Petrogenesis of Archean PGM-bearing chromitites and associated ultramafic-mafic-anorthositic rocks from the Guelb el Azib layered complex (West African craton, Mauritania)

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ContentslistsavailableatSciVerseScienceDirect

Precambrian

Research

j o u r n al hom ep age :w w w . e l s e v i e r . c o m / l o c a t e / p r e c a m r e s

Petrogenesis

of

Archean

PGM-bearing

chromitites

and

associated

ultramafic–mafic–anorthositic

rocks

from

the

Guelb

el

Azib

layered

complex

(West

African

craton,

Mauritania)

Julien

Berger

a,∗

_,

_Hervé

_Diot

b

_,

_Khalidou

_Lo

c

_,

_Daniel

_Ohnenstetter

d

_,

_Olivier

_Féménias

e

_,

Marjorie

Pivin

a

_{, Daniel}

_Demaiffe

a

_,

_Alain

_Bernard

a

_,

_Bernard

_Charlier

f

a_Département_des_Sciences_de_la_Terre_et_de_{l’Environnement}_&_FRS-FNRS,_Université_Libre_de_Bruxelles,_Belgium b_UMR_CNRS₆₁₁₂_{“Laboratoire}_de_{Planétologie}_et_de_{Géodynamique}_de_Nantes”,_Université_de_Nantes,_France c_Université_de_Nouakchott,_Mauritania

d_Centre_de_Recherches_{Pétrographiques}_et_{Géochimiques,}_CNRS,_F-54501_Vandoeuvre_lès_Nancy,_France e_IAMGOLD_Corporation_–_West_Africa,_Mali

f_Department_of_Earth,_Atmospheric_and_Planetary_Sciences,_{Massachusetts}_Institute_of_Technology,_USA

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received7March2012

Receivedinrevisedform4October2012 Accepted8October2012 Available online xxx Keywords: Anorthosite Layeredcomplex Archeantholeiite Granulites Mafic–ultramaficrocks

a

b

s

t

r

a

c

t

TheArcheanGuelbelAziblayeredcomplex(GAC)intheWestAfricancratonofMauritaniaiscomposedof anassociationofserpentinites,chromitites,amphibolitesandanorthositeswithfewfine-grained amphi-bolitedykes.Thecomplexformstectonicslicesin2.9–3.5GaTTGgneissterrainsincloseassociationwith supracrustalrocks(BIFs,impuremarbles,amphibolites).Itwasaffectedbyamaingranulite-faciesgrade metamorphism(upto900◦_C_at_5–6_kbar)_with_subsequent_{retrogression}_in_amphibolite_and_greenschist

faciesconditions.

Thepreservedigneousmacrostructures,themineralcompositionsandthenatureofrelicmagmatic assemblageshavebeenusedtoconstrainthecompositionoftheparentalmeltsandtheconditionsof crystallization.Accordingtopetrologicalobservationsandtocomparisonwithexperimentaldata,the formationofthecomplexcanbeexplainedbyfractionationofaslightlyhydroushigh-aluminabasaltic meltatlowpressure.Theearlyfractionationofolivineandtheabsenceofmassiveclinopyroxene frac-tionationbeforeplagioclasesaturationledtocrystallizationofhighlycalcicplagioclasewithFe-,Al-rich butCr-poorchromitefromahydroustholeiiticparentalmagma,similartoworldwideArcheantholeiites. ThecomplexsharesmanysimilaritieswithArcheananorthositelayeredcomplexes,possiblyformed inasupra-subductionzoneenvironmentaccordingtoresultsobtainedonsimilar2.9–3.0Gacomplexes fromGreenlandandIndia(namelyFiskenaessetandSittampundi).ThreephasesofPGEmineralization affectedtheGACchromitites:(i)igneouscrystallizationoflaurite;(ii)formationoflatemagmaticIPGE sulpho-arsenides(irarsite–hollingworthite)and(iii)hydrothermalPt–Pdmineralizationrepresentedby sperryliteandrustenburgite.

1. Introduction

TheassociationofhighlycalcicanorthositeandFe-rich chromi-titein ultramafic–maficlayeredintrusionsisalmostexclusively restricted to Archean terrains (Windley et al., 1981; Ashwal, 1993; Rollinson et al., 2002, 2010; Dutta et al., 2011). These ultramafic–mafic–anorthosite(hereafterUMA)layeredcomplexes aresystematically closely associated withsupracrustal rocksin stronglymetamorphosedanddeformedTTGterrains.Theyhave

∗Correspondingauthor.Presentaddress:ETHZurich,NOE59,Sonneggstrasse,5, 8092Zurich,Switzerland.Tel.:+41446328167;fax:+41446321030.

E-mailaddress:julien.berger@erdw.ethz.ch(J.Berger).

been recognizedand well studied in theNorth Atlanticcraton (the 2.97Ga Fiskenaessetcomplex and 2.98Naajat Kuuat com-plexinGreenland;Myers,1976;WindleyandGarde,2009;Polat etal.,2010;Rollinsonetal.,2010;Hoffmannetal.,2012),inthe IndianDarhwarcraton(the2.9GaSittampundiandrelated com-plexes;Duttaetal.,2011;DharmaRaoetal.,inpressandreferences therein),intheLimpopobeltlinkingtheZimbabweandKaapvaal cratons (the3.3GaMessina layeredintrusion;Hor etal., 1975; Barton,1996;Mourietal.,2009).Archeananorthosite–chromitite complexes in the Australian Pilbara craton have also probably similarorigins(HoatsonandSun, 2002).Accordingto petrolog-ical, geochemical and isotopic studies of the Fiskenaesset and Sittampundicomplexes,UMAarethoughttorepresentthe plu-tonicsectionofoceanicarccrustformedabovesubductingslabs

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Author's personal copy

J.Bergeretal./PrecambrianResearch224 (2013) 612–628 613

Fig.1. GeologicalmapsoftheWestAfricancraton(a)andtheAmsagaArea(b).ThemapoftheAmsagaandthegeochronologicalinformationsarefromPotreletal.(1998). GAC:GuelbelAzibComplex.

(Polatetal.,2009,2010;WindleyandGarde,2009;Rollinsonetal., 2010;Duttaetal.,2011;Hoffmannetal.,2012).Theirclose associ-ationwithTTG(tectonicorintrusiverelationships),interpretedas slabmelts(Martinetal.,2005)oraspartialmeltsfromthebaseof thickenedarccrust(Hoffmannetal.,2011),alsoarguesinfavourof asupra-subductionzoneorigin.

Hence, these UMA associations are markers to understand Archeanarc magmatismandcrustalrecyclingprocesses at sub-duction zones, and to characterize fluxes betweenmantle and islandarccrustinthelightofearlyoceanicandcontinentalcrustal growth.Disagreementspersistaboutthegeochemicalnatureofthe primitivemeltanditsevolutionwithdifferentiation(tholeiitic alu-minousbasalt,Mg-andAl-richultrabasicmagma;Weaveretal., 1981;Ashwal,1993;Rollinsonetal.,2010;Polatetal.,2011),the originofhighlycalcicplagioclaseandtheoccurrenceof metamor-phicversusigneousplagioclase–chromite–amphiboleassociations. Inthisstudy,wepresentnewdataontheArcheanGuelbelAzib complex,anewoccurrenceoflayeredanorthositecomplexlocated intheWestAfricancratonofMauritania.Itiscomposedofasuiteof ultramaficcumulates,chromitites,layeredgabbros,leucogabbros andanorthositesmetamorphosedunderamphiboliteandgranulite grades.Theprimitivemeltcompositionanditsliquidlineofdescent areestimatedinordertoexplaintherelationbetweenthedifferent rock-types,theoriginofhigh-Caanorthositesandtheirassociation withFe-richchromitites.Thedegreeandeffectofhigh-grade meta-morphismontheoriginaligneousmineralogyofthecomplexand thelinkbetweenPGEmineralizationandchromititesarealso dis-cussed.Finally,weemphasizetheimportanceofsuchcomplexesfor abetterunderstandingofArcheanintra-oceanicsubductionzone magmatism.

2. Regionalgeology

TheWestAfrican Craton(WAC)comprises twolargeshields where Archean and Paleoproterozoic terrains crop out at the southernandnorthernbordersoftheNeoproterozoicand Meso-zoicTaoudenibasin:theLeo-Manriseinthesouth(IvoryCoast, Guinea...)andtheReguibatriseinthenorthwesternAfrica,within theSaharadesert(Fig.1a).

