ORIGINAL PAPER

Geochemical studies and petrogenesis of ~2.21–2.22 Ga Kunigalmafic dyke swarm (trending N-S to NNW-SSE) from easternDharwar craton, India: implications for Paleoproterozoic largeigneous provinces and supercraton superia

Rajesh K. Srivastava & M. Jayananda &

Gulab C. Gautam & Amiya K. Samal

Received: 4 December 2013 /Accepted: 24 April 2014 /Published online: 9 May 2014# Springer-Verlag Wien 2014

Abstract The Archean eastern Dharwar craton is transactedby at least four major Proterozoic mafic dyke swarms. Wepresent geochemical data for the ~2.21–2.22 Ga N-S toNNW-SSE trending Kunigal mafic dyke swarm of the east-ern Dharwar craton to address its petrogenesis and forma-tion of large igneous province as well as spatial link tosupercontinent history. It has a strike span of about200 km; one dyke of this swarm runs ~300 km along thewestern margin of the Closepet granite. Texture and mineralcompositions classify them as dolerite and olivine dolerite.They show compositions of high-iron tholeiites, high-magnesian tholeiites or picrites. Geochemical characteris-tics of the sampled dykes suggest their co-genetic natureand show variation from primitive (Mg#; as high as ~76) toevolved (differentiated) nature. Although geochemicalcharacteristics indicate possibility of minor crustal contam-ination, they show their derivation from an uncontaminatedmantle melt. These mafic dykes are probably evolved froma sub-alkaline basaltic magma generated by ~20 % batchmelting of a depleted lherzolite mantle source and about15–30 % olivine fractionation. Paleoproterozoic (~2.21–2.22 Ga) mafic magmatism is recognized globally as dykeswarms or gabbroic sill complexes in the Superior, Slave,North Atlantic, Fennoscandian and Pilbara cratons.Possible Paleoproterozoic Dharwar–Superior–North-

Atlantic–Slave correlations are constrained with implica-tions for the configuration of supercraton Superia.

Introduction

Proterozoic mafic dyke swarms are key elements to under-stand ancient geodynamic processes of the planet Earth. Theirsystematic study is particularly important for understandingPrecambrian terrains and identification of large igneous prov-inces (LIPs) of that period (Coffin and Eldholm 2005; Bryanand Ernst 2008; Ernst and Srivastava 2008; Srivastava et al.2010). There are many criteria to identify LIPs, howeverprecisely-dated magmatic events are found to be very accurateto correlate different crustal blocks. The barcodes of regionaldyke swarms are successfully used to compare different crust-al blocks and reconstruct their connections in past continentsand supercontinents (e.g., Ernst and Buchan 1997, 2001a;Bleeker 2003, 2004; Bleeker and Ernst 2006; Ernst andBleeker 2010). Over the past two decades, short-livedmantle-generated magmatic events, and the information inher-ent therein, have emerged as the key to unraveling pre-Pangaea paleocontinental reconstructions (Bleeker and Ernst2006). Because of great vertical depth and lateral extent ofdolerite dyke swarms in shield areas, this may provide themost comprehensive record of short-lived, mantle-generatedmagmatic (LIPs) events through time and space (e.g., Halls1982; Halls and Fahrig 1987; Parker et al. 1990; Baer andHeimann 1995; Ernst and Buchan 2001a; Hanski et al. 2006;Bleeker and Ernst 2006; Ernst et al. 2010; Srivastava et al.2010; Srivastava 2011). Such robust records of LIPs can alsobe used to locate mantle plume centres, as well as, the properreconstruction of those crustal fragments (Bleeker and Ernst2006; Ernst et al. 2008; Ernst and Srivastava 2008).

Editorial handling: L. G. Gwalani

R. K. Srivastava (*) :G. C. Gautam :A. K. SamalCentre of Advanced Study in Geology, Banaras Hindu University,Varanasi 221005, Indiae-mail: rajeshgeolbhu@gmail.com

M. JayanandaCentre of Advanced Studies, Department of Geology, University ofDelhi, Delhi 110007, India

Miner Petrol (2014) 108:695–711DOI 10.1007/s00710-014-0327-5

The Indian shield is transected by mafic dykes and dykeswarms of different ages and compositions (Fig. 1); however,they are less prominent in the Proterozoic sedimentary basins(Halls 1982; Drury 1984; Murthy 1987; Devaraju 1995;Srivastava et al. 2008; Srivastava and Ahmad 2008, 2009;Srivastava 2011). On the basis of available geochronologicaldata on the magmatic events of the Indian shield duringProterozoic, Ernst and Srivastava (2008) reviewed the recordof Indian LIPs and LIP-‘fragments’ from the Dharwar, Bastar,Singhbhum, and Aravalli cratons and Eastern Ghats alkalineprovince. However, in recent years, some robust U-Pb geo-chronological results on the mafic dykes have been reportedwhich helped to recognize several discrete large igneous prov-inces (LIPs) in the central and southern Indian shield, partic-ularly in the Bastar and Dharwar cratons (Halls et al. 2007;French et al. 2008; Ernst and Srivastava 2008; Jayananda et al.2008a; French and Heaman 2010; Srivastava et al. 2011;Kumar et al. 2012a). These include the ~2.37 Ga Bangaloreswarm (Halls et al. 2007; French and Heaman 2010; Kumar

et al. 2012a), the ~2.21–2.22 Ga and ~2.18 Ga Pan-Dharwardyke swarms (French and Heaman 2010; Srivastava et al.2011), and the ~1.89 Ga Bastar-Dharwar LIP (French et al.2008).

