Geoscience Frontiers 5 (2014) 781e790

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Research paper

Petrogenesis of the crater-facies Tokapal kimberlite pipe, Indr�avatiBasin, Central India

N.V. Chalapathi Rao a,*, B. Lehmann b, B.K. Panwar a, Alok Kumar a, D. Mainkar c

aDepartment of Geology, Centre of Advanced Study, Banaras Hindu University, Varanasi 221005, IndiabMineral Resources, Technical University of Clausthal, 38678 Clausthal-Zellerfeld, GermanycDirectorate of Geology and Mining, Raipur, Chhattisgarh 492007, India

a r t i c l e i n f o

Article history:Received 10 October 2013Received in revised form20 November 2013Accepted 30 November 2013Available online 19 December 2013

Keywords:KimberliteTokapalColumbiaBastar cratonIndia

* Corresponding author. Tel.: þ91 9935647365.E-mail addresses: nvcr100@gmail.com, nvcraobhu@Rao).

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a b s t r a c t

New geochemical data of the crater-facies Tokapal kimberlite system sandwiched between the lower andupper stratigraphic horizons of the Mesoproterozoic Indr�avati Basin are presented. The kimberlite hasbeen subjected to extensive and pervasive low-temperature alteration. Spinel is the only primary phaseidentifiable, while olivine macrocrysts and juvenile lapilli are largely pseudomorphed (talc-serpentine-carbonate alteration). However, with the exception of the alkalies, major element oxides displaysystematic fractionation trends; likewise, HFSE patterns are well correlated and allow petrogeneticinterpretation. Various crustal contamination indices such as (SiO2 þ Al2O3 þ Na2O)/(MgO þ K2O) and Si/Mg are close to those of uncontaminated kimberlites. Similar La/Yb (79e109) of the Tokapal sampleswith those from the kimberlites of Wajrakarur (73e145) and Narayanpet (72e156), Eastern Dharwarcraton, southern India implies a similarity in their genesis. In the discriminant plots involving HFSE theTokapal samples display strong affinities to Group II kimberlites from southern Africa and central India aswell as to ‘transitional kimberlites’ from the Eastern Dharwar craton, southern India, and those from thePrieska and Kuruman provinces of southern Africa. There is a striking similarity in the depleted-mantle(TDM) Nd model ages of the Tokapal kimberlite system, Bastar craton, the kimberlites from NKF and WKF,Eastern Dharwar craton, and the Majhgawan diatreme, Bundelkhand craton, with the emplacement ageof some of the lamproites from within and around the Palaeo-Mesoproterozoic Cuddapah basin,southern India. These similar ages imply a major tectonomagmatic event, possibly related to the break-up of the supercontinent of Columbia, at 1.3e1.5 Ga across the three cratons. The ‘transitional’geochemical features displayed by many of the Mesoproterozoic potassic-ultrapotassic rocks, acrossthese Indian cratons are inferred to be memories of the metasomatising fluids/melts imprinted on theirsource regions during this widespread event.

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1. Introduction

Despite their volumetric insignificance kimberlites continue toattract wide global attention primarily due to their (i) derivationfrom greater depths than any other known rock type, (ii) entrained

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mantle- as well as crustal-xenoliths, and (iii) association with dia-mond (see Pearson et al., 2003; Pilbeam et al., 2013; Sparks, 2013;Tappe et al., 2013). Much of the kimberlite terminology and geneticmodels in vogue are influenced to a great extent by the southernAfrican occurrences and three variants of kimberlites, viz., Group I(kimberlite sensu stricto), Group II (or orangeite) and ‘transitional’type are known from the Kaapvaal craton of southern Africa. Theseare distinguished based on their differing mineralogy, geochem-istry and radiogenic (Sr, Nd and Pb) isotopes (e.g., Smith, 1983;Mitchell, 1995; Le Roex et al., 2003; Becker and Le Roex, 2006;Becker et al., 2007). Whereas Group I type (or archetypal) kim-berlites are known from Archaean cratons world-wide, the Group IIkimberlites were, for long, considered to be restricted only to theKaapvaal craton of southern Africa (Skinner, 1989; Mitchell, 1995)but such rocks have also been reported from (i) the Siberian

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Figure 1. (A) Generalised geological map of India showing the location of the Tokapalkimberlite in the Bastar craton, central India. The occurrence of other lamproites andkimberlites in various cratons (modified after Sahu et al., 2013) is also shown.CBF ¼ Chitradurga fault; CITZ ¼ Central Indian tectonic zone; CUB ¼ Cuddapah basin;CB ¼ Chhattisgarh basin; VB ¼ Vindhyan basin; SMB ¼ Singhbhum mobile belt;CIS ¼ Central Indian shear zone; W ¼ Wajrakarur kimberlite field; R ¼ Raichurkimberlite field; Np ¼ Narayanpet kimberlite field; T ¼ Tokapal kimberlite;M ¼ Mainpur kimberlite field; Dv ¼ Damodar valley; Mg ¼ Majhgawan; S ¼ Saptarshi;K ¼ Khadka; N ¼ Nuapada; G ¼ Garledinne; Vk ¼ Vattikod; Kr ¼ Krishna;Rd ¼ Ramadugu. (B) Geological setting of the Tokapal kimberlite system (adapted fromMainkar et al., 2004).

