Earth and Planetary Science Letters 277 (2009) 101–111

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Earth and Planetary Science Letters

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Deccan plume, lithosphere rifting, and volcanism in Kutch, India

Gautam Sen a,⁎, Michael Bizimis b, Reshmi Das b, Dalim K. Paul c, Arijit Ray c, Sanjib Biswas c

a Florida International University, Miami, FL 33199, USAb Department of Geological Sciences, University of South Carolina, Columbia, SC 29208, USAc Presidency College, Kolkata, India

Kutch

⁎ Corresponding author.E-mail address: seng@fiu.edu (G. Sen).

0012-821X/$ – see front matter © 2008 Elsevier B.V. Aldoi:10.1016/j.epsl.2008.10.002

a b s t r a c t

a r t i c l e i n f o

Article history:

Kutch (northwest India) exp Received 28 June 2008Received in revised form 6 October 2008Accepted 7 October 2008Available online 20 November 2008

Editor: R.W. Carlson

Keywords:Deccan Trapsmantle xenolithsplumevolcanismriftinglithosphere

erienced lithospheric thinning due to rifting and tholeiitic and alkalic volcanismrelated to the Deccan Traps K/T boundary event. Alkalic lavas, containing mantle xenoliths, form plug-likebodies that are aligned along broadly east–west rift faults. The mantle xenoliths are dominantly spinelwehrlite with fewer spinel lherzolite. Wehrlites are inferred to have formed by reaction between transientcarbonatite melts and lherzolite forming the lithosphere. The alkalic lavas are primitive (Mg#=64–72)relative to the tholeiites (Mg#=38–54), and are enriched in incompatible trace elements. Isotope and traceelement compositions of the tholeiites are similar to what are believed to be the crustally contaminatedDeccan tholeiites from elsewhere in India. In terms of Hf, Nd, Sr, and Pb isotope ratios, all except two alkalicbasalts plot in a tight cluster that largely overlap the Indian Ridge basalts and only slightly overlap the field ofReunion lavas. This suggests that the alkalic magmas came largely from the asthenosphere mixed withReunion-like source that welled up beneath the rifted lithosphere. The two alkalic outliers have an affinitytoward Group I kimberlites and may have come from an old enriched (metasomatized) asthenosphere. Wepresent a newmodel for the metasomatism and rifting of the Kutch lithosphere, and magma generation froma CO2-rich lherzolite mantle. In this model the earliest melts are carbonatite, which locally metasomatizedthe lithosphere. Further partial melting of CO2-rich lherzolite at about 2–2.5 GPa from a mixed source ofasthenosphere and Reunion-like plume material produced the alkalic melts. Such melts ascended along deeplithospheric rift faults, while devolatilizing and exploding their way up through the lithosphere. Tholeiitesmay have been generated from the main plume head further south of Kutch.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

Continental flood basalts represent significant accumulations ofpredominantly basaltic lava. The origin of such voluminous magmas iscommonly explained by one of two models: partial melting of deepmantle plumes (e.g., White and McKenzie, 1989; Richards et al., 1989;Duncan and Richards, 1991) or decompression melting of the shallownon-plume mantle, which could be the asthenosphere (e.g., King andAnderson, 1995; Sheth, 2005) or the sub-continental lithosphere (e.g.,Turner et al., 1996; Hawkesworth et al., 2000). Richards et al. (1989)proposed a “plume heads and tails” model in which an individualplume develops a large “head” by entraining shallower mantlematerials and a long and slender “tail” that anchors it to the core–mantle boundary. As the plume head rises to the base of thelithosphere, it melts to produce flood basalt magma; and the plumetail often produces a linear volcanic chain. Some authors alsosuggested that flood basalt volcanism caused the separation ofcontinental plates (White and McKenzie, 1989).

l rights reserved.

The Deccan Traps flood basalt province, which forms a thickblanket over western and central India, is often cited as an importantexample of the “plume heads and tails” model. The plume hypothesissuggests that the Deccan formed as India passed over a large plumehead, which was located where the Reunion hot spot is presentlysituated in the Indian Ocean (e.g., Mahoney et al., 2002). Theoccurrence of 72–73 Ma Reunion island-like alkalic lavas in Pakistan(Mahoney et al., 2002), 68.5Ma old alkalic intrusions in northern India(Basu et al., 1993), and younger (66–62 Ma) basalts further south (e.g.,Baksi, 1987; Widdowson et al., 2000; Sen, 2001, Sheth et al., 2001;Chenet et al., 2007) suggests that the igneous activity started inPakistan about 72–73 m.y. ago and then rapidly progressed along asoutherly path, with perhaps 95% of the lavas erupting in western andcentral India 65 (±1) m.y. ago.

The Deccan Trap lava province is predominantly composed oftholeiitic basalts and minor volumes of alkalic basalts and carbonatiticlavas and intrusions exposed mostly along rift zones. Our study area,Kutch, is located in the northwestern state of Gujarat and is an ancientrift basin thatwas active prior to and during theDeccan volcanic episode(Biswas 1987). Both alkalic and tholeiitic intrusions and lavas occur here,as do raremantle-derived xenoliths (e.g., De,1964, Krishnamurthy et al.,1989, Guha et al., 2005). As far as we are aware, there has not been any

Fig. 1. Simplified geological map of Kutch (bottom; Biswas 2005). Our area of study isaround Bhuj. There are several major roughly E–W faults (KHF: Kutch Highland fault,KMF— Kutch Mainland fault, NPF— Nagar Parker fault, IPF— Island Belt fault) that sliceup the geology. The xenolith-bearing alkalic bodies occur as plugs inside shallowmarineMesozoic sediments at Bhuj and along aWNW trending belt south of the KMF. The insetshows the location of Bhuj in India.

