Oceanic units in the core of the External Rif (Morocco): Intramargin hiatus or South-Tethyan remnants?

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ContentslistsavailableatScienceDirect

Journal of Geodynamics

j o ur na l h o me p a g e : h t t p : / / w w w . e l s e v i e r . c o m / l o c a t e / j o g

Oceanic units in the core of the External Rif (Morocco): Intramargin hiatus or South-Tethyan remnants?

Mohamed Benzaggagh

^a

, Abdelkader Mokhtari

^a

, Philippe Rossi

^b,c

, André Michard

^d,∗

, Abdelkhader El Maz

^a

, Ahmed Chalouan

^e

, Omar Saddiqi

^f

, Ech-Cherki Rjimati

^g

aUniversitéMoulayIsmail,FacultédesSciences,DépartementdeGéologie,BP11201BeniM’Hamed,Meknès,Morocco

bBRGM,BP36009OrléansCedex02,France

cCCGM,77rueClaude-Bernard,75005Paris,France

d10,ruedesJeûneurs,75002Paris,France

eUniversitéMohammedV-Agdal,FacultédesSciences,DépartementdesSciencesdelaTerre,AvenueIbnBatouta,BP1014Rabat,Morocco

fLaboratoireGéosciences,UniversitéHassanII-Casablanca,BP5366Maârif,Casablanca,Morocco

gMinistèredel’EnergieetdesMines,DirectionduDéveloppementMinier,DivisionduPatrimoine,BP6208RabatInstituts,Morocco

a r t i c l e i n f o

Articlehistory:

Received10March2013

Receivedinrevisedform1October2013 Accepted14October2013

Availableonline25October2013 Inmemoryofourprofessorandfriend MichelDurand-Delgawhoencouragedour researchintheareaoverseveralyears.

Keywords:

WestMediterranean Alpinebelts Maghrebides Morocco Suture Paleomargin Ophiolite Thrusttectonics Transcurrenttectonics

a b s t r a c t

Theaimofthispaperistodescribethemaficrocksthatcropoutinthecentral-westernMesorifZone (ExternalRifBelt),anddiscusstheirgeodynamicsignification.Basaltflows,olistolithsandbrecciasoccur inOxfordian–BerriasiandepositsofMesorifunitsascribedtothedistalpartoftheAfricanpaleomargin.

TheclimaxofvolcanicactivityisobservedatthenorthernborderofaKimmeridgiancarbonateplatform progressivelydismemberedduringtheTithonian–Berriasian.Inspiteofthealterationofthebasalts,their petrologicalandgeochemicalcharacterspointtoE-MORBafﬁnities.Thestudiedgabbromassifs(BouAdel, KefelRharwestandnorth)occurasrestrictedsliversorklippeswithintheSenhadjanappeormélange oftheinternalMesorif,whichoverliesthebasalt-bearingunitsandother,moreexternalMesorifunits.

Thecompositionsrangefromtroctoliticolivinegabbrotoferrogabbrowithfrequentortho-toheteradcu- mulatetextures;theydisplaytypicaltholeiiticaffinity.Thegabbromassifsarecrosscutbytrondjhemite dykesandoverlainbymetabasalts,fault-scarpbreccias,ophicalcites,marblesandradiolarites.Composi- tionfeaturinginitialnearliquidcomposition,displaymultielementspatternsclosetothoseofE-MORB, withaweakEunegativeanomalyandevidenceofslightcrustalcontamination.Thesegabbromassifswere regardedasJurassic–Cretaceousintrusions,locallydated(K–Ar)at166±3Ma.Conversely,weassume theyrepresentdiscretesamplesofaJurassic–Cretaceousoceanicbasement(ophiolites),emplacedtec- tonicallyintheSenhadjanappe(mélange)ofthecentralMesorif.Thecorrelationofboththesetypesof maficrockassociations(paleomarginbasaltsandophioliteklippes)withtheserpentinitesoftheeastern Mesorif(BeniMalek)andOranmountains(Algeria)isthenbrieflydiscussed.Weconcludethattheprevi- oushypothesisofanintramargin“Mesorifsuturezone”mustbereconsidered,beingchallengedbythat ofamajor,syn-collisional“Oran-MesorifStrike-SlipFault”.Inthelatterhypothesis,thenewlydescribed MesorifoceanicklippeswouldrepresentallochthonousremnantsoftheLigurian–Maghrebian(Tethyan) oceanicdomain.

1. Introduction

TheExternalZonesofanycollisionalbeltsarebydeﬁnitionout- sidethesuturezone(i.e.intheforelandsideoftheorogen)and, accordingly, theyareexpected tobe devoidof ophioliticunits, exceptintheformoftectonicklippesasinthePrealps(e.g.Schmid etal.,1996).IntheRifBelt,asinthewholeMaghrebides(Fig.1), theExternalZoneslieonthenorthernmarginoftheAfricanplate,

∗Correspondingauthor.Tel.:+33142360483.

E-mailaddress:andremichard@orange.fr(A.Michard).

fromwhichtheyderive(Durand-DelgaandFontboté,1980;Wildi, 1983;Favreetal.,1991).Theyareseparatedfromthedismem- beredInternalZones(AlboranDomain,Kabylias,Peloritan-Calabria unitswithEuropean/Alpineafﬁnities)bytheMaghrebianFlyschs suturezone(Bouillinetal.,1986;Guerreraetal.,2005).Thissuture iscurrentlyregardedastheresultoftheSE-toSW-wardretreat ofthesubductingLigurian–MaghrebianslabofwesternNeotethys (FrizondeLamotteetal.,1991;LonerganandWhite,1997;Jolivet andFaccenna,2000;SpakmanandWortel,2004;Carminatietal., 2012,andreferencestherein).However,ifthissuturezoneisrich in ophiolite remnants in the east (Ligurian nappes of Calabria, Sicily)asexpectable,suchremnantsbecomerareinthewest,with 0264-3707/$–seefrontmatter©2013ElsevierLtd.Allrightsreserved.

http://dx.doi.org/10.1016/j.jog.2013.10.003

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M.Benzaggaghetal./JournalofGeodynamics77(2014)4–21 5

Fig.1. LocationofthestudyareaintheMaghrebideBeltofNorth-Africa(A)andpaleogeographicsketchesshowingtheEoceneextensionoftheLigurianTethys(B)andthe MiddleMiocenecollisionalsettingoftheWestMediterraneanAlpinebelts(C).(B)and(C)refertothemostcommoninterpretationoftheWestMediterraneanevolution (seetextforreferences).

thelastserpentinite-gabbro-basaltassemblageseenbeneaththe LesserKabyliaMassif(Bouillinetal.,1997).IntheRifBelt,theonly maﬁcrocksassociatedwiththeMaghrebianFlyschsareafewolis- tolithsandsliversofE-MORBpillowbasaltsfoundintheeastern partofthesuturezone(Durand-Delgaetal.,2000).Thenaques- tionarises:wherearethesouthTethyan(Ligurian)ophiolitesinthe Alborantransect?