The Proterozoic mafic dyke swarms exposed in theDharwar craton are of special interest because this craton hasbeen considered as a principal constituent of several ancientsupercontinents (Bleeker 2003; Heaman 2008; Srivastavaet al. 2008; Pradhan et al. 2010; Srivastava 2008, 2011 andreferences therein). French and Heaman (2010) have present-ed little petrologic and geochemical data on the N-S to NNW-SSE trending mafic dykes. Kumar et al. (2012b) have alsopresented geochemical and paleomagnetic data on this dykeswarm and observed chemical homogeneity, which could bedue to limited fractionation, rapid emplacement and magmaderivation from a single homogeneous source rather than frommultiple sources. The purpose of this contribution is to presentpetrologic and whole rock geochemical data for the ~2.21–2.22 Ga N-S to NNW-SSE trending mafic dyke swarm of the

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Fig. 1 Generalized geologicaland tectonic map of the Indianshield (modified after Frenchet al. 2008). Ch, ChattisgarthBasin; CIS, Central Indian ShearZone; GR, Godavari Rift; M,Madras Block;Mk,Malanjkhand;MR, Mahanadi Rift; N, NilgiriBlock; NS, Narmada-Son FaultZone; PC, Palghat-Cauvery ShearZone; R, Rengali Province andKerajang Shear Zone; S,Singhbhum Shear Zone; V,Vindhyan Basin

696 R.K. Srivastava et al.

eastern Dharwar craton (Fig. 2) to understand their petroge-netic aspects. This study also aims to throw light on itspossible expansion across the entire eastern Dharwar craton.It would also help to correlate this N-S to NNW-SSE dykeswith other mafic dykes of similar ages exposed in otherancient cratons of the globe (Superior, North Atlantic andSlave cratons), and constrain paleocontinental reconstructionsat ~2.21–2.22 Ga.

Geological setting

The Archean Dharwar craton is exposed in large part of thesouthern peninsular India, which in turn, covered byMesoproterozoic sedimentary basins as well as by ca. 65 MaDeccan FloodBasalt Province to the north, the Godavari rift tothe northeast, Eastern Ghats Belt to the east, and the SouthernGranulite Terrain (SGT) in the south (Naqvi and Rogers 1987;

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Fig. 2 Simplified geological map of the Dharwar craton, showing theprincipal distribution of Proterozoic mafic dykes (modified from Frenchand Heaman 2010). Black dashed line demarcates the boundary betweenthe two blocks (Mahadevan 2008). Sampled dykes are red-marked. Site

locations of samples used for geochemical and U-Pb age determinationsin the present study and studied by French and Heaman (2010) are alsoshown

Petrogenesis of ~2.21–2.22 Ga Kunigal mafic dyke swarm 697

Ramakrishnan and Vaidyanadhan 2010) (see Figs. 1 and 2).The regional geology of the Dharwar craton is well document-ed in some recent publications (Mahadevan 2008; Chadwicket al. 2000; Chardon et al. 2008, 2011; Jayananda et al. 2000,2008b, 2013a, b; Ramakrishnan and Vaidyanadhan 2010 andreferences therein). The Archean basement of the Dharwarcraton comprises 3.40–2.56 Ga amphibolite to granulite faciesTTG-type gneisses (Meen et al. 1992; Peucat et al. 1993,2013; Chardon et al. 2011), two generations of supracrustalrocks viz. 3.35–3.20 Ga Sargur Group and 3.00–2.54 GaDharwar Supergroup (Balakrishnan et al. 1990; Nutmanet al. 1996; Jayananda et al. 2008b, 2013b) and 2.62–2.52 Ga calc-alkaline to potassic granites (Balakrishnanet al. 1999; Jayananda et al. 2000, 2006; Friend and Nutman1991; Mahabaleswar et al. 1995).

The craton is divided into two distinct blocks viz. westernDharwar craton (WDC) and eastern Dharwar craton (EDC)based on lithological and metamorphic patterns, age of crustalprovinces and crustal thickness. The thick mylonitic shearzone along the eastern boundary of the Chitradurga green-stone belt is considered a dividing line (Fig. 2. in Chardonet al. 2008). The WDC is dominated by abundant TTG-greenstone associations with relatively higher pressure low-T assemblages and minor high-potassic granites formed bylower crustal reworking whilst the EDC contain minor TTG-greenstones with preserved low pressure but high to ultra-hightemperature assemblages and most voluminous 2.56–2.52 Gajuvenile granitoids. Recent studies have shown that the WDCcratonized at 2.62 Ga, whereas the EDC cratonized close2.5 Ga (Jayananda et al. 2006, 2013b). Further, Chadwicket al. (2000) and Chardon et al. (2011) have suggested thatamalgamation ofWDC and EDC has probably occurred in thecontext westward convergence close to 2.5 Ga.

Mafic dykes and dyke swarms of different generations arewidespread in the eastern Dharwar craton and are well ex-posed all around the vicinity of the Cuddapah basin withdifferent trends (e.g. Ikramuddin and Stueber 1976; Drury1984; Murty et al. 1987; Murthy 1987; Devaraju 1995;Chatterjee and Bhattacharji 2001; Radhakrishana et al. 2007;Halls et al. 2007; Srivastava et al. 2008; French and Heaman2010; Srivastava 2011 and references therein). The N-S toNNW-SSE trending dyke swarm is one of the major dykeswarms recognized all over the EDC (Murthy 1987; Ernst andSrivastava 2008; French and Heaman 2010; Srivastava et al.2011 and references therein). A prominent N-S trending dykeswarm runs ~300 km along the western margin of the Closepetgranite (see Fig. 2). ID-TIMS (Isotope Dilution – ThermalIonization Mass Spectrometry) U-Pb baddeleyite age for thislong N-S dyke is recently presented by Srivastava et al.(2011), which yields 2,215.2±2.0 Ma. Another dyke of thisswarm also yields similar age, i.e. 2,211.7±0.9Ma (Srivastavaet al. 2011). A number of similar N-S trending mafic dykes arealso encountered from south and north of the Cuddapah basin

(see Fig. 2). A dyke of this swarm, located south of theCuddapah basin near Kondlamadugu, is dated by Frenchand Heaman (2010), which yields U–Pb baddeleyite age of2,220.5±4.9 Ma. Later, Kumar et al. (2012b) have presentedtwo whole rock-mineral Sm–Nd isochrons from one of thesedykes, which yield ages of 2,173±43 Ma and 2,190±51 Ma(2σ), respectively.