102 G. Sen et al. / Earth and Planetary Science Letters 277 (2009) 101–111

published information on the tholeiitic intrusions. An early jointexpedition by Indian and French scientists concluded that nine lavaflows occur in the Anjar area of Kutch (Courtillot et al., 2000). Thereoccurs a thin Iridium-rich sedimentary layer sandwichedwithin the lavapackage. According to Shukla et al. (2001) and Courtillot et al. (2000), allof the lavas are alkalic except for theuppermost lava (FlowF9),which is atholeiite. In a more recent field expedition by the Geological Survey ofIndia, Guha et al. (2005) found six tholeiite basalt flows forming anelongated blanket over theMesozoic sediments in KutchMainland; andonly at two places tholeiite flows were found to overlie alkalic flows.Later fieldwork by Paul et al. (2008) found six tholeiite lava flowscapping the alkalic flows. Aside from alkalic and tholeiitic basalts,carbonatite has also been found to occur asmelt inclusions in a recentlydiscovered lamprophyre body fromnorthernKutch (A. Ray, unpublisheddata).

Several studies have obtained geochronological information onthe Kutch lavas. Pande et al. (1989) obtained 40Ar/39Ar plateau ages of67.3±0.6 to 65±1 Ma on plagioclase separates on the xenolith-bearingalkalic lavas. Venkatesan et al. (as quoted by Shukla et al. (2001)obtainedwhole rock plateau ages from 68.7–65Ma for all flows exceptthe uppermost tholeiite flow F9, for which they obtained 61 Ma.Courtillot et al. (2000) pointed out that F9 date has an unacceptablelevel of uncertainty and is not trustworthy. Noting that the whole rock40Ar/39Ar dates are fraught with problems, these authors performedhigher resolution dating of carefully separated plagioclase crystals.They also carried out a simultaneous magnetic investigation of thesamples. Courtillot et al. (2000) concluded that the three alkalic flowsbelow the Ir-rich layer erupted at ∼66.5–67 Ma within the magneticchron C30N, and two alkalic flows above the Ir-rich layer gave and ageof ∼65 Ma (magnetic chron C29R). It is unfortunate that there are noreliable dates on the tholeiites (as Courtillot et al. pointed out) andtherefore, it is difficult to know how much of a time gap existsbetween tholeiite and alkalic eruption here.

Published chemical and petrological information on Kutch basaltsand xenoliths (e.g., Krishnamurthy et al., 1989; Simonetti et al., 1998;Courtillot et al., 2000; Karmalkar et al., 2000; Karmalkar and Rege,2002) is somewhat limited. Here we present new data on lavas andxenoliths and, in particular, new Nd, Sr, and Hf isotope composition ofthe lavas. Briefly, we find that the Kutch lavas span a large range ofcompositions that have a strong imprint of the non-plume astheno-sphere, the continental crust/lithosphere, and the Reunion plume. Wecombine the geological and isotopic constraints to present a newmodel for Deccan volcanism in Kutch.

2. A brief outline of the geology of Kutch

The geology of Kutch is dominated by an ancient east–west riftsystem that was active since the Late Triassic (Fig. 1; Biswas, 1987).Presently, the rift system shallows eastward with a broad, salt-cappedTertiary–Holocene sedimentary sequence filling the central basin orhalf-graben. Within the salt-filled basin occur “islands” (horsts?) builtof Mesozoic and Tertiary sedimentary rocks intruded by alkalic andlamprophyre bodies. This basin is bounded to the north by a majorfault (the Nagar Parker Fault) that juxtaposes Holocene sedimentsagainst Archean rocks to the north. The southern boundary is definedby another major fault, called the North Kathiawar Fault (Fig. 1;Biswas, 2005). There are also several southerly dipping, broadly east–west oriented normal faults, among which the Kutch Mainland Fault(KMF) is important.

The Kutch basin is asymmetric with maximum subsidence alongthe North Kathiawar Fault in the Gulf region. Seismic evidencesuggests that the thickness of Tertiary and Quarternary sediments,with a P-wave velocity of 2.92 km/s, increase from 0 on the KutchMainland to 6 km beneath themain rift basin (e.g., Chandrasekhar andMishra, 2002; Mandal, 2007). Seismic radial receiver function studiessuggest that the Moho occurs at 39–48 km beneath the Kutch

Mainland and that the crust below 3 km is an interlayered package ofigneous and sedimentary rocks (Mandal, 2006, 2007). However, thelocation of the Moho beneath the rift basin was not determined.

3. Analytical methods

Back-scattered electron imaging and microanalysis of the mineralphases in xenoliths were carried out with a JEOL 8400 electronmicroprobe at the Florida Center for Analytical Electron Microscopy.Analytical conditions were 15 kV accelerating potential, a 1 µm beam,and 10 nA sample current. These yielded reasonable count rates. Atleast 10 spots per grain of each phase in each xenolith were analyzed.All reported data are reproducible within ±2%.

The bulk rock major elements (Supplementary Table 1) wereanalyzed at a commercial analytical facility in National GeophysicalResearch Institute (Hyderabad, India). Trace element concentrations

Fig. 2. (a) Composition of olivines in Kutch wehrlite and lherzolite xenoliths arecompared. Filled circles— this study; and unfilled circles— Krishnamurthy et al. (1989).Also shown is a calculated equilibriummelting residue trend from a hypothetical source(circle with cross). The field for olivine phenocrysts in Deccan picrites from thenorthwest (Source: Krishnamurthy et al., 2000) is shown for comparison purpose. Thisplot suggests that primitive Deccan picrites, considered by many to be the parentalmagma to the tholeiites, may have been derived by about 10% partial melting oflherzolite. (b) Clinopyroxenes in the wehrlite and lherzolite xenoliths from Kutch arecompared (our data). Increased partial melting should result in strong depletion in Na2Owith increasing Mg# (i.e., Mg/Mg+Fe) in the residual clinopyroxenes. The lack of suchcorrelation in Kutch cinopyroxenes suggests that these are not simple products ofpartial fusion, and metasomatic enrichment in Na2O is offered as an explanation. Seetext for further discussion.

