Petrology, bulk-rock geochemistry, indicator mineral composition, and zircon U-Pb geochronology of...

Petrology, Bulk-Rock Geochemistry, IndicatorMineral Composition and Zircon U–PbGeochronology of the End-CretaceousDiamondiferous Mainpur Orangeites, BastarCraton, Central India

N. V. Chalapathi Rao, B. Lehmann, E. Belousova, D. Frei, and D. Mainkar

Abstract

The end-Cretaceous diamondiferous Mainpur orangeite field comprises six pipes (Behradih,Kodomali, Payalikhand, Jangara, Kosambura and Bajaghati) located at the NE margin of theBastar craton, central India. The preservation of both diatreme (Behradih) and hypabyssalfacies (Kodomali) in this domain implies differential erosion. The Behradih samples arepelletal and tuffisitic in their textural habit, whereas those of the Kodomali pipe haveinequigranular texture and comprise aggregates of two generations of relatively fresholivines. The Kosambura pipe displays high degrees of alteration and contamination withsilicified macrocrysts and carbonated groundmass. Olivine, spinel and clinopyroxene in theBehradih and the Kodomali pipes share overlapping compositions, whereas the groundmassphlogopite and perovskite show conspicuous compositional differences. The bulk-rockgeochemistry of both the Behradih and Kodomali pipes has a more fractionated naturecompared to southern African orangeites. Incompatible trace elements and their ratios readily

Electronic supplementary material The online version of thisarticle (doi:10.1007/978-81-322-1170-9_7) contains sup-plementary material, which is available to authorized users.

N. V. Chalapathi Rao (&)Centre of Advanced Study in Geology, Banaras HinduUniversity, Varanasi, 221005, Indiae-mail: [email protected]

B. LehmannMineral Resources, Technical University of Clausthal,38678, Clausthal-Zellerfeld, Germanye-mail: [email protected]

E. BelousovaGEMOC, Department of Earth and Planetary Sciences,Macquarie University, Sydney, NSW 2109, Australiae-mail: [email protected]

D. FreiDepartment of Earth Sciences, Stellenbosch University,Private Bag X1, Matieland, 7602, South Africae-mail: [email protected]

D. MainkarDirectorate of Mines and Geology, Chhattisgarh,Raipur, 492007, Indiae-mail: [email protected]

D. G. Pearson et al. (eds.), Proceedings of 10th International Kimberlite Conference,Volume 1, Special Issue of the Journal of the Geological Society of India,DOI: 10.1007/978-81-322-1170-9_7, � Geological Society of India 2013

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http://dx.doi.org/10.1007/978-81-322-1170-9_7

distinguish them from the Mesoproterozoic Wajrakarur (WKF) and the Narayanpetkimberlites (NKF) from the eastern Dharwar craton, southern India, and bring out theirsimilarity in petrogenesis to southern African orangeites. The pyrope population in theMainpur orangeites is dominated by the calcic-lherzolitic variety, with sub-calcic harzburg-itic and eclogitic garnets in far lesser proportion. Garnet REE distribution patterns from theBehradih and Payalikhand pipes display ‘‘smooth’’ as well as ‘‘sinusoidal’’ chondrite-normalised patterns. They provide evidence for the presence of a compositionally layeredend-Cretaceous sub-Bastar craton mantle, similar to that reported from many other cratonsworldwide. The high logfO2 of the Mainpur orangeite magma (DNNO (nickel-nickel oxide)of +0.48 to +4.46 indicates that the redox state of the lithospheric mantle cannot be of first-order control for diamond potential and highlights the dominant role of other factors such asrapid magma transport. The highly diamondiferous nature, the abundance of calcic-lherzoliticgarnets and highly oxidising conditions prevailing at the time of eruption make the Mainpurorangeites clearly ‘‘anomalous’’ compared to several other kimberlite pipes worldwide. U–Pbdating of zircon xenocrysts from the Behradih pipe yielded distinct Palaeoproterozoic ageswith a predominant age around 2,450 Ma. The lack of Archean-aged zircons, in spite of thefact that the Bastar craton is the oldest continental nuclei in the Indian shield with anEoarchaean crust of 3.5–3.6 Ga, could either be a reflection of the sampling process or of themodification of the sub-Bastar lithosphere by the invading Deccan plume-derived meltsduring the Late Cretaceous.

Keywords

Petrology�Geochemistry� Indicator minerals�U–Pb age�Zircon�Orangeite�Mainpur�Bastar craton � India

Introduction

For more than a century, Group II kimberlites (also termedas orangeites) are believed to have been restricted only tothe Kaapvaal craton of southern Africa (Skinner 1989;Mitchell 1995). However, recent discovery of diamondif-erous orangeites, synchronous with the eruption of Deccanflood basalts, from the Bastar craton of central India(Lehmann et al. 2010; Chalapathi Rao et al. 2011a) providesa rare opportunity to investigate the controls on genesis ofsuch magmas. In this study, we report new data involvingpetrology, bulk-rock geochemistry, indicator mineralchemistry (garnet, Cr-diopside and spinel xenocrysts) andU–Pb zircon geochronology of the Behradih, Kodomali,Payalikhand and Kosambura pipes from the Mainpurorangeite field, Bastar craton, Central India. We also esti-mate oxygen fugacity (fO2) of the Mainpur orangeite magmasby applying Fe-Nb oxybarometry to their perovskite chem-istry and attempt to assess the role of oxidation state in theirdiamond grade. The findings of this study provide additionalinsights into the (1) origin of these pipes, (2) diamond pros-pectivity, (3) nature and composition of the sub-Bastarlithosphere, and (4) in furthering our understanding on thepetrological and geochemical differences between theMainpur orangeites and well-studied Mesoproterozoic

kimberlites from the Wajrakarur (WKF) and Narayanpet(NKF) fields, Dharwar craton, southern India as well as thosefrom the Kaapvaal craton of southern Africa.

Geology of the Bastar Craton and the MainpurOrangeite Field

The Bastar craton is one of the oldest nuclei in the CentralIndian shield and is regarded to be a component of the earlyArchaean supercontinent ‘‘Ur’’ (Rogers and Santosh 2003).The basement consists largely of granitic rocks and maficdyke swarms which are of Meso-Neoarchaean age(Crookshank 1963; Ramakrishnan and Vaidyanadhan2008). A thickened continental crust (35–40 km) in theBastar craton since at least 3.6 Ga to the present day hasbeen documented from geochronological and geophysicalstudies (e.g., Rajesh et al. 2009; Jagadeesh and Rai 2008).The granitic basement is unconformably overlain by severalsupracrustal, intracratonic Meso- to Neoproterozoic sedi-mentary sequences, such as the Sabri Group (Sukma basin),the Indravati Group (Indravati basin), Pairi Group (Khariarbasin) and Chhattisgarh Supergroup (Chhattisgarh basin)from south to north (see Ramakrishnan and Vaidyanadhan2008).

94 N. V. Chalapathi Rao et al.

Three kimberlite/orangeite fields are so far known in theBastar craton: (1) the southern Indravati kimberlite field(IKF) represented by the C620-Ma non-diamondiferouskimberlite pipes intruded into the sedimentary rocks of theIndravati basin at Tokapal, Bejripadhar and Dunganpal(e.g., Mainkar et al. 2004; Lehmann et al. 2006, 2007); (2)the Dharambanda lamproite field in the Nawapara area atthe NE part of the Bastar craton (Patnaik et al. 2002); and(3) the Mainpur orangeite field comprising six diamondif-erous pipes at Behradih, Kodomali, Payalikhand, Jangra,Bajaghati and Kosambura at the NE part of the Bastarcraton (e.g., Newlay and Pashine 1993; Chatterjee et al.1995; Mainkar and Lehmann 2007; Fig. 1). The last twomentioned fields are located very close to the contactbetween the Bastar craton and the Eastern Ghats mobile belt(EGMB). This contact is characterised by ultra-high-tem-perature metamorphosed rocks of EGMB over-thrusted onthe Bastar cratonic footwall (Gupta et al. 2000).

The Archaean basement is not exposed in the Mainpurorangeite field. Bundeli granitoids (equivalents of nearby2,300 Ma Dongargarh granites) together with gabbroicrocks and dolerites constitute the oldest outcropping litho-units (Mishra et al. 1988). The Bundeli granitoids areoverlain by Meso-Neoproterozoic platformal sedimentaryrocks of the Khariar and Pairi basins in close westernvicinity of the thrust fault between the Bastar craton and theEastern Ghats Mobile belt (EGMB) (Fig. 1). The orangeitepipes of the Mainpur area, together with minor basalts ofDeccan Trap (Mainkar and Lehmann 2007; Chalapathi Raoet al. 2011b), constitute the youngest intrusives in thedomain and intrude the Bundeli granites in a spread of about19 9 6 km extending WNW-ESE (Fig. 1). The pipes havecircular to oval shape in outcrop with variable length in thelongest direction from *45 m (Bajaghati) to as much as*300 m (Behradih). The prominent NW–SE trendingSondhur fault zone dissects them into two clusters: (1) theeastern Payalikhand cluster (comprising Payalikhand andJangara pipes) and (2) the western Behradih cluster (com-prising Behradih, Kodomali Bajaghati and Kosamburapipes). A brief description of each of the orangeite pipes isprovided below:

Behradih orangeite (82�1206.300; 20�12054.500): This isthe first discovered diamondiferous primary host from theBastar craton (Newlay and Pashine 1993) located *15 kmsouth of Mainpur town. A thin veneer of bluish-black grittyeluvial soil horizon is present at places and rests over deeplyweathered, smectite-rich greenish clayey ‘‘yellow ground’’.Dimensions of the pipe are *300 9 300 m, making it thelargest pipe in the Mainpur field. Drilling data by Direc-torate of Mines and Geology (DGM), Chhattisgarh, revealsthat the Behradih pipe is carrot-shaped, filled by tuffisiticbreccia and contains both pyroclastic as well as diatremefacies rocks. Macrodiamonds up to 200 ct (Newlay and

Pashine 1993) as well as *486 microdiamonds have beenrecovered from treatment of pit material (Verma andSaxena 1997). Diamonds are mostly in the form of discreteoctahedrons and dodecahedrons with eclogitic and perido-titic inclusions reported (Jha et al. 1995). Drilling of thepipe was carried out to depths of 188 m via five bore holesby DGM and least weathered to fresh bluish-grey to black,hard and compact pipe rock has been generally encounteredbeyond 65 m in all of them. Compositions of xenocrystalpyrope, clinopyroxene and chromite from the Behradih pipeare presented in this paper.

