mineral compositions and petrogenetic evolution of...

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The Canadian MineralogistVol.48,pp.205-229(2010)DOI:10.3749/canmin.48.1.205

MINERAL COMPOSITIONS AND PETROGENETIC EVOLUTION OF THE ULTRAMAFIC-ALKALINE – CARBONATITIC COMPLEX OF SUNG VALLEY,

NORTHEASTERN INDIA

LeoneMeLLUSo§

Dipartimento di Scienze della Terra, Università di Napoli Federico II, I–80134 Napoli, Italy

RajeShK.SRIVaSTaVa

Department of Geology, Banaras Hindu University, Varanasi 221 005, India

VIncenzaGUaRIno

Dipartimento di Scienze della Terra, Università di Napoli Federico II, I–80134 Napoli, Italy

aLbeRTozaneTTI

Istituto di Geoscienze e Georisorse, CNR, I–27100 Pavia, Italy

anUpK.SInha

Department of Geology, Banaras Hindu University, Varanasi 221 005, India

abSTRacT

TheSungValleyalkalinecomplexisarelativelysmallintrusionofLowerCretaceousageemplacedslightlybeforeorduringthe India–Antarctica break-up. It consists of ultramafic rocks (dunites,wehrlites, clinopyroxenites, uncompahgrites),maficrocks(ijolitessensu lato),felsicrocks(nephelinesyenites)andcarbonatites.Thechemicalcompositionofthemaficmineralsindicatestheexpectedenrichmentinirontowardthefelsicrocks.Ontheotherhand,carbonatitesfeatureveryMg-richminerals,generallyCr-rich,indicatingthattheirgenesisiscompletelyunrelatedtothatofmaficandfelsicrocks(ijolitesandnephelinesyenites).Theparageneses indicate that thiscomplexwasformedbybatchesofprimitivemagmaswithadistinctmagmaticaffinity (olivinemelilitites andolivinenephelinites,basanites, andpossiblyalsocarbonatites)whichevolved independently,generatingtheobservedspectrumofintrusiverocks.Clinopyroxeniteshaveinterstitialalkalifeldsparandtitanite,indicatingthattheyformedfromevolvedfeldspar-normative(phonotephritic,tephriphonolitic)magmas.Thesequenceperovskite–titaniteandtitanite–garnetnotedinsomeijoliticrocksindicateschangesinthechemicalcompositionofcoexistingsilicatemeltsand,mostlikely,anincreasingf(O2).Thetrace-elementprofilesofcoexistingphasesininterestingassociationsinasampleofijoliteweredocumentedbymeansofLA–ICP–MSanalyses.

Keywords: ultramafic-alkaline rocks, carbonatite,mineral compositions,LA–ICP–MSdata, trace elements, garnet, titanite,perovskite,clinopyroxene,SungValleycomplex,India.

SoMMaIRe

LecomplexealcalindeSungValley,enInde,d’âgecrétacéinférieur,estunmassifintrusifrelativementpetitdontlamiseenplacealégèrementprécédéouaccompagnélarupturedusocleInde–Antarctique.Onytrouvedesrochesultramafiques(dunites,wehrlites,clinopyroxénites,uncompahgrites),mafiques(ijolitessensu lato),felsiques(syénitesnéphéliniques)etdescarbona-tites.Lacompositiondesminérauxmafiquestémoignedel’enrichissementenfertypiqueendirectiondesrochesfelsiques.Enrevanche,lescarbonatitescontiennentdesminérauxfortementenrichisenMget,engénéral,aussienrichisenCr,indicationqueleurfiliationseraitcomplètementdifférentedecellequiaproduitlesrochesmafiquesetfelsiques(ijolitesetsyénitesnéphéli-niques).Lesparagenèsesindiquentquececomplexeaétéformépardesvenuesdemagmaprimitifayantuneaffinitédistincte,

§ E-mail address:[email protected]

206 ThecanadIanMIneRaLoGIST

InTRodUcTIon

The SungValley ultramafic-alkaline–carbonatitecomplex (UACC) isoneof theCretaceous intrusionsemplacedwithintheShillongPlateau,anupliftedhorst-like feature in northeastern India (Chattopadhyay&Hashimi1984,Krishnamurthy1985,Rayet al.(2000),Srivastava&Sinha2004,Srivastavaet al. 2005, andreferences therein). Ray& Pande (2001) dated thecarbonatiteof this complexby the 40Ar–39Armethodandplaced itat107.2±0.8Ma.Theysuggested thattheplateau ages represent near-surface crystallization(or emplacement) ages (Ray&Pande 2001). Later,Srivastavaet al.(2005)providedaU–Pbperovskiteageof115.1±5.1MafortheijoliteSV58oftheSungValleycomplex,aswellasbulk-rockSr–Ndisotopedata.TheSungValleyijolitethusmayhaveaslightlyolderageof emplacement than the carbonatite, consistentwithfield relationships.The emplacement of theShillongPlateaualkalineprovinceisrelatedtotheinitiationoftheNinety-EastRidgevolcanismintheIndianOcean(e.g., Duncan 2002). In addition, strongly alkalinerocks (melilite-bearingultramafic lamprophyres)withroughly similar agesas thenortheastern Indian intru-sions(117–110Ma)arerecordedalsoinEastAntarctica(Foleyet al. 2002).These are likely counterparts ofthe northeast Indian alkaline rocks during the riftingevent(s),which split the two continents apart in theEarlyCretaceous (Duncan 1992, Storey et al. 1992,Royer&Coffin1992).

The SungValley intrusion consists of ultramaficrocks (clinopyroxenite, serpentinized peridotite, andmelilitolite), alkaline rocks (ijolite and nephelinesyenite) and carbonatites (Krishnamurthy1985,Sriv-astava&Sinha2004,Srivastavaet al. 2005,Fig. 1).Clinopyroxenite, serpentinized peridotite and ijolitesformmost of the complex,whereas the other rocktypes constitute less than 5%of the exposures.Theserpentinized peridotite occupies the central part ofthe complex, being surrounded by clinopyroxenite.Serpentinizedperidotiteandclinopyroxeniteareamongtheoldestrocksofthecomplex.Theijolitebodyformsaringstructure.Smalldykesofmelilitoliteintrudetheperidotiteandclinopyroxenite.Nepheline syeniteand

carbonatite occur in dykes, veins, stocks, and ovoidbodies, intruding the ultramafic rocks aswell as theijolites. Carbonatite is the youngestmember of thecomplex,asitintrudesalltheotherunits.

MineralcompositionsandevolutionarytrendsintheSungValleyintrusionhavebeennotadequatelystudied,except byViladkar et al. (1994).This is surprising,given the level of detail reached by age and isotopedeterminations andwhole-rock geochemistry. In thispaper,weaimtofillsuchagapbycharacterizingthemineral composition of themain lithotypes found inthiscomplex.Thechemicalsequenceandcompositionof thephasesareofprime importance indecipheringthe petrogenetic history of intrusive complexes. Inparticular,mineralsandparagenesesarebetterabletoidentify liquid linesofdescent,as thecompositionofintrusiverockscanbedeterminedbycumulusprocessesand not by closed-system crystallization ofmagmas.TherangeofcompositionsshownbytheSungValleyrocks is similar to those of nepheline–pyroxene-richintrusive complexesworldwide (Srivastava&Sinha2004, and see discussion).Yet, there are contrastinghypotheses to explain the petrogenesis of coexistingcarbonatites and ijolites (cf.LeBas 1977,Beccaluvaet al.1992)and,moregenerally, thatofcarbonatites,thesebeingeitherlateproductsofliquidimmiscibility(perhapsfractionalcrystallization)orearlyproductsofpartialmeltinginthemantle(Treiman&Essene1985,Bellet al.1998,Mitchell2005).

anaLyTIcaLTechnIqUeS

PolishedthinsectionswerepreparedforseventeenslabsofthemainlithologiesoftheSungValleycomplex(Fig.1).Morethan400analysesobtainedwithenergy-dispersivespectrometry(EDS),andwithback-scatteredelectron(BSE)images,havebeenperformedatCISAG,University ofNapoliFederico II, utilizing anOxfordInstrumentsMicroanalysisUnit.Thelatterisequippedwithan INCAX-actdetector anda JEOLJSM–5310microscopeoperatingat15kVprimarybeamvoltage,50–100mAfilament current, a 15–17mm spot sizeandanetacquisition-timeof50s.Measurementsweredonewith an INCAX-stream pulse processor.The

produisantuncortègedemélilititesàolivine,desnéphélinitesàolivine,desbasanites,etpossiblementaussidescarbonatites,quiauraientévoluédefaçonindépendante,générantainsilespectreobservéderochesintrusives.Lesclinopyroxénitespossè-dentunfeldspathalcalininterstitieletlatitanite,indicationqu’ellesontétéforméesàpartird’unmagmaévoluédecompositionphonotephritiqueoutephriphonolitiqueetàfeldspathnormatif.Laséquencepérovskite–titaniteettitanite–garnetprésentedanscertainesrochesijolitiquesindiquedeschangementscompositionnelsdesmagmassilicatéscoexistantset,toutprobablement,uneaugmentationenf(O2).Lesprofilsd’élémentstracesdansdesphasescoexistantesdanslesassociationsinteressantesd’unéchantillond’ijoliteontfaitl’objectd’analysesparlatechniqueLA–ICP–MS.

