1 Introduction

The Khairagarh Group is spread in approx. 1000 km2 area towards W, NW, and SW of Khairagarh (21°25′15″N, 80°58′50″E) in Central India (Survey of India toposheets 64 C/11, C/12 and C/15). It is a constituent of the NNE–SSW trending Dongargarh Kotri mobile belt (DKMB) (Ramakrishna and Vaidyanathan 2010), and located south of the Central Indian Tectonic Zone (CITZ) (Yedekar et al. 1990). The genesis of Paleoproterozoic (2.46–2.21 Ga) Khairagarh Group volcano-sedimentary succession has been attributed to rapid crustal growth of the north Bastar Craton (Sarkar 1957; Sarkar et al. 1994; Srivastava and Gautam 2009; Longjam and Talat 2012; Sensarma and Mukhopadhyay 2014; Manikyamba et al. 2016; Khanna et al. 2019) near Archean Proterozoic transition. Tectonomagmatic study of Khairagarh Group assumes special importance due to its temporal proximity to global Neoarchean Paleoproterozoic phenomena of onset of mature plate tectonics (c.f. Condie and Kroner 2008; Cawood et al. 2018; Hawkesworth and Brown 2018; Windley et al. 2021). Since Khairagarh volcanic formations exhibit alteration and very low-grade regional metamorphism (Asthana et al. 1996; Patel et al. 2008; Khanna et al. 2019; Khare and Asthana 2020; Khare et al. 2022), they provide a unique opportunity to undertake reliable geochemical and petrogenetic study to elucidate crust mantle processes responsible for Paleoproterozoic crustal evolution of north Bastar Craton. There is temporal and spatial proximity of Khairagarh volcanism to Malanjkhand (22°00′54″N; 80°42′36″E) Cu ± Mo ± Au metallogeny (Sikka and Nehru 1997; Kumar and Rino 2006; Pandit and Panigrahi 2012; Nehru and Sikka 2014; Pandit et al. 2014; Kumar and Asthana 2018; Arya et al. 2021). Therefore, it becomes imperative to undertake field geological, geochemical and petrogenetic study of Khairagarh volcanic formations and look for any evidence which indicates a relationship between Khairagarh volcanism and Malanjkhand metallogeny.

With this context, in this contribution, we present for the first time a detailed 1:50,000-scale geological map of Khairagarh Group, which is well exposed in Sitagota syncline. Our work presents a digital elevation map (DEM) of syncline prepared on ArcGIS software. We report the presence of faults in the syncline not reported earlier. Algoma-type BIF and intertrappean shale beds are reported in the Mangikhuta basalt of the Khairagarh Group. Magmatic oxide and sulphide mineralization is also reported in the Mangikhuta basalt. We report the presence of mafic enclaves in Dongargarh granite, which is the basement of the Khairagarh Group. Geochemical and petrogenetic study is presented for the Mangikhuta and Kotima volcanic formations, and a tectonomagmatic model presented for the genesis of the Khairagarh Group. A post-deposition orogenic model of Khairagarh Group is presented in this work.

2 Regional geology

Bastar Craton is an important Archean province of peninsular India, which lies to the south of Central Indian Tectonic Zone (CITZ) (Yedekar et al. 1990), and surrounded from the east and west by Singhbhum and Dharwar cratons, respectively (figure 1a) (Radhakrishna and Naqvi 1986; Naqvi and Rogers 1987; Mondal et al. 2006). The Dongargarh Supergroup, an important constituent of Bastar Craton is a Neoarchean–Paleoproterozoic succession of granite intrusions, bimodal volcanic formations, and volcano-sedimentary sequence spanning a 90 km wide and 150 km long NNE–SSW trending belt between Sakoli and Chhattisgarh basin in Central India (figure 1b) (Sarkar 1957, 1994; Mohanty 2015). The constituents of Dongargarh Supergroup are Nandgaon bimodal basalt and rhyolite formations, Dongargarh granite and Khairagarh Group (Rao 1981; Krishnamurthy et al. 1990; Neogi et al. 1996; Khanna et al. 2019; Santosh et al. 2020). Detailed chronostratigraphic succession of the Dongargarh Supergroup is given in table 1. The Khairagarh Group is around 12-km thick volcano-sedimentary sequence, which unconformably overlies above Dongargarh granite and Nandgaon bimodal volcanics. Sarkar (1957, 1958) published the geological map of Dongargarh System (figure 1c), which included map of Khairagarh Group from faulted western limb of Sitagota syncline (toposheet 64 C/12). However, a major portion of syncline towards north–northeast (toposheet 64 C/11 and C/15) remained unmapped due to inaccessible terrain. Rao (1981) published a regional map of Malanjkhand extension area which included the Sitagota syncline but detail mapping of Khairagarh volcanics and sediments towards centre of the Sitagota syncline (toposheet 64 C/11 and C/15) was not done. Later, several workers studied Dongargarh Supergroup (Sarkar et al. 1981; Krishnamurthy et al. 1990; Sarkar et al. 1994; Asthana et al. 1996, 1997, 2001; Neogi et al. 1996; Ghosh et al. 2006; Longjam and Talat 2012; Sensarma and Mukhopadhyay 2014; Khanna et al. 2019; Santosh et al. 2020). However, none of these studies reported detailed geology and field relations of volcanics and sediments from the centre of Sitagota syncline, which is part of our current work.

