In this article, novel geochronological (U–Pb SHRIMP) and geochemical data are presented from the lowermost sandstone unit (Par formation), basement granites of Gwalior Basin and sandstones from the Bhopal Basin, located along margins of the Archaean Bundelkhand Craton. The geochemical variation diagrams imply that sandstone units in the Gwalior and Bhopal Basins were deposited in rift-induced passive margin tectonic settings. In contrast to the magmatic features that are preserved in the zircons of granite of the Gwalior Basins, detrital zircons from sandstones of both basins are fragmentary and polymodal in size. The magmatic zircon grains from the basement granites yield a 207Pb/206Pb concordant age of 2538 ± 2 Ma. A group of detrital zircons from the sandstone of the Gwalior Basin with concentric magmatic zonation yield a weighted mean average age of 2564 ± 24 Ma. The detrital zircons from Gwalior Basin exhibit a patchy U-Th distribution overgrowing the magmatic zonation yield average age of 2044 ± 2 Ma. The detrital zircons from the Bhopal basin yield three distinct concordant ages of 2511 ± 5, 1694 ± 6, and 1355±9 Ma. The presence of ~2540 Ma concordant zircon population with concentric zonation in the sandstone of Bhopal Basin suggests their derivation from the granite of similar age. Therefore, an extension of Bundelkhand Craton granite below the Bhopal Basin is suggested. The 2500 Ma ages from the Gwalior granites are linked to global magmatic activity leading to the stabilization of extended Ur at ~2500 Ma. The 2048 and 1355 Ma ages from the Gwalior and Bhopal Basins, respectively, are concluded as the maximum depositional age (MDA) of the lowermost stratigraphic units within the basins. The MDAs are concluded to be the timings of passive margin basin formations along margins of the Bundelkhand Craton during extended Ur and Nuna or Columbia disintegration, respectively, during plume-driven tectonics.
Whole-rock geochemistry and geochronological data obtained from zircon, monazite, apatite, titanite, and other accessory phases from intrusive rocks are combined frequently to constrain the crustal assembly and dispersal processes [1-3]. In addition, detrital zircon geochronology rapidly developed as an essential tool because of the widespread occurrence of zircons in metasedimentary rocks [4-7]. Zircon is an isolated silicate mineral . Zircon is considered a physically and chemically robust mineral that can survive multiple episodes of erosion and deposition [4, 9, 10]. Thus, the U–Pb isotopic systematics of individual zircon grains can be traced back to the evolutionary history of their provenances [11-13]. Additionally, a statistical analysis of the age populations of the detrital grains and their correlations with global events can be correlated with the opening and closure of sedimentary basins as part of supercontinent dispersal and assembly processes [1, 6, 7, 14-17].
The cratons of India preserve geological evidences from Archean to the recent evolutionary history of the Earth and are considered key crustal blocks in ancient supercontinents [18-20]. The Bundelkhand Craton in north-central India preserves continental crust that has formed and has been recycled in phases between ~3.6 and 2.5 Ga [21-25]. The destruction of the Bundelkhand Craton in the Paleoproterozoic is evident from emplacements of the mafic dykes [26, 27] and the subsequent formation of sedimentary basins in the northern and southern margins of the craton [28, 29]. Marginal sedimentary basins surrounding the Bundelkhand Craton include the Gwalior basin at the northwest, the Bijawar, the Sonrai, and the Bhopal basins at the southeastern margins of the craton, while the Vindhyan Basin occurring along its northeast and southwestern margins [25, 30]. The origin of these rift-related basins is still a debated issue as opinion ranges from the existence of temporarily correctable large continuous pericratonic basins formed during craton destruction to the independent development and evolution of small basins as a part of the varying mantle plume activity (Basu and Brickford  and Ramakrishnan and Vaidhyanadan ).
It is thus imperative that the time frame of stabilization of the Bundelkhand craton and the evolution of its marginal basins are still debated. In this study, we present new U–Pb SHRIMP ages from the detrital zircons of the lowermost sandstone unit of the Par Formation of the Gwalior Basin that occurs at direct conformable contact with the basement granite. Similarly, we also present for the first time U–Pb detrital zircon ages from the sandstones forming the sole stratigraphic unit of the Bhopal Basin. Additionally, we constrain the age of the basement granite from the Gwalior Basin and suggest its possible linkage to the unexposed basement rocks of the Bhopal Basin. Geochronological data in this study are supported by geochemical analyses of the litho-units. Our results are critical to constrain the Precambrian tectonics from the northern and southern margins of the Bundelkhand Craton and, in broader aspects, their role in Proterozoic supercontinent reconstructions.
