/weathering of feldspar and a positive Eu anomaly is due to the origin of feldspar or prominent areas affected in hydrothermal vents (Rudnick 1992). The LREE/HREE ratio in felsic rocks is usually high and more pronounced to negative Eu anomaly while LREE/HREE ratio in mafic rocks is usually low and a few or no Eu anomalies.

3.3.4 Cerium (Ce) anomaly

Cerium anomaly is the concentration of Cerium in rock either enriched or depleted relative to other rare earth elements. If Cerium is depleted in a rock relative to other REE, the Ce anomaly is said to be negative and if the Cerium is enriched in a rock relative to other REE, the Ce anomaly is said to be positive. This only occurs in the marine environmental condition associated with the formation of nodulus (manganese). In marine conditions, when Ce3+ oxides to Ce4+, it separates as an insoluble phosphate. The result is distinctive Ce depletion in marine water conditions.

Cerium anomaly (Ce anomaly) is generally represented by Ce/Ce* and expressed as:

$$ {\text{Ce}}/{\text{Ce}}^{*}_{{{\text{chondrite}}\;{\text{normalized}}}} = \frac{{{\text{Ce}}_{{{\text{chondrite}}\;{\text{normalized}}}} }}{{0.5 \times {\text{La}}_{{{\text{chondrite}}\;{\text{normalized}}}} \times 0.5 \times {\text{Pr}}_{{{\text{chondrite}}\;{\text{normalized}}}} }}. $$

\( {\text{Ce}}/{\text{Ce}}^{*}_{{\text{chondrite}}\, {\text{normalized}}}\) bentonitized tuff samples (BKB1–BKB12) from Badakhetar range from 0.93 to 1.63 with an average value 1.12 indicating a positive Ce anomaly, but individual Ce anomaly values of samples BKB1–BKB2, BKB7–BKB12 ranges from 1.02 to 1.63 indicating a positive Ce anomaly and that of samples BKB3–BKB5, BKB9 ranges from 0.93 to 0.97 indicate a negative Ce anomaly.

The REE pattern of the samples clearly indicates enrichment of LREEs over HREEs with higher Ce/Yb ratios. This is quite similar to the REE content of most of the fine-grained sedimentary rocks. The sample BKB3, BKB5–BKB12 is showing further higher enrichment of LREEs suggesting a very high percentage of clays with very low biogenic carbonate and quartz. The chondrite normalized plot is showing a negative Eu anomaly. The negative anomaly is commonly observed in all the sedimentary rocks. The only important sedimentary rock type with no Eu depletion is the first cycle volcanogenic sediments deposited in forearc basins of Island arcs and derived mainly from andesites (Nauce and Taylor 1977).

The negative Eu anomaly in the present study may be due to the chemical fractionation and represent a reducing environment. The negative Ce anomaly is not a common phenomenon of the fine-grained sedimentary rocks; it may be due to chemical weathering in the acidic environment (Brown et al. 1955). Alternatively, these Ce and Eu anomalies may represent the source from rhyolitic/rhyodacitic to trachyandestic composition, which is more plausible in the present case. Furthermore, diagenetic changes in Eh also affect Ce and Eu because of their ability to exist in different oxidation states under geological conditions. A pronounced negative Eu anomaly in some samples indicates earlier crystallization of plagioclase (e.g., Huff et al. 1998). The lack of corresponding depleted HREE indicates the fractionation of phases such as garnet and clinopyroxene, did not play a major role in the evolution of these calc-alkaline magmas. In addition, a very slightly negative Ce and Er anomalies in all the samples suggest depletion due to diagenesis in an alkaline environment. A pronounced positive Tm anomaly is attributed to contribution from the facies and the alkaline depositional environment. The bentonitized tuff band samples have unique chemical signatures in having euhedral biotite and apatite grains and less abundant zircons and possess high field strength elements such as Zr, Y, Th and Nb. The uncommon zircon combined with the presence of apatite crystals and weak Eu anomaly implies that the source was not an oversaturated peralkaline melt (MacDonald and Spooner 1981).

Vitroclastic texture or even relic primary volcanic glass, phenocrysts such as clear quartz, sanidine, plagioclase, pyroxene, euhedral biotite, euhedral zircon, apatite and their distinct morphological feature, clay mineral assemblage all indicate that the Upper Siwalik bentonitized tuff band is derived from altered volcanic ash. Variations in colour and composition indicate that the layers may be comprised of several eruptions.