Figure 1
figure 1

A geological map prepared after detailed field investigation of Sitagota syncline. (a) shows the location of Bastar Craton (BC) in Peninsular India vis-a-vis CITZ and other Archean cratons (Radhakrishna and Naqvi 1986). (b) shows the overall geology of Bastar Craton and location of the Dongargarh Supergroup in Bastar Craton (Kumar and Asthana 2018). (c) is geological map of Dongargarh Supergroup after Sarkar (1957). This map shows unmapped NE portion of Sitagota syncline. (d) is a geological map of Sitagota syncline made during the course of current field study. Black circles show location of samples collected for geochemical studies.

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Table 1 Chronostratigraphic succession of Dongargarh Supergroup and Khairagarh Group in Sitagota syncline after Rao (1981) and Sarkar (1994). The geochronology data taken from Krishnamurthy et al. (1990), Manikyamba et al. (2016) and Khanna et al. (2019). The thickness of volcanic and sedimentary units in Sitagota syncline estimated during the course of the current study.
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3 Field geology of Sitagota syncline

The Nandgaon bimodal volcanics and Dongargarh granite form the basement of Khairagarh Group. The outcrops of Dongargarh granite (figure 2a) are well exposed in and around Dongargarh (21°10′37″N; 80°45′04″E). The Pitepani basalt (figure 2b) and Bijli rhyolite (figure 2c) of Nandgaon Group are well exposed in Pitepani (21º09′55″N; 80º35′12″E) and Bagrekasa (21º08′54″N; 80º34′26″E), respectively. Dark grey-coloured, micro-granular mafic enclaves are present in Dongargarh granite outcrops (figure 2d), which could be either remnants of underlying Pitepani basalt or a product of magma mixing and assimilation. One km north of Dhara (21º15′45″N; 80º51′03″E), a 50-m thick conglomerate horizon in lower portion of Bortalao Formation establishes an unconformable relationship between Dongargarh granite and Khairagarh Group (figure 2e). Conglomerate outcrops of Bortalao Formation are also seen near Bortalao (21º13′33″N; 80º36′57″E) along Dongargarh Darekasa Road (figure 2f). Khairagarh volcano-sedimentary sequence consists of Sitagota, Mangikhuta and Kotima basalt formations, respectively, overlying Bortalao, Karutola and Ghoghra sandstone formations (figure 1d). Bortalao Formation occurs as a prominent sandstone ridge near Dhara (figure 2g). The Sitagota basalt outcrops (figure 2h) are seen between Bortalao and Darekasa near Jamakudo (21º15′26″N; 80º35′36″E). During a traverse along Khairagarh Lanji Road, the outcrops of Mangikhuta basalt are exposed in the nallah section near Temri (21º23′39″N; 80º47′23″E). The Khairagarh volcanic formations occur as dispersed outcrops between sandstone ridges. They are fresh green to greenish grey coloured, non-schistose and devoid of pillow structures. Intertrappean shale of 5–10 m thickness is present in Mangikhuta basalt. Although flow layers are not evident in the field, quartz and calcite-filled vesicles and amygdules easily identify them (figure 2i). A 10-m thick Algoma-type banded iron formation (BIF) was discovered in intertrappean shales in Mangikhuta basalt near the contact of Karutola and Mangikhuta formations. The BIF extends for nearly 30–40 m and consists of alternate meso to micro clay and iron-rich bands. The maturity of Khairagarh sediments progressively increases from lower Bortalao to upper Ghoghra Formation. Field study indicates that the Ghoghra Formation pinches out laterally in both limbs of the syncline, and underlying Mangikhuta basalt continues without any overlying sediments into the Kotima basalt in the centre of the syncline (figure 1d). Khairagarh volcano-sedimentary sequence was gently folded (dips varying from 10º to 45º) to give rise to a regional NNE–SSW trending syncline structure spread in survey of India toposheets 64 C/11 and 64 C/15. The western limb of the Sitagota syncline was sinistrally displaced southwards (toposheet 64 C/12) by the ‘Great Darekasa Fault’ (figure 1d, Sarkar 1957).