103G. Sen et al. / Earth and Planetary Science Letters 277 (2009) 101–111

were determined on a Finnigan Element 1 ICPMS at FSU (e.g. Bizimiset al., 2007). Sample powders were prepared on an agate mortar andpestle on handpicked rock chips. Powders were dissolved with HF–HNO3 mixture and converted to 2% HNO3 at 100 ppm total dissolvedsolids. Indium at 1 ppb concentration was added as an internalstandard. Instrumental drift was corrected by double interpolation, byboth correcting each element for drift against the bracketing externalstandard and by correcting each sample to the internal In spike.Concentrations were determined using the USGS BHVO-1 as externalstandard using the concentrations reported by Willbold and Jochum(2005) and Eggins et al. (1997). The USGS rock standard BCR-1 wasalso prepared alongwith the samples andwas run as an unknown. Thereported concentrations are generally within 5% of the values reportedby Willbold and Jochum (2005) except for Nb, Rb and Pb, which arelower by about 15% in our data. Strontium, Nd and Hf isotopes weredetermined from a single dissolution of handpicked chips. Briefly,sample chips were leached in 6 N HCl for ∼20min and dissolved in 3:1HF:HNO3. Hafnium was separated from the bulk rock following themethod of Munker et al. (2001) and determined with the HOT-SIMStechnique (Salters, 1994) on the VG-ISOLAB at FSU. Strontium and Ndwere separated using conventional cation exchange techniques. Pbisotope compositions were determined on handpicked chips preparedas for the Sr–Ndmethod and separated using a two-columnprocedurefollowing Abouchami et al. (1999). Additional analytical details aregiven in the Table 2 caption.

4. Mantle xenoliths

Mantle xenolith bearing alkalic bodies (sills, dikes, and plugs)intrude Mesozoic sediments in Kutch Mainland and are alignedalong the east–west strike of the paleo-rift faults. The xenoliths(olivine — Fo88–90) are small (b3 cm), rounded, disk shaped, andcommonly exhibit protogranular to roughly equigranular textures.Most (75%) are spinel wehrlites; and the rest are spinel lherzolites(De, 1964, Krishnamurthy et al., 1989, Karmalkar et al., 2000, presentstudy, Supplementary Table 1). Two-pyroxene thermometry usingSen and Jones (1989) thermometer on the spinel lherzolites givesrelatively low temperatures 980°±50 °C, supporting the idea thatthese xenoliths are from the lithosphere (e.g., Karmalkar et al.,2000).

Fig. 2a shows that in terms of Ni vs.Mg# (=Mg/(Mg+Fe⁎) in olivine,the Kutch xenoliths (both wehrlites and lherzolites) plot along a trendthat would be expected of upper mantle residues of partial melting.During partial melting and melt extraction incompatible elements areextracted and refractory elements are left behind in the residue; andtherefore, a negative correlation would be expected when anincompatible element (or oxide, e.g., Na2O) is plotted against ageochemical indicator of the residual extent, such as Mg# (Fig. 2b).The lack of such correlation in Fig. 2b argues against a simple residualhypothesis and calls for a more complicated origin that involves aselective enrichment of these rocks in incompatible elements bymetasomatism.

Karmalkar and Rege (2002) carried out a detailed major and traceelement chemistry of the chrome-diopsides in both lherzolites andwehrlites from the same outcrops and noted that these diopsides areall variably enriched in Zr/Hf, Sr, and LREE. They proposed that thewehrlites are products of carbonatite metasomatism in the litho-sphere formed by a reaction that dissolved away orthopyroxene fromthe lherzolite wall rock:

CaMg CO3ð Þ2Carbonatite

+ 4MgSiO3opx wallð Þ

= 2Mg2SiO4 + CaMgSi2O6 + 2CO2Product minerals þ gas

There are two problems with this hypothesis: (1) The carbonatitemelt inclusions that have been found in Kutch are all calcitic and notdolomitic. (2) Karmalkar and Rege noted that the negative correlation

in Ti/Eu and La/Yb ratio shown by the diopsides is better explained ifthe reaction involved Ca-carbonatite melt rather than dolomiticcarbonatite. Thus, a more appropriate reaction for such metasomaticenrichment would be:

CaCO3Carbonite

+ 3MgSiO3opx wallð Þ

= Mg2SiO4 + CaMgSi2O6 + CO2Product mineralsþ gas

We note that such a reaction has not been verified in laboratoryexperiments but should be expected to occur at pressures of b2 GPa,where a calcic carbonate melt is stable but dolomitic carbonate melt isunstable.

Although the trace element data support the metasomatismhypothesis, the complete lack of any chemical correlation betweenthe lherzolites and wehrlites is still difficult to explain. In view of thevery small size of these xenoliths, it seems reasonable to suggest that

104 G. Sen et al. / Earth and Planetary Science Letters 277 (2009) 101–111

the modes are not meaningful representation of the larger sections ofthe lithosphere where such metasomatic reactions occurred.

Based on standard thermodynamic data tables (Robie et al. 1978)we calculate a molar volume change of −19% vs. −12% cm3 for thesolids in the above two reactions, respectively, as the gas escapes intothe walls. Such a volume change associated with explosive outgassingof CO2 was likely a very efficient way to develop closely spacedfractures within the lithosphere. We speculate that the disk shape andsmall size of the xenoliths are a result of tightly spaced fractures,metasomatism, and rounding off the edges during magmatictransport.

Unfortunately, their extremely small size and impregnation by thehost lava prevented us from analyzing the xenoliths for their isotopecomposition. Mohapatra and Murty (2002) analyzed N, Ne and Arisotope composition of olivine phencrysts in an alkalic basalt andolivine and clinopyroxene separates from five xenoliths from the samelocalities in Kutch. These authors found that the xenolith mineralsform a cluster aroundmixing lines between air andMORB (Mid-OceanRidge Basalt) end members on various isotope ratio plots andconcluded that the xenoliths contain a “MORB type component”.Note that the same authors found that the olivine phenocrysts inalkalic basalts have a Reunion plume-like noble gas isotopiccomponent. Thus, apparently, the alkalic basalts from Kutch werealready tapping Reunion plume like materials.

