Mafic xenoliths in Proterozoic kimberlites from Eastern Dharwar Craton, India:Mineralogy and P–T regime

S.C. Patel a,*, S. Ravi b, Y. Anilkumar a, A. Naik c, S.S. Thakur d, J.K. Pati e, S.S. Nayak b

a Department of Earth Sciences, Indian Institute of Technology, Powai, Mumbai, Maharashtra 400 076, Indiab Geological Survey of India, Bandlaguda Complex, Hyderabad 500 068, Indiac Department of Earth Sciences, Sambalpur University, Burla, Orissa 768 019, Indiad Wadia Institute of Himalayan Geology, 33 General Mahadev Singh Road, Dehra Dun 248 001, Indiae Department of Earth and Planetary Sciences, University of Allahabad, Allahabad 211 002, India

Keywords:EclogiteGarnet pyroxeniteKimberliteXenolith

a b s t r a c t

Mafic xenoliths of garnet pyroxenite and eclogite from the Wajrakarur, Narayanpet and Raichur kimber-lite fields in the Archaean Eastern Dharwar Craton (EDC) of southern India have been studied. The com-position of clinopyroxene shows transition from omphacite (3–6 wt% Na2O) in eclogites to Ca pyroxene(<3 wt% Na2O) in garnet pyroxenites. Some of the xenoliths have additional phases such as kyanite, ensta-tite, chromian spinel or rutile as discrete grains. Clinopyroxene in a rutile eclogite has an XMg value of0.70, which is unusually low compared to the XMg range of 0.91–0.97 for all other samples. Garnet inthe rutile eclogite is also highly iron-rich with an end member composition of Prp26.5Alm52.5Grs14.7Adr5.1-

TiAdr0.3Sps1.0Uv0.1. Garnets in several xenoliths are Cr-rich with up to 8 mol% knorringite component.Geothermobarometric calculations in Cr-rich xenoliths yield different P–T ranges for eclogites and garnetpyroxenites with average P–T conditions of 36 kbar and 1080 �C, and 27 kbar and 830 �C, respectively.The calculated P–T ranges approximate to a 45 mW m�2 model geotherm, which is on the higher sideof the typical range of xenolith/xenocryst geotherms (35–45 mW m�2) for several Archaean cratons inthe world. This indicates that the EDC was hotter than many other shield regions of the world in themid-Proterozoic period when kimberlites intruded the craton. Textural and mineral chemical character-istics of the mafic xenoliths favour a magmatic cumulate process for their origin as opposed to subductedand metamorphosed oceanic crust.

1. Introduction

Deep-seated xenoliths in kimberlites are of great interest togeologists for providing direct samplings of material from theupper mantle. Ultramafic rocks (lherzolite, harzburgite, wehrlite,dunite and pyroxenite) are the most common mantle-derivedxenoliths, whereas mafic xenoliths (garnet pyroxenite and eclog-ite) are less common. Garnet pyroxenite and eclogite are essen-tially biminerallic garnet-clinopyroxene rocks, but theirnomenclature is confused because there is often a compositionaltransition between them. The term ‘eclogite’ is normally usedwhen the clinopyroxene is a Ca–Na pyroxene (omphacite),whereas the rock with Ca pyroxene is called ‘garnet pyroxenite’.The IUGS Subcommission for the nomenclature of metamorphicrocks (SCMR) recently recommended the crystal-chemical classifi-cation of clinopyroxene by Morimoto et al. (1988) to be used fordefining omphacite (Desmons and Smulikowski, 2004 and up-grades). Following this scheme, Patel et al. (2006) identified eclog-

ite xenoliths from three kimberlite pipes of the WajrakarurKimberlite Field (WKF) in the Eastern Dharwar Craton of southernIndia (Fig. 1). In the present contribution we examine the mineral-ogical characteristics of garnet pyroxenite xenoliths from a numberof pipes of WKF, and a few pipes of the adjacent Narayanpet Kim-berlite Field (NKF) and Raichur Kimberlite Field (RKF). Some newlyfound eclogite xenoliths in the pipes are also included in the study.A xenolith geotherm has been derived for the region based onquantitative geothermobarometry.

2. Geology of Dharwar craton

The Archaean Dharwar craton is a typical granite-greenstoneterrane with a gneissic basement of tonalite-trondhjemite-grano-diorite (TTG) composition known as Peninsular Gneisses (Naqviand Rogers, 1987). The craton is bounded in the east by the Prote-rozoic Eastern Ghats Mobile Belt, in the northeast by the ArchaeanBastar craton, and is covered in the northwest by the Cretaceous-Tertiary lava flows of the Deccan Traps (Fig. 1) A striking featureof the craton is the N–S to NW–SE trending, �400 km long and20–30 km wide cluster of plutons known as the Closepet Granite,

Fig. 1. Generalised geological map of southern India modified after Drury et al. (1984) and Geological Survey of India (1998) showing kimberlite and lamproite fields in theEastern Dharwar Craton. BC = Bastar Craton; EDC = Eastern Dharwar Craton; KLF = Krishna lamproite field; NKF = Narayanpet kimberlite field; NLF = Nallamalai lamproitefield; TKF = Tungabhadra kimberlite field; WDC = Western Dharwar Craton; WKF = Wajrakarur kimberlite field.