Figure 2
figure 2figure 2

Pictures of geological features in and around Sitagota syncline. (a) Dongargarh granite batholith outcrops in Dongargarh. (b) A visual of fresh Pitepani basalt sample taken from Pitepani village. (c) Bijli rhyolite outcrops near Bagrekasa village. (d) Mafic enclaves seen in Dongargarh granite outcrop located in Dongargarh town. (e) Bortalao Basal conglomerates present near Dhara. (f) Conglomerate horizon near Bortalao railway station along Dongargarh Darekasa road. (g) Bortalao sandstone ridge seen from Dhara water tank. (h) Sitagota basalt outcrops near Dongargarh Darekasa road. (i) Vesicles seen in Mangikhuta basalt. (j) Chilpi shale outcrops along Khairagarh Lanji road near Lachhna.

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Towards the east, the syncline abuts against buff-coloured, NS trending and vertical dipping beds of the Chilpi Group (figure 2j, Rao 1981). This contact between Khairagarh Group and the Chilpi Group is a major fault, which is evident from occasional brecciated and silicified outcrops from Dhara to Karelagarh (figure 1d). Due to its prominent exposure near Lachhna, we name this fault as ‘Lachhna Fault’. A NS trending sinistral ‘Borla Fault’ was traced in the field, which runs parallel to Lachhna Fault (figure 1d).

4 Digital elevation map

Figure 3 is a digital elevation map (DEM) of Sitagota syncline, and its adjacent area prepared from United States Geological Survey (USGS) earth explorer digital data of terrain. The digital data was plotted on ArcGIS Arc Scene application to get a DEM of Sitagota syncline. DEM represents absolute vertical elevation of the earth surface that excludes any surface objects and has vertical accuracy of around 0.82 m (www.usgs.gov). DEM reveals volcano-sedimentary sequence underwent more than one phase of compression and orogeny post its deposition. Syncline is elevated towards the centre with a maximum elevation of around 800 m (2625 ft). The red-labelled elevations (>600 m) show a regional NS trend irrespective of formations of the Khairagarh Group, which indicates an EW compressive force probably acted on the volcano-sedimentary sequence giving rise to a NS trending dome/anticline structure.

Figure 3
figure 3

Digital elevation map of Sitagota syncline plotted on ArcGIS from terrain digital data downloaded from USGS earth explorer.

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5 Geochemistry of Khairagarh volcanics

5.1 Petrography

Petrography of Mangikhuta and Kotima volcanic samples collected from the centre of the syncline reveals the presence of quenched, sub-variolitic, and subophitic-to-ophitic relict igneous textures. Vesicles are filled with chalcedony and calcite (figure 4a). Some samples show pseudomorphs after olivine (?) in a matrix of quenched pyroxenes and plagioclase, indicating relict porphyritic textures (figure 4b). Flow top samples reveal relict quenched texture (figure 4c). Interior of the flow samples reveal subophitic to ophitic relict igneous textures where prismatic altered euhedral crystals of plagioclase are partially to completely enclosed by fresh magmatic clinopyroxenes, which indicates cotectic precipitation of clinopyroxene and plagioclase (figure 4d). Plagioclase occurs both as microphenocrysts and in groundmass, shows twinning and occasional sericitization. The glassy matrix is altered to secondary mineral aggregate consisting of chlorite, zoisite, clinozoisite, prehnite and pumpellyite. Few samples contain a good number of opaque minerals that occur randomly throughout the section. The reflected light study of opaque minerals indicates they are typical magmatic oxides predominantly Ti-magnetite, skeletal magnetite, vanadiferous magnetite, ilmenite, and hematite (figure 4e). Other than these typical magmatic oxide mineral assemblages, some sulfide minerals like pyrrhotite, pyrite and chalcopyrite grains are also seen in some samples (figure 4f) (c.f. Khare et al. 2024).