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Province, Russia (Bobrievich et al., 1959) and (ii) the Mainpur fieldof Bastar craton, central India (Lehmann et al., 2010; Chalapathi Raoet al., 2011; Fig. 1A). Likewise, the ‘transitional’ kimberlites havealso been reported from the Koidu Province, West Africa (Tayloret al., 1994), the KolaeKuloi craton (Beard et al., 2000) andNukyan Province, Russia (Agashev et al., 2008), the São Franciscocraton, Brazil (Bizzi et al., 1994), the Kimberley craton in north-western Australia (Edwards et al., 1992), the Guyana craton inVenezuela (Kaminsky et al., 2004), the Bundelkhand craton in northcentral India (Chalapathi Rao, 2005), and the Eastern Dharwarcraton of southern India (Haggerty and Birkett, 2004; ChalapathiRao and Dongre, 2009; Chalapathi Rao et al., 2012b).

The recent discovery of end-Cretaceous diamondiferous Group IIkimberlites synchronous with the eruption of the Deccan flood ba-salts, in the Mainpur field of the Bastar craton of central India(Lehmann et al., 2010) has given a new impetus to the genesis of theGroup II kimberlites as well as diamond exploration activities inIndia. This has alsonecessitated a re-look at the kimberlites reportedfrom various Indian cratons to ascertain their Group II kimberliticaffinities, if any, since the generation of the latter implies specificgeodynamic settings, viz., mantle-plume lithosphere interactions(Le Roex, 1986; Chalapathi Rao et al., 2011) or extensional eventsrelated to supercontinent breakup (Phillips et al.,1998). Even thoughthe crater-facies Tokapal pipe has been well-studied over the yearsin terms of its geology, petrology, geochemistry and diamond po-tential (Ramakrishnan, 1987; Mainkar et al., 2004; Lehmann et al.,2006; Chalapathi Rao et al., 2012a), little attempt was made to un-derstand its petrogenetic aspects especially in comparisonwith thewell-characterised kimberlites from the Wajrakarur, Narayanpetand Raichur fields of the Eastern Dharwar craton and the Group IIkimberlites from the Mainpur field, Bastar craton, (see Fig. 1A forlocations) which is the main purpose of this paper.

2. Geological set-up

The Tokapal kimberlite is a saucer-shaped volcanic sheet, with adiameter of w2.5 km, occurring in the western part of the Meso-proterozoic platformal sequence of the Indr�avati Basin on theBastar craton of central India (Fig. 1B). Together with its NE expo-sure at Bejripadar, which constitutes its satellite body, it is one ofthe world’s largest (>550 ha) crater-facies kimberlites (Mainkaret al., 2004; Lehmann et al., 2006). The kimberlite system isexposed at the peneplained surface (Fig. 2A), in dug-wells as well asin stream cuttings and excavations (Fig. 2B). The SE part of thekimberlite is capped by a thick laterite cover and the circular courseof the Bahar Nala (stream) marks its margin (Fig. 1B). The Tokapalkimberlite system is located stratigraphically between the lowerIndravati Group of sedimentary rocks of the Cherakur (comprisingmicaceous purple shale) and Kanger formations (white and greylimestone) and the upper Jagdalpur Formation (limestone, calcar-eous shale and sandstone intercalations) and represents volcanicactivity during intracratonic sedimentation denoting an eventbreak (Ramakrishnan, 1987; Mishra et al., 1988). Gravity, magneticand resistivity surveys over the kimberlite system revealed itsthickness to be >50 m at many places and well over 90 m at a fewsites (Das et al., 2005). In situ U-Pb dating on titanite from Bejri-padar tuff-facies kimberlite gave a minimum Neoproterozoic age of620 � 30 Ma (Lehmann et al., 2007). However, U-Pb ages ofmagmatic zircons from the rhyolitic tuff from the uppermost Jag-dalpur Formation of the Indr�avati Basin gave an age of 1000 Ma(Mukherjee et al., 2012) suggesting that the titanite age possiblyrepresents an overprinted age (Pan-African event?). Thus, theemplacement age of the Tokapal system needs to be in excess of1000 Ma and very likely coincides with the 1100 Ma widespreadkimberlite event recorded from the Eastern Dharwar craton (Anil

Kumar et al., 2007; Chalapathi Rao et al., 2013a) thereby consti-tuting the world’s known oldest crater-facies kimberlite system.