(TraduitparlaRédaction)

Mots-clés:rochesultramafiquesalcalines,carbonatite,compositionsdeminéraux,donnéesLA–ICP–MS,élémentstraces,grenat,titanite,pérovskite,clinopyroxène,complexedeSungValley,Inde.

eVoLUTIonofTheSUnGVaLLeycoMpLex,IndIa 207

followingstandardswereusedforcalibration:diopside(Mg),wollastonite(Ca),albite(Al,Si,Na),rutile(Ti),almandine (Fe),vanadium(V),Cr2O3 (Cr), rhodonite(Mn),nickel (Ni),orthoclase (K), zircon (Zr), apatite(P),barite(Ba),strontianite(Sr),galena(Pb),syntheticSmithsonian phosphates (La, Ce,Nd), and internalstandards(Nb,Ta,U,Th).AsubsetofthesampleswereanalyzedatIGAG–CNR,Rome,usingaCamecaSX50WDS electronmicroprobe, using techniques alreadydescribedelsewhere(e.g.,Mellusoet al.2005).

In situ trace-element analyses of clinopyroxene,perovskite, titaniteandgarnet fromijoliteSV58havebeenperformedon the same thin section as used forelectron-microprobeanalysisbymeansoflaser-ablation–inductivelycoupledplasma–massspectrometry(LA–ICP–MS)atIGG–CNR,UnitofPavia(Italy).ThelaserprobeconsistsofaQ-switchedNd:YAGlaser,modelQuantel(Brilliant),whosefundamentalemissioninthenear-IR region (1064 nm)was converted to 266 nmwavelengthusingtwoharmonicgenerators.Spotdiam-eterwastypically60mm.Eachspothasbeencheckedinordertoassessthehomogeneityoftheablatedareaandthe absence of contributions frommineral inclusionsandfluidinclusions.TheablatedmaterialwasanalyzedbyusinganElanDRC-equadrupolemassspectrometer.

Helium,usedasthecarriergas,wasmixedwithargondownstreamoftheablationcell.WeusedNISTSRM610 as an external standard,whereas 44Cawas usedas an internal standard. Precision and accuracywereassessed from repeatedanalysesof theBCR-2g stan-dard,usually resulting inaprecisionbetter than10%forconcentrationsattheppmlevel.

peTRoGRaphyandWhoLe-RocKcoMpoSITIonofTheSUnGVaLLeyRocKS

Images of the samples of this study are reportedinFigure2 and in the supplementaryplates 1 and2,placed in theDepositoryofUnpublishedDataon theMACwebsite[documentSungValleyCM48_205].TheparagenesisisreportedinTable1.

Peridotites

Dunite SV31 is a heavily serpentinized coarse-grained rock,with cumulus olivine, rare interstitialclinopyroxene, amphibole,magnetite and perovskite.WehrliteSV19 contains totally serpentinized olivine,still fresh diopside,magnetite, perovskite and rarecrystals of phlogopite.Both samples have aluminous

fIG. 1. Geologicalmapof theSungValley ultramafic-alkaline – carbonatite complex(modified after Srivastava et al. 2005).Nepheline syenite andmelilitolite dykesexposedaroundthevillagesSungandMaskutareverysmall,hencenotreportedonthemap.


spinel inmagnetite (Figs. 2a, b). SampleSV31 alsocontainsilmenite,foundcoexistingwithmagnetite,orasfinelamellaeinit.

Uncompahgrites

UncompahgriteSV33(amelilite-richintrusiverock)is a coarse-grained rock formedby cumulusmeliliteand clinopyroxene (the latter commonly shows verytiny black oriented inclusions of Fe–Ti oxide) (Fig.2c). Layers alternatively enriched in clinopyroxeneandmeliliteareobserved.Minormineralsaretitanianmagnetite,ayellowishtogreenishphlogopite,andrareinterstitial perovskite and sulfides.Amodal analysisindicates roughly identical amounts of the twomaincumulusminerals (46.3%melilite, 46.8% clinopy-roxene, 6.9%phlogopite + opaque oxides+ sulfides+perovskite).

Clinopyroxenites

The clinopyroxenites of theSungValley intrusionhaveveryvariablegrain-sizeandtextures.SampleSV6isamedium-grainedrockmadeupofzonedclinopy-roxene,withminorinterstitialalkalifeldsparandrareidiomorphic titanite. Sample SV7 is coarse grained,withzonedclinopyroxeneandrareinterstitialandfinelyexsolvedK-feldspar.Veins rich inalkali feldsparandvery rare pyrochlore cut across this lithotype.Alkalifeldsparisclearlyigneous.SampleSV8isaveryfine-grained sample, almost completelymadeupofgreen

clinopyroxene,withrareveinsrichingarnet,andminorinterstitial titanite,magnetite, apatite andgarnet (Fig.2d).SampleSV9ismainlycomposedofzonedgreenclinopyroxene,withsmallamountsoftitanite.

Ijolites

Thenepheline-rich rockshaveavariableparagen-esis. SampleSV58 is slightly heterogeneous in grainsize,andiscomposedofidiomorphicclinopyroxeneandsubhedralnepheline.Perovskitewithareactionrimoftitaniteandgarnet,likelyitselfareactionrimontitanite(Fig.2e),completetheparagenesis.Nomagnetitewasobserved.SampleSV14isapegmatiticijolite,mainlycomposed of large crystals of nepheline and zonedclinopyroxene,with lesseramountsofapatite, titaniteandmagnetite.Poikiliticgarnetisaninterstitialphase.SampleSV10isveryfine-grained,nephelineandclino-pyroxenecoexistwithsmallpatch-likeclustersricherinperovskite,titanite,micaandmagnetite.SampleSV83ismedium- to fine-grained and formed by euhedral,zonedcrystalsofclinopyroxene,withnepheline,zonedgarnet,titanite,andinterstitialnosean(Fig.2f).Garnetandtitanitearebothidiomorphicanddonotshowanyreactionrelationshipswherefoundincontact.

Nepheline syenites

SamplesSV22andSV25havea typical tinguaitic(fluidal) textureandare composedofopticallyzonedK-feldspar(witharimofalbite),interstitialnepheline(usually corroded by cancrinite), deep green clino-pyroxene, titanite,mica andFe–Ti oxides (Fig. 2g).SampleSV56 isfine grained, andmostly consists ofidiomorphicalkalifeldspar,subidiomorphicnepheline,green clinopyroxene,magnetite, very rare ilmenite,titaniteand interstitialcancrinite.SampleSV52Aisacompositerockconsistingofamorefine-grainedneph-eline syenitewith alkali feldspar, nephelineanddeepgreenclinopyroxeneincontactwithacoarser-grainedrock,mostlymade up of the samemineralswith, inaddition,Fe–Tioxidesandsomemica.