TheAmsagaarea(Fig.1b)(Barrère,1967),locatedwithinthe Reguibat rise,belongs tothe Choum-Ragel Abiodterrane(Key et al., 2008)which is characterizedby major magmatic events at 3.5, 2.99, 2.8 and 2.7Ga with a main tectono-metamorphic eventbracketedbetween2.95and 2.73Ga(Auvrayetal.,1992; Potrel, 1994; Potrel et al., 1996, 1998; Key et al., 2008). The northern part of the Amsaga area (Fig. 1b) is composed of supracrustal metasediments (metaquartzites, metagreywackes andmetapelites)intrudedby2.99Gametamorphosedcharnockitic plutons (Potrel,1994;Potrelet al.,1998).Thesouthern domain (Fig. 1b) is dominated by migmatitic, locally garnet-bearing, quartzo-feldspathicgneissesofTTGaffinitywithlayers,sheetsor lenses of amphibolite (former mafic dykes or lavas), banded iron formation (BIF), cipolin and chromitite/anorthosite-bearing mafic–ultramafic layered complexes dissected by Neoarchean shear zones. Post-metamorphic/late-kinematic formations (Fig. 1b) include the 2.73Ga Touijenjert–Modreïgue granite and the 2.7Ga Iguilid mafic intrusion (Potrel et al., 1998).

Except for volumetrically minor Paleoproterozoicto Jurassic mafic dykes, the Amsaga area underwent no major magmatic and/or metamorphic events since 2.7Ga. The formation and

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Fig.2.DetailedgeologicalmapoftheGuelbelAzibcomplexwiththelocationof chromititeoutcropsinthearea.TherectanglereferstoblocdiagramofFig.3.

reworkingoftheMauritanides,amixedPan-AfricanandVariscan orogenic belt, has ledto thrusting of tectonic sheets onto the ArcheanrocksinthesouthwesternpartoftheReguibatrise,but hasnotaffectedthestudiedarea.

3. ChromititesintheAmsagaareaandtheGuelbelAzib complex(GAC)

OurfieldworktogetherwiththeobservationofBarrère(1967) hasrevealedsevenmainoccurrencesofchromititesalignedalong a NE–SW strike. Chromitites are associated with serpentinites, ultramaficmeta-cumulates,maficrocks(coarse-grainedand fine-grained amphibolites) and anorthosites. The wholeassociation formscomplexesdissectedbylateArcheanN35–45◦_dextral_shear

zones(Fig.1b).

Thelargestcomplex(300–1500mwideand10kmlong)located about5kmsouthofGuelbelAzib(Figs.1band2)issurroundedby porphyroclasticgranodioriticandtonaliticorthogneisses, amphi-bolites and impure marbles(Fig.2). BIFs lodes haveonly been observedontheeasternsideofthecomplex,whereasfew leucoc-raticgarnet-sillimanitemyloniticorthogneissesoccurexclusively onitswesternside(Figs.2and3).Mostfoliationsareuprightwith down-dipstretchinglineations;somefoliationplanesarehowever onlygentlydippingawayfromthemajorshearzone.Late dex-tralstrike-slipmovementsareevidencedbyhorizontallineations andasymmetricmantledporphyroclastsinshearzonesofseveral tensofkilometreslength.Thislastductile eventisalsoattested byuprightfoldswithverticalaxisthatarewellexpressedinthe surroundingBIFs(Fig.3).

TheGACisanallochthonous,highlytectonized,layeredbody affected by high- and low-grade metamorphism. Its original igneous stratigraphy can therefore not be reconstructed. The geometriesofboundariesbetweenthedifferentlithologicalunits presentedonthegeologicalmap(Fig.2)aresimplified;theymore closelycorrespondtothosedrawnontheblockdiagramofFig.3.

Chromitites crop out aslenses of severalmetres width and tensofmetrelength(Fig.3).Theyoftenoccurwithinserpentinites andmetawebsterites.Chromititesareeithermassive,brecciated (Fig. 4b)ordisseminatedwithinthehostcumulates. Some lay-eredchromitepodshavealsobeenobservedwithinanorthosite andleuco-amphibolitelayers(Fig.4candsampleMA435),a com-monfeatureofArcheanultramafic–mafic–anorthositecomplexes (Rollinsonetal.,2010).

Serpentinites are deeply silicified, carbonated and oxidized (Fig.4d).Consequently,neitherhightemperatureanhydrous min-erals (olivine, pyroxenes...)nor primary magmatic textures or structures are preserved. Spinel veins (Fig. 4d) and rodingitic dykes(hydrogrossular–prehnite–chloriterocks)cutacrossthe ser-pentinites. Talc is often present either as small patches or as

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Author's personal copy

Fig.4.FieldphotographsofrocksformingtheGuelbelAzibcomplex:(A)layeredchromitite;(B)brecciatedandretrogressedchloritechromitite;(C)chromititepodina leuco-amphibolite/anorthositeblock;(D)spinelveinincarbonatedandoxidizedserpentinite;(E)layeredhornblendite/mela-amphibolite;(F)layeredleuco-amphibolite.

individualized spinel-talc rocks. Elliptical bodies of metaweb-sterite,hornblendite,amphibolite,anorthositeandchromititeare scatteredwithinthemainmassofserpentinite(Fig.3).

The ultramafic cumulates consist of partly serpen-tinized metawebsterites, metatroctolites, hornblendite and

mela-amphibolite, with some lenses of chromitites. Elliptical bodiesofmaficmaterialandserpentinitearealsocommonlyfound (Fig.3).Despitestrongdeformationandpervasivemetamorphism, igneouslayeringispreservedinbothmela-amphibolites(Fig.4e), ultramaficcumulatesandafewchromitites.

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Table1

Modalandtexturalpropertiesofchromititesamples.

Sample X Y Texture %Chromite Matrixmineral

MA17 643,562 2,242,634 Massive 95 Chlorite MA44 653,300 2,254,645 Disseminated 42 Chlorite–calcite MA226 657,930 2,274,050 Massive 78 Chlorite MA238 654,701 2,253,581 Massive 61 Chlorite MA239 654,656 2,253,739 Brecciated 87 Chlorite MA273 653,400 2,255,000 Massive 61 Amphibole MA400 654,028 2,256,615 Massive 87 Chlorite MA420 653,144 2,253,504 Massive 59 Amphibole MA422 652,474 2,253,808 Disseminated 21 Srp-Talc MA425 653,050 2,254,000 Layered 80–40 Amphibole MA436 653,165 2,254,250 Brecciated 65 Chlorite MA440 653,372 2,254,820 Massive 69 Amphibole

X–Y:UTMcoordinates(m),zone28,datumWGS84.

Anorthosites,leuco-amphibolitesandmeso-amphibolitesform adistinctunitdespitethepresenceofafewbodiesofserpentinite. Igneouslayeringcanstillbefound(Fig.4f)butbothmetamorphism andductiledeformationaffectedtheamphibolite.Scarce chromi-titepodsareinterlayeredwithinanorthosites/leuco-amphibolite.

TwogenerationsofbasicdykesarefoundwithintheGAC(Fig.3): theoldestoneofArcheanageisaffectedbyHTmetamorphismand deformation,whiletheyoungestonecutsacrossArcheanstructures andisobliquetofoliation.

4. Petrographicdescription 4.1. Chromitites

Chromititeshavebeensubdividedaccordingtothemodal pro-portion of spinels and the nature of the matrix silicate phase (Table1).

Massive chromitites contain more than 50vol.% spinels (Fig.5a–c).Themainhostsilicateispalegreenishchlorite(MA17, 226,238,400)orgreenhornblende(MA273,420,440).Chromite grainsareangularandcoarse(upto5mm)inthechlorite-bearing samples(Fig.5c),whiletheyaresmall(<1mm)androunded in theamphibolite(Fig.5aandb).Ferritchromitgenerallyforms a thinrimaroundprimaryspineloraroundchloriteandamphibole inclusions(Fig.5b).Othersilicateandoxideinclusionsarescarce; theyconsistofnearlypureanorthite,magnesianolivineandrutile. Smallshuiskite,Cr-pumpellyiteandCr-grossularhavebeenfound, respectively, in the matrix and as inclusions in chromite from sampleMA440.Minute(<7mm)sulphideandarsenideinclusions arecommon,thedominantspeciesbeingeuhedralmillerite(NiS), whichisfrequentlyfoundincloseassociationwithanorthiteand rutileinclusions(Fig.4d).Theprimarysulphidesfrequentlydisplay exsolutionorreplacementbandsofpyriteandpentlandite. Cov-elite(CuS),chalcopyrite(CuFeS₂)andasingleisolatedgersdorffite (NiAsS)werealsofoundwithinchromitegrains.

Brecciatedchromititesowetheiraspecttothedevelopmentof anetworkofchlorite-filledfractures.Thehostmatrixmineralis chloriteandspinelwithaspongytexturecharacterizedby numer-ousinclusionsofchlorite(Fig.5c).Theferritchromitrimislargely developed;itcanentirelyinvadesomechromite,surround chlo-riteinclusionandoutlinefractures.Anhydroussilicate,oxideand sulphideinclusionshavenotbeenobserved.

Theonly sample oflayered chromitite (MA 425)selected for detailedinvestigationisincloseassociationwithanorthositeand leucogabbro.Despiteitslayeredstructure,thissamplehasexactly thesamepetrographicalfeaturesasamphibole-bearingmassive chromitites,exceptforalargermeangrainsize(upto5mmwide) inthesilicate-richband.