5. Geochemistry of the lavas

5.1. Major and trace element composition

Bulk rock major element analyses of the lavas are given inSupplementary Table 2, and trace element analyses are presentedin Table 1. Kutch alkalic lavas are quite primitive (Mg#=100×Mg/(Mg+Fe2+)=64–72) relative to the tholeiites (Mg#=38–54). The

Table 1Trace elements in Kutch volcanics

Sample

Concentration BH 12-3 BH 13-1 BH 16-2 BH 17-1 BH 18-1 BH

(ppm)

Li 11.95 5.58 8.35 5.93 6.68Rb 103 27.7 25.5 10.8 29.4Sr 1516 606 572 141 694 8Y 27.7 19.4 20.8 26.9 21.5Zr 366 188 178 77 220 2Nb 118 40.6 34.3 4.70 49.2Cs 2.32 0.59 1.13 0.50 0.63Ba 2320 469 418 120 530 12La 87.11 28.91 27.54 7.39 36.42Ce 157.9 61.1 58.3 15.9 74.9 1Pr 17.49 7.52 7.02 2.07 8.97Nd 67.03 31.76 29.88 9.56 37.4Sm 11.61 6.63 6.31 2.71 7.64Eu 4.35 2.29 2.19 0.95 2.65Tb 1.62 0.96 0.92 0.57 1.13Dy 6.09 4.30 4.40 4.34 4.77Ho 1.00 0.72 0.76 0.94 0.8Er 2.37 1.77 1.94 2.88 1.95Yb 1.86 1.41 1.55 2.48 1.59Lu 0.23 0.18 0.21 0.38 0.2Hf 7.57 4.27 4.10 1.99 4.97Ta 6.32 2.63 2.15 0.31 3.24Pb 5.98 2.48 2.44 1.96 3.19Th 11.01 3.68 3.32 1.69 5.02U 2.41 0.82 0.73 0.35 1.16Sc 19.7 22.7 26.3 52.5 24.0Ti 22478 19676 16223 5784 16649 228V 225 307 299 363 297 2Cr 334 515 669 283 751 5Co 67.7 58.6 63.3 55 80.5Ni 374 252 323 116 554 5

alkalic lavas have steeper trace element concentration patterns thanthe tholeiites on a primitive mantle normalized compatibilitydiagram (i.e. “spidergram”, Fig. 3), with higher light ion lithophileelement and lower heavy rare earth element (HREE) concentrationsthan the tholeiites. Compared to the alkalic lavas, the tholeiites showvariable positive Pb spikes with low Ce/Pb ratios (6–12 vs. 23–26 inthe alkalic lavas) and negative Nb anomalies with high Th/Nb ratios(0.25–0.36 vs. 0.09–0.11 in the alkalic). These features are consistentof a tholeiite magma that has been contaminated by the continentalcrust (Fig. 3; Mahoney 1988). In turn, the alkalic basalts do notappear to be significantly contaminated as their Ce/Pb and Th/Nbratios overlap the “canonical” values of OIB with Ce/Pb=25(Hofmann et al., 1986; Willbold and Stracke, 2006) and Th/Nb=0.08–0.15 (Weaver, 1991) . Their trace element patterns ingeneral indicate a strong “ocean island basalt” (OIB) component(often taken to be from mantle plumes) in the source (Melluso et al.,1995; Peng and Mahoney, 1995; Karmalkar et al., 2000).

All alkalic basalts show strong fractionation inmiddle to heavy rareearth elements (MREE/HREE) (e.g. Dy/Ybpm=1.8–2.2, wherepm=chondrite normalized), which is a diagnostic character of meltsthat leave behind a residue containing garnet. Such a residue could bea garnet lherzolite, which is stable at pressure greater than 2.8 GPa orabout 85 km depth; or it could be a garnet pyroxenite, which has aminimum pressure of stability of about 1.5 GPa (e.g., Pertermann andHirschmann, 2003). Melting experiments on peridotitic as well asgarnet pyroxenite source rocks have shown that silica content of near-solidus melts can be an effective indicator of pressure (e.g., Falloonet al. 1988, Hirose and Kushiro 1993, Tuff et al. 2005; see Appendix).The presence of spinel lherzolite xenoliths and relatively undiffer-entiated chemical characteristics of the alkalic basalts suggest thatthey are pretty close to being primary mantle peridotite-equilibratedmelts. We therefore compare their silica contents with experimentallyproduced near-solidus melts from peridotites in an effort to estimate

19-3 BH 2 BH 3-1 BH 3-2 BH 7-1 N 1-1 BCR-1

SD

8.86 9.30 6.72 6.20 7.15 6.37 13.9355.2 37.3 22.5 20.6 22.3 42.1 41.2477 247 207 187 774 765 32425.8 45.3 40.4 36.4 33.2 22.4 35.490 262 153 133 280 206 18295 24.6 14.2 12.6 57.00 43.50 11.393.42 0.60 0.61 0.56 0.64 0.64 1.01

51 337 326 298 514 478 67565.41 34.34 19.59 16.72 44.76 36.55 23.9822.7 72.4 39.1 33.9 95.2 72.6 50.613.82 8.64 4.68 4.20 11.58 8.61 6.3755.29 35.61 19.27 17.59 48.68 36.11 28.1910.4 8.04 4.45 4.08 10.02 7.47 6.653.65 2.45 1.55 1.45 3.36 2.57 2.241.48 1.31 0.88 0.82 1.50 1.07 1.105.98 8.12 6.32 5.80 7.01 4.99 6.480.97 1.57 1.36 1.25 1.23 0.83 1.262.30 4.45 4.21 3.89 3.08 2.09 3.661.79 3.60 3.69 3.49 2.51 1.63 3.120.23 0.52 0.57 0.53 0.33 0.22 0.466.68 6.22 3.61 3.20 6.22 4.84 4.675.58 1.30 0.88 0.78 3.41 2.86 0.774.77 5.99 5.43 4.95 3.69 2.95 13.269.51 7.77 3.84 3.26 5.76 5.08 5.82.07 1.70 0.79 0.70 1.42 1.12 1.61

21.1 34.5 43.5 40.2 32.2 27.5 33.0175 17381 9330 8705 19336 16581 1298443 473 389 360 298 307 41823 125 6.73 5.73 511 712 9.6874.9 41.6 48.3 45.8 63.6 73.7 36.9116 55 29 26 270 381 8.66

Fig. 3. Primitivemantle (McDonough and Sun,1995) normalized trace element patterns in Kutch volcanics (our data). The alkalic rocks show prominent negative Pb anomalywhereastholeiites clearly showa positive Pb spike that is similar to the basalts from Saurashtra area, which is further south of our study area. The Saurashtra data are fromMelluso et al. (1995).

105G. Sen et al. / Earth and Planetary Science Letters 277 (2009) 101–111

their possible depth of equilibration in the mantle. We find that thebulk of the Kutch alkalic melts is similar to the near-solidusexperimental melts produced under volatile-free conditions at apressure of about 3 GPa (Appendix). However, this pressure would belowered by about 0.5–1 GPa and significantly lower temperature ifmelting occurred along a CO2-saturated peridotite solidus (discussedlater).