Carbonatites

Coarse- to fine-grained carbonatitic facies arepresent at SungValley. SampleSV49 isfine-grainedwithcoarserareas.Thefine-grainedareaismadeupofcalcite, in some caseswith the appearanceof pheno-crysticolivine,mostlyalteredtoserpentine,markedlyzonedphlogopite,with a darker (Fe-rich) core and aclear(Mg-rich)rim,clinohumite,apatite,Fe–Tioxidesandrarepyrochlore(Fig.2h).Thecoarse-grainedareasarealmostcompletelymadeupofcalcite.SampleSV68is coarse grained, andmainly composed of calcite,with rare crystals of dolomite, clear and idiomorphicphlogopite, ilmenite,magnetite and apatite (Fig. 2i).SampleSV73isalsocoarse-grainedandconsistingof

fIG. 2. Back-scattered electron (BSE) andmicroscopeimages of peculiar petrographic features of SungVal-ley rocks. a)BSE image of intergrowth of perovskite,amphibole andmagnetite, sampleSV31. b)BSE imageof patches ofAl-rich spinel in an otherwise chemicallyhomogeneous titanianmagnetite, sampleSV31.c) Inter-growths ofmelilite and clinopyroxene, uncompahgriteSV33,crossednicols.d)ClinopyroxeneandmagnetiteintheclinopyroxeniteSV8,parallelnicols.e)BSEimageofthesequenceperovskite!titanite!garnetintheijoliteSV58(seetext).Notealsotheidiomorphicclinopyroxeneand the late-crystallized nepheline.This is the area ana-lyzed for LA–ICP–MSdata.The numbers indicate thespot analysesmade, as reported inTable 11.Circles arelargerthantheactualsizeofthespot.f)Idiomorphicgar-net,greenclinopyroxeneandtitanite,withnephelineandnoseanintheijoliteSV83,parallelnicols.g)Earlyalkalifeldspar,subidiomorphicnepheline,aegirine-richclinopy-roxeneandmica,definingatinguaitictextureinnephelinesyeniteSV22,crossednicols.h)Zonedphlogopitewithaclear,moreMg-richrim,clinohumiteandcalcite.Olivine,apatiteandoxidesaremicrocrystals,incarbonatiteSV49,parallelnicols.i)BSEimageofcalcite,Fe–Tioxidesanddolomite,carbonatiteSV68.


calcite,subhedralolivine,sulfides,oxidesandapatite.Clinopyroxeneisabsentinthecarbonatites.

Bulk compositions

Major- and trace-element compositions of SungValleyrocksarereportedinSrivastava&Sinha(2004),andinthesupplementarytable,placedintheDepositoryofUnpublishedDataontheMACwebsite[documentSungValleyCM48_205].Oneshouldnotethatminer-alogy,textures,grainsizeandwhole-rockgeochemistrymakemost of these rocks unlikely representatives ofliquidcompositions.Theyaretheresultofaccumula-tionofmineralson thebottomor in thebordersof amagmareservoiremplacedintheEarth’suppercrust.Indeed,Srivastava&Sinha (2004) andSrivastavaet al.(2005)haddifficultytofindreliableindicationsofmagmacompositionsandliquidlinesofdescentfromthestudyofbulk-rockgeochemicaldata.Toovercomethis difficulty,we decided to focus on the chemicalvariationsofthecoexistingphases.

MIneRaLcoMpoSITIonS

Olivine and clinohumite

Olivine occurs in dunites,wehrlites and carbon-atites.OlivinerelicsintheserpentinizedduniteSV31haveanarrowrangeincomposition(Fo86–Fo87),withrelativelyhighCacontents(0.014–0.021apfu,basedon

fouratomsofoxygen).OlivineinthecarbonatitesSV73andSV49 has also a narrow range of compositions,butatmarkedlyhigherforsteritecontents(Fo94–Fo96),andwithmuchlowerCacontents(0–0.008apfu).Thisappears to be curious for amineral in equilibriumwith suchCa-rich rocks andminerals.Nevertheless,it is not unexpected, given the lack ofmonticellite–kirschsteinite solid solutions to bufferCa contents inforsterite–fayalitesolidsolutionsattheirhighestvalues(Sharpet al.1986).Conversely,theMnOcontentsarevery similar in both olivine compositions (Table 2).Olivine in the carbonatites has a core slightlymoreFe-rich than the rim, a featuremuchmoreevident inthecoexistingphlogopite(seebelow).Clinohumite isaminor, relatively late-crystallized primary phase ofcarbonatiteSV49. IthasTiO2rangingfrom2 to2.57wt%,which iswellwithin the rangeof values foundintheJacupirangacarbonatite(Brazil,TiO2upto5.96wt%,Mitchell 1978,Morbidelli et al. 1986,Gaspar1992).TheMg#ofthismineral(95–96)isidenticaltothatofcoexistingolivine.

Clinopyroxene

ClinopyroxeneoftheSungValleyintrusionshowsthecomplete range fromdiopside toaegirine (Fig.3,Table3).Theclinopyroxene inwehrliteand inclino-pyroxenites is diopside, but hasmarked composi-tional differences inminor elements such asTi andAl. Inwehrlite, the clinopyroxene is in the range


Ca50–52Mg40–42Fe6–8,with1.3–2wt%TiO2(0.03–0.05Tiapfu,basedonsixatomsofoxygen),3.6–5.6wt%Al2O3(0.15–0.25Alapfu),andMg#[atomicMg*100/(Mg+Fe)throughout]varyingfrom84to87.Thisisa typical diopside crystallized froman alkalinemelt.TheclinopyroxeniteSV8hasthemostpeculiarcompo-sitionalrange.Indeed,clinopyroxenehasavariableTi(0.35–1.91wt%TiO2,0.01–0.05Tiapfu),buthighAl(2.6to9.8wt%Al2O3,0.12–0.44Alapfu),atMg#vari-ablefrom54to63.Theotherclinopyroxenites,whichcontainalkalifeldspar,haveuniformlylow-Ti,low-Alclinopyroxene(TiO2from0.07to0.68wt%,0.002–0.02Tiapfu,andAl2O3from0.4to1.15wt%,0.02–0.05Alapfu),withMg#rangingfrom62to87.

InuncompahgriteSV33,thediopsideischemicallyindistinguishable from that inwehrliteSV19andhasa limited compositional range (TiO2 from 1 to 1.4wt%,0.03–0.04Tiapfu,Al2O3from2.6 to4.4.wt%,0.12–0.18Alapfu).TheMg#rangesfrom79to85.


fIG.3. a)ClassificationofclinopyroxeneofSungValleyrocks.b)andc)Elementcon-centrationsinclinopyroxeneversusMg#.

The ijolites contain diopside to increasingly sodicaegirine-augite(Na2Ofrom1to3.24wt%,0.07–0.23Naapfu),with relatively lowAl2O3 (1.1 to3.8wt%,0.05–0.17Alapfu) and variable, but generally low,TiO2 contents (0.14–1.8wt%,0.004–0.05Tiapfu) at

Mg#rangingfrom35to74.Thetitanite–garnet-bearingijoliteSV14hasthehighestconcentrationsofNaandTi;consequently,thedataplotinadifferentfieldfromtheother samples of ijolite.The ijoliteSV83has themostMg-poor clinopyroxene (35<Mg#<48),with


the highestNa and the lowestTi contents among alltheotherijoliticrocks.

Clinopyroxeneofthenephelinesyenitesvariesfromaegirine-augite to aegirine (Na2O from4 to 13wt%,0.35–0.94Naapfu,CaOfrom1to16wt%,0.05–0.65Caapfu,Mg#from4to32).Thistypeofclinopyroxenetypicallycrystallizesfrommeltsthatreachedperalka-lineconditions.TheAl2O3contentsarelowandalmostconstant at about 1wt%,whereasTiO2 is variable(0.2–1.4wt%,0.01–0.03Tiapfu),butdoesnot showanycorrelationwithNa,Mg,FeorCa.

Spinels and ilmenite

Titanianmagnetite is the dominant iron oxide oftheSungValleyrocks(Table4).Thecompositionsarepoor in the ulvöspinel component, being not higher

than25mol.%,withthehighestvaluesintheperidotiticrocksandthelowestinthenephelinesyenites.Spinelsofwehrliteanddunitehaveamong themostMg-richcompositions[MgOfrom2.7to6.2wt%,Mg#intherange19–27,whereMg#is100Mg/(Mg+Fe2+)].ThelackofCr-richcompositionsintheperidotites(Cr2O3


MgO).TheCr contents are higher than those of thespinelinperidotites.