2. General Geology and Sample Description
2.1. Gwalior Basin
The sedimentary rocks are exposed in an NE-SW trending fault-bounded basin in and around Gwalior and are continuous around 80–100 km. The original extent of the basin is again a debatable issue, as the Upper Vindhyan rocks conceal the extent of pre-Vindhyan rocks in the Gwalior Basin (Figure 1(b)).
Roy et al. , Absar [33, 34], Chakrabarty and Pant , and Chakrabarty and Paul  studied stratigraphy and depositional history of the pre-Vindhyan rocks exposed in the Gwalior Basin. Initially, Roy et al.  classified the Gwalior Group into the basal clastic sequence of sandstone-shale composite (Par Formation) and an upper chemogenic sequence of limestone–mafic intrusive–shale - Banded Iron Formation (Morar Formation). Absar et al.  studied the geochemical characteristics of the lower clastic formation. Absar et al.  suggest compositional maturity and extreme quartz enrichment in the Gwalior sandstone implying a slow upliftment and high-in-situ weathering of basement terrains before the deposition of clastic sequences in a shallow platform environment.
Geochronological data from the Gwalior group are scanty in the literature. Colleps et al.  suggest the unroofing of the Bundelkhand Craton is marked by the 2700 to 2500 Ma zircon peaks of basal sandstones from the Bijawar and the Gwalior Basins. Colleps et al.  also suggest that the subpopulation of 2200 and 2300 aged grains defines the maximum depositional age (MDA) range for the oldest units deposited in the craton. Absar et al. [33, 34] obtained Pb–Pb ages of ~1914 ± 120 Ma from the carbonates of the Morar Formation. The topmost Banded Iron Formation unit was dated 1866 ± 250 Ma (Pb–Pb age, Absar et al. ). Ages obtained from a series of intrusive mafic rocks are constrained between 1830 ± 200 Ma (whole rock Rb–Sr ) and 1733 ± 13 Ma (U–Pb baddeleyite age; Srivastava et al. ). Colleps et al.  further suggest that Bundelkhand Craton attend tectonic stability circa 2200 Ma. Following the tectonic stability, a combination of far-field stresses and marginal tectonism probably initiate the stages of broad subsidence within Bundelkhand Craton, within the strata of the Vindhyan rocks. The circa 2200 Ma tectonic stability of the craton contradicts the available geodynamic models of Bundelkhand Craton, where Joshi et al. , Nasipuri et al. , and many others document that Bundelkhand Craton stabilized at ~2500 Ma overlapping with the timing of Archean-Proterozoic transition.
Our field study area in the Gwalior Basin was along a roadcut section within a forest near Gwalior that extends for nearly 2 km (Figure 1(b)). Along the section, we observed conformable contact of the basement granite and the lowermost sandstone (Par Formation), where the sandstone overlies the basement granite (Figures 2(a) and 2(b)). For detailed geochemical analyses and zircon geochronology, we sampled the sandstone (LGJ5-7, 24) and basement granite (LGJ3, 4A, 48; Figure 1(b); Figures 2(a) and 2(b). More detailed fieldwork in the remote areas of Gwalior was not possible due to ultraleft political activities.
2.2. The Bhopal Basin
The exposed rocks in and around the city of Bhopal are either the Deccan-Rajmahal Volcanic Traps basalt or the sandstone of the Bhopal Basin . Traditionally, the sandstone of the Bhopal Basin is being correlated with the Bhander Group, Upper Vindhyan Sandstone [41-43]. Retallack  reported Dickinsonia fossils from the Maihar sandstone, Upper Bhander group, Bhimbetka, that constrained the depositional age of Maihar sandstone as Late Ediacaran (635 and 541 Ma). However, no radiometric ages are available from sandstones from the Bhopal Basin. For the present study, samples have been collected from sandstone exposures along the road-cuts around Bhopal (BPL1-3; Figure 1(b); Figures 2(c) and 2(d)). Along these sections, the sandstone is folded.
3. Analytical Methods
3.1. X-Ray Fluroscence (XRF) Analyses
Major element contents of the sandstone and granite samples have been determined by an X-Ray fluorescence spectrometer at the National Geophysical Research Institute Hyderabad. The analytical protocol, detection limits, and associated errors for major element oxides are after Krishna et al. .
3.2. Inductively Coupled Plasma Mass Spectrometer (ICPMS) Analyses
Trace elements and Rare Earth Elements (REEs) of the set of samples have been determined using an HR-ICPMS at the National Geophysical Research Institute Hyderabad. Analytical methods are after Satyanarayanan et al. .
