Seychelles alkaline suite records the culmination of Deccan Trapscontinental flood volcanism

T.M. Owen-Smith a,⁎, L.D. Ashwal a, T.H. Torsvik a,b,c, M. Ganerød c, O. Nebel d, S.J. Webb a, S.C. Werner b

a School of Geosciences, University of the Witwatersrand, Johannesburg, Private Bag 3, WITS 2050, South Africab Centre for Earth Evolution and Dynamics (CEED), University of Oslo, PO Box 1047, Blindern, 0316 Oslo, Norwayc Geodynamikk, Norges Geologiske Undersøkelse (NGU), Geological Survey of Norway, NO-7491 Trondheim, Norwayd Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia

a b s t r a c ta r t i c l e i n f o

Article history:Received 2 May 2013Accepted 21 September 2013Available online 1 October 2013

Keywords:Alkaline magmatismDeccan TrapsIsotope geochemistryRéunion mantle plumeSeychelles

Silhouette and North Islands in the Seychelles represent an alkaline plutonic–volcanic complex, dated at 63 to63.5 Ma by U–Pb zircon and 40Ar/39Ar methods. This magmatism coincides with the final stages of the cataclys-mic Deccan Traps continental flood volcanism in India (67 to 63 Ma), and thus a causal link has been suggested.Recent reconstructions have placed the Seychelles islands adjacent to the Laxmi Ridge and at thewesternmarginof the Réunionmantle plume at the time of formation of the complex. Here we present geochemical evidence insupport of the notion that the Seychelles alkaline magmatism was initiated by the peripheral activity of the Ré-union mantle plume and is thus part of the Deccan magmatic event. Positive εNd (0.59 to 3.76) and εHf (0.82 to6.79) and initial Sr of 0.703507 to 0.705643 at 65 Ma indicate derivation of the Seychelles alkaline magmasfrom a Réunion-like mantle source with an additional minor enriched component, suggesting entrainment ofsub-continental lithospheric mantle. The similarity in trace element composition between the Seychelles suiteand Deccan alkaline felsic and mafic rocks provides additional evidence for a common mantle source for theSeychelles andDeccanmagmatism. Furthermore, we demonstrate the role of fractional crystallisation in the evo-lution of the alkaline suite. Modelling using major elements suggests that fractional crystallisation and varyingdegrees of accumulation of olivine, plagioclase, ilmenite, clinopyroxene, alkali feldspar and apatite can describethe spectrum of rock types, from gabbro, through syenite, to granite.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

The Deccan Traps continental flood basalts record the peak of activ-ity of one of the largest magmatic events in Earth's history, when morethan 2000000 km3 of lava erupted onto the Indian sub-continent, themajority in a period of less than 1 million years, with catastrophicconsequences for life on Earth (Chenet et al., 2007; Courtillot andRenne, 2003). This event, which has been attributed to the impinge-ment of the Réunion mantle plume, culminated in the final phase ofrupture of the Gondwana supercontinent, the separation of theSeychelles from India (Collier et al., 2008; Mahoney et al., 2002). Thisposes the question: to what extent was the Seychelles affected by theplume before rifting moved it out of range?

The Seychelles microcontinent represents a fragment of Rodinian(Neoproterozoic) continental crust and the majority of the Seychellesislands thus consist of ~750 Ma granite (Ashwal et al., 2002; Torsviket al., 2001). However, Silhouette Island and North Island (Fig. 1) areexceptions, comprising alkaline felsic rocks with ages recentlyconstrained to between 63 and 63.5 Ma (Ganerød et al., 2011),

contemporaneous with the Deccan Traps volcanism (Collier et al.,2008). Furthermore, the latest reconstructions have shown that theSeychelles was situated west of India and in the vicinity of theRéunion plume at this time (Ganerød et al., 2011). It remains to beshown that the Seychelles alkaline magmas were indeed producedby the plume and that it is part of the Deccan magmatic event.Ultimately, this has implications for the original extent of the DeccanTraps Large Igneous Province (LIP) and therefore has relevance forglobal tectonic, magmatic and climatic events near the Cretaceous–Tertiary boundary.

2. Background

The tectonic evolution of the Seychelles microcontinent is well doc-umented in the studies of Plummer and Belle (1995), Subrahmanya(2001), Collier et al. (2008), Ganerød et al. (2011) and Armitage et al.(2011). During the later stages of Gondwana breakup, after 83.6 Ma(Torsvik et al., 2000), the Seychelles–India continental block separatedfrom Madagascar via the Mascarene Ridge and migrated northward(Collier et al., 2008; Devey and Stephens, 1992; Torsvik et al., 2001).In so doing, it passed over the surface expression of the Réunionplume and plume impingement on the lithosphere induced extensive

Lithos 182–183 (2013) 33–47

⁎ Corresponding author. Tel.: +27 76 177 2884; fax: +27 11 717 6579.E-mail address: trishya.owen-smith@students.wits.ac.za (T.M. Owen-Smith).

0024-4937/$ – see front matter © 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.lithos.2013.09.011

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magmatic activity, includingwhat is now theDeccan Traps, and also ini-tiated the rifting of the Seychelles away from India (Armitage et al.,2011; Collier et al., 2008; Courtillot et al., 1986; Lightfoot andHawkesworth, 1988; Mahoney et al., 2002; Plummer and Belle, 1995;Sen, 1995; Todal and Edholm, 1998). This rifting began in the Gop Rift,between India and the Seychelles–Laxmi block, between 70 and65 Ma (Armitage et al., 2011; Collier et al., 2008; Ganerød et al.,2011), but from 63.4 Ma the spreading centre shifted southward tothe Carlsberg Ridge, separating the Seychelles and Laxmi continentalfragments (Armitage et al., 2011; Collier et al., 2008; Ganerød et al.,2011).

Accordingly, Collier et al. (2008) identified three main phases ofactivity in the Deccan-Réunion magmatic event, as follows: An early,‘pre-Deccan’ phase of plume-related activity (78–67 Ma) relates toextension in the Gop Rift and is manifested in the Bibai basalts ofPakistan, alkaline intrusive rocks in north-western India, the Praslin(Seychelles) tholeiitic dykes and offshore mafic volcanic rocks on thesouthwest of the Seychelles plateau. This was followed by the mainDeccan event (67–63 Ma), which produced massive outpourings oftholeiitic basalt on the western margin of the Indian sub-continent.After the Seychelles began to rift away from India, a third ‘post-Deccan’pulse of magmatism (63–58 Ma) produced offshore mafic volcanic andintrusive rocks on the northern margin of the Seychelles plateau andalkaline volcanism in India.

Recent work by Ganerød et al. (2011) has provided precise geo-chronological and palaeomagnetic constraints on the timing of theSilhouette–North Island magmatism, summarised here. Dating by40Ar/39Ar step heating and U–Pb in zircons yielded weighted meanages of 63.1 ± 0.34 Ma and 63.27 ± 0.05 Ma (2σ errors), respectively,for all the North Island lithologies (syenite, microsyenite and gabbro).U–Pb dating for Silhouette Island lithologies gave a weighted meanage of 63.54 ± 0.06 Ma, while 40Ar/39Ar dates for these lithologies en-compass a larger range, from 66.8 ± 0.8 (trachyte), through syeniteand microsyenite, to 61.3 ± 0.57 (syenite). The U–Pb data indicatethat in general the Silhouette Island igneous units are older by 0.27 ±0.08 Ma than those on North Island. Whereas the previous range in

K–Ar mineral and Rb–Sr isochron ages for Silhouette and North Islandsby Dickin et al. (1986) had uncertainties too great for adequate correla-tion, these new ages clearly constrain this magmatism to the end of themain Deccan event. Ganerød et al. (2011) go on to show, via refinedplate reconstructions, that the Seychellesmicrocontinentwas still locat-ed adjacent to the Laxmi Ridge and above the deep mantle Plume Gen-eration Zone of Burke et al. (2008) at this time.

Speculation that the Silhouette–North Island alkaline complex mayhave formed as the Seychelles–India plate passed over the Réunionmantle plume is not new (see Courtillot et al., 1986; Devey andStephens, 1992), and indeed, preliminary geochemical comparisonshave been made. Devey and Stephens (1991, 1992) initially proposedthat the Silhouette–North Island alkaline rocks andmafic dykes of Praslin,Mahé and Felicité Island in the Seychelles showed a strong resemblanceto certain Deccan rocks in terms of geochemistry, petrology and rock re-lations, and took this as evidence for a common source for thesemagmas.However, the mafic dykes “of known Cretaceous/Tertiary age” (Deveyand Stephens, 1992, 276) from Mahé and Felicité used in comparisonhave been shown to be Precambrian (Ashwal et al., 2002; Hargravesand Duncan, 1990; Torsvik et al., 2001), while those from Praslin clearlyrelate to the pre-Deccan phase and their original compositions are ob-scured by significant contamination (Devey and Stephens, 1991; Dickinet al., 1986). For the alkaline felsic suite, the key comparisons in thestudy of Devey and Stephens (1992) were made with Deccan alkalinemafic and tholeiitic rocks. There was also a general paucity of geochemi-cal and isotopic data with which to compare. In light of the new, moreprecise geochronological and palaeomagnetic data of Ganerød et al.(2011), a comprehensive geochemical investigation into the origins ofthe Silhouette–North Island magmatism and its relation to the Deccanmagmatism and the activity of the Réunion Plume is essential, andforms the basis of this contribution.

3. Methodology

Samples were collected from Silhouette Island and North Island ontwo field expeditions in 1997 and 2008. Major and trace element (Ga,Nb, Zr, Y, Cr, Ni, Co, Sc, Cu, Zn, S) compositions of the 1997 sample set(10 samples prefixed LA and RT) were analysed by X-ray fluorescence(XRF) at the Department of Geology of the University of KwaZuluNatal, South Africa. Additional trace element compositions (Ba, Th, U,V, Pb, Hf, Ta, W, REE) were analysed using inductively coupled plasmaemissionmass spectrometry (ICP-MS)with a VG elemental Plasmaquadinstrument at the Département de Géologie, Musée royal de l'Afriquecentrale, Belgium. Details of these analyses and estimates of precisionmay be found in Ashwal et al. (2002).

Rb, Sr, Lu, Hf, Sm and Nd isotopes and elemental concentrationswere analysed at the Research School of Earth Sciences, AustralianNational University, Canberra. Details of the procedures are givenin Nebel and Mezger (2006) and Nebel et al. (2009, 2010). The Lu, Hf,Sm and Nd isotope ratios were analysed with a ThermoFinniganMC–ICP-MS. Measured ratios of Sm, Nd and Hf were internallycorrected for mass fractionation using 146Nd/144Nd = 0.7219 and179Hf/177Hf =0.7325. All samples are reported relative to the La Jolla(143Nd/144Nd =0.511859 after Lugmair and Carlson, 1978) and JMC-475 (176Hf/177Hf = 0.282160 after Blichert-Toft et al., 1997) isotopestandards, which yielded values during the analytical session of143Nd/144Nd = 0.511826 ± 17 and 176Hf/177Hf = 0.282152 ± 8.Rb isotopes were analysed using the method described in Nebel et al.(2005). Sr was measured on a FinniganMAT 261 in static mode. Instru-mental mass fractionation was corrected relative to a value of 86Sr/88Sr = 0.1194. The NBS-987 standard yielded a value of 87Sr/86Sr =0.710222 ± 12. Procedural blanks are 15 pg, 10 pg, 30 pg, 40 pg, 30 pgand 50 pg for Lu, Hf, Sm, Nd, Rb and Sr, respectively.