Disseminatedchromititescontainlessthan50vol.%spinels;their matrix iseither composed ofserpentine withtalc(MA 422) or

chloritewithcalcite(MA44).Chromitegrainsarecharacterizedby athickferritchromitrim,afrequentspongytexturedueto numer-ousmineralinclusions(samenatureasinthematrix),andthelack ofanhydroussilicate,oxideandsulphideinclusions(Fig.5e). 4.2. Ultramaficmetacumulates

Foursubgroupshave beendistinguishedonthe basis ofthe lithologicalnatureofthesample:olivine-amphibolerocks, spinel-amphibolite,mela-amphiboliteandhornblendite(Table2).

Olivine-amphibolerocks(Fig.6a)arecomposedofolivine,spinel andamphibole.Theyaredeeplyserpentinized(morethan60vol.% ofolivinewereserpentinized),butthepresenceofroundedolivine relicssurroundedbyamphibolecrystals,probablypseudomorphs afterformerpyroxenes,allowsustoidentifyaformerpoikilitic tex-ture.Greenspinelissurroundedortotallyreplacedbymagnetite.

Spinel-amphibolites (Fig. 6b) show the asso-ciation spinel–amphibole–anorthite (MA 264) or spinel–amphibole–olivine–orthopyroxene(MA25).Thesesamples areaffectedbyastrongdeformationoutlinedbylargeelongated amphibole prisms(upto1cm). Granularpolyhedralamphibole neoblasts crystallized at the borders of larger porphyroclasts indicateahigh-temperaturerecrystallizationprocess.Plagioclase grainsarepolyhedralanddonotshowevidenceofinternalstrain. Theyare,however,partlyalteredintoalbite–epidoteintergrowths. Green spinel and olivine (<2mm) are interstitial to amphibole porphyroclasts.

4.3. Amphibolitesandanorthosites

Mela-amphibolites (Fig. 6c) consist of brown amphi-bole+plagioclase±ilmenite±clinopyroxene±orthopyroxene. Twotypesoftextureshavebeenobserved:(i)undeformed poly-hedralplagioclasegrainswithlargeprismaticamphibole(MA262 and401),(ii)undulatoryplagioclasewithdeformationtwinsand smallgranularamphibolerepresentingtherecrystallization prod-uctoflargerprophyroclasts(MA28 and34).Formeranhydrous granulitic assemblages and granulartextures are still observed ina fewsamples(ex: MA401).Brownamphibolegrowsatthe expenseofclinopyroxenegrains(Fig.6c)indicatingthatamphibole isnotaprimaryigneousphase.Plagioclaseispartlyalteredinto associationofepidote,albiteandsulphides.

Onesampleofhornblendite(MA258)hasbeenselected;itis exclusivelycomposedoflarge(upto5cmlong)orientedprismatic brownamphibole(Fig.4e).

Theanorthosite MA424(Fig.6d)is exclusivelycomposed of plagioclase (with a mean size of 3mm) which displayslobate boundaries, internal strain evidenced by undulose extinction and deformation twins. The rock has been affected by both high-temperaturegrain-boundary-migration(GBM)andsubgrain

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Fig.5.MicrophotographsofchromititesfromtheGAC.(a)Roundedchromiteinamphibole-richmatrix(MA273).Notethelauriteinclusionintheinnerrimofachromite grain.(b)Roundedchromitegrainswithferritchromitriminacalcite–talc–serpentinematrixfromachloritechromitite(MA238).Sperryliteisexclusivelypresentinthe ferritchromitrim.(c)Spongychromitefromabrecciatedchloritechromite(MA239).Theferritchromitrimdevelopsaroundchloriteinclusions.(d)Sulphides(millerite replacedbypyriteandpentlandite),rutileandanorthiteinclusionswithinachromitegrainfromanamphibolechromitite(MA273).(e)Chromitegrainwithathick ferritchromitriminadessiminatedchromitite(MA422).chr:chromite,fct:ferritchromit,hbl:hornblende,chl:chlorite,cc:calcite,tlc:talc,srp:serpentine;rt:rutile;pl: plagioclase,mi:millerite,py:pyrite;pe:pentlandite.

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Table2

ModalcompositionofsilicaterocksfromtheGAC.

Ol Cpx Opx cAmp Antophyllite Pl Qz Zrc Ap Spl Ilm Sulfides %Alt

Olivine-amphibolerocks Ma426 62 25 13 35(srp) Ma43 63 29 8 60(srp) Ma259 75 17 8 75(srp) Spinel-amphibolite Ma25 7 67 20 6 + Ma264 23 1 63 13 15(Srp+chl) Hornblendite Ma258 100 + + + Mela-amphibolites Ma28 46 25 24 + 5 25(ant) Ma34 62 38 + + 2(ep+ab) Ma262 + 63 32 + + 4 8(ep,ab,msc) Ma401 15 26 28 31 + Leuco-amphibolites Ma29 23 77 + Ma27 14 86 + + Ma37 1 13 33 53 2(srp,chl) Anorthosites Ma424 1 99 2(ep,ab) Ma423 10 90 5(ep,ab) Fine-grainedamphibolite Ma256 10 66 24 + 7(ep,ab) Ma267 3 16 45 36 + 8(ep,ab) Ma408 4 15 47 34 + 11(ep,ab) Ma412 3 14 47 36 + 12(ep,ab) Ma435 27 41 32 1 + + +

%Alt:modalproportionofsecondarylowtemperaturephases(thenatureofthephasesisindicatedbetweenbrackets). +:accessory;srp:serpentine;ant:antophyllite;chl:chlorite,ep:epidote,ab:albite,msc:muscovite.

rotation (SGR) recrystallization, the latter being shown by the presenceofsmallpolygonalgrainsborderingthelarger porphyro-clasts.SampleMA423(Table2)isananorthositewith∼8vol.%of smallprismaticgreenamphibolepatchesreplacingaformerlarger clinopyroxene.Plagioclaseispolyhedralwithsharpstraightgrain boundaries.Itshowsevidenceforsubgrainrotation recrystalliza-tion(SGR),leadingtothepresenceofbothlargedeformedgrains (upto1cmlong)andsmallunstrainedneoblasts.

Leuco-amphibolites (Fig. 6e) are characterized by the associ-ation of green–brown amphibole (14–33vol.%) and plagioclase (53–86vol.%),sampleMA37showingadditionalgranular orthopy-roxene.Plagioclaseislargelydominantoveramphibole;nooxide hasbeenobserved.Texturalevidencesuggeststhatgreen amphi-boleprogressivelyconsumedbrownamphiboleduringretrograde phase.Twosamples(MA27and29)haveabimodalsizedistribution ofplagioclasewithslightlylobatestrainedpolyhedral porphyro-clasts surroundedby neoblasts formed by SGR. MA37 sample displaysastrongshapepreferredorientationofbothplagioclase aggregatesandamphibole,plagioclasebeingunstrainedand poly-hedral.Epidoteandalbitearecommonlow-temperaturealteration phasesofplagioclaseinthissample.

4.4. Fine-grainedamphibolites(amphibolites)

Samples MA 267, 408 and 412 show the metamorphic association plagioclase+brown amphibole+clinopyroxene+ orthopyroxene+ilmenite (Fig. 6f, Table 2). Former porphyro-clastsofclinopyroxene(upto1mm wide)aresurroundedby a fine-grainedgranulitic matrixofpolygonal plagioclase,granular orthopyroxene and brown amphibole blasts, the latter clearly consuminglargeclinopyroxene.SamplesMA238and435show a more equilibrated texturecharacterized bypolygonal plagio-claseandbrownamphibole.Fewrelicsofclinopyroxenearestill

observedinthecoreofamphibole.Quartz,apatiteandpyriteare commonaccessoryphasesinthesesamples.

5. Mineralchemistry

Forthefollowingsections,analyticalmethodsareprovidedas Supplementarymaterial Aandtablesofmicroprobeanalysesas SupplementaryfileB.

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.precamres.2012.10.005.

5.1. Chromiteandotherspinels

ChromitesfromGACchromititesdefinealargecompositional field,theFe2+_#_ranges_from₅₃_to₉₈_and_the₁₀₀_×_Cr/(Cr₊_Al)_ratio

from40to75(Fig.7a)withlowTicontents(max0.54wt%TiO2).

ComparedtoworldwideArcheanandPaleoproterozoicchromites, those from the GAC are characterized by high Fe# and low Cr/(Cr+Al)(Fig.7a) comparabletochromitesfromFiskenaesset (Rollinsonetal.,2010).Disseminatedandbrecciatedchromitites showlargecompositional variations(ex: Fe#:62–96in sample MA422) with grains surroundedby a ferritchromitrim. Three compositionaltrendscanbeobservedforbrecciatedand dissem-inatedchromites(Fig.7a):(i)astrongincreaseinCr#coupledto aslightincreaseofFe#towardsferritchromitcompositions(Fe#: 96,Cr#:99).(ii)AdecreaseofbothFe#andCr#towardsgreen spinelsfromspinel-amphibolites(Fe#:30,Cr/(Cr+Al):0).(iii)Large variationsinFe#(61–85)atconstantCr/(Cr+Al)ratio(∼61–64) with an endmember composition represented by the massive chlorite–chromititeMA226(Fe#:53–57;Cr/(Cr+Al):61–62)also characterizedbythedevelopmentofferritchromitrims.