3.3. U–Pb Zircon Analyses
Using hydraulic, magnetic, and heavy-liquid separation methods, zircons were separated from the sieved whole-rock power. Finally, the suitable zircon grains were handpicked under a binocular microscope. The handpicked zircons were mounted in an epoxy medium with a reference zircon standard, FC1 (1099 Ma; Paces and Miller ) and SL13 (Sri Lankan gem zircon; U = 238 ppm). Finally, the epoxy was polished to expose the internal structure of the zircon grains. Before SHRIMP analysis at Korea Basic Science Institute, the grains were inspected under an optical microscope and photographed. The Back-Scattered Electron (BSE) and Cathodoluminescence (CL) images of zircon grains were acquired using a JEOL (JSM 6610) SEM. The accelerating voltage was 15 kV, and the beam diameter was 60 µm. For the SHRIMP analysis, a 2.5 to 2.8 nA mass-filtered primary beam was focused to a spot of 25 µm diameter on the polished surface of the target grains. Each spot was roasted with the primary beam for about 120 seconds before the analysis to remove any common Pb effect and then analyzed five cycles with a single-electron multiplier. During one cycle, the magnet was stepped through nine peaks of 90Zr216O (counting time = 2 seconds), 204Pb (10 seconds), 206Pb (10 seconds), 207Pb (20 seconds), 208Pb (5 seconds), 238U (5 seconds), 232Th16O (2 seconds) and 238U16O (2 seconds), and 10 s for background-position.
The software program SQUID 2.50  was used to process the isotopic data. The online program, IsoplotR , has been used for weighted average ages and U–Pb Concordia plots. The corrections for common Pb were done using the 207Pb (for ages <1000 Ma) or by using 204Pb (for dates >1000 Ma) methods, respectively, based on the model by Stacey and Kramers .
The U–Pb data of zircons from the Par Formation sandstone samples, the basement granite sample of Gwalior Basin, and the sandstone unit from the Bhopal Basin are given in Tables 1–3, respectively.
The following section describes the results obtained from petrographic, geochemical, and geochronological studies from the Gwalior Basin and the Bhopal Basin.
4.1.1. Gwalior Basin
The Par Formation sandstone is characterized by 150–400 mm sized angular to subangular quartz clasts. K-feldspar was absent in Gwalior Basin sandstone. Fe–Ti oxide, white mica, and detrital zircons are the other accessory minerals present in the studied samples.
Quartz, alkali-feldspar-plagioclase feldspar constitute the basement granites. Most samples show the presence of mirmekite and antiperthite indicating substantial cooling from magmatic conditions. Chlorite is present as a retrograde mineral (Figures 3(d) and 3(e)).
4.1.2. Bhopal Basin
Quartz with volumetrically subordinate alkali feldspar constitutes more than >98% volume of sandstone samples from the Bhopal Basin. Muscovite, zircon, and Fe–Ti oxides were present as accessory minerals. In contrast to the Gwalior sandstone samples, the quartz clasts from the Bhopal sandstone are angular in shape and are characterized by the development of chess-board twinning and undulose extinctions. (Figures 3(b) and 3(c)).
The correlation coefficient between major element oxides for sandstone samples from the Gwalior Basin is shown in Supplementary Material 1. The SiO2 (wt%) contents of the sandstones from Gwalior and Bhopal Basins show a negative trend with all other major oxides that implied quartz as a dominant mineral phase in these rocks.
In the upper continental crust (UCC) normalized spider plot , the granite samples from the Gwalior Basins are characterized by negative Ba, K, Sr, and Ti and positive Rb, Tr, Hf, and Zr anomalies (Figure 4(a)). Similarly, the sandstone samples from the Gwalior and Bhopal Basins are characterized by negative K, Sr, and Ti and positive Th–U anomalies (Figures 4(b) and 4(c)).
The ΣREE content for the basement granite samples from the Gwalior Basin ranges from 64 to 94 ppm (Table 1). In a UCC-normalized spider diagram , the granite samples display variable (La/Yb)N values (0.45, 1.05) with Eu/Eu* constrained between 0.91 and 1.01 (Figure 4(d)). The ΣREE contents of sandstones from the Gwalior and Bhopal basins vary between 47–98 ppm and 44–60 ppm, respectively. In a UCC normalized spider diagram, the (La/Yb)N and (Eu/EU*)N of the sandstone units of the Gwalior and Bhopal basins vary between 0.76–1.35, 0.5–2.09, 0.95–1.1, 0.95–1.08, respectively (Figures 4(e) and 4(f)). The resemblance of UCC normalized patterns for the sandstone samples from the Gwalior Basin and its basement granite implies the latter to be the probable provenance for the sandstone unit.