Ilmenitehasbeenfoundinwehrlite,inthecarbon-atitesandinanephelinesyenite.Ilmeniteofthecarbon-atitescrystallizedclosetomagnetite,orisanexsolutionproduct(Fig.2i).IthashighMginsampleSV49(12–13wt%MgO,about47–49mol.%geikielite).Thesevaluesaremosttypicallyfoundinilmeniteofkimberliticrocks(e.g.,Wyattet al.2004,Mellusoet al.2008),butalsoincarbonatiteselsewhere(Mitchell1978,Gaspar&Wyllie1983).HighMgOcontentsarealsofoundin ilmeniteoftheduniteSV31(MgO15.5–15.7wt%,about52–54mol.%geikielite).TheilmeniteinthenephelinesyeniteSV56 is high inMnO (11.5wt%, about 30mol.%pyrophanite), as is typical of thismineral in evolvedsyeniticrocks(e.g.,Brotzuet al.1997).IlmenitehasasignificantNbcontent(upto3wt%Nb2O5incarbon-atiteSV49)(Table4).

Temperaturesandoxygenfugacitiesobtainedwithstandardgeothermobarometers(LePage2003,andrefer-encestherein,Sauerzapfet al.2008)giveanindicationoftemperaturesofsubsolidusequilibrationandoxygenfugacity above theNi–NiObuffer, as expected fromrockswith aegirine (such as the nepheline syenites).Carbonatiteshavecalculatedtemperaturesbetween700and550°C,marginallylowerthanthehighestcalculatedfor nepheline syenites (715°C).The low calculatedtemperatures are indication of extensive subsolidusre-equilibrationandwillnotbediscussedfurther.

Mica group

Almost all SungValley rocks have very smallamounts of primary phlogopite or annite (Fig. 4,Table5).PhlogopiteinthewehrliteSV19hasrelativelyhighAl(2.85–2.87apfu,basedon22atomsofoxygen)andMg (4.83–4.91), and lowTi (0.12–0.6 apfu),Fe (0.45–0.54apfu) andF contents,with highMg#(89–91).Similarly,inuncompahgriteSV33,phlogopitehasahighAl(2.60–3.09apfu)andMg(4.32–4.82apfu),andlowFe(0.67–0.95apfu),Ti(0.06–0.11apfu)andF(0–0.28apfu)contents.TheMg#rangesfrom82to88.Phlogopite inpyroxeniteSV9hashighAl(1.95–2.23apfu), Fe (1.13–1.31apfu) andMg (4.48–4.56apfu),and lowTi (0.06–0.07apfu) andF (0.16–0.17apfu)contents.TheMg#rangesfrom77to80.Theveryraremica in ijoliteSV10 isphlogopite (Mg# in the range56–61),with relatively highAl (2.53–2.60apfu), Fe(2.04–2.27apfu)andMg(2.99–3.29apfu),andlowTi(0.32–0.34apfu)andF(0.15–0.26apfu)contents.Micain nepheline syenites is a typical phlogopite–annitesolid solution (Mg# in the range31–60),withgener-ally highAl (1.87–2.13apfu), Fe (1.98–3.50apfu)andF(0.44–0.75apfu),andlowMg(1.67–3.39apfu),Na (0.04–0.11apfu) and awide range inTi contents(0.17–0.50apfu).The phlogopite in carbonatites hashighAl (1.88–2.90apfu) andMg (4.18–5.65apfu)contents (Mg# from77 to96), lowTi (0–0.24apfu),

Fe (0.22–1.22apfu) andF (0.01–0.38apfu) contents,indicatingstrongchemicalzoning.Thecoreisinvari-ablymoreFe-richthanthecleanrims.ThephlogopiteinthecarbonatitesalsocontainsthehighestNa(0.17–0.41apfu)andBa(0–0.15apfu)contents(Fig.4,Table5),despitetheverylowNacontentofthehostrocks.The


reversezoningofthephlogopitemayberelatedtothecocrystallizationofFe–Tioxides.

Themicaofnephelinesyeniteand,inafewcases,incarbonatite,showsverysmalldeficiencyinthesumofthecommontetrahedrallycoordinatedcations(i.e.,Si4++ IVAl3+<8); therefore, someFe3+ is present atthe tetrahedral site.This substitution is an indicationofachangeinoxygenfugacityoftheenvironmentinwhichtheycrystallized.TheIVFe3+–VIFe2+relationshipindicatesa fugacityofoxygenbetween theQFMandNi–NiObuffers.The increase inFe3+ is concomitant

with an increase inTi andMn, and a slight decreaseinAl.Micas inwehrlites,uncompahgrite,pyroxenite,ijolite andmostmicas of carbonatites show a smallexcessinAl.

Amphibole

Amphibole has been found in the dunite SV31(Table 5). Its composition is pargasite (IVAl ≥ Fe3+)(Mg#intherange79–82),accordingtotheclassificationschemeofLeakeet al. (1997).Thiscalcicamphibole

fIG.4. a)ClassificationofmicasofSungValleyrocks.b)andc)ElementconcentrationsinmicaversusMg#.


crystallizedadjacenttoperovskite(Fig.2a),thusgivingevidencethatthesetwophasesarenotmutuallyincom-patible. Perovskite and amphibole commonly coexistin kamafugites (Gibsonet al. 1995,V.Guarino&L.Melluso,unpubl.data)andcarbonatites(Chakhmoura-dian&Zaitsev2002).

Melilite

Asolidsolutionofåkermanite(Ca2MgSi2O7,48–64mol.%), “Fe-åkermanite” (Ca2FeSi2O7, 7–12mol.%)

and “sodamelilite” (CaNaAlSi2O7, 25–39mol.%) isthe characteristic phase of the uncompahgrite SV33.The composition of thismelilite has a significantrange in these threecomponents, ispoor ingehlenite(Ca2Al2SiO7, 0.6–4.3 mol.%), and devoid of the“Na-ferri-melilite”(CaNaFe3+Si2O7)(Fig.5,Table6).The100Mg/(Mg+Fe)valuerangesfrom82to89,andNa2O ranges from2.7 to 4.4.wt% (0.24–0.39apfu,based on seven atomsof oxygen).Themainmecha-nismofsubstitutionofthemineralisthusrelatedtotheåkermanite – “sodamelilite” solid solution (Fig. 5a).

fIG. 5. a)Melilite compositions of the SungValley uncompahgrite.Melilites of theroughly coeval ultramafic lamprophyres ofWestAntarctica (Foleyet al. 2002) areshown for comparison. b)TheSungValleymelilites plotwellwithin the field ofmagmaticmeliliteworldwide in theMg (apfu)versus (Na+K)/Al diagram, beingrelativelypoorofthegehlenitecomponenttypicalofmetamorphicmeliliteworldwide(acompilationofabout420analysestakenfrombothliteratureandunpublisheddataoftheseniorauthor).Theplaneåkermanite–“Fe-åkermanite”–“sodamelilite”inFig.5aisthelineat(Na+K)/Al=1.


Broadly similar compositions have been analyzed intheroughlycoevalultramaficlamprophyresatBeaverLake,Antarctica(Foleyet al.2002,Fig.5a).Itisclear

thatmeliliteintheSungValleysuiteplotswellwithinthe compositional rangeofmagmaticmeliliteworld-wide(Fig.5b).

Garnet

AcalcicgarnetisfoundintheSungValleyijolitesand clinopyroxenites. The garnet of ijolitic rocks(SV83, SV58, SV14) has a variable but gener-ally highTiO2 content, reaching values as high as14–15wt% (up to 0.96apfu, based on 12 atoms ofoxygen),indicatingsignificantamountsofschorlomite[Ca3Ti2(Si,Fe3+,Al,Fe2+)3O12, Chakhmouradian&McCammon2005] andmorimotoite [Ca3Ti(Mg,Fe2+)Si3O12] components (samples SV58 and SV14), aswellasandradite(Ca3Fe3+2Si3O12)(Table6).TheTiO2valuesareaslowas6.8wt%(0.41apfu)inSV83.Thegarnet also has significant amounts ofMg (0.2–1.1wt%MgO, 0.03–0.13apfu), decreasingwithTiO2.TheAl contents are very low (0.9–1.5wt%Al2O3,0.09–0.15apfu);thusthegarnetintheserockscanbeconsideredmainly as a solid solution of schorlomite,andraditeandmorimotoite(accordingtoLocock2008,Table 6).The zirconium contents reach values up to1.3wt% (asZrO2), indicating limited solid-solutiontowardkimzeyite[Ca3(Zr,Ti)2(Al,Si,Fe3+)3O12,upto2.7mol.%].GarnetoftheclinopyroxeniteSV8isdistinctlypoorer inTi (1.45–8wt%TiO2,0.09–0.5apfu),FeOt(20.8–16.55wt%, 1.1–1.47apfu) and significantlyricherinAl(4–9wt%Al2O3,0.43–0.85apfu);thusitismainlyasolidsolutionbetweenandraditeandgrossular(Ca3Al2Si3O12)(Table6).ThegarnetofSV8definesadifferentchemicaltrendwithrespecttothatfoundintheijolites(Fig.6),thusindicatingadifferentcompositionofparentalmagmafromwhichitcrystallized.

