Spatial variability of pyroxenite layers in the Beni Bousera orogenic peridotite (Morocco) and implications for their origin

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Spatial variability of pyroxenite layers in the Beni

Bousera orogenic peridotite (Morocco) and implications

for their origin

Kamar Chetouani, Jean-Louis Bodinier, C. J. Garrido, Claudio Marchesi,

Isma Amri, Kamal Targuisti

To cite this version:

Kamar Chetouani, Jean-Louis Bodinier, C. J. Garrido, Claudio Marchesi, Isma Amri, et al..

Spa-tial variability of pyroxenite layers in the Beni Bousera orogenic peridotite (Morocco) and

impli-cations for their origin. Comptes Rendus Géoscience, Elsevier Masson, 2016, 348 (8), pp.619-629.

�10.1016/j.crte.2016.06.001�. �hal-01468018�

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Tectonics,

Tectonophysics

Spatial

variability

of

pyroxenite

layers

in

the

Beni

Bousera

orogenic

peridotite

(Morocco)

and

implications

for

their

origin

Kamar

Chetouani

a

,

Jean-Louis

Bodinier

b,

*

,

Carlos

J. Garrido

c

,

Claudio

Marchesi

c,d

,

Isma

Amri

a

,

Kamal

Targuisti

a

LaboratoiredeGe´ologiedel’EnvironnementetRessourcesNaturelles,De´partementdeGe´ologie,Universite´ Abdelmalek-Essaaˆdi,Faculte´ desSciences,BP2121,Te´touan,Morocco

b

Ge´osciencesMontpellier,Universite´ deMontpellier,CNRSandUA,CampusTriolet,CC60,PlaceEuge`ne-Bataillon,34095Montpellier cedex05,France

c_Instituto_Andaluz_de_Ciencias_de_la_Tierra_(IACT),_CSIC_and_UGR,_Avenida_de_las_Palmeras_4,₁₈₁₀₀_Armilla,_Granada,_Spain d

DepartamentodeMineralogı´ayPetrologı´a,UGR,AvenidaFuentenuevas/n,18002Granada,Spain

1. Introduction

Orogenicperidotitescontainavarietyofpyroxene-rich mafictoultramaficlayers,oftencollectivelyreferredtoas ‘pyroxenites’, although they may also include garnet granulites and eclogites (Bodinier and Godard, 2014). Theserocksweregivenapeculiarattention,withseveral studies aiming to assess the suggestion by Alle`gre and Turcotte(1986)thatthemaficlayersrepresentelongated

strips of oceanic lithosphere recycled in theconvective mantle (the ‘Marble Cake’ model). The Beni Bousera orogenic peridotite, in the Rif mountains of northern Morocco,iswellknownforcontaininga widevarietyof pyroxenitelayers(Kornprobstetal.,1990;Pearsonetal., 1989,1993). A large proportionofthe publishedworks supporting a ‘Marble Cake’ origin for the orogenic pyroxenites are based indeed on samples from Beni Bouseraand, to a lesser degree, from theneighbouring Rondamassif,southernSpain(e.g.,Alle`greand Turcotte, 1986; Kornprobst et al., 1990; Morishita et al., 2003; PearsonandNowell,2004).However,severalstudiesofthe Beni Bousera pyroxenites also reported evidence for

ARTICLE INFO Articlehistory:

Received27February2016 Acceptedafterrevision7June2016 Availableonline9August2016 HandledbyMargueriteGodard Keywords: Crustalrecycling Melt–rockreaction Mantlepyroxenite Orogenicperidotite BeniBousera ABSTRACT

TheBeniBouseraperidotitecontainsadiversityofpyroxenitelayers.Severalstudieshave postulatedthatatleastsomeofthemrepresentelongatedstripsofoceaniclithosphere recycledintheconvectivemantle.Somepyroxeniteswere,however,ascribedtoigneous crystalsegregationormelt–rockreactions.Tofurtherconstraintheoriginoftheserocks, wecollected171samplesthroughoutthemassifandexaminedtheirvariabilityinrelation withthetectono-metamorphicdomains.Amajorﬁndingisthatallfaciesshowingclear evidence for a crustal origin are concentrated in a narrow corridor of mylonitized peridotites,alongthecontactwithgranuliticcountryrocks.Thesepeculiarfacieswere mostlikelyincorporatedatthemantle–crustboundaryduringtheorogeniceventsthat culminatedintheperidotiteexhumation.Theotherpyroxenitesderivefromadistinct protoliththatwas ubiquitousin themassif beforeitsexhumation. Theyweredeeply modiﬁedbypartialmeltingandmelt–rockreactionsassociatedwithlithosphericthinning. ß2016Acade´miedessciences.PublishedbyElsevierMassonSAS.Thisisanopenaccess articleundertheCCBY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/ 4.0/).

* Correspondingauthor.

E-mailaddress:bodinier@gm.univ-montp2.fr(J.-L.Bodinier).

ContentslistsavailableatScienceDirect

Comptes

Rendus

Geoscience

w ww . sc i e nce d i re ct . co m

http://dx.doi.org/10.1016/j.crte.2016.06.001

1631-0713/ß2016Acade´miedessciences.PublishedbyElsevierMassonSAS.ThisisanopenaccessarticleundertheCCBY-NC-NDlicense(http:// creativecommons.org/licenses/by-nc-nd/4.0/).

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igneousgarnetcrystallisationandsuggestedthatatleast partofthemoriginatedashigh-pressurecrystalsegregates inmagmaconduits,variablymodiﬁedbymetamorphicor metasomaticreactions(Gysietal.,2011;Kornprobstetal., 1990;Pearsonetal.,1989,1993).Thisinterpretationwas classically suggested for pyroxenite layers in other orogeniclherzolites(e.g.,BodinierandGodard,2014).