4.3. Zircon Morphology
4.3.1. Granites from the Basement of Gwalior Basin
The BSE images of representative zircon grains from the basement granite of Gwalior Basin with 207Pb/206Pb spot ages are shown in Figure 5(a). The zircons from the granite exhibit a large variation in their morphology and grain sizes. In the BSE images, the shape of the zircon grains varies from elongated (grains’ aspect ratios ranging between 2.44 and 0.40; grains 3, 4, 7,12, 16, 34, 46, 51, and 62) to stubby (grains 1, 2, 12, 22, 25, 36, 44, 45, 53, and 54). The margin parallel sharp concentric zoning is mostly preserved in larger crystals >200 μm (grains 4, 7, 21, 34, 46, and 48; Figure 5(a)). Such zoning patterns have been interpreted to represent the magmatic origin of zircons [51, 52]. However, the coarser and thick magmatic zonation in grains 4 and 46 probably implies crystallization from relatively SiO2-poor dioritic magma . Smaller and rounded zircon crystals (grains 6, 10, 24, 39, 42, 45, and 54) with aspect ratios 0.40–1.0 show either a diffused zonation or uniform.
The fifty-seven spot analyses from the granite sample yield a weighted mean average (207Pb/206Pb) age of 2540 ± 4 Ma (Mean Squared Weighted Deviation (MSWD ) = 1.6, Table 3; Figure 6(a)). In a U–Pb Concordia plot, U–Pb isotopic ratios of these grains define a linear trend with an upper intercept age of 2540 ± 4 Ma (MSWD 1.9, Figure 6(b)). Isotopic data from a single grain 31 exhibit a U–Pb concordia age of 2015 ± 68 Ma (MSWD 0.39). However, any definite conclusion from a single grain will be premature and is excluded from further discussion.
4.3.2. Sandstone Unit (Par Formation) Gwalior Basin
The BSE images of representative zircon grains with 207Pb/206Pb ages are given against each analyzed spot, shown in Figure 5(b). In the BSE images, the shapes of the zircon grains vary from rounded stubby to elongated, prismatic, with an aspect ratio between 0.16 and 4.85. The following zircon grains 1, 2, 4, 14, 15, and 18 exhibit margin parallel zonation implying an igneous source for them [51, 54]. The cores of zircon grains 6, 7, 8, 11, and 13 are rich in fluid and mineral inclusions. However, some of these grains exhibit ghost-like, uneven patchy zonation implying fluid interaction with zircons [55-57].
The Th and U contents of the current set of zircons vary widely between 34874–47 ppm and 9623–166 ppm, respectively (Table 2). Accordingly, the Th/U ratio varies widely between 4.85 and 0.16 (1σ = 1.1). The wide variation in U and Th and patchy zonation in grains 1, 2, 18, and fluid-inclusion-rich cores in grains 6, 7, 11, 13 imply fluid activity after crystallization of the magmatic grains .
4.3.3. Sandstone From Bhopal Basin
Zircons from the sandstone of Bhopal Basin exhibit a wide variation in the morphology and internal zonation patterns. The aspect ratio of these grains varies between 0.1 and 1.2. The zircon grains 14, 27, and 28 are rounded and exhibit recrystallized domains overgrowing earlier convolute zonation, implying high-temperature recrystallization . The second set of zircons (grains 12, 16, 20, 21, 26, 32, 33, 52, and 65) are elongated and exhibit an aspect ratio of 0.3–0.9. Zircon grains 6, 8, 52, and 53 exhibit margin parallel concentric zonation, indicative of magmatic origin for these grains (Figure 5(c)). Zircon grains 20, 32, 27, 42, and 46 exhibit inhomogeneous ghost-like patchy zonation (Figure 5(c)). The third group of zircon grains (8, 25, 30, 40, and 55) is of relatively smaller size (<100 mm) and irregular in shape with aspect ratios varying between 0.1 and 1.2. These zircon grains also show patchy and sector zoning in BSE images. The ghost-like patchy zonation in the second and third sets of grains imply fluid–zircon interaction.