Curiously, a serpentinite massif associated withmetabasites wasmappedsinceyears withintheExternalZonesoftheeast- ernRif,nexttoBeniMalekvillagewestoftheTemsamanemassif (Suter,1980a,b;Choubertetal.,1984).TheBeniMalekmassifwas interpretedsubsequentlyasamantlesliverdetachedfromanintra- marginhiatusdistinctoftheLigurianTethysitself,althoughbeing coevaltoit(Michardetal.,1992).Thesigniﬁcantextensionofthis sliveratdepthwascalibratedthroughmagneticanomalyinterpre- tation(Elazzabetal.,1997).Morerecently,Michardetal.(2007) emphasizedthattheallegedintramarginsuturecontinueseast- wardover200kmorso,uptotheOranregionatleast(Algeria), beingassociatedwithlow-grade,intermediatepressuremetamor- phismasin thenorthernTemsamaneunits(Negroetal., 2007, 2008).However, thewestwardcontinuation ofthis unexpected externalsutureremainedunknown,anditstectonicinterpretation poorlyascertained.

Theaimofthepresentpaperisfirstlytodescribethevarious occurrencesofJurassic–Cretaceousmaficassemblageswestofthe BeniMalekmassif,basedonnewfieldcampaignsandstratigraphic studies.Somehavebeenalreadydescribed(BenYaïchetal.,1989;

Benzaggagh,2011),andconsistsofbasaltflowsandclastsinpale- omarginunits.Othersare describedhereforthefirsttime, and correspondtosmallgabbromassifsassociatedwithoceanic-type cover sequences.We present thefirstpetrological–geochemical

descriptionofboththesemaficunits.Finallywebrieflydiscussthe tectonicinterpretationofthesecontrasting,paleomargin-typeand oceanic-typemaficunitswithintheRifExternalZone.

2. Geologicalsetting

In the Rif Mountains (Fig. 2), the Internal Zones constitute twopiecesofadismembered,Miocenemetamorphiccorecom- plex(García-Due ˜nasetal.,1992;Michardetal.,2006),namelythe NorthernRifandtheBokkoyamassifs.ThefamousBeniBousera mantleperidotitesandgranuliteslaywithinthelowerplateofthe complex,i.e.itslowestandmoremetamorphicunits(Sebtides).

The upperplate(Ghomarides) correspondsto severalsliversof low-gradePaleozoicrocksinterleavedwiththeirTriassic–Cenozoic coverrocks.TheMesozoic–Cenozoic“DorsaleCalcaire”unitsthat fringethesouth-westfrontoftheNorthernRifandconstitutemost oftheBokkoyaareremnantssouthernorsouth-westernpaleomar- ginoftheAlboranDomainnorthoftheMaghrebianOcean(Wildi, 1979,1983;Durand-Delga,1980;ElHatimietal.,1991;Blidiand Hervouët,1991;ElKadirietal.,1992;ChalouanandMichard,2004;

Guerreraetal.,2005;Durand-Delga,2006).

TheMaghrebianFlyschnappesareformedbythreemainthrust sheets (Mauretanian, Massylian, Numidian nappes) originating from thesedimentary inﬁll of theLigurian–Maghrebian Ocean.

TheMauretanianandMassylianJurassic-EarlyCretaceousbedsare associatedwithE-MORBpillowbasalts(Durand-Delgaetal.,2000) southof theeastern BokkoyaMassif(Fig.2).These nappesroot beneaththeInternalZonesandoverlietheExternalZones,except forsomeback-thrustunits(e.g.northernmostRifandKabylias).

Thus,thefaultthrustcontactbetweentheAlboranDomainandthe FlyschNappesrepresentsthemainsuturezoneoftheMaghrebide

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Fig.2.StructuralmapoftheRifBeltafterSuter(1980b)andChalouanetal.(2008),withlocationofFigs.3and4(framed),andoftheBeniMalek-TresForcasorMesorif SutureintheeasternRif(Michardetal.,2007).

Orogen.TheFlyschNappesareinturnobductedanddiverticulated ontotheuppermostunitsoftheExternalZones(TangierandHabt units)inthewesternRif,andontoslightlydeeperunitsofthesame zones(Ketama,Aknoul)inthecentralandeasternRif.

TheMaghrebideExternalZonesderivefromtheNorth-African paleomargininverted during theEarlyMiocene collision ofthe Internal Zones(Wildi, 1983;Favre et al., 1991;Guerrera et al., 2005;Crespo-BlancandFrizondeLamotte,2006).IntheRifBelt, theyaredivided,fromNE toSW and fromtop tobottom,into theIntrarif,MesorifandPrerifZones(Suter,1980a,b).Withineach ofthese zones,deeprooted, para-autochthonous unitscontrast withdiverticulated,gravity-driven nappesthrustoverthemore externaldomains.TheIntrarifzoneincludestheKetamaunit(Tri- assictoAlbo-Cenomanian),theTangierandLoukosunits,partly detachedfromtheKetamaunit,andtheHabtandAknoulnappes (LateCretaceous-Cenozoic),entirelydetached.TheMesorifZone shows allochthonous units including Triassic toPaleogene for- mations thrustover allegedlyautochthonous tectonicwindows whoseseriesendwithMiddle-UpperMioceneturbiditesandolis- tostomes.ThePrerifZoneasawholeconsistsofJurassic–Miocene unitsdetachedontheunderlyingTriassicevaporitesandthrust overtheUpper Mioceneforedeepdeposits(GharbBasin,Saiss).