3.4 Degree of weathering

To check the degree of weathering of a parent rock, it is essential to know element mobility during the change of volcanic tuff to bentonite. To understand the geochemical characteristics of source rock, physical reworking intensity and changes of igneous rocks, the felsic volcanic rocks ratio of TiO2/Al2O3 has been widely used (Zhou and Kyte 1988; Burger et al. 2002; He et al. 2014; Hong et al. 2019). The most immobile elements during the devitrification of volcanic ash to authigenic bentonite are Al and Ti (Slack and Stevens 1994; Hong et al. 2019). Ti has more mobility than Al which results in diminishing the TiO2/Al2O3 ratio progressively with increasing intensity of weathering. Hong et al. (2019) calculated the TiO2/Al2O3 values and classified the degree of weathering for felsic volcanic rocks are as under (i) if the TiO2/Al2O3 ratio is <0.055, it corresponds to primary ash compositions, (ii) if the TiO2/Al2O3 ratio is between 0.055 and 0.140, it suggests a moderate degree of weathering, and (iii) if TiO2/Al2O3 ratio is >0.140, it suggests strong degree of weathering.

All the samples of bentonitized tuff from the Badakhetar section possess TiO2/Al2O3 ranging between 0.0001 and 0.0230 with an average value of 0.01341 indicating primary ash composition/original composition of rock. Eu anomaly is commonly used to know the composition of source magma and is not influenced by reworking. Eu/Eu* vs. TiO2/Al2O3 is used to evaluate the reworking effect during the deposition of ash. All the samples are plotted on Eu/Eu* vs. TiO2/Al2O3 diagram that further highlights primary volcanic tuff nature near the composition of rhyolite/rhyodacite (figure 9).

Figure 9
figure 9

TiO2/Al2O3 vs. Eu/Eu* discriminate diagram parent rocks (blue star) (Hong et al. 2019). Rhyolite (in red square), andesite (in blue square) and basalt data (in Black Square) are from Laviano and Mongelli (1996). All the samples (BKB1–BKB12) fall in the domain of primary felsic tuffs.

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4 Source of bentonitized tuff band

During the Plio-Pleistocene times, there is no evidence of volcanic activity in north India. The presence of volcanic ash bed or bentonitized tuff band in the entire region of the Upper Siwalik of Jammu area suggests that the volcanic activity might have taken place during Plio-Pleistocene times. This band is extended up to Pinjor in Himachal Pradesh towards northeast in India and towards northwest, it extends into Pakistan and Afghanistan. In Pakistan, Johnson et al. (1982) identified the bentonitized tuff band and tuffaceous mudstones at four stratigraphic levels and these levels show variation in composition, which indicates multi-volcanic activity that took place during Plio-Pleistocene times.

The Dasht-e-Nawar volcanic complex of east-central Afghanistan (Pliocene–Quaternary centre for volcanism) has several large strato volcanoes, explosion centres with associated welded tuff, and explosion breccia and domes which are primarily andesites and rhyodacites. These are 50 independent volcanoes known in Dasht-e-Nawar area, but few are dated (Johnson et al. 1982). These rocks are composed of andesites, with cuspate and platy quartz shards, rhyodacites and lamprophyres having pyroxene, hornblende, fine microlites of feldspars, glass with gas cavities, pumice, biotite, sphene, apatite, zircon, tourmaline, etc.

The ash particles have blown with wind and cover approximately 1000 km distance. The bigger size and dense ash particles fall near to the source (near the place of eruption) and the small size and light ash particles were swept away from the source. The high sp. gravity ash particles settle down on the earth surfaces earlier than the low sp. gravity ash particles (e.g., the tuffaceous mudstone beds occurred at the base and composed of dark gray coloured of heavy minerals). The low sp. gravity and light-coloured minerals occurred above the tuffaceous mudstone and show grading towards top.

The present study points to the possibility that the bentonitized tuff band of Jammu might be the product of the volcanic activity that has taken place in Dasht-e-Nawar volcanic complex. This interpretation is based on the following three grounds: (1) The composition of Jammu bentonitized tuff band is same as that of Dasht-e-Nawar volcanic complex, (2) the time of deposition of the Jammu bentonitized tuff band and the eruption time of Dasht-e-Nawar volcanic complex is same, i.e., Plio-Pleistocene, and (3) the bentonitized tuff band has been reported in detached outcrops from Pakistan through Jammu and Kashmir to Himachal Pradesh at the same stratigraphic level.

5 Conclusion

Geochemistry of bentonitized tuff band from the Jammu area reveals their intermediate to felsic nature and their affinity towards rhyodacite–trachyandesite–andesite. Tectonic discrimination diagrams reveal volcanic arc granite plus syn-collisional tectonic setting. Chondrite normalized REE values indicate slight enrichment of LREE over HREE. Eu anomaly ranges from 0.56 to 0.93 indicating a mix of both positive and negative Eu anomaly. TiO2/Al2O3 values of samples are <0.055 suggesting their primary ash compositions. TiO2/Al2O3 vs. Eu/Eu* values suggest no reworking effect during deposition of bentonitized tuff band. The Dasht-e-Nawar volcanic complex of east-central Afghanistan might be the source of bentonitized tuff band of Jammu province. Variation in colour and grain size from bottom to top in different laminae indicates episodic eruption.