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Petrogenesis of lamprophyres from Chhota Udepur area, Narmada rift zone,and its relation to Deccan magmatism

N.V. Chalapathi Rao a,⇑, C.V. Dharma Rao b, Sanjay Das c

a Centre of Advanced Study in Geology, Banaras Hindu University, Varanasi 221 005, Indiab National Disaster Management Authority, Government of India, A-1 Safdarjung Enclave, New Delhi, Indiac Geological Survey of India, Operation Gujarat, Gandhinagar 382 043, India

a r t i c l e i n f o

Article history:Received 18 May 2011Received in revised form 22 August 2011Accepted 5 September 2011Available online 16 September 2011

Keywords:PetrologyGeochemistryLamprophyresChhota UdepurNarmada riftDeccan Large Igneous ProvinceIndia

a b s t r a c t

Geological setting, petrology and geochemistry of a new find of lamprophyre dykes, intruding the Pre-cambrian basement gneisses from the Chhota Udepur area, Narmada rift zone, Western India, are pre-sented. Of the three lamprophyre dykes, two of them display E–W trend paralleling that of theNarmada rift, extend up to �2.5 km in length and constitute the longest yet recorded lamprophyre dykesfrom the Indian shield. The Chhota Udepur lamprophyres (CUL) show several overlapping mineralogicaland geochemical characteristic features displayed by alkaline- and calc-alkaline lamprophyres and henceit is not straight forward to pigeon-hole them in existing lamprophyre classification schemes. They areporphyritic–panidiomorphic with euhedral to sub-hedral phenocrysts and microphenocrysts of olivine(Fo78.33–78.42 Fa21.24–21.37), diopsidic augite to titaniferous augite (Wo54.83–59.03En36.75–41.36 Fs0.1–5.66), bio-tite and amphibole (kaersutite). Feldspar (orthoclase perthite and plagioclase) is essentially confined tothe groundmass along with accessory phases such as apatite and spinel (ti-magnetite). Rare carbonateocelli are also present. In terms of major element geochemistry the CUL are nepeheline-normative andpredominantly sodic (Na2O > K2O); yet their K2O and SiO2 contents are sufficiently high to signal traitsof calc-alkaline lamprophyres. On the other hand, the CUL exclude negative spikes at Nb–Ta which areconsidered to be a characteristic of subduction-related (calc-alkaline) magmas, are highly evolved(Mg#: 27.82–55.45), enriched in incompatible trace elements (high La/Nb, Zr/Nb) and have fractionatedREE distribution patterns (La/YbN ratios: 35–44). Post-emplacement alteration and crustal contaminationhave had little influence on their geochemistry. Th/Yb, Ta/Yb, Nb/U and Ce/Pb ratios of the CUL imply anOIB type-enriched mantle source, similar to that displayed by other Deccan-related lamprophyres. Ametasomatised (enriched) garnet lherzolite mantle source with very low degrees of partial melting thathas subsequently undergone a large degrees of fractional crystallisation is capable of accounting observedREE geochemistry of the CUL. We infer the lamprophyres of Chhota Udepur area to be a part of the alka-line rock spectrum of the Deccan Large Igneous Province and their emplacement, together with that ofother alkaline complexes in the Chhota Udepur area, is envisaged to be strongly controlled by pre-exist-ing crustal weakness accentuated by the mantle plume–lithosphere interaction during the end-Creta-ceous in the Narmada rift zone.

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1. Introduction

Lamprophyres are small-volume, mesocratic to melanocratic,mostly hypabyssal (dykes, sills or pipes), volatile (H2O and CO2)-rich, sodic to potassic and mafic–ultramafic alkaline igneous rockswherein mineralogy is dominated by ferromagnesian minerals(olivine, clinopyroxene, biotite and amphibole) in general and hy-drous mafic silicates (biotite and hornblende) in particular. Lam-prophyres essentially display a panidiomorphic–porphyritictexture, lack feldspar and orthopyroxene as phenocrysts and feld-

spar is typically restricted as a groundmass phase only (e.g., Rock,1991; Woolley et al., 1996; Le Maitre, 2002).

Despite their relatively small-volume, increasing significance isbeing attached to the studies on lamprophyres owing to their (i)role in understanding of the large-scale geodynamic processessuch as continental extension (e.g., Larsen and Rex, 1992; Queenet al., 1996; Tappe et al., 2006), mantle plume–lithosphere interac-tions (e.g., Gibson et al., 2006; Orejana et al., 2008; Kerr et al.,2010), continental break-ups and collisions (e.g., Foley et al.,2002; O’Leary et al., 2009; Rocchi et al., 2009) and mantle-enrich-ment processes (e.g., Beard et al., 1996; Guo et al., 2004; Srivastavaand Chalapathi Rao, 2007), (ii) entrapped lower-crustal and mantlexenoliths which serve as snapshots to the lower portions of the

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⇑ Corresponding author. Mobile: +91 9935647365; fax: +91 542 2368174.E-mail address: nvcr100@gmail.com (N.V. Chalapathi Rao).

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Earth’s lithosphere (e.g., Coulson et al., 2003; Dessai et al., 2004;Nédli et al., 2010), (iii) diamond prospectivity (e.g., Lefebvreet al., 2005; Wyman et al., 2006) and (iv) temporal and spatialassociation (of the calc-alkaline lamprophyres) with mesothermalgold deposits (e.g., Rock and Groves, 1988; Duggan and Jaques,1996; Huang et al., 2002).

In this communication, we present new petrological and bulk-geochemical data for a recently discovered suite of lamprophyresfrom the Chhota Udepur area, NW India, evaluate their petrogene-sis, and suggest their links to the end-Cretaceous Deccan largeIgneous Province. As these lamprophyres intrude the Precambrianbasement, contrary to the all the hitherto known Deccan-relatedlamprophyres and a majority of the alkaline complexes whichare associated with the Deccan basalts, our work provides new in-sights into the origin of alkaline magmatism in the Deccan LargeIgneous Province.

2. Deccan Large Igneous Province, the associated Alkalinemagmatism and lamprophyres

The Deccan Large Igneous Province is one of the world’s well-known continental flood basalt provinces with an eruptive historyin three main volcanic events at 69–67 Ma, 65 Ma and a terminalevent at 61 Ma (e.g., Widdowson et al., 2000; Hofmann et al.,2000; Sheth et al., 2001; Chenet et al., 2007; Hooper et al., 2010;Lehmann et al., 2010; Chalapathi Rao et al., 2011b). Recent studies

show that the bulk of Deccan volcanism (�80%) of the 3500 m oflava flows, erupted during less than 800 ky in magnetic polarityC29r (64.8–65.6 Ma) and was likely a major contributor to themass extinction at K–T boundary (Saunders et al., 2007).

Alkaline magmatism represented by K-rich basalts, nephelinesyenites, ijolites, carbonatites, kimberlites and lamprophyres isan important, albeit volumetrically minor, component of the Dec-can Large Igneous Province and occur along the various rift zonessuch as E–W trending Narmada rift, Cambay rift and westerncoastal rifted margin (e.g., Subba Rao, 1972; Bose, 1980; Mahoneyet al., 1985; Gwalani et al., 1993; Simonetti et al., 1998; Ray andPande, 1999; Melluso et al., 2002; Dessai and Viegas, 2010; Kerret al., 2010; Chalapathi Rao et al., 2011a; Chalapathi Rao andLehmann, 2011).

Lamprophyres, mostly camptonite and monchiquite (belongingto alkaline variety) and rare kersantite (belonging to calc-alkalinevariety), either associated with the alkaline complexes or withthe basalts of western and north-western portion of Deccan Largeigneous Province are reported from the following locations: (i) thePhenaimata alkaline Complex (Durgadamath, 1984); (ii) the Girnaralkaline complex (Bose, 1973; Paul et al., 1977), (iii) the Kutchh(Jaitly et al., 1980; Sen et al., 2009), (iv) the Kanwad–Panwad re-gion and exposed along Heron River sections (Sukeshwala and Ava-sia, 1972), (v) the Chhaktalo and Bhakhatgarh–Phulmal, ChhoteUdepur areas (Hari, 1998; Hari et al., 2000; Randive et al., 2005),(vi) Murud Janjira area (Deshpande and Chakranarayan, 1973;

Fig. 1. Geological map of the area south west of Chhota Udepur showing the location of lamprophyres of this study (after Das and Srikarni, 2005). (For interpretation of thereferences to colour in this figure legend, the reader is referred to the web version of this article.)

