Geochemistry, Petrogenesis and Tectonic Setting of the Mafic Dykes from the Amgaon and Khairagarh Regions, Bastar Craton, Central Indian Shield: Constraints on the Precambrian Crustal Evolution

Mafic magmatism is a major contributor for the evolution of the crust from the early earth to the present. The nature of the magmatism changed from more mafic (komatiitic) to more basaltic and andesitic variants [1-4]. In the early history of the earth mafic magmatic rocks have been preserved in the granite greenstone sequences and dykes/dyke swarms to more recent large igneous provinces (LIP). It is known that the early earth was hotter and therefore mafic magmatic activities were rampant and various estimates indicate that nearly 80% of the crust was formed during the Archaean [3,5]. It is observed that the mafic volcanic rocks were preserved in the greenstone belts or Precambrian continental rift basins [6,7], but in most of the cratons where the basement gneisses are exposed, we observe only the dykes and dyke swarms and no volcanic equivalents are observed because of severe erosion to the deeper levels exposing the basement rocks [8-12]. It is possible that during the early earth there were numerous LIPs and their equivalents were present but these got eroded with time [13-16]. Because of the availability of higher temperature, more magma was generated from the evolving mantle that contributed to the formation of the crust [17-19]. Mafic and ultramafic magmatism poured mafic magma on the oceanic floor via the divergent plate boundaries, but rocks on the ocean floor are not older than 200 My [5,20,21]. Thus, we have to focus our studies on the continental crust, which preserves the oldest rocks to younger rocks produced at the continental arc, rift basins and LIPs [2,5].The Bastar Craton has preserved enormous dykes, dyke swarms and even major sills emplaced at deeper levels and now exposed along with the basement rocks as the supra-crustal rocks have disappeared because of long term erosion [12, 22-26], although some of the supracrustal rocks including volcanic flows are preserved in the rift basins such as the Sakoli belt volcanics and Khairagarh supracrustal volcanics [22,27,28]. We present here new geochemical data (major, trace including rare earth elements (REE) on the mafic dykes from the Amgaon and Khairagarh regions of the Bastar Craton (Figure 1). We have also reviewed the published data on the mafic magmatic rocks (dykes, sills and flows) and compared these with our studied rocks to put better constraints on the Precambrian crustal evolution of the Bastar Craton.

Figure 1 (a): Outline map of India showing the Central Indian Tectonic zone (CITZ) and mobile belts of the Indian shield. It is showing the study area marked as the Bastar Craton [29]. It is showing the study area marked as the Bastar Craton.

Figure 1 (b): Geological map of the part of Central Indian Shield (modified after [30,31]) showing the study area and the various lithologies.

The Central Indian Bastar Craton essentially comprises Archaean granitoid basement, with multiple supracrustal and volcanic sequences. The diverse lithological assemblages of Bastar Craton are made up of basement gneisses, namely, Tirodi, Amgaon, Bengpal, and Sukma gneisses, these gneisses have components of Tonalite-trondhjemite-granodiorite (TTG) and melts of diverse compositions [15,32-37]. Supracrustal sequences of Bastar Craton are preserved in Sausar, Dongargarh, Sakoli, Khairagarh and Mahakoshal Supergroup, comprising sequences of metavolcanic and metasedimentary rocks [28,38-40]. Granulites from Balaghat-Bhandara and Ramakona-Katangi Ganulite belts associated with Central Indian Suture (CIS) zone [41,28] and Bhopalpatnam and Karimnagar granulite belts are exposed in Western Bastar [36,42-46]. Mafic dyke swarms of Bastar Craton include NWSE trending Keshkal swarm, Bhanupratappur swarm, Dantewara swarm, and Bastanar swarm, NNW-SSE trending Sonakhan swarm, N-S trending Lakhna dykes, ENE-WSW trending Bandimal dykes are well studied [11,22,42,47-57]. Dongargarh, Malanjkhand, Abhujmar and Mul granites represent the diverse Proterozoic granites from Bastar Craton [37,56,58-62]. 

Hazarika et al., (2020) have discussed the geochemistry of mafic dykes from Amgaon area, particularly from southern margin of central Indian suture (CIS), with those of northern Bastar Craton Lakhna (1.46 Ga) and Bandimal (1.42 Ga) dykes and inferred a widespread mafic magmatic event across the Bastar Craton circa.1.42–1.46 Ga. The dykes from Amgaon region represent a subduction related outgrowth of Columbia supercontinent due to the accretion of continental margins [55,56]. 