The different interpretations may partly reﬂect the diversityofpyroxenitelayersinBeniBousera.Thestudies supportingtherecyclingscenarioweremostlyperformed onrelativelyrare,peculiarrocktypesincludinga graphi-tizeddiamond-bearing pyroxenite(Pearsonet al.,1989) and corundum-bearing aluminous pyroxenites ( Korn-probstet al., 1990). Gysi et al. (2011) studieddifferent types of pyroxenites, including more ‘classic’ garnet pyroxenitelayers,collectedalongtwotensofmeterslong river sections. However, none of the published studies embracesthefullrangeofthepyroxenitevariabilityand theapproachesgenerallydonotconsidertherelationships between pyroxenite layers and the petro-structural variationsin hostperidotites(Fretsetal.,2014). Studies ofpyroxenitesintheneighbouringRondaperidotitehave shownthattheirvariabilityisstronglycorrelatedwiththe peridotite tectono-metamorphic domains (Garrido and Bodinier,1999). RecentstudiesinBeni Bouseraindicate that the massif underwent an evolution comparable to Ronda, including extreme lithospheric thinning (Frets et al., 2014) and melt–rock interactions involving a subduction component (Gysi et al., 2011). During this evolution,whichculminatedinthrustingoftheperidotite bodies amidst continental crust, the massif may have incorporatedvariedcrustal components, including slab-derived melts possibly crystallized as high-pressure segregates (Pearson et al., 1993) or solid granulite componentsdelaminatedfromthecrustandintermingled withlithosphericperidotites(Gysietal.,2011).

Therefore,beforeconsideringthepyroxenitelayersof BeniBouseraandtheirhostperidotitesasacasestudyfor the convective ‘Marble Cake’ mantle, it is essential to assesstheeffectoflithosphericprocesses.Inthisstudy,we provideanoverviewofthedifferentpyroxenitefaciesin theBeni Bouseraperidotitebased on a large datasetof 171samplescollectedthroughoutthemassif.Weexamine their spatial distribution in relation with the tectono-metamorphic domains recently deﬁned by Frets et al. (2014)andusemajorandtraceelementstoconstraintheir origin. The aim was to determine the extent of the chemicalperturbationsattributabletothelate evolution-ary stages of the massif and evaluate the original heterogeneitydegreeoftheBeniBouseraparentbody. 2. TheBeniBouseraorogenicperidotite

The Beni Bousera peridotitemassif crops out in the Septidescomplex,inthelowerinternalzonesoftheAlpine Rifbelt,in northernMorocco (Kornprobst, 1974). Folia-tionsandlineationsareconsistentinperidotitesandtheir crustal hostrocks. According toKornprobst (1974), the peridotitebody recordsapolybaric evolutionstartingat depths>150km. A comparison of thestructures in the massifand in theoverlyingcrustal unitsledAﬁri etal.

(2011)toproposethattheperidotiteswereexhumedin the footwall of a lithospheric extensional shear zone. Detailedstructuralandpetrologicalmappingofthemassif byFretsetal.(2014)showedthatitiscomposedoffour tectono-metamorphicdomainswithconsistent kinemat-ics.Fromtoptobottom,thesedomainsinclude(Fig.1):(1) garnet-spinelmylonites,(2)Arie´gitesubfaciesﬁne-grained porphyroclastic spinel peridotites, (3) Arie´gite–Seiland subfacies porphyroclastic spinel peridotites, and (4) Seilandsubfaciescoarse-porphyroclasticto coarse-granu-lar spinel peridotites. Microstructures and crystal pre-ferred orientations point todeformation dominantly by dislocationcreepinalldomains,butcontinuousincreasein averageolivinegrainsizeindicatesdecreasingplasticwork rates fromtoptobottom.Thisevolutionindeformation conditionsisconsistentwiththechangeinsynkinematic pressure and temperature conditions, from 9008C at 2.0GPa in the garnet-spinel mylonites to 11508C at 1.8GPaintheSeilanddomain.Adiffuse dunitic-webste-ritic layeringsubparallel tothefoliationsuggests defor-mationinthepresenceofmeltintheSeilanddomain.To account for theconsistent kinematics and the tectono-metamorphicevolution,implyingatemperaturegradient ofc.1258Ckm 1_preserved_across_the_massif,_Frets_et_al.

(2014)proposedthattheentireperidotitebodywasa low-angleshearzone,afewkilometreswide,which accommo-datedexhumationofthebaseofthelithospherefrom90to 60kmdepth.

3. Classiﬁcation,petrographyandspatialdistributionof pyroxenitelayers

Kornprobstetal.(1990)recognizedtwomaintypesof garnetpyroxenitelayersintheBeniBouseramassif.TypeI is characterizedby relativelylow (<10wt%)and nearly constantAl2O3contentinbulkrocks,butvariableFeO/MgO

ratio(0.1–0.8).Incontrast,typeIIhasanarrowerrangeof FeO/MgOvalues(0.1–0.3)butvariableAl2O3content;most

samplesaremoreenrichedinalumina(upto15%)than type-Ipyroxenites.Type-Ilayerswereconsideredas high-pressure crystal segregates while type-II layers, which notably include corundum-bearing pyroxenites, were interpretedasmetamorphosedoceanicgabbros. Thereaf-ter, Pearson et al. (1993) reported a wide diversity of pyroxenitelithologiesbutitwasGysietal.(2011)whofirst proposed aclassificationof theBeni Bouserapyroxenite layersaimingtoembracetheirwholediversityrange.The classificationproposedhereisroughlycomparabletothat ofGysietal.(2011)andcomprisesfourmaingroups.Itis, however, based on a larger database of 171 samples collected throughout the massif and examined in thin sections (Fig. 1). Our sampling therefore includes rock faciesthatwereincompletelydocumentedbyGysietal. (2011), suchas the peculiar corundum-bearing pyroxe-nitesstudiedbyKornprobstetal.(1990).Table1inthe Supplementarymaterialgivesthemineralassemblagesof thedifferentgroupsandsub-groupsofourclassification and summarizestheir maincharacteristics.Fig.1shows thedistributionofthepyroxenitegroupswithrespectto thetectono-metamorphicdomainsdefinedbyFretsetal.

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(2014) on the basis of pyroxenite mineralogy and deformationmicrostructuresinperidotites.