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Dessai et al., 1990; Dessai and Viegas, 2010) and (vii) south of theTethyan suture zone, northeast of Quetta, Pakistan, where ultra-

mafic lamprophyres are located (Kerr et al., 2010). Recently, threelamprophyres are discovered about 4 km south of Chhota Udepur(Das and Srikarni, 2005) and their geological setting, petrologyand geochemistry constitute the subject of the ensuing sections.

3. Geological setting of lamprophyres from the Chhota Udepurarea

The Chhota Udepur area or Chhota Udepur subprovince (asalso referred by some workers, e.g., Gwalani et al., 1993) liesin the lower reaches of E–W trending Narmada rift valley, a ma-jor geo-tectonic feature, in western India (Fig. 1). Narmada rift(or Narmada lineament) extends for about 1300 km in CentralIndia and is widely regarded to be of Precambrian antiquitywhich got re-activated during the Phanerozoic (see Kumaret al., 2000). The Pre-Champaner granitic gneisses are the base-ment rocks in this area and are equivalent of such rocks inLower Aravalli craton, Rajasthan, NW India (see also Mamtaniand Greiling, 2005). However, no isotopic age data are availablefor these gneisses except for a metamorphic age (one of thestrongest deformational event) of 1168 ± 30 Ma (Srimal andDas, 1997). These gneisses are overlain by the calc-silicate rocksof Champaner Group (Mesoproterozoic) which are intruded byGodhra granite of 955 ± 20 Ma (Gopalan et al., 1979). The Deccanbasalts and associated alkaline rocks and sandstones (Bagh beds)of Late Cretaceous age rest on the Champaner and Pre-Champa-ner rocks. A generalised stratigraphic succession of the area isprovided in Fig. 1. Three lamprophyre dykes were recently re-corded whilst mapping the Precambrian terrane 4 km south ofChhota Udepur (Fig. 1; Das and Srikarni, 2005). These lampro-phyres (henceforth termed as CUL) cut across the gneissosityin the basement rocks and occur at three locations: Two of thesedykes are aligned along ENE–WSW and are located about 1.2–1.5 km north of Degala village in the forest, both of these arefairly extensive (2.5 km long and 20 m wide approximately),whereas the third one is located 1.2 km north-west of Singlavillage. It is only a few hundred metres long and 10 m widetrending NNW–SSE. Strike of the two major dykes coincides with

Fig. 2. (A) Fresh bouldary outcrop of lamprophyre occurring south of Piplaj, (B) aparticularly large �2 cm amphibole phenocryst in lamprophyre located south ofPiplaj and (C) a euhedral biotite phenocryst in lamprophyre occurring north of Degala.

Table 1Mineral chemistry (oxide wt%) of olivine from the lamprophyres of this study.

L-11 L-11 L-16 L-16

SiO2 36.61 36.62 36.66 36.79TiO2 0.05 0.00 0.05 0.05Al2O3 0.04 0.06 0.05 0.00Cr2O3 0.01 0.00 0.00 0.02FeO 19.75 19.86 19.74 19.78MnO 0.29 0.27 0.31 0.27MgO 40.62 40.83 40.87 40.75NiO 0.05 0.10 0.06 0.03CaO 0.65 0.43 0.36 0.43

Total 98.01 98.07 98.04 98.09

Cations for 4 oxygen atomsSi 0.972 0.969 0.970 0.973Ti 0.001 0.000 0.001 0.001Al 0.001 0.002 0.001 0.000Cr 0.000 0.000 0.000 0.000Fe(ii) 0.438 0.440 0.437 0.438Mn 0.006 0.006 0.007 0.006Mg 1.607 1.611 1.612 1.607Ni 0.001 0.002 0.001 0.001Ca 0.018 0.012 0.010 0.012

Total 3.045 3.042 3.039 3.038

Fo 78.33 78.34 78.42 78.37Fa 21.36 21.37 21.24 21.34Tp 0.31 0.293 0.336 0.292

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that of the Narmada (rift) lineament and the Deccan-relateddykes in the area. As all the three lamprophyre dykes areunmetamorphosed and display similar megascopic characterssuch as the presence of coarse grained amphibole and biotitephenocrysts (Fig. 2B and C) and a strikingly similar petrographyand geochemistry (below), we relate them to belong to the samesuite and to the Deccan Large Igneous Province.

4. Sampling and analytical methods

The sample locations are depicted in Fig. 1 and the study areafalls at the junction of four topo-sheets in the Survey of India toposheet No’s 46F/15, F/16, J/3 and J/4. Megascopically all collectedsamples are fresh, undeformed, unmetamorphosed, melanocraticand hard. Mineral compositions were determined by using a

Table 2Mineral chemistry (oxide wt%) of pyroxene from the lamprophyres of this study.

L-10 L-10 L-11 L-11 L-17 L-17 L-22 L-24 L-25 L-27

SiO2 44.22 44.47 44.47 42.97 43.36 45.56 44.06 46.07 44.40 44.30TiO2 2.45 2.28 2.35 3.05 2.91 2.17 2.53 2.03 2.48 2.48Al2O3 7.52 7.57 8.05 8.74 8.66 5.74 8.45 6.28 8.18 8.05Cr2O3 0.00 0.03 0.00 0.00 0.01 0.00 0.02 0.01 0.00 0.00FeO 8.16 7.21 6.85 8.21 8.03 8.99 7.95 7.28 8.28 8.82MnO 0.25 0.09 0.08 0.17 0.18 0.19 0.17 0.16 0.20 0.23MgO 11.99 12.15 11.73 10.99 10.80 12.01 11.09 12.44 10.69 11.09CaO 23.64 23.94 24.22 23.59 23.40 22.88 23.33 23.58 23.29 23.30Na2O 0.68 0.38 0.50 0.60 0.69 0.66 0.47 0.35 0.71 0.74K2O 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.00

Total 98.91 98.11 98.25 98.31 98.03 98.18 98.07 98.20 98.24 99.00

Cations for 6 oxygen atomsSi 1.661 1.682 1.679 1.631 1.650 1.730 1.676 1.743 1.687 1.669Ti 0.069 0.065 0.067 0.087 0.083 0.062 0.073 0.058 0.071 0.070Al 0.333 0.338 0.358 0.391 0.388 0.257 0.379 0.280 0.366 0.357Fe+3 0.255 0.195 0.186 0.217 0.196 0.206 0.159 0.144 0.170 0.217Cr+3 0.000 0.001 0.000 0.000 0.000 0.000 0.001 0.000 0.000 0.000Fe+2 0.001 0.033 0.030 0.044 0.060 0.079 0.094 0.086 0.093 0.060Mn 0.008 0.003 0.003 0.005 0.006 0.006 0.005 0.005 0.007 0.007Mg 0.672 0.685 0.660 0.622 0.613 0.680 0.629 0.701 0.605 0.623Ca 0.951 0.971 0.980 0.959 0.954 0.931 0.951 0.956 0.948 0.941Na 0.049 0.028 0.037 0.044 0.051 0.048 0.034 0.026 0.052 0.054K 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000

Total 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000 4.000

Wo 58.58 57.48 58.65 59.03 58.65 55.10 56.79 54.83 57.59 57.92En 41.36 40.57 39.53 38.26 37.67 40.22 37.57 40.23 36.75 38.36Fs 0.065 1.947 1.82 2.71 3.68 4.68 5.64 4.94 5.66 3.72

Table 3Mineral chemistry (oxide wt%) of amphibole from the lamprophyres of this study.

L-11 (core) L-16 (core) L-17 (core) L-17 (rim) L-22 (core) L-23 (core) L-23 (core) L-25 (core) L-27 (core)

SiO2 37.24 37.28 38.29 37.37 37.44 37.81 37.23 37.90 38.21TiO2 5.28 5.60 5.03 5.76 4.79 5.36 5.31 4.62 5.15Al2O3 13.99 14.17 12.17 12.64 12.14 14.23 13.99 12.14 12.06Cr2O3 0.00 0.01 0.01 0.01 0.00 0.02 0.01 0.01 0.02FeO 12.08 11.18 15.27 11.60 15.91 12.19 11.47 15.81 15.78MnO 0.19 0.15 0.33 0.11 0.36 0.13 0.16 0.35 0.35MgO 11.21 11.96 9.75 12.33 9.61 11.96 11.64 9.77 9.46CaO 12.42 12.45 11.73 12.11 11.69 11.18 12.44 11.79 11.74Na2O 2.06 2.04 2.39 2.08 2.39 2.08 2.02 2.48 2.39K2O 1.80 1.81 1.76 1.90 1.89 1.45 1.81 1.81 1.64