Hazarika, et al., (2019) identified mafic dykes of Boninitic compositions from Bastar Craton, earlier described by [63]. This boninite magmatism indicates the presence of remnant subduction-related signatures in the lithospheric mantle formed during the evolution of the Bastar Craton [56]. Dongargarh Supergroup records a significant portion of the evolutionary history of Bastar Craton. This supergroup comprises a sequence of metavolcanic and metasedimentary sequences of Nandgaon Group and Khairagarh Group overlying the basement of Amgaon gneisses. Nandgaon Group comprises of Bijli rhyolites and Pitepani volcanics, whereas Khairagarh Group comprises volcano sedimentary sequences [64,65]. The volcanic rocks of Khairagarh volcanic sequence are represented by variable Ti basalts [66] and basaltic andesite series that indicate partial melting of an enriched mantle source. The occurrence of the two contrasting sequences probably indicates generation in a hot Andean-type subduction zone for the High Magnesian Andesites (HMA) and Andean-type back-arc rifting for the basalt–basaltic andesite samples [28]. 

Present work is based on the geochemical study of Proterozoic mafic dykes from the Amgaon region, south of CIS and Khairagarh region of the Dongargarh Super group (Figure 1). These are doleritic dykes depict ophitic to sub-ophitic texture. Dominant mineralogy is plagioclases (andesine-labradorite) and pyroxene (dominantly augite) with minor olivine, opaques (magnetite-ilmenite), amphiboles and chlorite.

Major and trace elements (Table 1) were determined using WD-XRF (Pan Analytical- Axios) at the Department of Geology, University of Delhi, India. The accuracy of the analyses for the major elements is better than 1% for SiO₂ and 2% for other major elements, 2–5% for minor elements and better than 10% for trace elements. International rock standards used for calibrations are BHVO, JGB-1, JB-1A, JB-2, JB-3, JA-2, MB-H, AM-H, PM-S, GeoPT4 and GeoPT11 [28].  Some of the trace elements and the rare earth elements (REE: (Table-2) were analyzed by ICPMS (ELAN 6000 DRC-e, Perkin Elmer, Waltham, MA, USA) at the Indian Institute of Technology, Roorkee. Two USGS rock standards GSP-2 and AGVO-2 were used for calibration for the ICP-MS following the procedure described by [67]. 

Geochemical data on the studied dykes show groupings in terms of silica distribution, in the form of low and high silica groups (Table 1) (Figures 2 and 3). Majority of the samples fall in the low silica group where the SiO₂ contents vary from 48.32 to 54.19 wt%. The high silica samples show SiO₂ variation from 59.61 to 66.66 wt%, with a clear compositional gap. Al₂O₃ shows restricted distribution from 11.53 to 14.49 wt%. Fe₂O₃ shows large variation between 5.21 and 15.83 wt%. The high silica samples have restricted Fe₂O₃ between 5.21 and 8.61wt% and MgO varies between 3.98 and 9.87 wt%. The CaO contents for the low silica rocks varies between 6.76 and 11.58 wt% and for high silica rocks it is quite variable from 1.19 and 6.19 wt%. TiO₂ contents are low for the high silica rocks (0.46-0.81 wt %) and not so different for the low silica rocks (0.9-1.47 wt %) (Figure 2). As expected Ni is lower in the high silica rocks (45-149 ppm) but much higher for the low silica rocks, mostly in te range from 100-200 ppm, but overall it varies from 85 to 468 ppm.  Cr is quite variable from 95-~300 ppm for high silica but from nearly 300 to 930 ppm for the low silica rocks. Sc is lower for the high silica rocks (11-20 ppm) but higher for the low silica rock ranging from 24 to 50 ppm. As expected, Zr is much higher for the high silica rocks (225-492 ppm) compared to the low silica rocks having 72-314 ppm, mostly in the range of 72 to

Table 1: Major (wt%) and trace element (ppm) analyses for representative mafic dykes from the Aamgaon and Khairagarh region. Samples are classified as: Low Silica Group-1 (CP62, CP62A, CP142, CP128, CP114, CP28, CP36, and CP37); Low Silica Group-2 (CP306, CP7, CP147, CP22, CP19, CP47); and High Silica (CP109, CP55, CP18, CP113, CP29). LOI, loss on ignition; magnesium number, Mg# = Cationic (Mg*100/Mg+Fe).