GroupI:Corundum-garnet(Ia),garnet-spinel(Ib),and garnet-plagioclase(Ic)clinopyroxenites

Group I corresponds to the type-II pyroxenites of

Kornprobstetal.(1990).Thisgroupalsoincludesthe type-IVpyroxenitesofGysietal.(2011).GroupIiscomposedof pale-grey to greenish, cm- to tens of cm-thick layers occurring exclusively in the Grt–Sp mylonitic domain, alongthesouthwesternborderofthemassif(Fig.1).The layers are parallel tothemylonitefoliation. Theyshow sharp contacts to the host peridotite, highlighted by narrowrimsofbrightgreenCr-diopsidewebsterite,afew mms to a few centimetres thick. Corundum-garnet (subgroupIa–Fig.2a)andgarnet-spinel clinopyroxenites (subgroup Ib–Fig. 2b) occur in the core of thick layers (> 10cm) and grade outwards to garnet-plagioclase (spinel)clinopyroxenites(subgroupIc).However,several group-Ilayers,includingallthinlayers(<10cm),arefairly homogeneousandcomposedonlyofsubgroup-Ic clinopyro-xenites(Fig.2c).

Group-Ipyroxenitesaredistinguishedfromtheother pyroxenitesinBeni Bouserabya dominantgranoblastic microstructurewherelarge porphyroclastsof clinopyro-xene,garnet,corundumand/orAl-spinelareembeddedin a mosaic recrystallized matrix of clinopyroxene and garnet,plusvariablebut generallysubordinateamounts oforthopyroxene,plagioclase,spinel,sapphirine, amphi-boleandolivine(Fretsetal.,2012).Clinopyroxenecontains

upto20wt%Al2O3combinedwitharelativelylowNa2O

content,resultinginahighproportionoftheCa-Tschermak molecule(Kornprobstetal.,1990).Spinelisgreeninthin sectionandcontainslessthan1wt%Cr2O3.

Group II: Graphite-garnet websterites (IIa), garnet clinopyroxenites(IIb),andgarnet-spinelwebsterites(IIc)

Group II corresponds to the type-I pyroxenites of

Kornprobst etal. (1990)and thetype-III garnet-bearing pyroxenitesofGysietal.(2011).Group-IIpyroxenitesare found in the Grt–Sp mylonite, Arie´gite subfacies and Arie´gite–Seilandsubfaciestectonicdomains(Fig.1)where theyoccur asgreytodark-purplish,cm-tometer-scale layers parallel tothe peridotitefoliation (Fig. 2e). They showrathersharpcontactswiththehostperidotiteandare commonly isoclinally folded or boudinaged (Fig. 2f). Thinner(< 1cm) lenses of greenish garnet pyroxenites arealsoquitecommon, particularlyinmylonites where theywerestirredanddispersedbythedeformation.Thin layers (< 1 m) may occur in several meters-thick sequenceswherethepyroxenitesvolumetrically predom-inateoverthehostperidotite.Thethickerlayersareoften zoned, from subgroup-IIb garnet-rich clinopyroxenitein the central part tosubgroup-IIcwebsterite outwards, or from subgroup-IIc in the centre to group-III spinel websterite(below)intheexternalpart.Thegarnet-graphite websterites (subgroup IIa–Fig. 2d) are a rare rock type (Pearsonetal.,1989,1993),representedbyonlyacoupleof thick(>1m)layersoutcroppinginthesouthwesternpart ofthemassif,closetothecontactwithgranuliticcountry

Fig.1. GeologicalmapoftheBeniBouseraperidotitemassifshowingthetectono-metamorphicdomainsdeﬁnedbyFretsetal.(2014)andthelocationofthe pyroxenitessampledforthisstudy(seethetextandTable1forpyroxeniteclassiﬁcation).TheinsetshowsthelocationofthemassifintheAlboranrealm.

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rocks (Fig. 1). In contrast, the garnet clinopyroxenites (subgroupIIb–Fig.2e,f)areacommonandwidespreadrock typeoccurringthroughouttheGrt–Spmylonites andthe Arie´gite peridotite subfacies (Fig. 1). The garnet-spinel websterites (subgroup IIc–Fig. 2g) are speciﬁc of the Arie´gite–Seiland peridotite subfacies, deﬁned by Frets etal.(2014)asa narrowtransitiondomainbetweenthe Arie´giteandSeilanddomains.

Group-IIpyroxenitesarecharacterized bya porphy-roclasticmicrostructure.Coarsegarnets(upto1cm)are thepredominantporphyroclasts ingarnet clinopyroxe-nites(IIb),withonlysparseandsmaller(1mm)relictsof clinopyroxene porphyroclasts,whereas thewebsterites (IIaandIIc)alsocontainlargeporphyroclastsofaugitic clinopyroxene–upto2cmlonginsubgroupIIc,where theyarestronglydeformed.Garnetporphyroclastsoccur

either as isolated grains or as aggregates forming a compositionallayeringparalleltothefoliation.Garnetis either rimmed by kelyphite (IIa and IIb) or deeply kelyphitized(IIc).Kelyphitemineralogyincludes ortho-pyroxene, Al-spinel, amphibole and plagioclase. Clino-pyroxene pophyroclasts are rich in exsolutions, particularly in the websterites (IIa and IIb–Gysi et al., 2011).Porphyroclasticminerals are enclosedin a ﬁne-grained matrix(250–300

mm)

of recrystallized clinopy-roxene andgarnet(IIa andIIb) orin a coarser-grained (1 mm) assemblagedominated by ortho- and clinopy-roxene(IIc).SubgroupIIaisfurtherdistinguishedbythe presenceofasigniﬁcantamountofgraphite(>15wt%), containingrelictinclusionsofmicro-diamonds(ElAtrassi et al., 2011), and subgroup IIc by the presence of interstitialCr-spinel.