The 2008 sample set (14 samples prefixed TOS and TON) wasanalysed using a ThermoARL (Advant 'XP, model 9800XP) XRF instru-ment at Set Point Laboratories, Johannesburg, South Africa. Rock

Fig. 1. The location of Silhouette and North Islands with respect to the main Seychellesislands andwithin the IndianOcean (afterDickin et al., 1986; Plummer, 1995). The presentposition of the Deccan plume is below Réunion Island.

34 T.M. Owen-Smith et al. / Lithos 182–183 (2013) 33–47

powders were fused with lithium borate, except for analyses of Na, Sand C, which used pressed pellets. Detection limits are 0.01% forFe2O3, MnO, Cr2O3, V2O5, TiO2, CaO, K2O, P2O5, Cl and S, and 0.1% forSiO2, Al2O3, MgO and Na2O. Total instrumental uncertainties are at amaximum of 0.14% for Fe2O3, 0.24% for MnO, 0.25% for Cr2O3, 0.11%for V2O5, 0.22% for TiO2, 0.38% for CaO, 0.15% for K2O, 0.13% for P2O5,0.45% for SiO2, and 0.4% for Al2O3 and MgO. The trace elements wereanalysed on pressed pellets comprising 11.7 g of sample mixed with1.3 g of wax binder. The quality of analyses was monitored with inter-national reference standards NCSDC 73310, NCSDC 73334, SARM 2and SARM 1. Detection limits for this method are 3 ppm for Co, Zr, Nb,Y and Sr, 4 ppm for Cu, Zn, V, Ba and Rb, 2 ppm for Ni, 12 ppm for Crand 5 ppm for Ce.

These samples were further analysed by ICP-MS, using a Perkin-Elmer ELAN 6000, for a suite of trace elements, at the Department ofGeological Sciences, University of Cape Town. Calibration employedUSGS standard BHVO-2. Typical procedural error was better than 5%.Details of the preparation method and operating conditions can befound in Le Roex et al. (2001). Where trace element analyses wereduplicated by XRF and ICP-MS, the latter were used in preference, dueto their greater precision.

Energy-dispersive electron microprobe analysis was performed onselected pyroxenes, feldspars and amphiboles from 6 samples at theDepartment of Geology, University of Johannesburg, using a CamecaCamebax instrument. Details of operating conditions are as reportedin Ashwal et al. (2005).

4. Results

4.1. Field relations and lithologies

4.1.1. Silhouette IslandSilhouette Island consists predominantly of medium- to coarse-

grained (average 5 mm), grey-white to pale green syenite, which com-monly contains pegmatitic enclaves (Fig. 2a). Minor syenodiorite occurson the eastern side of the island. A central plug of granite in the GrandeBarbe amphitheatre is identifiedmainly by a change in soil texture, or ispresent as sparse, extremely weathered outcrop in river beds, butDickin et al. (1986) report an intrusive contact.

On the eastern margin of Silhouette Island, from Pointe Vareur toPointe Zeng Zeng, is a thin volcanic complex (Fig. 2a), consisting of

sub-horizontal black trachytic tuff units that contain fragments ofsyenite and earlier tuffs (Fig. 3a) and which show a prominent near-vertical fracture set (Ganerød et al., 2011) erroneously interpreted asflow-bounding bedding planes by Stephens and Devey (1992). Thenature and quantity of lithic fragments, the presence of phenocrystsand the matrix colour vary between flows. The tuffs are cut by at leasttwo generations of paler-coloured syenitic dykes (Fig. 3b). The cross-cutting relations confirm these to be intrusive and not volcanic, asproposed by Stephens and Devey (1992). The prefix “micro-” is usedhere to distinguish these dyke phases from their plutonic and volcaniccounterparts. The earliest dyke phase is a fine-grained (~0.02 mm)porphyritic microsyenite, which intrudes and brecciates the trachytetuffs, entraining fragments of the latter up to 1 m in size. The micro-syenite is intruded in turn by a medium fine-grained (~0.1 mm), lessporphyritic, quartz-rich microsyenite, which also forms a network ofsyenitic apophyses, some with pegmatitic, quartz-rich centres. Al-though not sampled in this study, Devey and Stephens (1992) recordedgranitic compositions for the latest phase of these, thus they are strictlymicrogranites. The dykes are near concordant with the lava flows in thenorth of the complex (thus better described as sills), but bend to almostvertical further south at Pointe Ramasse Tout. Two dolerite dykes, up to40 cm thick, intrude the tuffs at Pointe Vareur and Pointe Zeng Zeng(Fig. 2a), and both intrude and are cut by microsyenite. The volcaniccomplex is separated from the main body of syenite by an intrusivecontact, with the latter clearly cutting the former. The contact is highlyirregular, producing a relatively wide transition zone of contact breccia,and is associated with dense microsyenite apophysis networks.

4.1.2. North IslandMost of North Island (Fig. 2b), including the Congoment knoll,

consists of syenite, which is indistinguishable from that on SilhouetteIsland. A medium-grained syenodiorite occurs in the saddle of theisland. Gabbro is also present here as float (documented by Stephensand Devey, 1992), or as small xenoliths within the syenodiorite.

A small magmatic breccia complex is found on the northern point ofCongoment (Fig. 2b). This complex is an amalgamation of microsyeniteandmicrosyenodiorite dykes and apophyses, similar to those on Silhou-ette, although no tuffs are present. The earliest phase is syenodioritic,relatively fine-grained (~1 mm) and medium grey in colour. Thisphase is cross-cut and brecciated by a paler grey, quartz-bearing

Fig. 2.Updated geology of a) Silhouette Island (after Dickin et al., 1986), and b)North Island (after Stephens andDevey, 1992). Numbered points indicate the locations of analysed samples(TOS- and TON-); non-numbered points are other sampling and observation sites. (For interpretation of the references to colour in this figure, the reader is referred to the web version ofthis article.)

35T.M. Owen-Smith et al. / Lithos 182–183 (2013) 33–47

microsyenite. Entrained fragments of coarse-grained syenodiorite arealso present.

4.2. Petrography

These rock types are classified based mainly on modal primarymineralogy (Streckeisen, 1976; Supplementary Table 3), with lesserreliance on chemical discrimination tools such as total alkalis versussilica (Middlemost, 1994; Fig. 5) and its equivalent for altered rocks(Winchester and Floyd, 1977).

The syenite consists primarily (70–85%) of euhedral, finely- tocoarsely-exsolved perthitic alkali feldspar (orthoclase, microcline,anorthoclase; Supplementary Table 4), which is commonly zoned,shows recrystallised rims, and is heavily sericitised. All samples containsmall (0.5 to 4 mm) anhedral green clinopyroxene – either augite,aegerine-augite or hedenbergite – commonly replaced by Fe–Ti oxidesand biotite, or in some cases by secondary hornblende. In addition, theSilhouette syenites typically contain primary amphibole –green-browncalcic clinoamphibole (hornblende, ferro-actinolite, ferro-winchite) orbrown kaersutite – and euhedral titanomagnetite. On the Congomentknoll, the syenites contain sparse, altered fayalitic olivine. Some samplesexhibit minor interstitial quartz or small subhedral albite (b2%). Apatite,zircon and monazite are accessory phases.

The syenodiorite consists mainly of zoned, anhedral Na-plagioclase(An10–33) and lesser alkali feldspar (orthoclase,microcline, anorthoclase;0.5–1.5 mm), as in Fig. 4a. Pale green, anhedral calcic clinopyroxene

(up to 2 mm) is extensively replaced by biotite, Fe–Ti oxides and insome cases, actinolite. If present, quartz is minor (b1%). Euhedral zircon(up to 0.2 mm) and apatite (up to 1 mm) are accessory phases.

The microsyenite dykes and apophyses are similar in mineralogy tothe syenite. In general, the matrix of the microsyenite consists ofanhedral, finely perthitic, sericitised K-feldspar, fragmented blue-green to brown sodic amphibole (likely arfvedsonite; up to 0.3 mm),Fe–Ti oxides (~0.01 mm) and variable proportions of anhedral quartz(0–25%). Euhedral phenocrysts (up to 2 cm long) of perthitic zonedorthoclase (Fig. 4c)may be present. Several samples exhibit fragmentedgreen hedenbergite (up to 1 mm), replaced by amphibole and ilmenite.Some samples contain accessory disseminated chalcopyrite.

The microsyenodiorite dykes in the breccia complex are mineralogi-cally equivalent to their plutonic counterparts. They contain significantplagioclase (~55%), with less abundant orthoclase, as well as subhedralgreen hedenbergite (1 mm), biotite, minor subhedral Fe–Ti oxides andaccessory apatite and zircon.

The trachyte is strictly a matrix-supported trachytic lithic tuff. Itconsists predominantly of a dark grey, glassy ash matrix, with distinctbanding and flow textures (Fig. 4b). The rocks contain deformed pheno-crysts of orthoclase (up to 2 mm), fragmented augite (~0.4 mm) andclusters of small Fe–Ti oxides. Entrained pieces of earlier trachyte(cpx–feldspar–oxides; up to 22 mm) are angular to subrounded, andthe smaller of these are partially digested to form concentrations ofFe–Ti oxides with rims of recrystallised K-feldspar (Fig. 4b).

The hypersolvus alkali granite consists predominantly of anhedralquartz (10–35%) and heavily altered, coarsely perthitic alkali feldspar(host mainly orthoclase, with minor microcline; 50–75%), with intersti-tial Ti-poor biotite, anhedral Fe–Ti oxides and fragmented blue-green tobrown sodic amphibole, with or without minor plagioclase. Sparsesecondary riebeckite is associated with clay mineral alteration of thefeldspars. Euhedral zircon (up to 0.1 mm) is an accessory phase.

The dolerite consists of a homogeneous, fine-grained (~0.15 mm)matrix of weakly aligned subhedral plagioclase, Fe–Ti oxides andfragmented clinopyroxene. Euhedral, concentrically-zoned plagioclasephenocrysts (up to 1 mm) may be present, commonly with inclusionsof altered clinopyroxene. Sparse phenocrysts of altered subhedralclinopyroxene and cubic Fe–Ti oxides are evident in some samples.Agglomerates of anhedral recrystallised K-feldspar rimmed by clino-pyroxene represent partly digested syenitic xenoliths. Sample TOS7(Fig. 4d) in particular is highly altered, with extensive replacement ofclinopyroxene by fine-grained actinolite and amorphous biotite (up to20%), and minor relict olivine replaced by serpentine and Fe–Ti oxides.