The Th and U contents of the zircons vary between 307–5 ppm and 330–26 ppm (Table 4). Accordingly, the Th/U ratio is constrained between 4.2 and 0.1. The 207Pb/206Pb weighted average ages of 2540 ± 5 Ma (MSWD = 1, grains 14, 29, 47, and 53), 1673 ± 6 Ma (MSWD = 0.5, grains 6, 12, 16, 32, 34, 51, and 64), and 1375 ±12 Ma (MSWD =0.1, grains 8, 15, 40, and 41; Figures 6(e), 6(g) and 6(i)) are yielded by these zircons. Zircon groups with weighted average ages of 2540 ± 5 Ma and 1673 ± 6 Ma yield an upper intercept of 2541 ± 5 Ma and 1675 ± 8 Ma in U–Pb Concordia diagram (Figures 6(f) and 6(g)). The grains 8, 15, 40, and 41 define a U–Pb Concordia age of 1384 ± 7 Ma (Figure 6(j)). If the weighted average age 2540 Ma is constrained as the upper intersection in the U–Pb Concordia diagram, the lower intercept is constrained at 1394 Ma for the third group zircons which is similar to the weighted average age of 1375 Ma. If 1675 Ma age is anchored as the upper intercept in the U–Pb Concordia diagram, a lower intercept age of 1430 Ma is yielded, which is ~50 Ma older than the weighted average age of the third group of zircons. The collinearity of 2541 and 1375 Ma aged zircons in a U–Pb diagram implies the reworking of older zircon and the growth of newer zircons at ~1375 Ma. The zircons with 207Pb/206Pb ages <1200 Ma do not define any meaningful ages, probably due to fewer analyses or wanning phase of tectonothermal activities south of Bhopal Basin, and are excluded from further discussion.
5.1. Source and Tectonic Discrimination Diagram
Raza et al. , Joshi et al.  studied detailed geochemistry and geochronology of Bundelkhand granite. These studies indicate that the basement granites are ferroan, alkali-calcic, and silica-saturated peralkaline granites in chemical nature  and could have been derived from K-rich rocks .
The four sandstone samples from the Gwalior Basin (unfilled square) are classified as Fe-sandstone to sublith arenite, and three sandstone samples from the Bhopal Basin (marked as unfilled circles) are classified as quartz-arenites in a log (SiO2/ Al2O3) versus log Fe2O3/ K2O plot  (Figure 7(a)). The studied sandstone samples except the CaO-rich samples from Gwalior and Bhopal Basins show nearly similar Chemical index of alteration (CIA) (0.8, 0.9), Chemical Index of Weathering (CIW) (0.95, 0.99), and Plagioclase Index of Alteration (PIA; 0.97, 0.99) indices (Table 1). Nearly similar and overlapping indices imply moderate to strong weathering of source rocks for the generation of crustal materials parental to the sandstone samples from both basins (Table 1; Figures 7(b) and 7(c)).
The Al2O3/TiO2 ratio values for the sandstones from the Bhopal Basin are consistent between 8.07 and 11.79. In contrast, sandstones from the Gwalior Basin show some variation in the ratio, ranging between 10.23 and 49.11, although one sample with low SiO2 shows the lowest value of Al2O3/TiO2 ratio ~8.22. When plotted in SiO2 versus (Al2O3/TiO2) plot, sandstone samples from Gwalior and Bhopal Basins suggest a felsic source for most of the samples (Figure 8(a)). Only a single sample from the Gwalior Basin plots toward the mafic source field. The apparent discrepancy is attributed either to local variations in the source material (felsic with local mafic source, as documented from the zircon morphology). In a K2O/Na2O versus SiO2 plot , the sandstone samples from both the basins plot in a passive margin field (Figure 8, Roser and Korsch ). In the discriminant function plot, most sandstone samples plot in the passive margin settings, except a single sample plotted in the oceanic island arc field (Figure 8(c), Bhatia ). In the TiO2 versus (Fe2O3+MgO) and (Al2O3/ SiO2) versus (Fe2O3+MgO) diagrams, all sandstone samples plot in the passive margin field of Bhatia  (Figure 8(d)).
Verma and Armstrong-Altrin  suggest a modified discrimination function diagram to decipher the tectonic settings of sedimentation. The sandstone samples from both the basins with high silica contents ([SiO2]adj = >93%) plot in the collision and rift tectonic setting fields (Figure 8(e)). On the contrary, one sandstone sample from the Gwalior Basin with low silica content ([SiO2]adj = 42.53%) plots in collision setting (Figure 8(f)).
Trace element geochemistry of sedimentary rocks is a clue in understanding the provenances of sedimentary rocks (McLennan ). A TiO2 versus Zr plot can distinguish felsic, intermediate, and mafic rock types (Hayashi et al. ). In a TiO2 versus Zr plot for the sandstone units from the Bhopal and Gwalior Basins, most samples are scattered between the fields of felsic and intermediate source rocks and one sample plots in the mafic field (Figure 9(a)). In a binary plot of the Cr/V versus Y/Ni (after McLennan et al. ), the sandstone samples from both the basins plot in the felsic field (Figure 9(b)).