Figure 4
figure 4

Photomicrographs of Khairagarh volcanics in transmitted (a, b, c, d) and reflected (e, f) light. (a) Photomicrograph (×10) of Khairagarh volcanics showing presence of chalcedony as vesicle filling mineral. (b) Pseudomorphs after olivine (?) in a quenched groundmass forming relict porphyritic texture. (c) Photomicrograph (×10) of quenched textures obtained from Khairagarh volcanics flow top, formed due to rapid cooling of lava on surface that led to formation of dendritic pyroxene grains. (d) Grains of fresh unaltered pyroxenes, altered plagioclase and opaques showing sub-ophitic to ophitic relict textures (×10). (e) A reflected light photomicrograph showing typical euhedral magnetite grains exhibiting skeletal to exsolution features with laths of ilmenite occupying the octahedral cleavage planes of magnetite. (f) A reflected light photomicrograph showing magnetite grain along with ilmenite lamellae as an exsolution feature substituting the octahedral cleavage of magnetite. Other minerals include aligned pyrrhotite and chalcopyrite as ductile assemblages.

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5.2 Whole rock analytical techniques

Nine non-vesicular, unweathered, fine-grained and least altered Mangikhuta and Kotima volcanic samples collected from Sitagota syncline (location given in figure 1d) were chosen for whole-rock geochemical analyses. Major oxides and trace element concentrations were measured on a Wavelength Dispersive X-ray Fluorescence Spectrometer (WD-XRF; Bruker, Tiger S8) at the Wadia Institute of Himalayan Geology (WIHG), Dehradun, India. Precision of analysis is estimated to be ±2–3% for major oxides and ±5–6% for trace elements. Rare earth elements (REE) and selected trace element concentrations were analyzed with Perkin-Elmer SCIEX ELAN DRC-e Inductively Coupled Plasma Mass Spectrometer (ICP-MS) at WIHG, Dehradun. Accuracy from 2 to 12%, and precision varying from 1 to 8% were obtained. Details of whole rock XRF and ICP-MS analytical techniques are given elsewhere (c.f. Saini et al. 2000; Khanna et al. 2009). Whole rock major, trace, LOI, REE composition and CIPW norm of the Khairagarh volcanic samples are given in table 2.

Table 2 Whole rock major, trace and REE analysis data of Khairagarh volcanic samples collected from centre of syncline, location given in figure 2(d). Normative mineralogy is given in the table.
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5.3 Geochemistry

CIPW norm mineralogy reveals all Khairagarh volcanic samples chosen for analysis are quartz normative. Plagioclase, orthopyroxene, clinopyroxene, orthoclase, and Fe-oxide are the other major precipitating minerals inferred from norm mineralogy. Total alkali content of Khairagarh volcanics is low (Na2O + K2O < 4.17 wt%), indicating their sub-alkaline nature. SiO2 content from 51.19 to 55.25 wt% indicates basalt to basaltic andesite composition. On the TAS plot of Cox et al. (1979), Khairagarh volcanics plot in the field of sub-alkali basalt, basaltic andesite to andesite (figure 5). Low Nb (<10 ppm) and low Nb/Y ratios (0.1–0.4) corroborate their sub alkaline tholeiitic to calc-alkaline nature (Pearce and Cann 1973; Winchester and Floyd 1977; Hastie et al. 2007). On Nb/Y vs. Zr/Ti plot after Pearce (1996), Khairagarh volcanics plot in the field of sub-alkali basalt to basaltic andesite (figure 6). TiO2 content is overall low (0.48–0.76 wt%), which indicates they belong to the low-Ti suite of Asthana et al. (1996). MgO content is high (6.9–11.2 wt%) while Al2O3 is relatively low (9.15–14.01 wt%). On Al2O3 vs. MgO plot after Kersting and Arculus (1994), Khairagarh volcanics plot in the field of high-Mg basalt (figure 7), similar to Boninite like composition reported from modern day arcs (Murton 1989; Taylor et al. 1994; Li et al. 2022). CaO varies from 4.8 to 10.9 wt%, while FeO(T) varies from 10.2 to 11.6 wt%. On the AFM plot after Winchester and Floyd (1977), Khairagarh basalt shows a moderate iron enrichment trend indicating tholeiitic to transitional affinity (figure 8). However, on TYC plot after Davies et al. (1979), they show characteristic transitional to calc-alkaline trend (figure 9). Due to immobility of Cobalt and Th in altered rocks, Co vs. Th plot (Hastie et al. 2007) is drawn for Khairagarh basalt (figure 10). On this plot, which acts as proxy to K2O–SiO2 plot, Khairagarh volcanics plot in calc-alkaline field. Therefore, based on relatively immobile element plot findings, we assign transitional to calc-alkaline affinity to Khairagarh volcanics. Overall, the whole rock composition indicates sub-alkaline, low-Ti, high Mg tholeiitic, transitional to calc-alkaline basalt-basaltic-andesite to andesite composition of Khairagarh volcanics.