The gabbroic xenoliths hosted within the syenodiorite consistpredominantly of anhedral, zoned sodic plagioclase (An22–37; ~60%),large (up to 3 mm) deformed grains of strongly pleochroic biotite(~20%) and fragmented colourless clinopyroxene (diopside–augite;~10%), the latter typically replaced by Fe–Ti oxides and biotite. Theigneous texture, bulk chemical composition and isotopic composition(see later sections) indicate that these are fragments of gabbro, ratherthan sedimentary xenoliths, although they have clearly been extensivelydigested and metasomatised within the North Island magma chamber.

From the petrographic descriptions it is clear that there is a variablealteration across the suite and thus for geochemical comparisons, i.e. inall figures, the major and trace element compositions have beenrecalculated on a volatile-free basis.

4.3. Major and trace element geochemistry

Thewhole-rockmajor element compositions (Table 1; Fig. 5) clearlyreflect the alkaline, silica-saturated to slightly oversaturated nature ofthe suite. The lithologies from both islands show trends of decreasingTiO2, MgO, CaO and FeOT with increasing SiO2 (Fig. 9), from the mostprimitive units (gabbros and dolerites) to the least primitive ones(granites and microgranites), with a noticeable kink between 60 and65% SiO2. While there is some overlap between the microsyenite and

Fig. 3. a) Fragments of syenite and earlier trachyte,with reaction haloes, in porphyritic tra-chyte tuff, from Pointe Ramasse Tout, Silhouette Island. b) Microsyenite (paler) intrudingand brecciating trachyte tuff (darker), south of Pointe Ramasse Tout.

36 T.M. Owen-Smith et al. / Lithos 182–183 (2013) 33–47

syenite, the majority of microsyenite samples are more SiO2-rich andAl2O3-poor and trend towards granitic compositions.

In terms of trace element abundances, the sample set forms a coher-ent cluster (Fig. 6), although there is a relatively wide range of concen-trations, particularly in the large ion lithophile elements Ba and Sr, aswell asHf, Zr and P. Themarked negative Sr anomaly is themost strikingfeature of the suite. Dolerite TOS7 is the only sample distinctly differentfrom the Seychelles array. There is petrographic evidence of significantalteration and fluid flow through this rock and it is likely that the rockhas not retained its original trace element composition.

The Seychelles suite clearly exhibits light rare earth element (REE)enrichment, with variable heavy REE concentrations (Fig. 7). The sam-ples show parallel patterns (apart from dolerite TOS7), although witha wide range in concentrations. Positive Eu anomalies are evident inthe gabbros, dolerites, syenodiorites and two of the syenites, while theremainder of the syenite samples and all other lithologies exhibit nega-tive Eu anomalies.

4.4. Isotope geochemistry

The Sr-,Nd- and Hf-isotopic data for the Seychelles suite (Table 2)are combined with those of Dickin et al. (1986) for interpretation(Fig. 8). On the whole, the North Island initial 87Sr/86Sr values (takenat 65 Ma) are very similar to each other (0.703638–0.704078); theSilhouette samples less so (0.703507–0.705643). The εNd values are all

positive, ranging from 0.59 to 3.76. εHf compositions range between0.82 and 6.79 (Fig. 8b). Overall, the compositions for the two islandsoverlap. The suite plots between the compositions of modern dayRéunion plume basalts and Bulk Silicate Earth.

5. Discussion

5.1. Evolution of the Silhouette–North Island alkaline suite

5.1.1. The relative chronology of emplacementThe dating of Ganerød et al. (2011), summarised in Section 2, shows

that the Silhouette Island rocks are slightly older than the North Islandrocks, but the ages per rock type are essentially indistinguishableand therefore cross-cutting relations are used to determine the localchronology.

The trachyte tuff of the Silhouette volcanic complex is the earliestrock unit preserved, although the fragments preserved within it alludeto earlier syenitic phases. The main syenite–syenodiorite pluton likelyintruded these pre-existing volcanic cover rocks as a shallow-level stag-ing chamber, with negligible associated deformation. The increase inthe density of distribution of the microsyenite apophyses and dykes to-wards the contact with the main syenite body suggests that they werederived from it, asmultiple generations of feeders for surface eruptions.The trend in microsyenites is towards more quartz-rich, fluid-bearing,phenocryst-poor magmas. This, and the nature of the granite–syenite

Fig. 4. Photomicrographs viewed in crossed polars. a) Typical syenodiorite RT97N-3 (North Island) showing twinned plagioclase (plag) and alkali feldspar (ksp)with interstitial biotite (bi)and hedenbergitic clinopyroxene (cpx). b) Characteristic flow banding in the glassy groundmass of trachyte tuff LA97S-1A. Also evident are phenocrysts of augite, alkali feldspar andtitanomagnetite, as well as digested fragments of earlier trachyte (tr), with cores of titanomagnetite and rims of recrystallised alkali feldspar. c) Euhedral perthitic feldspar phenocrystin a fine-grained alkali feldspar matrix in microsyenite LA97S-1B. d) Euhedral twinned plagioclase phenocrysts in a plagioclase-actinolite-serpentine groundmass in porphyritic doleriteTOS7. (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.)

37T.M. Owen-Smith et al. / Lithos 182–183 (2013) 33–47

Table 1Major element (wt.%) and trace element (ppm) compositions for Seychelles alkaline rocks.

Silhouette Island

Sample TOS2 TOS7 TOS8 TOS9A TOS17 TOS21 TOS22 TOS25 TOS34 TOS35 LA97S-1A LA97S-1B LA97S-1C RT97SIL-1 RT97SIL-2

Rock type Msy. Dol. Msy. Msy. Sy. Dol. Tr. Sy. Dol. Sydi. Tr. Msy. Msy. Gr. Sy.

SiO2 63.0 48.4 69.1 64.2 61.5 49.4 60.4 63.4 52.9 57.0 64.6 68.6 67.3 70.0 62.0TiO2 0.43 0.62 0.31 0.44 0.49 1.84 0.80 0.37 2.38 1.77 0.50 0.38 0.43 0.29 0.74Al2O3 14.2 21.6 13.7 14.3 16.2 15.7 15.4 17.1 14.7 15.7 15.7 14.3 14.7 15.0 16.4Fe2O3T 5.11 9.66 4.66 5.44 4.80 11.91 9.03 4.12 10.90 8.77 5.76 4.64 5.13 3.08 6.64MnO 0.15 0.15 0.11 0.15 0.15 0.59 0.01 0.12 0.20 0.21 0.19 0.16 0.15 0.07 0.20MgO 0.8 3.6 0.3 0.2 0.5 5.0 0.4 0.4 2.9 1.9 0.4 0.1 0.1 0.2 0.4CaO 1.94 10.54 0.76 0.74 2.09 8.73 2.36 1.06 5.67 3.86 1.70 0.66 0.79 0.82 1.68Na2O 5.7 3.2 5.6 5.9 5.6 3.3 4.6 5.0 4.0 4.2 5.4 5.3 5.8 5.1 6.1K2O 5.30 0.51 5.04 5.31 6.01 1.97 5.06 6.25 2.73 3.30 5.36 5.18 5.33 5.35 5.78P2O5 0.04 0.07 0.06 0.01 0.06 0.72 0.25 0.02 1.12 0.72 0.09 0.02 0.04 0.06 0.15Cl 0.01 0.06 0.02 0.09 b0.01 0.02 0.03 b0.01 0.05 0.03 n.a. n.a. n.a. n.a. n.a.LOI 0.25 0.58 0.35 0.45 1.33 0.43 0.16 0.53 1.35 0.13 0.02 0.90 0.23 0.26 0.63Total 96.9 98.9 100.0 97.2 98.8 99.6 98.5 98.3 98.8 97.6 99.6 100.2 100.0 100.3 100.7Sc 9.68 28.1 n.a. 7.63 n.a. 34.3 27.3 8.10 27.3 23.8 17.4 9.5 6.5 4.9 9.1V 1.76 172 b4 0.70 b4 252 0.96 1.05 155 30.2 9.1 0.2 b0.1 4.40 b0.1Ni 2.01 72.8 7 1.34 b2 30.4 1.79 1.45 2.75 2.41 1.4 0 0 0 0Co 0.75 34.1 8 0.63 b3 28.9 1.94 1.17 16.7 3.77 1 0 69 80 44Rb 118 39.6 138 117 79 132 78.6 105 59.1 55.1 105 134 115 230 95.3Sr 6.31 366 5 11.0 8 542 64.2 7.04 520 442 50.4 9.15 13.2 49.8 12.05Y 66.3 14.7 90 62.1 22 43.3 61.3 16.7 54.1 46.0 72.5 83.7 68.7 68.1 31.1Zr 111 20.1 1163 67.6 147 61.5 73.4 56.0 91.4 35.5 780 1012 879 425 188Nb 193 6.01 232 128 65 70.5 115 79.9 101 91.4 161 221 96 184 104Ba 24.4 66.9 b4 207 b4 588 668 26.1 2225 9560 n.a. n.a. n.a. n.a. n.a.La 106 6.36 88 118 22 47.3 81.1 37.9 72.6 66.5 106 121 177 123 53Ce 192 13.8 158 214 14 80.4 149 68.2 137 126 190 209 292 198 102Pr 20.1 1.61 n.a. 22.3 n.a. 9.78 16.3 6.93 15.5 14.5 21.0 23.0 30.0 18.7 11.5Nd 72.7 7.07 n.a. 80.3 n.a. 39.4 63.0 24.7 62.9 57.8 70.0 73.0 98.0 56 41Sm 12.7 1.79 n.a. 14.5 n.a. 7.85 12.0 4.17 11.9 11.0 12.2 12.5 14.9 9 7Eu 0.74 0.80 n.a. 1.67 n.a. 2.58 3.17 0.40 4.50 7.23 1.89 0.54 1.7 0.69 0.51Gd 11.3 2.09 n.a. 12.8 n.a. 7.91 10.5 3.30 11.2 9.98 10.9 11.1 12.2 8.2 6.5Tb 1.91 0.36 n.a. 2.00 n.a. 1.17 1.68 0.50 1.59 1.45 n.a. n.a. n.a. n.a. n.a.Dy 11.9 2.43 n.a. 12.2 n.a. 7.29 10.7 3.04 9.47 8.62 11.0 11.9 10.7 9.3 5Ho 2.37 0.50 n.a. 2.27 n.a. 1.47 2.12 0.58 1.81 1.61 2.50 2.80 2.4 2.2 1.09Er 7.01 1.54 n.a. 6.48 n.a. 4.23 6.38 1.74 5.13 4.46 7.10 8.10 6.5 6.5 2.8Tm 1.00 0.22 n.a. 0.93 n.a. 0.57 0.92 0.26 0.66 0.57 n.a. n.a. n.a. n.a. n.a.Yb 6.68 1.51 n.a. 6.40 n.a. 3.73 6.08 1.96 4.09 3.73 7.80 9.50 7.3 7.6 3.1Lu 1.04 0.23 n.a. 1.01 n.a. 0.57 0.93 0.33 0.60 0.54 1.22 1.51 1.21 1.16 0.53Hf 3.96 0.76 n.a. 2.73 n.a. 1.70 2.88 1.48 2.24 1.37 16.7 22.2 16.3 12.5 4.4Ta 11.7 0.36 n.a. 7.70 n.a. 4.02 6.56 4.74 6.20 5.46 9.60 13.4 6.1 16.1 5.5Pb 10.1 14.8 n.a. 41.7 n.a. 8.07 13.6 10.6 14.5 14.7 21.0 8.10 17.1 12.8 10.1Cu 0 171 4 17 b4 b4 15 b4 12 b4 12 1.1 0 0 0Zn 0 134 120 131 62 358 122 55 124 120 142 136 123.6 59.7 100.6Cr 0 12 33 b12 19 17 b12 b12 b12 b12 1.5 15.3 0 0 0Ga n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. 27 27 26 23 22W n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. 3 3 361 424 200S n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. 29 263 383 23 381As n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. 7.00 0.00 3 0 2Th 16.4 0.57 n.a. 12.6 n.a. 4.06 9.33 4.40 7.10 7.00 18.4 29.0 16.8 44 4.9U 4.32 0.80 n.a. 2.40 n.a. 1.23 1.10 1.18 1.59 1.47 4.10 6.20 2.9 8.9 1.1

