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205
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]
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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.
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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.
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208 ThecanadIanMIneRaLoGIST
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eVoLUTIonofTheSUnGVaLLeycoMpLex,IndIa 209
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.
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210 ThecanadIanMIneRaLoGIST
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
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eVoLUTIonofTheSUnGVaLLeycoMpLex,IndIa 211
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.
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212 ThecanadIanMIneRaLoGIST
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
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eVoLUTIonofTheSUnGVaLLeycoMpLex,IndIa 213
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
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214 ThecanadIanMIneRaLoGIST
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
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eVoLUTIonofTheSUnGVaLLeycoMpLex,IndIa 215
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#.
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216 ThecanadIanMIneRaLoGIST
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.
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eVoLUTIonofTheSUnGVaLLeycoMpLex,IndIa 217
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
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218 ThecanadIanMIneRaLoGIST
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.
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eVoLUTIonofTheSUnGVaLLeycoMpLex,IndIa 219
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.
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220 ThecanadIanMIneRaLoGIST
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!
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eVoLUTIonofTheSUnGVaLLeycoMpLex,IndIa 221
-
222 ThecanadIanMIneRaLoGIST
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.
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eVoLUTIonofTheSUnGVaLLeycoMpLex,IndIa 223
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
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224 ThecanadIanMIneRaLoGIST
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
-
eVoLUTIonofTheSUnGVaLLeycoMpLex,IndIa 225
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-
-
226 ThecanadIanMIneRaLoGIST
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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