Fig.2. PhotographsofrepresentativesamplesandﬁeldoccurrencesoftheBeniBouserapyroxenites.A:Subgroup-Iacorundum-garnetclinopyroxenite;B: Subgroup-Ibgarnet-spinelclinopyroxenite;C:Subgroup-Icgarnet-plagioclaseclinopyroxenite;D:Subgroup-IIa graphite-garnetwebsterite; E&F: Subgroup-IIbgarnetclinopyroxenites;G:Subgroup-IIcgarnet-spinelwebsterite;H:Group-IIIolivine-spinelwebsterite;I&J:Group-IVCr-diopsidespinel websterites.SeethetextandTable1forpyroxeniteclassiﬁcation.

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GroupIII:Olivine-spinelwebsterites

Group-III olivine-spinel websterites (=Type-II spinel websteritesofGysietal.,2011)arecharacteristicofthe Seiland peridotite domain, where they represent the predominant pyroxenite facies (Fig. 1). They occur as greyish to dark-greenish, 2–10cm thick, single layers (Fig.2h)orinthecentreofcompositelayersrimmedby group-IVCr-diopsidewebsterites.Theymayalsobefound in thetransitionalArie´gite–Seilanddomain, intheouter partofgroup-IIlayers.GroupIIIisprimarilydistinguished fromgroupsIand IIbythelackofgarnetandkelyphite whereas aluminousspinel is amajorphase (5–10wt%). ComparedwithgroupsIandII,thecontactsofgroup-III pyroxenitestohostperidotitesaregenerallymorediffuse. Thisisparticularlytrueinlayerscharacterizedbyahigh proportion (up to 60wt%) of olivine near the contacts, where this mineral occurs as large (1 cm) crystals elongatedparalleltotheperidotitefoliation.

Group-III microstructure is coarse porphyroclastic. Pyroxenes occur as large, strongly deformed porphyro-clasts (up to 2cm for clinopyroxene) or as virtually undeformed aggregates with holly-leaf shaped spinel, plagioclaseandamphibole.Theaggregatesmaybealigned paralleltoperidotitefoliationandarecomparabletothe symplectitic aggregates described in the Ronda spinel websterites (type-Bof Garrido and Bodinier, 1999) and consideredtorepresentformergarnets.Pyroxene porphy-roclastsaresurroundedbyamatrixofmedium-sized(0.5– 1mm) anhedralpyroxene neoblasts, and both the por-phyroclastsand themedium-grained matrix are further embeddedinavery-ﬁnegrainedmatrixthatispresentin variableproportions.

GroupIV:Cr-diopsidespinelwebsterites

Group IVpyroxenitesaregenerallydistinguished by greenishtobright-greencoloursintheﬁeld.Thisgroupis mostly observed in the Seiland peridotite subfacies (Fig.1),especiallyinthelowerpartofthisdomainwhere itisfoundinassociationwithharzburgitesanddunites. Other types of bright-green pyroxenites, sometimes containing garnet, are observed locally in the other peridotitedomainswheretheyoccurasnarrowreaction rimsatthecontactbetweengarnetpyroxenitesandhost peridotites.IntheSeilanddomain,group-IVwebsterites typically occur as irregularly-shaped lenses and thin veinlets (a few mms to cms thick–Fig. 2i), sometimes forminganastomosednetworkscrosscuttingthe perido-titefoliation.Thecontactsarediffuseandtheveinsmay gradeintoperidotites‘impregnated’bysecondary pyrox-eneandCr-spinel.Group-IVCr-diopsidewebsteritesalso formtheouterpartofgroups-III/IVcompositelayersand may alternatively occuras singlelayersparallel tothe peridotite foliation and showing sharper contact with peridotitecomparedwiththelensesandveinlets(Fig.2j). AspostulatedforsimilarrocktypesinRonda(groupDof

GarridoandBodinier,1999),theselatterfaciesaremost likelyreplaciveaftergroup-IIIpyroxenites.

Group-IV microstructures are coarse granular to porphyroclastic,dominatedbyrelativelylarge orthopyro-xene(0.5–1cm)andsmallerCr-diopside(1–5mm) crys-talsassociatedwithanhedralchromiumspinelandolivine. Thesemineralsaresurroundedbyaﬁner-grainedmatrix,

where anhedral clinopyroxene and Cr-spinel tend to predominate.

4. Majorandtraceelementchemistry

Fiftypyroxenitesampleswereselectedtorepresentthe differentpetrologicalrocktypesandgrindedinanagate mortar for bulk rock analyses (Table 2, Supplementary material). Major elements were analysed by XRF at ‘InstitutoAndaluzdeCienciasdelaTierra’(Granada),with a sequential spectrometer Bruker S4 Pioneer. Trace-element contents were analysed on a quadrupole HP7700xICP-MSatGeosciencesMontpellier(AETEfacility, Montpellier) and on an Agilent 8800 ICP-QMS at IACT (Granada), following the procedure described by Ionov et al. (1992). Accuracy of the ICP-MS analyses can be assessedfromtheresultsobtainedfortheUBNand BIR internationalrockstandards(Table2).

The analysed samples show a wide range of major-element compositions correlated with the pyroxenite petrologicalfaciesandtheperidotitetectono-metamorphic domains. This feature is well illustrated by the Ca-Tschermak–forsterite–quartz (CaTs–Fo–Qz) ternary dia-gram (Fig. 3) showing an extended compositional array fromthefertile(highCaTs)group-Iclinopyroxenites(Grt– Spmylonitedomain)totherefractory(highFoand/orEn) groups-IIIandIVwebsterites(Seilanddomain).Indetail,the corundum-bearingclinopyroxenite (subgroupIa)has the highestCaTscontentandresemblesthe‘group-IV pyroxe-nites’ of Gysi et al. (2011), interpreted as metagabbros. AlthoughsimilartogroupIinseveralrespects,the‘maﬁc garnet granulites’ from Ronda (subgroup A1 of Garrido and Bodinier, 1999) are distinguished by more evolved compositions,oversaturatedinsilica(Fig.3)and character-ized by low mg-no. (55–85, compared with 81–89 for

Fig.3. Molarprojectionsfrom diopside(Di)into thepseudo-ternary diagramforsterite(Fo)–calciumTschermakpyroxene(CaTs)–quartz(Qz) oftheanalysedpyroxenites(symbolsforrocktypesasinFig.1),also showing the compositional ﬁelds of type-IV pyroxenites from Beni BouseraafterGysietal.(2011),group-A1maﬁcgarnetgranulitesand group-D Cr-richpyroxenites fromRonda after Garridoand Bodinier (1999),andBeniBouseraperidotites(unpublisheddatafromthisstudy). En:Enstatite.