North Island

Sample TON28 TON30 TON32 TON33 RT97N-3 RT97N-4 LA97N17A LA97N-17B LA97N-18

Rock type Sy. Sydi. Msydi. Sy. Sydi. Sy. Gb. Sydi. Sy.

SiO2 60.2 52.6 56.5 61.5 58.4 59.6 47.9 57.9 62.5TiO2 0.85 1.69 1.27 0.31 0.95 0.98 2.85 0.99 0.38Al2O3 17.2 18.1 18.5 16.8 17.6 17.1 17.5 18.1 17.2Fe2O3T 5.80 9.40 8.22 5.84 7.37 7.05 11.01 7.36 5.40MnO 0.14 0.16 0.15 0.17 0.16 0.18 0.18 0.15 0.17MgO 0.7 2.2 1.6 0.2 1.3 0.9 3.7 1.4 0.2CaO 2.02 6.10 4.18 1.44 3.65 3.27 8.30 3.99 1.77Na2O 5.1 3.9 4.3 6.0 5.8 6.0 4.2 5.9 6.1K2O 6.22 2.70 3.36 5.55 4.62 4.68 2.71 4.02 6.17P2O5 0.15 0.90 0.39 0.02 0.31 0.31 1.44 0.36 0.07Cl 0.01 0.04 0.03 b0.01 n.a. n.a. n.a. n.a. n.a.LOI 0.53 0.23 0.20 0.32 0.07 0.09 0.34 0.11 0.55Total 99.0 98.0 98.7 98.1 100.2 100.2 100.0 100.3 100.5Sc 11.1 16.5 9.68 4.74 7 9.5 20.5 1.8 3.5V 1.42 62.7 22.2 0.39 39 0.4 168 9 b0.1Ni 3.00 4.70 3.26 1.98 0 0 0 0 0Co 2.87 12.8 10.4 0.62 57 70 83 42 69Rb 57.7 69.4 92.9 180 96.75 100.95 64.85 89.05 116.45

38 T.M. Owen-Smith et al. / Lithos 182–183 (2013) 33–47

contact, suggests that granitewas the youngest phase emplaced into thecomplex.

The porphyritic dolerite dykes of Silhouette Island are describedhere for the first time and have not been dated, but from cross-cutting relations they are inferred to be contemporaneous withthe main microsyenite phase. Chemical similarities suggest thatthe dolerites are feeders from a Silhouette equivalent of the NorthIsland gabbro.

On North Island, the gabbroic xenoliths indicate that the gabbroicphase is earliest, predating the syenodiorite and syenite. The breccia com-plex reflects multiple generations of feeder dykes from the pluton— firstsyenodioritic, then syenitic in composition.

Thus Silhouette and North Island represent two eruptive centres in asingle magmatic system, which saw several generations of magmas ofdifferent compositions emplaced over a short time interval. There is ageneral trend, at least in the plutonic rocks, from gabbroic, tosyenodioritic, to syenitic and finally to granitic compositions.

5.1.2. Modelling the history of fractional crystallisationIt is evident from the range of mafic to intermediate to felsic rocks,

the trends in major element compositions, the presence of Eu anoma-lies, the range in trace element concentrations, the strong enrichmentin the highly incompatible trace elements and the cumulate texturesin these rocks that they are part of a single igneous suite, related byprocesses of magma differentiation. The major element variationsprovide a means to evaluate the role of fractional crystallisation inproducing the diversity of rock types observed.

Since these are cumulate rocks, a quantitative model is difficult toconstrain, as we cannot confidently assess the proportions of liquidsand accumulated minerals. In addition, the field relations indicate thatthere have been multiple phases of mafic and syenitic magmatism andthe major element trends of the suite thus reflect multiple, although

perhaps similar, fractional crystallisation paths. Nevertheless, a qualita-tive model can to some extent account for the compositional diversityand major element trends observed.

Fig. 9 shows arrays of major element abundances of the completesuite, with ranges of stoichiometric compositions of likely fractionatingminerals, trend lines for approximate evolving magma compositionsand suggested plausible fractionating mineral assemblages. The modelsuggests that the Silhouette and North Island suite overall can beexplained in simple terms by a 3-stage fractionation process from amafic parental magma. The first stage involves fractionation dominatedby forsteritic olivine, calcic clinopyroxene and ilmenite, driving theliquid from basaltic to intermediate compositions. There is alreadysome plagioclase fractionating at this time, as indicated by the Euanomalies in the gabbros, but its effect on the major element trend isminor. The second stage sees the introduction of calcic plagioclase as amajor fractionating mineral. In the third stage, olivine and ilmeniteare of minor importance and the major fractionating assemblage isclinopyroxene, plagioclase and alkali feldspar, driving the melt tomore silicic compositions. This is in line with the fractional crystal-lisation assemblages determined from experimental studies (e.g.Nekvasil et al., 2004) and modelling (e.g. Harris, 1983) of alkalinesilica-saturated volcanic suites.

The effects of crystal accumulation are also evident here. The K2Ovariations clearly show the trend of alkali feldspar accumulation in thesyenites, as suggested by the petrography. Similarly, plagioclaseaccumulation in the syenodiorites is apparent in the Al2O3, and tosome extent Na2O, variations. There are also indications of variablecalcic plagioclase accumulation in the dolerite samples, particularly inTOS7, as observed in the petrography of these rocks. The microsyenitedykes show a continuous progression from equivalent syenitic to equiv-alent granitic compositions. The dykes represent the evolving liquidtapped from the magma chamber, whereas the syenites represent

Table 1 (continued)

North Island

Sample TON28 TON30 TON32 TON33 RT97N-3 RT97N-4 LA97N17A LA97N-17B LA97N-18

Rock type Sy. Sydi. Msydi. Sy. Sydi. Sy. Gb. Sydi. Sy.

Sr 68.1 676 570 4.20 373.65 359.9 769.75 493.4 42Y 14.9 31.4 28.2 40.0 33.7 40.2 35.2 32.5 31.5Zr 68.3 80.9 38.5 463 392.9 300.9 109.1 313.2 198.5Nb 46.1 93.9 111 155 102.5 104.4 63 106.7 86.2Ba 1179 2026 1672 10.4 n.a. n.a. n.a. n.a. n.a.La 29.6 63.9 59.1 78.7 62 71.1 58 64.54 55.03Ce 52.2 115.7 104 141 103 120.16 106 107.97 93.71Pr 5.40 12.1 10.5 14.5 10.7 12.49 12.4 11.51 9.91Nd 20.6 46.5 38.8 52.5 36 42.6 47 40.32 33.41Sm 3.54 7.99 6.21 8.86 5.6 6.93 7.3 6.18 5.78Eu 3.60 3.92 2.80 0.30 2 2.62 3.9 2.75 1.89Gd 3.24 7.12 5.57 7.54 5.4 6.71 7.5 6.08 5.52Tb 0.47 0.97 0.84 1.17 n.a. n.a. n.a. n.a. n.a.Dy 2.80 5.79 4.92 7.29 5 6.31 5.7 5.4 5.05Ho 0.53 1.08 0.96 1.45 1.11 1.39 1.19 1.17 1.2Er 1.51 2.96 2.75 4.58 3.1 3.89 2.9 3.27 3.23Tm 0.20 0.38 0.37 0.70 n.a. n.a. n.a. n.a. n.a.Yb 1.38 2.45 2.38 5.32 3.3 4.07 2.4 3.31 3.72Lu 0.21 0.34 0.36 0.89 0.51 0.61 0.34 0.51 0.62Hf 1.64 1.82 1.20 10.4 8.7 8.1 2.5 0.5 6Ta 2.61 4.45 5.67 8.63 6.7 7.6 4.3 6.5 6.3Pb 6.55 6.95 9.14 9.38 7.9 16.8 3.3 10.6 10.5Cu b4 9 7 b4 2.9 0 0 0 0Zn 47 94 87 82 73.9 89.8 78.4 75.2 86.4Cr b12 b12 b12 b12 0 0 0 0 0Ga n.a. n.a. n.a. n.a. 21 20 19 21 20W n.a. n.a. n.a. n.a. 259 287 323 145 274S n.a. n.a. n.a. n.a. 1164 54 848 770 1217As n.a. n.a. n.a. n.a. 0 0 0 0 0Th 4.45 3.50 4.90 9.28 10.4 19.6 4.1 12 10.2U 1.09 0.84 1.19 1.98 2.3 3.7 0.9 2.7 2.2

Total Fe as Fe2O3. Italicised values indicate analysis by XRF rather than ICP-MS. All samples renormalised to volatile-free compositions. n.a. = not analysed. Sy. = syenite; Sydi. =syenodiorite; Msy. = microsyenite; Msydi. = microsyenodiorite; Tr. = trachyte; Gr. = granite; Dol. = dolerite; Gb. = gabbroic xenolith.

39T.M. Owen-Smith et al. / Lithos 182–183 (2013) 33–47

accumulation within the chamber. This further supports an origin forthe granite by progressive fractional crystallisation.

Trace element variations also give information on crystallisingphases. It is clear from the petrography that for the samples withpositive Ba anomalies relative to Rb and Th, the Ba resides in magmaticbiotite (5–15% in these rocks). Variations inHf andZr in the syenites andsyenodiorites likely record the influence of amphibole, and to a lesserextent, pyroxene. P reflects variable accumulation of apatite.