Figure 5
figure 5

TAS plot of Khairagarh volcanics after Cox et al. (1979). Brown data points in plot taken from Khare and Asthana (2020). Khairagarh volcanics are sub-alkaline and plot in the fields of basalt, basaltic andesite to andesite.

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Figure 6
figure 6

Nb/Y vs. Zr/TiO2 plot of Khairagarh volcanics after Pearce (1996). Brown data points in the plot were taken from Khare and Asthana (2020). Khairagarh volcanics plot in the field of basalt and basaltic andesite.

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Figure 7
figure 7

Al2O3 vs. MgO plot of Khairagarh volcanics after Kersting and Arculus (1994). Brown data points in the plot were taken from Khare and Asthana (2020). All samples plot in the field of high-Mg basalt.

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Figure 8
figure 8

AFM plot of Khairagarh volcanics after Winchester and Floyd (1977). Volcanics show moderate iron enrichment tholeiitic trend. Brown data points in the plot were taken from Khare and Asthana (2020).

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Figure 9
figure 9

TYC plot of Khairagarh volcanics after Davies et al. (1979). Volcanics show an overall trend transitional between tholeiitic and calc-alkaline. Brown data points in the plot were taken from Khare and Asthana (2020).

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Figure 10
figure 10

Co vs. Th plot of Khairagarh volcanics after Hastie et al. (2007). All samples plot in the field of calc-alkaline basalt. Brown data points in the plot were taken from Khare and Asthana (2020).

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The chondrite-normalized REE plot is drawn for Khairagarh volcanics (figure 11) with normalization values taken from McDonough and Sun (1995). The plot shows moderate LREE enrichment with respect to HREE. (La/Sm)N varies from 2 to 3.4, which indicates moderate enrichment of LREE with respect to MREE either due to magma source enrichment with an LREE-enriched melt/fluid or enrichment of LREE due to magma fractionation. (Gd/Lu)N varies from 1.3 to 1.8 indicating least impact of fractionation and partial melting on their relative abundance. Negative Eu anomaly indicates plagioclase fractionation from the melt, also evident from petrography and normative mineralogy.

Figure 11
figure 11

Chondrite normalized REE plot of Khairagarh volcanics with normalization values taken from McDonough and Sun (1995). Mild enrichment of LREE and negative Eu anomaly seen in the plot.

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6 Nature of mantle source

HFSE ratios remain unchanged due to various degrees of partial melting, fractional crystallization, alteration, and low-grade metamorphism; therefore, they give insights into the nature of mantle source. Table 3 presents HFSE ratios of Khairagarh volcanics, Fertile MORB Mantle (FMM), Primitive Upper Mantle (PUM), Enriched Upper Mantle (EUM) and Ocean Island Basalt (OIB) (c.f., McDonough and Sun 1995). Overall, HFSE ratios indicate mantle source with an elemental ratio close to PUM. CaO/TiO2 ratios vary from 6 to 23, while Al2O3/TiO2 vary from 15 to 22, which indicates a non-refractory fertile mantle source of Khairagarh volcanics.