Kodomali orangeite (82�14025.600; 20�11018.200): Located*4.8 km S-SE of the Behradih pipe. It is the only ‘‘hardebank’’ outcrop in the Mainpur field with dimensions of80 9 30 m and extending in E-W direction. The outcrop isdark greyish green, hard, compact and massive and forms agently elevated ([3 m) topography. An older gabbroic rockof *500 m length and with NNW-SSE trend is exposed ineastern close vicinity (Fig. 1). One surface sample weighing45 kg was collected during prospecting and yielded 6macro- and 1 microdiamond (ORAPA 2000). Compositionsof xenocrystal pyrope, clinopyroxene and chromite fromKodomali orangeite are provided in this paper.

Payalikhand orangeite (82�20033.700; 20�100800): Unex-posed, and located *15.4 km east of Behradih pipe. Dia-mond, pyrope, chromite, chrome diopside and phlogopitecontaining smectite-rich greenish soil comprising brownishand greyish shale, sandstone, grit and granite fragmentswere recovered from pits made by DGM, Chhattisgarh,during 1995–1997. Ground magnetic survey demarcated anE-W trending body occupying an area of 240 9 60 m withan irregular outline and concealed below a weathered zoneof 15–20 m thickness (Singh et al. 2000). No suitablesamples are available for petrographic and geochemicalstudies. Gravel collected over the orangeite yielded col-ourless, clear and octahedral/dodecahedral diamondsweighing 0.29 and 0.08 ct (Newlay and Pashine 1995).Compositions of only xenocrystal pyrope, which are avail-able to us, are provided in this paper.

Jangara orangeite (82�1903300; 20�100300): Located*14 km ESE of Behradih pipe and *2 km west of Paya-likhand pipe. It is also a concealed pipe occupying an areaof 50 m in diameter with a well-developed typical yellowground (Chatterjee and Jha 1994). No drilling has been donefor this pipe, and no samples are available for study.

Kosambura orangeite (82�12006.500; 20�10042.600):Located 3.7 km south of Behradih pipe and occurs as ahighly weathered and deeply altered 80-m long dyke havinga width of 0.5–1.0 m as delineated from ground electro-magnetic survey (Small and Vaidya 2002). No diamondsare reported from this pipe. Surface exposures are unsuit-able for petrography and geochemistry owing to their highlysilicified and weathered nature. However, compositions of

Petrology, Bulk-Rock Geochemistry, Indicator Mineral Composition 95

Fig. 1 Regional geological map of kimberlite/lamproite fields of thenorth-eastern Bastar craton which includes a part of Raipur district,Chhattisgarh district and Nuapada district, Orissa (geology after

Mishra et al. 1988; Ramakrishnan and Vaidyanadhan 2008; Mainkar2010). MKF = Mainpur kimberlite field; 1 = Behradih; 2 = Kodo-mali; 3 = Bajaghati; 4 = Kosambura; 5 = Payalikhand; 6 = Jangara


xenocrystal pyrope and clinopyroxene are presented in thispaper.

Bajaghati (Temple) orangeite (82�11013.300; 20�120

37.500): Located 1.6 km WSW of Behradih pipe with asurface area of 45 9 25 m in dimension as inferred fromground electromagnetic survey (Small and Vaidya 2002).The pipe is represented at surface by bluish-black alluvialsoil of unknown thickness. No samples are available forpetrography and geochemistry. A solitary gem-varietymacrodiamond of *4 mm diameter has been recorded fromthis pipe by the DGM, Chhattisgarh.

Earlier Work

The discovery of ‘‘yellow ground’’ at the Behradih pipe byNewlay and Pashine (1993) paved the way for the finding ofother pipes in the Mainpur area (Chatterjee and Jha 1994;Newlay and Pashine 1995). The earliest publications aremerely extended abstracts that deal with the geology,mineralogy and geochemistry of the pipes and mantle-derived xenocrysts including the nature of diamonds(Chatterjee et al. 1995; Jha et al. 1995). Reconnaissancepetrography and geochemistry of the Behradih pipe wasgiven by Mainkar and Lehmann (2007), and its detailedpetrological and petrogenetic aspects were provided inChalapathi Rao et al. (2011a). Petrology of the Kodomalipipe has been discussed by Fareeduddin et al. (2006)wherein the pipe rock is inferred to be an orangeite basedsolely on petrography and mineral chemistry. However, itshould be pointed out here that in a subsequent publication,Mitchell and Fareeduddin (2009) re-interpreted Kodomalipipe to represent a lamproite. Geochemical aspects ofKodomali pipe are also dealt by Paul et al. (2006) andMarathe (2010) wherein its kimberlite status has beendeduced. On the contrary, only a few studies reporting thecomposition of mantle-derived xenoliths/xenocrysts areavailable (e.g., Mukherjee et al. 2000).

39Ar/40Ar whole-rock dating of drill cores from theBehradih orangeite gave a weighted mean plateau age of66.7 ± 0.8 Ma (Lehmann et al. 2010). These ages areindistinguishable from in situ 206Pb/238 U perovskite ages of65.10 ± 0.80 Ma and 65.09 ± 0.76 Ma (2 r) obtained ontwo samples of the Behradih orangeite (Lehmann et al.2010). On the other hand, the Kodomali orangeite gaveslightly younger 39Ar/40Ar (whole-rock) ages of 62.1 ± 1.4and 62.3 ± 0.76 Ma and 206Pb/238 U (perovskite) ages of62.3 ± 0.8 Ma (Lehmann et al. 2010). It should be pointedout here that an older Pan-African age of 491 ± 11 Mareported earlier for Kodomali pipe (Chalapathi Rao et al.2007) is due to inherited excess 40Ar in the sampled chlo-ritised phlogopite megacryst (see Lehmann et al. 2010).These ages are much younger than the previously known

episodes of potassic alkaline magmatism at ca. 1.1–1.5 Gainvolving diamondiferous Mesoproterozoic kimberlites andlamproites from the Dharwar (Gopalan and Kumar 2008;Osborne et al. 2011) and Bundelkhand (Gregory et al. 2006;Masun et al. 2009) cratons and the ca. 117 Ma eventencompassing potassium-rich ultramafics from the Damo-dar valley, Chotanagpur Mobile belt, off Singhbhum craton,eastern India (Kent et al. 1998) and constitute the youngestyet known diamondiferous event in the Indian shield.

Analytical Techniques

Mineral compositions of groundmass phases from theKodomali and Behradih pipes reported in this study(Tables 1, 2, 3, 4) were determined by using a CAMECASX100 Electron Probe Micro Analyzer (EPMA) at theTechnical University of Clausthal, Germany. An acceler-ating voltage of 15 kV, a beam current of 20 nA and a beamdiameter of 2 lm was used. The analyses were carried outusing wavelength-dispersive spectrometers employing TAP,PET and LLIF crystals and a PAP online correction pro-gramme. Several in-house natural standards were used forcalibration. After repeated analyses, it was found that theerror on major element concentrations is \1 %. For theperovskite analyses (Table 5), an acceleration voltage of20 kV, beam current of 40 nA, beam diameter of 2 lm anda counting time of 60 s and a number of synthetic standardssuch as REE6-1, REE6-2, REE6-3, REE6-4, pure Nb andTh-G were used.

Whole-rock major elements of nine Behradih and oneKosambura samples (Table 6) were analysed by X-rayfluorescence spectrometry (XRF) at Bundesanstalt fürGeowissenschaften und Rohstoffe, Hannover, Germany.Relative uncertainties of the XRF analyses are B3 %.Whole-rock trace elements were measured with a Perkin-Elmer Sciex ELAN 6000 ICP-MS instrument with Mein-hardt-nebulizer at TU Clausthal, and the relativeuncertainties range from 10 to 30 %. Whole-rock analysesof the three Kodomali samples were carried out at thelaboratories of the National Geophysical Research Institute(NGRI), Hyderabad, India. Concentrations of major ele-ments were determined by X-ray fluorescence spectrometry(XRF) using a Philips MAGIX PRO Model 2440. Typicaluncertainties of the XRF analyses are\5 %. Concentrationsof trace, REE and HFSE were determined by ICP-MS usinga PerkinElmer SCIEX ELAN DRC II. UB-N (Frenchstandard) along with BHVO-1 and JB-2 were used as ref-erence materials following Balaram and Gnaneshwar Rao(2003). Overall, an accuracy of better than ±5 % wasobtained for most determinations with a precision of betterthan ±6 % RSD. Data on rare earth elements (REE) ongarnet concentrates from the Behradih and Payalikhand


pipes were acquired using a laser-ablation PerkinElmerSciex ELAN 5100 ICPMS (LAM) at the School of EarthSciences, Macquarie University, Australia, and are given inTable 7. The external standard was the NIST 610 multi-element glass, accuracy of the measurements is expected tobe within 1-sigma errors and the analytical conditions are asdescribed by Norman et al. (1996) and Girffin et al. (1999).

Zircon grains were set in epoxy resin mounts, sectionedand polished to approximately half of their thickness. Priorto U–Pb isotopic analysis, cathodoluminescence imageswere obtained for all grains using a scanning electronmicroscope (JEOL JXA 8900 RL instrument) at the Geo-logical Survey of Denmark and Greenland (GEUS),Copenhagen, Denmark (Fig. 9a). U–Pb geochronology ofzircons was performed using a Thermo-Finnigan Element IIsector-field ICPMS system coupled to a Merchantek/New-Wave 213-nm Nd-YAG laser system at the GeologicalSurvey of Denmark and Greenland (GEUS), Copenhagen,Denmark, and the results are given in Table 8. The methodapplied essentially followed that described by Frei andGerdes (2008). For the interpretation of the zircon data,analyses with 95–105 % concordance [calculated from 100x (206Pb/238U age)/(207Pb/235U age)] are considered to beconcordant and those with a discordance [10 % wererejected and consequently not considered.

Mineral chemistry of the indicator minerals (xenocrysts)is given in Electronic Supplementary Tables 1 to 6 and isfrom Mainkar (2010). Data of indicator minerals have beenprovided to one of us (DM, working with Directorate ofMines and Geology, Chhattisgarh) as proprietary informa-tion by exploration companies such as De Beers, ORAPA,B.Vijaya Kumar Chhattisgarh Exploration (BVCE) and RioTinto, and their analytical conditions could not be retrieved.For such data generated at Technical University of Claus-thal, the instrument and analytical conditions remain thesame as given earlier in this section.