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groupI–Fig.4).Group-IIpyroxenitesﬁllthegapbetween groupsIandgroupsIII–IV,with,however,acleardistinction between subgroups IIa and IIb on one hand (Grt–Sp myloniteandArie´gitedomains),andsubgroupIIcon the otherhand(transitionalArie´gite–Seilanddomain),thelatter being distinctively morerefractory. Finally, subgroupsIII and IVare also welldiscriminated inFig. 3: group III is clearly shiftedtowardstheFo apexandslightlyoverlaps with peridotite compositions, whereas, similar to their counterparts from the Ronda massif (group-D Cr-rich pyroxenites ofGarridoandBodinier,1999),the groupIV websteritestrendtowardstheEncomponent.Afew group-IVsamplesshowingintermediatecompositionsarelikely replaciveaftergroup-IIIwebsterites.

Themg-no.,andtheminorandtraceelements,arenot wellcorrelatedwiththesystematicsdeﬁnedbythe major-elementcomponents.Themg-no.increaseswithdecreasing Al2O3(i.e.withdecreasingCaTs/Foratio)fromgroupsIand

II to groups III and IV (Fig. 4). However, group I is distinguished by higher mg-no. values at a given Al2O3

content(mg-no.=81–89ingroupI,comparedwith76–88 ingroupIIb–c).Conversely,the group-IIagraphite-garnet pyroxenite showsthe lowestmg-no.(68) inour dataset. TiO2andYbareroughlycorrelatedwithAlingroupsIIand

III (Fig.4), butgroupsIand IVaredistinctively impove-rished in these elements, whereas the graphite-garnet websterite(subgroupIIa)ismarkedlyenrichedinYb.

Similar to the majority of pyroxenites from orogenic peridotites (Bodinier and Godard, 2014), most of the analysed samples show chondrite-normalizedRare Earth Elements(REE)patternssimilartoN-MORB,i.e. character-izedbyaratherﬂatheavy-REE(HREE)segment(Gd–Lu)and depletioninlightREE(LREE).Indetail,however,thedifferent rocktypestendtoshowdistinctivefeatures(Fig.5): aspreviouslynotedbyKornprobstetal.(1990)andGysi

etal. (2011),group-Ipyroxenitesaredistinguishedby positiveEuanomalies(Eu/Eu*>1.2–Fig.4),thehighest value (1.6) being observed in the corundum-bearing sample(subgroupIa);

GroupIIischaracterizedbyvariableHREEfractionation asillustratedbyitswiderangeof(Gd/Yb)Nvalues,from

0.3 in the HREE-enriched, graphite-garnet websterite (subgroupIIa)to2 insomegarnetclinopyroxenites (subgroupIIb);

inadditiontobeingdepletedinHREEcomparedtothe othergroups, someof group-IIIand mostof group-IV websteritesshowREEdistributionsthatclearly depart fromthe‘N-MORB’type,includingbothconvex-upward (‘hump-shaped’)andsomewhatconvex-downward (‘U-shaped’) REE patterns. The latter are observed in the mostREE-depletedgroup-IVsamples.

In spite of these variations, the studied pyroxenites showanoverallnegativecorrelationbetweenthedegreeof LREE depletion (La/Sm) and Al2O3 (Fig. 4). A notable

exception is the corundum-garnet clinopyroxenite (sub-groupIa),whichisLREE-enriched.Conversely,the graphite-garnet websterite (subgroup IIa)is strongly depleted in LREEcomparedwiththeothergroup-IIpyroxenites(Fig.5). The different pyroxenite groups can be fairly well discriminatedbyaseriesofselected,Primitive Mantle-normalized inter-element ratios, as illustrated in

Figs.4and6:

GroupIischieﬂycharacterizedbyhigh(Sr/Nd)Nvalues

positivelycorrelated withEu/Eu*(Fig.6). LikeEu/Eu*, (Sr/Nd)Nshowsitshighestvalue(7)inthe

corundum-bearingsample(subgroupIa).Comparedwiththeother garnetandgarnet-spinelclinopyroxenites(groupIIb–c), groupIisfurtherdistinguishedbylower(Zr/Hf)N(<1),

andhigher(Pb/Ce)Nand(Th/La)Nvalues.The

corundum-garnet clinopyroxenite (subgroup Ia) is also distin-guished from all the other pyroxenites, except the graphite-garnet websterite (subgroup IIa), by higher, superchondriticNb/Lavalues((Nb/La)N=2.4);

within group II, the graphite-garnet websterite (sub-group IIa) is clearly distinguished from all the other pyroxenites by elevated (Nb/La)N and (U/Th)N ratios

(20and64,respectively),andverylow(Zr/Hf)Nvalues

(0.36)(Figs.4and6).ThissamplealsoshowsPb/Ceand Th/Lavalues>PM,wellabovethevaluesobservedinthe othergroup-II pyroxenites. The latter show relatively widerangesofinter-elementratios,butthevaluesare generallydispersed aroundPMvalues (e.g.,Pb/Ce), or belowinthecaseofNb/LaandTh/La(Fig.6);

GroupIIIisdistinguishedfromgroupIIonlybyhigherU/ Thvaluesinseveralsamples(Fig.4)andthetendencyofa fewsamplestooverlapwithgroup-IVwebsteritesforZr/ Hf,Ti/Gd,Pb/Ce,Th/La,andNb/La;

GroupIV showsseveral distinctive features, including low,subchondriticZr/Hfandhigh,superchondriticTi/Gd, Pb/Ce, and Th/La ratios. In this respect, group-IV websteritesshowsomesimilaritieswiththe graphite-garnet websterite (subgroup IIa), in spite of the contrastedmineralogicaland major-element composi-tionsofthesetworocktypes(Table1,Fig.3),andtheir well-distinctstructuraloccurrences(Fig.1).