The La/Th versus Hf diagram is useful to distinguish the sediment contributions from the ocean-island sources – mixed basic and felsic sources (Casas ). In a La/Th versus Hf tectonic setting discrimination diagram, the chemical composition of sandstone samples plot in the andesitic arc to felsic source fields (Floyd and Leveridge ). The higher Hf concentrations in a few samples that plot in the felsic source field indicate contributions from older crustal components (Figure 9(c)).
The Th/Sc versus Zr/Sc plot is useful to constrain the provenance and degree of sediment recycling processes (McLennan et al. ). In the Th/Sc versus Zr/Sc plot, all sandstone samples plot in the UCC field with significant sediment recycling due to the addition of zircons (Figure 9(d)). The Th/U versus Th plot is utilized to constrain the source area of weathering. Zaid  suggests surface-weathering elevates Th concentration and increases Th/U values above the average UCC. The Th/U ratios of the sandstone samples from Gwalior and Bhopal Basins vary between 3.1 and 4.5, indicating a low to moderate degree of chemical weathering for the sandstones in their source areas (Figure 9(e)).
5.2. Zircon U–Pb Ages
The weighted mean average age of 2564 ±2 4 Ma is yielded from the cores of two zircon grains (grains 4 and 14; Figure 5(b)) of the lowermost stratigraphic unit of the Gwalior Basin. These grains preserve well-defined margin parallel zonation. The younger weighted mean average age of ~2044 ± 2 Ma is yielded either from zircons that show strong evidence of interaction with fluid or occur as rims overgrowing older cores (grains 4 and 14; Figure 5(b)). Samon et al.  reported 2100 Ma aged mafic sills from the Gwalior Basin. Ahmad et al.  suggest the occurrence of a large mega-plume beneath the Aravalli-Bundelkhand Craton during Paleoproterozoic time. These lines of evidences imply a different set of zircon growth circa 2045–2100 Ma in the Bundelkhand granites.
The moderate to high CIA, CIW, and PIA values of the sandstone unit is indicative of moderate weathering of basement granites. When combined, the results of the tectonic discrimination diagram and geochemical weathering indices (Figure 9) imply that the formation of the Gwalior Basin was initiated at ~2100 Ma in an extensional setting. Thus, the integrated zircon morphology, geochronology, and geochemical data presented in this study help to constrain the extensional tectonics related to the early stages of evolution of the Gwalior Basin at ~2100–2000 Ma .
The oldest zircon population from the sandstone unit of the Bhopal Basin displays concentric zonation as of igneous zircons and yields a 207Pb/206Pb Concordia age ~2540 Ma (Figures 6(e) and 6(f)). This age is comparable with the crystallization ages of the Gwalior Basin basement granite and the Bundelkhand Craton’s potassic granite . This zircon population’s morphology and zonation patterns imply that the ~2500 Ma granite of Bundelkhand craton extends till the Bhopal basin. The younger weighted mean average and Concordia ages yielded by the detrital zircons of the sandstone unit are ~1675 and ~1384 Ma (Figures 6(i) and 6(j)). The ~1675 Ma Concordia age is correlated with major tectonic events recorded from the Proterozoic mobile belts warping the southern margin of the Bundelkhand craton. These belts include the Mahakoshal Belt in north, Betul Belt in center, and the Central Indian Tectonic Zone in south (CITZ; at the northern margin of Bastar Craton; Figure 1(a)). Within the Mahakoshal belt and Betul Belt, the period of ~1700–1600 Ma is marked by extension when A-type granites were emplaced .
The southern margin of the CITZ at the contact of Bastar Craton and the Chotanagpur Gneissic Complex (CGC) in eastern CITZ experienced low-pressure granulite facies metamorphism under arc-type settings at ~1650–1600 Ma [76, 77]. The third set of zircons with patchy zonation yield the youngest Concordia age of ~1384 ± 7 Ma (Figure 6(j)). We infer that this youngest Concordia age of ~1384 ± 7 Ma represents the MDA of the sandstone unit of Bhopal Basin (Figure 6(j)). Further analysis reveals that the 2540 Ma zircons and youngest 1375 Ma zircon plot in a linear array in a U–Pb Concordia diagram. Sequeira et al.  report 1400 Ma A-type granite emplacement in 1600 Ma metamorphosed terrains in the CGC, located north-east of the Bhopal Basin along the eastern segment of the CITZ. Sequeira et al.  correlate the 1400 Ma event with an extensional tectonic setting leading to the disintegration of Columbia. Since the presence of 1385 Ma zircons in the sandstones from the Bhopal Basin can be correlated with the rifting and disintegration of Columbia, we suggest the opening of the Bhopal Basin in continuation of rifting in the CGC. The patchy zonation of the zircon populations yielding 1675 and 1384 Ma concordia ages implies their origin during metamorphic processes for which the high-grade terrains of Mahakoshal Belt, CITZ, and the CGC acted as provenances.