Petrography and Mineral Chemistry

The Kodomali occurrence is the best exposed and leastaltered pipe of the Mainpur field. The studied samples havea distinct inequigranular texture dominated by macrocrysts(0.5–1 mm) and microphenocrysts of fresh and unalteredolivine set in a fine-grained groundmass dominated byphlogopite, clinopyroxene, spinel, perovskite and apatite(Fig. 2a). Microlites of clinopyroxene, present as acicularlaths, are a characteristic feature of the pipe along withclusters of spinel and perovskite (Fig. 2b). Serpentinisationof olivine and chloritisation of phlogopite is minimal. Thegroundmass displays magmatic flow layering, and theolivine macrocrysts display occasional evidence of

corrosion. Autoliths of earlier erupted pipe rock are alsoreported (Chatterjee et al. 1995).

Rounded to sub-rounded olivine macrocrysts and pelletallapilli set in a very fine-grained glassy to cryptocrystallineserpentine-chlorite matrix dominated by olivine micro-phenocrysts are a characteristic feature of the Behradih pipe(Fig. 2c). The mesostasis is dominated by phlogopite,clinopyroxene, perovskite, spinel and apatite (Fig. 2d). Themacrocrysts as well as the matrix phases are affected bypervasive carbonate–talc–serpentine alteration. Fresh andunaltered olivine is not observed and has been reported onlyfrom autoliths (Chalapathi Rao et al. 2011a). The mode ofoccurrence of clinopyroxene as microlitic laths is identicalto that observed in the Kodomali pipe. Petrography of theBehradih pipe has been reported by Mainkar and Lehmann(2007) and Chalapathi Rao et al. (2011a).

The Kosambura pipe is highly silicified with fragmentaltexture wherein completely altered macrocrysts are set infine-grained altered and indistinguishable groundmasswhich constitutes a major proportion. Relict textures of theserpentinised, talcose and carbonated remnants are promi-nent along with secondary silica, chlorite and clay minerals.

New microprobe data for minerals from the Kodomalipipe and for perovskite from Behradih are given inTables 1, 2, 3, 4, 5 and discussed below.

Olivine

Phenocrystal olivine data from the Kodomali pipe are givenin Table 1. The olivine compositions (Fo86–90) of this studyare similar to those (Fo87–91) reported earlier from theKodomali pipe (Fareeduddin et al. 2006) and overlap withthe olivine phenocryst compositions available for the Beh-radih pipe (Fo83–90; Chalapathi Rao et al. 2011a). However,NiO contents of the Kodomali olivines vary from 0.36 to0.45 wt% and are higher than those from the Behradih pipeand also from the NKF and WKF pipes in southern Indiashown for comparison (Fig. 3a).

Phlogopite

Compositional data for groundmass phlogopite from theKodomali pipe are given in Table 2. In the TiO2 versusAl2O3 (Fig. 3b) plot, its composition is similar to thatreported for lamproites from Smoky Butte and Leucite Hills,USA, and contrasts with that of the Behradih pipe which hasdistinctly lower TiO2 and a far wider range in Al2O3 contents(Fig. 3b). In the FeOT versus Al2O3 (Fig. 3c) plot for dis-criminating between groundmass phlogopites from variouspotassic–ultrapotassic alkaline rocks, the Behradih phlo-gopites show two distinct clusters—macrocrystal and


microphenocrystal—with the latter compositionally closerto the phlogopite from Kodomali samples and anotherplotting away from them. The mica compositions from WKFand NKF display a far greater range in composition in eitherof the plots (Fig. 3b, c).

Spinel

Groundmass spinel composition of the Kodomali samples isgiven in Table 3. A distinct compositional difference existsin MgO-Al2O3 space amongst the groundmass spinels fromKodomali and Behradih (Fig. 3d). The Kodomali spinel isrelatively Mg–depleted and shows far greater variation inthe Al2O3 content than the spinel from Behradih and issimilar to the spinel composition in WKF and NKF kim-berlites (Fig. 3d). Macrocryst spinel from the Tokapalkimberlite located in the Indravati field in the Bastar cratonhas an altogether different composition. Groundmass spinelfrom kimberlites has been shown to define two distincttrends (Mitchell 1986): (1) a magnesio–ulvöspinel–

magnetite trend (trend 1; characteristic of Group I kimber-lites) and (2) a Ti-magnetite trend (trend 2 which is char-acteristic of orangeites and is also similar to zoning ofspinel in basalts; Roeder and Schulze 2008). Spinel from theKodomali as well as from Behradih pipes shows a strongaffinity towards trend 2 (Fig. 3e).

Clinopyroxene

Clinopyroxene in the Kodomali samples is diopsidic(Table 4) with a considerable range in composition ofWo34–44 En35–46 Fs11–13 Ac7–15. The data reported in thisstudy are distinct from the Fe-poor clinopyroxene(Wo50–52En44–47Fs0.9–4.3) reported earlier by Fareeduddinet al. (2006). In Al-Ti (atomic) space (Fig. 3f), clinopy-roxene from the Mainpur pipes overlaps with the compo-sitional fields of diopside from the NKF, Krishna lamproitesand orangeites from southern Africa as well as those ofworldwide lamproites (Mitchell 1995).

Fig. 2 Backscattered electron (BSE) images depicting the petro-graphic features of the Mainpur pipes of this study. a Macrocrystaltexture of the Kodomali pipe showing two generations of fresh andunaltered subhedral to euhderal olivines set in a very fine-grainedgroundmass. b High magnification image showing the microlites ofclinopyroxene which is a dominant phase in the groundmass of theKodomali pipe; also seen are clusters of perovskite and spinel in closeassociation with each other. c Rounded to sub-rounded olivine

macrocrysts set in a finer groundmass dominated by clinopyroxene,perovskite and spinel are a characteristic feature of the Behradih pipe;and d Phlogopite microcrysts with apatite blebs are a characteristicfeature of some of the Behradih drill core samples. Note thatserpentine is also an important component of the mesostatis(Ol = olivine; Cp = clinopyroxene; Ph = phlogopite; Sr = serpen-tine; Ap = apatite; Pv = perovskite; Sp = spinel)


Perovskite

The composition of perovskite from the Behradih andKodomali pipes is presented in Table 5. Their CaO(Behradih: 36.5–37.3 wt%; Kodomali: 35.3–37.2 wt%) andTiO2 (Behradih: 51.8–52.9 wt% Kodomali: 52.8–54.0wt%) contents show little variation. However, their Fe2O3*(Behradih: 2.51–2.90 wt% and Kodomali: 1.24–2.53 wt%;both re-calculated from FeO) and Nb2O5 contents (Beh-radih: 0.70–0.73 wt%; Kodomali: 1.04–1.64 wt%) displayconsistent differences. On the other hand, their LREE2O3

(4.24–5.29 wt%) and SrO (0.44–1.17 wt%) show a tightcompositional range. Na2O (0.22–1.45 wt%) is present insubstantial concentrations. LREE2O3 contents of the per-ovskites are higher than those reported for NKF(0.88–0.97 wt%) and WKF (2.57–5.37 wt%) kimberlites.Likewise, Nb2O5 contents are also higher than those fromNKF pipes (which range from 0.22–0.33 wt%) but aresimilar to those from the WKF pipes (0.34–2.33 wt%; datasources: Chalapathi Rao et al. 2004; Chalapathi Rao and

Dongre 2009; Chalapathi Rao et al. 2012). The Fe and Nbcontents of perovskite from the Mainpur orangeites arefurther used for evaluating the redox conditions of themagma (below).

Bulk-Rock Geochemistry

Whole-rock major, trace (including REE) and radiogenicisotope (Sr and Nd) data for drill core samples of Behradih(Mainkar and Lehmann 2007; Lehmann et al. 2010;Chalapathi Rao et al. 2011a) and Kodomali (Paul et al.2006; Fareeduddin et al. 2006; Marathe 2010; Lehmannet al. 2010) are available. There are no published geo-chemical data for the other pipes from the Mainpur field. Inthis study, we generated new bulk-rock geochemistry dataon nine drill core samples from the Behradih pipe, threesurface samples from the Kodomali pipe and one drill coresample from the Kosambura pipe (Table 6). These new datatogether with earlier data sets are utilised in this study.

Table 1 Mineral chemistry (wt%) of olivine from the Kodomali orangeite

Oxide wt% KDK-2 KDK-2C KDK-3 KDK-3 KDK-4 KDK-4

SiO2 41.38 41.79 40.92 40.66 41.77 41.95

TiO2 0.01 0.00 0.04 0.02 0.04 0.01

Al2O3 0.00 0.03 0.04 0.02 0.02 0.05

Cr2O3 0.02 0.03 0.00 0.02 0.06 0.08

FeO 7.46 7.35 12.71 12.60 9.22 9.34

MnO 0.09 0.10 0.09 0.08 0.10 0.10

MgO 49.65 49.33 45.64 45.34 48.12 47.75

NiO 0.42 0.40 0.41 0.43 0.36 0.45

CaO 0.03 0.02 0.05 0.07 0.13 0.16

Total 99.04 99.07 99.91 99.24 99.82 99.91

Cations for 4 oxygens

Si 1.013 1.021 1.016 1.017 1.022 1.026

Ti 0.000 0.000 0.001 0.000 0.001 0.000

Al 0.000 0.001 0.001 0.001 0.001 0.001

Cr 0.000 0.000 0.000 0.000 0.001 0.002

Fe(ii) 0.153 0.150 0.264 0.263 0.189 0.191

Mn 0.002 0.002 0.002 0.002 0.002 0.002

Mg 1.811 1.796 1.690 1.690 1.755 1.741

Ni 0.008 0.008 0.008 0.009 0.007 0.009

Ca 0.001 0.001 0.001 0.002 0.003 0.004

Total 2.988 2.979 2.984 2.984 2.980 2.976

End-members

Fo 92.15 92.18 86.41 86.44 90.20 90.02

Fa 7.76 7.71 13.50 13.47 9.69 9.87

Tp 0.09 0.11 0.10 0.09 0.11 0.11


Major Element Geochemistry

The SiO2 (41.8–47.2 wt%) and MgO (22–27.2 wt%) con-tents of the Behradih as well as those of the Kodomali pipe(SiO2: 46.5–49.2 wt%; MgO: 22.1–26.3 wt%) are clearlydifferent from the Mesoproterozoic Wajrakarur (WKF) andNarayanpet (NKF) kimberlites, Eastern Dharwar craton,southern India, whose overall SiO2 contents (\43 wt%) arelower and MgO contents (18–33 wt%) display a muchwider range. The extent of negative correlation betweenSiO2 and MgO implies magmatic fractionation processes.When compared to the archetypal kimberlites and orange-ites of southern Africa, the Mainpur pipes as well as kim-berlites from NKF and WKF clearly have a morefractionated nature (Fig. 4a). On the other hand, the Fe2O3

*

contents of the Behradih (6.47–8.28 wt%) and Kodomali(7.54–9.27 wt%) pipes show strong affinities towardsorangeites, different from the WKF and NKF pipes.