Theparticular,InternalPrerifZonecorrespondstoslicesofJurassic- EarlyCretaceouslimestonesthatformastringofsteephills(“sofs”) betweentheMesorifand ExternalPrerifZones. Theearly(Tor- tonian)fold andthrustcontactsaresealedbythetransgression of Upper Tortonian–Messinian conglomerates and sandy marls (molasses), that have been subsequently folded (e.g. Taounate

“post-nappe”syncline;Samakaetal.,1997).

ThetectonicstructuresobservedintheRifExternalunitsand overlyingMaghrebianFlyschoutliersshowanoverallsouthwest- warddisplacement(Frizonde Lamotteet al.,1991, 2004).Two majorNE-trendingleft-lateralfaults,namelytheJebhaandNekor

Faults(Fig.2),giveadditionalevidenceoftheobliquityofthecol- lisionoftheAlboranDomainagainstAfrica(LeblancandOlivier, 1984;FrizondeLamotte,1985).Thisisaccountedforbythecurrent modelsofslabroll-backoftheLigurian–Maghrebiansubduction beneath theEuropeanlithosphere(Spakmanand Wortel,2004;

Jolivetetal.,2008).

3. Basalt-bearingpaleomarginunits

Inthissectionwepresentthevolcanicandvolcano-sedimentary formationsthatoccurmostlyintheKimmeridgian–Berriasiancar- bonateformationsoftheMesorifZone.Thestudiedoutcropsare locatedintwoareasoftheExternalRif(Fig.2),i.e.theOuezzane areaofWesternRif(Fig.3A),andtheTaounate-Tainesteareaof CentralRif(Fig.3B).InthewesternMesorifZone,thevolcanicevent hasbeenascribedtotheBathonian–Callovian (BenYaïch etal., 1989; BenYaïch,1991), whereas it wasregarded asBarremian inthecentralMesorifZone(Vidal,1979,1983a,b).Newaccurate datingsbasedonplanktonicassemblageshavebeenpublishedby Benzaggagh(2000),BenzaggaghandHabibi(2006)andBenzaggagh (2011).

Thecarbonateformationsoccureverywhereontopofathick (1000–1500m)Callovian–Oxfordianturbiditeformationknownas the“Ferrysch”that sealstheLower-MiddleJurassic tilted-block andhemigrabenstructuresofthepaleomargin(Wildi,1981;Favre, 1992).TheFerryschdepositsarebroadlyidenticalallovertheInter- nalPrerif,MesorifandIntrarifZones,butthickensfromthePrerifto theMesorif.FromtheKimmeridgianuptotheBerriasian,carbon- atefaciesdevelopeverywhereabovetheFerryschturbidites.Inthe InternalPrerif,theyconsistbasicallyofpelagicfacies(ammonite- bearingthin-beddedlimestonesandammoniticorossofacies).In contrast,intheMesorifZone(e.g.JebelTaharBouZhaier,Fig.4A), thecarbonatesequencebeginswiththicklayersofcalciruditesor

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Fig.3.LocationofthestudiedoutcropsintheMesorifframework(seeFig.2formaplocation).NumbersrefertotheanalyzedsamplesofTable2.(A)Basalt-bearingoutcrops ofthewesternMesorifZone(Ouezzanearea).SketchcontoursofthestructuralzonesafterSuter(1964a,1980b)andBouhdadi(1999).(B)Basalt-bearingoutcropsandmaﬁc oceanicunitsofthecentralMesorifZone(Taounate-Taïnestearea).SketchcontoursofthestructuralzonesafterSuter(1980b)andLeblanc(1983).Framed:detailmapsFigs.6 (BouAdel)and9(KefelRhar).

monogeniccarbonatebreccias,moreorlesssiliceous(Fig.4B),of Kimmeridgianageaccordingtotheirbenthicforaminifercontent (BenzaggaghandAtrops,1997).TheMesorifcarbonatesequence continuesupwardintheTithonianwithvariedfaciesofbreccias, either massive layers of carbonate breccias or chaotic breccias withrareclayeymatrix(Fig.4C).Ammonitesarescarce,anddat- ingsarebasedontheplanktonicassemblages(Benzaggagh,2000;

BenzaggaghandHabibi,2006).Theamountofclayandsiltincreases intheBerriasiandeposits,whichalsocontainsbrecciasinsomeof thestudiedsections(Figs.4Dand5A,B).

Mostoftheelementsofthebrecciasareshallowwater,plat- formcarbonates(upto60–80cminsize)datedasKimmeridgianor lowerTithonianintheUpperTithonianbrecciasandUpperTitho- nianintheBerriasianbreccias(Benzaggagh,2011).Likewise,the layeredlimestonesfrequentlycontainreworkedalgaeandbenthic foraminifers(Fig.5BandC).Thissuggestsredepositionbymass ﬂowsandturbiditycurrentsrelatedtocannibalismphenomena, andaccountsforthefrequentandimportantvariationsinthethick- nessoftheUpperJurassiccarbonatesequence(Fig.5),asalready quotedbySuter(1965).

Besidesofthedominantcarbonateclasts,allofthestudiedout- crops(Fig.3)showanalmostconstant occurrenceofmagmatic clastswithin thechaotic breccias. Theyspansfrom a few cen- timeterstoseveralmetersinsize,withmostfrequentblocksand boulders(Fig.4EandF).Theirlithologyrangesfromtypicalbasalts todolerites,tofinegrainedgabbros(seeSection5.1).Somebasalt flowsorfragmentsoflavaflowarepreservedbyplace(Fig.5A,C andD).Theearliestmagmaticeventisrecordedbypyroclasticlay- ersbytheveryendoftheOxfordian(Fig.5E),whereasthelatest wouldbeUpperTithonianorBerriasianinage.Theoccurrenceof abasaltdykeintheFerryschformationofSidiKassem(Fig.3)next totheKerkorvolcano-sedimentarybreccias(Fig.5D)confirmsthat theUpperJurassic–Berriasianvolcaniccenterswerelocatedinthe MesorifZone.