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dated 2.51 Ga (Friend and Nutman, 1991). Sediments of Meso- toNeoproterozoic intracratonic sedimentary basins such as the Cud-dapah basin unconformably overlie the granite – greenstone ter-rane. The craton is divided into two sub-provinces – EasternDharwar Craton (EDC) and Western Dharwar Craton (WDC) withChitradurga Boundary Fault located along the eastern margin ofthe Chitradurga schist belt as the boundary between them (SwamiNath et al., 1976; Drury et al., 1984; Chadwick et al., 2000). Someworkers believe that the Closepet Granite, which is located�50 km east of the Chitradurga Boundary Fault represents theboundary between the EDC and WDC (Naqvi and Rogers, 1987;Gupta et al., 2003; Moyen et al., 2003). Although the actual bound-ary between the two cratonic blocks remains debatable there arenotable differences in lithology and metamorphism of the twoblocks. The WDC is dominantly occupied by TTG gneisses (3.0–3.4 Ga) with minor schist belts of Sargur age (3.0–3.3 Ga), majorschist belts of Dharwar age (2.9–2.6 Ga) containing predominantplatformal sediments, and a few Late Archaean granitoid plutons

dated in the range of 2.60–2.65 Ga (Jayananda et al., 2006 and ref-erences therein). On the other hand the EDC is characterised byvoluminous Late Archaean granitoids (2. 51–2.75 Ga) (the ‘‘Dhar-war batholith” of Chadwick et al., 1996, 2000) with minor TTGgneisses and thin volcanics-dominated schist belts of Dharwarage. The schist belts in the craton are metamorphosed to greens-chist to amphibolite facies regional metamorphism. The profusionof granitoids in the EDC is responsible for low pressure regionalmetamorphism (andalusite-sillimanite type) in this block in con-trast to the intermediate pressure regional metamorphism (kya-nite-sillimanite type) in the WDC.

2.1. Kimberlite fields

Kimberlites discovered in southern India till now are restrictedto the EDC and are distributed in four fields, viz. WKF, NKF, RKF,and Tungabhadra Kimberlite Field (TKF) (Fig. 1). The WKF contains28 kimberlite pipes spread over four clusters, namely Wajrakarur

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(13 pipes; P1–P13), Chigicherla (5 pipes; CC1–CC5), Kalyandurg (6pipes; KL1–KL6) and Timmasamudram (4 pipes, TK1–TK4) (Nayakand Kudari, 1999; Srinivas Choudary et al., 2007) (Fig. 2). There are30 pipes in the NKF in four clusters, which are Narayanpet (10pipes; NK1–NK10), Maddur (11 pipes; MK1–MK11), Bhima (BK1–BK3) and Kotakonda (6 pipes; KK1–KK6) (Rao et al., 1998). TheRKF has 6 pipes out of which 3 pipes (SK1–SK3) occur in Siddanpal-li cluster, and other three pipes (RK1–RK3) are dispersed (Sridharet al., 2004). The TKF is the most recently discovered kimberlitefield, which has two pipes (MNK1 and MNK2) (Ravi et al.,2007b). Most of the WKF pipes are diamondiferous, but the NKFand RKF pipes have not yet been proved to be diamondiferous(Neelakantam, 2001). Detailed studies on RKF and TKF kimberlitesare yet to be attempted. In addition to the four kimberlite fieldsthere are two lamproite fields in the EDC, viz. Krishna LamproiteField and Nallamalai Lamproite Field.

Available radiometric ages for the kimberlites of WKF rangefrom 840 to 1150 Ma, whereas those of NKF range from 1080 to1400 Ma (Anil Kumar et al., 1993; Chalapathi Rao et al., 1996,1999). Chalapathi Rao et al. (1999) have suggested that theemplacements of Kotakonda kimberlite in the NKF, and Chelimalamproite in the NLF were contemporaneous (�1400 Ma) and thatthese pipes are older than the WKF kimberlites (�1090 Ma). How-ever, the older phlogopite K–Ar and Ar–Ar ages reported by Chala-

Fig. 2. Geological sketch map of Wajrakarur kimberli

pathi Rao et al. (1999) are not borne out by the phlogopite Rb–Srisochron ages of Anil Kumar et al. (2001). Therefore, it can be safelyconcluded that the kimberlites of EDC erupted episodically close to1090 Ma.

3. Mafic xenoliths

Xenolith samples include hand specimens (several centimetersin size) collected from pits and boreholes, and subrounded mineralaggregates, also termed nodules (2–5 mm across) in heavy mineralconcentrates (Ravi et al., 2007a). In most kimberlites of EDC, maficxenoliths are greatly subordinate in numbers to ultramafic xeno-liths (Ganguly and Bhattacharya, 1987; Nehru and Reddy, 1989).However, the KL2 pipe of WKF is unusual as eclogites constitutemore than 95% of the xenolith population (Rao et al., 2001). In bothgarnet pyroxenite and eclogite xenoliths, garnet and clinopyroxenetogether constitute P80 vol%. Some mafic xenoliths are observedto contain additional phases such as kyanite, enstatite, chromianspinel and rutile as discrete grains which constitute 620 vol% ofthe rock (Table 1). The mineral abbreviations used in this paperare: adr, andradite; alm, almandine; cpx, clinopyroxene; grs, gros-sular; grt, garnet; ilm, ilmenite; kn, knorringite; omp, omphacite;prp, pyrope; rt, rutile; sps, spessartine; uv, uvarovite.

te field modified from Nayak and Kudari (1999).

Table 1Mafic xenoliths in kimberlite pipes of EDC

Xenolith type Kimberlitepipe

Hand specimens from pits andboreholes

Nodules fromheavy mineralconcentrate

Kyaniteeclogite

KL2 KL2SL1 (omphacite totallyaltered), KL2SL5, KL2SL12,KL2MUP, KL2-7 (garnet-freepart)

–

Enstatiteeclogite

KL2 – KL2N9a

Rutile eclogite P3 P3Xe/86 –Chromian

spineleclogite

CC4 – CC4N8b

Bimineralliceclogite

KL2 – KL2N9b, KL2N9c

P2 P2N4a, P2N5aP10 – P10N7b, P10N7cMK8 – MK8N1a

Enstatitegarnetpyroxenite

P12 – P12N6b

Chromianspinel-garnetpyroxenite

P12 – P12N6c

Biminerallicgarnetpyroxenite

P2 – P2N5b, P2N5c

P3 P3MXe/86, P3Xe11/86 –NK3 – NK3N2a,

NK3N2bRK3 – RK3N5a,

RK3N8b

Pipes KL2, P2, P3, P10, P12, CC4 are in WKF, pipes MK8 and NK3 in NKF, and pipeRK3 in RKF.

Fig. 3. Photograph (a) of hand specimen of biminerallic eclogite from KL2 pipe, and pholight. (a) Graded layering is defined by decrease (white arrow) in the size of garnet grainshydrous Ca–Al silicate. Small divisions in scale are millimeters. (b) Omphacite-garnet assDiscrete grain of rutile with exsolution lamellae of ilmenite.