Petrography

A brief petrography of N-S to NNW-SSE trending maficdykes is presented by French and Heaman (2010) andKumar et al. (2012b). French and Heaman (2010) have clas-sified N-S trending dyke samples as dolerite with ophitic/sub-ophitic texture, whereas a dyke trending in NW-SE and hav-ing slightly younger age (2,209.3±2.8 Ma) was defined asolivine gabbro-norite. The latter contains almost equal amountof plagioclase, orthopyroxene and clinopyroxene withsubordinate olivine. On the other hand, Kumar et al. (2012b)observed that N-S dykes contain plagioclase, augite and oc-casionally hypersthene with poikiltic (? probably ophitic)texture; no olivine presence is noticed. However, in the pres-ent study slightly different petrographic features are observed.

Most of the studied mafic dyke samples are very fresh,medium-to coarse-grained and composed of subhedral grainsof plagioclase and pyroxenes. Ophitic and sub-ophitic texturesdisplayed by plagioclase enclosed by pyroxene grains arecommon (Fig. 3); however, at places, a few samples alsodisplay hypidiomorphic texture. Augite is commonclinopyroxene present in majority of the studied samples. Acouple of samples contain few small grains of hypersthene,otherwise orthopyroxene (opx) is absent in most of the studiedsamples. However, due to pinkish nature coupled with faintpleochroism of many augite grains often misleads toorthopyroxene. Hypersthene grains are identified on the basisof pleochroic scheme from dark to light pink and havingalmost straight extinction and low relief. On the other hand,titan-augite grains have feeble pink pleochroism with compar-atively high relief and are twinned and have high extinctionangle (>35°) (Fig. 3c). Olivine grains are noticed in manysamples (Fig. 3e, f). Medium-grained olivines are anhedralrounded/sub-rounded to euhedral. Serpentinization along thecracks is noticed in these olivine grains, which suggest low-temperature late hydrothermal alterations. This leads tocrystallize chlorite, amphibole and biotite in some samples.Clouding of feldspar due to saussuritization is also ob-served in a number of samples (Fig. 3d). Ilmenite, apatiteand zircon/baddeleyite are common accessory mineralspresent in these samples. On the basis of observed miner-alogy and textures, these dyke rocks may be classified asdolerites and olivine dolerites.

698 R.K. Srivastava et al.

Sampling and analytical techniques

Every attention has been paid in the present study to collectfresh samples for geochemical analyses. Samples for chemicalanalyses were collected almost middle of the dyke withmassive-homogenized characteristics and devoid of any vein,alteration or pegmatitic texture tominimize any contaminationor alteration. Most of the samples were collected from N-S toNNW-SSE trending dykes exposed in the southern portion ofthe eastern Dharwar craton (Fig. 2). Special attention is givento a very long (>250 km long and>30 m wide) N-S to NNW-SSE trending dyke emplaced very close to, and parallel to thewestern margin of the Closepet granite (see Fig. 2). It liesabout 125 kmwest of the earlier-studied N-S dykes by Frenchand Heaman (2010). Locations of analyzed samples aremarked on Fig. 2. All the analyses were done at theActivation Laboratories Ltd., Ancaster, Ontario, Canada. AnICP (Model: Agilent 735) was used to analyze major elementsby lithiummetaborate/tetraborate fusion method and accuracyis much better than 1–2 %. An ICP-MS (Model: Perkin ElmerSciex ELAN 9000) was used to determine trace elementconcentrations and, at 100x the detection limit, accuracy isbetter than 5 %. Several standards, such as MRG1, W2,

DNC1, STM1 and SY3, were run to check accuracy andprecision. All the chemical analyses are presented in Table 1.

Geochemistry

Following the TAS (total alkali-silica diagram; after Irvine andBaragar 1971; Le Maitre 2002), the studied rocks classify assub-alkaline basalt (Fig. 4a). Few samples (EDD09/1,EDD09/4 and DC12/4) have relatively higher contents ofMgO than the normal basalt derivatives; therefore, they arealso plotted on extended TAS diagram. It is recommended thatan igneous rock having MgO >12 %, SiO2 between 30 and52 % and Na2O+K2O <3 % may be classified as picrite(Fig. 4b; Le Bas 2000; Le Maitre 2002). Accordingly twosamples (EDD09/1 and EDD09/4) of present study are clearlyclassified as picrites. One sample (JEF-99–11) studied byFrench and Heaman (2010) also shows similar nature. It isimportant to note that these samples contain appreciableamount of olivine, showing a probable primitive nature; Mgnumbers (Mg#) vary between 76.01 and 68.07. On the otherhand, two other samples [one from the present study (DC08/4)and other from the French and Heaman (2010) (JEF-00–55)]

(e)

(d)

(b)

(c)

(f)

(a)Fig. 3 Photomicrograph ofstudied dolerite and olivinedolerite samples. Mostmicrophotographs prominentlyshow ophitic texture; sub-ophitictexture is also noticed (Fig. d).Twinning is observed in a numberof augite grains (Fig. c). Figure dalso shows saussurization offeldspar grains. Olivine grains areanhedral rounded/subrounded toeuhedral in shape and showserpentinization along the cracks(Fig. e and f). Width of eachphotograph is equivalent to 4 mm

Petrogenesis of ~2.21–2.22 Ga Kunigal mafic dyke swarm 699

have very close geochemical characteristics; both plottingtogether and having higher iron and alkali contents than theother samples. Probably these two are most differentiated

samples as also reflected from their lower Mg#, i.e. 37.08and 41.42, respectively. On Jensen’s cation plot (Fig. 4c;Jensen 1976), which is successfully used for igneous rocks

Table 1 Whole rock major oxides (wt%), trace and rare-earth element compositions of 2.21–2.22 Ga mafic dyke swarm from the eastern Dharwarcraton, India