Figure 2: MgO versus major elements Binary plot of the studied rocks (Low Silica Group-1, Low Silica Group-2 and High Silica).

 Figure 3: MgO versus trace elements Binary plot of the studied rocks (Low Silica Group-1, Low Silica Group-2 and High Silica).

 Figure 4: Zr versus trace elements Binary plot of the studied rocks (Low Silica Group-1, Low Silica Group-2 and High Silica).

Table 2: Rare Earth Element (ppm) analytical data for representative mafic dykes from the Aamgaon and Khairagarh region. Classification of samples are same as described in (Table-1).

In terms of the rare earth elements (REEs), the high silica are much more enriched compared to the low silica rocks. Whereas the high silica rocks have enrichment of LREE from ~250 to >400 times chondrite, the low silica rocks have LREE enrichment from ~50 to ~100 times chondrite (Fig. 5a: normalising values after Sun and Mc Donough, 1989). The (La/Sm)_N, (La/Gd)_N, (La/Yb)_N and (Gd/Yb)_N for the high silica rocks vary from 5.6 to 5.1, 11.58 to 9.07, 55.16 to 28.79 and 4.7 to 3.17 respectively.

Figure 5a: Chondrite-normalized rare earth elements (REEs) plot shows enriched characteristics for the studied rocks. High Silica rocks shows higher abundances compared to Low Silica Group-1 and Low Silica Group-2 samples. Normalizing values are from [68].

For the low silica rocks, these ratios are 2.55 to 4.72, 2.68 to 6.99, 3.59 to 9.45 and 1.33 to 1.36 respectively.  Thus, it is very clear that the high silica rocks have much higher LREE enrichment and much higher LREE/HREE ratios compared to the low silica rocks, probably indicating their derivation from different sources or these two rock types have undergone very different petrogenetic histories (Figure 5a). The multi-elements spidergram patterns for the studied samples show strong negative anomalies for Nb, P, Ti and equally strong positive anomalies for Pb and to some extent Zr (Figure 5b). Like the REE patterns (Figure 5a) the multi-element spidergram patterns show that the high silica samples have much higher abundances of incompatible trace elements compared to the low silica samples (Figure 5b).

Figure 5b: Primitive mantle-normalized multi-element plot shows overall enriched pattern with strong negative anomalies for Nb, P, Ti and strong positive anomalies for Pb. Normalizing values are from [68].

Geochemically majority of the studied samples are basalt to basaltic-andesite composition, but a few samples with higher SiO₂ classify as andesites based on TAS (Total Alkali vs Silica: [69,70] diagram (Figure 6a). The high SiO₂ andesitic samples plot separately with a compositional gap. In terms of the trace elements ratios (Nb/Y vs Zr/TiO₂: [71]) the studied samples again classify as sub-alkaline basalt to basaltic andesite and the high SiO₂ samples classify as rhyodacite (Figure 6b). Like figure 6a the high silica samples plot separately with a compositional gap with higher Zr/Ti ratios (Figure 6b). For comparison we have considered published data on Proterozoic mafic magmatic rocks (dykes and flows) from the adjoining areas (Sakoli volcanics: [72], Amgaon Gneissic Complex basaltic and boninitic dykes [55,56], Amgaon Gneissic Complex amphibolitic and doleritic dykes [38] and Khairagarh volcanics [28]. Texturally and mineralogically they are similar to the mafic magmatic rocks of the adjoining areas, as described by the earlier workers [22,26,28,38,55,56,72-74]. Geochemically these mafic magmatic rocks from the adjoining areas depict subalkaline basalt-basaltic andesite to andesite-rhyolite as observed in our studied samples (Figures 6c and 6d), however, volcanics the samples from the adjoining regions depict much larger compositional variations, especially the Sakoli bi-modal, because they represent different tectonic [72].

Figure 6a: SiO₂ versus Na₂O + K₂O (TAS diagram) [69] shows majority of the studied samples (Low Silica Group-1 and Low Silica Group-2) are basalt to basaltic- andesite while High Silica samples classify as andesites.

Figure 6b: Nb/Y versus Zr/TiO₂ plot [75] shows studied samples (Low Silica-Group-1 and Low Silica Group-2) classified as sub-alkaline basalt to basaltic andesite while High Silica samples are classified as rhyodacite.

Figure 6c: SiO₂ versus Na₂O + K₂O (TAS diagram) shows that mafic magmatic rocks from the adjoining areas represent basalt-basaltic andesite to andesite- rhyolite as observed in our studied samples.