Total 96.28 96.64 96.72 95.91 96.22 96.40 96.07 96.66 96.80

Cations for 23 oxygen atomsSi 5.688 5.648 5.900 5.725 5.837 5.725 5.682 5.871 5.896Ti 0.606 0.638 0.583 0.664 0.562 0.610 0.609 0.538 0.598Al 2.518 2.530 2.210 2.282 2.231 2.539 2.515 2.216 2.193Cr 0.000 0.001 0.001 0.001 0.000 0.003 0.001 0.001 0.002Fe(iii) 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000Fe(ii) 1.543 1.416 1.967 1.486 2.074 1.543 1.464 2.048 2.036Mn 0.024 0.019 0.043 0.014 0.048 0.017 0.021 0.046 0.046Mg 2.553 2.702 2.240 2.816 2.233 2.700 2.649 2.256 2.176Ca 2.033 2.021 1.936 1.988 1.952 1.813 2.033 1.956 1.941Na 0.611 0.600 0.715 0.619 0.721 0.609 0.599 0.745 0.715K 0.351 0.350 0.346 0.371 0.375 0.280 0.352 0.358 0.323

Total 15.928 15.924 15.941 15.965 16.034 15.839 15.926 16.034 15.927

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CAMECA SX100 Electron Probe Micro Analyzer (EPMA) at the Tech-nical University of Clausthal, Germany. An accelerating voltage of15 kV, a beam current of 20 nA and a beam diameter of 1 lmwas used. The analyses were carried out using wavelength disper-sive spectrometers employing TAP, PET and LLIF crystals and a PAPonline correction programme. Several in-house natural standardswere used for calibration. After repeated analyses it was found thatthe error on major element concentrations is <1%. EPMA data ispresented in Tables 1–7.

Major and trace element geochemical analyses of the eight sam-ples (L-11, L-16 and L-17 from Singla; L-22, L-23, L-24, L-25 and L-27 from Degala) were carried out at Activation Laboratories,Ancaster, Canada, by ICP and ICP-MS (inductively coupled plasmamass spectrometry) after lithium metaborate/tetraborate fusion.Several international geochemical reference material samples wererun along with these samples. Precision was approximately 5% formajor oxides and 5–15% for trace elements, respectively. The pre-

cision and accuracy of the data can be judged from the Quality con-trol data provided in Electronic supplementary Table 1. The whole-rock geochemistry of the eight samples is given in Table 8 alongwith their CIPW norms calculated on a volatile-free basis.

5. Petrography and mineral chemistry

All the studied CUL samples display similar porphyritic–panid-iomorphic texture, typical of lamprophyres, wherein clinopyrox-ene, amphibole, biotite and olivine constitute the phenocrystsand microphenocrysts within a groundmass of pyroxene, amphi-bole, biotite, feldspar, apatite, opaque oxides and carbonates(Fig. 3). The order of predominance in the phenocrysts is clinopy-roxene > biotite > amphibole > olivine and a considerable variationin their grain size is noticed. Clinopyroxene constitutes �50 vol% ofthe phenocrysts and varies in grain size from 0.2 mm to >1 mm and

Table 4Mineral chemistry (oxide wt%) of biotite from the lamprophyres of this study.

L-11 (core) L-11 (core) L -16 (core) L-16 (core) L-17 (rim) L-22 (core) L-23 (core) L-25 (core) L-27 (core)

SiO2 32.38 33.07 32.44 32.76 34.11 33.08 33.01 33.13 33.57TiO2 2.22 2.46 2.81 8.27 7.62 7.10 7.65 7.63 8.06Al2O3 18.55 17.66 16.77 15.06 14.70 14.38 14.43 14.68 14.51Cr2O3 0.01 0.01 0.00 0.03 0.00 0.00 0.01 0.00 0.01FeO 28.56 27.74 28.84 15.74 17.68 19.71 20.50 19.82 19.64MnO 0.60 0.59 0.63 0.26 0.31 0.45 0.45 0.44 0.41MgO 1.82 2.59 2.50 11.41 10.80 9.88 9.21 9.37 9.37CaO 0.07 0.10 0.10 0.04 0.06 0.06 0.07 0.06 0.07Na2O 0.09 0.53 0.14 0.69 0.66 0.44 0.59 0.51 0.53K2O 9.95 9.36 9.83 8.15 8.76 8.98 8.45 8.50 8.29

Total 94.25 94.10 94.06 92.39 94.69 94.08 94.37 94.14 94.46

Cations for 22 oxygen atomsSi 5.297 5.381 5.341 5.116 5.245 5.195 5.175 5.184 5.218Ti 0.273 0.301 0.348 0.971 0.881 0.839 0.902 0.898 0.943Al 3.576 3.387 3.253 2.771 2.662 2.663 2.667 2.708 2.658Cr 0.001 0.001 0.000 0.002 0.000 0.000 0.001 0.000 0.001Fe(ii) 3.906 3.774 3.969 2.055 2.273 2.589 2.688 2.594 2.552Mn 0.083 0.081 0.088 0.034 0.040 0.059 0.060 0.058 0.054Mg 0.444 0.627 0.613 2.657 2.476 2.314 2.152 2.186 2.172Ca 0.011 0.018 0.017 0.006 0.009 0.011 0.012 0.010 0.011Na 0.029 0.167 0.046 0.209 0.197 0.133 0.179 0.154 0.160K 2.075 1.944 2.065 1.623 1.717 1.799 1.690 1.697 1.644

Total 15.695 15.681 15.739 15.445 15.500 15.601 15.526 15.489 15.414

Table 5Mineral chemistry (oxide wt%) of feldspar from the lamprophyres of this study.

L-11 L-16 L-16 L-16 L-17 L-17 L-22 L-25 L-27

SiO2 61.99 62.62 60.87 64.56 62.58 62.98 63.49 64.17 51.25Al2O3 19.55 19.58 24.56 19.52 19.34 19.51 19.87 19.85 30.46FeO 0.38 0.19 0.07 0.22 0.39 0.05 0.21 0.22 0.43CaO 0.28 0.19 0.25 0.17 0.06 0.02 0.33 0.31 12.23Na2O 3.01 2.44 5.72 2.68 1.68 0.12 3.33 4.37 4.02K2O 13.00 13.99 7.95 13.31 14.37 16.29 12.00 10.96 0.59

Total 98.21 99.00 99.42 100.45 98.42 98.97 99.23 99.86 98.99

Cations for 32(O)Si 11.643 11.684 11.061 11.801 11.742 11.785 11.709 11.724 9.421Al 4.328 4.305 5.260 4.204 4.278 4.302 4.319 4.273 6.598Fe(ii) 0.059 0.030 0.010 0.033 0.061 0.008 0.033 0.033 0.066Ca 0.056 0.038 0.049 0.034 0.013 0.004 0.065 0.060 2.408Na 1.098 0.882 2.014 0.948 0.612 0.043 1.190 1.546 1.433K 3.114 3.329 1.842 3.102 3.440 3.887 2.822 2.554 0.139

Total 20.298 20.268 20.237 20.122 20.145 20.029 20.138 20.19 20.066

An 1.31 0.90 1.26 0.83 0.31 0.11 1.60 1.44 60.51Ab 25.72 20.76 51.57 23.22 15.06 1.09 29.19 37.16 35.99Or 72.98 78.34 47.17 75.95 84.63 98.80 69.21 61.40 3.50

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occurs as zoned phenocrysts and as prismatic crystals. It is palegreen to dark green in colour and slightly pleochroic. At places,glomeroporphyritic clusters of clinopyroxene are also observed(Fig. 3B). Biotite is highly pleochroic (light brownish yellow to darkbrown) and constitutes �25–30 vol% of the phenocrysts and ishomogeneously distributed. It displays maximum size variationamongst the phenocrysts (0.2 mm to >5 mm) and many grains dis-play optical zoning (Fig. 3C). Amphibole is pleochroic (greenishbrown to pinkish brown) and constitutes up to 25% of the pheno-crysts and occurs as elongated and prismatic crystals of up to6 mm size. Some of the amphiboles display resorption along theirmargins. Olivine occurs as a minor constituent (<10 vol%) only asphenocrysts and many of the grains are serpentinised (Fig. 3F).Mantling of olivine by clinopyroxene (Fig. 3F) is also noticed in athin section. Feldspar is sub-hedral in nature and is the predomi-nant holocrystalline fine grained groundmass phase (Fig. 3D–F)

together with microlites of ferromagnesian minerals. Apatite oc-curs as a ubiquitous accessory and displays acicular (needle shapedwith a high aspect ratio of 1:75) or in rare prismatic habit. Opaqueminerals are widespread in the groundmass and are seen as poik-ilitic inclusions in earlier formed phenocrysts (Fig. 3). Secondarycarbonate ocelli varying from 2 mm to 10 mm are also noticed atplaces.