1 2 3 4 5 6 7 8 9DC08/4 DC08/10 DC08/12 EDD09/1 EDD09/4 DC12/1 DC12/4 JEF-99–11 JEF-00–55

SiO2 49.53 51.14 50.49 50.72 50.89 51.49 51.09 49.42 49.01

TiO2 1.29 0.77 0.66 0.35 0.48 0.68 0.58 0.51 1.60

Al2O3 13.13 14.41 14.85 13.10 13.01 16.50 13.77 9.79 13.82

Fe2O3 17.51 13.40 12.29 10.38 11.67 11.70 11.90 11.44 14.93

MnO 0.21 0.20 0.18 0.16 0.18 0.17 0.18 0.18 0.21

MgO 5.21 6.83 6.79 13.95 12.56 6.22 10.29 15.51 5.33

CaO 8.91 10.55 10.87 10.01 10.24 11.36 10.69 9.43 9.32

Na2O 2.43 1.89 1.92 1.32 1.46 2.09 1.54 1.08 2.39

K2O 0.83 0.44 0.26 0.24 0.37 0.39 0.42 0.45 0.90

P2O5 0.18 0.08 0.05 0.04 0.07 0.06 0.06 0.06 0.23

LOI 0.21 0.62 0.20 0.00 0.00 0.26 0.40 1.23 0.86

Total 99.44 100.33 98.56 100.27 100.93 100.92 100.92 99.10 98.60

Mg#* 37.08 50.23 52.25 72.69 68.07 51.29 63.13 76.01 41.42

Cr 80 180 160 1,170 890 160 640 1,220 48

Ni 80 110 110 330 300 120 230 298 47

V 386 271 247 192 203 245 220 187 337

Rb 49 18 7 7 12 14 14 25 20

Ba 208 73 67 75 97 89 111 66 342

Sr 176 137 139 92 98 156 103 88 229

Nb 4 2 2 1 2 1 1 1.9 4.9

Ta 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.4 0.6

Zr 96 52 43 27 44 47 49 40 96

Hf 2.7 1.4 1.1 0.8 1.3 1.2 1.1 1.0 2.4

Y 27 15 14 10 12 13 15 11 24

Ga 19 17 16 11 11 15 12 8 14

Th 2.4 1.4 1.1 0.9 1.5 1.2 1.8 1.3 1.7

U 0.7 0.4 0.3 0.2 0.3 0.3 0.4 0.4 0.4

La 12.00 8.10 6.40 4.10 7.00 5.50 7.40 4.42 11.80

Ce 25.90 16.20 13.00 8.50 14.50 11.80 15.30 9.02 25.00

Pr 3.31 2.08 1.69 0.99 1.64 1.39 1.80 1.09 3.12

Nd 14.10 8.20 6.80 4.30 6.80 6.30 7.30 4.79 14.00

Sm 3.70 2.30 1.90 1.10 1.60 1.90 1.90 1.27 3.55

Eu 1.16 0.76 0.68 0.38 0.55 0.69 0.56 0.41 1.19

Gd 4.40 2.70 2.40 1.30 2.00 2.20 2.20 1.49 3.79

Tb 0.80 0.50 0.40 0.20 0.40 0.40 0.40 0.27 0.65

Dy 5.20 3.00 2.60 1.60 2.40 2.50 2.50 1.67 3.90

Ho 1.10 0.60 0.50 0.30 0.50 0.50 0.50 0.35 0.78

Er 3.20 1.80 1.60 1.00 1.40 1.40 1.50 1.07 2.26

Tm 0.50 0.27 0.24 0.15 0.22 0.23 0.24 0.16 0.33

Yb 3.40 1.70 1.50 1.00 1.50 1.50 1.70 0.99 2.02

Lu 0.53 0.27 0.24 0.17 0.24 0.25 0.26 0.15 0.31

Sample numbers JEF-99–11 and JEF-00–55 are from French and Heaman (2010); *Mg number (Mg#) calculated by SINCLAS computer program(Verma et al. 2002)

700 R.K. Srivastava et al.

that underwent post-magmatic processes like hydrothermalalterations and low-grade metamorphism, two most differen-tiated samples show high-iron tholeiitic nature, whereas twomost primitive samples show high-Mg derivatives (basaltickomatiite). Other samples show high-magnesian tholeiiticnature.

A number of variation diagrams are examined to evaluatecrystallization behavior and co-genetic nature of the studiedsamples. Major oxide plotted against MgO show very limitedvariation; most sample show good fractionated trends sug-gesting their co-genetic nature (Fig. 5). All the plotted majoroxides increase with differentiation (i.e. with decreasingMgO). Two samples (DC08/4 and JEF-00–55) show slightlydifferent nature on MgO-SiO2 and MgO-Al2O3 plots. Thismight be due to some post-magmatic modifications. Few traceelements, belonging to transition elements and large-ionlithophile elements (LILEs) groups, also suggest co-geneticnature of the studied samples (Fig. 6). Except Cr and Ni(compatible elements), all the other elements show negativecorrelation with MgO, i.e. these trace elements increase withdifferentiation. It is important to note that LILEs also showgood crystallization trends. LILEs are generally mobile duringthe post-magmatic processes like late hydrothermal alterationsand metamorphism and show wide range of their concentra-tion; also difficult to observe any good trend on variationplots. However in the present case they show very limitedscatter and indicate absence of any significant post-magmaticalteration.

As some minor effect of alteration evident in petrographicdata, it is essential to evaluate any effect due to this alterationon geochemistry. High-field strength elements (HFSEs),which also include rare-earth elements (REEs), are thoughtto be relatively immobile under medium grade metamorphicconditions, hydrothermal alteration, and sea-floor alteration(Pearce and Cann 1971, 1973; Winchester and Floyd 1976,1977; Floyd and Winchester 1978; Verma 1992; Jochum andVerma 1996). HFSEs (Ti, P, Zr, Y, Ce and Nd), plotted againstMgO, show obvious crystallization trends and suggestingtheir derivation from a single magma batch (Fig. 7). Anotherimportant point noticed on these plots (Figs. 6 and 7) is thattwo samples (DC08/4 and JEF-00–55), which are supposed tobe most differentiated, show higher concentration of traceelements than the other samples. Increase of these trace ele-ments during differentiation also suggests crystallization ofminor minerals like opaques, zircon/baddeleyite and apatite atthe later stages.