3. Analytical techniques

Nine kimberlite samples, free from visible crustal contamina-tion, have been collected from various exposed domains of the

Figure 2. (A) Tokapal kimberlite exposed (marked by dotted lines) in a stream cutting. Note the gently undulating peneplained surface in the background. (B) Tokapal tuff exposedin a pit. (C) Photomicrograph of the Tokapal pipe showing olivine macrocrysts (Ol) and pelletal lapilli (Plp) (under plane polarised light). (D) Back scattered electron (BSE) image ofthe Tokapal pipe showing olivine macrocrysts (Ol), titanite (Ti) and spinel (Sp).

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kimberlite system and their geographical co-ordinates are providedin Table 1. Microscopic studies were carried out under an OlympusBX51 polarising research microscope at Department of Geology,BHU, India, whereas back-scattered electron (BSE) imaging wascarried out on a CAMECA SX100 Electron Probe Micro Analyzer(EPMA) at the Mineral Resources Unit, Technical University ofClausthal, Germany. Geochemical analyses of the samples werecarried out at Activation Laboratories Ltd., Ancaster, Ontario, Can-ada. Major elements were analysed by ICP (Model: Agilent 735)after lithium metaborate/tetraborate fusion and accuracy is betterthan 1e2%. Trace elements were analysed by ICP-MS spectrometry(Model: Perkin Elmer Sciex ELAN 9000) with an accuracy betterthan 5% at 100� the detection limit. International standards, suchas NIST 694, SY-4, BIR-1, W-2A, and WMG-1, were run to checkaccuracy and precision. The analytical procedure is detailed by Galeet al. (1997) and details are available at the Activation LaboratoriesLtd website (http://www.actlabs.com).

4. Petrography

The Tokapal kimberlite system was subjected to extensive low-temperature serpentinisation and carbonate alteration (seeMainkar et al., 2004). Two olivine generations, comprising sub-hedral to rounded macrocrysts (up to 15 mm), and euhedral fine-grained microphenocrysts (<0.5 mm size), as well as juvenilelapilli characterise the kimberlite. The shape and form (amoeboidto ovoid) of the lapilli are highly variable. The olivine is completelypseudomorphed by serpentine/talc and the groundmass is

dominated by serpentine, carbonate and accessory phases such asspinel and titanite (Fig. 2C and D). Spinel is widespread is, and infact, the only preserved primary mineral of petrogenetic signifi-cance. Some spinel crystals have chemical features of the diamondfacies field (Chalapathi Rao et al., 2012a). Rare garnierite (a Ni-Mg-bearing hydrous silicate phase), derived from serpentinisation ofolivine, has also been documented from parts of the Tokapalkimberlite (Chalapathi Rao et al., 2013b). Reconnaissance micro-diamond testing on two 18 kg samples by the Directorate ofGeology and Mining, Chhattisgarh, India, proved their non-diamondiferous nature (Mainkar et al., 2004). The classification ofthe Tokapal system as kimberlite is based on (i) its strikingly similarpetrography of the crater-facies Fort à la Corne (FALC) kimberlites,Saskatchewan, Canada, and other crater-facies kimberlites asdocumented by Mitchell (1997); (ii) presence of atoll-texturedspinels which are a characteristic feature of kimberlites(Chalapathi Rao et al., 2012a); and (iii) geochemical data (Mainkaret al., 2004).

5. Bulk-rock geochemistry

In view of their friable nature, tuffaceous kimberlites/lamproitesare highly susceptible to alteration and other secondary processesand, therefore, their geochemical studies are very limited in com-parison to those from the root zones or even from diatreme por-tions. Nevertheless, geochemical studies on such crater-facies rockshave been documented in the literature as in the case of kimberlitesfrom the North Alberta Province (Eccles et al., 2004) and NW

Table 1Bulk-rock geochemistry of samples from the Tokapal diatreme.