Eu anomalies are clearly controlled by the behaviour of plagioclase.This is evident in the correlation between the magnitudes of Euanomalies and normative anorthite contents, as well aswith plagioclaseAn contents (from microprobe analyses). The Sr2+ shows similarplagioclase-controlled partitioning. In general the dolerites, gabbroand syenodiorites have positive Eu anomalies and high Sr concentra-tions, indicating plagioclase accumulation. This is supported by thepresence of discrete, cumulus, calcic plagioclase in these samples. Onthe other hand, the negative Eu anomalies and relatively low Sr in thesyenites, microsyenites, trachytes and granites indicate plagioclaseremoval during fractionation. These samples contain sodic plagioclasewithin perthite. The apparent discrepancies in trace element composi-tions (particularly Sr, Ba and Eu) between the two islands previouslyidentified by Dickin et al. (1986) and Devey and Stephens (1992) areclearly due to the differential distribution of rock types.

5.2. The source of the Seychelles alkaline magmas

5.2.1. Similarities between Seychelles and Réunion Plume mantleThe isotopically-depleted nature of the Seychelles alkaline suite

(Fig. 8) clearly indicates a mantle, rather than crustal, source for thesemagmas, in line with the basaltic parental magma inferred fromfractionation modelling. But is this source related to the Réunionplume?

The Seychelles Sr- and Nd-isotopic values (and Pb-isotopic values ofDickin et al., 1986) showsomeoverlapwith thefield ofmodernRéunionbasalts, i.e.material supplied, uncontaminated, by the plume today. Thisis consistent with the original derivation of these rocks from a Réunionplume source, itself comprising OIB-like plume asthenosphere andentrained shallow MORB-like asthenosphere.

However, it is also evident, especially from the Hf-isotopic data(Fig. 8b), that the Seychelles cluster (mean εHf = +3.4) is displacedto slightly more enriched values than Réunion (mean εHf = +10.6).Furthermore, it appears from the Sr- and Nd-isotopic compositionsthat, although there is substantial overlap, the Silhouette Island samples(mean ISr = 0.704534; εNd = +2.4) generally trend to more enrichedvalues than North Island (mean ISr = 0.703841; εNd = +3.6). This

suggests that an additional, minor continental crustal or enriched sub-continental lithospheric mantle (SCLM) component is involved, andthat it has affected the earlier Silhouette magmas to a greater extent.

In many cases, oxygen isotopes may be useful in distinguishingcrustal from lithospheric mantle input. Dickin et al. (1986) recordedδ18O values between +1.98 and +7.27‰ for the Silhouette and NorthIsland rocks. Normally, the upper end of this rangewould be consideredconsistent with mantle derivation of these magmas without any assim-ilation of continental crust, while the trend to low δ18O values is indica-tive of subsolidus fluid interactions (Eiler, 2001). However, Harris andAshwal (2002) showed that the δ18O compositions of the SeychellesPrecambrian granitoid crust – the only potential crustal contaminant

Fig. 6. Multi-element trace element diagrams for samples from Silhouette and NorthIslands, normalised to the primitive mantle values of McDonough and Sun (1995). a) Tra-chyte and microsyenite; b) syenite, syenodiorite and granite, and c) dolerite and gabbroicxenolith.

Fig. 5. Classification of the Seychelles alkaline rocks analysed in this study by the total al-kalis versus silica diagram for plutonic rocks ofMiddlemost (1994). Both plutonic and vol-canic rocks are plotted for the sake of comparison.

40 T.M. Owen-Smith et al. / Lithos 182–183 (2013) 33–47

here – are unusually low for crustal material, in the range −1.2 to+7.5‰. The effects of crustal assimilation may therefore not bediscernable.

Other isotopesmay shedmore light on the picture. The Pb-isotopic ra-tios of the Seychelles suite (206Pb/204Pb = 17.81–18.41, 207Pb/204Pb =15.52–15.57 and 208Pb/204Pb = 37.85–38.55 at 65 Ma; Dickin et al.,1986) trend from Réunion-like compositions to unradiogenic values(equivalently, North Island to Silhouette Island). Although Pb-isotopicdata are sparse for Seychelles Precambrian crust, and non-existant forSCLM in this region, crustal contamination would be expected to displacecompositions in the opposite direction. This points to a SCLM rather thancrustal component.

The majority of the ~750 Ma Seychelles granitoid crust would havehad very low εNd (−13.27 to +1.18 at 65 Ma) and high ISr (0.706491to 1.226946; Ashwal et al., 2002). Any small component of this crustwould affect the Seychelles alkaline suite significantly, so only verysmall degrees of assimilation would be required for a large shift in theisotopic values. We have modelled the evolution of a basaltic plume-derived parent by simple mixing with a crustal contaminant, aswell as by assimilation in combination with Rayleigh fractionalcrystallisation (AFC; Fig. 10). We have used typical Réunion olivinebasalt RE603 of Fisk et al. (1988) as a proxy for the mafic parent andthe Seychelles Precambrian granitoids of Ashwal et al. (2002) as crustalassimilants,with the ratio of assimilant to fractional crystallisation at 0.2(the best-fit rate). The proportions of fractionating minerals werederived from modelling of the crystallisation of this basaltic parentusing the MELTS program of Ghiorso and Sack (1995) at a pressure of1 kbar and crystallisation temperature of 1252 °C under theQFMbuffer.Mineral partition coefficients for Sr and Nd used to calculate the bulkmineral–melt distribution coefficients (D) are from the compilation ofErsoy and Helvacı (2010) for a basic system.

Certainly, the major element modelling suggests that the suite had amuchmore complex fractional crystallisation and accumulation historythan this model and there are many assumptions in the conditions ofassimilation and fractionation, however even this simplemodel demon-strates some important results. Firstly, the typical Seychelles crust(represented by the average of all granitoid samples) is far too radio-genic to explain the displacement of any of the Silhouette and NorthIsland samples by assimilation. If a less extreme end member is used,such as the Mahé granites LA9725A or LA97M26B, which may beplausible since this is the closest crust geographically, part of the suitemay be explained by a maximum of 25% assimilation. However, themajority of the samples have initial Sr ratios too low to be explained

Fig. 7. Rare earth element abundances for Seychelles samples from this study, normalisedto the chondritic values of Anders andGrevesse (1989). a) Trachyte,microsyenite, doleriteand gabbroic xenolith. b) Syenite, syenodiorite and granite.

Table 2Rb–Sr, Sm–Nd and Lu–Hf isotopic data for Seychelles alkaline rocks.

Sample Rock type Rb (ppm) Sr (ppm) 87Rb/86Sr 87Sr/86Sr ISr 65 Sm (ppm) Nd (ppm) 147Sm/144Nd 143Nd/144Nd εNd 65

SilhouetteLA97S-1A Trachyte 103 45.9 6.533 0.709916 ± 18 0.703990 ± 18 13.9 71.4 0.1175 0.512714 ± 17 2.1 ± 0.3LA97S-1B Microsyenite 131 6.07 62.73 0.762466 ± 22 0.705643 ± 22 14.0 73.9 0.1147 0.512697 ± 17 1.8 ± 0.3LA97SIL-1C Microsyenite 113 8.86 36.90 0.736937 ± 17 0.703507 ± 17 17.0 99.3 0.1034 0.512707 ± 18 2.1 ± 0.4RT97 SIL-1 Granite 230 55.2 12.04 0.714916 ± 15 0.704012 ± 15 10.3 58.9 0.1054 0.512746 ± 24 2.9 ± 0.5RT97 SIL-2 Syenite 95.0 7.95 34.50 0.735820 ± 37 0.704567 ± 37 7.97 44.0 0.1096 0.512752 ± 25 2.9 ± 0.5

NorthRT97 N-3 Syenodiorite 97.5 370 0.7621 0.704586 ± 20 0.703896 ± 20 6.95 36.9 0.1139 0.512795 ± 17 3.8 ± 0.3LA97 N-17A Gabbroic xeno. 68.1 763 0.2583 0.704106 ± 19 0.703872 ± 19 9.28 45.7 0.1228 0.512785 ± 18 3.5 ± 0.4

Sample Rock type Lu (ppm) Hf (ppm) 176Lu/177Hf 176Hf/177Hf εHf 65

SilhouetteLA97S-1A Trachyte 1.18 15.9 0.0106 0.282879 ± 5 4.3 ± 0.2LA97S-1B Microsyenite 1.47 20.5 0.0102 0.282949 ± 5 6.8 ± 0.2LA97SIL-1C Microsyenite 1.15 14.1 0.0116 0.282885 ± 6 4.5 ± 0.2RT97 SIL-1 Granite 1.14 11.9 0.0136 0.282814 ± 7 1.9 ± 0.2RT97 SIL-2 Syenite 0.528 3.55 0.0212 0.282817 ± 13 1.7 ± 0.5

NorthRT97 N-3 Syenodiorite 0.494 6.17 0.0114 0.282869 ± 4 3.9 ± 0.1LA97 N-17A Gabbroic xeno. 0.338 2.43 0.0198 0.282791 ± 15 0.8 ± 0.5

Decay constants for age corrections are: λ87Rb = 1.393 × 10−11 yr−1 (Nebel et al., 2011), λ147Sm = 6.54 × 10−12 yr−1 (Lugmair and Marti, 1978), λ176Lu = 1.865 × 10−11 yr−1

(Scherer et al., 2001). Epsilon values calculated using CHUR parameters 147Sm/144Nd = 0.1967 (Peucat et al., 1989), 143Nd/144Nd = 0.512638 (Goldstein et al., 1984), 176Lu/177Hf =0.0336 (Bouvier et al., 2008) and 176Hf/177Hf = 0.282785 (Bouvier et al., 2008). Uncertainties reported on measured ratios are 2 standard errors of the mean (2σ).

41T.M. Owen-Smith et al. / Lithos 182–183 (2013) 33–47

by any plausible variation in the endmembers or mixing parameters ofthese processes. This trend of enrichment in isotopic Ndwith negligiblechange in isotopic Sr indicates mixing with an EM1-like end member,which suggests assimilation of lithospheric mantle rather than crust.

Trends in Th/Yb and Nb/Yb ratios (Fig. 11) for the suite furthersupport derivation from a plume source. The range of modern Réunionbasalts represents plume asthenosphere with variable incorporation ofentrained depleteduppermantle, but the Seychellesmagmas are clearlydominated by plume material. In addition, displacement of the suite isin the direction of fractional crystallisation effects and not crustalcontamination. This favours enrichment within the mantle instead, assuggested by the isotopes, although the involvement of SCLM cannoteasily be distinguished here, due to insufficient data for sub-Deccanmantle.

We have shown that the Seychelles alkaline magmas have strongsimilarities with modern Réunion basalts. A further test of derivationis comparison with known products of the Réunion plume — rocksfrom the Deccan LIP.