Figure 6d: Nb/Y versus Zr/TiO₂ plot [75] of the mafic magmatic rocks from the adjoining areas depict alkaline basalt to sub-alkaline basalt, basaltic andesite to andesite and Rhyodacite as observed in our studied samples.

In a series of simple but informative bi-variant plots using MgO wt% and Zr ppm concentrations against other major and trace elements are presented here for the studied sample (Figures 2-4). These elements are used as indices of fractionation to understand the relationships between various elements during petrogenetic processes in basaltic magma. Zircon is least mobile trace element that does not get perturbed during the alteration and low grade metamorphism that these rocks have undergone and is essentially incompatible in basaltic systems [76]. In the MgO vs major elements plots (Figure 2), we observe at least three sub-trends, one for the high silica group and two sub-trends for the low silica basaltic rocks, probably indicating that all the studied rocks were not derived from the same parental melt, rather they represent independent magma batches or they represent heterogeneous sources (Figure 2). Most of the elements show negative trends against MgO except CaO which shows positive trends, probably indicating coprecipitation of olivine and clinopyroxene. Na₂O and K₂O show somewhat scattered trends, which could be partly related to alteration effect and partly source heterogeneity. The MgO vs compatible trace elements Ni, Cr, V and Sc show positive trends probably indicating coprecipitation of mafic phases (olivine, pyroxene, amphiboles) and MgO vs incompatible trace elements Zr, Nb, Y, Sr and Rb show negative trends, indicating little effects of alteration on these trace elements (Figure 3). To further assess the possibility of alteration on the studied rocks, we have plotted various trace elements against Zr, which is considered robust during alteration and low grade metamorphism, thus categorized as high field strength elements (HFSE). The plots of P, Ti, Y, Nb, Rb, Ba, La and Nd show positive trends and Ni shows the expected negative trend against Zr, indicating less perturbed nature of these trace elements (Figure 4).  

The binary plots of MgO wt% and Zr ppm against compatible and incompatible trace elements indicate little effect of secondary alteration processes on the studied samples and depict sub-trends as observed in the MgO vs major elements (Figure 2), indicating source heterogeneity or their derivation from independent parental magma. The Zr and Ni concentration vary from less than 100 ppm to about 500 ppm Zr and from ~470 ppm to 45 ppm Ni in the studied samples (Figures 3 and 4). If all the samples were derived from the same or similar parental melt, then at least 80% fractional crystallization will be required but no such evidence is reflected in our petrographic studies. We observe for example wide range of concentrations for Al₂O₃ and CaO for a given MgO concentration, similarly we observe wide variation in the concentration of Ni and Cr for similar Zr levels, all these observations indicate that the studied samples were derived from different batches of primary magmas with different levels of MgO and Zr or these were derived from heterogeneous sources (Figures 2-4). The incompatible trace elements ratios for example Zr/Nb, Nb/La, Sm/Nd, Zr/La etc. do not change with moderate degrees of crystal fractionation but are quite sensitive to varying degrees of partial melting, source heterogeneity and to some extent crustal contamination. On the other hand, the ratios of compatible/incompatible trace elements e.g. Ni/Zr, Cr/Zr, V/Zr etc. are quite sensitive to fractional crystallization, our data indicate that the studied samples have undergone varying degrees of partial melting of heterogeneous sources, followed by varying extent of fractional crystallisation of gabbroic assemblages [7]. 

In the plot of Zr/Y vs Zr (ppm) the studied samples and those from the adjoining areas are plotted to understand their source characteristics, degrees of partial melting and evolutionary history of the parental melts to give rise to derivative melts through fractional crystallization in different combinations. These diagrams will also allow us to compare the studied samples with the adjoining areas to understand regional scenarios (Figures 7a and 7b). In this diagram (Figures 7a and 7b) two calculated partial melting curves are shown, after [76].

Figure 7a: Binary plots of Zr versus Zr/Y [78]. The two partial melting curves are after [76] and vectors for fractional crystallization. Our samples from the Low Silica Group-1 plot between the two melting curves, Low Silica Group-2 samples plot along  the partial melting curve II and the High Silica samples plot at higher levels of Zr as well as Zr/Y ratios.