Massive and layered chromitites (Fig. 7b), either amphibole or chlorite-bearing, do not show compositional variation from

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Author's personal copy

Fig.6.MicrophotographsofsomesilicaterocksfromtheGAC:(A)olivine–amphibolerockMA426,notetheformerpoikilitictexture;(B)spinelamphiboliteMA25;(C) mela-amphiboliteMA262showingtheprogressivereplacementofclinopyroxenebyhornblende;(D)anorthositeMA424;(E)leuco-amphiboliteMA29;(F)fine-grained opx-bearingamphiboliteMA435.

grainto grainin a given sample (ex: MA 273, Fe#: 59.2–60.8; Cr/(Cr+Al):51.8–52.8)andnochemicalzoningwithinindividual grains.Chromiteinthesesamplesarecharacterizedbyslight vari-ation in Cr/(Cr+Al)ratio (46–53) withFe# ranging from59 to 78;withthemassivechloritechromititeMA238showing maxi-malvaluesforbothFe#andCr/(Cr+Al)ratio(85–88and54–56, respectively).Chromiteinmassiveandlayeredchromititesisthe

mostsimilartothosefromArcheanlayeredanorthistecomplexes (Fig.7b).

On theFe3+_–Cr–Al_plot₍_Fig.₈_),_brecciated _and _disseminated

chromitesarelocatedbetweenmassivechromititesandeither fer-ritchromit(Al#:0–10,Fe3+_#:_35–40)_or_Mg–Al_green_spinels_(Al#:

90–100).Asawhole,massiveandlayeredchromititesare compa-rabletoFiskenaessetchromitesthoughhavinglowFe3+_#_(4–10).

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0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90100 2+ 2+

Fe /(Fe +Mg)

C

r/

(C

r+

A

l)

Chromitites Massive 50-70% 70-90% 90-100% Brecciated Layered Disseminated Ultram. metacumulates Spl-amphibolites Chl Amp Srp Metamorphic spinel from meta-troctolites Ferritchromit Fiskenaesset Komatiites Greenstone Cont. Intrusions 10 20 30 40 50 60 70 80 90 100 MA 400 Sittampundi

A

B

Fig.7.Fe#vsCr#plotfortheGACchromitesandspinels.(A)PlotofallspinelsandchromitesfromtheGACwiththethreemetamorphictrends(thickblackarrows)described inthetext.(B)PlotofGACchromiteswithpreservedigneouscompositionandtrend(thickgreyarrow)comparedtochromitesfromthosefromArchean/Paleoproterozoic terrains.ComparisonfieldsarefromBarnesandRoeder(2001)exceptforFiskenaesset(Rollinsonetal.,2010)andSittampundichromites(Duttaetal.,2011).Thenatureof theMA400chromites(metamorphicvsigneous)isdiscussedinthetext.Chl,AmpandSrprefertochlorite-,amphibole-andserpentine-chromitites,respectively.

OntheMg#vsAlplotfromchromites,thethreetrendsobserved forbrecciatedanddisseminatedchromitesarewelldefined.The massivechromititesdefineatrendofconstantAlfordecreasing Mg#andthesamplewithlowestMg#ischaracterizedbylowest Alcontent(Fig.9).

5.2. Plagioclaseandamphibole(Figs.10and11)

Plagioclase compositions range from An50 to An98 (Fig. 10)

in the whole Guelb el Azib complex and are unzoned (less than2mol.%Andifferencebetweencoreandrim,exceptintwo retrogressedfine-grainedandonemela-amphibolite)withno com-positional difference between large strained grains and small

Al

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_B

Ferritchromit Fiskenaesset Komatiites Greenstone Cont. Intrusions

Fig.8.Fe3+_–Al–Cr_plot_from_the_GAC_spinels_and_chromites_compared_to_chromites

fromArchean/Paleoproterozoicsettings.SamereferencesasinFig.7.

polygonal newgrains.ThemostAn-rich plagioclaseis foundin thespinel-amphiboliteMA25(An97–98);itisassociatedwith

alu-minousspinelandpargasiticamphibole(Mg#:90–93;upto0.6 (Na+K)Aa.p.f.u.).Theplagioclase frommela-amphibolitesvaries

fromAn56toAn92,themostcalcicbeingfoundinthesampleMA

401whichalsohasthemostNaandAl-richamphibole(upto1.9

IV_Al_a.p.f.u.,_Fig.₁₀_)._Amphibole_from_this_group_is_highly_variable

incomposition fromonesample toanother(Mg#:43–78,IV_Al:

0.15–1.9a.p.f.u.;Fig.11).Again,plagioclasecrystalsareunzoned

Mg#

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Fe-Mg exchange trend

Fig.9. Mg#vsAlplotforchromitesfromtheGAC.Thegreyfieldrepresentsthe compositionofpreservedigneouschromites.

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50 60 70 80 90 100 An (mol.%) µ-amphibolite Amp-anorthosite Hornblendite Leuco-amphibolite Mela-amphibolite Spl-amphibolite Anorthosite 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 Al in amphibole (p.f.u.) Ma 34 8 2 a M

Fig.10.RelationshipbetweencompositionofamphiboleandplagioclaseintheGAC silicaterocks.Thelinesarejoiningplagioclase–amphibolepairsfromasamesample.

andlimitedchemicalvariations(upto4mol.%An)fromgrainto grainare observedwithinasinglerock.Leuco-amphibolitesare characterizedbyplagioclasewithcompositionalrangeAn75–93,the

mostcalcic plagioclase (MA 37)beingassociated withNa- and Al-richtschermakitic/pargasiticamphibole(Mg#:85–87;(Na+K)_A upto0.5).Themono-mineralicanorthosite(MA 424)hascalcic plagioclase (An85–87), but the amphibole anorthosite(MA 423)

hasplagioclasewithdistinctlylowerCacontents(An50–52)

com-pared to classical Archean anorthosites (Phinney et al., 1988), andisassociatedwithaAl-andNa-poorMg-hornblende(0.1–0.2 (Na+K)_A;IV_Al:_∼0.8_p.f.u.)._The_different_fine-grained_amphibolites

showcalcicplagioclase(An64–70)andAl-richMg-hornblende(Mg#:

54–68)exceptforsampleMA435whichischaracterizedby Al-andNa-pooramphibole(Mg#:78–80).Amphibolescomposition fromchromititeslargelyoverlapstheonefromspinel–amphibolite andolivine–amphibolerocks, theyaremagnesiohornblendeand

5.5 6.0 6.5 7.0 7.5 8.0 40 50 60 70 80 90 100 (Na+K)A 0.0 0.2 0.4 0.6 0.8 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 µ-amphibolite Amp-anorthosite Hornblendite Leuco-amphibolite Mela-amphibolite Spl-amphibolite Meta-ultramafic actinolite tremolite magnesiohornblende ferrohornblende ferroactinolite tschermakite (Na+K) <0.5 or pargasite (Na+K) >0.5 ts c h e rm a k it e s u b . . b u s e t i s a g r a p edenite sub. ferropargasite Si (p.f.u.) IV A l (p .f .u .) M g # Chromitite

Fig.11.CompositionofamphibolesfromtheGAC.(A)Classificationdiagramfrom Leakeetal.(1997).(B)(Na+K)AvsIVAlplotshowingthetrendsproducedbyvarious

substitutions.

tschermakite(Mg#:84–96;IV_Al:_0.9–1.7_p.f.u.). _Amphibole_from

the hornblendite MA 238 has a Fe-rich composition (Mg#: 60) compared to other ultramafic samples (Mg#>85; Fig. 11). A few amphiboles from mela-amphibolites and chromitites in chlorite-richmicrodomainsareactinoliteortremoliteformed dur-inglow-temperaturelatestagealteration.

5.3. Olivine

OlivineshavearestrictedrangeofcompositionfromFo83toFo84

bothinspinel-amphiboliteMA264andmetawebsterites43and 426.TheolivinewithinchromititesisMg-richcomparedtosilicate rocks:Fo91insampleMA17andFo94insampleMA400.TheCr

contentishighinolivinefromchromitites(upto1.04wt%Cr₂O₃) comparedtoolivinefromsilicaterocks(<0.04wt%Cr2O3). 5.4. Pyroxenes

Clinopyroxenes from mela-amphibolites and fine-grained amphibolites are augites withMg# ranging from 0.70 to0.80; they have low contents of non-quadrilateral elements (Al: 0.03–0.06a.p.f.u.andTi:0.002–0.004a.p.f.u.).Orthopyroxenefrom the spinel amphibolite MA 264 is the most magnesian (Mg#: 0.84).Withinthegroupofmaficrocks,theolivine-bearing leuco-amphibolitehasthemostMg-richorthopyroxene(Mg#:0.76–0.78) compared to mela-amphibolite (0.59–0.62) and fine-grained amphibolites(0.52–0.66).TheAlcontentincreaseswithdecreasing Mg#inMg-richsamples(from0.050–0.067to0.074–0.098p.f.u.); it is significantly lowerin mela- and fine-grained amphibolites (0.011–0.028p.f.u.).