Oxide (wt%) TP1/1 TP1/2 TP2/1 TP2/2 TP3/1 TP3/2 TKP/1 TKP/2 TKP/3

N19�1030.200

E81�51028.800N19�1030.200

E81�51027.500N19�1034.900

E81�52022.200N19�1034.900

E81�52023.400N19�1034.900

E81�52022.600N19�1040.400

E81�52028.700N19�0102200

E81�52026.600N19�1034.900

E81�52022.200N19�1034.900

E81�52023.200

SiO2 40.64 41.22 47.12 47.81 35.68 35.35 39.96 35.91 36.47TiO2 3.033 3.256 2.59 2.562 1.763 2.009 1.388 2.073 1.539Al2O3 4.2 4.42 3.42 3.37 3.37 2.44 7.32 3.73 5.48Fe2O3

t 10.65 11.55 12.92 12.31 10.5 10.5 7.88 9.79 8.15MnO 0.04 0.034 0.018 0.018 0.196 0.222 0.139 0.144 0.103MgO 26.95 25.21 25.37 25.3 33.82 34.51 22.3 27.74 22.11CaO 1.16 1.31 0.53 0.56 0.63 0.72 8.8 5.52 11.07Na2O 0.03 0.03 0.04 0.04 0.02 0.01 0.04 0.03 0.03K2O 0.01 0.01 0.01 0.01 0.01 0.01 0.02 0.02 0.01P2O5 0.76 0.8 0.44 0.46 0.43 0.51 0.44 0.59 0.43LOI 10.81 10.39 6.52 6.53 12.61 12.48 10.83 13.33 13.39Total 98.28 98.22 98.98 98.97 99.04 98.76 99.11 98.88 98.78Mg# 83.4 81.2 80 81 86.5 86.7 84.9 84.9 84.3Si/Mg 1.17 1.27 1.44 1.46 0.82 0.79 1.39 1.00 1.28CI 1.66 1.81 1.99 2.02 1.15 1.10 2.12 1.43 1.90Trace elements (ppm)Sc 18 19 13 13 14 13 11 13 10V 69 63 99 96 60 55 275 60 150Cr 1540 1720 1680 1720 930 1020 610 1010 680Co 90 89 84 90 86 72 44 78 62Ni 2840 2970 1260 1390 1430 1750 800 980 950Cu 50 70 50 50 30 30 40 70 90Zn 220 250 200 220 80 50 80 90 50Ga 10 11 9 9 8 6 11 9 10Rb 2 2 2 2 2 2 2 4 2Sr 26 30 29 15 21 22 130 118 233Y 31 33 16 14 11 13 17 15 16Zr 227 246 187 176 113 126 171 180 172Nb 154 176 118 121 82 97 71 103 76Cs 0.5 0.5 0.5 0.5 0.5 0.5 0.5 2.9 0.5Ba 125 111 65 31 27 27 46 118 92Hf 5.4 6.2 4.4 4.5 2.9 3.1 4.3 4.4 4.2Ta 11 12 8 8.2 5.2 6.4 4.3 6.7 4.6Pb 16 19 21 23 12 8 5 38 5Th 18.5 19.6 13.5 13.8 8.1 9.3 24.7 15.5 25.5U 1.8 1.8 2.5 2.5 1.4 1.6 4.7 4.7 4.3La 164 175 104 92.5 63.9 70.1 157 85.8 112Ce 258 267 176 173 116 125 251 158 185Pr 29.2 31.3 22 19.3 11.9 13.9 23.3 16.2 17Nd 114 121 83.5 72.8 44.2 52.9 78.2 58.8 57Sm 17.7 19.4 13.8 11.7 7.1 8.4 10.7 9.6 8.9Eu 4.66 5.13 3.55 2.86 1.87 2.24 1.87 2.27 1.68Gd 12.3 13.5 8.1 7 4.7 5.5 6.4 6.3 5.7Tb 1.3 1.5 1 0.8 0.6 0.7 0.9 0.8 0.7Dy 5.9 6.5 4.2 3.7 2.6 3.2 4.6 3.4 3.8Ho 1 1.1 0.7 0.6 0.4 0.5 0.8 0.6 0.6Er 2.3 2.5 1.7 1.5 1.1 1.2 1.9 1.5 1.6Tm 0.27 0.27 0.2 0.19 0.13 0.14 0.23 0.17 0.2Yb 1.5 1.5 1.1 1 0.8 0.8 1.4 1 1.2Lu 0.2 0.2 0.15 0.14 0.11 0.11 0.2 0.14 0.18

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Territories (Nowicki et al., 2008), Canada, and the Argyle lamproite,western Australia (Jaques et al., 1989), wherein first-order petro-genetic interpretations were made after carefully evaluating themobility of elements in the bulk-rock samples either due to crustalcontamination and/or alteration.