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Hand specimens of eclogite from the KL2 pipe are characterizedby honey brown to dark brown garnets of 1–5 mm size, which aredistributed in a pale greenish grey to white matrix. The matrix is ahydrous Ca–Al silicate derived by secondary alteration of ompha-cite. Some samples show mineralogical banding on 1–3 cm scale,with transition from kyanite eclogite to kyanite-free eclogite. Thelatter is commonly marked by layering from garnet-rich to gar-net-poor bands, with conspicuous graded layering in a few samples(Ravi et al., 2007a) (Fig. 3a). In hand specimens of both garnetpyroxenite and eclogite which do not show gross inhomogeneities,the amount of garnet and clinopyroxene is 30–50 vol% each.

Despite extensive secondary alteration the original outlines ofomphacite grains in the matrix are recognizable in thin sections.The matrix comprises anhedral to interstitial grains of omphaciteof 0.5–4 mm size in which subhedral to rounded grains of garnetare set. There is often a patchy distribution of the phases in therocks. In omphacite-rich portions straight or curved grain bound-aries and 120� angles at many triple junctions can bee seen. Garnetcommonly shows semi-opaque, kelyphitic alteration rim, whichconsists of an aggregate of fine-grained phlogopite, K-feldsparand hydrous Cal-Al silicate. In samples where omphacite in thematrix is completely altered, fresh omphacite is occasionally pre-served as subhedral to subrounded inclusions in garnet. Microfrac-turing is very common in all types of xenoliths (Fig. 3b). All thenodules of eclogite and garnet pyroxenite from heavy mineral con-centrate of different pipes are medium grained (1–5 mm size) withpink garnet and green omphacite or Ca pyroxene as the principalminerals. Modal proportions of garnet and clinopyroxene in thenodules vary widely, but the nodules are clearly related to otherrocks in the mafic xenolith suite.

In some of the eclogite xenoliths garnet grains are characterizedby microscopic triangular arrays of exsolution needles of rutile

tomicrographs (b–d) of rutile eclogite from P3 pipe of WKF under plane polarized(dark grey) in a garnet-rich layer. Omphacite is completely altered to grey to white

emblage with numerous fractures. (c) Fine exsolution needles of rutile in garnet. (d)

Table 2Microprobe analyses of omphacite, Ca pyroxene and enstatite (n = number of points; blank = not analysed)

Omphacite Ca pyroxene Enstatite

Rutileeclogite

Cr-spineleclogite

Bimineralliceclogite

Enstatite garnetpyroxenite

Cr-spinel garnetpyroxenite

Biminerallic garnet pyroxenite Enstatite garnetpyroxenite

P3Xe/86n = 4

CC4N8b MK8N1a P12N6b P12N6c P2N5b P2N5c P3MXe/86n = 6

P3Xe11/86n = 6

NK3N2a NK3N2b RK3N5a RK3N8b P12N6b

SiO2 55.61 51.27 54.87 53.84 54.60 54.94 54.04 53.79 54.12 53.11 54.22 53.40 52.58 55.56TiO2 0.21 0.10 0.10 0.12 0.14 0.15 0.04 0.00 0.10 0.20 0.13 0.17 0.12 0.02Al2O3 8.71 4.49 3.88 3.70 3.77 4.88 5.70 3.60 7.12 2.67 3.39 2.62 3.71 1.29Cr2O3 0.40 3.41 3.95 2.77 2.92 0.66 0.25 1.08 0.14 0.17 0.16 0.15 0.46FeO 7.35 3.03 1.80 1.69 1.86 3.40 1.59 2.80 1.00 3.21 2.28 2.40 2.60 4.66MnO 0.05 0.07 0.07 0.06 0.09 0.07 0.06 0.12 0.02 0.08 0.06 0.00 0.00 0.16MgO 8.19 14.44 14.60 14.85 15.08 15.35 14.42 15.83 14.16 16.30 15.69 16.63 16.71 35.08NiO 0.03 0.02 0.05 0.04 0.08 0.08 0.09 0.06 0.12 0.07 0.13 0.04CaO 13.42 18.95 15.77 19.91 18.95 18.94 20.72 19.39 20.69 24.45 23.05 22.82 22.07 0.64Na2O 6.20 3.15 3.34 2.82 2.64 2.81 2.14 2.54 2.46 0.90 0.97 0.99 1.23 0.12K2O 0.00 0.05 0.01 0.00 0.00 0.02 0.08 0.00 0.01 0.00 0.00 0.00 0.00 0.00Total 100.17 98.98 98.44 99.80 100.13 101.30 99.13 99.21 99.94 101.16 100.08 99.03 99.17 98.03Cations per 6 oxygensSi 2.007 1.897 1.996 1.953 1.967 1.958 1.956 1.961 1.936 1.924 1.961 1.955 1.924 1.948Ti 0.006 0.003 0.003 0.003 0.004 0.004 0.001 0.000 0.003 0.005 0.004 0.005 0.003 0.001AlIV,a 0.000 0.103 0.004 0.047 0.033 0.042 0.044 0.039 0.064 0.076 0.039 0.045 0.076 0.052AlVI 0.371 0.093 0.162 0.111 0.127 0.163 0.199 0.116 0.236 0.038 0.105 0.068 0.084 0.001Cr 0.011 0.100 0.114 0.079 0.083 0.019 0.007 0.031 0.004 0.005 0.005 0.004 0.013Fe2+ 0.222 0.094 0.055 0.051 0.056 0.101 0.048 0.085 0.030 0.097 0.069 0.073 0.080 0.137Mn 0.002 0.002 0.002 0.002 0.003 0.002 0.002 0.004 0.001 0.002 0.002 0.000 0.000 0.005Mg 0.441 0.796 0.792 0.803 0.810 0.815 0.778 0.860 0.755 0.880 0.846 0.908 0.911 1.833Ni 0.001 0.001 0.001 0.001 0.002 0.002 0.003 0.002 0.003 0.002 0.004 0.001Ca 0.519 0.751 0.615 0.774 0.731 0.723 0.803 0.758 0.793 0.949 0.893 0.895 0.865 0.024Na 0.434 0.226 0.236 0.198 0.184 0.194 0.150 0.180 0.171 0.063 0.068 0.070 0.087 0.008K 0.000 0.002 0.000 0.00 0.000 0.001 0.004 0.000 0.000 0.000 0.000 0.000 0.000 0.000Total 4.014 4.068 3.980 4.022 4.000 4.024 3.995 4.036 3.996 4.041 3.996 4.019 4.034 4.023

Mg/(Mg + Fe) 0.67 0.89 0.94 0.94 0.94 0.89 0.94 0.91 0.96 0.90 0.92 0.93 0.92 0.93

a AIIV = 2-Si.