5.2.2. Similarities between Seychelles and Deccan rocksThe Seychelles Sr–Nd cluster overlaps closelywith the Ambenali and

Mahabaleshwar tholeiitic basalt suites of the Deccan Traps (Fig. 8a),which is strong evidence for a common mantle source. The AmbenaliSuite is considered the least crustally-contaminated basalt suite in theDeccan and the most similar in composition to the original materialfed by the Réunion plume (Fisk et al., 1988; Lightfoot andHawkesworth, 1988; Peng and Mahoney, 1995; Peng et al., 1994). TheAmbenali–Panhala–Mahabaleshwar trend of compositions has beeninterpreted as plume-derived mantle melts with a contribution fromold, enriched SCLM, and not the assimilation of crustal material(Lightfoot and Hawkesworth, 1988; Lightfoot et al., 1990). This issupported by mantle xenolith studies (Lightfoot and Hawkesworth,1988). This distinction is most evident in the Pb-isotopic ratios, wherethe trend is towards unradiogenic Pb, in the opposite direction of thecrustally-contaminated Deccan suites (Lightfoot and Hawkesworth,1988). This is identical to what is observed in the Seychelles alkalinesuite. Since the Seychelles and India were originally part of the samecontinent, it is likely that the SCLM below the Seychelles alkalinecomplex was similar to that which imparted the enriched mantlesignature on the Mahabaleshwar magmas.

There are also Sr–Nd isotopic and Th/Yb vs Nb/Yb affinities with theuncontaminated Deccan alkaline intrusive complexes (Figs. 8a; 13),particularly the slightly more enriched group (Mundwara, Barmer)interpreted as sourced from Réunion plume mantle and plume-metasomatised lithosphere (Simonetti et al., 1998).

Trace element comparisons between equivalent rock types, i.e. rockswith a similar fractionation history, can also reveal similarities inmagma source. The Deccan province exhibits both tholeiitic and alkalibasalts, as well as volumetrically minor alkaline and carbonatitic intru-sions and lavas (Basu et al., 1993; Gwalani et al., 1993; Ray et al., 2000;Sen et al., 2009; Simonetti et al., 1998). Of these alkaline rocks, themajority are silica-undersaturated, such as nephelinites, basinites andtephrites, while alkaline silica-saturated rocks, such as trachytes andsyenites, form a very minor component (Gwalani et al., 1993; Rayet al., 2000; Simonetti et al., 1998). The Deccan counterparts to theSeychelles alkaline rocks are clearly this latter group, although there isa paucity of trace element data for comparison. Nevertheless, Fig. 12aand b shows that there are strong similarities in the REE patterns ofthe Seychelles trachytes and microsyenites, and syenites andsyenodiorites, with their Deccan counterparts. The considerable rangein the magnitudes of Eu anomalies in the Seychelles rocks is an obviousdifference, suggesting more extreme fractionation and accumulation.

The dolerite dykes of Silhouette (this study) and gabbro of NorthIsland (Devey and Stephens, 1992) represent Seychelles mafic litholo-gies that may be compared with the Deccan mafic suites. There isvariability in the absolute abundances of the REE between thesesamples (Fig. 7), a reflection of differential fractionation and accumula-tion, but the shape of the REE patterns is similar. The REE patterns forthe Seychelles dolerites (TOS21 and TOS34) overlap with those of typi-cal Deccan alkali basalts (Fig. 12c). The Seychelles gabbro has beenaffected by the accumulation of plagioclase. Dolerite TOS7 is quitedistinct from the Deccan basalts, but shows equivalent low levels ofREE enrichment similar to offshore basalts on the Seychelles plateau,which have been linked with the later stages of Deccan magmatism(Collier et al., 2008).

Comparison between the Seychelles rock types and the Deccan felsicand mafic alkaline components is also possible for a wider spectrum oftrace elements (Fig. 13), although the Deccan data in this case areeven more sparse. Once again, the Seychelles trachytes and micro-syenites, syenites and diorites, and gabbros and dolerites generally fallwithin the range of their Deccan equivalents. This further supports aDeccan affinity for the Seychelles suite.

The presence of mantle-derived, silica-saturated alkalinemagmas inthe Seychelles, with no evidence of crustal contamination, is a point ofinterest. As we have discussed, this is in contrast to the predominantly

Fig. 8. a) Sr–Nd isotopic compositions of the Seychelles alkaline rocks (Dickin et al., 1986;this study), local Precambrian granitoid crust (Ashwal et al., 2002), modern Réunion ba-salts (Fisk et al., 1988), Deccan tholeiitic basalt suites (Lightfoot and Hawkesworth,1988; Peng et al., 1994, 1998), and Deccan alkaline intrusive complexes and lavas(Gwalani et al., 1995; Melluso et al., 2002; Sen et al., 2009; Simonetti et al., 1998), allcorrected to 65 Ma. Depleted Mantle and Bulk Silicate Earth (dashed lines) are fromWorkman and Hart (2005). Indian Ocean MORB is from Mahoney et al. (1989) andChauvel and Blichert-Toft (2001). GLOSS = average global subducting sediments (Plankand Langmuir, 1998). b) Hf- and Nd-isotopic compositions for the Seychelles alkalinesuite (this study), in relation to the field of modern Réunion plume basalts (Paul et al.,2005) and alkali and tholeiitic basalts from Kutch (north-western India) in the DeccanTraps (Sen et al., 2009). Approximate EMI, EMII andHIMU compositions are from the com-pilation of Faure andMensing (2005). DepletedMantle and Bulk Silicate Earth are as in (a).Indian Ocean MORB is from Salters (1996) and Chauvel and Blichert-Toft (2001). SCLMfield is from Salters and Hart (1991). GLOSS and the mantle array are from Chauvel et al.(2008). The Pacific-Indian mantle boundary is from Pearce et al. (1999).

42 T.M. Owen-Smith et al. / Lithos 182–183 (2013) 33–47

Fig. 9.Major element compositions for the complete Seychelles suite (this work; Devey and Stephens, 1992) shown with a range of ideal compositions of likely fractionating minerals. Kvalues indicate the proportion of K relative to Na in alkali feldspar; Di values the proportion of Mg relative to Fe in calcic clinopyroxene. The solid trend lines represent the inferredapproximate evolution path of a hypothetical basaltic liquid undergoing fractionation dominated by the mineral assemblage indicated.Where applicable, the actual average compositionof cumulus alkali feldspar in the syenites (from microprobe analyses; Supplementary Table 4) is shown.

43T.M. Owen-Smith et al. / Lithos 182–183 (2013) 33–47

silica-undersaturated alkaline Deccan lithologies on the Indian subcon-tinent. The relativelyminor silica-oversaturated Deccan lithologies havelargely been interpreted either as partial melts of Deccan tholeiiticbasalt, or attributed to assimilation of crustal material (Chatterjee andBhattacharji, 2001; Gwalani et al., 1993; Lightfoot et al., 1987). How-ever, this suite is not unique in the scheme of plume magmatism, bothin a continental setting (e.g. the ‘transitional series' of the East AfricanRift System; Kampunzu and Mohr, 1991) and oceanic setting (e.g.lavas of Ascension Island; Harris, 1983).While there is still much debateover the origin of silica-saturated alkaline suites, most workers empha-sise the importance of fractional crystallisation (e.g. Nekvasil et al.,2004).

We propose that the Seychellesmagmatism resulted from shallowermelting than the generally early, low-degree melting under the Indian

continent that produced the silica-undersaturated Deccan magmas,but lower degrees of partial melting than that which produced themain episode of Deccan tholeiitic flood volcanism, due to its positionat the periphery of the plume. This produced a mildly alkalic olivinebasalt parental melt that underwent substantial fractional crystal-lisation in crustal magma chambers.

5.3. Implications for the Deccan LIP

The Silhouette–North Islandmagmatismmarks the last phase of Ré-union plume activity in the Deccan event before continental break-up.While the earliest age for the onset of rifting at the Carlsberg Ridgehas been recorded as 63.4 Ma, from sea-floor magnetic anomalies(Collier et al., 2008), the mean age of 63.54 Ma for Silhouette Islandlithologies suggests that extension was in action at least from thistime. It is also evidence of plume involvement in the final break-upevent.

In addition, this study confirms that Deccan products are notrestricted to the Indian side of the rift, and thus significantly increasesthe original areal extent of the Deccan flood basalt province. This hasimplications for calculations of the original volume of material erupted,which themost recent estimates place at up to 8.6 × 106 km3 (Eldholmand Coffin, 2000, in Courtillot and Renne, 2003), and thus for the associ-ated impact on global climate.

6. Conclusions

The Seychelles plutonic–volcanic alkaline complex, on Silhouetteand North Islands, consists of a main plutonic body of alkali gabbro,syenite, syenodiorite and alkali granite, representing the subvolcanicchamber, and remnants of the volcanic cover, as trachyte tuffs, cut bymicrosyenite, microsyenodiorite and dolerite feeder dykes. Fractionalcrystallisation of plagioclase, olivine, clinopyroxene, ilmenite and K-feldspar was responsible for the diversity of rock types present.

From the isotopic and trace element compositions of the Seychellesalkaline suite, it may be concluded that these magmas were derivedfrom a Réunion plume source, equivalent to that of the Deccan basalts,with an additional minor contribution from an enriched source, likelysub-continental mantle lithosphere. Dated at 63 to 63.5 Ma, thisSeychelles alkaline magmatism coincides with the culmination of theDeccan magmatic event and the initiation of rifting between the

Fig. 11. Th/Yb versus Nb/Yb for the Seychelles alkaline suite (this work; Devey andStephens, 1992), compared to the field of modern-day Réunion plume basalts (Fisket al., 1988; Paul et al., 2005), Deccan tholeiitic basalts (Chatterjee and Bhattacharji,2001; Lightfoot and Hawkesworth, 1988; Sheth et al., 2004), Deccan alkaline mafic rocks(Karmalkar et al., 2005) and Deccan alkaline felsic rocks (Chatterjee and Bhattacharji,2001). N-MORB, E-MORB and OIB (Ocean Island Basalt) compositions are from Sun andMcDonough (1989); SCLM from McDonough (1990); GLOSS from Plank and Langmuir(1998). Numbered vectors approximate the direction of 1) crustal contamination and2) fractional crystallisation. Dashed lines indicate the mantle array.

Fig. 10. a) Modelled mixing and AFC curves for the evolution of a basaltic plume-derived parent, represented by typical Réunion basalt RE603 (Sr = 324 ppm, Nd = 20 ppm, ISr =0.704057, εNd = +4.46, DSr = 0.14, DNd = 0.33; Fisk et al., 1988), assimilating Precambrian granitoid crust — samples LA97M26B (Sr = 407 ppm, Nd = 33 ppm, ISr = 0.706491,εNd = −5.05), LA97M25A (Sr = 141 ppm, Nd = 44 ppm, ISr = 0.712264, εNd = −5.21) or average Seychelles granite (Sr = 181 ppm, Nd = 49 ppm, ISr = 0.794389,εNd = −5.87) from Ashwal et al. (2002). Tick marks on mixing lines represent intervals of 10% (up to 90% in the case of AFC). Silhouette and North Island isotopic compositionsare from this work and Dickin et al. (1986). b) Zoomed in view of the same scenario for greater detail.

