Geochemical study of Ambaji − Sendra granitoids and mafic dykes from the South Delhi fold belt, NW Indian shield: Implications for magma generation, emplacement and geodynamic evolution

4.1 Element mobility assessment

The primary idea about the mobilization of various elements is adduced from LOI (loss on ignition values). Secondary minerals like epidote, zoisite, chlorite etc contain significant H₂O or carbonate minerals like calcite, dolomite etc release CO₂, both enhance LOI values. Rocks considered as being ‘weathered’ or ‘metamorphosed’ possess signatures such as abnormally high water contents or high LOI values (Hastie et al., 2007). The LOI values of all the granitoids range between 1.13 and 1.39 wt% except one sample each of Jaitpura (1.66 wt%) and Balaram (1.43 wt%) granitoids (Table 1B), which are consistent to their primary mineralogy (amphiboles and micas.).

Values of correlation coefficients of major oxides with SiO₂ are highly consistent in individual granitoids i.e. either positive or negative. For example, r coefficient of Al₂O₃ with SiO₂ is +0.99 in Tarpal, Jaitpura, Chitar, Chang and −0.89 to −0.99 (other granitoids). Na₂O and K₂O follow similar trend ruling out their any significant mobility. LILEs are also suspected for their mobility. When correlated with immobile element Zr, these elements also show their positive or negative variation within permissible limits −0.99 to +0.47 in consonances to their major oxides. Another noteworthy correlation is between Zr – Sc. Except Chitar granitoids all other granitoids have highly positive values (0.86 to 0.99). These correlations suggest that they are the source characteristics (Supplementary Table 1). Presence of allanite and zircon may increase concentration of REE and U, Th and Hf. Zr shows positive covariation with Hf, U, Th and Yb and Y with Ce in Borwar, Chitar and Chang granitoids. Despite such variations Th, U, HREE or Ce concentrations in these granitoids are not proportionally (anomalously) high with compatible concentrations of Zr or Y contents. It suggests that primary concentrations of REEs, U or Th have not been appreciably enhanced.

4.2 Geochemistry of AST granitoids

Tarpal, Balaram and Chiknawas granitoids show slightly wide range of all major oxides e.g. 61.80–70.92 wt%_SiO2,11.13–15.96 wt%_Al2O3(Tarpal),70.76–74.65 wt%_SiO2,11.28–14.63 wt%_Al2O3 (Balaram) and 70.28–76.54 wt% _SiO2,12.89–16.58 wt%_Al2O3(Chiknawas) whereas Jaitpura, Borwar, Seliberi, Chitar and Chang granitoids have very narrow range of major oxide contents, in particular SiO₂:73.66–77.34 wt% and Al₂O₃:11.82–13.89 wt%. Tarpal, Rankapur and Jaitpura granitoids are characterized by low Ga/Al ratio (1.61–2.81) coupled with small concentration of HFSE (Nb + Y:14.31–6.68) except Jaitpura possessing high (Nb + Y) content (113.4–158.2). Ga/Al ratio and (Nb + Y) content of Borwar, Seliberi, Chang, Chitar and Chicknawas granitoids are relatively high (3.11–3.91) and (129.61–242.35) respectively except Chiknawas which has intermediate (Nb + Y) content (43.97–65.52).

4.3 Classification of AST granitoids

The TAS and IUGS-based QAP classification schemes classify all AST granitoids as granites except Ranakpur and one sample of Tarpal as diorite and granodiorite respectively.