Figure 7b: Binary plots of Zr versus Zr/Y [78]. The two partial melting curves are after [76] and vectors for fractional crystallization. Samples from the adjoining areas i.e. from Sakoli mafic volcanics and Western Bastar dykes plot along the melting Curve I and the Sakoli acidic volcanics plot separately with much higher Zr.

The two melting curves correspond to two sets of mineralogy for the sources; curve I is calculated for sources with 60% olivine, 20% orthopyroxene, 10% clinopyroxene and 10% plagioclase; and curve II is calculated for  sources with 60% olivine, 20% orthopyroxene, 10% clinopyroxene and 10% garnet for Archaean mantle sources [76,77]. The Zr/Y ratio for the melting curve II is higher because Y is held in garnet, thus on melting of such sources the resulting melt will have lower Y and therefore higher Zr/Y ratios, for curve I on the other hand, the Zr/Y ratio is higher as all the Y from the source region is released to the melt. Zr being incompatible for the basaltic system, its abundances are higher for lower degrees of partial melting and its abundances decrease in the derivative melts as the degrees of partial melting increases. Also shown in this diagram are the fractional crystallization curves for olivine + plagioclase, clinopyroxene and amphiboles. Fractionation of olivine + plagioclase does not affect the Zr/Y ratios although the Zr abundance increases in the derivative melts as the fractional crystallization processes proceed. The clinopyroxene and amphibole fractionation increase the Zr/Y ratios, more drastically for amphibole fractionation. Like the olivine + plagioclase fractionation, clinopyroxene and amphibole fractionation also causes increase in the Zr abundances as the latter is essentially incompatible for the basaltic system [7,75]. In (Figure 7a), the low silica group I samples plot between the two melting curves but more closer to the partial melting curve I, samples of the low silica group II plot along the partial melting curve II and the high silica samples plot at higher levels of Zr as well as Zr/Y ratios. Incidentally the studied high silica samples plot closer to the highly differentiated Garhwal dykes with similar Zr levels but the studied samples also have higher Zr/Y ratios. The low silica group I samples depict large variation in Zr abundances indicating their derivation from very low to higher degrees of partial melting at shallower levels compared to the low silica group II samples which probably got generated at deeper levels where garnet was stable in the source region. The low silica group I and high silica samples show increasing Zr/Y ratios with increasing Zr abundances, which probably indicate that these samples have undergone clinopyroxene + amphibole fractional crystallization. The Low silica group II probably represents higher degrees of partial melting which was followed by olivine + plagioclase fractionation, as the samples with higher Zr do not show any increase in the Zr/Y ratios as shown by the other groups (Figure 7a). Also the low silica group II plot is closer to the Garhwal flows which are shown to have undergone olivine + plagioclase fractionation of melts generated from sources from garnet stability field [7,79]. Samples from the adjoining areas particularly those from Sakoli mafic volcanics and Western Bastar dykes plot along the melting Curve I and the Sakoli acidic volcanics plot separately with much higher Zr, as seen in the case of highly differentiated Garhwal dykes (Figures 7a and 7b). The sample of the Amgaon amphibolites plot closer the melting curve II and the samples of the Khairagarh basalt plot along both the curves with majority of them plotting between the two melting curves. Samples of the high magnesian andesites (HMA) from Khairagarh and the Amgaon boninitic dykes plot closer to the melting curve II (Figure 7b). Some of the boninite samples plot much above the melting curve indicating that these samples have undergone extensive amphibole fractional crystallization, probably because of the presence of water, the same may be true for the studied high silica samples. The samples of the Amgaon amphibolite show large variation in Zr but no change in their Zr/Y ratios probably indicating that these samples have undergone olivine + plagioclase fractionation. All the other samples from the adjoining areas, probably have undergone dominantly clinopyroxene fractional crystallization [72,55].

The multi-elements patterns of the studied samples depict enrichment of the incompatible trace elements, like the rare earth elements patterns (Figures 5a and 5b). However, the multi-elements patterns show strong negative anomalies for Nb, P and Ti but they also show positive Pb anomalies; these features are commonly observed in subduction related arc magmatism [7,71,78]. Thus, the incompatible trace elements and the rare earth elements patterns suggest that the studied samples represent arc magmatism (Figures 5a and 5b). The studied samples are plotted in AFM diagram [80], they follow the tholeiitic to calc alkaline trend (Figure 8a). The plots indicate that the low silica rocks represent an immature arc while the high silica rocks follow the calc-alkaline trend indicating their derivation when the arc has matured. In the same diagram (Figure 8b), the mafic magmatic rocks from Khairagarh (basalts and high magnesian andesite) follow the tholeiitic trend although they plot separately. It has been shown by [28] that the HMA were derived from Andean type subduction environment and the basaltic rocks were emplaced in the back arc environment of the same arc. The Amgaon Boninite also plotted along with the Khairagarh HMA, indicating a similar environment of their emplacement as suggested by [55]. The Sakoli volcanics plot towards the high iron end of the tholeiitic trend and have been shown to have been emplaced in a rift tectonic environment with the supra-crustal sequences (Figure 8b).