Table 3 HFSE ratios of Khairagarh volcanics compared with HFSE ratios of fertile MORB mantle (FMM), Primitive Upper Mantle (PUM), Enriched MORB (EMORB) and Ocean Island Basalts (OIB). The HFSE values taken from McDonough and Sun (1995).
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7 Discussion

The beginning of the Proterozoic era in the northern Bastar Craton was marked with widespread Nandgaon bimodal volcanism, Dongargarh granite intrusion, followed by mafic magmatism in the Khairagarh basin. U–Pb dating by Manikyamba et al. (2016) has revealed near similar ages of Dongargarh granite (~2.5 Ga) and Bijli rhyolite (~2.48 Ga). U–Pb zircon geochronology by Khanna et al. (2019) has revealed the 2463 ± 45 Ma age of the Bortalao Formation, which marked the first phase of sedimentation in the Khairagarh basin. Bortalao Formation and its basement have near similar age, which indicates a major rapid crust formation event in northern Bastar Craton at around 2.4–2.5 Ga, which led to intrusion of Dongargarh granite, bimodal Bijli rhyolite volcanism and rifting of continental crust, resulting in formation of Khairagarh basin. U–Pb dating of Khairagarh sediments by Khanna et al. (2019) has revealed ~2453 Ma age of Karutola sandstone and ~2210 Ma age of Ghoghra sandstone, which indicates that sedimentation and volcanism in Khairagarh basin continued for around 243 million years.

7.1 Tectonomagmatic model

Our geochemical work reveals quartz normative, sub-alkaline, transitional to calc-alkaline, high-Mg basalt to basaltic andesite composition of Mangikhuta and Kotima volcanic formations. NMORB normalized incompatible element plots drawn for Khairagarh volcanic samples (figure 12) reveal negative anomaly of Nb with respect to Th, U and K2O. Enrichment of LILE (K2O, U, Th) and LREE (Ce, Nd, Sm) compared to HFSE of similar compatibility (Nb, Zr, Ti and Y) indicates enrichment of LILE and LREE in the mantle source of Khairagarh volcanics, which could be attributed to either fluid flux enrichment of the mantle source or to metasomatic and magma mixing processes. Slight enrichment of LREE with respect to HREE in REE plot (figure 11) can be attributed to both arc and continental rift setting. On Co vs. Th plot (figure 10), Khairagarh volcanics plot in calc-alkaline field indicating their genesis either in supra-subduction zone or due to magma mixing. On Zr vs. Ti plot (figure 13), Khairagarh volcanics plot in field of arc lava. However, on Ti vs. V plot (figure 14), Khairagarh volcanics plot in field transitional between arc and MORB. Ti/V in some samples is more than 20, which indicates their calc-alkaline character. From such contrasting geochemical signatures, we conclude that tectonomagmatic inferences solely drawn from geochemical data of volcanics can lead to misleading inferences, if not substantiated by field relations. Besides, tectonomagmatic models for the genesis of Phanerozoic igneous provinces cannot be superimposed on Archean Paleoproterozoic provinces. Several previous workers (Asthana et al. 1996; Neogi et al. 1996; Khanna et al. 2019) have inferred rift basin, back-arc basin to Andean type supra-subduction zone tectonic environment for Khairagarh Group. Neogi et al. (1996) inferred stable continental margin environment of Khairagarh sediments, which is distinct from trench/arc sediments. Field investigation of the Sitagota syncline reveals the presence of intertrappean ferruginous shales and thin Algoma-type BIF beds in Mangikhuta basalt, besides the presence of sulphide and oxide mineralization in volcanics. Such BIFs are chemically precipitated sediments with alternate thin layers of Fe minerals and chert (Gourcerol et al. 2016). Local deposition of Algoma-type BIF is a common feature of Eoarchean Paleoproterozoic volcano-sedimentary sequences in greenstone belts. Such a BIF deposition model corresponds to semi-closed Khairagarh basin with extensive volcanism and hydrothermal fluids (c.f. Bekker et al. 2010; Gourcerol et al. 2016). Radiating mafic dyke swarms reported in this area have geochemical and geochronological similarities with the Khairagarh volcanics. Such dykes indicate a rift rather than subduction zone tectonic environment for Khairagarh basin. Therefore, based on geochronology, field geological and geochemical data, we infer rifting in stable continental crust led to the genesis of the Khairagarh basin, which led to the deposition of continent-derived sediments and massive mafic magmatism (figure 15a). The presence of calc-alkaline and quartz normative composition of Khairagarh volcanics is consistent with an evolved or late stages of a continental rift. Arc-related geochemical signatures of Khairagarh mafics could be due to its spatial and temporal proximity to Dongargarh Kotri mobile belt, which some workers infer as a paleo subduction zone. From a modern plate tectonics perspective, the rapid opening of stable continental crust along a continental margin could be due to the formation of a back-arc basin, although the nature of plate tectonics, size of plates, and prevailing geothermal gradients in Archean Proterozoic times vis-a-vis Phanerozoic are little understood.