K2O contents (1.42–2.92 wt%) of the Mainpur pipes arehigher than the corresponding Na2O contents (0.57–1.18wt%; Table 6) and display the potassic nature of the pipes.As K is the most mobile of the large-ion lithophile elementsand Ti is highly immobile as a high-field strength element, aK2O-TiO2 plot is widely used to discriminate kimberlitesand orangeites (Smith et al. 1985; Mitchell 1995). Such aplot (Fig. 4b) clearly brings out the higher K2O and lowerTiO2 contents of the Behradih and Kodomali pipes com-pared to those from the WKF and NKF. The solitary drillcore sample from the Kosambura pipe shows very highdegrees of contamination as is evident from its elevatedSiO2 (74.2 wt%) and depleted MgO (3.08 wt%), andtherefore, its geochemistry serves little purpose in makingany meaningful geochemical inference and hence is notconsidered in the plots.

Trace Element Geochemistry

Normalised multi-element plots (Fig. 4c) reveal thestrongly enriched trace element abundances of the Behradihand Kodomali samples compared to primitive mantle. Allpatterns are parallel to sub-parallel suggesting petrogeneticsimilarity (Fig. 4c). Conspicuous negative spikes areobserved at K, Sr and Ti for all samples and negative Ta forsome samples of the Kodomali pipe, whereas positive Baspikes are noticed in many of the samples (Fig. 4c). Recentstudies have shown that incompatible element abundances(Ba, Nb, La and Rb) can be used to discriminate betweensouthern African kimberlites and orangeites (Donnelly et al.2011). Lower Nb and higher Ba contents of the Behradihand Kodomali pipes distinguish them from many of theWKF and NKF kimberlites (Fig. 4d) of which some are alsodemonstrated to display ‘‘transitional’’ characteristics(Chalapathi Rao and Srivastava 2009; Chalapathi Rao and

Fig. 3 a Variation of Fo [Mg/(Mg ? Fe)] and NiO (wt%) contents in olivine from the Mainpur orangeites. Data sources: Kodomali (this work);Behradih (Chalapathi Rao et al. 2011a); NKF and WKF (Chalapathi Rao et al. 2011b; Chalapathi Rao and Dongre 2009; Chalapathi Rao et al.2004). b TiO2 (wt%) versus Al2O3 (wt%) and c FeOT versus Al2O3 (wt%) of groundmass phlogopite from this study. Data sources: Kodomali(this work); Behradih (Chalapathi Rao et al. 2011a); NKF, WKF and Cuddapah and Krishna lamproites from Chalapathi Rao et al. (2011b),Chalapathi Rao and Dongre (2009), Chalapathi Rao and Srivastava (2009), Chalapathi Rao et al. (2004, 2010) and Paul et al. (2007); Fields ofkimberlite, Leucite Hills madupite, West Kimberley and Smoky Butte lamproites are taken from Dawson and Smith (1977), Gibson et al. (1995).Arrows (evolutionary trends of mica composition) in (b) are taken from Mitchell (1995). Symbols are the same as in Fig. 3. d MgO (wt%) versusAl2O3 (wt%) of the groundmass Kodomali spinel. Data sources: Kodomali (this study); Behradih (Chalapathi Rao et al. 2011a); WKF and NKF(Chalapathi Rao et al. 2011b; Chalapathi Rao and Dongre 2009; Chalapathi Rao and Srivastava 2009; Chalapathi Rao et al. 2004); southernAfrican kimberlites (Scott-Smith and Skinner 1984). Data for spinel from crater facies Tokapal kimberlite, Central India, are from theunpublished data set of the authors e Fe2+/(Fe2++Mg2+) versus Ti/(Ti ? Cr ? Al) (mol fraction) for groundmass Kodomali spinel projected ontothe front face of the ‘‘reduced’’ spinel prism. Magmatic trends 1 and 2 exhibited by southern African spinel and spinel trend from orangeites arefrom Mitchell (1995). Data sources for Behradih, WKF and NKF are same as in 3A. Symbols are the same as in Fig. 3a. f Al total versus Ti total

expressed as atoms per formula unit for clinopyroxene for the Kodomali (this work) and Behradih (Chalapathi Rao et al. 2011a). Also shown arethe data for clinopyroxene from the Krishna lamproites (open squares; Paul et al. 2007; Chalapathi Rao et al. 2010) and Narayanpet kimberlites(open circles; after Chalapathi Rao et al.). Other fields are taken Mitchell (1995). g Oxygen fugacity (fO2) (DNNO) conditions of the Behradihand Kodomali orangeites (this study) compared with those recorded by cratonic mantle lithosphere, mantle-derived magmas and globalkimberlites (adapted from Canil and Bellis 2007). Data for Dutoitspan kimberlites are from Ogilvie-Harris et al. (2009); NKF kimberlites(Chalapathi Rao et al.)

b

Fig. 3 continued


Dongre 2009; Chalapathi Rao et al. 2012). Lower La con-tents are characteristic of the Behradih pipe, whereasKodomali has relatively higher La contents with bothsharing indistinguishable Rb abundances (Fig. 4e).

Indicator Mineral Chemistry

Various indicator minerals, including diamond, wereobtained from the Mainpur orangeites by dense mediaseparator (DMS) processing by the Directorate of Minesand Geology, Chhattisgarh (Mainkar 2010). Whereasdetailed studies on diamond are addressed separately in thisvolume, the present paper discusses the composition ofpyrope garnet (Electronic Supplementary Tables 1 to 3),chrome diopside (Electronic Supplementary Tables 4) andchromite (Electronic Supplementary Tables 5 and 6) fromBehradih, Payalikhand and Kosumbura orangeites.

Pyrope crystals are mostly \3 mm in size and rarelylarger (up to 5 mm), and most of them are characterised byconchoidal fractures. The composition of the analysedpyropes from Behradih, Payalikhand, Kodomali andKosambura pipes is plotted (Fig. 5a–d) in the garnet clas-sification scheme for Cr2O3 (wt%) and CaO (wt%) given byGrutter et al. (2004) which is widely followed for inferringdifferent paragenesis. A predominant proportion of theBehradih garnets corresponds to the calcic-lherzolitic vari-ety (i.e., G9 type of Dawson and Stephens 1975) and\5 %belong to the high-interest sub-calcic harzburgitic category(i.e., G10 type of Dawson and Stephens 1975) and theremainder, to other fields (Fig. 5a). The Payalikhand dataare limited (n = 7) and most garnets plot in the G5 and G9fields (Fig. 5b). Likewise, in the case of Kodomali, most ofthe pyropes are classified into the G9 variety and some inG5 with only one solitary grain plotting in the harzburgiticG10 category (Fig. 5c). Pyropes of the silicified Kosambura

Table 2 Mineral chemistry (oxide wt%) of clinopyroxene from the Kodomali orangeite

KDK-2 KDK-2 KDK-3 KDK-3 KDK-4 KDK-4

SiO2 52.23 53.20 53.15 52.39 51.71 51.71

TiO2 1.94 0.87 1.80 1.27 2.20 3.18

Al2O3 1.27 0.27 0.43 1.95 0.71 0.50

Cr2O3 0.28 0.02 0.01 0.04 0.00 0.02

FeO 8.39 6.93 8.05 7.29 6.48 7.45

MnO 0.09 0.14 0.16 0.11 0.13 0.13

MgO 13.31 13.52 12.70 16.54 16.65 13.75

CaO 18.18 22.34 20.07 16.96 19.13 20.94

Na2O 4.20 2.04 3.07 2.12 1.70 2.31

K2O 0.83 0.04 0.04 0.37 0.22 0.25

Total 100.74 99.40 99.57 99.06 98.94 100.25

6 oxygens

Si 1.882 1.967 1.962 1.918 1.904 1.901

Ti 0.053 0.024 0.050 0.035 0.061 0.088

Al 0.054 0.012 0.019 0.084 0.031 0.022

Cr 0.008 0.001 0.000 0.001 0.000 0.001

Fe(ii) 0.253 0.214 0.249 0.223 0.199 0.229

Mn 0.003 0.004 0.005 0.003 0.004 0.004

Mg 0.715 0.745 0.699 0.903 0.914 0.754

Ca 0.702 0.885 0.794 0.665 0.755 0.825

Na 0.293 0.146 0.220 0.150 0.121 0.165

K 0.038 0.002 0.002 0.017 0.010 0.012

Total 4.000 4.000 4.000 4.000 4.000 4.000

Wo 35.86 44.42 40.44 34.271 37.90 41.812

En 36.53 37.42 35.63 46.496 45.92 38.196

Fs 12.62 10.83 12.73 11.487 10.09 11.642

Ac 14.99 7.34 11.19 7.7467 6.092 8.3496


pipe are similar to those of the Behradih pipe in mostaspects and plot significantly into the G9 and with *5 % inthe G10 category.

REE abundances of garnet concentrates from Behradih andPayalikhand could only be analysed in this study (Table 7),and their chondrite-normalised patterns are presented inFig. 6. Data from low-Cr lherzolite pyropes from the Finschmine, Kaapvaal craton, southern Africa (Gibson et al. 2008)are also provided in Fig. 6 for comparison. Two broad typesof garnet REE distribution patterns are recognised fromstudies from the Kaapval craton, southern Africa and else-where (e.g., Shimizu 1975; Stachel et al. 1998; Burgess andHarte 2004; Lehtonen 2005; Gibson et al. 2008):(1) ‘‘smooth’’ chondrite-normalised patterns without anykinks with a strong LREE depletion relative to MREE andHREE and a gradual enrichment from SmN to YbN which isconsidered typical of Ca-saturated lherzolitic garnets and(2) ‘‘sinusoidal’’ chondrite-normalised REE patterns showingpeaks at Nd which are characteristic of sub-calcic (harz-burgitic) garnets and also of the rare Ti-poor lherzolitic class.The Behradih pyropes display both types of REE patternswith a clear predominance of garnet with ‘‘smooth’’ patterns,whereas the Payalikhand garnets lack ‘‘sinusoidal’’ patterns.