6. P–Tcalculationsandthermodynamicmodelling

Despite preserved igneous macrostructures, such as layer-ing, scarce igneous textures and minerals, the rocks from the GuelbelAzibcomplex,arestronglymetamorphosedunder gran-ulitic grade with subsequent retrogression under amphibolite andgreenschistfaciesgrade.TheP–Tcalculationswillthusonly constrainthemetamorphicconditionsregisteredbytheGAC com-plex.Accordingtopetrographicalobservations,samplesfromthe GACweresubjecttoanhydrousgranuliticgrademetamorphism thatisevidencedbygranularopx–cpx–plagassemblagesinmafic rocksandolivine–spinel–pargasiteinultramaficrocks.Itwasthen affectedbypervasivehydrousrecrystallizationinthelowT gran-ulitetoamphibolite-faciesconditions(growthofbrownandgreen amphibole)andthensubjecttolocalrecrystallizationinthe green-schistfacies(rodingites,chlorite,epidote).

Amphibole–plagioclase thermometry of Holland and Blundy (1994) wasused tocomputeequilibration temperature in GAC rocks. Asrecommended by the authors, plagioclase–amphibole pairscharacterizedbyX_Anlargelyabove0.9(MA25:0.97–0.98) were excluded of the calculations. The thermometer was applied to amphibole–anorthosite, fine-grained amphibolites, leuco-amphibolites and mela-amphibolites. Pressure has been fixed at 5kbar (see below) but this calibrationis only slightly pressure-dependentwithatemperatureincreaseof7◦_C/kbar._The

calculatedtemperaturesvaryfrom650to960◦_C₍_Fig.₁₂_),_the

mela-amphibolitesshowingthehighestvaluesandmosttemperatures fallwithintherange750–850◦_C_(granulitic_conditions)._The

pla-gioclaseandamphibolecompositionsfrommela-amphiboliteMA 401havemineralcompositionsclosetothelimitsofapplicability ofthisthermometer(XAn:0.89–0.93,withmostvaluesat0.91;IVAl

inamphibolebetween1.66and1.84a.p.f.u.).Computed tempera-turesrangefrom960to1000◦_C_in_this_sample_but_the_uncertainty_is

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Fig.12.Barplotoftemperaturescalculatedwithhornblende–plagioclase thermom-etry(HollandandBlundy,1994)forthevariousrocktypesoftheGuelbelAzib complex.Pressureisfixedat5kbar.

probablylargerthantheonerecommendedbyHollandandBlundy (1994)(±40◦_C).

Pressureisdifficulttoestimatefor themetamorphic assem-blages lacking baro-dependent minerals. One olivine leuco-amphibolite sample (MA 37) has been selected to draw a fixedcompositionphase diagrambecauseit hasa lowvariance assemblage(pl+amph+ol+opx)comparedtoothersamples.The pseudosection (Fig.13)hasbeenbuiltwithPerpleX (Connolly, 2005) usingthethermodynamic datasetof Hollandand Powell (1998,updatedin2003)andthesolutionmodelsofDieneretal. (2007)foramphibole,ofHollandandPowell(1996)forortho-and clino-pyroxeneandofHollandandPowell(2003)forplagioclase. Thestabilityofthepl+amph+opx+olassemblageoccupiesasmall portionofthegrid:below6.6kbarat800–910◦_C._Mineral_isopleths

havebeendrawnfortheMg#oforthopyroxene;theisopleththat

4.0 5.2 6.4 7.6 8.8 10.0 P(kbar) opx amph pl sp cpx opx amph pl sp opx amph pl ol sp opx pl ol cpx H2O opx amph pl sp cpx H2O opx pl sp cpx H2O opx am ph pl o l H2O opx amph pl ol sp H2O opx amph pl ol sp cpx opx amph pl ol cpx H2O opx pl ol sp cpx H2O opx amph pl ol sp cpx H2O 8 7 n E 7 7 n E mph pl px H2O opx pl cpx H 7 m px pl H 77

MA 37: olivine leuco-amphibolite (ol pl amph opx)

SiO AlO FeO MgO CaO NaO HO 45.57 25.59 5.57 8.42 13.22 1.13 0.50 (wt%) Range of T° calculated using hbl-pl thermometry 760 820 880 940 1000 T(°C) 700

Fig.13.PseudosectionbuiltwithPerple XforsampleMA37.Thegreyshadedarea representsthetemperaturerangemeasuredwithhornblende–plagioclase ther-mometryforthesamesample.Thestabilityfieldfortheassemblageobservedin sampleMA37isoutlinedbyathickline.

matchesthecompositionmeasuredbymicroprobe(En77)crosses

thepl+amph+opx+olfieldinthehighestpressurepartofthe sta-bilityfield.Whencombiningtheresultsofhornblende–plagioclase thermometry(shadedgreyareainFig.13)andthosefromphase diagramcalculation,thebestfitforpressureisbetween5.2and 6.4kbar.Thispressureestimationisingoodagreementwith pre-viousstudiesonmetapeliticrocksoftheAmsagaarea(5±1kbar; Potreletal.,1998)butthemaximaltemperatureestimationsfrom thisauthorarelower(800±50◦_C)_compared_to_ours_(880–910◦_C).

Thediagramalsosupportthatthegrowthofbrownamphiboleat theexpenseofpyroxeneisaretrogradereactionlinkedtoboth tem-peraturedecreaseandhydrationwithinthegranulite–faciesspace (Fig.13),asdeducedfrompetrographicobservations.

7. Concentrationandspeciationofplatinum-group elements

Platinum-groupminerals (PGM)haveonly beenobserved in massiveandlayeredchromitites.ThemostabundantPGMis lau-rite(RuS2)whichoccursassmall(<5mm,mostly2–3mmwide)and euhedralinclusions(Fig.14aandb)inthecoreandtherimofthe chromite.

Quantitative electron microprobe analyses have not been undertakenduetothesmallsizeoftheinclusionsthatapproaches the diameter of microprobe beam. Semi-quantitative analyses with energy dispersive spectrometer (EDS) show that irid-ium and osmium are the most common elements substituted to Ru into laurite: the range of measured composition is (Ru0.80–0.87Os0.09–0.12Ir0.04–0.08)S2.Sulfoarsenidesoftheirarsite(Ir,

Rh)AsS–hollingworthite (RhAsS) solid solution are the second mostfrequent PGMin theAmsaga chromitites.Theyarefound assmall overgrowthson laurite(Fig. 14a and b) oras isolated anhedralgrains(<2mmwide)sometimesassociated withrutile and anorthite(Fig.14c). Thecomposition of irarsites measured byEDSinsamplesMA420and226correspondstotheformula Ir0.60–0.69Rh0.31–0.40AsS. Sperrylite (PtAs2) is the coarser (up to

15mmlong)PGMfound(Fig.14d)andislocatedintheferritchromit rim of spinel from samples MA 273 and 238. Minute (<1mm) rustenburgite(Pt3Sn)hasalsobeenobservedformingsmallgrains

overgrowinglauriteinsampleMA226.

SixbulkchromititessampleshavebeenanalysedforbulkPGE content.Threesubgroupscanbedistinguishedonthebasisofthe chondrite-normalizedPGEpattern(Fig.15):

-MA226,400and422showapronouncedRupeakwithlowPPGE (Rh,Pt,Pd)comparedtoIPGE(Ir,Ru):i.e.low(Pt/Rh)Nratioswith

(Rh/Ir)Nclosetounity(0.5–1.9)andhigh(Pt/Pd)Nratio(3.2–3.4).

-Thetwosamplescontainingsperrylite(MA273andMA238)are characterizedby highPGEcontentscompared totheprevious groupandhighPPGE/IPGEratios((Rh/Ir)N:3.5–4.9).

-Thelayeredchromitite MA425(found withinananorthositic unit)hasthelowestPGEcontentswithamoreorlessflatprofile andaslightpositivepeakforRu.

8. Discussion

8.1. Impactofmetamorphismonchromitecompositionsand mineralassemblages

Four different trends can be observed in Fig. 7a and b for chromitecompositions.TheincreaseinbothFe#andCr/(Cr+Al) ratiotowardsferritchromitcompositioncanbeascribedto reequi-librationduringlow-temperaturehydrousmetamorphism.Indeed, many chromites from the samples following this trend show spongy textures characterized by (i) numerous inclusions of

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Fig.14.BackscatteredelectronimagesofPGMenclosedinchromitegrains:(a)euhedrallauritewithirarsiteovergrowth(MA420);(b)euhedrallauritewithhollingworthite bud(MA226);(c)laurite(white)associatedwithrutile(grey)andplagioclase(black)inachromite(darkgrey)(MA420);(d)sperrylitegrainsurroundedpyPdoxideswithin theferritchromitrim(sampleMA273).

chlorite, (ii) thickrim of ferritchromitcomposition, (iii) strong compositional variation from graintograin. Ferritchromitrims or grains are often interpreted as low grade reequilibration of chromiteunderanoxidizingenvironment(seeMukherjeeetal., 2010).Chromitelyingalongthistrendcannotbeusedtoinferthe compositionoftheirparentalmagma.