Further, geochemical nature of the studied samples is eval-uated on the primordial mantle normalized multi-element(ME) spidergrams and chondrite normalized rare-earth ele-ment (REE) patterns (Fig. 8). All the plotted elements showenrichment with respect to the primordial mantle and chon-drite. Two differentiated samples show higher concentrationof all the trace elements than the other samples. ME

35 45 55 65 750

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High-Fetholeiite

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Fig. 4 (a) Total-alkali silica (TAS) classification diagram (after LeMaitre2002); dotted line discriminates between sub-alkaline basalts and alkalinebasalts (after Irvine and Baragar 1971). (b) TAS classification (the shadedportion in Fig. 4a) for high-Mg mafic rocks (Le Bas 2000; Le Maitre2002). (c) Jensen’s cation plot (Jensen 1976). Symbols: ♦ dyke samples ofpresent study; Δ and □ two samples (JEF-99–11 and JEF-00–55) aretaken from French and Heaman (2010)

Petrogenesis of ~2.21–2.22 Ga Kunigal mafic dyke swarm 701

spidergrams show prominent negative Nb-Ta anomalies.Comparatively small negative anomaly is noticed for Baand Ti. Sr shows mixed nature, some samples show slightlynegative anomalies and others show slightly positive anom-alies; this is probably due to slight fractionation of plagio-clase during course of crystallization and slight accumula-tion of plagioclase at later stages, respectively. It is difficultto observe any significant anomaly for Hf-Zr. REE patternsalso corroborate co-genetic nature of the studied samples;all show similar patterns. Slight enrichment in light-REE(LREE) is noticed; however, middle-and heavy-REE(MREE and HREE) show almost flat patterns. The sample(JEF-00–55) studied by French and Heaman (2010) alsoshows similar ME and LREE patterns, however slight dif-ference in HREE is noticed. It is rather difficult to explainthis without any further geochemical data, particularly ra-diogenic isotope data, however, probably it indicate somelater modification.

Discussion

A number of features observed in the studied N-S to NNW-SSE mafic dyke samples during the present study, i.e. (i) thepetrography classify them as dolerite/olivine dolerite, (ii)geochemical features suggest their nature as sub-alkalinebasalt (high-iron tholeiites, high-magnesian tholeiites andpicrites), (iii) major and trace element data suggest theirgenetic relationship with each other, and (iv) they show

variation from primitive nature (Mg#>~ 76) to evolvedtype (low Mg#), which probably suggest their derivationfrom a single melt. Available geochronological data suggestthat these mafic dykes were emplaced ~2.21 Ga and have anacross-strike span of at least 200 km. Before discussingabout its petrogenetic aspects, it is worth to present anyrole of contamination, particularly crustal, during emplace-ment of this melt.

Crustal contamination

From the geochemistry, it is observed that studied dyke sam-ples, except two evolved samples, have very close geochem-ical similarities and show low-to moderate-variations. Theyhave almost parallel ME and REE patterns suggesting theirco-genetic nature (Fig. 8). Behavior of Ta-Nb and Hf-Zr aresuccessfully used to identify involvement of crustal contami-nation. The ME spidergrams (Fig. 8a) of the studied maficdykes show prominent negative Ta-Nb anomaly but no sig-nificant anomaly noticed for Hf-Zr. Negative Ta-Nb anomalyindicate possibility of crustal contamination, howevercrustally contaminated samples must also show negative Hf-Zr anomaly, which is absent in these samples (Zhao and Zhou2007; Cai et al. 2010). Additionally, crustally contaminatedsamples normally show prominent enrichment in LREE withflat HREE patterns. Although, studied samples have slightenrichment in LREE but it is not adequate to confirm crustalcontamination (Fig. 8b). Therefore, observed geochemical

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702 R.K. Srivastava et al.

characteristics of studied dykes are not straightforward tosuggest involvement of crustal contamination during theiremplacement.

A combination of compatible and incompatible traceelements and their ratios are used to perceive any role ofcrustal contamination (Rollinson 1993). It is important tonote that both LILEs (Ba, Rb, Sr and K) and HFSEs (Ti, P,Zr, Y, Ce and Nd) show noteworthy trends on variationplots (Figs. 6 and 7) suggesting absence of any seriousalteration. Few trace element ratios like Ti/Y, Ba/Nb, Ba/Zr, Zr/Y and Nb/Y ratios also indicate similar results(Fig. 9). All the studied mafic dyke samples on Ti/Y, Ba/Nb and Ba/Zr ratio diagram plot close to the mantle derivedderivatives like N-MORB, E-MORB and primordial mantleand away from the average lower crust (Fig. 9a and b).Similar results are observed on Zr/Y and Nb/Y ratio plot;studied samples show close geochemical similarity with N-MORB and primordial mantle and there is no indication ofany average continental crust contribution (Fig. 9c). Thesegeochemical features obviously suggest absence of any

serious role of crustal contamination in these samples. Itis also known that normally contamination process occursmore at the margins of the dykes, however central part ofintrusion is suppose to be unaffected (Gill and Bridgwater1979; Kalsbeek and Taylor 1986). Further, Tarney andWeaver (1987) suggested that the degree of crustal contam-ination in dykes is absolutely less, and sometimes evenabsent, in comparison to volcanic rocks. Therefore, it maybe concluded that studied mafic dyke samples have veryless possibility of crustal contamination and probably de-rived from an uncontaminated mantle melt.