Inadditiontothecarbonateandvolcanicclasts,thechaoticbrec- ciasalsocontainpebblesorcobblesofsandstonewithFerrysch-like facies.Theabundanceandsizeofsuchelementsseemstoincrease upward. Largeolistoliths oftypical Ferryschfacies occurinthe BerriasiansequenceatJ.Mazoura(Fig.5C).

4. Theexoticgabbromassifs

In thissectionwe describetheexotictectonicunitsthatwe deﬁneherefortheﬁrsttime intheCentralMesorifatBouAdel andKefElRhar(Fig.3Bforlocation).Theyarecharacterizedby theoccurrenceofathickgabbrobasementoverlainbyvaried,low- grademetavolcanitesandmetasedimentaryrocks.Weparticularly developtheBouAdelcasestudy.

4.1. BouAdelunit

TheBouAdelgabbrocropsoutonabout2km² (Fig.6)inthe deeply incised Oued Azrou valley where it is exposed in steep slopesupto150mhigh(Fig.7A).Theplutonicbodywasregarded asaPaleozoicgranitesliverincludedinthe“Senhadjanappe”by Suter (1964b, 1965),whereas Vidal(1983a) recognizeditsgab- broic nature in the freshoutcrops of theconcave slope of the valley.ThelatterauthorclassiﬁedthemassifasaCretaceous(Bar- remian?)intrusion. However,revisionof themapcontours and detailed petrological data (see below)enable us todiscard the intrusion hypothesis and deﬁne herea tectonic unit (Bou Adel unit)includingaplutonicbasementanditsvolcanic-sedimentary cover.

The tectonic unitsoverlying the BouAdel unit (Fig.6) con- sist dominantly of platform carbonates classiﬁed as Lower to UpperLiassicbySuter(1964b)andVidal(1983a,b),althoughsome couldbeUpperJurassic(Bulundwe, 1987;Papillon,1989).Any- way,thesecarbonateunitsoverlaintheBouAdelunitthrougha thrustcontactastheyaredevoidofthelow-grademetamorphism thataffectstheBouAdelunit,andshowdistinctfoldstructures.

Theywould correspondtoa stack oftwo largeunitsseparated by sliversofFerrysch (Callovian–Oxfordian) andsurroundedby Middle-UpperMiocenepebblymarls(Tortonianmélangematrix;

Vidal,1983a).

TheplutonicpartoftheBouAdelunitiscomposedofdiffer- enttypesofgabbro(Table2).Thedominantfaciesisacoarseto mediumgrainedtroctoliticgabbrothatdisplaysanigneouslayer- ingdipping40±10^◦totheNW(Figs.6and7B),revealedbythin (lessthanonecentimeterthick)lightlayersofplagioclasethatcan

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Fig.4.UpperJurassic–Berriasiansuccessionandvolcano-sedimentarybreccias.(A)GeneralviewontheJ.TaharBouZhaiermountainslope,lookingeastward(seeFig.3Bfor location).(B)WeatheredsurfaceofaKimmeridgiancarbonatebreccia(J.Alebra;seeFig.3Aforlocation).(C–F)TithonianfromtheJ.TaharBouZhaier.(C)Clast-supported chaoticbreccia(LowerTithonian).Noticethelargelimestoneslabbeneaththehammer,andtheoccurrenceofabasaltcobble(arrow).(D)Chaoticbrecciawithanincreased proportionofmarlymatrix(UpperTithonian)toppedbyLowerBerriasianmarlsandlimestones.(E)Closeviewofabasaltblockinamassivecarbonatebreccia.(F)Massive boulderofdoleriteorﬁne-grainedgabbro.

befollowedforsomemeters.Pegmatiticveinsorpocketscanbe observedelsewhere(Fig.7D).Typicaltroctolitesamples(BA1,BA6) werecollectedalongtheOuedAzrou(Fig.6).

Fromthecentraltothesouthwesternpartofthemassifthegab- broisreddishduetoweathering.TypicalsamplesareBA2,BA3,BA5, BA7(Fig.6)withacompositionofTi–Fegabbro,hereaftercalledfer- rogabbro(Fig.7E).Layeringshowsaratherconstantstriketrending N70Ewhereasdipmaychangefrom30^◦NWtosubvertical.Thin leucocraticdykes,sometimesenrichedindarkmineralsoneach sideandaffectedbysheardeformation,crosscuttheferrogabbro.

Fieldsurveydidnotprovideopportunitytoobservethetransition betweentroctoliticandferrogabbroiczonesbecausescreeandsoils hidealargepartoftheoucrops.Howeversomeobservationscan bemadeonthewesternbankoftheOuedAzrourivernearthe highestpartofthemaﬁccomplex(N34,53529;W4,50551)where onemetricenclaveofpossibletroctoliteisincludedinferrogabbro.

Closetothislastoucropa1m-thick,N100E-trendingdykeofpla- giogranite(BA9)crosscutssharplythegabbroatthevicinityofthe volcanic-plutoniccontact.Greenschist-faciessecondaryminerals arewidespread(e.g.epidote,Fig.7D).

Atboth thewestern and eastern tips of the massif(Fig.6), thevolcanic cover sequence of thegabbro massifis preserved.

Itbasicallyconsistsofspilitizedbasaltsanddolerites.Thewest- ernoutcropsarethemostinterestingastheyinclude,nexttothe metabasalts(Fig.7A):(i)amassofcoarsevolcanic-gabbroicbreccia withchlorite-serpentinitematrix,extendingoverca.100m×20m at thebottom of the slope (Fig. 7F); we interpret this outcrop asasubmarinefault-scarpbreccia; (ii)typicalophicalcitefacies (Fig.7G)insomeblocksofthiscoarsebreccia;(iii)whitemarbles transgressiveontocarbonate-cementedvolcanicbreccias(Fig.7H andJ);(iv)bandedvolcaniclasticmarblescloselysimilartoophi- oliticsandstone layers in thesenseof Lagabrielle and Lemoine (1997)(Fig.7I).Noticethatblocksofwhitemarblesarescattered intheadjoiningfault-scarpbreccia.Conversely,detritalelements of gabbro, doleriteand basalt can be foundwithin the marble facies(Fig.7J).Insomecase,aclearmetamorphicfoliationcanbe observed(Fig.7J),whichhasbeenaffectedbylate-metamorphic folding(Fig.7K).