5.2. Isotopic composition

We analyzed 7 alkalic basalts (mostly alkali olivine basalts andbasanites) from different plugs and 4 tholeiite lava flows from Kutchfor their Pb, Nd, Sr, and Hf isotopic composition (Table 2). Previously,Simonetti et al. (1998) analyzed a few alkalic basalts from Bhuj fortheir Nd, Sr, and Pb isotope composition. Here we present new Hfisotope data and additional Nd, Sr, and Pb isotope data on more plugs,including those near Bhuj. These data are plotted in Fig. 4a–c alongwith fields for other well-known sites in the Deccan and with the

Table 2Isotopic composition of Kutch volcanic rocks

Sample 87Sr/86Sr 2σe 87Sr/86Srin.

143Nd/144Nd2σe

143Nd/144Ndin.

ɛNd T=65Ma

BH 12.3 0.704515±7 0.704333 0.512607±9 0.512576 0.42BH 13.1 0.703790±7 0.703668 0.512895±6 0.512862 6BH 16.2 0.703753±6 0.703634 0.512919±13 0.512885 6.45BH 17.1 0.705870±7 0.705664 0.512694±9 0.512621 1.3BH 18.1 0.703643±7 0.70353 0.512900±7 0.512867 6.1BH 19.3 0.704071±6 0.703903 0.512650±7 0.512617 1.23BH 2 0.709340±19 0.708936 0.512414±10 0.512377 −3.46BH 2 (dupl) 0.512407±5 0.51237 −3.6BH 3.1 0.706434±7 0.706143 0.512546±7 0.512506 −0.94BH 7.1 0.703750±6 0.703673 0.512872±9 0.512839 5.55N 1.1 0.703629±9 0.703482 0.512916±6 0.512882 6.39

Strontium, Nd and Pb isotope compositionswere determined on a FinniganMAT 262 TIMS at FSUagainst the measured value of the E&A standard: 87Sr/86Sr=0.708000±14 (2SD, n=11). Nd isagainst themeasured value of the La Jolla standard: 143Nd/144Nd=0.511846±11 (2SD, n=8). The208Pb/204Pb=36.60±0.04 (n=18) and the reportedPb isotope ratios are corrected for fractionatiomeasured at 176Hf/177Hf=0.282185±19 (2SD, n=11) and the Hf isotope compositions are reisotope ratios, and ɛNd and ɛHf values are calculated at 65Ma, using theRb/Sr, Sm/Nd and Lu/Hf ra147Sm/144Nd=0.1967, 176Hf/177Hf=0.282772, 176Lu/177Hf=0.0332.

fields of Reunion island lavas and of basalts from the Central IndianRidge (CIR). Amongst the various Deccan fields, noteworthy is the fieldfor the Barmer alkalic lavas, which are characterized by high, Reunion-like, 3He/4He ratio (Basu et al., 1993). Ambenali field represents theleast contaminated, thickest, and most widespread Ambenali Forma-tion basalts (Cox and Hawkesworth, 1985; Lightfoot and Hawkes-worth, 1988; Peng et al., 1994, 1998; Widdowson et al., 2000].

In the ɛNd–ɛSr isotope space, all but two alkalic basalts form a tightcluster that falls largely within the CIR field but cuts across into theReunion field (Fig. 4a). Two alkalic lavas with the highest light REE/heavy REE ratios (samples BH12-3 and BH19-3) fall below the CIR-Reunion array having less radiogenic Nd for a given Sr isotopiccompositions (Fig. 4a). In 207Pb/204Pb vs. 87Sr/86Sr isotope space theyoverlap with the other alkali basalts and lavas from the Ambenaliformation (Fig. 4b). The tholeiites have high and variable 207Pb/204Pband 87Sr/86Sr, consistent with crustal contamination (Fig. 4b).

In Hf/Nd isotope space (Fig. 4c), the main cluster of Kutch alkalicbasalts fall well within the IndianMid-Ocean Ridge Basalts (MORB) field,

176Hf/177Hf2σe

176Hf/177Hfin.

ɛHf T=65Ma

206Pb/204Pb 207Pb/204Pb 208Pb/204Pb

0.282705+8 0.2827 −1.09 19.02 15.53 40.180.283114+7 0.283106 13.3 18.83 15.56 39.070.283137+12 0.283128 14.06 18.37 15.5 38.370.282977+19 0.282943 7.53 18.71 15.62 39.060.283116+8 0.283109 13.38 18.68 15.52 38.750.282686+5 0.28268 −1.79 18.58 15.55 39.140.282697+8 0.282682 −1.71 20.09 15.83 41.090.282697+6 0.282682 −1.710.282814+9 0.282786 1.96 18.31 15.61 38.750.283116+10 0.283107 13.31 18.94 15.55 39.020.283105+5 0.283097 12.97 18.64 15.53 38.76

. Sr isotope ratioswere corrected for fractionation using 86Sr/88Sr=0.1194 and are reportedotope ratios are corrected for fractionation using 146Nd/144Nd=0.7219, and are reportedNBS-981 Pb standardwasmeasured at 206Pb/204Pb=16.90±0.02, 207Pb/204Pb=15.45±0.02,n relative to theNBS-981 values reported by Todt et al. (1996). The JMC475Hf standardwasported relative to the widely accepted JMC value of 176Hf/177Hf=0.282160. Initial (in.)tios from the trace element data, and present day values for CHUR: 143Nd/144Nd=0.512638,