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Table 3Microprobe analyses of garnet (n = number of points; blank = not analysed)

Rutileeclogite

Cr-spineleclogite

Bimineralliceclogite

Enstatite garnetpyroxenite

Cr-spinel garnetpyroxenite

Biminerallic garnet pyroxenite

P3Xe/86 n = 3 CC4N8b MK8N1a P12N6b P12N6c P2N5b P2N5c P3MXe/ 86n = 4

P3Xe11/ 86n = 6

NK3N2a NK3N2b RK3N5a RK3N8b

SiO2 39.34 41.28 42.17 41.45 41.71 41.19 41.39 41.00 41.65 39.91 41.10 39.94 40.27TiO2 0.08 0.27 0.12 0.01 0.06 0.08 0.07 0.38 0.04 0.05 0.03 0.04 0.03Al2O3 20.65 16.53 20.55 20.73 21.32 24.25 23.91 21.22 23.67 23.96 24.38 23.82 23.05Cr2O3 0.02 8.30 5.71 4.68 3.54 0.61 0.26 1.76 0.22 0.38 0.24FeO 24.95 8.04 6.11 8.32 7.37 11.04 6.93 10.22 7.78 13.88 12.38 12.56 11.98MnO 0.42 0.38 0.31 0.56 0.35 0.27 0.30 0.54 0.16 0.45 0.34 0.20 0.36MgO 6.63 18.00 21.24 22.13 21.01 19.62 16.38 20.21 16.28 16.21 16.37 18.22 17.84NiO 0.02 0.00 0.02 0.01 0.01 0.04 0.00 0.05 0.00 0.04 0.04CaO 7.00 8.37 4.53 2.94 4.76 3.96 10.40 4.27 9.33 5.81 6.22 4.87 6.03Na2O 0.05 0.41 0.06 0.08 0.05 0.05 0.03 0.07 0.17 0.03 0.01Total 99.16 101.58 100.82 100.91 100.18 101.11 99.67 99.72 99.30 100.72 101.11 99.65 99.56Cations per 12 oxygensSi 3.065 2.997 2.984 2.947 2.973 2.922 2.972 2.964 3.001 2.900 2.947 2.903 2.934Ti 0.005 0.015 0.006 0.001 0.003 0.004 0.004 0.021 0.002 0.003 0.002 0.002 0.002Al 1.896 1.414 1.714 1.737 1.791 2.028 2.024 1.808 2.010 2.052 2.060 2.040 1.979Cr 0.001 0.476 0.319 0.263 0.200 0.034 0.015 0.101 0.013 0.022 0.014Fe 1.626 0.488 0.362 0.495 0.439 0.655 0.416 0.618 0.469 0.843 0.742 0.763 0.730Mn 0.028 0.023 0.019 0.034 0.021 0.016 0.018 0.033 0.010 0.028 0.021 0.012 0.022Mg 0.770 1.948 2.241 2.346 2.233 2.075 1.754 2.178 1.749 1.756 1.750 1.974 1.938Ni 0.001 0.000 0.001 0.001 0.001 0.002 0.000 0.003 0.000 0.002 0.002Ca 0.584 0.651 0.343 0.224 0.364 0.301 0.800 0.331 0.720 0.452 0.478 0.379 0.471Na 0.008 0.058 0.008 0.011 0.007 0.007 0.004 0.010 0.024 0.004 0.001Total 7.984 8.070 7.997 8.059 8.032 8.044 8.007 8.067 7.998 8.062 8.017 8.073 8.076End member percentageAdr 5.1 4.9 0.0 2.6 1.6 0.6 0.0 5.2 0.0 1.1 0.0 2.7 4.1TiAdr 0.3 0.7 0.3 0.0 0.1 0.2 0.2 1.0 0.1 0.1 0.1 0.1 0.1Uv 0.1 16.0 11.3 4.7 9.9 1.7 0.8 4.6 0.7 1.1 0.7 0.0 0.0Grs 14.7 0.0 0.0 0.0 0.3 7.4 25.8 0.0 23.7 12.4 15.2 9.6 11.1Kn 0.0 7.7 4.9 8.2 0.0 0.0 0.0 0.4 0.0 0.0 0.0 0.0 0.0Prp 26.5 57.0 70.7 68.8 73.8 68.4 58.7 71.0 59.3 57.5 58.5 64.2 63.0Sps 1.0 0.8 0.6 1.1 0.7 0.5 0.6 1.1 0.3 0.9 0.7 0.4 0.7Alm 52.5 12.9 12.2 14.5 13.4 21.1 13.9 16.8 15.9 26.8 24.8 23.0 21.0

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Table 4Microprobe analyses of chromian spinel, rutile and ilmenite (blank = not analysed)