44 T.M. Owen-Smith et al. / Lithos 182–183 (2013) 33–47

Seychelles and Indian continental plates. The Seychelles thus recordsthe last phase of Deccan magmatism from the western margin of theRéunion Plume, before rifting moved it out of range.

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.lithos.2013.09.011.

Acknowledgements

The authors wish to thank Prof. Anton Le Roex of the University ofCape Town for the ICP-MS analyses. We are also indebted to Patrick

Samson of Seychelles Petroleum Company Limited and Linda Vanherckand the staff of North Island Lodge for their assistance with fieldworkand access. L. Melluso, A. K. Baksi, R. B. Trumbull and G. Sen contributedconstructive reviews of the paper. Funding for this project came fromresearch grants to T.H. Torsvik from the Norwegian Research Counciland the Norwegian Geological Survey, and to L.D. Ashwal and T.M.Owen-Smith from the South African National Research Foundationand the Mellon Postgraduate Mentoring Programme.

Fig. 12. REE patterns for selected Seychelles (this work; Devey and Stephens, 1992) andequivalent Deccan lithologies: Salsette Island (Lightfoot et al., 1987), Sirawasan-Dughdaand Phenai Mata complexes (Gwalani et al., 1995), Saurashtra dykes (Chatterjee andBhattacharji, 2001), Mt. Girnar complex (Paul et al., 1977), Kutch alkaline mafic rocks(Karmalkar et al., 2005; Sen et al., 2009), AmbaDongar complex (Ray et al., 2000) andDec-can tholeiitic basalts (Lightfoot and Hawkesworth, 1988; Lightfoot et al., 1990; Peng et al.,1994).

Fig. 13.Multi-element trace element diagrams for Silhouette andNorth Islands (thiswork;Devey and Stephens, 1992), compared to equivalent rock types from the Deccan province:alkalinemafic rocks fromKutch and Rajpipla (Karmalkar et al., 2005;Mahoney et al., 1985;Sen et al., 2009), Amba Dongar complex and surrounds (Gwalani et al., 1993), Saurashtradykes (Chatterjee and Bhattacharji, 2001), Phenai Mata and Mt. Girnar complexes(Gwalani et al., 1993; Paul et al., 1977) and Deccan tholeiitic rocks (Lightfoot andHawkesworth, 1988; Melluso et al., 2006; Peng et al., 1994). All samples are normalisedto the primitive mantle values of McDonough and Sun (1995).

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References

Anders, E., Grevesse, N., 1989. Abundances of the elements: meteoritic and solar.Geochimica et Cosmochimica Acta 53, 197–214.

Armitage, J.J., Collier, J.S., Minshull, T.A., Henstock, T.J., 2011. Thin oceanic crust and floodbasalts: India–Seychelles breakup. Geochemistry, Geophysics, Geosystems 12,Q0AB07.

Ashwal, L.D., Demaiffe, D., Torsvik, T.H., 2002. Petrogenesis of Neoproterozoic granitoidsand related rocks from the Seychelles: the case for an Andean-type arc origin. Journalof Petrology 43, 45–83.

Ashwal, L.D., Webb, S.J., Knoper, M.W., 2005. Magmatic stratigraphy in the BushveldNorthern Lobe: continuous geophysical and mineralogical data from the 2950 mBellevue drillcore. South African Journal of Geology 108, 199–232.

Basu, A.R., Renne, P.R., DasGupta, D.K., Teichmann, F., Poreda, R.J., 1993. Early and lateigneous pulses and a high-3He plume origin for the Deccan flood basalts. Science261, 902–906.

Blichert-Toft, J., Chauvel, C., Albarède, F., 1997. Separation of Hf and Lu for high-precisionisotope analysis of rock samples by magnetic sector-multiple collector ICP-MS.Contributions to Mineralogy and Petrology 127, 248–260.

Bouvier, A., Vervoort, J.D., Patchett, P.J., 2008. The Lu–Hf and Sm–Nd isotopic compositionof CHUR: constraints from unequilibrated chondrites and implications for the bulkcomposition of terrestrial planets. Earth and Planetary Science Letters 273, 48–57.

Burke, K., Steinberger, B., Torsvik, T.H., Smethurst, M.A., 2008. Plume generation zones atthemargins of large low shear velocity provinces on the core-mantle boundary. Earthand Planetary Science Letters 265, 49–60.

Chatterjee, N., Bhattacharji, S., 2001. Origin of the felsic and basaltic dikes and flows in theRajula-Palitana-Sihor Area of the Deccan Traps, Saurashtra, India: a geochemical andgeochronological study. International Geology Review 43 (12), 1094–1116.

Chauvel, C., Blichert-Toft, J., 2001. A hafnium isotope and trace element perspective onmelting of the depleted mantle. Earth and Planetary Science Letters 190, 137–151.

Chauvel, C., Lewin, E., Carpentier, M., Arndt, N.T., Marini, J.-C., 2008. Role of recycledoceanic basalt and sediment in generating theHf–Ndmantle array.Nature Geoscience1, 64–67.

Chenet, A.L., Quidelleur, X., Fluteau, F., Courtillot, V., Bajpai, S., 2007. 40K–40Ar dating ofthe Main Deccan large igneous province: Further evidence of KTB age and shortduration. Earth and Planetary Science Letters 263, 1–15.

Collier, J.S., Sansom, V., Ishizuka, O., Taylor, R.N., Minshull, T.A., Whitmarsh, R.B., 2008. Ageof Seychelles–India break-up. Earth and Planetary Science Letters 272, 264–277.

Courtillot, V.E., Renne, P.R., 2003. On the ages of flood basalt events. Comptes RendusGeoscience 335, 113–140.

Courtillot, V., Besse, J., Vandamme, D., Montigny, R., Jaeger, J.-J., Cappetta, H., 1986. Deccanflood basalts at the Cretaceous/Tertiary boundary? Earth and Planetary ScienceLetters 80, 361–374.

Devey, C.W., Stephens, W.E., 1991. Tholeiitic dykes in the Seychelles and the originalspatial extent of the Deccan. Journal of the Geological Society (London) 148,979–983.

Devey, C.W., Stephens, W.E., 1992. Deccan-related magmatism west of the Seychelles–India rift. In: Storey, B.C., Alabaster, T., Pankhurst, R.J. (Eds.), Magmatism and theCauses of Continental Break-up. Geological Society, London, Special Publications, 68,pp. 271–291.

Dickin, A.P., Fallick, A.E., Halliday, A.N., Macintyre, R.M., Stephens, W.E., 1986. An isotopicand geochronological investigation of the younger igneous rocks of the Seychellesmicrocontinent. Earth and Planetary Science Letters 81, 46–56.

Eldholm, O., Coffin,M.F., 2000. Large igneousprovinces andplate tectonics. In: Richards,M.,Gordon, R., Van der Hilst, R. (Eds.), The history and dynamics of global plate motions.American Geophysical Union Geophysical Monograph, 121, pp. 309–326.

Eiler, J.M., 2001. Oxygen isotope variations of basaltic lavas and upper mantle rocks. In:Valley, J.W., Cole, D.R. (Eds.), Stable isotope geochemistry. Reviews in Mineralogyand Isotope Geochemistry, 43. Mineralogical Society of America, Washington,pp. 319–364.

Ersoy, Y., Helvacı, C., 2010. FC–AFC–FCA and mixing modeler: A Microsoft Excel spread-sheet program for modeling geochemical differentiation of magma by crystalfractionation, crustal assimilation andmixing. Computers & Geosciences 36, 383–390.

Faure, G., Mensing, T.M., 2005. Isotopes: Principles and Applications, 3rd ed. John Wiley,New Jersey (897 pp.).

Fisk, M.R., Upton, B.G.J., Ford, C.E., White, W.M., 1988. Geochemical and experimentalstudy of the genesis of magmas of Reunion Island, Indian Ocean. Journal of GeophysicalResearch 93 (B5), 4933–4950.

Ganerød, M., Torsvik, T.H., Van Hinsbergen, D.J.J., Corfu, F., Werner, S., Owen-Smith, T.M.,Ashwal, L., Webb, S., Hendriks, B.W.H., 2011. Palaeoposition of the Seychellesmicrocontinent in relation to the Deccan Traps and the Plume Generation Zone inLate Cretaceous–Early Palaeogene time. In: Van Hinsbergen, D.J.J., Buiter, S.J.H.,Torsvik, T.H., Gaina, C., Webb, S.J. (Eds.), The Formation And Evolution of Africa: ASynopsis of 3.8 Ga of Earth History. Geological Society, London, Special Publications,357, pp. 229–252.

Ghiorso, M.S., Sack, R.O., 1995. Chemical mass transfer in magmatic processes IV. A re-vised and internally consistent thermodynamic model for the interpolation and ex-trapolation of liquid-solid equilibria in magmatic systems at elevated temperaturesand pressures. Contributions to Mineralogy and Petrology 119, 197–212.

Goldstein, S.L., O'Nions, R.K., Hamilton, P.J., 1984. A Sm–Nd study of atmospheric dustsand particulates from major river systems. Earth and Planetary Science Letters 70,221–236.

Gwalani, L.G., Rock, N.M.S., Chang, W.J., Fernandez, S., Allégre, C.J., Prinzhofer, A., 1993.Alkaline rocks and carbonatites of Amba Dongar and adjacent areas, Deccan IgneousProvince, Gujarat, India: 1. Geology, petrography and petrochemistry. Mineralogyand Petrology 47, 219–253.

Gwalani, L.G., Fernandez, S., Karanth, R.V., Demeny, A., Chang, W.J., Avasia, R.K., 1995.Alkaline and tholeiitic dyke swarm associated with Amba Dongar and Phenai Matacomplexes, Chotta Udepur Alkaline sub-province, western India. In: Devaraju, T.C.(Ed.), Dyke Swarms of Peninsular India. Memoirs of the Geological Society of India,33, pp. 391–423.

Hargraves, R.B., Duncan, R.A., 1990. Radiometric age and palaeomagnetic results fromSeychelles dykes. In: Duncan, R.A., Backman, J., Peterson, L.C., et al. (Eds.), Proceedingsof the Ocean Drilling Program. Scientific Results, 115. College Station, Texas,pp. 119–122.

Harris, C., 1983. The petrology of lavas and associated plutonic inclusions of AscensionIsland. Journal of Petrology 24, 424–470.

Harris, C., Ashwal, L.D., 2002. The origin of low δ18O granites and related rocks from theSeychelles. Contributions to Mineralogy and Petrology 143, 366–376.

Kampunzu, A.B., Mohr, P., 1991. Magmatic evolution and petrogenesis in the East Africanrift system. In: Kampunzu, A.B., Lubala, R.T. (Eds.), Magmatism in ExtensionalStructural Settings. Springer-Verlag, Berlin, pp. 85–136.