Petrographic observations reveal that none of the samples un-der this study contain any visible crustal xenoliths. This is wellreflected in low SiO2 (35.4e41.2 wt%; with the exception of twosamples having an excess of 47 wt%) and Al2O3 (2.44e4.42 wt%;with two samples in excess of 5.48 wt%). These values are indis-tinguishable from those (SiO2: 34.6e52.4 wt%; Al2O3: 1.88e3.91 wt%) reported by Lehmann et al. (2006). The crustal contaminationindex (C.I.) (Clement, 1982; C.I. ¼ (SiO2 þ Al2O3 þ Na2O)/(MgO þ K2O)) for the samples under study ranges from 1.10 to 2.12and is close to the cutoff value (1.51) prescribed for uncontaminatedkimberlites. Likewise, Si/Mg values of the samples range from 0.79to 1.46 and are within or close to the threshold value of 1.2 (Fesq

et al., 1975) prescribed for uncontaminated kimberlites and addi-tionally exclude significant crustal contamination. Mg numbers ofthe samples range from 80.0 to 86.7 and demonstrate their prim-itive and ultramafic nature. Extreme impoverishment of alkalies(K2O < 0.02 wt% and Na2O < 0.04 wt%) and trace elements such asRb (<4 ppm) and Cs (<2.9 ppm) is a characteristic feature of theTokapal samples (Table 1) and reflects the effects of low-temperature alteration which has markedly affected the large ionlithophile element (LILE) concentration in the samples. On theother hand, titanium displays a tight range (TiO2: 1.76e3.25 wt%)reflecting its relative immobility in response to such alteration.With respect toMgO, the major oxides such as SiO2, Al2O3, TiO2 andFe2O3 display systematic variations (Fig. 3A to D) which can beattributed to fractionation. The high degree of correlation displayedbetween various (i) major-oxide and compatible (MgO - Ni), andincompatible (TiO2 - Nb) trace elements, and (ii) HFSE, e.g., Nb-Ta,Zr-Hf (Fig. 3EeH) is particularly striking. The previously published

Figure 3. Major and trace element variation diagrams of the samples from the Tokapal kimberlite. Published data for the Tokapal kimberlite as well as that of the Bejripadar pipe isalso depicted in the plots. High correlations displayed between major oxides as well as trace elements demonstrate that weathering and alteration undergone by the kimberlite didnot significantly influence its bulk-rock geochemistry.

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N.V. Chalapathi Rao et al. / Geoscience Frontiers 5 (2014) 781e790786

data for the Tokapal and Bejripadar systems has also been plotted inFig. 3; they are indistinguishable from the data set generated in thisstudy. High Loss on Ignition (LOI) contents (up to 13.4 wt%) testifyto the hydrous mineralogy dominated by serpentine/talc and tosome extent by carbonate. High abundances and varying transitionmetal contents, i.e., Ni: 800e2970 ppm, Cr: 610e1720 ppm and V:55e150 ppm (Table 1), imply their control by varying modal olivineproportions.

The Tokapal samples under study show highly fractionatedchondrite-normalised REE distribution patterns, with La abun-dances ranging from 250 to 700� chondrite (Fig. 4A), and HREEabundances of 7 to 10� chondrite. The HREE abundances areslightly higher (HREE: 3e8� chondrite) than those from thewell-studied kimberlites from the WKF and NKF but indistin-guishable from those of the RKF (HREE: 9e12� chondrite),Eastern Dharwar craton, southern India (see Chalapathi Rao et al.,2010). All samples display similar patterns which are indistin-guishable from the published data which is also shown forcomparison in Fig. 4A. The La/Yb ratios (79e109) of the samplesunder study are similar to those from the kimberlites of WKF(73e145) and NKF (72e156), Eastern Dharwar craton, southern

Figure 4. (A) Chondrite-normalised (after Evenssen et al., 1978) rare earth element (REE) disfrom Lehmann et al. (2006) and Paul et al. (2006). Note their overall similarity. (B) PrimitivTokapal samples under study. Negative anomalies of various magnitudes are noticeable for

India (Chalapathi Rao et al., 2004; Chalapathi Rao and Srivastava,2009; Paton et al., 2009).

Primitive-mantle normalised multi-element patterns (Fig. 4B)reveal that incompatible trace elements are substantially enrichedcompared to primitive mantle. Pronounced negative spikes areshown at Rb, Ba, K and Sr whereas slight to moderate negativespikes are displayed at U and Zr. Negative anomalies are conven-tionally interpreted to reflect a number of possible causes such as(i) hydrothermal or secondary alteration, (ii) the presence of re-sidual phases, and (iii) source characteristics (Gibson et al., 1995;Mitchell, 1995; Le Roex et al., 2003; Tappe et al., 2013). Most ofthe negative spikes for LILE can be attributed to pervasive low-temperature hydrothermal alteration and lateritic weathering. Onthe other hand, lower CI and Si/Mg values together with goodcorrelations, especially between HFSE, imply that the negative Zrspike could be source related (e.g., residual zircon).