Figure 8a: AFM diagram [80] shows the tholeiitic to calc alkaline trend for our studied samples (Low Silica Group-1, Low Silica Group-2 and High Silica).

Figure 8b: AFM diagram [80] follows the tholeiitic trend and plot separately for the magmatic rocks from the Khairagarh (basalts and high magnesian andesite).

In the FeO/MgO vs SiO₂ [81], whereas the low silica rocks plot within the tholeiite field, the high silica rocks plot within the calc-alkaline field (Figure 9a). This implies that the low silica rocks were derived from dominantly mantle components of the arc sources and the high silica rocks were derived from dominantly crustal sources or they represent mantle derived magma that interacted extensively with the crustal components, as the arc matured [82,83]. In this diagram, the samples from Sakoli belt, Amgaon mafic dykes and amphibolites, western Bastar dykes, and basaltic volcanics from Khairagarh belt plot within the tholeiitic series field, on the other hand the low-Ti basalt and HMA and boninite dykes plot in the calc-alkaline series along with the high silica rocks of the studied samples (Figure 9b). Like the AFM diagram (Figure 8) the FeOt/MgO vs SiO₂ diagram (Figure 9) clearly distinguishes two lineages, tholeiitic samples representing continental rift volcanics and dykes and those from back arc basin.

Figure 9a: SiO₂ versus FeO/MgO plot [81], shows Low Silica Group-1 and Group-2 rocks plot within the tholeiite field whereas the High Silica rocks plot within the calc-alkailine field.

Figure 9b: SiO₂ versus FeO/MgO plot [81], the samples from the adjoining areas plot within the tholeiitic series field while the samples from the low-Ti basalt, HMA and boninite dykes plot in the calc-alkaline series.

The boninitic dykes from Amgaon gneissic complex and the HMA and low Ti basalt of the Khairagarh basin plot in the calc alkaline series field. In the triangular plot of 2Nb - Zr/4 - Y [83], all the studied samples plot in the field of volcanic arc basalt (Figure 10a). All the mafic rocks samples from the adjoining areas plot in the fields of volcanic arc basalt and within plate tholeiites. Sakoli volcanics are shown to be rift related volcanics [72], Amgaon amphibolites and dykes are again rift related [38] this is confirmed by their plot in within-plate tholeiites field (Figure 10b).

Figure 10a: The Nb-Zr/4- Y discrimination diagram for basalts [84]. The fields are defined as follows: Al- WPAB; All- WPAB/WPT; B- E-type MORB; C- WPT/ VAB; D- N-type MORB/VAB. All the studied samples (Low Silica Group-1, Group-2 and High Silica) plot in the field of volcanic arc basalt. (WPAB- within plate alkali basalts; E-type MORB- Mid Ocean Ridge Basalt; N-MORB- Normal MORB; VAB- volcanic arc basalts; WPT- within plate tholeiites). 

Figure 10b: The 2Nb - Zr/4 - Y discrimination diagram for basalts [84]. All the mafic rocks samples from the adjoining areas plot in the fields of volcanic arc basalt and within plate tholeiites.

Western Bastar dykes and the boninites are shown to be subduction related magmatism with arc tholeiite and boninitic affinities. HMA and low Ti basalt indicate generation in a hot Andean-type subduction zone for the HMA and Andean-type back-arc rifting for the basalt–basaltic andesite samples [11,28,48,63]. 

The tectonic discriminant Ti-V diagram after [85] is also being used to understand the tectonic setting of the studied samples. The basis of this plot is the variation in the crystal / liquid partition coefficients for V, which ranges with increasing oxygen fugacity from >1 to << 1. This is because V can exist in reducing conditions (V³⁺) or oxidizing conditions (V⁴⁺ and V⁵⁺) in a natural magma system. Since the partition coefficient for Ti is almost always <<1, the depletion of V relative to Ti is a function of the ƒO₂ of the magma and its source region. Based on the Ti/V ratios rocks from different settings (Figure 11) are classified, although there are some overlaps for the rocks from continental flood basalt and back arc and MORB, the continental flood basalt field is restricted to higher V and Ti abundances with similar Ti/V ratios between 20 and 50. In this diagram the studied samples plot dominantly in the arc tholeiite field, although some samples straddle in the BAB field (Figure 11a).