Emerald green to bright green macrocrystic clinopy-roxene (essentially diopside) is of *2 mm size in theKodomali and Kosambura pipes. No information isavailable on the macrocrystic clinopyroxene from otherorangeites of the Mainpur field. In terms of composition inAl2O3-Cr2O3 wt% space, both localities are similar andpredominantly have been derived from garnet peridotitewith very few of them transgressing into the spinel peri-dotite field (Fig. 7). Jha et al. (2002) also reported chromediopsides of similar composition from the Payalikhandpipe.

Chromites collected from the Behradih and Kodomalipipes mostly occur as euhedral and octahedral crystals.Behradih spinels have Cr2O3 varying between 27–66 wt%with most of the data having[52 wt% Cr2O3. About 55 %chromites are high chromium (58–66 wt% Cr2O3), and outof these 26 chromites, most have between 10.5 to 13.5 wt%MgO and low TiO2 (\ 1 wt%) and fall in the diamondinclusion and intergrowth fields (Fig. 8a, b). This is alsoduplicated by about 30 % of the Kodomali chromites whoseCr2O3 (35–65 wt%), MgO (7–18 wt%) and TiO2 (7.5 wt%;but mostly \3 wt%) contents extend into the diamond sta-bility field (Fig. 8c, d). The chemistry of *75 % chromitesfrom the Kosambura pipe belongs to the high-chromevariety Cr2O3 (55–67 wt%), and MgO (mostly 10.0–13.5wt%) and TiO2 (mostly \1.15 wt%) fall into the diamondinclusion and intergrowth fields suggesting encouragingdiamond potential (Fig. 8e, f).

U–Pb Zircon Geochronology

A number of zircon grains have been recovered whilstprocessing samples from the Behradih pipe. They areessentially of euhedral to subhedral shape, and their grainsize mostly varies from 100–150 lm (Fig. 9a–l) and isessentially \500 lm. Sub-rounded to rounded zircons arenot observed suggesting a lack of magmatic resorption. It iswell known that zircon in kimberlites can be of crustal aswell of mantle (megacrystic) origin and the chemicalcomposition plays an important role in their discrimination(e.g., Konzett et al. 2000; Belousova et al. 2002). The ele-vated contents of U (113–1096 ppm) in the studied samples(Table 8) exclude their mantle derivation since megacrysticmantle-derived zircons are known to have U contents\60 ppm (see Page et al. 2007).

U–Pb dating (see Table 8 for results and Fig. 9m forgraphical representation) demonstrates that the zircon pop-ulation is essentially of Palaeoproterozoic age (*2450 Ma)and provides compelling evidence for their crustal-derivation.

Table 3 Mineral chemistry (wt%) of phlogopite from the Kodomaliorangeite

Oxide wt% KDK-2 KDK-2 KDK-3 KDK-3 KDK-4

SiO2 40.12 38.43 37.62 39.868 39.96

TiO2 6.29 6.08 5.37 6.11 6.09

Al2O3 7.76 9.73 10.92 7.98 8.24

FeO 10.34 10.08 10.26 10.30 10.39

MnO 0.06 0.08 0.12 0.07 0.08

MgO 18.83 18.09 19.29 18.64 18.97

CaO 0.01 0.14 0.04 0.00 0.03

Na2O 0.55 0.32 0.32 0.55 0.44

K2O 9.53 9.29 8.52 9.55 9.12

Total 93.49 92.23 92.46 93.07 93.32

Cations for 22 oxygens

Si 6.012 5.829 5.675 6.003 5.982

Ti 0.709 0.693 0.610 0.691 0.686

Al 1.371 1.739 1.940 1.416 1.453

Fe(ii) 1.296 1.279 1.294 1.297 1.301

Mn 0.007 0.010 0.016 0.009 0.011

Mg 4.206 4.090 4.337 4.185 4.233

Ca 0.002 0.023 0.007 0.000 0.005

Na 0.160 0.093 0.092 0.160 0.127

K 1.822 1.798 1.640 1.834 1.741

Total 15.59 15.554 15.55 15.554 15.5537


Ta

ble

4M

iner

alch

emis

try

(wt%

)of

grou

ndm

ass

spin

elfr

omth

eK

odom

ali

oran

geit

e.F

e2+

and

Fe3

+re

dist

ribu

tion

done

bysp

inel

stoi

chio

met

ry

Oxi

de(w

t%)

KD

K-2

KD

K-2

KD

K-2

KD

K-3

KD

K-3

KD

K-3

KD

K-3

KD

K-4

KD

K-4

KD

K-4

KD

K-4

KD

K-4

KD

K-4

SiO

20.

080.

110.

160.

080.

020.

070.

080.

120.

020.

030.

050.

070.

05

TiO

21.

951.

811.

962.

074.

702.

212.

711.

951.

720.

501.

861.

881.

89

Al 2

O3

6.70

5.76

4.06

6.50

1.53

3.29

2.93

5.21

28.3

437

.43

4.40

4.14

5.59

Cr 2

O3

55.0

554

.17

51.0

953

.37

38.2

651

.76

48.1

455

.89

22.8

427

.67

54.9

555

.58

55.7

1

Fe 2

O3

5.33

7.05

12.3

06.

5723

.53

11.3

614

.47

5.72

14.2

82.

907.

607.

225.

86

FeO

22.7

123

.26

24.4

222

.89

28.2

223

.90

25.0

123

.221

.31

17.5

224

.12

23.4

323

.29

MnO

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

MgO

7.83

7.21

6.63

7.64

5.47

6.70

6.27

7.29

11.4

213

.55

6.46

6.95

7.26

CaO

0.14

0.19

0.17

0.13

0.22

0.17

0.17

0.15

0.07

0.03

0.26

0.14

0.16

Tot

al99

.80

99.5

510

0.81

99.2

510

1.95

99.4

799

.79

99.6

010

0.00

99.6

399

.72

99.4

199

.81

Cat

ions

for

32ox

ygen

s

Si

0.02

20.

031

0.04

50.

021

0.00

60.

020

0.02

30.

032

0.00

50.

006

0.01

50.

021

0.01

4

Ti

0.40

10.

375

0.40

90.

430

0.99

40.

469

0.57

70.

406

0.30

80.

088

0.39

10.

395

0.39

2

Al

2.15

91.

876

1.32

82.

112

0.50

71.

093

0.97

51.

693

7.95

110

.232

1.45

01.

363

1.81

7

Cr

11.8

9611

.843

11.1

9811

.625

8.51

011

.521

10.7

5012

.191

4.86

45.

075

12.1

3912

.285

12.1

53

V0.

000

0.00

00.

000

0.00

00.

000

0.00

00.

000

1.26

70.

000

0.00

00.

000

0.00

00.

000

Fe(i

ii)

1.09

71.

469

2.56

71.

362

4.98

22.

409

3.07

60.

000

2.55

90.

507

1.59

91.

521

1.21

7

Fe(i

i)5.

193

5.37

95.

662

5.27

46.

639

5.62

65.

907

5.36

84.

242

3.39

95.

636

5.47

95.

373

Mn

0.00

00.

000

0.00

00.

000

0.00

00.

000

0.00

00.

000

0.00

00.

000

0.00

00.

000

0.00

0

Mg

3.18

92.

972

2.74

03.

138

2.29

42.

811

2.64

12.

998

4.05

24.

687

2.69

32.

895

2.98

6

Ca

0.04

10.

055

0.05

10.

039

0.06

70.

051

0.05

20.

046

0.01

90.

008

0.07

70.

041

0.04

7

Tot

al24

2424

2424

2424

2424

2424

2424

Ti/

(Ti

?C

r?

Al)

0.03

0.03

0.03

0.03

0.10

0.04

0.05

0.03

0.02

0.01

0.03

0.03

0.03

Fe2

/(F

e2?

Mg)

0.62

0.64

0.67

0.63

0.74

0.67

0.69

0.64

0.51

0.42

0.68

0.65

0.64


Ta

ble

5M

ajor

oxid

e(w

t%)

ofpe

rovs

kite

sfr

omB

ehra

dih

and

Kod

omal

ior

ange

ites

.Not

eth

atto

tal

iron

isex

pres

sed

asF

e 2O

3in

all

the

anal

yses

.DN

NO

3is

log

fO2

rela

tive

toN

NO

(nic

kel–

nick

elox

ide)

buff

er

Beh

radi

hpi

peK

odom

ali

pipe

Oxi

dew

t%B

HS

2-4

BH

S10

-12

BH

S15

-18

BH

S36

-37

BH

S36

-37

BH

S36

-37

BH

S52

BH

S61

KD

K-

2K

DK

-2

KD

K-

2K

DK

-3

KD

K-

3K

DK

-3

KD

K-

4

Na 2

O0.

240.

220.

290.

240.

270.

250.

270.

280.

640.

550.

60.

970.

591.

150.

73

Al 2

O3

0.28

0.29

0.27

0.26

0.28

0.31

0.28

0.29

0.10

0.09

0.13

0.07

0.13

0.05

0.07

SiO

20.

150.

440.

120.

250.

100.

250.

090.

090.

220.

160.

170.

280.

130.

150.