Garnet is a typical accessory phase in themildlyto strongly evolved alkaline intrusive and volcanicrocks (e.g.,Dawsonet al.1995,Mellusoet al.1996,Flohr&Ross 1989).The garnet of the SungValleyijolitesisvirtuallyidenticaltothatfoundintheAmbaDongar (melilite-free) nephelinites, northernDeccan,India (Gwalani et al. 2000, and references therein),whereas the garnet of clinopyroxenite SV8 ismostsimilar togarnetofnephelinesyenitesandphonolitesinthealkalineigneousprovinceofsouthwesternBrazil(Brotzuet al.1997,2005,2007,andreferencestherein,Morbidelliet al. 1997).Thegarnet in theTuriy suiteinRussia (data fromDunworth&Bell2003) revealsbothhigh-Tiandlow-Ticompositions,asatSungValleyandMt.Vulture,inItaly,thismineraloccurringinbothmelilitites and phonolites (Melluso et al. 1996, andunpubl.data)(Fig.6).

Feldspathoids

Nepheline,noseanandcancrinitehavebeenobservedintheSungValleyrocks.Nephelineoftheijoliticrockshas low excess of silica component and has a rather


homogeneous composition,Ne68–80Kls16–22Qtz1.6–4.3,withmolarNa/(Na+K)from0.78to0.85.Nephelineofnepheline syenites is slightlymoreenriched in thesilica component [Ne73–76Kls17–20Qtz5–7.3,withmolarNa/(Na+K)from0.81to0.84](Fig.7).ThelevelofCaOislowerthan0.6wt%(Table7).NoseanhasbeenfoundintheijoliteSV83.ItcontainsK2Ointherange3.2–5.7wt%andNa2O,from12.8to15.9wt%(Table7).Sulfur(asSO3)rangesfrom7.1to7.5wt%.Highlybirefringent cancrinite developed at the expense ofnephelineinthesyenites,likelyatthesubsolidusstage.IthasalmostnoK,andCaOvariesfrom3.2to4.3wt%(Table7).Membersofthecancrinite–vishneviteseriesarealsolocallyfound.

Alkali feldspar

PlagioclaseisabsentamongtheSungValleyrocks.Alkali feldspar is the mainmineral of nephelinesyenites, but it also occurs inmany clinopyroxenites

as an interstitial phase, in some cases concentratedin veins that cut across the rocks. Its composition ispotassicinthenephelinesyenites(Or70–96)andidenticalintheclinopyroxenites(Or82–96)(Fig.7,Table7).Albiteusuallyrimsalkali feldspar,most likelyasaresultofsubsolidusresetting.Thealkalifeldsparmaybefinelyperthitic;BaandSraregenerallylow(BaOupto0.45wt%,SrOupto0.77wt%).

Perovskite, titanite and pyrochlore

Perovskite is an accessory phase of peridotites,uncompahgrite and ijolites.The perovskite shows acomposition close to theCaTiO3 component (88 to99mol.%).Minor Sr,Na,Th,REE andNb substi-tute as tausonite (SrTiO3, 0–0.6mol.%), loparite(Ce0.5Na0.5TiO3,0–5.2mol.%),thorutite(Th0.5TiO3,0–1mol.%), latrappite (CaNb0.5Fe3+0.5O3, 0–5.5mol.%),lueshite(NaNbO3,0–4.1mol.%)and“Ce-orthoferrite”(CeFe3+O3, 0–1.2mol.%) components.As seen in

fIG.6. GarnetcompositionsoftheSungValleyrocks.ThedifferenttrendsofgarnetinijolitesandclinopyroxeniteSV8areclearlyvisible.ThegarnetcompositionsofTuriy(asterisks:Dunworth&Bell 2003),Mt.Vulture (small lines:Mellusoet al. 1996),Brazilianalkalinecomplexes(blacktriangles:Brotzuet al.1997,2007,andreferencestherein,Morbidelliet al. 1997) andAmbaDongarmelilite-free nephelinites, India(crosses:Gwalaniet al.2000)arereportedforcomparison.


Figure 8, in the peridotites, clinopyroxenites anduncompahgrite,perovskiteismuchlowerinNaandNbthanperovskiteofijolites,demonstratingavariationofcompositionof thismineralwithdegreeofmagmaticevolution(Fig.8,Table8).IntheMurudolivineneph-elinites,inthewesternDeccanTraps,theperovskiteischemicallydifferent,richinNa(from1.5to3.5wt%Na2O) but poor inNb (lower than 1.1wt%Nb2O5),consistentwith the anomalously lowNb contents oftheMurudnepheliniticrocks(Mellusoet al.2002,andunpubl.data).Perovskiteisabsentfromthenephelinesyenites,andoccurstogetherwithamphiboleinduniteSV31.

Titaniteischemicallyuniformthroughoutthevariousrock-types, and has lowerNb,REE andNa contentsthancoexistingperovskite,asexpected(Table8,andseebelow).Itisfoundwithmagnetite(TiO20.6–7.4wt%).Titaniteisassociatedwithmagnetite(TiO25–6wt%)inpyroxenites,andwithTi-poormagnetite(TiO20.6–1.5wt%)andilmeniteinnephelinesyenites.Trace-elementdistributionsbetweencoexistingtitaniteandperovskitearereportedinthenextchapter.

Pyrochlore-groupminerals occur in carbonatitesample SV48 and in a syenitic vein in clinopyrox-eniteSV7.Theyareveryvariableincomposition,thepyrochloreofcarbonatitebeingmuchricherinTh,CaandNbandmuchlowerinPb.Thepyrochloreofthesyenitic vein has highU, Pb and relatively lowNb(Table8).Allcompositionsplotinthepyrochlorefield(Hogarth 1977).Vacancies at theA site are probablycaused bymetamictization, due to high contents ofuraniumandthorium,asalsoevidencedbythepositivecorrelationofU+ThandTi+Zr.

Carbonates

Calciteandveryminordolomitearepresentinthecarbonatites(Figs.2g,i,Table9).Bothphasescontainappreciable amounts of Sr, higher in calcite than incoexisting dolomite (0.36–0.81wt%SrO in calciteversus0.29–0.4wt%SrOindolomite).BariumandNaareinvariablyveryloworabsent,asareFeandMn.

Apatite, sulfides

Apatiteisatypicalaccessorymineral.ItisrelativelyrichinSr(1.1–2.1wt%SrO)inalllithologies.IjolitesandcarbonatiteshaveapatitewithlowcontentsofREE,UandTh.ApatiteislowinF(0.3–0.5wt%);onlyafew

fIG.7. Nephelineandalkali feldsparcompositions in thenepheline–kalsilite–silicadiagram(weight%).Thefieldsofnephelinesyeniteandijolitewhole-rockcomposi-tionsarealsoreported.ThenephelinecompositionsoftheMurud–Janjiramelilite-freenephelinites(Mumbairegion,DeccanTraps,India)arereportedforcomparison(datafromMellusoet al. 2002).

fIG. 8. Concentrations ofNa versus Nb +Ta (apfu) inperovskiteofSungValley.Thegoodcorrelationinvolvingtheseelementsandaclearcompositionalchange linkingperovskiteofthevariouslithotypesarevisible.


carbonatitesampleshavehighervalues(2.8–3.4wt%).(Table10)The“tetrahedralsitesubstitutionindex”ofapatite[TSSI=100*(Si+S+C)/P],whichrepresentsameasureofthedegreeofsubstitutionattheTsite,islow(0.5–3.2%).Thesevalues indicateanequilibriumenvironmentofcrystallization.AsingleapatiteanalysisinaclinopyroxeniterevealsahighTSSI(5.3%).