5. Discussion

Pyroxenitemeltingandmelt–rockreactions

TheBeniBouserapyroxenitesshowﬁrst-order varia-tionsoftheirmajor-elementcompositionscorrelatedwith pyroxenite petrological facies and peridotite tectono-metamorphic domains (Fig. 3). These variations are reminiscentofthesystematicsobservedinthe neighbour-ingRondaperidotite(southernSpain),wherepyroxenite compositionsandrocktypesarecorrelatedwiththemajor structuraldomainsrecognizedinthemassif(Garridoand Bodinier,1999).InRonda,thesevariationswereascribedto partialmeltingofthepyroxenitesandreactionofresidual

Fig.4.Al2O3(wt.%)covariationdiagramsformg-no.,TiO2(wt.%),andPrimitiveMantle-normalizedYbcontentandLa/Sm,Gd/Yb,U/Th,Pb/Ce,Eu/Eu*,Zr/Hf,

andTi/Gdratiosfortheanalysedpyroxenites.Fieldsofgroup-DCr-richpyroxenitesandgroup-CAl-richpyroxenitesfromRondaafterGarridoandBodinier (1999)andBodinieretal.(2008).mg.no.=100Mg/Mg+Fecationicratio,withFe=totalironasFe2+

.Eu*=EuN/[(SmN+GdN)/2].Normalizingvaluesafter McDonoughandSun(1995).

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pyroxenites and/or peridotites with peridotite+ pyroxe-nite partial melts, or with subduction-related melts (Bodinieretal.,2008;GarridoandBodinier,1999;Lambart etal.,2012;Marchesietal.,2013).Theseprocesseshave beenascribedtothinningand heatingof subcontinental lithosphereinaback-arcsetting,aprocessthatculminated inthemassifexhumation(Hidasetal.,2013;Lenoiretal., 2001;Marchesietal.,2012;).Recently,Fretsetal.(2014)

proposed extreme lithospheric thinning to explain the tectono-metamorphiczoningoftheBeniBouseramassif, andGysietal.(2011)suggestedtheorigin(or modiﬁca-tion)ofcertainBeniBouserapyroxenitesasaresultofmelt inﬁltration,includingsubduction-derivedmelts.

Indetail,themajor-andtrace-elementsignaturesofthe studiedsamplespointtothreedistinctprocessesinvolving in-situorinﬁltratedpartialmelts:

(1)thevariationfromsubgroupIIbtosubgroupIIc,parallel to the CaTs-En cotectic line and correlated with retrogression of garnet clinopyroxenites (IIb) into garnet-spinel websterites (IIc) can be accounted for by partial melting of a fertile (high CaTs) garnet-clinopyroxeniteprotolith.VariableHREEfractionation (Figs. 4–5) suggests open-system conditions in the stabilityﬁeldofgarnet;

(2)thetransitionfromsubgroupIIctogroupIIIandthe trenddefinedbygroupIIItowardstheperidotitefield coincides with compositions experimentally deter-mined by Lambart et al. (2012) for reaction of peridotiteswithpyroxenitepartialmelts.This transi-tion marks the boundary between the Arie´gite and Seilandperidotitesubfacies,anditis worthyof note that in Ronda this boundary coincides witha sharp peridotitemeltingfront(GarridoandBodinier,1999). Acrossthefront,majormicrostructuralandchemical changesoccurwithinabout200masaresultof feed-backrelationshipsbetweencrystalgrowthandpartial melting(Lenoiretal.,2001;VanderWalandBodinier, 1996).Such anetmeltingfrontwasnotobservedin Beni Bousera, but Frets et al. (2012, 2014) noted microstructuralchangesinperidotitesandpyroxenites attheArie´gite–Seilandtransition,involvingenhanced recrystallizationandgraincoarsening.Therefore,the offsetofgroup-IIIolivine-spinel pyroxenitestowards theFocomponentandperidotitefieldonfigure3likely resultsfromreactionofpre-existinggroup-II pyroxe-nites(mostly subgroupIIc) withperidotite+ pyroxe-nitepartialmelts,followingthescenariosuggestedfor Ronda spinel pyroxenites by Garrido and Bodinier (1999), Lambart et al. (2012), and Marchesi et al. (2013).Furtherevidenceforgeneralizedmeltinginthe Beni Bousera Seiland domain includes the diffuse contactsbetweengroup-III spinel-olivinewebsterites and their host peridotites,as well asthe noticeable abundanceofrefractoryperidotites(harzburgitesand dunites)inthisdomain;

(3)in theory, the composition of group-IV Cr-diopside spinelwebsterites,whichplotclosetotheEn compo-nentinfigure3,couldreflectinteractionofperidotites withasilica-richmeltderivedfrommaficpyroxenites (Lambart et al., 2012). However, this scenario is

0.01 0.10 1.00 10.00 100.00 0.01 0.10 1.00 10.00 100.00 1.00 10.00 100.00 0.01 0.10 1.00 10.00 100.00

BaTh_{U NbTa La}CePbPrSr_{Nd Zr}_{Hf SmEu Ti GdTb}DyHoErTmYb Lu 0.01 0.10 0.01 0.10 1.00 10.00 100.00

Fig.5.Chondrite-normalizedRare-EarthElements(REE)patternsofthe analysedpyroxenites(symbolsforrocktypesasinFig.1).Normalizing valuesafterMcDonoughandSun(1995).