The volcanic cover of the plutonicmassif shows basically a domeshape.Accordingtothelayeringstrikeanddipinthegabbro,

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Fig.5.StratigraphiccolumnsofsomeofthestudiedMesorifsections(seeFig.3forlocation).NoticethealmostconstantassociationofthebasaltoccurrenceswithKim- meridgiantoUpperBerriasianchaoticbreccias.Theearliestbasaltsarealsoassociatedwithresedimentedbeds(“Ferrysch”,layeredlimestoneswithalgaeandbenthic foraminifers).

basicallyENEandtotheNW,respectively,thelessdifferentiated troctoliticgabbroislocatedtothetopofthesequence,whereasthe moredifferentiatedferrogabbroremainssituatedinlowerposition.

Ifoneconsidersthatthemassifresultsmainlyofasimpleprocess offractionalcrystallizationitcouldbeconcludedthatthecomplex wastiltedbeforebeingoverheadbyitsvolcanicandsedimentary cover.

4.2. KefelRharunits

ThreemassifsofgabbrocropoutclosetotheKefelRharvillage, 20kmeastofBouAdel(Fig.3).Thetwolargest(KefelRharmas- sifsproperlysaid)areexposedabout1.5kmnorthofthevillage (Fig.8),whereastheDarBouAzamassifcropsoutsome6kmwest ofit.Thedimensionsandgeologicalframework(Senhadja“nappe”) ofthesethreemassifsaresimilartothatoftheBouAdelmassif,and theywerealsoclassiﬁedasLowerCretaceousintrusionsbyVidal (1983a,b).However,liketheBouAdelmassif,theyshowremnants ofmetavolcanitesandmetasedimentarycoverformationssuggest- inganoriginfromalostoceanicseaﬂoor.

ThepoorlyexposedDarBouAzamassif,5kmwestofKefelRhar, issurroundedbyMiocenemarlsandconglomerates,exceptonits southernboundary,whichisatectoniccontactbetweenthegab- broandanoverlying“Flyschnoir”unit,accordingtoVidal(1983a).

Wefoundatthenorthernboundaryofthemassifanoutcropof marblesthatoverlay directlythelow-grademetagabbroicrocks

andcontainscatteredpebbleofmetabasaltsandmetagabbros,and boudinagedcherts(Fig.9A).Thesebedsarecloselysimilartothe BouAdelvolcaniclasticmarbles(Fig.7H–J).

TheKefelRharmassifss.str.formapairofklippesorslivers surroundedbyMioceneclasticmarls(mélangematrixaccordingto Vidal,1983a,b)andoverhangedbyvariouscarbonateunits(Fig.8).

Athickvolcaniccover,locallyassociatedwithmarbles,isassociated tothesouthernklippe.Remarkably,thenorthernklippepreserves awell-developed,low-grademetasedimentarycovermadeupof palegreen,pinkorredradiolarites(Fig.9B–D).Theseradiolarites arefollowedupwardbyblacklimestones-blackshalesalternations whosefaciesarewidespreadintheLowerCretaceousExternalRif sequences.Thestructuresobservedintheradiolarites(Fig.9D)are consistentwithanupper-greenschistfaciesmetamorphism,which alsoaffecttheunderlyinggabbroitself.

5. Petrographyandgeochemistry

5.1. Materialandmethods

Six basaltic rocks sampled in the Upper Jurassic–Berriasian formationsoftheIzzareneWindowandBouHaddoudNappe(pale- omarginunits)havebeenanalyzed(Table1andFig.3).Intheexotic gabbrounits,the9analyzedsamples(Table2)wereallcollectedin theBouAdelmassif(Fig.6).

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Fig.6. GeologicsketchmapoftheBouAdelmassifbasedonthemapsbySuter(1964b)andVidal(1983a),modiﬁed.SeeFig.3Bforlocation.Severalmetabasaltblocksare foundamidsttheslopeformationsbeneaththeJ.Keilcarbonatecliffs.Layeringstrikeanddiparevisibleonthewesternpartofthegabbromassif.

ThinsectionswereproducedandstudiedbothattheDepart- mentofGeology,MeknèsUniversityandtheBureaudeRecherches Géologiques et Minières (BRGM, Orléans, France). The samples wereanalyzed at theBRGM: majorelementsby X-rayﬂuores- cence(XRF),transitionelementsbyparInductivelyCoupledPlasma

Atomic Emission Spectroscopy (ICP-AES), and Rare Earth and otherelementsbyInductivelyCoupledPlasmaMassSpectroscopy (ICP-MS)NEPTUNEandchemicalmineralanalysesonaCAMECA SX 50 Electron Probe Micro-Analyser (EPMA) equipped with ﬁve wavelength-dispersivespectrometers using an acceleration Table1

Locationandstratigraphicageoftheanalyzedbasaltsamples(seemapsFig.3AandB)withlossonignition(L.O.I.)toestimateweathering.

Sample# Location Stratigraphicposition L.O.I

2 Harrara,100mnorthofthevillage DykewithintheFerrysch(Callovian–Oxfordian) 7.7

4 Zendoulapathabovethevillage(lavaﬂow) EndofLowerTithonianlimestones 3.6

5 BouHaddoudNappe,J.TaharBouZhaier Beckeri/Hybonotumzones(baseofLowerTithonian) 12.2

6 EastofKerkor,lavaﬂow LowerTithonian 7.0

8 OuedMarticha,Msilaarea LowerTithonian 18.5

9 EastofJ.Mguedrouz Tithonian-LowerBerriasian 14.1

Table2

LocationandpetrographiccharactersofthenineanalyzedsamplesfromtheBouAdelgabbromassif(seemapFig.6),withlossonignition(L.O.I)toestimateweathering.

SampleR1(troctolite)islocatedveryclosetoBA1.

Sample# Location Petrographictype L.O.I.