11

CaO

37.2

336

.95

37.2

437

.06

37.2

336

.46

37.2

537

.09

37.1

736

.71

36.9

236

.69

36.4

435

.29

37.2

0

TiO

251

.80

52.8

552

.33

52.3

251

.99

52.4

452

.53

52.7

053

.41

53.3

653

.03

53.4

852

.86

53.7

454

.02

Fe 2

O3

2.84

2.57

2.51

2.59

2.90

2.68

2.78

2.65

2.18

2.27

2.53

2.23

2.20

1.48

1.91

SrO

0.44

0.47

0.45

0.45

0.48

0.44

0.42

0.46

0.32

0.58

0.79

0.48

0.41

1.03

0.57

Nb 2

O5

0.70

0.73

0.70

0.78

0.75

0.70

0.68

0.73

1.08

1.04

1.38

1.04

1.07

1.64

1.05

La 2

O3

1.06

1.00

1.07

0.96

1.09

1.02

1.01

1.07

0.97

1.11

1.07

0.88

1.14

1.21

0.96

Ce 2

O3

3.08

2.62

2.94

2.50

2.95

2.78

2.97

2.83

2.35

2.73

2.53

2.46

2.87

2.80

2.51

Pr 2

O3

0.29

0.26

0.31

0.18

0.28

0.26

0.29

0.24

0.17

0.24

0.25

0.21

0.29

0.20

0.24

Nd 2

O3

1.01

0.86

0.95

0.84

0.97

0.97

0.99

0.92

0.75

0.81

0.76

0.81

0.88

0.77

0.76

Ta 2

O5

0.10

0.04

0.13

0.13

0.09

0.10

0.09

0.09

0.02

0.18

0.02

0.01

0.15

0.07

0.05

ThO

20.

390.

250.

330.

250.

310.

300.

370.

270.

220.

170.

120.

220.

240.

260.

21

Tot

al99

.60

99.5

399

.63

98.7

999

.70

98.9

499

.99

99.7

199

.60

100.

0110

0.28

99.8

399

.39

99.8

510

0.38

DN

NO

34.

282.

932.

763.

034.

463.

573.

973.

300.

711.

201.

881.

010.

933.

080.

48


Ta

ble

6B

ulk-

chem

istr

y(w

t%)

ofB

ehra

dih,

Kod

omal

ian

dK

osam

bura

oran

geit

esfr

omth

eM

ainp

urfi

eld

Beh

radi

hK

odom

ali

Kos

ambu

ra

Oxi

dew

t%B

HS

-2-4

BH

S-1

0-12

BH

S-1

5-18

BH

S-2

3-25

BH

S-3

6-37

BH

S-3

9B

HS

-52

BH

S-5

4B

HS

-61

KD

K-2

KD

K-3

KD

K-4

MK

O

SiO

245

.344

47.2

45.8

4241

.843

.346

.544

47.3

446

.47

46.5

374

.24

TiO

21.

251.

071.

081.

021.

211.

141.

131.

021.

231.

291.

421.

280.

50

Al 2

O3

5.53

4.58

4.92

4.83

4.57

4.48

5.34

5.38

4.92

6.57

6.57

6.4

2.44

Fe 2

O3*

7.59

7.06

7.22

7.39

8.28

7.54

7.71

7.35

8.15

7.58

7.75

7.54

5.67

MnO

0.12

0.13

0.1

0.11

0.12

0.13

0.13

0.11

0.13

0.11

0.11

0.11

0.23

MgO

2223

24.7

24.6

27.2

25.5

24.2

23.7

23.9

22.1

322

.14

22.6

63.

08

CaO

6.66

7.48

4.63

5.33

4.85

7.01

6.3

4.97

6.47

87.

687.

862.

67

Na 2

O0.

60.

780.

730.

780.

570.

740.

660.

951.

180.

890.

871.

090.

01

K2O

2.92

2.15

1.42

1.63

2.19

2.18

2.41

2.15

2.92

1.6

2.11

1.75

0.13

P 2O

50.

701

0.59

70.

319

0.23

40.

235

0.42

40.

275

0.27

40.

263

1.6

2.11

1.75

1.78

LO

I6.

518.

466.

977.

518.

18.

497.

76.

95.

753.

813.

693.

998.

26

Tot

al99

.675

99.8

0199

.837

99.7

8599

.738

100.

1499

.672

99.8

7899

.76

100.

9210

0.92

100.

9699

.09

Mg#

85.2

86.6

87.1

86.8

86.7

87.0

86.1

86.5

85.3

84.5

84.8

84.3

Sr

926

909

643

739

801

1010

895

774

1040

683.

5864

4.78

600.

3238

4

Rb

132

101

54.8

66.8

181

133

161

68.8

162

81.4

787

.287

.22

7

Cu

2633

2931

3435

3429

3648

.39

44.6

442

.684

15

Ni

696

875

859

950

1070

945

829

966

1040

1131

.211

31.5

1114

860

Cr

495

410

470

440

515

490

550

450

530

2487

.615

56.6

1532

.321

65

Co

4451

4956

6056

4854

5972

.88

72.8

873

.19

50

V64

8090

7058

5462

116

7613

8.75

137.

8913

8.66

80

Sc

12.5

10.5

1010

1111

129.

511

.516

.89

16.9

316

.17

20

Y18

.114

.313

.513

.815

.914

.916

.113

.615

.715

.39

15.3

115

.15

24

Zr

206

128

156

160

226

207

195

154

217

264.

8126

2.17

262.

6636

9

Nb

8455

48.5

4863

.565

.572

51.5

59.5

94.1

107

122

319

Ba

1510

1380

940

1560

1430

2260

1540

2290

4130

2325

.320

88.1

2278

.196

7

Hf

4.67

4.68

4.64

Ta

5.7

3.1

3.3

3.2

4.2

4.4

43.

54

1.11

1.21

1.23

11

Pb

159

911

1013

1613

127.

666.

997.

4813

Th

27.3

15.6

18.3

15.8

18.7

17.9

2316

.222

.88.

258.

538.

354

Ga

12.9

712

.79

13.1

4

U4.

553.

253.

152.

13.

353.

43.

22.

74.

71.

351.

331.

326

(con

tinu

ed)


Ta

ble

6(c

onti

nued

)

Beh

radi

hK

odom

ali

Kos

ambu

ra

Oxi

dew

t%B

HS

-2-4

BH

S-1

0-12

BH

S-1

5-18

BH

S-2

3-25

BH

S-3

6-37

BH

S-3

9B

HS

-52

BH

S-5

4B

HS

-61

KD

K-2

KD

K-3

KD

K-4

MK

O

Zn

6248

4455

5751

6148

5611

0.99

116.

0611

6.72

54

La

153

102

109

103

122

117

153

108

126

94.2

296

.14

93.8

237

5

Ce

261

180

184

175

209

201

247

179

213

159.

4616

3.65

159.

556

3

Pr

18.8

19.3

518

.91

Nd

91.8

64.1

69.4

6578

.474

.685

65.9

77.7

66.7

769

.04

67.1

721

3

Sm

12.2

8.95

9.3

8.85

10.7

10.3

11.3

8.8

10.9

9.77

10.1

9.65

32

Eu

2.85

2.2

2.2

2.1

2.5

2.4

2.65

2.1

2.55

3.42

3.43

3.09

Gd

6.78

5.66

5.62

5.58

6.3

6.12

6.89

5.42

6.44

10.4

710

.610

.34

Tb

0.97

0.99

0.96

6

Dy

3.26

3.2

3.16

Ho

0.6

0.6

0.58

Er

1.63

1.65

1.63

Tm

0.19

0.19

0.19

Yb

1.3

1.05

10.

951.

11.

051.

20.

951.

050.

961.

010.

99

Lu

0.2

0.16

0.16

0.14

0.14

0.14

0.18

0.14

0.16

0.15

0.15

0.14


Fig. 4 Whole-rock variations of a MgO (wt%) versus SiO2 (wt%) andb K2O (wt%) versus TiO2 (wt%) for the Mainpur pipes. Data sources:Behradih (earlier work; Chalapathi Rao et al. 2011a; Lehmann et al.2007); Kodomali (earlier work; data from Fareeduddin et al. 2006;Paul et al. 2006; Lehmann et al. 2006); WKF and NKF (ChalapathiRao and Dongre 2009; Chalapathi Rao and Srivastava 2009; Chala-pathi Rao et al. 2004). Various kimberlite and orangeite fields are from

Donnelly et al. 2011 and the references therein. c Primitive-mantle-normalised (Sun and McDonough 1989) multi-element spidergram forthe Behradih and Kodomali orangeite samples. (x = Kodomali earlierwork; Fareeduddin et al. 2006; Paul et al. 2006; Lehmann et al. 2006).Variation of d Nb (ppm) versus Ba (ppm) and e La (ppm) versus Rb(ppm). The symbols and the data sources are the same as in Fig. 4a.Various fields are taken from Donelly et al. (2011)


Discussion

The geology of the Mainpur orangeites reveals that theypreserve diatreme (Behradih) as well as hypabyssal facies(Kodomali) and implies differential erosion in this domain.Except for the Behradih and Kodomali pipes, all otheroccurrences in the Mainpur field are highly altered, con-taminated and silicified. Radiometric age data reveal thatthe Behradih and Kodomali pipes constitute the youngestyet recorded diamondiferous magmatic event in the Indianshield, straddling the Cretaceous-Tertiary (K-T) boundary,synchronous with the eruption of Deccan flood basalts (seeLehmann et al. 2010). The Mainpur orangeites have alsorecently been inferred to be a part of the kimberlite–lamp-rophyre–carbonatite–alkaline rock spectrum in the DeccanLarge Igneous Province with variable lithospheric thicknesscontrolling the geographic distribution of the different rockvariants (Chalapathi Rao and Lehmann 2011).

In terms of mineral chemistry, similarities as well asdifferences have been noticed between the Kodomali andBehradih pipes. Olivine, spinel and clinopyroxene in bothpipes have overlapping compositions, whereas the

groundmass phlogopite (TiO2) and perovskite (Nb2O5 andFe2O3) show marked differences. Distinctness in the com-position of clinopyroxene of the Kodomali samples of thisstudy and those reported by earlier workers (Fareeduddinet al. 2006) raises the possibility of multiple eruptionswithin the same pipe as autoliths of earlier kimberliteintrusions were also recorded (Chatterjee and Jha 1994). Asmultiple kimberlite eruptions (up to 8) within a short spanof a few million years have been recorded from the Prieskaregion, southern Africa (Smith et al. 1994) and the Fort á LaCorne field, Canada (Kjarsgaard et al. 2009), radiometricdating on mineralogically distinct samples is in progress toascertain such episodes within the Kodomali pipe. Never-theless, mineral compositions of the Behradih and Kodo-mali pipes together show considerable differences to theMesoproterozoic kimberlites of WKF and NKF, southernIndia, and resemble the southern African orangeites in thisregard.