Petrogenesis

The rare-earth element patterns are very useful to under-stand melting conditions and nature of mantle composi-tions; at several instances degree of melting of a sourcemay be reflected in REE patterns. Frequently REE patternsrepresent the original composition of the unaltered parental

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Petrogenesis of ~2.21–2.22 Ga Kunigal mafic dyke swarm 703

magma (Cullers and Graf 1984). It is established that sub-alkaline tholeiitic magma may be generated by 15–30 %melting of upper mantle peridotite, whereas any meltingless than 10 % of peridotitic mantle yield alkali-rich basalticmelt (Green and Ringwood 1968; Green 1973; Hirschmannet al. 1998). Melts generated at low degree of melting showan enriched LREE pattern, whereas LREE concentrationdecreases rapidly with increased melting up to 30 %; be-yond this there is no significant change in the REE concen-tration (Cullers and Graf 1984; Hirschmann et al. 1998).REE patterns of the studied samples (Fig. 8b) show verysmall LREE enrichment which probably suggests nearly15–20 % melting of a mantle source.

A petrogenetic model based on Ni–Zr concentrations dem-onstrates that different extents of melting of the same sourcemay produce different compositions and should have a similartrend to melting curves (Fig. 10a; Rajamani et al. 1985). Thevalues for the mantle source (7.8 ppm Zr and 2,000 ppm Ni)are taken from Taylor and McLennan (1981) (source mode:55 % ol, 25 % opx and 20 % cpx; melting mode: 20 % ol,

25 % opx, 55 % cpx). Batch melting curves I and II, presentedin Fig. 10a, are calculated at 1,850 °C, 50 kb and 1,575 °C,25 kb, respectively. Other two curves III and IV are olivinefractionation trends at one atmosphere with ticks, markingincrements of 5 % olivine fraction from previous tick. Thestudied samples plot near the batch melting curve II andfollow the fractionation trend IV. This suggests that thesesamples are derived from a melt generated by ~20 % batchmelting of a depleted mantle at approximately 1,575 °C and25 kb and about 20–30 % olivine fractionation. This petroge-netic model is slightly modified by Condie et al. (1987). Theyconsidered lherzolite mantle with 2,000 ppm Ni and 11 ppmZr composition as a source and batch melting curve calculatedat 1,500 °C (1 atm equivalent) (Fig. 10b). Again, almost,similar picture is noticed for the present samples. This clearlysuggests that these dykes are fed from a sub-alkaline basalticmagma generated by 20–25 % batch melting of a depletedlherzolite mantle source and about 15–30 % olivine fraction-ation. In both the diagrams, one most differentiated sample(DC08/4) show highest olivine fractionation.

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in wt% and other elements inppm) variation diagrams for thestudied mafic dyke samples

704 R.K. Srivastava et al.

Large igneous province

Proterozoic mafic dyke swarms form an excellent tool toidentify the Large Igneous Province (LIP) records and theseare a key factor for establishing any meaningfulpaleocontinental reconstructions (Bleeker 2003, 2004; Ernstet al. 2005; Ernst 2007; Ernst and Bleeker 2010; Srivastavaet al. 2010). The LIP may be defined as a magmatic provincewith an aerial extents >1 million km2, igneous volumes>0.1million km3 and maximum life-spans of ~50 Ma. Moreover, aLIP has an intraplate tectonic settings or geochemical affini-ties, and it is characterized by igneous pulse (s) of shortduration (~1–5 Ma), during which a large proportion(>75 %) of the total igneous volume has been emplaced(Bryan and Ernst 2008). Recently, on the basis of U-Pbgeochronology, a number of LIP records were identified inthe southern Indian Shield (Halls et al. 2007; French et al.2008; Ernst and Srivastava 2008; French and Heaman 2010;Srivastava et al. 2011); this includes ~2.37 Ga Bangalore LIP,~2.18 Ga Mahbubnagar (North Dharwar) LIP, ~2.21 Ga Pan-Dharwar LIP and ~1.89 Ga Bastar-Dharwar LIP. Here it is

important to mention that Pan-Dharwar LIP, recognized byFrench and Heaman (2010), includes two major dyke swarmsi.e. at ~2.18 Ga and ~2.21 Ga. These authors interpreted theseas two real age groupings and indicate two closely spaced yetdistinct periods of short-lived (∼5–10 m.y.) dyke

eltnaMlaidro

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Petrogenesis of ~2.21–2.22 Ga Kunigal mafic dyke swarm 705

emplacement or it could be part of a protracted (∼40 m.y.)period of 2.2 Ga Pan-Dharwar dyking.

The name Pan-Dharwar LIP is based on Pan-African over-prints in the Dharwar craton which is recognized on the basisof U-Pb zircon and baddeleyite lower intercepts on concordiadiagrams from the three diabase dykes, one emplaced in thewestern Dharwar craton and other two in the eastern Dharwarcraton, suggesting they were influenced by an∼500 Ma Pan-African overprint (French and Heaman 2010). Such Pan-African thermal imprint is also documented by U-Pb zirconconcordia of greenstone felsic volcanics and granitoid plutonsof EDC (Jayananda et al. 2013a). There is a small confusion innaming dyke swarms of this event. If we consider Pan-Dharwar LIP as a protracted (∼40 m.y.) period of 2.2 Ga

Pan-Dharwar dyking, its recognition as Pan-Dharwar isvery appropriate. However, it surely contains two closelyspaced short-lived (∼5–10 m.y.) dyke emplacements andnaming one of them again as ~2.2 Ga Pan-Dharwar dykeswarm is not fruitful; it should be named with somedifferent name. On the basis of samples dated andconcentration of dyke emplacements, French et al. (2004)and Ernst and Srivastava (2008) have recognized ~2.18 Gaswarm as Mahabubnagar swarm. Later, French andHeaman (2010) recognized this swarm as NorthernDharwar dyke swarm. However, as most of the dykeswarms of southern region are spread all over theDharwar craton, it is better to name them with some localpopular names. Therefore, we prefer to name this ~2.18 Gaswarm as Mahbubnagar swarm. For other swarm, i.e.~2.21 Ga swarm mostly trend in N-S to NNW-SSE, weprefer to name this as Kunigal swarm. This way there willbe no confusion in recognizing the two distinct swarms ofclosely spaced short-lived swarms, i.e. Mahbubnagarswarm for ~2.18 Ga dykes and Kunigal swarm for~2.21 Ga dykes.