Pyrite,pyrrhotiteandaPb–Fesulfide(identifiedbyEDSspectra)havebeenfoundincarbonatites,nephelinesyenitesandclinopyroxenites.

TheTRace-eLeMenTcoMpoSITIonofcoexISTInGphaSeSInIjoLITeSV58

The chemical composition of clinopyroxene,perovskite, titanite andgarnet fromSV58 ijolitewasinvestigatedinmoredetailbyaddingLA–ICP–MSdata

(Table11).Thisisaparticularlyinterestingsituation,aspetrographyshowsoneor twoperitecticrelationshipsbetween accessory phases occurring during or laterthanthecrystallizationofclinopyroxene(perovskite!


titanite,andthentitanite!garnet),withnephelineasthe lastphase tocrystallize. Inparticular, the texturalrelationship between perovskite, titanite andTi-richgarnet(Fig.2e)canberelatedtothechemicalreaction:perovskite+silica!titanite,andtitanite+magnetite!garnetreportedbyDawsonet al.(1995)intheirstudyofplutonicnodulesofOldoinyoLengai,orCaTiSiO5+SiO2+Fe2O3+2CaO!Ca3Fe2TiSi2O12byVuorinen&Hålenius (2005) atAlnö, Sweden.Barbosa et al.(2008)notedthissequenceofmineralsinrocksoftheSalitrecomplex,AltoParanaíba,Brazil.

Asexpectedonthebasisofpreviousinvestigationsonperalkaline rocks (Onumaet al. 1981,Dawsonet al.1994,Mellusoet al.2008,Arzamastsevet al.2009,Yanget al.2009),aswellasexperimentalinvestigationsontrace-elementpartitioning(Corgne&Wood2005),perovskiteshowsalargeREEfractionation(LaN/YbNup to390,where the subscript “N”meanschondrite-normalized,Fig.9a),beingthemostimportantreposi-toryofLREE(lightrare-earthelements)(LaN~20000).

PerovskitealsocontainslargeamountsofSr,Nb,Ta,UandTh(withUN/ThNintherangeof1.7–2.0),aswellassignificantamountsofPb,ZrandHf.Thevanadiumcontentsaremoderate(270–300ppm).

Clinopyroxene (in textural equilibrium withperovskite)containsrelativelylowlevelsofREE(e.g.,LaN in the range 10–25),HFSE (high field-strengthelements) andLILE (large-ion lithophile elements),exceptZrandHf,whichareatthesamelevelofconcen-trationasperovskiteanddefinemarkedpeaks(Fig.9b).EnrichmentinZrandHfhasbeendocumentedinclino-pyroxenefromalkalinecumulatesinmantlexenolithsofseverallocalities,beingmainlyascribedtotheoccur-rence of relatively large clinopyroxene-melt partitioncoefficientsforsuchelements(Raffoneet al.2009).Inparticular, large clinopyroxene–liquid partition coef-ficients forZrandHfhavebeencommonlyobservedincarbonatitesystems(Adam&Green2001),and inourrocksisfavoredbythelargesizeexpectedforthe[6]-fold-coordinatedsite(M1),becauseofthelargeFe

fIG.9. a)Chondrite-normalized (CIchondriteofAnders&Grevesse1989) rare-earthelementdistributioninthecoexistingmineralsofijoliteSV58.Thebulk-rockcomposi-tionofsampleSV58isalsoplottedforreference.b)Multi-elementdistributioninthesameminerals.Elementorderisgivenaccordingtomantle–liquidpartitioncoefficients.


content.Obertiet al. (2000)discussedtherelationshipsbetweenthepartitioncoefficientsforHFSEandthesizeandelasticfeaturesoftheoctahedralsites.Chondrite-normalizedREE patterns have a sinusoidal shape,characterizedby convex-upwardLREE fractionation,relativedepletionintheMREEandenrichmentintheheaviestREE(Fig.9a).The inversionof theslope intheHREEregionisinstrikingcontrastwiththesteadyHREEdepletionshownbyperovskite.Thisfeaturehasbeendocumentedalready inmagmatic clinopyroxenesegregatedfromperalkalinemelts(e.g.,Vuorinenet al.2005,Arzamastsevet al. 2009), and is interpreted asevidenceforHREEuptakeincationsite(s)smallerthanthe[8]-foldcoordinatedM2(Foleyet al. 2001,Fedeleet al. 2009).TheREE andSr decrease found in therimofclinopyroxeneislikelytheresultofprogressivedepletionoftheseelementsinthemelt–orsolid–solidchemicalexchangeaftercrystallization.Thevanadiumcontents of clinopyroxene (320–430ppm) are higherthanthoseinperovskite(Table11).

Titanite(crystallizingthroughreactionofperovskitewithmelt) showsTh, Nb, Ta andHREE contentssimilartothoseofperovskite,lowerlevelsofU,lightandmediumREE,andSr,butsignificantlyhigherZr,HfandV.SimilarvalueshavebeenfoundbyDawsonet al. (1994) for perovskite–titanite pairs in ijolite oftheOldoinyoLengaivolcano.Theyarealsoconsistentwith those expected on the basis of present experi-mentalknowledgeaboutthetrace-elementpartitioningof perovskite, titanite andmelt (Tiepoloet al. 2002,Corgne&Wood 2005, Prowatke&Klemme2005).Thus, the compositional features of perovskite andtitanite from ijoliteSV58 likelyapproachedchemicalequilibriumwithasimilarmelt.

Garnet from ijolite SV58 has by far the largestvanadium(1400–1520ppm)andHREEcontents(e.g.,YbNup to675),whichare, however, associatedwithlargeconcentrationsof lightandmediumREE(up to1300timeschondrite):thecombinationofthesefeaturesproduced slightly convex-upwardREEpatterns.Thegarnet in SV58 hasZr,Hf,U andTh contents fromslightly lower toslightlyhigher thanthose in titanite,but slightly lowerNb,Ta andTi (withNbN/TaN stillabove1).SimilarREEpatternshavebeendocumentedbyVuorinenet al.(2005)forgarnetinmelteigiteandijolite ofAlnö Island, Sweden. Broadly consistentHFSE–REE and (U,Th)–REE fractionation has beenfound in garnet from alkaline plutonic rocks fromtheHighAtlas (Markset al. 2008). It is noteworthythatclinopyroxeneassociatedwithgarnet in theAlnölithologieshaslargerLREEandlowerHREEcontentsthanthoseinSV58;thegarnetinSV58thuscrystallizedinthepresenceofadifferent,probablymoreevolved,meltwithrespecttotheparentliquidofclinopyroxene(andperovskite).

Inconclusion, levelsoftraceelementsinmineralssupport the petrographic observations; they indicatethat: (i) perovskite and clinopyroxenewere the early

minerals tocrystallize,wellbeforegarnet, (ii) titanitegrowth occurred in the presence of amelt not verydifferentintermsoftraceelementswithrespecttotheparentalmeltofclinopyroxeneandperovskite,and(iii),conversely, garnet crystallized from amore evolvedcompositionofmelt.

dIScUSSIon

SungValley isaclassicshallow-levelplagioclase-free ultramafic alkaline intrusion. It hasmany inter-esting petrological features: a) presence ofmelilite,whichisastablephaseoflarnite-normativemagmasatlowtomoderatepressure,b)presenceofperovskiteinmanylithotypes,insomecasesaccompaniedbyamphi-bole, again indicatingabundanceof larnite-normativemagmasformingthiscomplex,c)thepresenceofijoliticrocks,indicatingthatnepheliniticmagmaswerepresentduring the evolution of the complex, d) presence ofnephelinesyeniteswithnearlycotecticproportionsofnepheline and alkali feldspar,which plot in the low-pressurephonoliticminimumofthenepheline–kalsilite–silicadiagram,e)thepresenceofcarbonatiticintrusiverockscontainingMg-richminerals,andf)anextremescarcityofhydrousminerals.