BA1 SofBouAdelsprings,rightbankofAzrouriver Troctolite 1.5

BA2 EofBouAdelsprings,rightbankofAzrouriver Ferrogabbro 0.6

BA4 SofBouAdelsprings,leftbankofAzrouriver Diorite(?)dyke 2.0

BA6 SEofBouAdelsprings,alongtheAzrouriver Troctolite 1.3

BA8 EofBouAdelsprings,rightbankofAzrouriver Leucocraticvein 2.0

BA9 SofBouAdelsprings,leftbankofAzrouriver Trondjhemitedyke 0.8

BA10 SEtipofthemassif,leftbankofAzrouriver Congelationmargin(?) 3.0

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Fig.7.BouAdelunitoutcropsandtypicalrockfacies.(A)ViewofthewesterntipofthegabbromassiffromthenorthernslopeoftheOuedAzrouvalley(seeFig.6forlocation).

(B)Freshoutcropsoflayeredgabbrosalongtheunpavedroadonthenorthernslopeofthevalley.(C)Trondjhemitedykesintheeasternpartofthemassif.(D)Pegmatitic trondjhemitepocket,about500meastof(B).(E)Freshferrogabbrofromthesameareaas(C).(F)Coarsegabbro-basaltbreccia(probableoceanicfaultscarpbreccia)atthe uppermostBouAdelspring(seephotoAforlocation).(G)Ophicalcite(infloatclosetoF).(H)Stratigraphiccontactbetweenvolcanicbrecciaandwhitemarble,justabovethe breccia(F)alongthepaththatclimbsonthesouthslopeofthevalley(seephotoA).(I)Limestoneswithvolcaniclasticsandstonelayerssourcedfrommaficrocks;thisfacies cropsoutalongthewesternslopeofthegabbromassif,southoftheBouAdelsprings.(J–K)TwofreshblocksinfloatintheOuedAzrouriverbedwith(J)Foliatedmarblewith thin,boudinagedvolcaniclasticlayersincludingalargedolerite/gabbropebble,and(K)foliated,impuremetamorphiclimestonewithminorfolds.

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Fig.8. GeologicalsketchmapofthenorthernKefelRharmassifbasedonthemap byVidal(1983a),modiﬁed.Heavyblacklinesareshallow-dippingtectoniccontacts (teethtowardupperunitwhenrecognized).

voltageof20kVandabeamcurrentof100nA;aPaPcorrection programwasusedtocorrectmatrixeffects.

5.2. Basaltsofthepaleomarginunits 5.2.1. Petrography

Theserockssufferedgenerallystrongalteration(spilitisation andp.p.weathering)andthinsectionexaminationdidnotprovide criticalmineralinformationwiththeexceptionofonekeydoleritic basaltsample(#6)fromtheeastofKerkor.Inthisdoleriticbasalt, interbeddedinLowerTithonianseries,examinationrevealsinthe plagioclaselattice:augite,pigeoniteandscarceolivine(Fig.10A).

Pigeonite was demixed into clinopyroxene and orthopyroxene arranged according toa typicalherringbone texture(Fig. 10B).

Matrixismainlyformedbylatticeofsubautomorphicplagioclase andrare(secondary?)quartzandTi–Feoxydes.Thepresenceof pigeonitewasusedbyKuno(1968)todeﬁnethe‘pigeoniticseries’, nowadaysknownastholeiiticseries.

5.2.2. Geochemistry

Lossonignition(L.O.I)ofbasalts(Table3)iscomprisedbetween 7and18.5%andrevealsastrongalteration(spilitisationandweath- ering).Majorelementanalyseswererecalculatedwithoutwater.

Unfortunately,thepartofthesample#6thatwasanalyzedwasnot thesamewherethethinsectionwasrealized.Inthesampling,there isnoanalysiscorrespondingtoanoriginalmagmaticcomposition.

Alargepartofthemajorelementswasenrichedordepleted.Among traceelements,thesumCr+Ni∼250ppmandtheratioZr/Hf=31 arecharacteristicofabasalticcomposition.Plotofanalysesonto aWinchesterandFloyd(1977)diagramfallsfranklyintheﬁeldof subalkalinebasalt(Fig.11).

Analyseswereplottedinamulti-elementsdiagramnormalized toprimitivemantle(SunandMcDonough,1989)anddisplaytwo maingroupsoneachsideofTa(Fig.12).TheﬁrstgroupfromRbto

Table3

AnalysesofbasaltsamplesfromtheMesorifpaleomarginunits(Table1andFig.3A andBforlocationandstratigraphiccontext).