Composition of each of the minerals in the CUL is now discussedseparately and their comparison with similar data, where avail-able, from other lamprophyres from the Deccan LIP is made.

Olivine: Olivine is compositionally a forsterite with Fo78.33–

78.42 Fa21.24–21.37 and its CaO content varies from 0.36 to 0.65 wt%(Table 1). Composition of the CUL olivine is similar to that of oliv-ine from Chhaktalao lamprophyres (Fo78.3–78.6 and CaO 0.35–0.46 wt%; Hari, 1998) but different from that of olivine from MurudJanjira lamprophyres (Fo76.5 and CaO 0.20 wt%; Melluso et al., 2002and Fo82–86 and CaO 0.13–0.74 wt%; Dessai and Viegas, 2010; Elec-tronic supplementary Fig. 1).

Clinopyroxene: Clinopyroxene is a diopsidic augite to titanifer-ous augite and shows a considerable variation in composition(Wo54.83–59.03 En36.75–41.36 Fs0.1–5.66 Table 2). It is quite distinct inits chemistry from the diopside reported from Murud Janjira lam-prophyres (Wo47–50 En33–39 Fs10–19; Melluso et al., 2002; Dessai andViegas, 2010), Phenai mata and Panwad-Kawant lamprophyres(Wo52–56 En25–44 Fs4–11; Rock et al., 1994) and Chhaktalao lampro-phyres (Wo48–48 En35–37 Fs13–14; Hari, 1998) from the Deccan LIP.CUL clinopyroxene shows very good positive correlation betweenAl and Ti implying a fractionation trend (Electronic supplementaryFig. 2).

Biotite: Wide chemical variation is displayed by the CUL micas(Table 3) and all of them are compositionally biotite sensu stricto(Electronic supplementary Fig. 3) similar to those of the micas fromChhaktalao lamprophyres (Hari, 1998). Their Mg/(Mg + Fe) rangefrom 0.1 to 0.6 and demonstrate varying melt composition of theirparental magmas. Their K2O content range from 8.15 to 9.95 (wt%)close to the pristine composition. In comparison, the micas fromMurud Janjira lamprophyres are clearly phlogopites and also aremore primitive [Mg/(Mg + Fe) up to 0.88] in nature (Mellusoet al., 2002) (Electronic supplementary Fig. 3).

Table 6Mineral chemistry (oxide wt%) of spinel from the lamprophyres of this study.

L-11 L-11 L-16 L-17 L-22 L-25 L-25 L-27 L-27

TiO2 13.62 12.70 13.43 14.28 13.01 13.53 13.09 13.51 13.60Al2O3 5.81 5.93 6.64 3.86 4.78 6.61 4.63 4.81 4.73Cr2O3 0.06 0.04 0.09 0.03 0.01 0.03 0.09 0.03 0.05FeO 72.98 72.62 72.57 73.83 74.95 73.23 74.33 73.29 73.26MnO 1.02 0.94 0.64 1.53 1.28 0.75 1.77 1.65 0.75MgO 1.35 2.05 3.31 0.76 0.53 3.34 0.23 1.43 2.61CaO 0.02 0.01 0.31 0.10 0.01 0.11 0.08 0.02 0.08

Total 94.85 94.28 96.98 94.38 94.58 97.61 94.22 94.73 95.07

Fe2O3 35.24 37.13 37.43 35.73 37.29 35.29 35.58 36.83 37.63FeO 41.27 39.21 38.89 41.68 41.40 41.47 42.32 40.15 39.39

98.49 98.03 100.78 98.57 98.36 100.47 98.51 98.47 98.90

32 (O)Ti 3.030 2.824 2.867 3.235 2.933 2.967 2.946 3.020 3.003Al 2.024 2.065 2.220 1.369 1.689 2.272 1.633 1.686 1.637Cr 0.013 0.010 0.021 0.008 0.002 0.006 0.022 0.008 0.011V 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000Fe(iii) 7.841 8.259 7.995 8.100 8.412 7.743 8.013 8.238 8.311Fe(ii) 10.207 9.692 9.232 10.499 10.381 10.111 10.592 9.980 9.669Mn 0.256 0.236 0.154 0.390 0.326 0.826 0.449 0.415 0.186Mg 0.593 0.902 1.402 0.341 0.238 0.049 0.102 0.634 1.142Ca 0.005 0.002 0.095 0.031 0.004 0.004 0.024 0.005 0.024

Total 24 24 24 24 24 24 24 24 24

Fe2/(Fe2 + Fe3) 0.57 0.54 0.54 0.56 0.55 0.566 0.57 0.55 0.54Fe3/(Fe3 + Fe2) 0.43 0.46 0.46 0.44 0.45 0.434 0.43 0.45 0.46

Table 7Mineral chemistry (oxide wt%) of apatite from the lamprophyres of this study.

L-11 L-11 L-17 L-22 L-25

CaO 55.11 54.50 54.83 55.12 55.30Na2O 0.11 0.04 0.00 0.12 0.04FeO 0.18 0.18 0.49 0.32 0.39MnO 0.05 0.05 0.03 0.06 0.05MgO 0.11 0.09 0.14 0.09 0.10P2O5 40.86 39.97 39.83 40.56 40.17SiO2 0.29 0.30 0.95 0.40 0.37

Total 96.71 95.13 96.27 96.67 96.42

No. anions on basis of 25 O, OH, Cl, FCa 10.071 10.141 10.087 10.094 10.176Na 0.036 0.013 0.000 0.040 0.013Fe 0.026 0.026 0.070 0.046 0.056Mn 0.007 0.007 0.004 0.009 0.007Mg 0.028 0.023 0.036 0.023 0.026P 5.900 5.877 5.790 5.869 5.841Si 0.049 0.052 0.163 0.068 0.064OH 1.000 1.000 1.000 1.000 1.000

Total 17.117 17.140 17.151 17.148 16.182

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Amphibole: Amphiboles in all the samples show a narrow rangeof compositional variation (Table 4). They display reverse zoningwith titanian-depleted cores compared to their titanian-enrichedrims as observed in the amphibole from Murud Janjira (see Dessaiand Viegas, 2010). As per the existing International MineralogicalAssociation (IMA) classification (Leake et al., 1997) they come

under kaersutite category (Electronic supplementary Fig. 4) similarto the amphiboles from the Murud Janjira lamprophyres (Dessaiand Viegas, 2010). Amphibole is conspicuously absent in theChhaktalao lamprophyres of Deccan LIP (Hari, 1998).

Feldspar: Both potassium as well as plagioclase feldspar arepresent with the former clearly predominating over the latter in

Table 8Bulk-rock geochemistry of the lamprophyres from Chhota Udepur area. The data published by Das and Srikarni (2005) is also provided. Mg# = Mg/Mg + Fe2+ re-calculated on avolatile-free basis. ⁄ = Total iron. n.d. = Not determined.

Oxide wt% This study Das and Srikarni (2005)

Sample L-11 L-16 L-17 L-22 L-23 L-24 L-25 L-27 D4122 D43454 D4355 D4356

SiO2 47.88 43.88 44.78 47.38 47.55 47.16 47.3 46.47 43.50 45.00 46.02 46.50TiO2 1.523 2.126 2.122 1.67 1.63 1.945 2.00 1.791 3.00 2.63 2.40 2.21Al2O3 17.74 15.73 16.85 17.68 17.71 15.00 15.00 17.2 19.00 21.00 18.00 13.70Fe2O�3 7.90 10.73 10.13 8.38 8.53 8.16 8.31 8.90 3.55 4.37 3.85 3.20FeO 6.05 4.03 7.20 12.00MnO 0.19 0.192 0.20 0.20 0.20 0.19 0.19 0.20 0.22 0.22 0.09 0.06MgO 2.59 4.32 4.32 2.70 2.79 2.89 3.05 3.16 4.70 3.50 2.55 2.75CaO 6.77 9.85 9.35 7.06 7.10 7.48 7.41 7.59 10.00 7.00 9.80 9.15Na2O 4.42 3.10 4.35 4.67 4.72 4.23 4.41 4.33 3.50 4.90 4.20 3.75K2O 4.96 3.50 3.90 4.73 4.65 4.44 4.31 4.42 2.00 4.00 3.60 3.25P2O5 0.60 0.87 0.83 0.61 0.66 0.68 0.63 0.70 0.40 0.40 0.22 0.52LOI 4.84 4.71 3.08 3.98 2.83 6.21 5.61 3.42 n.d. n.d. n.d. n.d.Mg# 39.81 44.82 46.24 39.39 39.75 41.67 42.63 41.73 53.27 55.45 36.04 27.82