The timing of the closure of the Bhopal Basin is debated. McKenzie et al. [79, 80] suggest a U–Pb Concordant age could mark the closure of the sedimentary basin. Accordingly, the Concordia U–Pb age, 1298 Ma, might mark the closing of the Bhopal Basin. Retallack et al.  report Dickinsonia from Bhimbetka Rock Shelters, Bhopal, which constrains an age of 550 Ma for the Bhimbetka sandstone. However, Meert et al.  contradict the claim of Dickinsonia from Bhimbetka. In the absence of statistically more robust geochronological data and recent criticism by Meert et al. , any comment regarding the closure of the Bhopal Basin will be premature.
5.3. Supercontinent Connections of the Paleo- to Mesoproterozoic Tectonics Constrained From the Bhopal and Gwalior Basins in This Study
There is a general agreement that the supercontinent cycles govern the thermal evolution of different Archean blocks in the Earth [21, 82, 83]. The Ur is the oldest known supercontinent in the Archean, which stabilized by assembling the East Dharwar, West Dharwar, Bastar, and Singhbhum cratons of South India Block [19, 30], Bundelkhand Craton of north-central India [21, 23], the Kaapvaal Craton of South Africa, and the Pilbara Craton of Western Australia [18, 19]. Rogers and Santosh  and Mohanty  suggest that the Yilgran Cratons and the Zimbabwe Craton assembled with the original Ur at 2500 Ma by forming extended Ur. Magmatism associated with high-temperature metamorphism at Limpopo Belt , Zimbabwe Craton , high-pressure low-temperature metamorphism followed by widespread granite magmatism along central Bundelkhand Craton [21, 23], East and West Dharwar Cratons [87, 88], and Bastar Craton  marked the stabilization of extended Ur.
Radhakrishna et al. , Pradhan et al. [26, 27], French et al. , and Saha et al.  suggest that the Archaean East and West Dharwar, Bastar, and Bundelkhand cratons evolved as a coherent block within Ur and extended Ur configuration . Disintegrations of these cratonic blocks between 2450 and 1860 Ma are attributed to mantle superplume activities leading to long-lived igneous activities [91, 92]. Thus the formation of Gwalior Basin (~2100; this study) at the north-west margin of Bundelkhand Craton, Bijawar Basin (~2300 Ma ;) at the south-east margin of the Bundelkhand Craton , the Cuddapah Basin (~1900 Ma ) at East Dharwar Craton also marked the prolonged period of superplume activities.
Immediately younger to the extended Ur is the Nuna or Columbia supercontinent that assembled in the Paleoproterozoic with maximum packing of cratonic blocks accreted along different orogenic belts by ~1800–1700 Ma [18, 82, 93]. The position of India in the configuration of Columbia is being debated. Zhao et al.  proposed that India was connected with North China Craton and East Antarctica. Santosh  and Hou et al.  established a link between North China Craton and eastern margin of India. These authors proposed a linkage between the CITZ and Trans-North China Orogeny. On the contrary, Mohanty  proposed a correlation of the CITZ with that of the Capricorn and Mangaroon orogenies of Western Australia during the assembly of Columbia. Kaur et al.  further established a linkage between the north-western margin of the Aravalli-Delhi Fold Belt (ADFB) with that of the ~1850 Ma high-grade terrains of the eastern Cathaysia block (South China Craton), Korean peninsula. Although the configuration of Columbia is debated, the linkage of cratonic blocks of peninsular India with its assembly is established along the multiple orogenic belts like the CITZ and ADFB (Figure 1(a)). Columbia breakup was initiated at ~1600 Ma and ended at ~1300 Ma. In central and southern India, the period of ~1650–1400 Ma has been characterized by the formation of craton margin basins like the Vindhyan Basin along margins of the Aravalli and Bundelkhand cratons (~1630 Ma [37, 43]), the Khariar (~1500 Ma ), and the Chhattishgarh Basin (~1400 Ma [99, 100]) at the margin of Bastar Craton. The age of formation of Bhopal Basin at ~1384 Ma is constrained in this study. Extensive intrusions of mafic dykes between ~1590 and 1400 Ma occurred along the Bundelkhand Craton. This further implies that plume-driven extensional tectonics linked to the disintegration of the Columbia between ~1600 and 1400 Ma were dominant in central and southern segments of peninsular India.