Fig. 4. (a). Initial Nd–Sr isotope compositions of Kutch basalts are compared with Deccan basalts from northwestern India and some select formations (Ambenali, Mahabaleshwar,Thakurvadi) from theWestern Ghats (Cox and Hawkesworth, 1985; Lightfoot and Hawkesworth, 1988; Peng et al., 1994; Peng and Mahoney, 1995). Reunion hot spot generated lavas andCentral Indian Ocean Ridge basalts data are also shown for comparison (source of data: GEOROC). Initial ɛSr is calculated using present day bulk earth 87Sr/86Sr=0.7047 and 87Rb/86Sr=0.08168, and ɛNd using chondritic earthwith 143Nd/144Nd=0.512638 and 147Sm/144Nd=0.1967. (b). Pbvs Sr isotope plot for the Kutch volcanics comparedwith other basalts. (c). InitialɛHf–ɛNd diagram comparing the Kutch volcanics with Indian MORB, Reunion and Mauritius lavas, and Group 1 kimberlites. Kutch tholeiites plot over a wide area that covers much of theIndianMORBfield andDeccan basalts fromother areas (not shown). Two alkalic basalts plot inside the kimberlite field and perhaps represent very small degrees ofmelts from an enrichedsublithospheric source. Other alkalic basalts plot between Indian MORB and Reunion lavas, suggesting that they were derived from a mixed source of depleted asthenosphere and theReunionplume.Data sources: Reunion: Bosch et al. (2008); IndianMORB:Meyzenet al. (2007) and theGEOROCdatabase;Mauritius: Paul et al. (2005); G1kimberlites: Nowell et al. (2004).

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Fig. 5. Schematic blockdiagramshowing inferredgeological relationshipsbetween themain structural elements ofKutchrift zoneandDeccanvolcanism (the structural elements aremostlybased on Biswas, 1987, 2005). The subsurface Deccan Trap ridge at the center of the diagram is based on gravity data interpretations (Chandrasekhar and Mishra, 2002). We consider thelithosphere to be about 90–100 km thick on either side of this paleo-rift zone, andunder such conditions it shouldhave a garnet peridotite lower layer. Such a layer ismissing in beneath therift zone as evident from the absence of garnet peridotite xenoliths in the alkalic basalts. Therefore, we suggest that the rift was already extended and thinned during prior Late Triassic–Jurassic rifting. Isotopedata suggest that thealkalicbasaltswereproduced fromamixedasthenospheric (IndianMORB-like)andplume (Reunion-like) source (this is shownasblack “blobs”).

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while the two most light ion element-enriched alkalic basalts fall withinthe field of Group I kimberlites, having relatively unradiogenic εHf for agiven εNd. The tholeiites have both unradiogenic Hf and Nd isotopiccompositions and fall along the terrestrial array, again consistent withcrustal contamination.

Basu et al. (1993) obtained 87Sr/86Sr and 3He/4He analyses of somealkaline rocks from other locations around northern Deccan, includingan alkaline complex from the Barmer area, which occurs furthernortheast fromKutch (Fig.1). They found these rocks to have a high 3He/4He ratio similar to that of Reunion lavas and thereforemade a case for alink between DeccanTraps volcanism and the Reunionplume. There areno 3He/4He data for Kutch lavas. Instead, we compare our Nd, Sr, Pbisotope data with the isotopic analyses obtained by Simonetti et al.(1998) on Barmer rocks in Fig. 4a,b, which show that the bulk of thealkalic rocks from Kutch are isotopically somewhat less enriched thanthe enriched samples from both Barmer and Reunion. Note that theKutch alkalic lavas overlap the least depleted (or most enriched, plumeinfluenced?) end of the CIR field. One possibility is that both Kutch andBarmer lavas came from a mixture of depleted asthenosphere andplume, with the former tappingmore of the asthenospheric componentthan the latter. Alternatively, one could suggest that the Reunion plumeitselfwas aheterogeneousmixture of isotopically depleted and enrichedcomponents; and the Kutch alkalics largely tapped the depleted com-ponents whereas Barmer magmas were derived from the enrichedcomponents. Similar conclusions were reached by previous workers

studying lavas in Saurashtra, just to the east of Kutch (Peng andMahoney, 1995; Melluso et al., 2006). The two highly enriched alkaliclavas, however, require a different source. Their displacement towardsunradiogenic Hf isotope compositions is similar to that of Group 1kimberlites from South Africa (e.g., Nowell et al., 2004). Such a mantlesource requires low time-integrated Lu/Hf relative to Sm/Nd. Someauthors have attributed these characteristics to a source that iscomposed of ancient (N1 Ga), recycled oceanic crust, trapped in theconvecting asthenosphere beneath continents (Salters andWhite,1998;Nowell et al., 2004). Alternatively, it is possible that some deepasthenospheric melts formed in the presence of garnet that metasoma-tized the lithospheric mantle beneath Kutch some time ago (e.g., 1 Ga).These metasomatized compositions evolved with time to fall below thearray in Hf–Nd isotope space, similar to that of subducted oceanic crust.Subsequent melting, related to the arrival of the Reunion plume andrifting, generated melts from this metasomatized subcontinental litho-spheric mantle.

The above two scenarios essentially generate identical Hf–Ndisotope systematics. However it is generally accepted that recycledoceanic crust that was processed through a subduction zone willdevelop with time radiogenic Pb isotope compositions (e.g. HIMU-type; e.g. Hofmann and White, 1982). The vast majority of HIMU-typelavas have 206Pb/204PbN20, (e.g. compilation in Stracke et al., 2003)while our two enriched lavas have 206Pb/204Pbb19.1. Also, Willboldand Stracke (2006) showed that HIMU-type lavas have certain

Fig. 6. A geodynamic model of magma generation in Kutch is presented based on peridotite-CO2 melting relations (Presnall and Gudfinnsson, in press). In all three figures the bottom diagramshowsaschematicgeological cross-section (north is approximately to the rightandsouth is to the left), and the topshowsmagmaproduction inpressure–temperaturephasediagrams. In all threefigures volatile free (gray) and CO2-saturated (black) lherzolite solidi are shown as solid lines and geothermal gradient is shown as a dashed curve. The position of the geotherm changes inresponse to rifting and laterondue to arrival of deeper, hotter, Reunion-likebodies.Magmageneration is shown in three stages (I, II, and III) as initial rifting (stage I), arrival andmeltingofCO2-richperidotitic blobs (stage II, alkalicmelt production), andgenerationof tholeiitic picrites from themainplumehead (stage III). Lithospheric thinningdue to rifting causesCO2-bearingasthenosphereto rise and cross the volatile bearing peridotite solidus, generating carbonatiticmelt. As thesemelts rise through the lithosphere they freeze, releasing CO2-rich vapor. Thesemelts and associatedvapormetasomatizes the lithosphere, converting spinel lherzolitewall rock to spinel wehrlite along veins used by such fluids. Stage II shows the arrival of Reunion-like bodies that break off theleading edge ofmain Deccanplume head and begin tomelt once their solidus is crossed around∼75–90 km. In this stage the geotherm rises higher and produces alkalicmagmas, which ascendalong pathways created by deep lithospheric rift faults and erupt to form small bodies distributed along the strike ofmajor east–west fault systems. In the lowermost diagram the plume head isshown as thermally zoned with a hot core that produces tholeiitic magmas and a cooler rim, which feeds the alkalic magmas. The thick arrow shows direction of plate movement. Stage IIIrepresents production of picritic tholeiite magmas from the hotter core of the plume head as the volatile-free solidus is crossed by the hot plume geotherm. It is speculated that the tholeiitemagmas were not generated at Kutch but the lavas arrived from elsewhere further south.