Total 99.41 99.01 99.91 99.06 98.37 98.39 98.22 98.18 95.92 97.05 97.93 97.09

CIPW norms calculated on a volatile-free basisOrthoclase 30.74 21.67 23.76 29.08 28.27 27.89 26.93 27.03 12.31 24.35 21.72 19.77Albite 11.88 5.61 1.61 10.25 10.77 14.51 14.90 9.49 13.80 10.43 7.51 14.25Anorthite 14.59 19.56 15.39 13.84 13.79 9.38 8.88 14.94 31.49 24.19 20.04 11.27Nepheline 14.81 11.85 19.68 16.72 16.43 12.76 13.30 15.40 9.24 17.48 15.58 9.98Diopside 13.84 21.36 22.27 15.46 15.10 21.14 21.34 16.12 14.06 6.93 23.28 27.20Olivine 7.03 9.97 8.72 6.91 7.29 4.85 5.07 7.90 7.21 4.05 0.99 7.15Magnetite 1.80 2.45 2.27 1.90 1.91 1.89 1.91 2.00 5.36 6.26 5.70 4.78Ilmenite 3.03 4.23 4.15 3.29 3.18 3.93 4.02 3.52 6.13 5.15 4.65 4.32Apatite 1.46 2.11 1.98 1.47 1.57 1.68 1.54 1.68 0.97 0.95 0.52 1.24

Trace Elements (ppm)Sc 8 16 16 8 8 11 12 11V 151 256 247 165 163 192 198 185Cr 80 110 130 120 120 110 100 140 71 43 41Co 17 29 31 19 19 20 19 21Ni <20 30 70 <20 <20 <20 <20 <20 60 50Cu 30 60 70 40 40 70 50 40Zn 110 130 220 110 110 130 120 110Ga 20 20 21 20 20 21 21 21Rb 109 99 96 123 121 123 119 115Sr 1350 1666 1607 1577 1590 1229 1214 1366Y 27 31 30 27 28 27 28 28Zr 394 371 381 388 388 471 459 386Nb 152 137 147 149 150 137 137 147Cs 2.1 1.9 2.5 2.1 2.1 20.9 20.2 2.2Ba 2461 3585 2270 2528 2512 1624 1577 2240

La 143 145 184 149 152 124 125 148 208 210 159Ce 222 233 287 237 241 208 212 238 326 317 240Pr 22.4 25.1 28.6 23.8 24.5 22.1 22.5 24Nd 73 88.2 94.3 79 80.9 77.5 79.2 81.1 33 97 76Sm 10.2 13.1 13.8 10.9 11.5 11.3 11.9 11.4 20 18 13Eu 2.66 3.37 3.11 2.92 2.99 2.94 3.09 3 5.4 4.7 3.4Gd 6.5 11.1 10.1 7.3 7.9 7.3 9 8.6Tb 0.9 1.2 1.2 1 1 1 1 1 1.8 1.1 1.2Dy 5 6.2 6.3 5.3 5.4 5.3 5.5 5.6Ho 0.9 1.1 1.1 1 1 0.9 1 1Er 2.6 3.1 2.6 2.7 2.9 2.5 2.6 2.9Tm 0.36 0.4 0.43 0.37 0.38 0.34 0.34 0.39Yb 2.6 2.8 2.9 2.6 2.7 2.3 2.4 2.7 3.1 3.4 3.5Lu 0.44 0.47 0.52 0.45 0.46 0.41 0.4 0.46 0.47 0.49 0.39

Hf 6.7 7.2 7.6 6.9 7.1 8.7 8.7 7 9.9 12 6.7Ta 9.7 8.9 9.9 9.6 9.8 8.7 9 9.5 14 20 10Pb 22 18 22 22 22 23 23 20Th 35.5 29 28.3 34.3 35 29.7 29.1 33.7 31 28 23U 6.5 4.6 5.5 6.2 6.4 6 5.8 6Ag 1.1 1.1 1.4 1.1 1.1 1.3 1.2 1

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all the samples. Composition of K-feldspar range from pureorthoclase to orthoclase perhite (Or98.8Ab1.09An0.11 toOr47.17Ab51.57An1.26) whereas plagioclase has a composition ofAb35.99An60.51Or3.5 Table 6). Fe contents in the feldspar are low(<0.43 wt%).

Spinel: All the analysed spinel have similar chemistry and areCr2O3- (<0.09 wt%) and MgO- (<3.34 wt%) depleted and are compo-sitionally titano-magnetites (Table 7).

Apatite: Compositionally apatite of different paragenesis issimilar (Table 8) and their FeO (<0.49 wt%) and SiO2 (<0.95 wt%)contents are very low.

6. Whole-rock geochemistry

Major and trace element geochemistry of CUL samples (Singlaand Degala occurrences) is presented in Table 8 together withthe published data for four samples from Piplaj lamprophyre re-ported by Das and Srikarni (2005). Their CIPW norms and Mg#[(Mg+(Mg + Fe2+)], on a volatile-free basis re-calculated to 100%,are also presented. Samples from all the three CUL display broadlysimilar and overlapping bulk-rock geochemistry consistent with

their broadly similar petrography. Their SiO2 (43.88–47.88 wt%)and Al2O3 (13.70–17.74 wt%) contents show variation in their com-position. All of them are titania-rich (TiO2 essentially >1.5 wt%)which is an alkaline character. Even though there is not much var-iation in the potassium and sodium contents in the CUL they aremarkedly sodic (Na2O > K2O) than potassic (K2O > Na2O). In theTAS diagram (Le Bas et al., 1986; not shown) the high total alkali(up to 9 wt%) and low SiO2 (43.88–47.88 wt%) content of the CULplots them in the phono-nephrite field of the alkaline series whichis further attested by the presence of nepheline (up to 17.48%)in all of their norms (Table 8). The CaO contents are high(up to 10 wt%) and reflect high Clinopyroxene, amphibole as wellas presence of carbonate ocelli. Their P2O5 contents are variable(0.22–0.87 wt%) along with their modal apatite content. High LOIcontents (3.08–6.21 wt%) correspond to the modal proportions ofmica, amphibole as well as carbonate ocelli (Table 8).

The Mg# values of CUL show a wide variation from 27.82 to 55.45(Table 8) and their moderate to low values indicate a highly evolvednature of their magmas. This is also well depicted in the empiricalplot for distinguishing calc-alkaline (shoshonitic) and alkaline lam-prophyres based on normative parameters (Rock, 1977) wherein theCUL samples show distinct calc-alkaline affinities compared to the

Fig. 3. Microphotographs and Back Scattered Electron (BSE) images of the Chhota Udepur lamprophyres. Clinopyroxene is the dominant phenocryst (A; L-11) in all studiedlamprophyres and at places aggregates to form glomeroporphyritic texture (B; L-17). Biotite is strongly zoned and shows considerable variation in grain size from 1 mm lathsto <0.5 mm sized grains (C; L-11). Amphibole is also a widespread phenocryst and similarly varies in grain size; feldspar is essentially confined to the groundmass wherein itoccurs as a predominant phase (D; L-22). Spinel is an important accessory mineral and is up to 0.25 mm (E; L-17). Olivine is rare and is usually very coarse grained (�2 mm)and at places shows rimming by clinopyroxene (F; L-21). Cp = clinopyroxene; Bt = biotite; Am = amphibole; Fs = feldspar; Sp = spinel; Ol = olivine.

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other Deccan lamprophyres at Murud Janjira and Chhaktalaowhich inturn show alkaline affinities (Fig. 4). Positive correlationbetween MgO, Fe2O�3 and CaO and a negative correlation betweenMgO and SiO2 and Al2O3 is observed (Fig. 5). The CUL have relativelyhigher contents of SiO2, Al2O3 and lesser or similar contents ofMgO, Fe2O�3 and CaO in comparison to the more primitive alkalinelamprophyres (Murud Janjira and Chhaktalao), ultramafic lampro-phyres (south of the tethyan suture) from the Deccan LIP andlamprophyres with calc-alkaline affinities from Villany Mountains,Hungary (Nédli and TÓth, 2007) and Taihang Mountains, NorthChina craton (Chen and Zhai, 2003) provided in Fig. 5 for acomparison. A good positive correlation displayed in various inter-element plots (K2O vs Rb and Zr vs Hf; Fig. 5) highlights that thepost-emplacement alteration and crustal contamination have hadlittle influence on the geochemistry of the CUL.