Some ultramafic metacumulates have preserved olivine–amphibole assemblages with an aluminous spinel. Thecompositionof thesespinelsand theassociationof high-Al amphibole and green aluminous spinel witholivine aretypical ofhightemperature(granulitetoamphibolitesfaciesconditions) meta-troctolites(Tenthoreyetal.,1996;Bergeret al.,2010; see Figs.7aand8)andsakenites(Giulianietal.,2006;Raithetal.,2008). Green spinelsfromspinel–amphibolite andolivine–amphibolite are strongly aluminous and depleted in Cr. Their composition suggeststhat high-grademetamorphismhasledto enrichment inthespinelend-memberwithleachingofCr,ashiftcomparable tothereplacementtrenddescribed byRollinsonetal.(2002)in metamorphosed Archean chromitite–anorthosite associations fromGreenland.Atvariancewithourinterpretation,theseauthors

interpret the replacement trend as a result of late magmatic interactionbetweenhigh-Crspinelsandevolvedinterstitialmelt. Recent studies on chromitites from the Archean Sittampundi complex,inIndia(Duttaetal.,2011)alsoproposeametamorphic originforthealuminousgreen–bluespinel.Chromitecomposition trending towardstheseMg–Al spinelsin Fig.7a (i.e.brecciated chromititeMA436)areinterpretedtohavebeenpartially equili-bratedduringhighgrademetamorphismandtheywillnotbeused tocalculateparentalmagmacompositions.

The strongvariations in Fe# for a constant Cr/(Cr+Al)ratio observed for disseminatedchromitites (Fig.7a) also character-izedbythedevelopmentofferritchromitrimsaremoredifficult tointerpret. Because thesesamplesshow strongcompositional variationfromgraintograinwiththefrequentdevelopmentofa ferritchromitrim,onecaninferthattheircompositionhasbeen modifiedbymetamorphism.Oneofthesesample(MA422)isa serpentine-bearingdisseminatedchromititeprobably represent-ingaformerolivine–chromitite.Fe–Mgexchangebetweenspinel andolivineisaquickreactionthatledtopartial reequilibration of thesetwophases atPT conditions(Ozawa,1983).Asolivine

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Ir Ru Rh Pt Pd 0.001 0.01 0.1 1 10 MA 226 MA 238 MA 273 MA 400 MA 422 MA 425 C h o n d ri te n o rm a lis e d c o n c e n tr a ti o n s

Fig.15.PGEpatternofselectedchromititesfromtheGAC.Normalizationvaluesare fromNaldrettandDuke(1980).

containsonly minoramountsofAland Cr,reequilibrationwith spinelwillnotmodifytheCr/(Cr+Al)ratioofthelatterbutwill stronglyaffectthepartitionofFeandMg.Thedisseminatedandthe massivechromitite(MA226)lyingontheFe–Mgexchangetrend withconstantCr/(Cr+Al)ratiosrepresentchromitethathasbeen reequilibratedwithalowAlandCrbuthighFe–Mgphase,probably olvineand/orclinopyroxene.

Themassivechromititeshavechromiteasmainphase(upto 95vol.%).Reequilibrationwithaminorhostsilicatephasewillthus haveminoreffectsonthecompositionofthesechromites. More-over,massive chromitites(exceptMA226)showhomogeneous chromitecompositionsinagivensample,theydonotshow fer-ritchromitrims,theyarenotaffectedbylow-temperaturegrowth ofchloriteasinbrecciatedchromititesandtheyarevery compa-rabletoFiskenaessetandSittampundichromiteswithpreserved igneouscompositions(Fig.7b;Rollinsonetal.,2010;Duttaetal., 2011).Thelasttrendobservedformassivechromitecanthusbe interpretedasapreservedigneousfeature.

Silicate rocks have also been affected by HT and LT meta-morphism. The composition of their minerals cannot be used to estimate compositions of their parental melts The granu-laropx–cpx–plag assemblagesin fine-grainedamphibolites and mela-amphibolitesargueforapeakHT–MPgranulitegrade meta-morphism:upto 910◦_C _at _5–6_kbar_in _anhydrous _environment

(Fig. 13). Pyroxenes were nearly totally consumed to amphi-boleduringaretrogressivehydrouseventintheamphiboliteand lower granulite facies conditions (900–650◦_C). _A _few _samples

havepreserved evidencesfor direct replacementof clinopyrox-enebyamphibole(Fig.6c),whichisconcordantwitharetrograde evolutioninthepseudosectionbuiltforMA37bulkcomposition (Fig.13).

ThereisarelationshipbetweentheAncontentofplagioclaseand theNaandAlcontentsofco-existingamphibolefromfine-grained, leuco-andmela-amphibolites,themostcalcicplagioclasebeingat equilibriumwithaNa-andAl-richamphibole(Fig.10).This cer-tainlyindicatesametamorphicoriginfortheamphiboleinthose rocks,theNaenclosedinformerigneousplagioclasehasbeen trans-ferredintoamphibolethankstoa pargasitesubstitutionduring clinopyroxenebreakdown(thelattercannotdeliverlarge quanti-tiesofNa).TheAncontentoftheplagioclaseinamphibole-bearing rocksthusnomorereflectstheoriginaligneouscomposition,as attestedbynearlypureanorthitecomposition(0.97–0.98Anmol.%; Fig.10)ofplagioclaseinspinel-amphiboliteMA25.Exceptionsare

observedin Fig.10for two mela-amphibolites(MA 28 andMA 34).Thisanomalycannotbeexplainedbydifferentequilibration temperatures inthesesamples(800–850◦_C) _compared_to_other

amphibolites(720–930◦_C)_but_could_reflect,_however,_the

preser-vation of igneousamphibole that hasbeenreequilibrated with plagioclaseduringgranulite-faciesmetamorphism.

As discussed above, olivine–amphibole rocks and spinel amphibolites share many characteristic with meta-troctolites metamorphosedunderHTamphibolitetogranulitegrade.

Theoriginofchromite–amphiboleassociationinsome chromi-titesismoreambiguous.Thesesamplesarefoundinclosespatial associationwithanorthosites.IntheFiskenaessetcomplex, chromi-titesinanorthositeshaveamphiboleasmatrixphase(seeRollinson etal.,2010).Onthebasisofcomparisonwithworldwide unmeta-morphosed amphibole–chromitite occurrences, Rollinson et al. (2010)proposedthat amphibolefromchromititesis ofigneous origin.Duttaetal.(2011)howeverobservedtexturalevidencefor clinopyroxenereplacementbyamphiboleinchromititesfromthe Sittampundi complex.In oursamples,there is no textural evi-dencepreservedbuttheamphibolecompositionmatchestheone ofspinel–amphibolemetamorphicrocks(seeFig.11).Wetherefore proposethatamphiboleinchromititeisofmetamorphicoriginin theGuelbelAzibcomplex.

Asaconclusion,onlythemono-mineralicrocks(ornearlyso)are expectedtohavepreservedtheiroriginaligneousmineral compo-sitions.Thisismostprobablythecasefortheanorthositesandthe chromitesfrommassiveandlayeredchromitites.

8.2. Determinationofthechromititeparentalmelts

Kamenetskyetal.(2001)haveshownthattheAlandTicontents ofigneouschromitesarelinearlycorrelatedtothatofthemelt. MaurelandMaurel(1983)proposedanequationtocomputethe FeO/MgO ratio in the parental melt of igneouschromites. The valueofthisratiointhespinel ishoweverstronglysensitiveto variationsoxygenfugacityand,asaconsequence,largeerrorsare attachedtothedeterminationoftheFeO/MgOratiointhemelt. Only primary chromitesfollowing the proposed igneous trend (Fig. 7b)and showingnopetrographicalnorchemical evidence for hydrothermal alteration, oxidation or highgrade metamor-phismwereusedforthecalculations.Fewrutileexsolutionswere observedinchromites,theTiO2 contentoftheequilibriummelt

ismostprobablyunderestimated(seeRollinsonetal.,2002)and willthereforenotbeusedasadiscriminatingfactor.Furthermore, evolvedchromiteswithhighFe#showastrongdecreaseoftheirAl content.Theyhaveprobablyco-crystallizedwithplagioclaseand theirAlcontenthasbeenbufferedbythefeldspar.Consequently, onlytheresultsforthechromitescrystallizedduringandbefore theAlpeak(Fig.9)areplottedintheFeO/MgOvsAl2O3diagram

(Fig.16).Themeltinequilibriumwiththemostprimitivechromite (highestMg#,MA273)has15wt%Al2O3andplotsbothinthefield

ofArcheantholeiites(Fig.16).ThereisalmostnoincreaseofAl2O3

contentswithincreasingFeO/MgOratioinmeltsthatcrystallized chromitites,upto15–16wt%forthemostevolvedchromite (low-estMg#,MA238)parentalmelts,confirmingthattherewasno massivefractionationofplagioclaseduringthissequenceof differ-entiation.AllcalculatedmeltsfallinthefieldofArcheantholeiite inFig.16.