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characteristic and very reproducible trace element ratios (e.g. Ce/Pb∼30 Th/Ub4, Ba/Lab8) consistent with dehydration of subductedoceanic crust in the arc setting, while the average ratios in the twolavas are 26, 4.6, 23, respectively. It therefore appears that the

observed unradiogenic 176Hf/177Hf for a given 143Nd/144Nd in the twoenriched alkalic lavas is not compatible with an origin as recycledoceanic crust and an origin from a long lived metasomatizedsublithospheric mantle is more likely.

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6. Discussion: A model for the lithosphere and magma generation

We begin with a summary of the pertinent observations madeabove: First, the Kutch basin has undergone multiple rifting eventsincluding the one that witnessed xenolith-bearing alkalic volcanism.Second, the mantle xenoliths came from the lithosphere thatexperienced carbonatite metasomatism (Karmalkar and Rege, 2002,and present study). The finding of carbonatite melt inclusions fromKutch suggests that such melts existed in the lithosphere duringalkalic volcanism. Third, MORB-like rare gas isotope composition ofxenoliths suggests that the lithosphere beneath Kutch rift zone wasalready oceanic in character. Fourth, the trace element and isotopiccomposition of the alkalic lavas shows that they may have beenderived from Indian Ocean-like asthenosphere or from amixed sourceconsisting of Indian Ocean asthenosphere and the Reunion plume. Thetholeiites show evidence of strong crustal contamination. Finally, rareearth element composition of the basalts indicates their generationfrom a garnet-bearing source.

6.1. The lithosphere beneath Kutch

Fig. 5 is a schematic model of the Kutch lithosphere based on theinferences on themantle xenoliths and on the geological interpretationsmade by Biswas (2005), who showed that Kutch is built of broadly east–west half-grabens. Biswas reviewed all existing geophysical and fieldstratigraphic data to show that in the north a single, near-vertical fault,called theNagar Parker fault, forms the boundary between theKutch riftsediments and the Precambrian rocks. In the south and middle part ofthe Kutch basin there are several roughly east–west running faults.Biswas inferred that although at the surface these main rift faults havesteep dips, their dips shallow at depth. Gravity data show that there is aridge with a density of 2.7 g cm−3 located near the middle of the Kutchbasin, whichwas interpreted by Chandrasekhar andMishra (2002) to bea basaltic “upwelling”. It is possible that this “upwelling”may representthe arrested development of a true ocean basin in the sense that thecrustal part of the lithosphere did not separate to allow the growth of anoceanic crust at Kutch.

The mantle xenoliths offer a window into the deeper lithospherebeneath Kutch. Phase equilibrium studies indicate that at about1000 °C, which is recorded by the Kutch xenoliths, spinel lherzolitecan be stable over a pressure range of 1–1.6 GPa (i.e., about 30–50 km).Unfortunately, there is no reliable barometer for spinel lherzolites;and therefore, we cannot pinpoint the depths of origin of eachindividual xenolith. At the present time the available seismic data inthis area cannot unequivocally locate the base of the lithosphere (e.g.,Kumar and Mohan, 2005). Nevertheless, one can surmise that a spinelperidotite layer existed at ∼30–50 km beneath the Kutch mainland.

The presence of garnet signature in the trace element compositionof the alkalic basalts suggests that these magmas were produced at apressure of N1.6 GPa. As we noted earlier, garnet-bearing xenoliths donot occur, which suggests either that a garnet-bearing layer did notexist or if it did exist it was perhaps too thin to be sampled by theascending magmas. One may also argue that the absence of garnetbearing xenoliths is simply reflective of selective sampling of thelithosphere caused by vapor saturation of the magmas: the risingmagmas reached CO2 vapor saturation at ∼1.6 GPa, fractured theoverlying rocks (spinel peridotites), and then picked up the wall rockfragments as xenoliths. An experimental study by Presnall andGudfinnsson (in press) indicates that carbonatite magma is generatedat about ∼1000 °C but at a pressure greater than 2 GPa — close to acusp on the CO2-saturated lherzolite solidus (Fig. 6). Note thatsuch carbonatite magma would freeze and release a CO2-rich fluid(i.e., vapor saturation) as it crosses the “ledge” on the CO2-saturatedsolidus.

We believe that the two inferences are not mutually exclusive andsuggest that xenolith “picking” was forced by vapor saturation but it

was the lithosphere thinning that perhaps allowed alkalic volcanism.This conclusion is broadly consistent with the oceanic nature of thexenoliths, as evident from theMORB-like rare gas isotope compositionof the xenoliths. The Indian lithosphere is 100 km thick beneath theshield areas (Kumar et al., 2007; Mahadevan et al., 2007); andtherefore, it appears that the lithosphere beneath the Kutch rift zonewas ∼50% thinner than the normal lithosphere during alkalicvolcanism. Thinning of the lithosphere seems reasonable beneath arifted continental margin, as has been determined for the opening ofthe Red Sea.

6.2. Magma generation

Based on our earlier discussion the following sequence of eventscan be inferred: rifting and lithosphere thinning started pre-Deccan;and carbonatite metasomatism of the lithosphere and alkalic volcan-ism occurred during Deccan time (67–65 Ma). In this connection, notethat Baker et al. (1998) found evidence of carbonatitic and hydrousmelt induced metasomatism of the lithosphere, as recorded inxenoliths, above the Afar plume during the Yemen flood basalteruptions. The voluminous tholeiites may be slightly younger orcontemporaneous with the alkalic eruptions. Isotope data indicatethat the alkalic lavas came from a mixed source — one of which isReunion-like plume source and the other a Central Indian Ridge-likelithospheric/asthenospheric source. It seems likely that the alkaiclavas of Kutch tapped the edge of the Deccan–Reunion plume.