Karmalkar, N.R., Rege, S., Griffin, W.L., O'Reilly, S.Y., 2005. Alkaline magmatism fromKutch, NW India: implications for plume–lithosphere interaction. Lithos 81, 101–119.

Le Roex, A.P., Späth, A., Zartman, R.E., 2001. Lithospheric thickness beneath the southernKenya Rift: implications from basalt geochemistry. Contributions to Mineralogy andPetrology 142, 86–106.

Lightfoot, P., Hawkesworth, C., 1988. Origin of Deccan Trap lavas: evidence fromcombined trace element and Sr-, Nd- and Pb-isotope studies. Earth and PlanetaryScience Letters 91, 89–104.

Lightfoot, P.C., Hawkesworth, C.J., Sethna, S.F., 1987. Petrogenesis of rhyolites andtrachytes from the Deccan Trap: Sr, Nd, and Pb isotope and trace element evidence.Contributions to Mineralogy and Petrology 95, 44–54.

Lightfoot, P.C., Hawkesworth, C.J., Devey, C.W., Rogers, N.W., Van Calsteren, P.W.C., 1990.Source and differentiation of Deccan Trap Lavas: implications of geochemical andmineral chemical variations. Journal of Petrology 31 (5), 1165–1200.

Lugmair, G.W., Carlson, R.W., 1978. The Sm–Nd history of KREEP. Proceedings of the 9thLunar and Planetary Science Conference, Houston, Texas.

Lugmair, G.W., Marti, K., 1978. Lunar initial 143Nd/144Nd: differential evolution of thelunar crust and mantle. Earth and Planetary Science Letters 39, 349–357.

Mahoney, J.J., Macdougall, J.D., Lugmair, G.W., Gopalan, K., Krishnamurthy, P., 1985. Originof contemporaneous tholeiitic and K-rich alkalic lavas: a case study from the north-ern Deccan Plateau, India. Earth and Planetary Science Letters 72, 39–53.

Mahoney, J.J., Natland, J.H., White,W.M., Poreda, R., Bloomer, S.H., Fisher, R.L., Baxter, A.N.,1989. Isotopic and geochemical provinces of the western Indian Ocean spreadingCenters. Journal of Geophysical Research 94 (B4), 4033–4052.

Mahoney, J.J., Duncan, R.A., Khan,W., Gnos, E., McCormick, G.R., 2002. Cretaceous volcanicrocks of the South Tethyan suture zone, Pakistan: implications for the Réunionhotspot and Deccan Traps. Earth and Planetary Science Letters 203, 295–310.

McDonough, W.F., 1990. Constraints on the composition of the continental lithosphericmantle. Earth and Planetary Science Letters 101, 1–18.

McDonough, W.F., Sun, S.-S., 1995. The composition of the Earth. Chemical Geology 120,223–253.

Melluso, L., Sethna, S.F., D'Antonio, M., Javeri, P., Bennio, L., 2002. Geochemistry andpetrogenesis of sodic and potassic mafic alkaline rocks in the Deccan Volcanicprovince, Mumbai Area, India. Mineralogy and Petrology 74, 323–342.

Melluso, L., Mahoney, J.J., Dallai, L., 2006. Mantle sources and crustal input as recorded inhigh-Mg Deccan Traps basalts of Gujarat (India). Lithos 89, 259–274.

Middlemost, E.A.K., 1994. Naming materials in the magma/igneous rock system. Earth-Science Reviews 37, 215–224.

Nebel, O., Mezger, K., 2006. Reassessment of the NBS SRM-607K-feldspar as a highprecision Rb/Sr and Sr isotope reference. Chemical Geology 233, 337–345.

Nebel, O., Mezger, K., Scherer, E.E., Münker, C., 2005. High precision determinations of87Rb/85Rb in geologic materials by MC–ICP-MS. International Journal of MassSpectrometry 246 (1–3), 10–18.

Nebel, O., Morel, M.L.A., Vroon, P.Z., 2009. Isotope dilution concentration analyses of Lu,Hf, Zr, Ta and W, and Hf-isotope compositions of NIST SRM-610 and SRM-612 glasswafers. Geostandards and Geoanalytical Research 33 (4), 487–499.

Nebel, O., van Westrenen, W., Vroon, P.Z., Wille, M., Raith, M.M., 2010. Deep mantlestorage of the Earth's missing niobium in late-stage residual melts from a Hadeanmagma ocean. Geochimica et Cosmochimica Acta 74, 4392–4404.

Nebel, O., Scherer, E.E., Mezger, K., 2011. Evaluation of the 87Rb decay constant by agecomparison against the U–Pb system. Earth and Planetary Science Letters 301,1–8.

Nekvasil, H., Dondolini, A., Horn, J., Filiberto, J., Long, H., Lindsley, D.H., 2004. The originand evolution of silica-saturated alkalic suites: an experimental study. Journal ofPetrology 45, 693–721.

Paul, D.K., Potts, P.J., Rex, D.C., Beckinsale, R.D., 1977. Geochemical and petrogenetic studyof the Girnar igneous complex, Deccan volcanic province, India. Geological Society ofAmerica Bulletin 88, 227–234.

Paul, D., White, W.M., Blichert-Toft, J., 2005. Geochemistry of Mauritius and the origin ofrejuvenescent volcanism on oceanic island volcanoes. Geochemistry, Geophysics,Geosystems 6, Q06007. http://dx.doi.org/10.1029/2004GC000883.

Pearce, J.A., Kempton, P.D., Nowell, G.M., Noble, S.R., 1999. Hf–Nd element and iso-tope perspective on the nature and provenance of mantle and subduction com-ponents in Western Pacific arc-basin systems. Journal of Petrology 40 (11),1579–1611.

Peng, Z.X., Mahoney, J.J., 1995. Drillhole lavas from the northwestern Deccan Traps, andthe evolution of Réunion hotspot mantle. Earth and Planetary Science Letters 134,169–185.

Peng, Z.X., Mahoney, J., Hooper, P., Harris, C., Beane, J., 1994. A role for lower continentalcrust in flood basalt genesis? Isotopic and incompatible element study of the lower

46 T.M. Owen-Smith et al. / Lithos 182–183 (2013) 33–47

six formations of the western Deccan Traps. Geochimica et Cosmochimica Acta 58,267–288.

Peng, Z.X., Mahoney, J.J., Hooper, P.R., Macdougall, J.D., Krishnamurthy, P., 1998. Basalts ofthe northeastern Deccan Traps, India: isotopic and elemental geochemistry andrelation to southwestern Deccan stratigraphy. Journal of Geophysical Research 103(B12), 29843–29865.

Peucat, J.J., Vidal, P., Bernard-Griffiths, J., Condie, K.C., 1989. Sr, Nd, and Pb isotopic systematicsin the Archean low- to high-grade transition zone of southern India: syn-accretion vs.post-accretion granulites. The Journal of Geology 97 (5), 537–549.

Plank, T., Langmuir, C.H., 1998. The chemical composition of subducting sediment and itsconsequences for the crust and mantle. Chemical Geology 145, 325–394.

Plummer, P.S., 1995. Ages and geological significance of the igneous rocks fromSeychelles. Journal of African Earth Sciences 20 (2), 91–101.

Plummer, P.S., Belle, E.R., 1995. Mesozoic tectono-stratigraphic evolution of the Seychellesmicrocontinent. Sedimentary Geology 96, 73–91.

Ray, J.S., Ramesh, R., Pande, K., Trivedi, J.R., Shukla, P.N., Patel, P.P., 2000. Isotope and rareearth element chemistry of carbonatite ± alkaline complexes of Deccan volcanicprovince: implications to magmatic and alteration processes. Journal of Asian EarthSciences 18, 177–194.

Salters, V.J.M., 1996. The generation of mid-ocean ridge basalts from the Hf and Ndisotope perspective. Earth and Planetary Science Letters 141, 109–123.

Salters, V.J.M., Hart, S.R., 1991. The mantle sources of ocean ridges, islands and arcs: theHf-isotope connection. Earth and Planetary Science Letters 104, 364–380.

Scherer, E.E., Münker, C., Mezger, K., 2001. Calibration of the lutetium–hafnium clock.Science 293, 683–687.

Sen, G., 1995. A simple petrologic model for the generation of Deccan Trap magmas.International Geology Review 37, 825–850.

Sen, G., Bizimis, M., Das, R., Paul, D.K., Ray, A., Biswas, S., 2009. Deccan plume, lithosphererifting, and volcanism in Kutch, India. Earth and Planetary Science Letters 277,101–111.

Sheth, H.C., Mahoney, J.J., Chandrasekharam, D., 2004. Geochemical stratigraphy ofDeccan flood basalts of the Bijasan Ghat section, Satpura Range, India. Journal ofAsian Earth Sciences 23, 127–139.

Simonetti, A., Goldstein, S.L., Schmidberger, S.S., Viladkar, S.G., 1998. Geochemical and Nd,Pb, and Sr isotope data from Deccan alkaline complexes — inferences for mantlesources and plume–lithosphere interaction. Journal of Petrology 39, 1847–1864.

Stephens, W.E., Devey, C.M., 1992. Seychelles and the fragmentation of Gondwana:evidence from the igneous rocks. In: Plummer, P.S. (Ed.), Proceedings of the FirstIndian Ocean Regional Seminar on Petroleum Exploration, Seychelles. SeychellesNational Oil Company, Victoria, Seychelles, pp. 211–222.

Streckeisen, A., 1976. To each plutonic rock its proper name. Earth-Science Reviews 12,1–33.

Subrahmanya, K.R., 2001. Arabian Sea — the prime witness to the drama of India–Madagascar–Seychelles separation. Gondwana Research 4, 792–793.

Sun, S.-S., McDonough, W.F., 1989. Chemical and isotopic systematics of oceanic basalts:implications for mantle composition. Geological Society, London, Special Publications42, 313–345.

Todal, A., Edholm, O., 1998. Continental margin off Western India and Deccan largeigneous province. Marine Geophysical Researches 20, 273–291.

Torsvik, T.H., Tucker, R.D., Ashwal, L.D., Carter, L.M., Jamtveit, B., Vidyadharan, K.T.,Venkataramana, P., 2000. Late Cretaceous India–Madagascar fit and timing ofbreak-up related magmatism. Terra Nova 12, 220–224.

Torsvik, T.H., Ashwal, L.D., Tucker, R.D., Eide, E.A., 2001. Neoproterozoic geochronologyand palaeogeography of the Seychelles microcontinent: the India link. PrecambrianResearch 110, 47–59.

Winchester, J.A., Floyd, P.A., 1977. Geochemical discrimination of different magma seriesand their differentiation products using immobile elements. Chemical Geology 20,325–343.

Workman, R.K., Hart, S.R., 2005. Major and trace element composition of the depletedMORB mantle, DMM. Earth and Planetary Science Letters 231, 53–72.

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