6. Discussion

Combined petrography and geochemistry of the samples fromthe Tokapal kimberlite system reveal that they have been subjected

tribution patterns for the Tokapal samples of this study. Published data has been takene-mantle normalised (after Sun and McDonough, 1989) multi-element patterns for theRb, Ba, K, Sr and Zr (see text).

Figure 5. (A) Ce (ppm) versus Pb (ppm) bi-variate plot. Pb contents of >15 ppm and Cecontents of >200 ppm are characteristic of many southern African orangeites(Mitchell, 1995); (B) La/Nb versus Th/Nb incompatible element ratio plot to distinguishbetween kimberlites and orangeites (adapted from Coe et al., 2008), and (C) Zr (ppm)versus Nb (ppm) bi-variate plot. The field for WKF and NKF kimberlites is from thedatabase published in Chalapathi Rao et al. (2004), Chalapathi Rao and Dongre (2009)and Chalapathi Rao and Srivastava (2009). The symbols in the above three plots are thesame as in Fig. 3. The open triangle denotes data of Behradih Group II kimberlite(central India) taken from Mainkar and Lehmann (2007) and Chalapathi Rao et al.(2011). Note the close similarity between the Tokapal and Behradih samples in allplots.

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to low-temperature alteration and other secondary processeswhichhave significantly affected their LILE inventory, but their HFSEconcentrations and other immobile element inter-relationshipsremain robust and permit petrogenetic considerations. As theTokapal systemhas erupted in argillaceous and calcareous lithounitsof the Indr�avati Basin, via a granitic basement, assimilation of felsicminerals should have resulted in elevated silica and alumina con-tentswhich, however, is not the case. High La/Yb ratios coupledwiththe absence of positive Eu spikes, further argue against significantassimilation of crustalmaterial. Similarities in thenormalisedREE aswell as multi-element patterns preclude significant crustalcontamination as systematic similarities or differences cannot beattributed to the latter. It is pertinent to point out here that a low tomoderate degree of crustal contamination of the samples from theTokapal system and their similarity to global uncontaminatedkimberlites was highlighted earlier by Lehmann et al. (2006, p. 11).Therefore, compatible as well as incompatible (especially HFSE)trace element geochemistry of the Tokapal systemcanbeutilised forderiving petrogenetic inferences. Our results also additionallydemonstrate that even strongly weathered kimberlites have theirspecific chemical features particularly metals (Ni, Cr) and highlyfractionated chondrite-normalised REE distribution pattern.

High Ni (up to 2970 ppm), Cr (up to 1720 ppm) andMg# (>86.7),and low Al2O3 contents (<3.91 wt%) of the Tokapal samples implythe involvement of a refractory garnet-bearing mantle sourcesimilar to depleted harzburgite (e.g. Mitchell and Bergman, 1991;Beard et al., 2000). In order to account for the high abundance ofincompatible elements, low-degree melting of metasomatisedmantle is required (e.g. Gaffney et al., 2007; Paton et al., 2009; Tappeet al., 2013). Lower normalised HREE relative to LREE contentsnecessitate a source that has experienced previous melt depletion(e.g. Tainton and McKenzie, 1994). This scenario of initial depletionand subsequent enrichment is analogous to that observed in a ma-jority of continental lithospheric peridotite xenoliths (e.g., Carlsonand Irving, 1998; Beard et al., 2007) entrained in kimberlites, andis also invoked in the genesis of kimberlites from the EasternDharwar craton, southern India (Chalapathi Rao et al., 2004;Chalapathi Rao and Srivastava, 2009) and elsewhere (e.g., Le Roexet al., 2003; Becker and Le Roex, 2006; Donnelly et al., 2011).

A number of bi-variate discriminant plots involving criticalincompatible trace element pairs and their ratios (see Le Roexet al., 2003; Becker et al., 2007; Coe et al., 2008; Paton et al.,2009) are used to further evaluate the geochemical affinity ofthe Tokapal samples. Two such plots, involving Ce and Pb (Fig. 5A)and La/Nb and Th/Nb (Fig. 5B), are equivocal in the sense thatsamples from Tokapal display affinities to Group I as well as GroupII kimberlites. However, in plots involving Zr and Nb (Fig. 5C),Th/Nb vs Ce/Pb (Fig. 6A) and La/Sm vs Gd/Lu (Fig. 6B), the samplesfrom the Tokapal system show (i) strong affinities to the Group IIkimberlites from southern Africa in general and those from theBehradih Group II kimberlite of the Mainpur field, central India, inparticular, and (ii) slight affinity to the field of kimberlites fromWKF and NKF, southern India and southern Africa. Crucial factorssuch as degree of melting, depth of melting and control of garnetin the source could have brought out the observed variations in theTokapal samples (see Eccles et al., 2004) especially given the largevolume and aerial extent of the kimberlite system. It should bementioned here that the Group II kimberlitic affinities of theTokapal kimberlite were also recently highlighted by Dhote et al.(2013).