2 4 5 6 8 9

SiO2 46.2 51.3 41.5 48.1 34.7 38.8

Al2O3 16.5 14.8 15 16.2 10.2 13.4

Fe2O3t 8.9 9.9 9.5 9.1 6.3 10.7

MgO 13.1 9.7 4.7 11 10.1 5.6

CaO 0.9 3.7 10.8 2.7 18 12.2

MnO 0.04 0.14 0.05 0.1 0.07 0.11

K2O 4.77 1.25 0.11 1.3 0.03 0.12

Na2O 0.4 4.9 4.9 3.9 1.3 4.3

TiO2 1.09 1 1.01 1.01 0.68 1.12

P2O5 0.13 0.11 0.12 0.11 0.08 0.13

L.O.I. 7.7 3.6 12.2 7 18.5 14.1

Total 99.73 100.4 99.89 100.52 99.96 100.58

SiO2 50.20 53.00 47.33 51.43 42.60 44.87

Al2O3 17.93 15.29 17.11 17.32 12.52 15.49

Fe2O3t 9.67 10.23 10.83 9.73 7.73 12.37

MgO 14.23 10.02 5.36 11.76 12.40 6.48

CaO 0.98 3.82 12.32 2.89 22.10 14.11

MnO 0.04 0.14 0.06 0.11 0.09 0.13

K2O 5.18 1.29 0.13 1.39 0.04 0.14

Na2O 0.43 5.06 5.59 4.17 1.60 4.97

TiO2 1.18 1.03 1.15 1.08 0.83 1.30

P2O5 0.14 0.11 0.14 0.12 0.10 0.15

100.00 100.00 100.00 100.00 100.00 100.00

U 0.5 0.3 0.4 0.4 0.7 1.2

Th 6.2 4.1 4.9 4.1 4.2 4.5

Ta 0.5 0.4 0.4 0.4 0.4 0.4

Nb 8.3 6.6 6.1 6.7 6.1 8.1

Hf 2.8 2.3 1.8 2.3 1.8 1.7

Zr 86 70 50 72 55 45

Cr 209 165 139 298 123 206

Co 36 41 33 37 26 47

Ni 72 78 67 85 53 83

Rb 50 29 2 14 0.9 0.9

Sr 41 131 129 74 86 160

Ba 239 98 42 117 9 36

Y 28 19 13 16 10 17

La 17 7 5.7 3.8 4 13.5

Ce 38 16 14 12 9.3 30

Pr 4.7 2.2 2 1.7 1.2 4.1

Nd 20 10 9.2 7.6 5.5 18

Sm 5 2.7 2.3 1.9 1.5 4.2

Eu 1.3 0.8 0.4 0.7 0.4 0.7

Gd 5.4 3.2 2.5 2.3 1.6 4.3

Tb 0.9 0.5 0.4 0.4 0.3 0.6

Dy 5.9 3.5 2.3 2.8 1.9 3.4

Ho 1.2 0.7 0.5 0.6 0.4 0.7

Er 3.1 2.1 1.5 2 1.3 1.7

Tm 0.4 0.3 0.2 0.3 0.2 0.2

Yb 2.3 1.9 1.1 2 1.3 1.2

Lu 0.3 0.3 0.2 0.3 0.2 0.2

Kshowsnormalizedvaluescomprised(foralargepart)between 20and100,butwithstrongvariationswhereasthesecondgroup fromTatoYbshowssmoothervariationsandnormalizedvalues thatremainequalorunder10.Curvesofanalyses#2,#8and#9 thatshownumerousandsharpvariationswillnotbeconsidered hereafter.

Intheﬁrstgroup,RbtoKelementspatternﬁtswithcontinen- tal(RudnickandGao,2003)crustorvolcanicarcs(Mauryetal., 1998),butstrongandantithetic(e.g.K),variationscouldberelated tospilitisationand/orweathering.Inthesecondgroupvariations arenotsosharpandREEpatterns(Fig.13)aresmoothedandtheir shaperevealsthatREEwerenotmobile.Thepossibleoriginofthese basaltswillbediscussedfurther.

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Fig.9.MetasedimentsoverlyingthegabbrosoftheKefelRharregion(Fig.3Bforlocation).(A)IntenselydeformedmarblesontopoftheDarBouAzamassif.Noticethe boudinagedchertnodules,thefragmentsofwhicharerotated.CoordinatesN34^◦3012,W04^◦1851.(B)Low-grademetasedimentarysuccessionontopofthemainKef elRhargabbromassif(detaillocationinFig.8).Theradiolarites(totalthicknessca.25m)arealternativelypalegreenorred.Theunderlyinggabbroisnotvisibleinthephoto.

(C)Closeviewofthin-bedded,pinkradiolarites.(D)Closeviewofanotherradiolariteoutcroprichinslumpfolds(markedwithwhitedashes)withsuperimposedtectonic structures.Thelattercomprise(i)S/Cstructuresassociatedwithapenetrativecleavage(S0-1,paralleltothestratiﬁcationplaneS0)intheclayeymatrixaroundthesiliceous hinges,and(ii)conjugateextensionalfaults.(Forinterpretationofthereferencestocolorinthisﬁgurelegend,thereaderisreferredtothewebversionofthearticle.)

5.3. BouAdelgabbromassif 5.3.1. Petrography

In thin section (Fig. 10D), the troctolite displays an ortho- to heteradcumulate texture. Grains of olivine are often grouped as clusters of about ten crystals and are surrounded by automorphic, weakly zoned plagioclase (labrador). Both olivine and plagioclase are embedded in large poikilitic diop- side clinopyroxene. Opaque mineral are mainly composed of ilmenite.

Chemicalanalyses(EPMA) ofolivine,clinopyroxeneandpla- gioclasearegiveninTable4.InthetroctoliteBA1,plagioclaseis composedofalargecore(An64)andsmallerrim(An60).Olivine compositionvariesfrom:Fo70±0.3toFo63±0.8and,insample R1,fromFo72±0.6toFo68±0.1.Thecompositionofclinopyrox- eneisratherconstant(Wo45.6,En42.9Fs11.4)inthediopsideﬁeld, andCr₂O₃contentcanreach1%.Ti-richbiotite(0.64<XMg<0.70) is scarce, its composition plots toward the phlogopite corner.

Ferrogabbrodisplaysorthocumulatetexture.Olivine(BA5:Fo 53.2±0.2;BA2:Fo53.2±0.24)istheearliestmineralsurrounded byautomorphicplagioclasecomposedofalargeunzonedcoreand

athinnerrim(e.g.BA5,core:An59–56–rimAn51orBA2:core An50–rimAn47).Theclinopyroxeneisnotpoikiliticandisin automorphiccrystals(Wo44,En40, Fs16).It isinsomeplace partly transformedinamphibole ofedenite composition(Leake etal.,1997).Ilmenite(dominantonmagnetite)isdispersedinthe matrix.Ti-richbiotite(0.56<X_Mg<0.60)surroundsmostfrequently ilmenite.

In spite oflack of textural evidencebecause of postsolidus low-temperaturealteration,thethreefollowingsampleswillbe furthertested ascandidates tolookfor a possibleinitialliquid ofthemaﬁccomplex:(i)BA4fromadykeonthewesternbank of the Oued Azrou, (ii) BA8 from a dyke in the center of the massif, and (iii) BA10 sampledat thetop of thegabbro at the basisofthevolcaniccover,whichcouldbeacongelationcumu- late.

To summarize, when magmatic texture is preserved, one can observe the crystallization of every type of gabbro is characterized by an early appearance of a medium An content plagioclase at liquidus, more or less in the same time as Fo rich olivine, which is a typical tholeiitic sequence of crystallization according to Kuno’s (1968) original deﬁni- tion. Experimental data in the system forsterite-fayalite show

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Fig.10.Thinsections(A)BasaltfromtheKerkorlavaﬂowwithlargecrystalofdemixedpigeonite;sampleBA6,Table2.(B)Closeviewoftheexsolutionlamellae.(C) TrondjhemitefromaBouAdeldyke;sampleBA9.(D)Troctolitewithcumulativeplagioclase,olivinegrainsandpoikilicclinopyroxene.