Cr-spineleclogite

Cr-spinel garnetpyroxenite

Rutile eclogite

CC4N8b P12N6c P3Xe/86

Cr-spinel Cr-spinel Rutile Ilmenite

L1a L2

SiO2 0.14 0.11 0.00 0.00 0.00TiO2 2.66 0.47 97.69 51.95 53.15Al2O3 32.70 14.98 0.22 0.61 2.75Cr2O3 29.49 54.05 0.03 0.00 0.02V2O5 0.57 0.31 0.32Fe2O3

b 4.40 2.39 0.12FeO 15.18 15.25 43.94 42.04MnO 0.31 0.26 0.01 0.00 0.05MgO 15.75 12.63 0.01 1.47 1.28CaO 0.14 0.00Total 100.77 100.14 98.65 98.28 99.61Oxygens 4 4 2 3 3Si 0.004 0.003 0.000 0.000 0.000Ti 0.058 0.011 0.990 0.988 0.979Al 1.112 0.560 0.003 0.018 0.079Cr 0.673 1.357 0.000 0.000 0.000V 0.005 0.005 0.005Fe3+ 0.096 0.057 0.001Fe2+ 0.366 0.405 0.929 0.861Mn 0.008 0.007 0.000 0.000 0.001Mg 0.678 0.598 0.000 0.055 0.047Ca 0.004 0.000Total 2.999 2.998 0.999 1.995 1.972Fe/(Fe + Mg) 0.35 0.40 0.94 0.95Cr/(Cr + Al) 0.38 0.71

a L1 and L2 are two different lamellae exsolved from rutile.b Fe2O3 in chromian spinel recalculated following the method of Barnes and

Roeder (2001). All iron assumed as Fe2O3 in rutile and as FeO in ilmenite.

jadeite

NaAlSi O (Jd)2 6

aegirine-augiteomphacite

aegirine

Quad

NaFe Si O (Ae)3+

2 6

Q (Wo, En, Fs)

20 20

80 80

50

Bimineralliceclogite

Kyanite eclogite

Enstatite eclogite

Biminerallicgarnet pyroxenite

Chromian spineleclogite

Enstatite-garnetpyroxenite

Chromian spinel-garnet pyroxenite

Rutile eclogite

N = 25

Fig. 4. Clinopyroxene compositions plotted on end-member triangular diagram ofMorimoto et al. (1988). 12 Omphacite compositions are from Patel et al. (2006) forKL2, P2 and P10 pipes of WKF; 3 omphacite compositions and 10 quadrilateralpyroxene compositions are from this study. Q = quadrilateral pyroxene; Wo = wol-lastonite; En = enstatite; Fs = ferrosilite; Jd = jadeite; Ae = aegirine.

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which are 2–10 lm thick and 10–200 lm long (Fig. 3c). These nee-dles always show inclined extinction. Recent experiments haveshown that Ti solubility in garnet depends on P–T conditions.Zhang et al. (2003) reported increasing solubility of TiO2 (0.8–4.5 wt%) in garnet with increasing P and T in the experimental con-ditions of 50–150 kbar and 1000–1400 �C. On the other handKawasaki and Motoyoshi (2007) observed that TiO2 content of gar-net increases with temperature and decreases with pressure in theP–T range of 7–20 kbar and 850–1300 �C. The results of these stud-ies show that rutile exsolution in garnet can be the result ofdecompression and/or cooling.

Discrete grains of rutile in an eclogite xenolith show two sets oforiented ilmenite lamellae which are nearly perpendicular to eachother and uniformly distributed throughout the rutile (Fig. 3d). Thelamellae are 0.5–5 lm thick and 20–200 lm long, are most likelythe result of a primary exsolution phenomenon (e.g. Rudnicket al., 2000). The limit for solid solution of ilmenite in rutile is7 wt% at 1050 �C (Basta, 1959). Liu et al. (2004) reported exsolutionof ilmenite from rutie in eclogite and attributed it to decompres-sion from a pressure greater than 60–70 kbar. Zhao et al. (1999)highlighted the role of oxygen fugacity in rutile-ilmenite assem-blages in the mantle. However, the relative effects of pressure,temperature and oxygen fugacity, and of other constituents, suchas Al and V on the solubility of FeTiO3 in TiO2 are unknown.

4. Mineral chemistry

Chemical compositions of minerals were determined by JEOL-JXA-8600M electron microprobes (EMP) at the Indian Institute ofTechnology, Roorkee, and the Geological Survey of India, Hydera-bad. The operating parameters were: acceleration voltage of15 kV, probe currents of 20–50 nA, and beam diameter of �2 lm.

Standards include both natural and synthetic minerals and datareduction was done using the ZAF correction procedure.

Mineral chemistry of one sample each of rutile eclogite, Cr-spi-nel eclogite, biminerallic eclogite, enstatite garnet pyroxenite andCr-spinel garnet pyroxenite, and several samples of biminerallicgarnet pyroxenite from the P3, P12 and CC4 pipes of WKF, MK8and NK3 pipes of NKF, and RK3 pipe of RKF are given in Tables2–4. The mineral chemistry of other xenoliths listed in Table 1 suchas kyanite-, enstatite- and biminerallic eclogites from KL2, P2 andP10 pipes of WKF can be found in Patel et al. (2006). The primaryminerals in the mafic xenoliths do not show discernible zoningwithin individual grains or significant compositional variationamong grains in the same xenolith.

4.1. Clinopyroxene

The clinopyroxenes generally have low total iron content(<3.5 wt%) except for the rutile eclogite sample P3Xe/86, in whichthe value is 7.4 wt% FeO (Table 2). In order to derive a clinopyrox-ene formula from a chemical analysis, it is desirable to have Fe2+

and Fe3+ values. In microprobe analyses, only total iron is deter-mined from which Fe2+ and Fe3+ values can be calculated from stoi-chiometry. However, for clinopyroxenes with low total ironcontent such as those in the present study, the calculation is verysensitive to analytical error, especially of SiO2 due to its majorabundance and +4 charge (e.g. Sobolev et al., 1999). It was there-fore decided to choose the most iron-rich clinopyroxene as a refer-ence for the calculation of Fe3+/Fetot ratio, and then use this ratio tocalculate Fe2+ and Fe3+ contents in clinopyroxenes of all other sam-ples. The most iron-rich clinopyroxene occurs in the rutile eclogitewhich gives a Fe3+/Fetot ratio of 0.17. This is closely comparable tothe published values of Fe3+/Fetot ratio for eclogitic clinopyroxenesbased on different analytical methods such as Mössbauer spectros-copy (0.08–0.14, McCammon et al., 1998), Mössbauer milliprobespectroscopy (0.22–0.23, Sobolev et al., 1999) and micro-XANESanalysis (0.25–0.30, Schmid et al., 2003). After calculation of Fe2+

and Fe3+ contents in all clinopyroxenes using this ratio, their chem-istry is plotted in a triangular diagram with components Ca–Mg–Fe

CB

A

FeO+MnO MgO

CaO

55%

MgO

30%

MgO

Bimineralliceclogite

Kyanite eclogite

Enstatite eclogite

Biminerallicgarnet pyroxenite

Chromian spineleclogite

Enstatite-garnetpyroxenite

Chromian spinel-garnet pyroxenite

Rutile eclogite

Fig. 5. Garnet compositions on the CaO–(FeO + MnO)–MgO diagram of Coleman etal. (1965).