Transition metal content (Ni = <20 to 70 ppm; Cr = 41–140 ppmand Co = 17–29 ppm) of the CUL is significantly lower (Table 8)than those in Deccan-related alkaline lamprophyres from MurudJanjira (Ni = 117–284 ppm; Cr = 111–991 ppm and Co = 57–75 ppm; Melluso et al., 2002; Dessai and Viegas, 2010), Chhaktalao(Ni = 139–167 ppm; Cr = 169–240 and Co = 44–55 ppm; Hari,1998) and ultramafic lamprophyres from south tethyan region(Ni = 384–548 ppm; Cr = 614–1230 ppm and Co = 54–69 ppm;Kerr et al., 2010) and further supports their relatively more differ-entiated nature. All the CUL samples share identical chondrite nor-malised Rare Earth element (REE) and primitive mantle normalisedmulti element distribution patters (Fig. 6A and B) indicating originfrom similar source. LREE are significantly enriched over the HREEresulting in a sub-parallel, steep and fractionated (La/YbN = 35–44)These rocks also show significant enrichment in LILE relative toHFSE (Fig. 6B), such as high Ba/Nb and Rb/Zr contents.

The Ba/Nb ratios of the CUL vary from 11.5 to 16.9 and are sim-ilar to those from Chhaktalao lamprophyres (Ba/Nb = 15–17; Hari,1998) but are considerably higher than those (Ba/Nb = 6.2–7.5) inMurud Janjira lamprophyres (see Melluso et al., 2002). However,REE as well as multi element distribution patterns of CUL are sim-ilar to that of Murud Janjira lamprophyres and even Chhaktalaolamprophyres implying a broad similarity in their genesis(Fig. 6A and B). But CUL are quite distinct in their character fromthe Deccan-related ultramafic lamprophyres from south of the

Tethyan zone (Kerr et al., 2010) with the latter characterised byprominent negative spikes in their Hf–Zr contents (Fig. 6A and B).

7. Discussion and petrogenesis

Their mode of occurrence (dykes), petrography (porphyritic–panidiomorphic texture), mineralogy (presence of hydrous maficsilicates and restriction of feldspar essentially as a groundmassphase) and geochemistry (alkaline nature, enrichment in LILE, highLa/Yb, etc.) indeed attest the samples under study from ChhotaUdepur as lamprophyres. Whereas they are clearly distinct fromthe ultramafic lamprophyres of the Deccan LIP (Kerr et al., 2010),they do share characteristic features of the Deccan-related alkalinelamprophyres from the Murud Janjira and the Chhaktalao (Mellusoet al., 2002; Dessai and Viegas, 2010; Hari, 1998) as well as calc-alkaline lamprophyres which are commonly considered to be prod-ucts of subduction-related processes (Rock, 1991). The similaritieswith alkaline lamprophyres include presence of carbonate ocelli,kaersutite variety of amphibole, high Al–Ti-biotite, normativenepheline, high-titania (bulk rock) contents and overall sodi-potas-sic (Na2O > K2O) nature. On the other hand, the calc-alkaline char-acters are well portrayed in their major element (SiO2 and K2O) aswell as in the normative ternary discriminant plot (Fig. 4). How-ever, CUL conspicuously lack the negative Nb and Ta spikes in theirprimitive-mantle normalised multi-element plots (Fig. 6B) whichare characteristic of calc-alkaline lamprophyres derived fromsources that experienced subduction (see also Chen and Zhai,2003; Guo et al., 2004; Nédli and Tóth, 2007; O’Leary et al.,2009) and in fact mimic the alkaline lamprophyres from MurudJanjira and Chhaktalao in their incompatible trace element pat-terns and concentrations (Fig. 6A and B). As magmas with calc-alkaline affinities are recorded (e.g., Hawkesworth et al., 1995)even from extensional settings, such as Basin and Range province,USA, the tectonic connotation of such geochemical signals shouldbe exercised with caution (see also Sheth et al., 2002; Scarrowet al., 2009) and any such consideration should encompass otheravailable geochemical, geological and geophysical evidences aswell.

Before constraining the petrogenesis of the CUL dykes, the ex-tent of post-magmatic alteration and crustal contamination are as-sessed and the nature of their mantle source regions are evaluated.

7.1. Evaluation of post-magmatic alteration and crustal contamination

Presence of optical and compositional zoning in ferro-magne-sian minerals reveal that samples under study were not subjectedto metamorphism. This is further supported by the lack of any dis-cernable alteration in minerals and absence of low temperaturemetamorphic mineral assemblages such as chlorite and talc. Sys-tematic correlation portrayed between MgO and all other majoroxides indicate an important role played by fractional crystallisa-tion of their magmas en route the surface and exclude significantpost-magmatic alteration and/or crustal contamination (Fig. 5A–D).It is also well established that LILE (large ion lithophile elements;Rb, K, Ba and Sr) concentrations are more susceptible to post-magmatic hydrothermal alterations than that of HFSE (high fieldstrength elements: Nb, Ta, Ti, Hf, etc.; Rollinson, 1993). Therefore,correlations between various LILE and HFSE elemental pairs(Fig. 5E and F) further support the argument for a lack of significantpost-magmatic alteration effects in these rocks.

We did not find any field or petrographic evidence for incidenceof crustal xenoliths or xenocrysts in the CUL. Also their concentra-tions of trace elements such as Sr (1214–1666 ppm), Zr (371–471 ppm) and Nb (137–152 ppm; Table 8) are considerably higherthan those in continental crust (Sr = �360 ppm; Zr = �130 ppm

Fig. 4. Emperical plot for distinguishing calc-alkaline (shoshonitic) and alkalinelamprophyres based on normative parameters (Rock, 1977). Two dashed linesdefine the field of transitional characteristics. Note that the Chotta Udepurlamprophyres (d) of this study as well as those studied by Das and Srikarni(2005) show strong affinities to calc-alkaline, rather than alkaline, lamprophyres.Composition of Murud Janjira lamprophyres (D) (Dessai and Viegas, 2010) andChhaktalao lamprophyres (+) (Hari, 1998) is also provided for comparison.

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and Nb = �17 ppm; Rudnick and Gao, 2003) to account their deri-vation from the latter. Furthermore, their strongly enriched LREEpatterns (up to 800� chondrite) and low HREE (e.g.,Lu = <0.47 ppm) and Y (�31 ppm) compels us to opine that theCUL samples are not significantly contaminated by the continentalcrust.

7.2. Nature of mantle source regions

High abundances of incompatible trace element abundances,LILE and HFSE, in the CUL samples suggest that their mantle sourceregions to have been enriched (metasomatised) similar to that hasbeen invoked to explain the genesis of several world-wide lampro-

phyres and potassic-magmas (e.g., Foley et al., 1987; Beard et al.,1996; Guo et al., 2004; Srivastava and Chalapathi Rao, 2007)including those from the Deccan (Melluso et al., 2002; Hari,1998; Dessai and Viegas, 2010; Kerr et al., 2010). Strong negativespikes of Sr and Eu in chondrite normalised patterns are normallyinterpreted to be related to plagioclase fractionation; but theirabsence in CUL (Fig. 6B) implies that Sr depletion may be relatedto source characters that may involve the presence of residualclinopyroxene or depletion of Sr during an earlier melt extraction.Negative spikes at K and Ti present in CUL (Fig. 6B) may be explainedby presence of residual phlogopite and titanite respectively andsuch features are recorded from several world-wide lamprophyres(Rock, 1991). The absence of negative spikes at Nb and Ta (Fig. 6B)

Fig. 5. Bi-variate variation plots for the Chhota Udepur lamprophyres (d) of this study and those reported by Das and Srikarni (2005) involving (A) MgO (wt%) vs SiO2 (wt%),(B) MgO (wt%) vs Al2O3 (wt%), (C) MgO vs Fe2O�3, (D) MgO (wt%) vs CaO (wt%), (E) K2O (wt%) vs Rb (ppm) and (F) Zr (ppm) vs Hf (ppm). Possible fractional phases duringmagma evolution are also shown (after Guo et al., 2004). The data for Chhaktalao lamprophyres (+) reported by Hari (1998); Murud Janjira lamprophyres (D) (after Dessai andViegas, 2010), Réunion plume related Late Cretaceous ultramafic lamprophyres (x) (Kerr et al., 2010), Taihang lamprophyres (o) from North China craton (Chen and Zhai,2003) and Villany Mountains Lamprophyres (h), Hungary (Nédli and TÓth, 2007) are also shown for comparison.

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exclude the major role of crustal contamination and/or subduc-tion-related sources in their genesis (e.g., Rogers et al., 1987).The overall uniformity in incompatible trace element distributionsin CUL suggests that fractionation of minerals enriched in thesetrace elements must be either absent or very limited (see Foleyet al., 2002). The CUL, however, are likely to have been derivedfrom shallower depths than the tethyan lamprophyres owing totheir relatively lower HREE contents (Fig. 6A) which can be inter-preted as a lesser control of residual garnet in their source regions(Le Roex et al., 2003).