Figure 11a: Ti-V basalt discrimination diagram [85] shows the studied samples (Low Silica Group-1, Group-2 and High Silica) plot dominantly in the arc tholeiite field, although some samples straddle in the Back-arc basin (BAB) field.

Figure 11b: Ti-V basalt discrimination diagram [85], the samples from the adjoining areas plot dominantly in the fields of continental flood basalt field, arc tholeiite field and MORB, BAB field.

The samples of the Amgaon amphibolites and western Bastar dykes plot dominantly in the continental flood basalt field. The samples of the Khairagarh basalt and HMA plot dominantly in the arc tholeiite field, although some of the Khairagarh basalt plots in the continental flood basalt field. Samples of the Amgaon boninite dykes plot with the arc tholeiite field (Figure 11b). The observed tectonic settings for the samples from the adjoining areas are consistent with the earlier studies [31], Amgaon Gneissic Complex basaltic and boninitic dykes [22,55,56], Amgaon Gneissic Complex amphibolic and doleritic dykes [38] and Khairagarh volcanics [28]. The present studied samples are similar to the Sakoli volcanics [31] plotting dominantly in the arc tholeiite field but a few samples plot in the BAB field. 

The U−Pb zircon age of 3.56Ga for a tonalite from the central part of the Bastar Craton was first described by [33]. Subsequently a U–Pb SHRIMP age of 3.6 Ga for a K-rich granite from Bastar Craton was reported by [11]. Our data indicate that the extraction age of the protoliths for the Amgaon Gneissic Complex vary between 3396 and 2550Ma [72] and the extraction age for the Tirodi Gneissic Complex vary from 2960 to 2122 Ma respectively [86]. The depleted mantle model ages for the Khairagarh volcanics vary from 2489 to 2984 Ma [28] and the depleted mantle model ages for the Sakoli mafic volcanics vary from 2000 to 2275 Ma [31]. Thus, it appears that the Bastar Craton evolved through bimodal volcanics right from the beginning in the form of basement gneiss (Amgaon and Tirodi Gneiss) and subsequently as rift and arc volcanism as the Craton evolved.

The studied dyke samples from the Bastar Craton represent Proterozoic mafic dykes from Amgaon and Khairagarh regions, belonging to the Dongargarh Supergroup. These dykes intrude the basement rocks known as the Amgaon Gneissic Complex with no apparent volcanics related to these dykes are present in the basement rocks. However, there are exposure of the Khairagarh mafic volcanics and HMA exposed as part of the supracrustal belt [28,64,66]. In the regional framework there are mafic volcanics recorded from the Sakoli supracrustal belt, where volcanic rocks are exposed with the rift related sediments. There are also reports of arc volcanism in the form of Pitepani belt volcanics and those from the Kotri-Dongargarh mobile belt [31,64]. However, in the adjoining areas dominantly dykes and sills are present in the basement rock - Amgaon and Tirodi gneiss [11,38,55,56]. On the basis of our field observations from the Bastar Craton and those from the other parts of the world, we conclude that because of deep erosion all the evidence of supracrustal rocks including the volcanics are gone and only dykes-sills and dyke swarms acted as plumbing system for the emplacement and eruption of mantle derived melts [46,87,88]. As the temperature of the early earth was much higher, there could have been enormous lava flows on the surface of the early earth, equivalent of Mesozoic Large Igneous Plutons (LIPs) but they are not preserved and their presence is indicated by the dykes, dyke swarms, sills and other mafic plutonic bodies which are exposed because of deep erosion and exposure of plutonic bodies and basement gneisses [9,62,89,90].

HC thanks the Director, Inter- University Accelerator Center (IUAC), New Delhi for mentorship and support all through this work. University Grants Commission (UGC), DS Kothari fellowship, application number- ES/19-20/0011 is gratefully acknowledged by HC to carry out this research work. TA thanks the JC Bose Fellowship, SERB New Delhi for the financial support and Director, Wadia Institute of Himalayan Geology, Dehradun for the infrastructural facilities.

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