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unlikely because this group is distinguished by a specifictrace-elementsignatures(lowZr/Hf,highTh/ Laand Pb/Ceratios -Fig. 6)that is notobserved in fertile Beni Bousera pyroxenites, except for the graphite-garnet websterite (subgroup IIa) and, to a lesserdegree, forthegroup-Ialuminouspyroxenites. Theserock-typesoccur onlyin (orverynear to)the Grt–Spmylonitedomain,farfromthemain occurren-ces of group-IV pyroxenites in the Seiland domain (Fig.1). AsproposedbyGysietal.(2011),the trace-elementsignature ofgroup-IV pyroxenitespointsto infiltrationof a subduction-related melt component. WorksonsimilarCr-rich,pyroxenitesfromRondahave suggestedtheiroriginbycrystalsegregationorreactive replacementofpre-existingpyroxenitesbyrefractory meltofboniniticaffinity(GarridoandBodinier,1999). Cr-rich pyroxenites represent the latest stage of igneous mantle activity, both in Ronda and Beni Bousera,implyingthattheyrecordarecent evolution-arystageofthemassifsinaCenozoicsupra-subduction setting,shortlybeforetheiremplacementinthecrust (Gysietal.,2011;Marchesietal.,2012).

Threedistinctpyroxeniteprotoliths

Whilepyroxenitemeltingandmelt–rockreactionsmay account for the ﬁrst-order major-element systematics observedintheCa–Ts-Fo–Qzternaryplot(Fig.3),the mg-no, minor- and trace-element systematics (Figs. 4–6)

cannot be explained by these processes. Combined together, structural, petrological and geochemical data indicate that the Beni Bousera pyroxenitesderive from threedifferentprotolithsthatwerealsomarkedlydistinct intermsofrelativeabundanceandspatialdistribution: (1)GroupI- Basedontheir fertilecompositionnearthe

high-CaTsapexinﬁgure3,group-Ipyroxenitesmightbe considered as the ultimate protolith of most Beni Bouserapyroxenites.Sincetheotherpyroxenitesshow increasingimprintofmeltingandmelt–rockreactions fromtheGrt–SpmyloniteandArie´gitedomainstothe Seilanddomain,theoccurrenceofgroup-Ipyroxenites onlyintheGrt–Spmylonitedomain(Fig.1)isatﬁrst sight consistent with this scenario. However, this hypothesis is ruled out by the mg-no., minor- and trace-elementsystematicsshowingthatgroup-IIgarnet clinopyroxenitescannotderivefromgroupIbypartial meltingaswouldbeexpectedinthisscheme.Asnoted above,groupIisnotablydistinguishedfromgroupIIby highermg-no.,andmuchlowerTiandHREEcontents (Fig.4).Kornprobstetal.(1990)andGysietal.(2011)

pointedoutthedistinctivepositiveEuanomaliesandSr enrichments of group-I pyroxenites(Fig.6), and the implication of this feature for their origin as low-pressure plagioclase-bearing cumulates. Kornprobst et al. (1990), and Morishita et al. (2003)for similar pyroxenitesfromtheRondamassif,interpretedgroup-I

Fig.6.PrimitiveMantle-normalizedcovariationdiagramsforNb/La,Pb/CeandTh/Laratiosvs.Zr/Hf,andEu/Eu*vs.Sr/Nd.Fieldsofgroup-DCr-rich pyroxenitesfromRondaafterGarridoandBodinier(1999)andBodinieretal.(2008).Eu*=EuN/[(SmN+GdN)/2].NormalizingvaluesafterMcDonoughand Sun(1995).

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pyroxenitesasformeroceaniccrustcumulatesrecycled intheconvectivemantle.Thishypothesisis,however,at oddswiththeobservationthatthisparticulartypeof layer is found only in the narrow Grt–Sp mylonite domain,alongthecontactwithgranuliticcountryrocks (Fig.1).GroupIisfurtherdistinguishedfromgroupIIby signiﬁcantly higherTh/La andPb/Ce ratioscombined with lower Zr/Hf and variable Nb/La values (Fig. 6), suggestingasubductionsettingforitsorigin.Basedon theirresemblancewithislandarccumulates,Gysietal. (2011)interpretedgroup-Ipyroxenitesasdelaminated lowerarccrust.However,thisinterpretationdoesnot well account for the concentration of subgroup-Ia, corundum-rich (i.e.former plagioclase-rich)facies in thecentreofsymmetrically-zonedlayers.Thisfeatureis suggestiveofigneouscrystalsegregationandthe group-I layers may rather represent magmatic sills under-plated at the crust–mantle boundary in a supra-subduction setting.In both scenarios, however, they were entrained down to 60km depth (Kornprobst et al., 1990), most likely by subduction (see below), beforebeingexhumedbacktocrustallevelsasresultof lithosphericthinning(Fretsetal.,2014).

(2)SubgroupIIa-Inadditiontothepresenceofgraphitized diamondpseudomorphs(ElAtrassietal.,2011;Pearson et al., 1989, 1993), the graphite-garnet websterite (subgroupIIa)showsseveralmajor-andtrace-element distinctivefeatures.Thissampleisthemost differenti-atedintermsofmg-no.(Fig.4)andshowsanegative anomalyofEu(Figs.4,5).Itis,however,themost LREE-depleted (Fig. 5) and, compared to the other Beni Bouserapyroxenites,isfurtherdistinguishedbystrong enrichments in HREE, Nb-Taand U,and lowerZr/Hf values.ItisalsocharacterizedbyelevatedPb/CeandTh/ La ratios comparable to the values of group-IV websterites (Figs. 4 and 6). Subgroup-IIa represents thereforeapeculiarpyroxeniteprotolith,likelya high-pressure(garnet-anddiamond-bearing)crystal segre-gatefromapartialmeltderivedfrom hydrothermally-alteredoceaniccrust(hemipelagicsediments-Pearson etal.,1991a,b,1993).However,subgroupIIa,wasfound onlynearthecontactwithcrustalgranulites,similarto group-Ialuminouspyroxenites(Fig.1).Asaconsequence, itsgeochemicalsignaturecannotbeusedtoinfera‘Marble Cake’originforthewholeoftheBeniBouserapyroxenites. (3)SubgroupIIb -The garnetclinopyroxenitesoccurring throughouttheArie´giteandGrt–Spmylonitedomains (subgroupIIb)representtheprotolithofsubgroup-IIc garnet-spinelwebsterites, andthereforetheultimate protolithofgroup-IIIandreplacivegroup-IV webste-rites.Thisimpliesthatsubgroup-IIbgarnet clinopyro-xenites were present throughout the Beni Bousera peridotiteattheonsetoflithosphericthinning,partial melting andmelt–rockreactions thatresultedinthe individualisationofthedifferenttypesofpyroxenites observedintheArie´gite–SeilandandSeilanddomains (Fig.1). Kornprobstetal.(1990) suggestedanorigin of subgroup-IIb garnet clinopyroxenites as high-pressurecrystalsegregates.However,incontrastwith the graphite-garnet websterite (subgroup-IIa), sub-group-IIbgarnetclinopyroxenitesdo notdisplay the