It is well established that trace element ratios (e.g., Th/Yb,Ta/Yb, Nb/U and Ce/Pb) are not affected by magmatic processes andreflect that of their source regions and therefore, serve as petroge-netic indicators (e.g., Rogers et al., 1987; Hofmann, 1997). As theCUL show certain calc-alkaline characters in terms of their norma-tive major element composition, their trace element ratios (Figs. 7and 8A and B) are exploited to provide additional insights as totheir mantle source regions. The following inferences can be drawnin this regard: (i) the Chhota Udepur lamprophyres, together withother Deccan-related lamprophyres from Murud Janjira andTethyan suture zone, essentially plot in an array defined by non-subduction generated magmas such as MORB and within plate

basalts, (ii) they exclude contamination by either continental crustor by subduction-related components such as pelagic sediments,unlike in the case of 120 Ma calc-alkaline lamprophyres fromTaihang Mountains, North China, which show some evidences ofcrustal contamination (Fig. 8B) and (iii) all of them display a strongaffinity towards OIB type-enriched mantle source.

In view of the strikingly similar geochemical behaviour be-tween Nb and Ta, the ratio of Nb/Ta is hardly affected by processessuch as fractional crystallisation and partial melting unless a sig-nificant proportion of modal rutile or amphibole is present in themantle source region (Tiepolo et al., 2001; Foley et al., 2002). Sim-ilar behaviour is also displayed by Zr and Hf and their ratio remainsconstant during magmatic processes unless subjected to carbon-ate-rich/fluid-metasomatism (e.g., Rudnick et al., 1993; Furmanand Graham, 1999; Tappe et al., 2008). Zr/Hf and Nb/Ta fraction-ation displayed by the Deccan-related lamprophyres in generaland CUL in particular in Fig. 9 very likely implies the role of carbon-ate-rich and rutile-rich metasomatic imprints on their source re-gions. In order to further evaluate the role of accessory rutile, wehave plotted Nb/La and Ta/La vs a differentiation index viz., Mg#(Fig. 10A and B). Increasing Mg# and concomitant decrease inNb/La and Ta/La (Fig. 10) further suggests that accessory rutile,

Fig. 6. (A) Chondrite-normalised (after Sun and McDonough, 1989) REE distribution patterns for Chhota Udepur lamprophyres (continuous solid lines) compared with thosefrom Murud Janjira (Dessai and Viegas, 2010), Chhaktalao (Hari, 1998) and Réunion plume related Late Cretaceous ultramafic lamprophyres from the tethyan zone (Kerr et al.,2010). (B) Primitive-mantle normalised (Sun and McDonough, 1989) multi-element spidergrams for Chhota Udepur lamprophyres (continuous solid lines) compared withthose from Murud Jhanjira (Dessai and Viegas, 2010), Chhaktalao (Hari, 1998) and Réunion plume related Late Cretaceous ultramafic lamprophyres from the tethyan zone(Kerr et al., 2010). Note the strong similarity between Chhota Udepur and Murud Janjira lamprophyres.

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albeit in minor proportions, could well be holding back Ta and Nb.Occurrence of the nearby (<50 km) Amba Dongar carbonatite com-plex gives additional support for the carbonate-rich metasomatismin this domain.

7.3. Petrogenesis of Chhota Udepur lamprophyres

A variety of models are in vogue for the generation of lampro-phyres. These include (i) low-degree partial melting processes ofmetasomatised continental lithospheric mantle (e.g., Owens andTomascak, 2002; Scarrow et al., 2009; Rocchi et al., 2009), (ii) ex-treme differentiation of a volatile-rich basic magma (e.g., Currieand Williams, 1993), (iii) asthenospheric melts directly derivedfrom a mantle-plume carapace (Kerr et al., 2010), (iv) mixing ofultrabasic magma with crustal material (e.g., McDonald et al.,1985) and (v) mixing of basic and alkaline melts (Xu et al., 2007).

Primary lamprophyre magmas are characterised by Mg# = 65–80; Sc = 15–30 ppm; Cr = 200–500 ppm; Co = 25–80 ppm andNi = 90–700 ppm (Rock, 1991). The Mg# = 27–55; Sc = 8 = 16 ppm;Cr = 40–140 ppm; Co = 17–31 ppm and Ni = <20–60 ppm of theCUL clearly preclude their primary status. Their highly fractionatednature is also apparent in the major element variation diagrams(Fig. 5). The absence of any crustal contamination (Figs. 7 and 8)negates generation of CUL from mixing of ultrabasic magma withcrustal material. Likewise, the absence of any field relationshipswith basic magma, and basic xenoliths together with lack of othergeochemical aspects such as abrupt increase of SiO2 content of themelt (see Xu et al., 2007) preclude us to invoke genesis of CUL frommixing of basic and alkaline melts.

Highly fractionated La/Yb coupled with low HREE contents ofthe CUL suggest the presence of residual garnet in their source re-gion (e.g., Mitchell and Brunfelt, 1975; McKenzie and O’Nions,1991). In order to evaluate this supposition, the observed REE con-tents of CUL are compared with the non-modal batch meltingcurves (after Nédli and TÓth, 2007) for spinel and garnet-lherzolitesource compositions (Fig. 11). Lower degrees of partial melting of aprimitive garnet lherzolite mantle followed by greater amounts (atleast 40–60%) of fractional crystallisation can account for the ob-

served La/Yb and Yb contents of CUL. Petrogenesis of CUL is strik-ingly similar in many aspects to that of the Upper Cretaceousalkaline lamprophyres Villány Mountains, Hungary (Nédli and

Fig. 7. Ta/Yb vs Th/Yb diagram (after Menzies and Kyle, 1990) for Chhota Udepurlamprophyres (d). Also shown is the data for Murud Janjira lamprophyres (D)(Dessai and Viegas, 2010), Réunion plume related Late Cretaceous ultramaficlamprophyres (x) (Kerr et al., 2010), Chhaktalao lamprophyres (+) (Hari, 1998),Taihang lamprophyres (o) from North China craton (Chen and Zhai, 2003) andVillany Mountains Lamprophyres (h), Hungary (Nédli and TÓth, 2007). TH = tho-leiitic, CA = calc-alkaline, SHO = shoshonitic boundaries for arc basalts; vectors:s = subduction zone enrichment; c = crustal contamination; w = within-plateenrichment and f = fractional crystallisation (from Nédli and TÓth, 2007).

Fig. 8. (A) Ce (ppm) vs Ce/Pb plot for the Chhota Udepur lamprophyres (d). Data forMurud Janjira lamprophyres (D) (Dessai and Viegas, 2010), Chhaktalao lampro-phyres (+) (Hari, 1998) and Taihang lamprophyres (o) from North China craton(Chen and Zhai, 2003) is also plotted. Dashed lines (indicate range of mantle values)and all other compared fields are taken from Owen (2008). (B) Nb (ppm) vs Nb/Uplot for the Chhota Udepur lamprophyres (d). Data for Murud Janjira lamprophyres(D) (Dessai and Viegas, 2010), Chhaktalao lamprophyres (+) (Hari, 1998), Taihanglamprophyres (o) from North China craton (Chen and Zhai, 2003) and Réunionplume related Late Cretaceous ultramafic lamprophyres (+) (Kerr et al., 2010) arealso plotted along side. Note the OIB affinity of all the Deccan-related lamprophyres.

Fig. 9. Zr/Hf vs Nb/Ta plot for Chhota Udepur lamprophyres (d) showing possiblecarbonate-enrichment processes in the source. Data for Murud Janjira lamprophy-res (D) (Dessai and Viegas, 2010), Chhaktalao lamprophyres (+) (Hari, 1998) andRéunion plume related Late Cretaceous ultramafic lamprophyres (+) (Kerr et al.,2010) are also plotted. Various other fields and data sources are from Guo et al.(2004).

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TÓth, 2007) and to some extent with the Chhaktalao lamprophy-res. On the other hand, the Murud Janjira lamprophyres seem tohave been derived from a similar, but more enriched and less frac-tionated, garnet peridotite mantle sources (Fig. 11).