The Paleoproterozoic magmatic rifting and rift basin formation are widely reported from the periphery of the Bundelkhand and Aravalli cratons (Paleoproterozoic Aravalli rocks, , Lower Vindhyan Basins, Ray ), the Bastar Craton (the Chhattisgarh and other small sedimentary basins [99, 102, 100, 103, 104]), and Eastern Dharwar Craton (the Cuddapah Basin [91, 105]). Basu and Bickford  compiled the available geochronological data and grouped them into an older Paleoproterozoic basin (Cuddapah basin in Eastern Dharwar Craton), Lower Vindhyan rocks of the Gwalior-Bijawar basins at the margins of Aravalli and Bundelkhand Cratons, Mesoproterozoic rocks in the Bastar craton [99, 106], Upper Vindhyan rocks at the margins Aravalli and Bundelkhand Cratons , and the Marwar basin [107, 108] in the Aravalli craton. However, the extent of spatial continuity or isolation of these basins, especially along a single craton margin, remains debated due to the lack of chronological correlation. This is because the lithology preserved in the smaller basins is supposed to represent erosional remains of nearby larger basins .
The U–Pb zircon ages obtained from the Gwalior and Bhopal Basins across the Bundelkhand Craton have been compared with the global zircon database to correlate the regional tectonics with global events (Figure 10). As discussed in the preceding paragraphs, the formation of passive margin basins occurred at 2.1–2.0 Ga and 1.35 Ga at the north-western the south-eastern margins of the Bundelkhand craton. The oldest of the two events, that is, 2.0 Ga, correlates with the mega Large Igneous Provinces (LIP) formation event during the breakup of the extended Ur before the formation of Columbia (Figures 10(a) and 10(b)). The northern margin of Bundelkhand craton records such a rifting event prior to the deposition of the Vindhyan group of sediments and affirms that Gwalior Group represents a pre-Vindhyan sequence. In addition to the 2.5 Ga aged zircons, the presence of high-temperature-metamorphic zircons (~1.69 and 1.35 Ga) in studied samples from Bhopal Basin implies high-temperature metamorphic events prior to its deposition. The 1.6 and 1.3 Ga old zircons of Bhopal Basins are concomitant with the granulite facies events from the southern part of the CITZ and CGC, respectively. A comparison of the 207Pb/206Pb KDE values (Figure 10) also implies that the initiation of extensional tectonics and basin opening occurred during different time intervals at different parts of Bundelkhand Craton.
For the present study, the exposed basement sandstone cover relationship in the Gwalior Basin (Figures 2(a) and 2(b)) implies its deposition over ~2500 Ma Bundelkhand granites . Though, the basement rocks of the Bhopal Basin are not exposed, the presence of ~2500 Ma zircons from the sandstone of the Bhopal Basin also implies its derivation from ~2500 Ma rocks. Additionally, the presence of ~2000 and ~1385 Ma aged zircon populations from Gwalior and Bhopal basins, respectively, implies their opening under extensional tectonic settings . The ~2000 Ma zircon ages from the Par formation imply the timing of rifting in the northern margin of the Bundelkhand Craton leading to extended Ur disintegration and assembly of Nuna. Sequeira et al.  correlate the A-type magmatism in CGC with the disintegration of Nuna. Accordingly, we suggest the relatively younger Bhopal Basin opened during rifting and A-type granite magmatism in the CGC. The study demonstrates that the thick cratonic landmasses, once formed in the Archean, can be destroyed by Paleo-Mesoproterozoic diachronous mantle plume activities operated at different parts of cratons.
Authors acknowledge Prof. A. Tamer and an anonymous reviewer for their constructive suggestions to prepare the manuscript. A.K. Shrivastava and M.B. Raza acknowledge their respective funding agencies like UGC, India and IISER Bhopal for sponsoring field trips during the study. A part of the fieldwork was carried out by using partial funding from Professional Development Account (PDA) of P. Nasipuri. L. Saha acknowledges Faculty Initiation Grant, IIT Roorkee for funding field work and sample preparation. No grant number available in each case.
Conflicts of Interest
The authors declare that they have no conflict of interest.
The petrographical thin sections and locations for the samples utilized for this study can be available from the corresponding author on personal request.