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pyroxene, jadeite and aegirine (Fig. 4). Clinopyroxene compositionsfor eclogites of KL2, P2 and P10 pipes of WKF are taken from Patelet al. (2006) and plotted in this figure for comparison. Clinopyrox-ene falling in the omphacite field is classified as omphacite,whereas that falling in the Quad field is Ca pyroxene. It can be seenthat there is a compositional transition from omphacite (in eclog-ite) to Ca pyroxene (in garnet pyroxenite). Jadeite component inomphacite is up to 45 mol% which is very unusual for deep-seatedxenoliths world-wide (Sobolev et al., 1999).

The XMg (=Mg/Mg + Fe2+) value of all clinopyroxenes except onefalls in the range of 0.91–0.96. The exception is the iron-richomphacite in the rutile eclogite which has an XMg value of 0.70.TiO2 content in all clinopyroxenes is <0.2 wt%. MnO and K2O con-tents are invariably low (60.1 wt%). Cr2O3 content is mostly below1 wt%; but several clinopyroxenes have 3–4 wt% Cr2O3, which isless than that in associated garnet.

4.2. Garnet

Garnets in the xenoliths of garnet pyroxenite and eclogite havewide variations in Ca, Fe and Mg (Table 3). They contain only smallconcentrations of Mn (0.2–0.6 wt% MnO) and Ti (0.01–0.4 wt%TiO2), and are virtually devoid of Ni (<0.05 wt% NiO). Na2O contentis mostly below 0.1 wt%, although in one analysis it is as high as0.41 wt%. Cr2O3 content in most garnets is below 1 wt%, althoughin a few samples it is high (1.8–8.3 wt%). End member calculationsfollowing the method of Sobolev et al. (1973) for Cr-rich garnetsgive the following values (in mol%) for all samples except rutileeclogite: pyrope (57–74), almandine (12–27), grossular (0–26),spessartine (61), andradite (0–5), Ti-andradite (61), uvarovite(0–16) and knorringite (0–8). The garnet of rutile eclogite is highlyiron-rich with end member composition of Prp26.5Alm52.5Grs14.7

Adr5.1TiAdr0.3Sps1.0Uv0.1.The chemistry of all garnets is summarised in the CaO–

(FeO + MnO)–MgO ternary plot after Coleman et al. (1965) alongwith the data points for eclogites of KL2, P2 and P10 pipes ofWKF from Patel et al. (2006) (Fig. 5). Garnets of all samples exceptrutile eclogite of P3 pipe and kyanite eclogites of KL2 pipe fall inthe Group A field. Garnet of rutile eclogite belongs to Group C,whereas garnets of kyanite eclogite fall in the fields of Group Band C. Garnets of enstatite eclogite and enstatite garnet pyroxenite

are relatively the most magnesian, and garnets of kyanite eclogiteare reltively the most calcic in composition.

4.3. Chromian spinel, rutile and ilmenite

Chromian spinel in an eclogite has XFe (=Fe2+/Mg + Fe2+) value of0.35, and XCr (=Cr/Cr + Al) value of 0.38 (Table 4). In a garnet pyrox-enite chromian spinel has XFe = 0.38 and XCr = 0.71. Discrete rutilegrains in rutile eclogite, and ilmenite lamellae exsolved from themare somewhat aluminous and V-rich. Al2O3 content of rutile is0.2 wt%, whereas that in ilmenite ranges from 0.6 to 2.8 wt%.V2O5 contents are 0.6 wt% and 0.3 wt% in rutile and ilmenite,respectively. MgO content of ilmenite is up to 1.5 wt%.

5. Geothermobarometry

Geobarometry of eclogites and garnet pyroxenetites has been aproblem world-wide because of the high thermodynamic varianceof the assemblages. Nimis and Taylor (2000) formulated a Cr-in-clinopyroxene barometer which is applicable to clinopyroxeneswith Cr2O3 contents between 0.5 and 5 wt% and with sufficient cal-cium to form CaCr-Tschermak’s (CaCrVIAlIVSiO6) component. Thesecompositional criteria are satisfied by only a few samples of eclog-ite and garnet pyroxenite. P–T conditions for these samples havebeen deduced from the intersection of the Cr-in-clinopyroxenegeobarometer with the garnet-clinopyroxene Fe–Mg exchangegeothermometer.

The application of Fe3+ corrections in the temperature calcula-tions in geothermobarometry has long been a subject of contro-versy. As demonstrated by Canil and O’Neill (1996), Sobolev et al.(1999) and Proyer et al. (2004), the errors introduced by estimatingthe Fe3+ content of clinopyroxenes from EMP analyses are large andoften unacceptable for geothermobarometry. Sobolev et al. (1999)studied the effects of various Fe3+/Fetot values for clinopyroxeneand garnet on calculated temperatures, and found that for clinopy-roxene, T decreases with increasing Fe3+/Fetot whereas for garnet, Tincreases with increasing Fe3+/Fetot. They concluded that due tothis compensation effects between garnet and clinopyroxene theFe3+ corrections in EMP analyses do not greatly affect temperatureestimates in eclogites. Therefore, in the present study tempera-tures have been calculated assuming Fe2+ = Fetot for both clinopy-roxene and garnet.