An OIB-type enriched mantle source, geochemically similar tothe Réunion plume source, is suggested to be one of the majorcomponents in the Deccan-age alkaline magmatism from theKutchh region (Karmalkar et al., 2005). Furthermore, the crypticmetasomatism recorded in the mantle xenoliths (spinel lherzo-lites) from the Kutchh lithosphere is related to influence of theRéunion plume (Karmalkar et al., 2009). A strong OIB-affinity dis-played by their trace element ratios of the CUL samples compelsus to attribute the metasomatism (enrichment) experienced bythe source regions of the CUL to the upwelling melts from the Réu-nion mantle plume, which is widely considered to be responsiblefor the origin and evolution of the Deccan Large Igneous Province(see Saunders et al. (2007) and Chalapathi Rao and Lehmann(2011) for recent reviews).

7.4. Implication for alkaline magmatism in the Deccan Large IgneousProvince

As discussed before, the Deccan basalts were erupted as threemain volcanic events at 69–67 Ma, 65 Ma and a terminal event at61 Ma. Alkaline complexes in the NW India at Mundwara and Sar-nu Dandali at Rajasthan are of 68.5 Ma (Basu et al., 1993) whereasthe Amba Dongar carbonatite complex and Phenai Mata alkalinecomplex located in the Chhota Udepur subprovince, within theNarmada rift and the kimberlites (orangeites) of the Mainpur fieldin Bastar craton are of 65 Ma (Basu et al., 1993; Ray and Pande,1999; Chalapathi Rao and Lehmann, 2011). The phlogopite sepa-rates from the Murud Janjira lamprophyres are dated to be of64.9 ± 09 Ma by Rb–Sr method (Sahu et al., 2003) whilst the K–Arage of the litho-units of the Girnar alkaline complex range from63.8 to 56.2 Ma (Paul et al., 1977). Thus, the alkaline magmatismin the Deccan LIP has preceded, succeeded as well as was synchro-nous with the main basalt eruptions.

Various models proposed for the generation of the Deccan LargeIgneous Province can be treated under three broad categories: (i)plume-head models (e.g., Richards et al., 1989; Campbell and Grif-fiths, 1990); (ii) plume-rift models (e.g., White and McKenzie,1989); and (iii) non-plume models (e.g., Elkins-Tanton and Hager,2000; Anderson, 2007). In the Chhota Udepur subprovince, the40Ar–39Ar dating of the basalt exposed inside the ring dyke at AmbaDongar and phonolite from an alkaline plug (located at Tawa at thenorthernmost edge of the province) yielded plateau ages of68.5 ± 0.9 and 65.2 ± 0.7 Ma, respectively (Ray et al., 2003). Theseages demonstrate that basalt outcroppings in the Amba Dongarcomplex predate the latter and that entire alkaline magmatism inthe Chhota Udepur subprovince (an area of �1200 km2) was sug-gested to be contemporaneous at 65 Ma (Ray et al., 2003). A closesimilarity between the isotopic data from the various Deccan alka-line complexes viz., Amba Dongar, Mundwara, Sarnu-Dandali andBhuj and those for present-day volcanics from Réunion hotspotsuggests a significant involvement of plume-type mantle in thegeneration of these alkaline rocks (Simonetti et al., 1998).

There is no isotopic data for the lamprophyres of this studywhich makes it difficult to pin point the absolute chronology oftheir emplacement. Although they also do not show any field rela-tionships with the Deccan basalts and other alkaline plugs in theChhota Udepur Province, their structural trends (ENE–WSW andNNW–SSE) are parallel to those of other Deccan-related alkalinecomplexes and dykes (see Subba Rao, 1972; Bose, 1980; Basuet al., 1993). Therefore, we infer them to be of �65 Ma along withother alkaline complexes of the Chhota Udepur Province and hencean integral component of the Deccan Large Igneous Province. This

Fig. 10. Mg# (Mg/(Mg + Fe) vs (A) Nb/Ta and (B) Ta/La for the Chhota Udepurlamprophyres (d). Slight decrease in Nb/La and Ta/La with concomitant increase inMg# suggests involvement of accessory rutile in the source region.

Fig. 11. La/Yb vs Yb (ppm) plot for the Chhota Udepur lamprophyres (d). Data forMurud Janjira lamprophyres (D) (Dessai and Viegas, 2010) and Chhaktalaolamprophyres (+) (Hari, 1998) is also shown. Melting curves for spinel and garnetperidotite calculated using non-modal batch melting equation on the initial mantlemodal source are from Nédli and TÓth (2007). Melting phases for garnet lherzolite[olivine (1.4%) + orthopyroxene (8.7%) + clinopyroxene (36%) + garnet (54%) ] andspinel lherzolite [olivine (1.2%) + orthopyroxene (8.1%), clinopyroxene(76.4%) + spinel (14.4%)]. Degree of partial melting (%) is indicated along themelting curves. Mineral–matrix partition coefficients are from McKenzie andO’Nions (1991). Sources: primitive mantle from Sun and McDonough (1989);enriched asthenospheric mantle from Seghedi et al. (2004). Fractional crystallisa-tion lines for melt are generated by 0.1% melting of primitive mantle and 0.5%melting of enriched asthenospheric source. Percent of fractional crystallisation isindicated in italics along lines. Hypothetical crystallising phases: olivine(40%) + clinopyroxene (40%) + plagioclase (20%). The field for Upper Cretaceouslamprophyres from Villany Mountains, Hungary, is from Nédli and TÓth (2007).

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inference is strongly supported by the OIB-type trace element sig-nature which is similar to that of Deccan-related lamprophyres andalkaline rocks. As CUL intrude the Precambrian basement, contraryto all the hitherto known Deccan-related lamprophyres and also amajority of the alkaline complexes which are associated with theDeccan basalts, their OIB-type geochemical signature assumesimportance as it provides further evidence for the derivation ofDeccan Large Igneous Province from a Réunion type mantle plumesource. Geophysical support for plume–lithosphere interaction inthis domain of the Narmada rift zone is provided by the anoma-lously high seismic velocities that are attributed to under-platedmafic material at shallower depths of 1.5–9 km (Kumar et al.,2000).

The occurrence of CUL as 2.5 km long dykes is quite unusual andmake them the longest yet recorded lamprophyres from the Dec-can LIP as well as from the Indian shield. Pre-existing rift zonesare regarded as the regions of maximum uplift and extension atthe time of plume impact and also the sites of alkaline magma gen-eration in a Large Igneous Province (Gibson et al., 2006). Therefore,the CUL emplacement, together with that of other alkaline com-plexes in the Chhota Udepur Province, is attributed to pre-existingcrustal weakness accentuated by the mantle plume–lithosphereinteraction during the end-Cretaceous in the Narmada rift zone.

Various aspects such as nature of mantle source regions, degreeof metasomatism (enrichment) and fractional crystallisation ap-pear to have played a decisive role in bringing out conspicuous pet-rological and geochemical differences between the CUL and otherDeccan-related lamprophyres.

8. Conclusions

Recently recorded lamprophyres occurring south of the ChhotaUdepur area, NW India, within the Narmada rift zone, extend up to�2.5 km in length, and constitute the longest yet recorded lamp-rophyre dykes from the Indian shield. Their mineralogy and geo-chemsitry show several overlapping characteristics of alkaline- aswell as calc-alkaline lamprophyres. Low Mg#, incompatible traceelement-enrichment and highly fractionated chondrite-normalisedREE distribution patterns are hall-marks of these rocks. Post-emplacement alteration and crustal contamination exercised littleinfluence on their geochemistry. Their incompatible trace elementgeochemistry implies involvement of an OIB type-enriched mantlesource, similar to that displayed by other Deccan-relatedlamprophyres..

A metasomatised garnet lherzolite mantle that has undergonevery low degrees of partial melting and large degrees of fractionalcrystallisation is inferred to be the source rock of the Chhota Ude-pur lamprophyres. We envisage the lamprophyres of Chhota Ude-pur to be a part of the alkaline rock spectrum of the Deccan LargeIgneous Province and their emplacement, together with that ofother alkaline complexes such as Phenai Mata Complex and AmbaDongar Complex in the Chhota Udepur area, is strongly controlledby pre-existing crustal weakness accentuated by the Réunion man-tle plume–lithosphere interaction during the terminal-Cretaceousin the Narmada rift zone. Various factors such as nature of mantlesource regions, depth of generation, degree of metasomatism andfractional crystallisation appear to have played a decisive role inbringing out conspicuous petrological and geochemical differencesbetween the Chhota Udepur lamprophyres and other Deccan-re-lated lamprophyres.

Acknowledgements

N.V.C.R. thanks the Head, Centre of Advanced Study of Geology,B.H.U., Varanasi, for encouragement and for providing necessary

facilities. Constructive reviews by Georg Zellmer and an anony-mous reviewer and editorial suggestions by Bor-ming Jahn led toa significant improvement of this paper and we express our sincerethanks to all of them.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.jseaes.2011.09.009.

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