The major element contents viz., SiO2, MgO, Fe2O3*,K2O and TiO2 of the Behradih and Kodomali pipes areclearly different from bulk rocks of the WKF and NKFwhich have relatively lower SiO2 and a much wider range inMgO. Furthermore, when compared to the archetypal

Fig. 5 Cr2O3 versus CaO (wt%) variation plots of pyrope garnet separates from orangeites (Mainkar 2010). The fields are adopted from Grutteret al. (2004). Source of analyses is also given


Ta

ble

7R

are

eart

hel

emen

t(R

EE

)co

nten

tsof

garn

etse

para

tes

from

the

Beh

radi

h(B

EH

seri

es)

and

Pay

alik

hand

(PA

Yse

ries

)or

ange

ites

ppm

BE

H-A

-07

BE

H-A

-08

BE

H-A

-13

BE

H-A

-18

BE

H-A

-19

BE

H-A

-26

BE

H3-

01B

EH

3-02

BE

H3-

04B

EH

3-05

BE

H3-

07B

EH

3-08

BE

H3-

10B

EH

3-12

La

0.03

0.03

0.03

0.03

0.04

0.06

0.05

0.04

0.10

0.12

0.11

0.07

0.06

0.04

Ce

0.12

0.16

0.21

0.16

0.10

0.26

0.76

0.30

1.06

1.05

0.98

0.71

0.82

0.25

Pr

0.04

0.07

0.09

0.07

0.04

0.08

0.30

0.12

0.30

0.29

0.28

0.34

0.31

0.06

Nd

0.42

0.60

0.78

0.83

0.52

0.41

3.02

1.06

1.96

2.04

1.81

2.8

2.4

1.18

Sm

0.37

0.65

0.72

0.65

0.58

0.25

1.98

0.77

0.70

0.62

0.62

1.89

1.79

0.62

Eu

0.25

0.35

0.36

0.39

0.27

0.07

0.95

0.35

0.11

0.09

0.23

0.98

0.99

0.19

Gd

0.89

1.28

1.47

1.23

1.42

0.33

4.32

1.04

0.55

0.23

0.37

4.06

3.96

0.92

Dy

1.38

3.28

1.98

2.25

2.8

0.55

6.32

1.39

0.70

0.30

0.57

6.57

6.36

2.04

Ho

0.39

0.73

0.40

0.50

0.66

0.21

1.46

0.45

0.07

0.13

0.10

1.45

1.40

0.46

Er

1.51

2.15

1.29

1.8

1.89

0.60

4.47

1.28

0.62

0.45

0.48

4.84

4.56

1.79

Yb

1.73

2.67

1.74

1.79

2.35

1.20

6.32

2.3

0.61

0.61

0.43

6.37

6.03

2.21

Lu

0.30

0.41

0.29

0.31

0.36

0.23

1.01

0.31

0.08

0.09

0.11

1.07

0.98

0.3

ppm

BE

H3-

15B

EH

3-16

BE

H3-

21B

EH

4-07

BE

H4-

09P

AY

-A-0

2P

AY

-A-1

3P

AY

-A-1

5P

AY

2-05

PA

Y1-

03P

AY

1-04

PA

Y1-

07

La

0.05

0.07

0.12

0.03

0.04

0.04

0.05

0.05

0.04

0.08

0.04

0.03

Ce

0.24

0.88

0.98

0.09

0.31

0.27

0.12

0.12

0.27

0.77

0.17

0.26

Pr

0.14

0.31

0.29

0.03

0.19

0.10

0.04

0.04

0.10

0.13

0.07

0.09

Nd

1.31

3.06

1.95

0.37

1.37

0.83

0.46

0.46

1.33

0.93

0.69

1.14

Sm

0.51

2.2

0.66

0.56

1.2

0.52

0.51

0.51

0.82

0.77

0.97

0.92

Eu

0.18

0.94

0.14

0.31

0.63

0.24

0.30

0.30

0.41

0.33

0.39

0.56

Gd

0.66

3.89

0.48

1.45

1.94

0.93

1.18

1.18

1.57

1.12

1.71

1.85

Dy

0.46

6.13

0.54

3.26

2.18

1.82

2.53

2.53

2.57

1.85

1.66

2.76

Ho

0.06

1.37

0.07

0.75

0.51

0.44

0.51

0.51

0.65

0.49

0.39

0.69

Er

0.41

4.54

0.26

2.47

1.31

1.33

1.73

1.73

1.72

1.69

1.17

1.78

Yb

0.59

6.27

0.43

2.72

1.41

1.41

1.88

1.88

2.03

1.7

1.23

1.76

Lu

0.08

1.04

0.08

0.42

0.21

0.24

0.27

0.27

0.32

0.21

0.17

0.31


kimberlites and orangeites of southern Africa, the Mainpurpipes as well as kimberlites from NKF and WKF clearlydisplay a more fractionated nature and show strong affinitiestowards orangeites. Prominent negative spikes at K, Sr, P,Ta (for some samples from Kodomali) and Ti and positiveBa spikes are noticed in the primitive-mantle-normalisedplots (Fig. 4c), and the samples under study can be utilisedto infer the nature of their mantle source regions. Spikes inthe multi-element plots are variably interpreted to reflectresidual phases, fractional crystallisation or even hydro-thermal alteration. Negative K-Ta-Sr-P-Ti spikes in south-ern African kimberlites have been attributed to be a primaryfeature of the source rock (Becker and Le Roex 2006),whereas negative K-Ti in southern African orangeites havebeen linked to the fractionation of phlogopite (Coe et al.2008). Positive Ba spikes are also a characteristic feature ofsouthern African orangeites and have been explained byconcentration of Ba in phlogopite (Howarth et al. 2011).

Similar explanations can be invoked in the case of samplesunder this study.

The depth of generation of kimberlite magmas is amatter of debate with sources ranging from sub-continentallithospheric mantle (SCLM) (e.g., Le Roex et al. 2003;Chalapathi Rao et al. 2004; Becker et al. 2007; Donnellyet al. 2011), convecting (asthenospheric) upper mantle (e.g.,Mitchell 2006; Woodhead et al. 2009), recycled oceaniccrust at the transition zone or lower mantle (e.g., Ringwoodet al. 1992; Paton et al. 2009), core-mantle boundary (e.g.,Haggerty 1999; Torsvick et al. 2010; Collerson et al. 2010)and even multiple reservoirs (Tappe et al. 2011). On theother hand, there is general agreement for SCLM to be thesource for orangeites (e.g., Skinner 1989; Mitchell 2006;Coe et al. 2008; Chalapathi Rao et al. 2011a). Incompatibletrace element ratios of Ce/Y, La/Yb and Zr/Nb can beexploited to infer the degree of melting involved in thegeneration of kimberlites and orangeites, and increasing

Fig. 6 Chondrite-normalised(Sun and McDonough 1995) rareearth element distributionpatterns for pyrope garnetseparates from Behradih (a) andPayalikhand (b). The field ofFinsch low-Cr lherzolite pyropes,shown for comparison, is fromGibson et al. (2007)


La/Yb is interpreted to represent lower degrees of partialmelting of a peridotitic source (Mitchell 1995; Howarthet al. 2011). Figure 10a, b, involving these ratios, suggeststhat (1) the Behradih and Kodomali pipes have undergonedegrees of partial melting similar to the unevolvedKroonstad orangeites from the Kaapvaal craton, southernAfrica, (2) are typified by higher Zr/Nb ratios than thosefrom WKF and NKF kimberlites, Dharwar craton and(3) their compositions cannot be derived by single-stagepartial melting of a peridotite mantle source. We havecompared the observed REE ratios of Behradih and Kodomalias well as those of others from the WKF and NKF (the latterdata were taken from Chalapathi Rao et al. 2004; ChalapathiRao and Srivastava. 2009; Chalapathi Rao and Dongre 2009),with the melting trajectories of inferred southern AfricanGroup I and orangeite source regions presented by Becker andLe Roex (2006), in an effort to test whether the southernAfrican model is applicable for the Indian samples. Resultspresented in Fig. 10c show that a simple melting trajectory,although slightly away from the assumed southern Africanorangeite, can account for the observed REE compositionsof the Behradih and Kodomali samples. On the other hand,the composition of both the NKF and WKF samples can bebetter explained by a kimberlite source or a combination ofkimberlitic and orangeitic sources.

The bulk-rock compositions of the Kodomali andBehradih samples are also compared (Fig. 11) with theexperimental data obtained from a multi-component (natu-ral) bulk system at 10 GPa to constrain the generation anddifferentiation of orangeite magma (Ulmer and Sweeney2002). Their compositions are consistent with the differen-tiation of an orangeite composition by olivine–garnet–orthopyroxene fractionation (Fig. 11).

The pyrope garnet population in the Behradih, Kodo-mali, Payalikhand and Kosambura pipes is dominated bythe calcic-lherzolitic variety, with \5 % belonging to thehigh-interest sub-calcic harzburgitic category and theremainder to other fields such as eclogitic types (Fig. 5).The lherzolitic trend is suggestive of garnet in equilibriumwith clinopyroxene (see Gibson et al. 2008) which is con-sistent with the garnet-peridotite affinities of the Mainpurdiopside xenocrysts (Fig. 7). The findings of our study arealso consistent with those reported earlier by (1) Jha et al.(2002) wherein the pyrope population in the Mainpur pipeshas been inferred to be dominated by the calcic-lherzoliticvariety corresponding to G9 and some eclogitic (G3, G4 andG5) and iron–titanium low calcium (G1 and G2) garnetswith the sub-calcic harzburgitic category (G10) pyropegarnet population between 1 and 5 % and (2) Mukherjeeet al. (2000) who reported garnets of eclogitic paragenesisfrom the Behradih pipe.