Available 87Sr/86Sr(i) (0.689472e0.703239) and εNd(i) (1.7 to 2.9)isotopic data (t ¼ 1100 Ma) for the Tokapal system (Lehmann et al.,2006) imply derivation from a OIB type mantle source that hadexperienced a long term depletion in LREE similar to that recordedfrom archetypal kimberlites. This is very different from the

radiogenic Sri > 0.7070 and low εNd(i) of �5 typical of Group IIkimberlites and lamproites (Mitchell, 2006; Chalapathi Rao et al.,2011). Thus, the radiogenic isotopic signatures of the Tokapal sys-tem are decoupled from their geochemical signals in terms of

Figure 6. (A) Incompatible element Th/Nb versus Ce/Pb ratio plot for samples understudy. Group I and II kimberlite cut-off values of Ce/Pb ¼ 20 and Th/Nb ¼ 0.14 are fromBecker et al. (2007) defined for South African kimberlites. (B) La/Sm versus Gd/Lu bi-variate for the samples under study. The fields for Group I (Kimberley) and II (Starand Swartruggens) kimberlites are from Coe et al. (2008). Symbols are the same as inFig. 5.

N.V. Chalapathi Rao et al. / Geoscience Frontiers 5 (2014) 781e790788

Group II kimberlitic affinities. Such decoupling has also beendocumented from some of the kimberlites from the Eastern Dhar-war craton, southern India (Haggerty and Birkett, 2004; Paul et al.,2006; Chalapathi Rao and Dongre, 2009) as well as those from thePrieska and Kuruman provinces of the Kaapvaal craton, southernAfrica (Skinner et al., 1994; Donnelly et al., 2011). Even though themineralogical-genetic classifications have for long been consideredappropriate for the nomenclature of the alkaline mafic potassic-ultrapotassic rock spectrum (Mitchell, 1995; Le Maitre, 2002;Mitchell and Tappe, 2010) the robustness of such schemes isquestionable. For example, Taylor and Kingdom (1999) demon-strated pit-falls in their application to the Jagersfontein kimberlite

Table 2Nd isotopic compositions and TDM model age of the kimberlites from various Indian crspecified.

Name of the kimberlite field/pipe Emplacement age(absolute/inferred)

Wajrakarur kimberlite field (WKF), Eastern Dharwar craton 1100 MaNarayanpet kimberlite field (NKF), Eastern Dharwar craton 1100 MaRaichur kimberlite field (RKF), Eastern Dharwar craton 1100 MaMajhgawan pipe, Bundelkhand craton 1100 MaTokapal kimberlite, Bastar craton 1100 MaBejripadar kimberlite, Bastar craton 1100 Ma

and stressed the need to place reliance on appropriate traceelement and isotopic discriminators. Likewise, Francis andPatterson (2010) argued that the mineral chemistry of the inter-stitial groundmass phases would only characterise the residualliquids, whereas the bulk-rock composition of the olivine-richrocks, such as kimberlites, is ideally suitable for constraining thecomposition of parental magmas. In light of the above, we opinethat the orangeitic geochemical signals portrayed by the Tokapalsystem merit significance.