ChromitesfromMA400arenotinequilibriumwithcommon Archean basic–ultrabasicmelts (Fig. 16).Theyalsoplot outside theigneoustrendformedbyothermassivechromitites(Fig.9). Although there is no petrographic evidence for metamorphic reequilibration,thechromitefromthissampleprobablyunderwent metamorphicreequilibrationwithhostphase.Itisindeedthemost Fe3+_-rich_spinel_of_all_massive_chromitites_(up_to_0.17_a.p.f.u.).

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Fig.16.CompositionofthemeltinequilibriumwithGACchromiteswithpreserved igneouscomposition(Fig.7b)comparedtobasaltickomatiites,Archeantholeiites, Izu-BoninandCascadesarcmagmas(datafromtheonlineGEOROCdatabase).The mostevolvedchromitefromsampleMA226hasnotbeenplottedinthisdiagram becauseithasprobablybeencrystallizingalongwithplagioclase.

8.3. Inferencesontheprimarymagmaandtheconditionsof crystallization

TheigneouschromitewithhighestMg#crystallizedfromamelt havingaMg#around47.Thisistoolowtobeinequilibriumwitha classicalmantleperidotitewithpyroliticordepletedharzburgitic composition.Themeltthathascrystallizedtheigneouschromite waseitherevolvedorithasbeenformedbyapertubatedFe-rich mantle.TheGACalsopresentlargevolumesofultramaficrocksin theformofserpentinite(formerspinel-bearingolivine-richrocks) andultramaficmetacumulates(formerolivine–plagioclase–spinel rocks). It is not possible to estimate the composition of their parentalmeltsbecauseofthestrongimprintofbothHTgranulitic metamorphismand late greenschist facies alteration.However, theirpresenceindicatesatleastthattheprimitivemagmawasmore Mg-richcomparedtotheonethatcrystallizedchromites.

Usinginformationonrockswithpreservedigneousmineralogy and/ormineralcompositionandonthenatureoftheigneous pre-cursorsofmetamorphicrocks,theprimarymagmaandisliquidline ofdescentmustfittothefollowingobservations:

(i)The melt has crystallized primitive ultramafic olivine-rich cumulates (olivine–pyroxenes–spinel±plagioclase rocks), chromites,mela-andleuco-gabbros(plagioclaseand pyrox-ene,±amphibolerocks)andhighlycalcicanorthosites(Fig.4). Thehornblendites(amphibolecumulatesormeta-pyroxenite) formonlysmallellipticalbodieswithinultramaficunitsand areconsequentlynotamajorconstituentoftheGAC.

(ii)Theanorthositeandsomeleucogabbroshaveahighlycalcic plagioclase.Theformationofsuchrocksrequiresanaluminous parentalmeltwithhighCa/Naratio.Massivefractionationof clinopyroxenedidcertainlynotoccurbeforeplagioclase sat-urationbecauseitwouldhavestronglydecreasedtheCa/Na ratioofthemeltparentaltotheanorthosite.

(iii) Thecompositionaltrendsobservedforchromitites(decrease inAlcontentforthemostFe-richigneouschromites),together withfieldobservation,suggestthatplagioclasesaturationin themeltwasreachedduringchromitecrystallization(Fig.9). (iv)ComparedtoArcheanand Paleoproterozoicchromitesfrom

komatiites,greenstonebeltsandlayeredintrusions,thosefrom GuelbelAzib havehigher Fe# andlower Cr#mean values

(Figs. 7 and 8). The parental melt to chromitites has thus highFe/Mgratioand/orlargeamountof a phase withlow Fe/Mgratio (olivine)hasbeenfractionatedfromahigh-Mg meltbeforechromitesaturation(assuggestedbythepresence ofultramaficolivine-richcumulatesinGAC).

Consideringtheselinesofevidence,theprimitivemeltshould havebeenanMg-rich,highaluminabasalticmelt.Sucha compo-sitionexplainsthepresenceofformerolivine-richmeta-igneous rocksco-existingwithlargevolumeofplagioclase-rich metagab-brosandanorthosites.Inaddition,experimentshaveshownthat increasingwater contents inthemelt promotesthe crystalliza-tionofCa-richplagioclase(seeGroveandBaker,1984;Sissonand Grove,1993;Takagi etal.,2005;Feigetal.,2006).Thisis com-patible withthe probablepresence of few igneous amphiboles in mela-amphibolites MA 28 and 34 and withthe occurrences of hornblenditethat arecommonly formedfromhydrous mag-masinsimilarcontext(see Polatetal.,2012).Increasingwater contentleadstotheenlargementofthetemperatureintervalof pyroxenecrystallizationandreducesthewindowwhere plagio-claseandchromiteco-precipitate(Berndtetal.,2005;Feigetal., 2006;HamadaandFujii,2008).Theprimitivemeltwasthusnot saturatedwithwater,itwasslightlyhydrous.

Experimentalstudiesofsubalkalinemeltcrystallization demon-strate that increasing pressure tends to stabilize pyroxenites overolivine–plagioclasecumulatesandleadstotheformationof igneousgarnet(Munteneretal.,2001;Villigeretal.,2004,2007; Alonso-Perezetal.,2009).Pyroxenitesareabsentorminorinthe GAC,garnetwasnotobservedneitherasigneousnormetamorphic phasewhilemetamorphosedolivine–plagioclaserocksarepresent. Wecanthusinferalowpressureofcrystallization,atdepth corre-spondingtoupperormiddlecrust.

FormationoftheGACthroughlowpressurecrystallizationofa hydrousmagmaisinagreementwithpreviousstudiesonsimilar complexes(Weaveretal.,1981;Rollinsonetal.,2010;Polatetal., 2011;Duttaetal.,2011).Themeltbecamesubsequentlyenriched inAlthroughfractionalcrystallizationofolivineandchromiteand reachedacompositionmatchingthatofArcheantholeiites(Fig.16, seebelow)justbeforeplagioclasesaturation.AnArchean tholei-itecompositionfortheArcheananorthositeparentalmeltagrees withresultsobtainedbyHendersonetal.(1976),Ashwal(1993) andRollinsonetal.(2010)onthebasisofinvertedREEcontentsof plagioclase.

8.4. PGMcrystallizationandPGEfractionation

PGMco-crystallizedalongwithchromitesandbasemetal sulp-hides.Detailedobservationswiththeelectronmicroscopeandbulk PGEpatternshaveshownthatthreephasesofPGEmineralization canbedistinguishedinchromitites.

(i)Theearliestoneisevidencedbytheformationofeuhedral lau-riteandmillerite(Fig.14aandb).Theshapeoflauritetogether withexperimentalresults(BrenanandAndrews,2001)show thatlauriteisahigh-temperatureigneousPGMintheGACas in ophioliticchromititeswhereitis entrappedduring mag-matic growthofchromite(Augé,1985).Thismineralization phasehascharacteristicPGEpatternswithhighIPGEcontents compared toPPGE(samples MA226, 400and 422;Fig.15) whichfitsrelativelywellwiththatofchromititesfrom ophio-lites,komatiitesandlayeredintrusions(Cabri,2002;Naldrett etal.,2012;Pagéetal.,2012).Chromitecrystallizedin komati-itic and tholeiitic melts concentrates IPGE. These elements may be present in chromite as solid solution (Pagé et al., 2012;Brenanetal.,2012)orco-crystallizedwithIPGE min-eralsduetotheirlowsolubilitiesinbasicmagmas(Pagéetal.,

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Table3

Majorandtraceelementcompositionofbulkchromititesamples.

MA226 MA238 MA240 MA241 MA273 MA400 MA422 MA425 Majorelements(wt%) SiO2 3.02 8.94 3.33 12.85 12.14 1.65 19.71 10.23 TiO2 0.31 0.36 0.29 0.24 0.54 0.38 0.17 0.44 Al2O3 16.23 17.11 23.54 22.17 20.56 11.77 7.29 22.12 Fe2O3 27.54 25.13 30.59 24.39 19.58 24.59 26.66 23.57 MnO 0.51 0.69 0.61 0.69 0.32 0.44 0.50 0.37 MgO 10.03 10.30 4.92 11.53 11.68 8.39 19.86 8.02 CaO 0.10 0.09 0.12 0.18 2.79 0.13 1.21 2.54 Na2O 0.14 0.21 0.16 0.22 0.37 0.16 0.09 0.32 K2O 0.02 0.03 0.02 0.14 0.08 0.02 0.02 0.07 P2O5 0.00 0.00 0.00 0.00 0.01 0.01 0.02 0.01 Cr2O3 40.45 29.58 35.13 21.04 32.61 50.35 18.32 32.34 LOI −0.3 6.02 0.3 3.9 −0.9 −0.25 6.05 −1.11 98.06 98.46 99.01 97.34 99.77 97.65 99.90 98.93 Trace-elements(ppm) Co 169 132 214 176 103 120 128 136 Cu 29 27 37 49 23 36 26 21 Ni 1597 1295 847 1188 1334 833 2237 969 Zn 1372 3207 2780 2919 556 2615 1115 1969 V 439 772 648 555 987 342 391 940 PGE(ppb) Ir 42 241 75 63 28 10 Ru 329 583 242 709 355 45 Rh 8 442 97 37 20 6 Pt 6 783 1700 42 22 21 Pd 1 463 418 6 10