There are several calibrations of the garnet-clinopyroxene geo-thermometer and the most widely used ones for eclogites and gar-net pyroxenites are those by Ellis and Green (1979), Powell (1985),Krogh (1988) and Krogh Ravna (2000). For the xenoliths understudy the calibrations of Krogh (1988) and Krogh Ravna (2000)yield similar temperatures. The temperatures obtained from thecalibrations of Ellis and Green (1979) and Powell (1985) are alsosimilar, but significantly higher than those from Krogh (1988)and Krogh Ravna (2000) for most of the xenoliths (Table 5). For agiven calibration, eclogites record higher P–T conditions than gar-net pyroxenites. The Cr-spinel eclgoite sample CC4N8b yieldsanomalously high temperatures by all the four calibrations of thegarnet-clinopyroxene geothermometer and most likely are an arte-fact due to the high Cr and Ca contents of garnet in this samples.

Enstatite is present in two samples, one of which is a garnetpyroxenite (sample P12N6b) and the other is an eclogite (sampleKL2N9a). The presence of enstatite allows calculation of pressurefrom the Al-in-orthopyroxene (coexisting with garnet) geobarom-eter. Brey and Kohler (1990) gave a calibration of this geobarome-ter along with a calibration of the two-pyroxene geothermometer.P–T values obtained from the simultaneous solution of the Al-in-orthopyroxene geobarometer and either two-pyroxene geotherm-meter or garnet-clinopyroxene geothermometer for the two sam-

Table 5Results of geothermobarometry for the mafic xenoliths from the kimberlite pipes of EDC. Mineral analyses used for sample KL2N9a, P10N7b and P10N7c are taken from Patel et al.(2006) and those for other samples is from this study

Sample no. Remark T (EG) T (Powell) T (Krogh) T (KR) T (BK) T (BK) T (EG) T (Powell) T (Krogh) T (KR)P (NT) P (NT) P (NT) P (NT) P (NT) P (BK) P (BK) P (BK) P (BK) P (BK)

P12N6b Enstatite garnet pyroxenite 885 859 730 741 696 681 858 830 682 63531.3 30.5 26.8 27.1 25.9 14.3 23.6 22.1 14.3 12.0

P3MXe/86 Biminerallic garnet pyroxenite 998 976 889 89537.3 36.4 32.9 33.1

P2N5b Biminerallic garnet pyroxenite 1015 995 899 86929.4 28.5 24.5 23.3

P12N6c Cr-spinel garnet pyroxenite 1001 981 915 88532.8 32.3 30.4 29.6

MK8N1a Biminerallic eclogite 1108 1093 1036 104137.4 37.0 35.3 35.5

P10N7c Biminerallic eclogite 1153 1141 1085 109435.2 34.8 33.2 33.4

P10N7b Biminerallic eclogite 1169 1157 1128 114041.1 40.7 39.7 40.1

CC4N8b Cr-spinel eclogite 1272 1269 1323 126538.9 38.8 40.4 38.7

KL2N9a Enstatite eclogite (Garnet host andexsolved ortho- and clinopyroxenes)

833 1208 1192 1058 122527.8 47.1 46.2 39.1 48.1

KL2N9a Enstatite eclogite (Omphacite hostand exsolved garnet and clinopyroxene)

867 971 943 790 84536.4 41.9 40.4 32.5 35.3

T in �C and P in kbar. P (NT) = Cr-in-cpx geobarometer of Nimis and Taylor (2000); P (BK) = Al-in-opx geobarometer of Brey and Kohler (1990). T (BK) = two-pyroxenegeothermometer of Brey and Kohler (1990); other temperatures are from garnet-clinopyroxene geothermometer of Ellis and Green, 1979 (EG), Powell (1985), Krogh (1988),and Krogh Ravna, 2000 (KR).

344

ples are given in Table 5. The enstatite garnet pyroxenite satisfiesthe criteria for Cr-in-clinopyroxene geobarometry in addition toits suitability for the Al-in-orthopyroxene geobarometry. A com-parison of pressures calculated from the two geobarometers in thisrock shows that the pressure yielded by the Al-in-orthopyroxenegeobarometer is invariably much less than that by the Cr-in-clino-pyroxene geobarometer. Considering all the geothermobarometriccalculations it is seen that the equilibration pressures and temper-atures of the xenoliths fall mostly in the ranges of 20–48 kbar and700–1225 �C, respectively.

6. Xenolith geotherm

Continental geotherm for a given region is conventionally esti-mated on the basis of a number of parameters including surface

60

80

70

1000

30

1200 1400 1600600 800

T (˚C)

P(K

bar)

50

40

20

10

Diamond

30 mW/m2

Graphite

40 mW/m2

50 mW/m2

60 mW/m2

0.85 Tm

0.90 Tm

45 mW/m2

P (NT) - T (Krogh)

P (NT) - T (Powell)

P (BK) - T (BK)

P (BK) - T (Krogh)

P (BK) - T (KR)

P (BK) - T (EG)

P (BK) - T (Powell)

P (NT) - T (EG)

P (NT) - T (KR)

P (NT) - T (BK)

Fig. 6. Empirical geotherm (thick solid line) for mafic xenoliths. Conductive modelgeotherms (dashed lines) for different surface heat flow values, and mantle solidii(Tm) are from Pollack and Chapman (1977). Abbreviations are as in Table 5.

heat flow, radiogenic heat source distribution, variation of thermalconductivity and mode of heat transfer within the lithosphere.Generally several assumptions regarding these parameters aremade while calculating model geotherms which introduce signifi-cant error. However, independent estimates of pressures and tem-peratures obtained from mantle xenoliths allow the constructionof empirical temperature–depth curves (O’Reilly and Griffin, 2006and references therein), which can be compared with modelgeotherms.