Tholeiites are considerably fractionated and have completelydifferent isotope and trace element chemistry. They occur only inthe southern fringes of Kutch. Although tholeiitic intrusives have beensuggested to be present in Kutch by previous authors (Guha et al.,2005), not a single tholeiitic intrusive can be pinpointed as feeders ofthese tholeiites. Consideration of these three facts together leads us toconclude that the tholeiites, which represent the dominant phase ofDeccan volcanism, came from elsewhere — perhaps from Saurashtraor even further south where main plume head had impacted thelithosphere (Peng and Mahoney, 1995; Melluso et al., 2006).

We present a model for the generation of carbonatites, alkalicbasalts, and tholeiites that requires melting of a carbonatedperidotite source (Fig. 6). We show this in terms of three distinctthermal evolutionary stages represented by the dT/dP curves, I, IIand III, in Fig. 6 for brevity; but we recognize that the change from Ito II was likely a gradual evolution. This type of thermal evolutionwould be expected in a rifting environment, where extension andthinning of the lithosphere would allow the asthenosphere topassively rise to shallower levels (Stage I). Arrival of small meltbodies (Stage II) would further increase the temperature close to thelithosphere/asthenosphere boundary. However, the temperaturehigher up in the lithosphere would not change so quickly becauseat such level heat transfer would occur via conduction, which is anextremely slow process. Stage III dT/dPmay only occur when a muchlarger and hotter plume impacts the base of the lithosphere.However, we do not believe Stage III type dT/dP curve was everreached at Kutch. This stage occurred much further south, perhapsnear the Western Ghats, where picritic magmas (parent to thetholeiites) were generated from the main Deccan–Reunion plumehead (Peng and Mahoney, 1995).

In Stage II, alkalic basaltic magmas were produced by partialmelting of the edge of the Deccan–Reunion plume-head (Fig. 6). Basedon the Presnall and Gudfinnsson (in press) diagram, we believe thatthis melting occurred at a minimum pressure of 2 GPa (Fig. 6), wheregarnet would be a stable residual phase. The presence of garnet as aresidual phase in the mantle is indicated by the REE patterns of thealkalic lavas.

Stage III represents the production of picritic magmas that wereperhaps parental to the tholeiites (Melluso et al., 1995; Sen, 2001 andrefs therein). The final melting depths may have been as shallow as

110 G. Sen et al. / Earth and Planetary Science Letters 277 (2009) 101–111

45kmat siteswhere the lithospherewas already thinnedbyprior riftingevents (during the separation of Gondwana continents), such as theNarmada rift in Satpura area (Sen and Cohen,1994). Elsewhere, the bulkof themelting occurred close to thebase of the 100 kmthick lithosphere.

7. Summary of conclusions

Rifting caused thinning of the sub-Kutch lithosphere to a thicknessof about 50 km prior to alkalic volcanism. This rifting started beforeand continued through Deccan volcanism (Biswas, 1987). Bulk of theKutch alkalic lavas have come from an Indian Ridge mantle sourcerather than the Reunion plume, which ∼65 Ma may have resembledthe Barmer alkalics (Basu et al., 1993). The Kutch lithosphere wascomposed of spinel lherzolite that was largely converted to spinelwehrlite by carbonatite metasomatism. Such carbonatite melts weregenerated by decompression melting of the asthenosphere during itsrise to shallower levels in response to extension and thinning of thelithosphere. Mantle xenolith bearing alkalic magmas were mainlygenerated from a mixture of dominated by asthenospheric materialand edge of the Deccan–Reunion plume. These melts ascended, whilepicking up xenoliths, along pathways created by deep rift faults.Tholeiitesmayhave come fromelsewhere in the south, from the hotterpart of the Deccan plume head.

Acknowledgments

The authors thank A.R. Basu, J. Mahoney, D.W. Peate, and R. Carlsonfor their detailed official reviews. Their comments really helped inimproving the presentation of the final manuscript. Sen thanks A.Acosta, M. Borges, T. Beasley, K. Chau, and I. Sen for their help. Wegratefully acknowledge the analytical support from FCAEM at FIU andfrom FSU's geochemistry laboratories. This work was supported byNSF-OISE grant# 0352948.

Appendix A

Volume change of the opx-dissolution reaction involving carbo-natite liquid and wall rock orthopyroxene was calculated using thethermodynamic data tables in Robie et al. (1978). The olivine/liquidpartition coefficients for NiO and MgO are from GERM websitedatabase (http://earthref.org/GERM). The batch melting equationsused can be found in any textbook.

The pressure of generation of alkalic melts along volatile-free (“dry”)lherzolite solidus can be determined to a first order from the SiO2

content. Based on experimental data of Falloon et al. (1988) and Hiroseand Kushiro (1993) we know that silica content of melts produced at 0–4 GPa negatively correlates with pressure of generation of near-solidusmelts from “dry”peridotite (not shownhere). This is also true of volatile-free garnet pyroxenitemelting based on the data inTuff et al. (2005).Weobtained a best-fit linear equationwith a R2=0.9: P (GPa)=0.1⁎(174.43−3.1757×(wt.% SiO2)) for “dry”peridotitemelts; and for garnet pyroxenitemelts it is P (GPa)=((53.014−(wt.% SiO2))/4.5668) with a R2=0.99. Thefact that the Kutch alkalic lavas contain mantle xenoliths and have veryhigh MgO values suggests to us that these are near primary magmasgenerated from a peridotitic source. Using the peridotite-liquidbarometer above, we obtain a pressure range of 2.5–3.8 GPa for thelastmantle equilibrationofKutch alkalicmagmas.Aswe state in the text,any involvement of CO2 could affect such a calculated pressure fromvolatile-free equation. Based on the xenolith temperature constraints,we prefer the production of alkalic basalts along the CO2-bearinglherzolite solidus.

Appendix B. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.epsl.2008.10.002.

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