2012).ThepositivecorrelationbetweenCuandRucontentsin chromititesandtheabsenceofcorrelationbetweenPGEand CrorSi(Table3)contents suggestthatthePGE(and espe-ciallyIPGE)havebeenentrappedbysulphidephasesthatnow forminclusionswithinchromitegrains.TheigneousPGE min-eralizationmostprobablyoriginatedthroughseparationofa sulphidefractionfromthesilicatemelt,inagreementwiththe morecompatiblebehaviourofIPGErelative toPPGEin sul-phidemeltscomparedtosilicatemagmas(Sattarietal.,2002). Thesulphidemelt/silicatemagmaimmiscibilityeventcould havebeentriggeredeitherbymixingbetweenevolvedand primitivemagmas(Irvine,1977)orbyassimilationof enclos-ing gabbros by the fractionatingmelt (Bédard and Hébert, 1998; Gervilla et al., 2005). Considering the strong meta-morphicimprintontheGAC,moredetailedgeochemicaland isotopic analysesareneeded tochoosebetweenthese two processes.The layeredchromitite (MA 425) associatedwith anorthosite–leuco-amphibolitesamplesalsoshowsaPGE pat-ternwithapeakinRubutwithlowerbulkPGEcontentsand lowerRu/RhratiocomparedtoMA226–400–422.This deple-tion in bulkPGE and in Ru relative toother PGEcouldbe explainedbystrongdepletionofRuand,toalesserextent,bulk PGEcontentsintheparentalmelt.Indeed,onthebasisof sim-ulationswithMELTSandaccordingtomineralcompositions, itisstressedthatthechromititeinanorthositehascrystallized fromevolvedaluminousmeltsincomparisontomore primi-tiveMg-richandAl-poorchromitites.Hence,theparentalmelt toevolvedchromiteswasalreadydepletedinPGE,especially IPGEoverPPGE,duetotheigneoussegregationoflauritewith moreprimitivechromites.

(ii)Thesecondphaseofmineralizationisevidencedbythe over-growthofanhedralirarsite-hollingworthitegrainsonlaurite orasisolatedgrainswithinchromite(Fig.14).Thiseventof sulpho-arsenidecrystallizationisnotmarkedinbulkPGE pat-ternsasitremobilizesthePGEinaclosedsystembutitislinked toanincreaseinarsenicactivity.Afewirarsiteand hollingwor-thitewereobservedalongwithrutileandanorthite(Fig.14c), anigneousorlate-magmaticoriginismoreplausible.

(iii) Thethirdmineralizingphaseconsistsofthecrystallizationof sperryliteintheferritchromitrimofchromitegrainsandscarce rustenburgite overgrowthat therimof laurite.A preferen-tialincorporationofPPGEisobservedfrombothmineralogy andbulkPGEpatternsofsamplesMA238and273(Fig.15) which are characterizedby higher Rh, Pt and Pd contents (upto1700ppbPt).Theevolution fromIPGEtonearlyflat PGMpattern duetoPPGEenrichment(Fig.15)issimilarto thatobservedinPaleoproterozoicferropicritesfromPechenga (Brügmann et al., 2000). Since ferritchromit preferentially developsaroundinclusionsofchloriteandalongtheborderof chromitegrains,thisphaseofPt–Pdmineralizationsis inter-pretedasaresultofhydrothermalactivityunderhigharsenic activities.SimilarconclusionshavebeendrawnforPt–Pd min-eralization in hydrothermally altered ophioliticchromitites (Leblanc, 1991;Prichard etal., 2008)and for theevolution ofthePGMfromsulphides,tosulphoarsenidesandarsenides followed byhydrothermal intermetallicalloysthat arealso observedintheTwoDucklakeintrusionfromtheColdwell complex(WatkinsonandOhnenstetter,1992).

8.5. TheGuelbelAzibcomplex:themetamorphosedequivalentof Archeananorthositelayeredbodies

The GAC shares many similarities withArchean anorthosite complexes.ItisoccurringwithinTTGterrainsinclosespatial asso-ciationwithsuprecrustals(impuremarbles,amphibolites,BIF),it ischaracterizedbyhighlycalcicanorthositeandFe-richchromite anditisstructuredaslayeredsequencesofolivine-richcumulate rocks,formergabbrosandanorthositeswithminorpyroxenitesand fewhornblenditesbodies(seeWindleyandGarde,2009;Rollinson etal.,2010;Polatetal.,2011,2012).

ItisdifficulttoascribeatectonicsettingtotheGACduetostrong recrystallizationundergranulitetogreenschistfaciesconditions. Butseverallinesofevidencespointtoasubductionzoneorigin:

(i)Thehighlycalciccompositionofplagioclaseinanorthosites and thehydrous natureof the primitive meltsare characteris-tic ofmodern hydrous arc magmasand xenoliths (Arculus and

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Author's personal copy

Wills,1980;Takagietal.,2005).(ii)AspointedoutbyRollinson etal.(2010),theassociationamphibole–calcicplagioclase–ferrian chromitesisalsofoundinarcxenoliths(ArculusandWills,1980), eveniftheprimaryorsecondaryoriginofamphiboleisdebated in layered anorthosite complexes (Owens and Dymek, 1997; Rollinsonetal.,2010;Duttaetal.,2011;thisstudy).(iii)The inter-pretationofspinelcompositionsintermsoftectonicsettingisnot straightforward. Indeed,compositional fields ofchromitesfrom variousmodernPhanerozoictectonicsettingsarelargely overlap-ping(BarnesandRoeder,2001)andmostGACchromitesplotin thefieldsofmodernfloodbasaltsintheFe#–Cr#plotbutfallsin theophiolitefieldinthetrivalentionplot.Rollinsonetal.(2010) moreovernoticedthatsomespinelsinmodernarctholeiiteshave compositionclosetothoseanalysedinUMAcomplexes.

TheFeO/MgOratioandAl₂O₃contentofthemeltsinequilibrium withigneouschromitesareverycomparabletomoderncontinental (Cascades)andoceanic(Izu-Bonin)arcbasalts(Fig.16,comparison datafromtheonlineGEOROCdatabase).

TheGACchromiteswithpreservedigneouscompositionsare comparabletothoseanalysedintheArcheanFiskenaessetand Sit-tampundiUMAcomplexes (Figs. 7band 8).Asupra-subduction zoneoriginisproposedbyPolatetal.(2009,2010,2011,2012) fortheFiskenaessetcomplexonthebasisoftrace-elementand iso-topicdata(negativeNbanomaliesinmostsamples,positiveinitial« Nd).Althoughthefewpetrologicalevidencesconvergetoa supra-subductionzonesettingfortheGAC,moredetailedgeochemical analysisareneededtoconfirmthishypothesis.

9. Conclusions

TheArcheanGuelbelAzibcomplexintheWestAfrican cra-tonisametamorphosedequivalentoffamousArcheananorthosite bodies.Despitegranulite(upto900◦_C,₅_kbar)_to_greenschist

meta-morphic eventsthat have affected thecomplex, few preserved igneousmineralcompositionsandthelithologicalnatureofthe igneousprecursorsbeforemetamorphismlead tothefollowing conclusions:

•Theprimitivemeltwasaslightlyhydroushighaluminabasaltic melt. It evolved towards Archean tholeiite-like composition throughthemassivefractionationofolivineandchromite gener-atingasequenceofultramaficcumulatesnowtransformedinto serpentinitesandolivine–amphibole–spinelrocks (metatrocto-lites).

•Fe-rich,Cr-poorchromites,An-richanorthositeand metamor-phosedgabbrosformedfromanArcheantholeiiteparentalmelt. The wide crystallization window of chromite and the calcic natureoftheplagioclasearepromotedbythelackofmassive fractionationofclinopyroxene.

•IPGEmineralssuchaslauritewereprecipitatedduringigneous crystallizationof chromititesand IPGEwereremobilized dur-inglate-magmaticcrystallizationofhollingworthiteandirarsite. APt–Pdphaseofmineralization,representedbysperryliteand rustenburgiteislinkedwithlatelow-temperaturehydrothermal metamorphism.

•ThemainpetrologicalcharacteristicsoftheGACarecompatible withlow-pressureofcrystallizationatdepthcorrespondingto upperormiddlecrust.Geochemicalanalysesareneededto pre-cisethetectonicsettingoftheGACbut,bycomparisonwithother Archeananorthositecomplexes,itcouldhavebeenformedina supra-subductionzonesetting.

Acknowledgements

Thisstudywasfunded bya FRS-FNRSgranttoJB.Wewould liketothankMohamedDahmada,OusmaneN’Diaye(deceasedin

November2010), MaloumBaba, MedSalem andthe OMRGfor theirsupportandforthefantasticfieldtripintheAmsagaduring November2008.ReviewsmadebyHughRollinsonandan anony-mousrefereetogetherwiththeeditorialhandlingofGuochunZhao weregreatlyappreciated.

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