The ~2.21–2.22 Ga N-S to NNW-SSE trending Kunigaldyke swarm has an across-strike span of at least 250 km (seeFig. 2). Barcode based on available precise U-Pb agesmatching to present dated dykes from the Dharwar cratonand other parts of the world are presented in Fig. 11 for agecomparison. Four precise ID-TIMS U-Pb baddeleyite ages areavailable for the Kunigal dyke swarm. French and Heaman(2010) have dated two mafic dykes from the southeasternquadrants of the eastern Dharwar craton; one is N-S trendingKondlamadugu dyke (2,220.5±4.9 Ma) and another is NNW-SSE (N330°) trending Somala dyke (2,209.3±2.8 Ma).Srivastava et al. (2011) have also dated two samples; one froman extremely long semi-continuous curvilinear dyke whosestrike length possibly exceeds 250 km, trending N-S in thesouth to NNW-SSE in the north (N345º) and another from ashorter N-S dyke segment near the southern extension of theClosepet batholiths (see Fig. 2 for exact locations), whichprovide ages of 2,215.2±2.0 Ma and 2,211.7±0.9 Ma, respec-tively. Both ages reflect the timing of igneous emplacement.Recently, Kumar et al. (2012b) have presented two wholerock-mineral Sm–Nd isochrons for the long N-S dyke at2,173±43 Ma and 2,190±51 Ma (2σ) respectively. Theseages are slightly younger to the U-Pb ages obtained for thisswarm.

French and Heaman (2010) have also provided two moreU-Pb ages for mafic dykes of EDC which show younger agesthan the Kunigal dyke swarm. One of these is WNW-ESEtrending Bandepalem dyke (2,176.5±3.7 Ma) and other isNW-SE trending Dandeli dyke (in western Dharwar craton)(2,180.8±0.9 Ma), identified as Northern Dharwar dykeswarm. On the basis of four ages presented, two for N-S toNNW-SSE dyke swarm and other two for WNW-WSE to

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Fig. 10 Trace elements modeling based on Ni vs. Zr. (a) after Rajamaniet al. (1985). I and II: calculated batch melting curves at 1,850 °C, 50 kband 1,575 °C, 25 kb, respectively, and are marked with percentage ofmelting. III and IV: olivine fractionation trends at one atmosphere withticks, marking increments of 5 % olivine fraction from previous tick.Values for the mantle source (7.8 ppm Zr and 2,000 ppm Ni) are takenfrom Taylor and McLennan (1981) (source mode: 55% ol, 25 % opx and20 % cpx; melting mode: 20 % ol, 25 % opx, 55 % cpx). (b) after Condieet al. (1987). 1. Batch melting curve at 1,500 8C (1 atm equivalent) withdegrees of melting noted in percent. Melting relation of assumedlherzolite mantle source (11 ppm Zr and 2,000 ppm Ni) is as given byRajamani et al. (1985). 2, 3, and 4: olivine fractionation curves withpercent of olivine removal noted in 5 % increments

706 R.K. Srivastava et al.

NW-SE dyke swarm, French and Heaman (2010) have divid-ed them into two age groups at 2,221–2,209 and 2,181–2,177 Ma and suggested that these two age groupings are realand indicate two closely spaced yet distinct periods of short-lived dyke emplacement. It is also important to note that boththe groups have distinct geochemical characteristics and trendin different directions (French and Heaman 2010). On thesedistinct features, we are also in opinion that both the swarmshave their separate identity; therefore, only ages close to~2.21 Ga are taken in consideration for further discussion.

Geodynamic implications

Mafic magmatic event at ~2.21 Ga is well described for thedifferent early Precambrian cratons of the Canadian Shield; inparticular, Superior Province has experienced ~2.21 Ga maficmagmatism as dykes (Senneterre dyke swarm; Buchan et al.1993, 1996, 1998) and sills (Nipssing sill complexes; Corfuand Andrews 1986; Noble and Lightfoot 1992). TheSenneterre dyke swarm has been emplaced at 2,214–2,216 Ma within the southern Superior Province, whereas

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2220.5±4.9 Ma

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Mackay dyke swarm

Southern WestGreenland Md2 Dyke

2209.2±4.6 Ma

Fig. 11 U–Pb age correlationbarcode records with their errorbars for the worldwidedistribution of 2,209–2,220 Mamafic magmatic events of similarage to those preserved in theeastern Dharwar craton. Source:DC08/12 and EDD09/4(Srivastava et al. 2011); JEF-00–55 and JEF-99–11 (French andHeaman 2010); Senneterre swarm(Buchan et al. 1998); Nipissingsills (Gowganda Castle Mine,Kerns and Triangle Mountain)(Corfu and Andrews 1986; Nobleand Lightfoot 1992); the Klotzdyke swarm (Buchan et al. 1998);southern West Greenland BN1dyke (Nutman et al. 1995);southern West Greenland MD2dyke (Nilsson et al. 2013);Mackay dyke swarm (Ernst andBuchan 2001b)