Pollack and Chapman (1977) computed model conductive geo-therms in the lithosphere for surface heat flow values in the rangeof 30–150 mW m�2 which are shown in Fig. 6. All the calculated P–T values from the xenoliths of eclogite and garnet pyroxenite in thekimberlites of EDC are plotted in this figure. The P–T values arescattered, but most of them fall between the 40 and 50 mW m�2

model geotherms, and some fall above the 50 mW m�2 model geo-therm. An important constraint on geotherm is provided by thediamondiferous nature of most of the studied kimberlite pipes. Itimplies that the geotherm must intersect the graphite-diamondtransition curve below the mantle solidus. This condition is satis-fied in Fig. 6 if the geotherm is 645 mW m�2. With this constraintthe P–T ranges obtained from the mafic xenoliths approximate to a45 mW m�2 model geotherm. The P–T conditions of the xenolithsshow that they have equilibrated outside of the diamond stabilityfield. But since the kimberlites are diamondiferous it is obviousthat the transporting magmas must have originated at greaterdepths than recorded by the xenoliths. P–T values for the sampleCC4N8b fall above the 0.85Tm mantle solidus because of the anom-alously high temperature yielded by this sample.

Ganguly and Bhattacharya (1987) and Nehru and Reddy (1989)calculated mantle geotherms using garnet-clinopyroxene Fe–Mgexchange geothermometer of Råheim and Green (1974), and Al-inorthopyroxene geoarometer of Lane and Ganguly (1980) and Per-kins et al. (1981) on ultramafic xenoliths from the P3 pipe ofWKF. These geotherms are linear in nature, and a comparison withmodel geotherms shows that they fall between 42 and 50 mW m�2

model geotherms. Thus the xenolith geotherm of 45 mW m�2 de-rived in the present study is broadly consistent with the findingsof Ganguly and Bhattacharya (1987) and Nehru and Reddy (1989).

345

It is well known that xenolith geotherms are strongly dependent onthe geothermobarometers used (Grutter and Moore, 2003). Never-theless empirically constructed xenolith geotherms provide reli-able constraints on geothermal models since they areindependent of the uncertainties of model geotherms (Cull et al.,1991).

7. Discussion

7.1. Implication of xenolith geotherm on heat flow

Gupta et al. (1991) reported mean heat flow values of40 ± 3.4 mW m�2 for the EDC and 31 ± 4.1 mW m�2 for the WDC.Such heat flow variations in cratonic regions reflect variations inradiogenic heat produced in the crustal column and heat con-ducted into the crust from the underlying mantle. Senthil Kumarand Reddy (2004) measured K, U and Th abundance throughin situ gamma-ray spectrometry at numerous sites covering allmajor rock formations of both EDC and WDC. From the crustal heatcontribution models they concluded that mantle heat flow of theEDC is higher (17–24 mW m�2) relative to the WDC (7–10 mW m�2).

The xenolith geotherm of 45 mW m�2 obtained in the presentstudy for the EDC is towards the higher side of the typical rangeof xenolith/xenocryst geotherms (35–45 mW m�2) for several Ar-chaean cratons in the world (Finnerty and Boyd, 1987; O’Reillyand Griffin, 2006). This leads us to believe that the EDC was hotterthan many other shield regions of the world in the mid-Proterozoicperiod when kimberlites intruded the craton. This can be attrib-uted, following the present-day heat flow model of Senthil Kumarand Reddy (2004), to high mantle heat flow beneath the EDC in themid-Proterozoic time.

7.2. Lithospheric thickness

The finding of formerly supersilicic garnet in an enstatite eclog-ite xenolith from the KL2 pipe of WKF led Patel et al. (2006) to sug-gest that the minimum peak pressure for the rock is 50 kbar sincesupersilicic garnet is experimentally stable at pressures in excessof 50 kbar (Ringwood and Major, 1971). This pressure translatesto a minimum lithospheric thickness of 150 km beneath the EDCduring the mid-Proterozoic period. This value is in agreement withthe result of several workers who have estimated lithosphericthickness beneath the Dharwar craton using different methods(Pandey and Agrawal, 1999 and references therein). Estimatesbased on heat flow values include lithospheric thickness of148 km (Pandey and Agrawal, 1999) and P200 km (Gupta et al.,1991) for the Dharwar craton as a whole. From magnetotelluricstudies Gokarn et al. (1998) estimated a lithospheric thickness of180 km for the craton. Based on geobarometric calculations inultramafic xenoliths from the P3 pipe of WKF, Ganguly and Bhat-tacharya (1987) concluded that the lithosphere was at least185 km thick below the EDC during the mid-Proterozoic period.

7.3. Origin of mafic xenoliths

Mantle eclogites and garnet pyroxenites found in different partsof the world represent a rather heterogeneous group of rocks be-cause the wide range of possible solid solution in the garnet andclinopyroxene structures can accommodate a variety of bulk com-positions. Therefore, it is not reasonable to postulate a single originfor all mafic xenoltihs. Two contrasting petrogeneses have beenpostulated for the origin of mantle eclogites (review by Godard,2001). They represent either (1) high pressure magmatic cumu-lates which occur as magma chambers or dykes within upper man-

tle (e.g. Schmickler et al., 2004) or (2) subducted andmetamorphosed oceanic crust (e.g. Barth et al., 2001). This ques-tion is still debated nowadays. Both types of eclogites may occurin the same kimberlite.

Textural and mineral chemical characteristics favour a mag-matic cumulate origin for the mafic xenoliths in the Dharwar kim-berlites. Graded layering observed in hand specimens (Fig. 3a), andmicrotextural features such as garnet necklace and garnet-kyanitecluster (Patel et al., 2006) must have resulted from gravitativeaccumulation of early-formed crystals of garnet. The compositionof clinopyroxene shows transition from omphacite in eclogites toCa pyroxene in garnet pyroxenites (Fig. 4). Such transition demon-strates the cogenetic relationship of eclogites and garnet pyroxe-nites, and favours a high pressure igneous origin of these rocks.However, since eclogite xenoliths record higher pressure than gar-net pyroxenite xenoliths (Table 5) the former must have been de-rived from a greater depth than the latter.

Acknowledgements

The Dy. D.G., Geological Survey of India (Southern Region) isthanked for permission to SR to carry out research work at I.I.T.,Bombay. Tamal Ghosh of IIT-Roorkee is thanked for help in EPMAanalyses. Constructive reviews by Kuo-Lung Wang and an anony-mous reviewer were extremely helpful in improving the qualityof the paper.

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