Fig.11.PlotofthecompositionsofUpperJurassicvolcanicrocksfromtheIzzarene windowandBouHaddoudnappe(circles)ontoWinchesterandFloyd(1977)dia- gram.PossibleinitialliquidoftheBouAdelcomplex(sampleBA10)isalsoplot (square).

Fig.12.Multi-elementsdiagramofUpperJurassicbasaltsfromtheIzzarenewindow andBouHaddoudnappe(seeTable1forlocation).E-MORBcomposition(dotted blackline)valuesandnormalizationtoprimitivemantleusingvaluesfromSunand McDonough(1989),continentalcrustcomposition(dottedredline)afterRudnick andGao(2003).(Forinterpretationofthereferencestocolorinthisﬁgurelegend, thereaderisreferredtothewebversionofthearticle.)

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M.Benzaggaghetal./JournalofGeodynamics77(2014)4–21 15 Table4

Chemicalanalyses(EPMA)ofolivine(4A),clinopyroxene(4B)andplagioclase(4C)fromBouAdelgabbrosamples(S.D.:standarddeviation).

Sample BA1 R1 BA5 BA2

Med.3anal. S.D. Med.8anal. S.D. Med.7anal. S.D. Med.3anal. S.D. Med.4anal. S.D. Med.3anal. S.D.

a:Olivines

SiO2 38.28 0.11 37.22 0.17 38.59 0.20 37.65 0.24 35.61 0.49 35.31 0.25

FeO 27.00 0.29 32.00 0.64 25.41 0.60 28.16 0.04 40.01 1.16 39.25 0.09

MnO 0.41 0.03 0.49 0.06 0.38 0.08 0.41 0.02 0.67 0.02 0.60 0.05

MgO 35.94 0.16 31.47 0.49 37.08 0.44 34.76 0.09 24.78 1.23 25.40 0.31

Total 101.62 0.17 101.18 101.46 100.98 101.062 100.56

Si 1.000 1.002 1.002 0.997 1.000 0.924

Fe²⁺ 0.590 0.721 0.552 0.624 0.940 0.924

Mn 0.090 0.010 0.008 0.009 0.016 0.014

Mg 1.400 1.263 1.436 1.373 1.037 1.066

Fo 70.04 0.27 63.33 0.76 71.93 0.68 68.44 0.07 52.04 0.13 53.19 0.24

Sample BA1 BA2 BA5

Med.3anal. S.D. Med.4anal. S.D. Med.3anal. S.D.

b:Clinopyroxenes

SiO2 50.92 0.24 53.01 0.52 51.48 0.35

Al2O3 3.74 0.18 1.16 0.82 2.50 0.20

FeO 6.77 0.05 9.45 0.74 8.75 0.64

MgO 14.69 0.05 14.12 0.35 14.03 0.02

CaO 21.71 0.12 22.36 0.51 22.36 0.25

Na2O 0.50 0.04 0.45 0.06 0.48 0.05

MnO 0.16 0.09 0.20 0.09 0.11 0.12

TiO2 1.59 0.05 0.65 0.46 1.26 0.22

Cr2O3 0.88 0.01 0.04 0.06 – –

Total 100.94 101.44 100.99

Si 1.871 1.956 1.906

AlIV 0.129 0.044 0.094

AlVI 0.033 0.006 0.015

Fe2+ 0.208 0.292 0271

Mg 0.804 0.777 0.775

Ca 0.854 0.884 0.887

Na 0.036 0.032 0.035

Mn 0.005 0.006 0.003

Ti 0.044 0.018 0.035

Cr 0.025 0.001 –

Wo 45.63 45.15 4579

En 42.97 39.69 39.98

Fs 11.37 15.20 14.17

Sample BA1 R1 BA5 BA2

C:Plagioclases

SiO2 52.23 52.60 51.62 52.42 52.92 54.35 55.13 56.26 56.00

Al2O3 30.38 29.35 30.62 30.36 29.36 28.97 28.46 27.50 27.91

CaO 13.34 12.80 13.93 12.82 12.72 12.02 11.05 10.00 10.71

K2O 0.13 0.20 0.12 0.25 0.25 0.18 0.26 0.33 0.31

Na2O 4.07 4.52 3.86 4.45 4.82 4.93 5.58 5.80 5.70

FeO 0.17 0.23 0.17 0.21 0.27 0.21 0.09 0.12 0.16

Total 100.31 99.71 100.32 100.52 100.33 100.66 100.58 100.00 100.78

Si 2.365 2.395 2.341 2.369 2.398 2.444 2.473 2.528 2.505

Al 1.621 1.575 1.637 1.617 1.568 1.535 1.504 1.457 1.471

Ca 0.647 0.624 0.677 0.621 0.618 0.579 0.531 0.481 0.513

K 0.007 0.012 0.007 0.014 0.014 0.010 0.015 0.019 0.017

Na 0.357 0.399 0.339 0.390 0.423 0.430 0.485 0.505 0.494

Fe²⁺ 0.006 0.009 0.007 0.008 0.010 0.008 0.003 0.005 0.006

Ab 35.29 38.56 33.18 38.06 40.12 42.18 47.06 50.24 48.21

An 63.99 60.30 66.16 60.55 58.52 56.83 51.48 47.88 50.09

Or 0.71 1.13 0.66 1.38 1.36 0.99 1.46 1.88 1.70

that mineral composition is not affected by pH2O varia- tion, whereas in the system albite-anorthite water controls the An content of the plagioclase. A plot of the Fo content of olivine against the An value of the plagioclase (after Smith et al., 1983) improves the discrimination between the calc-alkaline (“wet”) and the tholeiitic (“dry”) series (Fig.14).

5.3.2. Geochemistry

PlotoftheanalysesoftheBouAdelsamples(Table5)ontothe Miyashiro(1974)diagramfranklydiscriminatesBouAdelmaﬁcs inthetholeiiticﬁeld(Fig.15)inaccordancewithpetrographicand chemicalmineraldata.

RareEarthelement(REE)patterns(Fig.16)werenormalizedvs.

mantle(normalizingvaluesafterSunandMcDonough,1989).The