systematic HREEenrichment and Zr/Hffractionation thatwouldbeexpectedingarnet-bearingsegregates. Instead, they show geochemical features, notably a highTiO2contentassociatedwithLREEdepletion,that

are reminiscent of pyroxenites formed by igneous refertilization(e.g.,groupCofRondaafterGarridoand Bodinier, 1999 - Fig. 4). A few samples from this subgroupshowasubtlepositiveEuanomalyassociated with slight Sr enrichment (Fig. 6). This may be consideredasevidencethattheirformationinvolved melting of a recycled, originally plagioclase-bearing source,asproposedbyMontaniniandTribuzio(2015)

forLiguriangarnetpyroxenites.SubtleanomaliesofEu, either positive or negative, were also observed by

Pearsonetal.(1991a)ingarnetpyroxenitesfromthe Arie´gitedomainofBeniBousera.Basedontheelevated

d

18_O _values _found_in_several _samples,_these_authors

suggested their crystallization from partial melts of hydrothermally-alteredoceaniccrust.Similartoother orogenic garnet pyroxenites, subgroup-IIb garnet pyroxeniteswillclearlydeservefurtherinvestigations toelucidatetheoriginoftheirprotolith.

6. Conclusion

SeveralstudieshavereportedevidencefortheMarble Cake model based on the crustal and/or subduction geochemical signatures observed in pyroxenites from the Beni Bousera massif (e.g., Kornprobst et al., 1990; PearsonandNowell,2004).Ournewtrace-elementdata conﬁrmtheseearlyobservations.However, these signa-turesareobservedonlyinpeculiarrocktypes,notablyin aluminous garnet clinopyroxenites (group I), including corundum-richfacies (Kornprobst etal.,1990), andin a raregraphite-garnetwebsterite(subgroupIIa)containing graphitizeddiamonds(ElAtrassietal.,2011;Pearsonetal., 1989, 1993). Most importantly, these rock types are concentratedin anarrow(<100m)corridor of myloni-tized peridotites, along the contact with the granulitic country rocks. These peculiar facies therefore do not represent recycled components from the convective mantle.Theywerelikelyincorporatedinthemantlealong a major tectonic discontinuity in an earlyevolutionary stage ofthe massif,perhaps atthe interface between a subductingslabandtheoverlyingmantlewedge.Thenthis discontinuitywouldhavebeenre-usedduringlithospheric thinning,allowing‘recycled’,high-pressurepyroxenitesto beexhumedalongthemantle–crustboundary.

Theothertypesofmaﬁclayersderivefromaclearly distinctgarnetclinopyroxeniteprotolith(groupIIb)that was ubiquitous in the peridotite body at the onset of lithosphericthinning.However,theselayersdonotshow clearevidenceforacrustalor/andsubductionoriginand thusdonotlendsupporttotheMarbleCakemodeleither. Theiroriginisnotfullyunderstoodbutlikelyoccurredvia igneous refertilization mechanisms, as suggested for pyroxenitelayeringinotherorogenicperidotites( Bodi-nier et al., 2008; Lambart et al., 2012), and possibly involved partial melts froma crustal source (Marchesi etal.,2013;MontaniniandTribuzio,2015;Pearsonetal., 1991a). Thereafter,the original garnet-clinopyroxenite

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protolith was deeply modiﬁed by partial melting and melt–rock reactions associated with lithospheric thin-ning.Thiseventprobablyoccurredinasupra-subduction setting(Gysietal.,2011;Marchesietal.,2012)andgave rise to a variety of reacted refractory pyroxenites (subgroup IIc, groups III & IV) comparable to those observedintheneighbouringRondaperidotite(Garrido andBodinier,1999).

Acknowledgements

FundingforthisstudywasprovidedbyMAE(France), MESRSFC(Morocco),andCNRS/INSU(France)throughthe cooperation project #042/STU/09 in the frame of the ‘VolubilisHubertCurien’program(2010–2013)andthree INSU research grants (‘Actions coordonneés’ 2008, and SYSTER 2010 and 2011). C.M. acknowledges funding by Ramoń y Cajal Fellowship RYC-2012-11314 by MINECO. This study benefited from the FP7-PEOPLE-2013-IRSESprojectMEDYNA,fundedunderGrant Agree-ment PIRSES-GA-2013-612572, from the International Lithosphere Program CC4-MEDYNA, and from grants funded bytheSpanishMINECO (CGL2013-42349-P)and ‘Junta de Andalucıá’ (RNM-131). This research hasalso benefited from EU Cohesion Policy funds from the European Regional Development Fund (ERDF) and the EuropeanSocialFund(ESF).WethankChristopheNevado (GM) and Rosario Reyes (IACT) for the preparation of samples and high-quality thin sections, and Chantal Douchet (GM) for assistance during ICP-MS analyses. Jean-Marie Dautria is thanked for his help during the petrographic studyand AndreáTommasiforstimulating discussions on the significance of the Beni Bousera mylonite domain. We gratefully acknowledge thorough reviewsbyTheodorosNtaflosandRiccardoTribuzio.

AppendixA. Supplementarydata

Supplementarydataassociatedwiththisarticlecanbe found,intheonlineversion,athttp://dx.doi.org/10.1016/j. crte.2016.06.001.

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