In regards to Fe number {FeO^T/ (FeO^T + MgO), Tarpal granites are magnesian (0.42–0.65), Ranakpur, Jaitpura, Seliberi, Chang and Balaram granitoids are ferroan (>0.8) and Borwar, Chitar and Chiknawas granites are magnesian to ferroan (0.78–0.90). Tarpal granites are Calcic and Alkali calcic (Na₂O + K₂O-CaO:3.83–8.25), Jaitpura granites are Calcic to Calc-alkalic (Na₂O + K₂O-CaO:5.79–7.11), Borwar, Seliberi, and Chang granites display Calc-alkalic (Na₂O + K₂O-CaO:5.83–7.88) nature, Chitar and Ranakpur granites are of Calcic character (Na₂O + K₂O-CaO:5.47–5.33), Balaram granites are Alkali calcic (Na₂O + K₂O-CaO:7.06–8.21) and Chiknawas granites posses Alkalic, Alkali calcic and Calcic varieties (Na₂O + K₂O-CaO: 5.77–10.94). According to alumina saturation index (A/CNK,) Jaitpura, Borwar, Seliberi, Chiknawas, Chitar granites are peraluminous (A/CNK:1.05–1.23), Tarpal and Chang granites are metaaluminous to peraluminous (A/CNK:0.55–1.26) and Balaram and Ranakpur granitoids are metaaluminous (A/CNK:0.88–0.95 Fig. 2, Table 1B).

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Fig. 2

(a)FeO^T/(FeO^T + MgO) versus SiO₂ (b) Modified alkali-lime index: (Na₂O + K₂O-CaO) versus SiO₂ (Frost et al. 2001) indicating majority of AST granites are alkali-calcic to calc-alkalic. (c) Aluminium Saturation Index plot (after Maniar & Piccoli, 1989) classifying AST granites as largely peraluminous.

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In Ga/Al versus Zr and Nb diagrams (Whalen et al., 1987) majority of the samples of Ambaji – Sendra Terrain granites (ASTG) indicate their A-type nature (Fig. 3a,b). The (Zr + Nb + Ce + Y)–(Na₂O + K₂O)/CaO and (Zr + Nb + Ce + Y)-FeO^T/MgO diagrams also broadly confirm A-type nature of ASTG (Fig. 3c, d). The ASTG are classified as A2 based on their high concentrations of Y and Ce relative to Nb and high ratios of Ce/Nb and Y/Nb (>1.2) indicating presence of arc-like geochemical signatures (Eby, 1992). ASTG also have high Ta contents and thus low Yb/Ta ratio suggesting significant fluid interaction with the granitic melts (Table 1B).

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Fig. 3

Classification of AST Granites in (a) 10000*Ga/Al versus Nb (b) Zr, (c) (Zr + Nb + Ce + Y) versus (K₂O + Na₂O)/CaO and (d) (Zr + Nb + Ce + Y) versus FeO^T /MgO diagrams indicating their broadly unfractionated A – type nature (after Whalen et al., 1987). Legends as in Fig. 2.

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Close scrutiny of elemental ratios particularly involving traces elements and REEs reveals that mineralogical division of ASTG into two groups is justified. ASTG1 (foliated granites, syn-orogenic), display least to moderately fractionated REE profiles (La/Yb_N:0.91–3.83) with moderate sink at Eu (Eu*: 0.24–0.51). ASGT2 (porphyritic granites, likely post −orogenic) possess significantly fractionated REE patterns (La/Yb_N:1.32–31.55) coupled with steep sink to positive kink at Eu (Eu*: 0.17–1.39) (Fig. 7). In synchronicity with their REE fractionation, ASTG1 possess lower values of Th/U, Rb/Sr, Th/Sc, La/Sc, ASI and Ga/Al ratios compared to ASGT2. However, Y/Nb ratio of the ASGT1 is higher than ASGT2 (Table 1B). Despite similar shapes of PM profiles of ASTG1 and ASTG2, differences in the abundance of various elements are clearly reflected in the magnitude of negative anomalies at Ba, Nb, Sr, P and, Ti and corresponding positive anomalies of preceding and succeeding elements. (Fig. 7). The mineralogical and geochemical grouping of ASTG is further confirmed by their trace element relationships. Selected TTE, LILE and HFSE show moderate to strong positive covariation with Zr in ASTG1 and crude positive variation in ASTG2.

Immobile trace element based Nb–Y diagram indicates that crustal fluids played role in modifying the composition of ATSG as significant number of samples (Ranakpur and Tarpal) plot in arc field (Fig. 4).

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Fig. 4

Nb versus Y diagram suggesting arc to within plate setting to AST Granites (Whalen and Hildebrand, 2019). Legends as in Fig. 2.