Petrogenetic features

If compared to other strongly alkaline intrusions,such as Jacupiranga and Juquiá, inBrazil (Melcher1966,Beccaluvaet al.1992,Rubertiet al.2005),TuriyandKovdor,inRussia(Ivanikovet al.1998,Veksleret al.1998,Verhulstet al.2000,Dunworth&Bell2003),Gardiner, inGreenland (Nielsen 1980, 1981, 1994),IronHillandMagnetCove,inColoradoandArkansas,USA,respectively[Nash(1972),Flohr&Ross(1989),andreferencestherein],theSungValleycomplexsharesmanycompositionalfeatures,includingthepresenceofolivine-rich intrusive rocks, clinopyroxenites,melili-tolites and ijolite-series rocks.These intrusions havebeen commonly considered to involve crystallizationof a highly silica-undersaturated ultramaficmagma,followed by formation of cumulate intrusive rocks,changing inmineralogy andmodal composition inresponsetocompositionofthemagmasthatfilledthechamber.Ivanikovet al.(1998)proposedtheformationofolivineclinopyroxenite,uncompahgriteandmelilitepyroxenitecumulates inorder tomodel the transitionfromanolivinemelilititetoamelilitenephelinite.Theassociated carbonatiteswere proposed to be productsof immiscibility ofmixed silicate–carbonatemagmasatvariousstagesofevolution.Inothercases,carbon-atitesmayhavehadacommonoriginwithmelilitolites(Verhulstet al.2000),ormayhaveformedbyimmis-cibility processeswith nepheline syeniticmagmas(e.g.,Beccaluvaet al.1992).Nielsen(1994)suggestedthe derivation of evolvedmagmas (trachyandesites,trachytes,phonolitesandmelilite-bearingrocks)froma


singleolivinenephelinitecompositionofmagmahavingvariablecontentsofvolatilesandH2O:CO2ratios.Ontheotherhand,Rass(2008)notedthedevelopmentofdistinctmagma-evolutiontrendsinmelilite-bearingormelilite-freeintrusivecomplexes.

InordertoidentifypossibleliquidlinesofdescentintheSungValleycomplex,wegivemajorimportancetotheparagenesisofthevariousrock-types,toconfirmor exclude genetic links between themagmas fromwhichtheseintrusiverockscrystallized.Thepresenceofperovskiteinperidotites,uncompahgriteandijolitesis relevant to the petrogenesis of the SungValleycomplex.Itiswellknownthatperovskitecancrystallizeonly fromhighly silica-undersaturated, feldspar-freeorthomagmatic rocks, such as kimberlites, ultramaficlamprophyres,melilititesandmelilitenephelinites(forCa-richparamagmatic rocksor skarns, inwhichveryCa-richplagioclase,gehleniticmeliliteandperovskitecan coexist, seeMelluso et al. 2003).Taking intoaccount the very similarmajor- andminor-elementcomposition of clinopyroxene (and perovskite) inwehrliteanduncompahgrite,andtheveryclosepositionofthesetwolithotypesinthecomplex,wesuggestthatthemagmasfromwhichdunitesandwehrlitesformedresembled olivine (±melilite) nephelinite or olivinemelilitite in composition.Thesemagmas graded toolivine-freemelilitites,fromwhichmeliliteandclino-pyroxene cocrystallized along a cotectic, forming theuncompahgrite.Thisrockthuswasderivedbycumulusprocesses in a fairlyprimitivemagma,given that theMg#of themaincoexistingminerals (melilite, clino-pyroxeneandphlogopite)rangesbetween80and90.

Thecontemporaneouspresenceofmelilite-bearingandalkali-feldspar-bearingintrusiverocksinthesameigneous complex is alwaysworthy of interest. It haslongbeenknownthatmagmasofmelilititicaffinity(i.e.,feldspar-free)donothaveappropriatecompositionstoevolvetowardphonoliteinaclosedmagmaticsystem(Yoder1973,Wilkinson&Stolz1983).Inexperiments,melilite-bearingcompositionsalsofailedtocrystallizefeldspar at very low residualmelt fractions (Gupta&Lidiak 1973,Guptaet al. 1973), reaching insteadminimumcompositionswithmelilite+leucite+neph-eline±clinopyroxene(e.g.,Pan&Longhi1989).Noreliableexceptionshavebeendemonstratedtodateinnaturalmagmas.Instead,examplesofevolvedmelilite-bearingrockshavebeenfoundatKaiserstuhl,Germany(Keller et al. 1990),Nyiragongo, in theDemocraticRepublicofCongo(Sahama1976),OldoynioLengai,inTanzania(Donaldson&Dawson1978),Mt.Vulture,inItaly(Mellusoet al.1996)andMt.Etinde,inCameroon(Nkoumbou et al. 1995). Therefore, the presenceof nepheline syenites,which plot in the area of thephonoliteminimuminthenepheline–kalsilite–silicaphasediagram(Fig.8)andtheirhighfeldsparcontent,indicate that highly silica-undersaturated feldspar-bearing(orfeldspar-normative)basanitescouldrepre-senttheparentalmagmaforsomeoftheselithotypes.

Examples of the derivation of peralkaline phonolites(nephelinesyenites) fromabasaniticparentalmagmaarenumerous(e.g.,Coombs&Wilkinson1969,Brotzuet al.19832007,Aurisicchioet al.1983,LeRoexet al.1990,Thompsonet al.2001,Mellusoet al.2007).Evidence ofmagmas derived by cumulus processesin silica-undersaturatedmagmas is also given by thepresence of alkali-feldspar-bearing clinopyroxenites(a sort of ultramafic shonkinites). Clinopyroxeniteswith relatively Fe-rich clinopyroxene compositionandinterstitialalkalifeldspar(plusaccessories)implymajor fractional crystallization (andaccumulation)ofclinopyroxene and rapid attainment of alkali feldsparsaturationbeforeplagioclase(orsimplyneverreachingplagioclase saturation).This cannot be a feature ofevolvedalkalibasaltic,hawaiiticormugeariticcompo-sitions(whichsystematicallyhaveCa-richplagioclaseontheliquidus),or,forthatmatter,nepheliniticmagmas(where interstitial nepheline is to be expected, ratherthanalkalifeldspar),butcanbeobservedinphonoteph-riticortephriphonoliticmagmas(cf.Brotzuet al.2007).ThegenerallylowMg#ofclinopyroxeneintheserocksismoreevidenceof the formationof these lithotypesfromevolvedmelts.

The significanceof the ijolitic bodies in theSungValleycomplexisevenmoreinteresting.Intrusiverocksformedofclinopyroxeneandnephelineareintermediatecompositions betweenmelilite-bearing and feldspar-bearingrocks(cf.LeBas1977,Pan&Longhi1989),andthequestioniswhethertheyhavebeengeneratedbycumulusprocessesinmagmaswiththesamelinkagetomelilitites,whethertheyaretobeconsideredasformedbymagmas that later evolved to nepheline syenites,or, lastly,whether the ijolites are tobe considered asderived from another independent batch ofmagma.TheinformationavailableontheSungValleyrocksiscontradictory.Thereactionrimoftitaniteonperovskitefoundinmanyijolites(e.g.,sampleSV58,seeabove),andthepresenceoftitanitewithoutperovskiteinotherijolites (samplesSV14andSV83) indicate that silicaactivityof these ijoliticmagmaswashigher than thatneeded for perovskite stability, favoring the crystalli-zationoftitanite.Thisgivesanapproximatevaluefora(SiO2)intherange0.47–0.6at900–1000°C(Barker2001). The reaction rim of titanite on perovskite,linkedtotheothermajorchemicaldifferencesincoex-istingminerals, demonstrates that the uncompahgritecannot be formed frommagmasmore evolved thanthosewhichformedtheijolites.Atthesametime,thestabilityoftitaniteinbothijolitesandfeldspar-bearingrocks (nepheline syenites) could indicate a geneticlinkbetweentheserocks.Ifwealsotakeintoaccountthe smooth trend displayed by ijolites and nephelinesyenites in Figure 3,with nepheline syenites havingthemore sodicandFe-richcompositions, thegenesisoftheselatterrockswouldhaveasimpleexplanation.On the other hand,we did not find interstitial (late-crystallized)alkalifeldsparinijolites,norarethereany


rockswithachemicalcompositiontransitionalbetweenijolitesandnephelinesyenites(juvites).Finally,alkalifeldsparcrystallizedbeforeoralongwithnephelineinthe syenites (seeFig. 2g and supplementaryFig. 2),thereverseofwhatwecouldhaveexpectedincaseofderivationfromnepheliniticmagmas(thepositionoftheijolitesinthenepheline–kalsilite–silicadiagram,Fig.7,broadlycorrespondstotheirnephelinecompositions).At thesame time,wedidnotfindany traceofneph-eline in the uncompahgrite (thus a gradation towardsturjaite)noranytraceofmeliliteintheijolites,thoughtheperovskitecompositionsshowaclearcompositionalrange in the transition from peridotites and uncom-pahgritetoijoliticrocks.