The presence of ‘‘sinusoidal’’ REEN patterns in Behradihgarnets of this study (Fig. 6) is significant since such pat-terns are regarded to result from a two stage processinvolving an (1) initial extensive komatiite melt extractionevent which results in extreme LREE and HREE depletionleading to a depleted lithosphere followed by (2) a secondstage involving fluid metasomatism wherein repeated pulsesof fractionated melt with low HREE and variable LREE/MREE react with the depleted lithosphere (Stachel et al.2004; Creighton et al. 2010). The sub-calcic harzburgiticgarnets displaying sinusoidal REEN are only confined todepths \175 km and temperatures of 1150 �C, whereaslherzolitic garnets and Ti–rich pyrope megacrysts originatefrom depths in excess of 175 km (e.g., Stachel et al. 1998,2004; Lehtonen 2005). Therefore, the present study

Fig. 7 Cr2O3 (wt%) versus Al2O3 (wt%) plots of Cr-diopside xenocrysts from the Kosambura (a) and Kodomali (b) pipes (Mainkar 2010). Thefields are adapted from Ramsay (1992)


Fig. 8 Cr2O3 (wt%) versus MgO (wt%) and Cr2O3 (wt%) versus TiO2

(wt%) variation plots of Behradih (A and B), Kodomali (c, d) andKosambura (e and f) chromite xenocrysts (Mainkar 2010). Diamond

inclusion and intergrowth field is after Fipke et al. (1995). The plotssuggest that the source region of most of the indicator chromitecrystals from the Mainpur pipes is from the diamond stability field


identifies, for the first time, the presence of a composi-tionally layered mantle in the end-Cretaceous sub-Bastarcraton similar to that reported from other cratons elsewheresuch as Kaapvaal craton (e.g., Gregoire et al. 2003; Gibsonet al. 2008), western Guyana shield (e.g., Schulze et al.2006), Slave craton (e.g., Griffin et al. 1999; Kopylova andCaro 2004), Siberian craton (e.g., Ashchepkov et al. 2010),

North Atlantic craton (Sand et al. 2009) and Karelian craton(Lehtonen et al. 2004).

Some of the chromites from the Behradih, Kodomali andKosambura orangeites are compositionally similar to thosefound as inclusions in diamonds (Fig. 8), implying theirderivation from the diamond stability field, which also findssupport in the diamondiferous nature of these pipes. It is

Fig. 9 Backscattered electron (BSE) images of zircon xenocrystsfrom the Behradih pipe depicting their varied morphology. Thecircles and their inscribed numbers correspond to their U–Pb isotopic

analysis provided in Table 8. Figure 9m U–Pb concordia diagram ofzircon analyses in (a–l). Data point ellipses are given as 2-sigmauncertainties


Ta

ble

8U

–Pb

isot

opic

data

ofzi

rcon

grai

nsfr

omth

eB

ehra

dih

pipe

.M

orph

olog

yof

mos

tof

the

anal

ysed

grai

nsis

pres

ente

din

Fig

.9a

toL

207P

bU

aP

baT

ha207P

bb2

rd

206P

bb2

rdrh

oc207P

be2

rd

207P

b2

r206P

b2

r207P

b2

rC

oncf

Cry

stal

#A

naly

sis

#(c

ps)

(ppm

)(p

pm)

U235U

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now well established that the occurrence of diamonds intheir primary source rock is a resultant of three major fac-tors: (1) presence of diamondiferous zones in the sub-con-tinental lithospheric mantle sampled by a kimberliticmagma, (2) a magmatic process that can disrupt lithosphericwall rock (various peridotites and eclogites) and incorporatesome portion into the magma as xenoliths and (3) relativelynon-destructive transport of diamonds to the surface byrapid ascent (e.g., Pearson et al. 2003; Scott-Smith andSmith 2009; Gurney et al. 2010). It has also been well

established that with increasing fO2, diamond in kimberlitemagma oxidises to CO2 and that prolonged residence timesin high fO2 conditions increase diamond resorption (e.g.,Fedortchouk et al. 2005), resulting in a decrease in diamondgrade of kimberlites. From experimental studies, Bellis andCanil (2007) proposed that diamond stability dependsmainly on the fO2 of the kimberlite magma and calibratedan empirical oxygen barometer based on Fe3+ content ofperovskite to estimate oxygen fugacity (fO2) during thecrystallisation and emplacement of kimberlites.

Fig. 10 a La/Yb versus Zr/Nb for the samples under study. Otherkimberlite and orangeite fields are after Becker and Le Roex (2006).Sover North and Pniel/Postmasburg represent fields for evolvedorangeites (Mitchell 1995) from southern Africa, whereas Kroonstadrepresents recently documented data for an orangeite field involvingthe Lace, Voorspoed and Besterskraal pipes from southern Africa(Howarth et al. 2011). b Zr/Nb versus Ce/Y and c La/Sm versus Gd/Ybof the Mainpur orangeites of this study. The symbols and the other datasources are the same as in Fig. 4. (b) Solid curved line indicatescompositions of melts formed by various degree (%) of equilibriumpartial melting of a peridotite containing 1.4 ppm Ce, 3.45 ppm Y,

8.51 ppm Zr and 0.54 ppm Nb and is taken from Tainton (1992).La/Sm versus Gd/Yb for Behradih and Kodomali as well as for theother kimberlites from the WKF and NKF. The symbols and the datasources are the same as in Fig. 4. c Illustrated curves (from Becker andLe Roex 2006) represent melting trajectories of inferred Group I and IIkimberlite source regions having a residual mineralogy as follows: GrpI, ol:opx:cpx:gt = 0.67:0.26:0.04:0.03; Grp II, ol:opx:cpx:gt =

0.67:0.26:0.06:0.01. Numbers shown represent the degree of melting.Fields for Kimberley (Grp I) and Swartrugens and Star (Grp II)kimberlites are from Becker and Le Roex (2006).The symbols and thedata sources are the same as in Fig. 4


The oxybarometry of Bellis and Canil (2007) has beenapplied to perovskite from the Kodomali and Behradihpipes of this study, and their DNNO estimates (see Table 5)exhibit a range from +0.71 (Kodomali) to +4.28 (Behradih)with Kodomali showing a much greater variation amongstthe two (Fig. 3g). Our results highlight that (1) the DNNOconditions of the Mainpur pipes (Table 5) are higher thanthose from non-prospective NKF pipes as well as otherprospective diamondiferous kimberlites located elsewheresuch as Dutoitspan, southern Africa, Somerset Island,Canada, and (2) are indistinguishable in the redox condi-tions of the highly diamondiferous Lac de Gras kimberlites,Canada (Fig. 3g). We therefore conclude that oxidationstate cannot explain the high incidence of diamonds in theMainpur field and other factors very likely have played asignificant role. The Mainpur orangeites are thus indeed‘‘anomalous’’, compared to many other kimberlites(Fig. 3g), in terms of their high diamond incidence con-sidering the preponderance of calcic-lherzolitic garnets andthe relatively oxidising conditions at the time of theireruption.

All zircons recovered from the Behradih pipe are crustal-derived xenocrysts and not mantle-derived megacrysts, asrevealed by their composition and U–Pb ages (Table 8).However, the absence of Archaean age data is surprisingsince the Bastar craton is regarded as the oldest continentalnuclei in the Indian shield with an Eoarchaean crust asevidenced by the 3.50–3.6 Ga zircons from tonalites andgranites (Sarkar et al. 1993; Ghosh 2004; Rajesh et al. 2009)and a thickened modern-day crust of 35–40 km (Jagdeeshand Rai 2008). Therefore, the lack of Archaean-aged zir-cons could well be a reflection of the sampling. Alternately,it may also represent modification of the sub-Bastar litho-sphere by the invading Deccan plume-derived melts duringthe end-Cretaceous synchronous with the eruption of the

Mainpur orangeites (see Chalapathi Rao and Lehmann 2011for a detailed discussion). Further studies involving theU–Pb dating of zircons from the Mainpur field are expectedto provide further clues in this direction.

Conclusions

Following major conclusions can be drawn from this study:• Contrasting textures in case of Behradih (pelletal and

tuffisitic habit) and Kodomali (inequigranular texturewith two generations of fresh olivine) orangeites imply adifferential erosion in this domain.

• Olivine, spinel and clinopyroxene in the Behradih and theKodomali orangeites share overlapping compositions,whereas the groundmass phlogopite and perovskite showconspicuous compositional differences.

• Incompatible trace elements and their ratios readily dis-tinguish Mainpur orangeites from the MesoproterozoicWajrakarur (WKF) and Narayanpet kimberlites (NKF)from the eastern Dharwar craton, southern India. How-ever, there is an overall similarity in petrogenesisbetween the Mainpur and the southern Africanorangeites.

• The Mainpur orangeites are dominated by calcic-lherzo-litic variety of pyrope population in comparison with thatof sub-calcic harzburgitic and eclogitic variety of garnets.

• ‘‘Smooth’’ as well as ‘‘sinusoidal’’ chondrite-normalisedREE patterns are displayed by garnets from the Behradihand Payalikhand orangeites. Such contrasting patternsprovide evidence for the presence of a compositionallylayered end-Cretaceous sub-Bastar craton mantle.

• Perovskite oxybarometry indicates that the redox state ofthe lithospheric mantle cannot be of first-order control for

Fig. 11 Composition of the Behradih and Kodomali orangeites incomparison with summary of liquid compositions (solid diamonds)obtained by Ulmer and Sweeney (2002) in comparison with mantlecarbonatite composition obtained by Sweeney (1994) and pseudoter-nary (wt%) diagram after Freestone and Hamilton (1980) projectedfrom CO2 ? H2O. Open circles = compositions of principal mineralphases used in the system; Gar = Garnet, Opx = Orthopyroxene;

Ol = Olivine and Phl = Phlogopite. Grey-filled diamonds = locationof starting orangeite (GPII) composition, MAR = MARID andCAR = carbonatite. Dashed lines = direction of differentiation ofGroup II composition by olivine–garnet–opx fractionation to producealkali-dolomitic carbonatitic liquids. Note that the Mainpur samplesexclude differentiation towards a MARID composition


diamond potential and implies the dominant role of otherfactors viz., rapid magma transport.

• Mainpur orangeites are clearly ‘‘anomalous’’ compared toseveral other kimberlite/orangeite pipes worldwide inview of their (1) highly diamondiferous nature, (2)abundance of calcic-lherzolitic garnets, and (3) highlyoxidising conditions prevailing at the time of eruption.

• U–Pb dating of zircon xenocrysts from the Behradih pipeyielded distinct Palaeoproterozoic ages. The lack ofArchean-aged zircons could either be a reflection of thesampling process or of the modification of the sub-Bastarlithosphere by the invading Deccan plume-derived meltsat ca. 65 Ma.

Acknowledgments NVCR thanks the Head, Department of Geology,Centre of Advanced Study, Banaras Hindu University for providinglogistics for undertaking field trip to Mainpur area and to the Hum-boldt Foundation for support. DM thanks Directorate of Mines andGeology, Chhattisgarh for support, permission and encouragement forcarrying out studies on the Mainpur pipes and their xenocrysts. Chi-ranjeeb Sarkar and Graham Pearson are thanked for their helpfulreviews.

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