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4.4 Petrogenesis of AST granitoids

A-type granites is a group of granitic rocks that is characterized by low water content and lack of orogenic or transitional tectonic fabric. They are classified in the S – I – A – M- or alphabetic system where ‘A’ stands for anorogenic or anhydrous. Compositionally A-type granites are highly ferroan and potassic and enriched in incompatible trace elements. They are also depleted in Sr and Eu, have high alkalies, high FeO/MgO and TiO₂/MgO ratios, and have high field strength elements (HFSE).

A-type granites are thought to be dominantly of non – crustal in origin. They are typically formed in stable intra-plate, back arc, or post collisional settings and be found in a variety of tectonic settings, including subduction zones, intracontinetal settings, and orogenic belts.

Nevertheless, the petrogenesis of A-type granites is still debated particularly for the nature of the potential source, e.g. granulitic sedimentary/igneous residue in the lower crust (Huang et al., 2011), underplated tholeiitic basalts and their derivatives (Wang et al., 2010) or a quartzo-feldspathic quartz–doritic–tonalitic– granodioritic source, as well as the role of the mantle (Wang et al., 2014).

Eby (1992) has shown that the Y/Nb ratio remains nearly constant during fractionation of A type magmas and thus serves as a key to identify the sources of magma. Granitic magmas with (Y/Nb < 1.2) are derived from the mantle source whereas higher ratios (Y/Nb > 1.2) imply solely crustal sources or mixed sources. Both groups of ATSG have Y/Nb > 1.2 suggesting their crustal origin. However, ASTG1 have very high Y/Nb ratios (2.27–14.4) compared to ASGT2 (1.88–4.56) advocating for their unequivocal crustal origin. This interpretation is well supported by various element ratios, A/CNK and trace element correlations with Zr in ASTG1 (Table 1B). Whereas, ASGT2 contain some mantle component as Zr does not vary positively with majority of elements except Sc advocating mafic mineral control over their composition.

It is thus interpreted on the basis of following points that metasomatized mantle-derived magmas also contributed in the generation of AST granites:

• Since Nb and Ta are less incompatible than Ce, Y and Yb during partial melting, melts generated in lower continental or upper continental crusts alone, should have higher Ce/Nb, Y/Nb and Yb/Ta ratios. ASTG1 and ASTG2 exhibit variably higher /lower or similar Ce/Nb, Y/Nb and Yb/Ta ratios compared to lower continental and upper continental crusts (Fig. 5c,d), preclude their derivation from a singular crustal source.

• REE contents in conjunction with trace element patterns differ in magnitude form continental crust. AST are enriched in HREEs, water soluble elements (i.e. Pb) and LILE which prescribe to them a source metasomatized by the fluid originated from subducting slab.

• Nb/Ta ratio of ASTG is relatively low (ASTG1:2.5–9.26, ASTG2:1.94–9.7), compared to Cl-chondrite (17.57) (Sun and McDonough, 1989) and continental crust (Gao et al., 1998), indicating that low Nb/Ta ratio is their source characteristic. Such a source is expected to contain melt/fluid derived from the deep and high temperature subducted slab. It has been suggested that dehydration of hotter subducted slabs produce fluids which have subchondrite Nb/Ta ratios (Ding et al., 2009). The metasomatized mantle by low Nb/Ta bearing fluids, above the subducted slab, sustains its low Nb/Ta character. ATSG display Nb and Ti troughs in PM profiles (Fig. 7). Such Nb and Ti depletions, and other similar arc-type geochemical features in felsic rocks, have also been ascribed to anatexis of rocks derived through subduction processes.

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Fig. 5

Discrimination diagrams for A 1 and A 2-type granites (Eby, 1992). In (a) and (b) dashed line corresponds to Y/Nb ratio of 1.2. OIB −Oceanic island basalt, IAB − Island arc basalt. Log plots of Eu versus Ba (e) and Sr (f) for AST Granites indicating their generation by the fractionation of K – feldspar. Legends as in Fig. 2.