Petrogenesis of ~2.21–2.22 Ga Kunigal mafic dyke swarm 707

the age of the Klotz dyke swarm, emplaced in the northernSuperior Province, is 2,209±1.0 Ma (Buchan et al. 1998).Besides dyke swarms, many sill complexes, known asNipissing sills and exposed within the southern SuperiorProvince, have also similar ages. These mafic sills exposedat Gowganda Castle Mine, Kerns and Triangle Mountain aredated and placed at 2,219.4±3.5, 2,217.2±4 and 2,209.6±3.5 Ma, respectively (Corfu and Andrews 1986; Noble andLightfoot 1992). A few smaller dyke swarms including Anuc(2,220±1 Ma) and Kogaluk bay (2,212±3 Ma) are also re-ported from the Superior province (Maurice et al. 2009). TheNorth Atlantic craton (NAC) also experienced ~2.21 Ga maficmagmatic event. Nutman et al. (1995) have reported 2,214±10 Ma age for the N-S trending BN1 dyke swarm exposedin the southern West Greenland. Recently, Nilsson et al.(2013) have dated a NE-SW trending dyke from the MD2swarm exposed in southern West Greenland and placed it at2,209±5 Ma. A similar mafic magmatic event is also re-corded in the Slave craton (Canadian Shield); the E-Wtrending Mackay dyke swarm has been dated 2,210 Ma(Ernst and Buchan 2001b). Besides these well-constrainedages, there are also reports of similar ages from theFennoscandian Shield, Finland, e.g. 2.21 Ga Karjalite dykeswarm (Vuollo et al. 1995) and Pilbara craton, westernAustralia, e.g. 2.21 Ga Hamersley dykes (Martin et al.1998; Mueller et al. 2004). The barcode records based onU-Pb ages of ~2.21 Ga mafic magmatic events are present-ed in Fig. 11. These ages clearly indicate a large igneousprovince with a maximum life-span of ~25 Ma with 3 to 4igneous pulses of short duration (~5 Ma).

The global ~2.21–2.22 Ga mafic magmatic event readilysupport breakup of a late Archean supercontinent or multiplecontinents, also termed supercratons (Bleeker 2003; Ernst andBleeker 2010). It is suggested that a number of ancient cratonswere together at the end of Archean and across the Archean-Proterozoic boundary as a supercraton (Bleeker 2003). Mostof these Archean cratons display rifted margins of Proterozoicage and well-marked by major mafic dyke swarms, and byinference, LIPs (Bleeker and Ernst 2006; Bryan and Ernst2008; Ernst and Srivastava 2008; Srivastava et al. 2010;Ernst and Bleeker 2010). Although, there are reliable radio-metric ages are available for the ~2.21 Ga dykes and sills, it isdifficult to establish any straightforward paleocontinentalreconstruction; however French and Heaman (2010) havetried to establish a correlation between the Dharwar, Slaveand Superior cratons. They suggested, particularly for theDharwar and Slave cratons, that these were part of thesuprcraton Sclavia (it is a late Archean supercraton thoughtto be parental to the Slave craton) which experienced riftingand breakup at∼2.21 Ga. Later, Kumar et al. (2012b) sug-gested that Dharwar, Slave, Superior and Rae cratons werejuxtaposed during Paleoproterozoic but, they mentioned, dueto lack of reliable Neoarchean paleomagnetic data from the

Dharwar, tracing its ancestry to either of the NeoarcheanSupercratons: Sclavia or Superia is unclear.

Recently, Nilsson et al. (2013) have correlated NE-SWtrending 2,209±5 Ma old tholeiitic (MD2) and N-S trending2,214±10Ma old boninitic–noritic (BN1) mafic dyke swarmsof the NAC (from the southern West Greenland) with maficdykes of similar ages and compositions from the Dharwarcraton and supposed whether the pair of ca. 2.2 Ga swarmscan be linked in a reconstruction. Bleeker (2003) found sim-ilarities between the Slave and Dharwar cratons, together withWyoming and Zimbabwe cratons, and suggested that thesemay all be part of the Sclavia supercraton. He further men-tioned that Sclavia stabilized ca. 2.58 Ga or shortly thereafter,well after stabilization of the another supercraton: Superia.The Superia is supposed to consist of Superior, NAC andSlave cratons as they share similar magmatic events(Bleeker 2003; Ernst and Bleeker 2010). Available U-Pb ageson the mafic dykes of the EDC probably indicate that it wasalso part of the supercraton Superia. U–Pb age correlationbarcode records of the N-S to NNW-SSE trending maficdykes of the EDC that are well comparable with mafic dykesof similar ages from the Superior, NAC and Slave cratons (seeFig. 11). However, due to lack of robust ages of dykes fromthe Slave craton, it is difficult to establish any definite time-space relationship with the Dharwar craton. More geochrono-logical ages of this global magmatism is needed for anymeaningful paleocontinental reconstruction.

Conclusions

& A N-S to NNW-SSE trending mafic dyke swarm,emplaced at ~2.21 Ga, is distributed throughout the east-ern Dharwar craton and identified as Kunigal mafic dykeswarm. From mineralogy and petrography, the dyke sam-ples can be classified as dolerite or olivine dolerite.Geochemically, they span from high-iron tholeiites tohigh-magnesian tholeiites and to picrites. Geochemicalcharacteristics suggest their co-genetic nature with a var-iation from primitive nature to evolved nature.

& Nb-Zr contents of these dykes suggest that they are de-rived from a sub-alkaline basaltic magma generated by20–25 % batch melting of a depleted lherzolite mantlesource and about 15–30 % olivine fractionation.

& Available precise U-Pb dates for the eastern Dharwarcraton provide evidence that the N-S trending dyke swarmhas an across-strike span of at least 200 km. An extensivealong-strike length for the swarm is also suggested.

& The barcode records based on U-Pb ages of ~2.21 Gaglobal mafic magmatic events clearly indicate existenceof a large igneous province with a maximum life-spanof ~25 Ma with 3 to 4 igneous pulses of short duration(~5 Ma).

708 R.K. Srivastava et al.

& A significant correlation of U–Pb age barcode records (ofthe N-S to NNW-SSE trending mafic dykes of the EDCwith mafic dykes of similar ages from the Superior, NACand Slave cratons) is established. This indicates that theEDCwas part of the supercraton Superia together with theSuperior, NAC and Slave cratons as they share similarmagmatic events.

Acknowledgements The Ministry of Earth Sciences (MoES), Govern-ment of India, New Delhi is thankfully acknowledged for the financialsupport through a Research Scheme MoES/16/10/11-RDEAS. Authorsare also thankful to the Head of the Department of Geology, BanarasHindu University, for extending all necessary facilities during this work.Authors are thankful to Piero Comin Chiaramonti and one anonymousreviewer for their constructive comments, which improved the MSconsiderably.

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