ClinopyroxeniteSV8isalarnite-normativesample,being formedbyAl-richand relativelySi-poorclino-pyroxene,butisalsomelilite-andperovskite-freeandtitanite-bearing (Fig. 2d,Table 1).TheTiO2 contentof clinopyroxene in this sample is also among thehighest found in theSungValley rocks.TheMg#ofclinopyroxeneisrelativelylow(54–63);thereforethismineralwasinequilibriumwithanevolvedmagma.Thecompletelydifferentchemicalcompositionofthegarnetinthissamplewithrespecttothegarnetoftheijolitesisalsonotable,giventhatthecompositionofthismineralismoreakintothatinphonolites(nephelinesyenites)elsewhere. To date, the petrogenetic relationshipsbetweenthissampleandtheotherrocksareunclear.

Carbonatite genesis: liquid immiscibility, fractional crystallization, mantle melting?

The carbonatites contain magnesian ilmenite,Cr-bearingmagnetite,olivineveryrichintheforsteritecomponent, and a highMg# in phlogopite rims (anddolomite aswell).TheseMg- andCr-richmineralsmusthavecrystallizedfromequilibriumliquidsmuchmore primitive than nepheline syenites (phonolites)ormagmaswith a similar degree of evolution.Thisexcludesanyreasonablehypothesisofliquidimmisci-bilitybetweenanySungValleycarbonatiteandliquidequivalents of nepheline syenites.As noted above,theSungValleynepheline syenitesplotveryclose totheir pertinentminimum-melt composition (Fig. 7),thus being the result of extreme fractional crystal-lization processes, and are evidently devoid of anyCr-andMg-richphases.Nonetheless,derivationoftheSungValleycarbonatitesby liquid immiscibilitywith“ijolitic”(nephelinitic)magmasalsoisdifficult,inthatalltheijoliticrockshavemineralsmuchmoreFe-richthan those found in thecarbonatites.Howandwhyacarbonate-bearingliquid(andthecoexistingminerals)canbecomemore(ormuchmore)Mg-andCr-richafterliquidimmiscibilityneedstobeexplained.Inaddition,thesamephasescoexistingwithconjugateimmisciblemelts should have the same composition (in theory),but this is not verified in any of themicas, olivineor ilmenite in carbonatites and silicate rocks present

at SungValley.Finally, the presence of suchFo-richolivine andMg-rich phlogopite in the SungValleycarbonatites(seeabove)isnotcommoninanysilicate-dominatedmagmatic rocksworldwide, though it iscommonincarbonatites(cf.Mitchell1978,Morbidelliet al.1986).

ThestronglyvariablechemicalcompositionsoftheSungValley carbonatites in terms of trace elements(see the supplementary table, deposited) is clearlyrelatedtothepresenceofminor-element-richminerals(particularly pyrochlore). It is difficult to link thesecarbonatitestofractionalcrystallizationprocesses,andalsotoconsidertheircompositionasrepresentativeofliquids.Thepossibility of liquid immiscibility of theSungValleycarbonatitesfromamoreprimitiveliquidcompositionisapossibility tobeseriouslytakenintoaccountand,incommonwithSrivastavaet al.(2005),wealsobelievethatliquidimmiscibilityisoneofthepossible petrogenetic processes. Srivastava&Sinha(2004) andSrivastavaet al. (2005) indeed proposedpartialmelting of a carbonatedmantle for the originof carbonatiticmagmas. In light of its lowviscosity,thismeltmoved upward and interactedwith perido-tite to formmetasomatic clinopyroxene and olivine,whichprogressivelychangedthelherzolitetoalkalinewehrlite,withconcomitantreleaseofCO2fluids.Thismodel satisfies field relationships and petrological,geochemical,andisotopiccharacteristicsobservedfortheSungValleycomplex.Weinferthatanalkalisilicatemagmawas generatedfirst (as supported by the agedeterminations on theSungValley rocks,cf. Srivas-tavaet al.2005)andemplacedbeforethecarbonatites.Carbonatitesareconsideredtohavebeenderivedfromameltoriginatingatgreaterdepthsthanthe“metasome”fromwhichsilicatecomponentsarederived.Thisdiffer-ence in thenatureof the source region in themantlethatwasresponsibleforcarbonatiticmagmasaccountsforthelessenrichedSr–Ndsignature(Srivastavaet al.2005)andthemoreMg-andCr-richcompositionthanthesilicatecomponents.

concLUSIonS

We contend that pulses of chemically differentparentalmagmas led to the SungValley intrusion.Thesepulses formedcumulitic intrusive rockswhoseparagenesis andmineral chemical compositions helpdistinguishthefollowingseries:

1) The ultramafic olivine-bearing rocks of thenorthwesternoutcropsareperovskite-bearing;therefore,they formed by accumulation ofmafic phases froma primitive olivine nephelinitic or olivinemelilititicmagma.Onebatchofthismagmacouldhaveevolvedalong a clinopyroxene–melilite cotectic to form theuncompahgrites,whichhave the same clinopyroxenecompositionasthewehrlite.

2)Theijoliteringintrusion,thoughheterogeneousinpetrographiccharacteristics,isformedbyperovskite-


bearingmagmatransitionaltotitanite-bearing(±Ti-richgarnet) clinopyroxene–nepheline-richmagma.Theformationoftheserocksfromanolivine-andmelilite-free nepheliniticmelt is straightforward, but it is notclearwhetherthisnepheliniticmeltcouldhaveformedfrom an even less evolvedmelilite nephelinitic orsimplyfromolivine-andperovskite-bearingnephelin-iticparentalmagmas.

3)Thefeldspar-bearingrocksareclinopyroxenitesandnephelinesyenites.Weproposethatthederivationof these rocks involved feldspar-normativemagmassuchasphonotephriticandtephriphonoliticmelts(fromwhichabundantclinopyroxene,thenfeldsparandfinallyfeldspathoidcancrystallize),asdeducedfromthecrys-tallizationsequenceandchemicalevolutionofminerals.

4)The carbonatites do not provide any evidenceof formation by liquid immiscibility of carbonated“nepheline syenitic” or carbonated “ijolitic”meltsor their equivalents.The verymagnesian nature ofolivine,phlogopiteandilmenite(plusclinohumiteanddolomite), and some relatively highCr contents ofspinel,make these rocks either products of immisci-bilityofmagnesiansilicate–carbonatemagmas(hencepoorlyevolvedmagmas, suchas thosegivingolivinenephelinites or olivinemelilitites), or direct productsofmantlemelting.This latterhypothesis seems tobereasonablealsofromindependentconsiderations,suchasthecross-cuttingrelationshipsofthecarbonatites(seeSrivastavaet al.2005).

All these rocks seem to have crystallized fromoxidizedmagmas at relatively shallow depth, andthere is evidence of concentration of incompatible-element-richmineralsinsomelithotypesasaresponsetotheincompatible-element-richnatureoftheinferredparentalmagmas(seealsoSrivastava&Sinha2004).

acKnoWLedGeMenTS

Roberto deGennaro (CISAG,Napoli) is thankedfor his kindhelp in the electron-microprobedetermi-nations andback-scattered electron images,MarcelloSerracino andMicheleLustrino (IGAG,Rome) alsocontributed valuable analytical assistance atRome’selectronmicroprobe.MicheleLustrinoisalsogratefullythanked for his further technical and scientificwork.IvanaRocco assisted in the laser-ablationwork.EricEssenepatientlypointedoutveryclearlycertaindraw-backsofaninitialversion,andhisadvicewasdeeplyappreciated.ThethoughtfulreviewsofTroelsNielsenandWilhelmVerwoerd and editorial comments ofRobertF.Martinhelpedverymuchtoimproveanearlyversionofthemanuscript.AdditionalcommentsofJohnLonghiwere alsomuch appreciated.This paper hasbeensupportedbyFondiperlaRicercaDipartimentale2008toL.Melluso,andbyCSIR,NewDelhi(SchemeNo.24(0251)/01/EMR–II)toRajeshK.Srivastava.

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Received July 10, 2009, revised manuscript accepted Decem-ber 8, 2009.

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