Apart from mantle-plume models that are widely in vogue toaccount for the genesis of southern African Group II kimberlites(Le Roex, 1986; Coe et al., 2008), the onset and cessation ofGroup II kimberlite emplacement has also been attributed toextensional events related to the commencement and break-upof the Gondwana supercontinent during the early Cretaceous(Phillips et al., 1998). The depleted-mantle (TDM) Nd model ages(Table 2; which are minimum ages) of 1380e1470 Ma of theTokapal kimberlite system (Lehmann et al., 2006) suggest thatthe timing of its source enrichment is indistinguishable from (i)the perovskite depleted-mantle (TDM) Nd model ages(1.3e1.5 Ga; Chalapathi Rao et al., 2013a) of the NKF and WKFkimberlites, Eastern Dharwar craton, southern India, (ii) bulk-rock depleted-mantle (TDM) Nd model ages (1.4e1.5 Ga) of thediamondiferous Majhgawan diatreme of Bundelkhand craton(Lehmann et al., 2006), and (iii) emplacement ages(1.37e1.43 Ga) of some of the lamproites from within andaround the Palaeo-Mesoproterozoic Cuddapah basin, EasternDharwar Craton (Chalapathi Rao et al., 2013c). This synchroneityimplies the involvement of a widespread tectonomagmaticevent in the Eastern Dharwar, Bastar and Bundelkhand cratonsof the Indian shield, possibly related to the break-up of theColumbia Supercontinent (Rogers and Santosh, 2009; Meert,2012). Columbia is widely regarded to be the first supercontin-ent in Earth’s history whose amalgamation took place between2.0 and 1.7 Ga and it remained as a “quasi-integral continentallid” till at least 1.3 Ga followed by its transformation to thesupercontinent of Rodinia (see Roberts, 2013 and the referencestherein). The final break-up of Columbia at 1.3e1.2 Ga is thoughtto be represented by 1271 Ma Mackenzie mafic dyke swarm andco-eval flood basalts in North America and other mafic dykeswarms of similar age in Australia, southern Africa and Ferro-scandia possibly related to multiple hotspots which were activeglobally during that time-frame (Goldberg, 2010) or even to aprolonged superplume (Peng et al., 2013 and the referencestherein). The ‘transitional’ geochemical features strikingly dis-played by many of the Mesoproterozoic potassic-ultrapotassicrocks (Chalapathi Rao, 2005; Chalapathi Rao and Dongre,2009), including those of the Tokapal system, across variousIndian cratons could well be memories imprinted upon theirsource regions by the metasomatising fluids/melts derived fromthis regional tectonomagmatic event(s) associated with theColumbia tectonics.

atons. Material on which Nd isotopes were measured (mineral/whole-rock) is also

εNd(t) TDM model age Reference

þ2.08 to þ2.92 (perovskite) 1385 to 1438 Ma Chalapathi Rao et al. (2013a)þ0.68 to þ1.99 (perovskite) 1450 to 1590 Ma Chalapathi Rao et al. (2013a)þ1.72 (perovskite) 1490 Ma Chalapathi Rao et al. (2013a)þ0.3 to þ0.4 (whole-rock) 1488 to 1524 Ma Lehmann et al. (2006)þ1.2 to þ2.9 (whole-rock) 1494 to 1593 Ma Lehmann et al. (2006)þ2.4 (whole-rock) 1398 Ma Lehmann et al. (2006)

N.V. Chalapathi Rao et al. / Geoscience Frontiers 5 (2014) 781e790 789

7. Conclusions

The main conclusions of this study are:

(1) Combined petrography and geochemistry of the samples fromthe Tokapal system reveal a minimal role of crustal contami-nation whereas low-temperature serpentinisation and car-bonate alteration is a dominating factor;

(2) With the exception of alkalies and LILE, the major element andHFSE concentrations of the Tokapal kimberlite system display ahigh order of correlation and fractionation trends demon-strating their robustness for petrogenetic inferences;

(3) The La/Yb ratios (79e109) of the Tokapal kimberlite system aresimilar to those of the kimberlites of WKF (73e145) and NKF(72e156), Eastern Dharwar craton, southern India, therebyimplying their similar genesis;

(4) In bi-variate discriminant plots involving Zr vs Nb, Th/Nb vs Ce/Pb and La/Sm vs Gd/Lu, the samples from the Tokapal systemdisplay strong affinities to the Group II kimberlites fromsouthern Africa and central India as well as to ‘transitionalkimberlites’ from the Eastern Dharwar craton, southern India,and those from the Prieska and Kuruman provinces of southernAfrica;

(5) A striking similarity between the (i) depleted-mantle (TDM) Ndmodel ages of the Tokapal kimberlite system, Bastar craton, thekimberlites from NKF and WKF, Eastern Dharwar craton, theMajhgawan diatreme of Bundelkhand craton, and the (ii)emplacement age of some of the lamproites from within andaround the Palaeo-Mesoproterozoic Cuddapah basin, southernIndia, implies a widespread regional metasomatic as well astectonomagmatic event, possibly related to the break-up of thesupercontinent of Columbia, across these three cratons;

(6) The ‘transitional’ geochemical features strikingly displayed bymany of the Mesoproterozoic potassic-ultrapotassic rocks,including those of the Tokapal system, across various Indiancratons may be a direct consequence of similar source enrich-ment process(es) during this regional event.

Acknowledgements

We thank the Head, Department of Geology, BHU for his supportand encouragement. BKP and AK thankfully acknowledge UGC andCSIR, respectively, for the award of research fellowships.Constructive reviews by Felix Kaminsky and an anonymousreviewer and editorial suggestions by Nick Roberts are thankfullyacknowledged.

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