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Moreover, ratios like Ti/Sc, Nb/Ta etc of the ATSG show narrow variations, suggesting that the basement to the granites was largely monotonous or, alternatively, reflecting smaller amounts of crustal contamination implying intrusion through thin crust. The Nb/Ta and Ti/Sc ratios in ATSG are (1.94–9.3) and (133.6–644.5) respectively (Table 1B). As Sc is more compatible in the mantle than Ti, mantle sources typically produce higher Ti/Sc values than one would expect from a more felsic source. Furthermore, almost flat to slightly inclined HREE pattens of ASTGs in conjunction with high (La/Yb)_PM and Zr/Y ratios and a pronounced negative Eu anomaly, preclude a garnet-rich, plagioclase-free residual phase (Zhang et al., 2023).

Log plots of Eu versus Ba and Sr for AST granites show gently inclined linear trends i.e. decrease in Ba and Sr contents with decreasing Eu concentration (Fig. 5e, f) which suggests constrained fractionation of feldspar in the evolution of these magmas wherein varying proportions of K-feldspar and plagioclase segregated through crystal fractionation processes. The evolution of A-type granites through feldspar fractionation is a global phenomenon because high temperature and in turn low viscosity promotes crystal fractionation in A-type magmas (Dall'Agnol et al., 2017).

5.1 Geochemistry of AST dykes

ASTD have very constrained values of loss on ignition (1.11–1.46 wt%). These values, in combination with field and petrographic observations, suggest that these rocks have not undergone any significant degree of alteration. The relationships between the resistant element Zr and LILE, TTE and HFSE are largely magmatic which further testifies that the chemistry of the dykes is almost undisturbed. Robust molecular ratio plots (figure not shown) devised to constrain elemental mobility of metavolcanics overwhelmingly indicate primary chemistry of ASTD even for alkalies.

SiO₂ values of ASTD, range from 49.81 to 53.94 wt% except Ranakpur and Seliberi dykes (56.86 wt% and 61.77 wt% respectively). Al₂O₃ generally varies from11.11 to 14.31 wt% except Seliberi (9.67 wt%) and a sample (VAD 3) from Vagdadi (16.35). Fe₂O₃ and MgO contents show inverse relationship in ASTD. Ranakpur, Seliberi and Borwar dykes are characterized by lower Fe₂O₃ (<7 wt%) coupled with higher MgO (>7 wt%) whereas in rest of the samples Fe₂O₃ is more (>13 wt%) and MgO is less (< 4 wt%). CaO content is significantly high in Ranakpur, Borwar and Seliberi dykes (11.04–18.68 wt%) compared to Vagdadi and Chiknawas dykes (6.13–7.26 wt%). Na₂O concentration is generally more than K₂O but is reverse in Seliberi and Chiknawas dykes. A significant feature of ASTD, particularly of Borwar, Vagdadi and Chiknawas dykes is their high TiO₂ and P₂O₅ contents which indicates presence of accessory minerals like ilmenite, rutile and apatite. This observation is duly endorsed by their petrography. The Mg# of ASDT (0.47–0.85) indicates their fractionated to evolved nature, corresponding to their transition from magnesian tholeiite (Mg# = ∼80) to Fe-rich tholeiite (Mg# = ∼50).

The relationships between immobile elements and Mg# and Zr suggest that ASDT are not primary melts and have experienced fractionation prior to emplacement in olivine + pyroxene + plagioclase sequence.

ASTD has variable but relatively high concentrations of TTE indicating control of mafic minerals such as pyroxenes. Sr and Ba contents are high and HFSEs are in moderate concentrations compatible with basalt chemistry. REE profiles of ASTD are nearly identical in shape with variable LREE enrichment (La/Yb_N:0.91–29.32). It is worth mentioning here that REE profile of two Borwar samples is like MORB (La/Yb_N:0.91) whereas its another sample is highly HREE fractionated (La/Yb_N:29.32). All ASTD samples show moderate to modest sink at Eu (Eu/Eu*:0.61–0.85) except VAD1 (Eu/Eu*:1.18) indicating variable plagioclase fractionation (Fig. 7).

PM normalized diagrams of ASTD show general enrichment of Rb, Th, Ta, Pb coupled with depletion in Ba, Nb, K and Ti of variable magnitude. Some samples of Ranakpur, Borwar and Seliberi dykes show significant depletion of P₂O₅ whereas all other samples are P₂O₅ enriched (Fig. 7).