Figure 12
figure 12

NMORB normalized incompatible element plot of Khairagarh volcanics. Th and U are enriched with respect to Nb. Ce is enriched with respect to Sr. Ti is depleted with respect to elements of similar compatibility. Normalization values are taken from Pearce and Parkinson (1993). Brown curves are plotted from Khare and Asthana (2020).

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Figure 13
figure 13

Khairgarh volcanics plotted on Zr vs. Ti plot after Pearce (1982). Ti shows coherent increasing trend with fractionation. All samples plot in the field of arc lava. Brown data points in plot taken from Khare and Asthana (2020).

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Figure 14
figure 14

Ti vs. V plot of Khairagarh volcanics after Shervais (1982). Brown data points in plot taken from Khare and Asthana (2020).

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Figure 15
figure 15

Tectonomagmatic model of Khairagarh basin illustrating (a) rifting, sedimentation, magmatism and formation of radiating dykes, (b) closure of Khairagarh basin due to EW compressive forces of Dongargarh Kotri mobile belt and (c) formation of regional Sitagota syncline structure due to Mesoproterozoic central Indian tectonic zone (CITZ).

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7.2 Post emplacement deformation

Our current and previous work reveals the ubiquitous presence of chlorite, zoisite, prehnite, and pumpellyite mineral aggregates in Khairagarh volcanics, which, along with the absence of schistosity and foliations in them, indicates their very low-grade regional metamorphism. Previous work by Patel et al. (2008) and Khanna et al. (2019) has established alteration and very low-grade metamorphism of Khairagarh volcanics. However, field investigation and regional structures inferred from DEM indicate several phases of orogeny undergone by volcano-sedimentary succession post its deposition in the Khairagarh basin. Khairagarh sediments gradually mature from the lower Bortalao to the upper Karutola and Ghoghra formations. The lower Bortalao Formation consists of conglomerates, arenites, arkosic sandstone, arenites, upper conglomerates, and tuffaceous sandstones (Sarkar 1957, 1994) whereas upper Karutola and Ghoghra sandstone formations consist of quartz arenite and sub-arkosic arenite (Chakraborty and Sensarma 2008; Khanna et al. 2019). Sarkar et al. (1994) and Sensarma (2007, 2011) reported relation of Mangikhuta and Sitagota volcanics through fractionation, although several previous workers (Sarkar 1957; Rao et al. 1981; Longjam and Talat 2012) classified each volcanic unit as a separate lithostratigraphic unit. Sensarma and Mukhopadhyay (2014) reported the absence of any discordance between the volcanic and sedimentary formations of the Khairagarh Group. Khanna et al. (2019) inferred that Khairagarh volcanics formed in a few tens of million years, although no geochronology data exists for Khairagarh volcanics. Our previous relict clinopyroxenes chemistry study (Khare and Asthana 2020; Khare et al. 2022) indicates the formation of Mangikhuta high-Mg andesite from fractionation of Kotima high Mg-basalt, which supports the findings of Khanna et al. (2019). It also validates the findings of Sensarma and Mukhopadhyay (2014) that volcanic are related by fractionation. With this perspective, we present a tectonomagmatic model (figure 15) for the genesis of Khairagarh volcano-sedimentary basin. This model proposes rapid opening and closure of continental crust in the northern Bastar Craton from ~2.46 to 2.2 Ga. EW trending extension tectonics and rifting in the northern portion of Bastar Craton around 2.46 Ga led to the formation of the Khairagarh basin in which Bortalao sediments were deposited near the shore, whereas Karutola and Ghoghra sediments were deposited away from the shore (figure 15a). The Bortalao and Karutola sediments give overlapping U–Pb zircon ages (around 2.51–2.4 Ga and 2.48–2.4 Ga, respectively). The sediments were derived from a continental basement made of mafic and felsic rocks (Sarkar 1957, 1994; Sensarma and Mukhopadhyay 2014). Magmatism from the centre of the basin led to the deposition of primitive Kotima basalt towards the centre and fractionated Mangikhuta basaltic andesite towards the shore (c.f. Khare and Asthana 2020; Khare et al. 2022). The discordance between sediments and volcanics is not reported by any worker. Rifting in continental crust was followed by volcanism and sedimentation. Later onset of EW compression led to the closure of the Khairagarh basin (figure 15b). We correlate this opening and closure of Khairagarh basin to a NS trending Paleoproterozoic Dongargarh Kotri mobile belt in the Bastar Craton (c.f. Ramakrishnan and Vaidyanathan 2010). Later, the second phase NS trending compressive regime led to the formation of NE–SW trending Sitagota syncline with the previous generation folds forming its two limbs (figure 15c). The second phase compression led to the deformation and development of 12 major NS trending faults affecting the limbs and hinges of the syncline (Sarkar 1957). The NS trending Darekasa fault in the western limb of syncline (Sarkar 1957) and another prominent NS trending Lachhna fault towards the east of syncline indicate a regional NS compressive force operating in northern Bastar Craton, which was probably due to the collision of north Bastar Craton with Bundelkhand Craton manifested as a prominent EW trending Mesoproterozoic Central Indian Suture Zone (Yedekar et al. 1990; Acharyya 2003). Ghosh et al. (2006) performed microstructural analyses and grain-scale measurements of Sitagota syncline sediments and reported two sets of cleavage fabrics peculiar to a low-grade low-strain belts. The ubiquitous presence of very low-grade metamorphic mineral assemblages (prehnite, pumpellyite, chlorite) in Khairagarh volcanics indicates that the Mangikhuta, Kotima and lower Sitagota volcanic formations did not undergo high P–T transformation, and the alteration and very low-grade metamorphism took place at near surface conditions.