5.2 Classification of AST dykes

Total Alkali-Silica, Zr/TiO₂-Nb/Y and YTC (Y:Y + Zr, T:TiO₂*100, C:Cr) diagrams classify ASTD as subalkaline rocks of basaltic to basaltic-andesite character with tholeiite to calcalkaline affinity indicating some role of subduction mechanism in their magma genesis.

Conventional discrimination diagrams give conflicting results about tectonic setting of ASTD varying from continental rift, oceanic basalt, continental arc to oceanic arc affinity (Fig. 6). Effectiveness of these discrimination diagrams have been questioned and debated by many researchers. Doucet et al. (2022), who observed that primary reason for such shortfalls stems from the fact that BABB, OFB, CFB and ARC-C environments exhibit mixed geochemical signatures due to localized factors, such as crustal and/or lithospheric contamination as well as their plate tectonic based origin i.e. arc settings. When ASTD data is plotted in more robust diagrams devised by Zhang et al. (2019), a broad IAB setting results for ASTD (Fig. 6e,f). Integration of the inferences adduced from Zhang’s diagrams and conventional discrimination diagrams e.g. Nb/Yb − Th/Yb (Pearce, 2008) and La/ Yb-Nb/La (Hollocher et al., 2012) (Fig. 6g,h) a continental arc setting for ASTD appears to be most convincing interpretation. PM normalized REE profiles of ASTD is another testimony for their continental arc character (Fig. 7).

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Fig. 6

Tectonic setting diagrams of AST Dykes. (a)Ta/Hf–Th/Hf (Wang et al. 2001): I. Divergent plate margin MORB; II. Convergent plate margin basalts; III. Oceanic within-plate basalts; IV. Continental within plate basalts; IV1: Intra continental rift + continental margin rift tholeiites; IV2: Intra continental rift alkali basalts; IV3: Continental extensional zone/initial rift basalts); V. Mantle plume basalts, (b) TiO₂/Yb (Pearce, 2008), (c) Zr − Zr/Y (Pearce, 1983), (d)Y/La-Nb/Yb-Th/Ta (e)Nb/Y vs K₂O × 100/Cu, (f)Sr/V vs (Na₂O × 100/Ga)(g) Nb/Yb − Th/Yb (Pearce, 2008) and(h) La/ Yb − Nb/La (Hollocher et al., 2012) showing within plate to arc character of AST Dykes.

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Fig. 7

Chondrite normalized REE diagrams and Primitive mantle (PM) normalized multielement diagrams for two groups of AST Granites and dykes. Normalizing values after McDonough and Sun (1989).

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Verma (2009) has pointed out that the Nb value is critically influenced by the tectonic setting. (Nb/Nb*)_PM values, [{2×(Nb_sa)/Nb_pm)/{(Ba_sa)/Ba_pm)} + {(La_sa)/(La_pm)}], where sa: concentration of element in sample, PM: concentration in the primitive mantle] in ASTD are 0.23–2.91. In intra-plate basalts this ratio is 0.25–0.70. Geochemical parameters, such as TiO₂ ≥ 2 wt% and P₂O₅ ≥ 1 wt% contents and (Nb/Y)/(Zr/P₂O₅ × 10000) ratio >1 indicate an intra-plate setting for ASTD (Winchester & Floyd, 1975). The within-plate affinity in conjunction with subduction signatures suggests that AST volcanism took place in a post-collisional continental geodynamic setting. Volcanism was probably driven by mantle convection processes which, in turn, might have been triggered by a late stage slab steepening or break-off beneath the Phulad ophiolite zone.

5.3 Petrogenesis of AST dykes

ASTD can also be divided into two groups. ASTD1 (Ranakpur, Seliberi and Borwar) are distinct from ASTD2 as displayed by their markedly lower concentrations of Fe₂O₃^T (4.33–6.93 wt%), TiO₂ (0.48– 0.63 wt%) and P₂O₅ (0.15–0.18 wt%), higher MgO (6.97–8.11 wt%), CaO (11.04––18.68 wt%), and silica contents (52.34–61.77 wt%). ASTD1 is also depleted in TTE, LILE (except Th, U) and HFSE but contains higher concentrations of REEs. Whereas ASTD2 (Borwar – BOD1, Vagdadi and Chiknawas) are characterized by lower SiO₂ (49.81–53.94 wt%), MgO (2.97–6.1 wt%), CaO (6.13–11.57 wt%), and significantly higher Fe₂O₃^T (13.17–16.8 wt%), TiO₂ (2.17–3.02 wt%), Al₂O₃ (11.11–16.35 wt%), Na₂O (1.35–2.3 wt%), K₂O (0.8–2.76 wt%) and P₂O₅ (0.65–2.86 wt%). Contrary to ASTD1, rocks of ASTD2 possess higher concentrations of TTE, LILE (i.e. Rb, Sr, Ba). These features of ASTD2 are similar to the Fe-Ti basalts which are characterized by iron- and titanium enrichment (FeO^T > 12 wt%, TiO₂ > 2 wt% and FeO^T/MgO > 1.75) coupled with silica depletion (Qian et al., 2006). The high Fe-Ti-P basaltic magma may be produced by several mechanism ranging from unmixing of magma, basaltic magma getting assimilated by Fe-Ti-P enriched rocks during its upward ascent, melting of pyroxenite mantle to produce Fe-Ti- rich primary magma which was later assimilated by P-rich rocks.

Since the ASTD do not have Fe-Ti oxides phenocrysts, the high concentrations of iron and titanium were not caused by the cumulus Fe-Ti oxides, and the Fe-Ti basalts may be interpreted to be products of moderate to high degree of Fenner trend differentiation of basaltic magma at low oxygen fugacity (Qian et al., 2006), to which a high P – rich rock or strange magmatic pulse enriched in P got assimilated. The cumulative product of this magmatic combination is the rocks ASTD2. However, ASTD1 possess unequivocal subduction signatures like negative Nb–Ta anomaly and enriched LILEs. When compared with primordial mantle, except Zr/Hf ratio, all other ratios of both groups are significantly higher or lower. Furthermore, despite possessing high MgO contents, ASTD1 are depleted in TTE but enriched in REEs compared with ASTD2 (Table 1C). These compositional differences between the two groups of ASTD refute their cogenetic origin either by fractional crystallization or crustal contamination of a common parental magma.

Sc is incompatible in olivine, plagioclase, and spinel, but is compatible in pyroxene, amphibole, and garnet. The presence of compatible minerals in the source would reduce Sc concentration in the magma. Niobium is a highly incompatible element in the mantle, but is highly compatible in rutile during plate subduction, which plausibly explains the typical Nb-Ta negative anomalies in IAB. Furthermore, IAB magmas are water rich which lowers the solidus line of plagioclase and increases that of amphibole, so that amphibole crystalizes earlier than plagioclase (Alonso-Perez et al., 2009). All these signatures of IAB are nearly present in ASTD1. On the other hand ASTD2 possess some chemical features generally found in within-plate rocks.

Although it is difficult to define any chronological/stratigraphic hierarchy of the two groups of dykes, it appears convincing to believe that ASTD2 are relatively younger as they are disposed close to Kumbalgarh sediments (Delhi Supergroup) and associated with post orogenic porphyritic granites (ASTG2) whereas ASTD1 are in the vicinity of Phulad ophiolite in association with synorogenic foliated granite (ASGT1). Phulad ophiolite is considered to represent oceanic crust formed in island arc environment (Khan et al., 2019). It implies that ASTD1 and their host granites may be an extensional activity of oceanic closure. Alternatively, the continental arc, a possible tectonic setting adduced from the chemistry for ASTD, developed in the back arc setting. It seems reasonable to infer that ASTD1 was derived from magmas formed when the base of the previously metasomatized, volatile-mineral bearing subcontinental lithospheric mantle (Phulad mantle wedge) was heated by an upwelling mantle plume. ASTD2 were formed by partial melting of the rising plume head at a depth where garnet was stable, perhaps > 80 km and got emplaced when back arc basin closed (i.e. a post collisional set up) as explained in Fig. 8.

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Fig. 8

Generalized sketch map illustrating the genesis of AST Granites and dykes and present-day distribution of various litho-components of Aravalli craton and Trans Aravalli Region (age brackets from multiple sources).

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