7.3 Global implications

Previous workers (Rogers and Santosh 2003; Longjam and Talat 2012) relate Paleoproterozoic rapid crustal growth of Bastar Craton to its association with Ur supercontinent. However, paleomagnetic dyke swarm data of neighbouring Singhbhum Craton in the Indian shield indicates its ancestry to Vaalbara supercontinent (Kumar et al. 2017). Rifting and mafic magmatism similar to Khairagarh basin is reported from coeval Lower Wyloo Group in Pilbara Craton and Hekpoort and Ongeluk basalt formations of Transvaal basin in Kaapvaal Craton (Eglington and Armstrong 2004; Humbert et al. 2018, 2020), which constitute Vaalbara supercontinent. Therefore, we propose areal extent of Vaalbara supercontinent (de Kock et al. 2009; Smirnov et al. 2013; Kampmann et al. 2015; Kumar et al. 2017) from Kaapvaal Craton in south Africa through Bastar Craton and Singhbhum Craton in Indian shield to Pilbara Craton in Australia in the Neoarchean Paleoproterozoic (2.8–2.1 Ga) times.

8 Conclusion

The following conclusions are drawn from the geological study of Khairagarh Group rocks in Sitagota syncline.

  1. 1.

    The volcanic rocks towards centre of syncline are sub-alkaline, low-Ti, high-Mg basalt to basaltic andesite with geochemical characteristics varying from tholeiitic, transitional to calc-alkaline. The mantle source of volcanics was enriched to primitive upper mantle.

  2. 2.

    Fe-oxide and Cu-sulphide mineralization is reported in Mangikhuta basalt formation of Khairagarh Group. Intertrappean shale and Algoma-type BIF beds are present in this volcanic formation. Tectonomagmatic model indicates rifting of stable continental crust led to sedimentation, massive mafic magmatism, and genesis of Khairagarh Group.

  3. 3.

    The Sitagota syncline is elevated towards the centre with a maximum elevation of around 800 m. The Khairagarh basin underwent EW and NS compression and orogeny probably related to Paleoproterozoic DKMB and Mesoproterozoic CITZ, respectively. NS compression led to the genesis of sinistral NS trending Lachhna, Borla and Darekasa faults. However, Khairagarh volcanic formations did not undergo high P–T transformation and most of the alteration and very low-grade metamorphism took place at near surface conditions.

  4. 4.

    The areal extent of Vaalbara supercontinent extended from Kaapvaal Craton in south Africa through Bastar Craton and Singhbhum Craton in Indian shield to Pilbara Craton in Australia in the Neoarchean Paleoproterozoic (2.8–2.1 Ga) times.