Petrology,
Geochemistry
Mafic
dikes
at
Kahel
Tabelbala
(Daoura,
Ougarta
Range,
south-western
Algeria):
New
insights
into
the
petrology,
geochemistry
and
mantle
source
characteristics
Abderrahmane
Mekkaoui
a,b,
Nace´ra
Remaci-Be´naouda
a,*
,
Khadidja
Graı¨ne-Tazerout
ca
De´partementdessciencesdelaTerre,Laboratoire«Ge´oressources,environnementetrisquesnaturels»,FSTU,Universite´ d’Oran2, MohamedBenAhmed,Oran,Algeria
b
De´partementdel’hydraulique,Universite´ deBe´char,Be´char,Algeria
cLaboratoiredeme´talloge´nie,USTHB,Alger,Algeria
1. Introduction
Maficdikeswarmsareacommonexpressionof mantle-derivedmagmagenerationincontinentalsetting charac-terizedbyextensionaltectonicregimesinpost-collisional or intraplate extensional settings (Rock, 1991). They resultedfromaconsiderableextensionofthecontinental lithosphere(Halls,1982;TarneyandWeaver,1987).They
canthusprovideimportantinformationforunderstanding notonlythemantlesourceofthemagmas,but alsothe tectonicevolutionoftheorogenicbelt(GorringandKay, 2001;Yangetal.,2007).
Mafic dikes, distributed in southwestern Algeria, outcrop in the Ougarta Range, the Tindouf basin, the Eglab,theRegganeandAbadlaregions.Theseoccurrences have been widely investigated, especially in Morocco (Bensalah et al., 2011; Bertrand and Westphal, 1977, ChoubertandFaure-Muret,1974;ElAoulietal.,2001;El Maidani et al., 2013; Hafid et al., 1998; Hollard, 1973; Leblanc,1973;MahmoudiandBertrand,2007;Veratietal.,
ARTICLE INFO Articlehistory: Received21April2017
Acceptedafterrevision27June2017 Availableonline25August2017 HandledbyMarcChaussidon Keywords: KahalTabelbala OugartaRange Algeria Maficdikes Petrology-Geochemistry Sr–Ndisotopes Mantleplume ABSTRACT
Newpetrological,geochemicalandSr–NdisotopicdataoftheLate TriassicandEarly
Jurassic Kahel Tabelbala (KT) mafic dikes (south-western Algeria) offer a unique
opportunity to examine the nature of their mantle sources and their geodynamic
significance.AnalkalinepotassicGroup1ofbasalticdikesdisplayingrelativelyhighMgO,
TiO2,CrandNi,La/YbN15,coupledwithlow87Sr/86Sri0.7037andrelativelyhigheNd(t)
+3,indicatesminor olivineandclinopyroxenefractionationandtheexistence ofa
depletedmantleOIBsource.Theirparentalmagmawasgeneratedfrompartialmeltingin
thegarnet–lherzolitestabilityfield.AtholeiiticGroup2ofdoleriticdikesdisplayinglow
MgO,CrandNicontents,La/YbN5,positiveBa,SrandPbanomalies,theabsenceofa
negativeNbanomalycoupledwithmoderate87Sr/86Sri
0.7044andloweNd(t)0
(BSE-like), indicatesa contaminationof amantle-derivedmagmathatexperiencedcrystal
fractionationofplagioclaseandclinopyroxene.Thissecondgroup,similartothelow-Ti
tholeiiticbasaltsoftheCentralAtlanticMagmaticProvince(CAMP),wasderivedfrom
partialmeltingintheperidotitesourcewithinthespinelstabilityfield.LowerMesozoic
continentalrifting could have been initiatedbya heterogeneousmantle plume that
suppliedsourcecomponentsbeneathDaoura,intheOugartaRange.
C 2017Acade´miedessciences.PublishedbyElsevierMassonSAS.Allrightsreserved.
* Correspondingauthor.
E-mailaddress:remaci.nacera@lycos.com(N.Remaci-Be´naouda).
ContentslistsavailableatScienceDirect
Comptes
Rendus
Geoscience
ww w . sci e nc e di r e ct . com
http://dx.doi.org/10.1016/j.crte.2017.06.003
2007).IncontrastwithMorocco,afewstudieshavebeen carried outinsouthern andsouthwesternAlgeria ( Bois-sie`re, 1971; Chabou et al., 2007; Chabou et al., 2010; Conrad,1972;Djellitetal.,2006;Fabre,1976;Mekkaoui, 2015;MekkaouiandRemaci-Be´naouda,2014;Menchikoff, 1930;Sebaietal.,1991).
Thispaperpresentsanewstudyofthepetrologyand geochemistry of these Mesozoic mafic dikes in the Ougarta Range, southwestern Algeria. New major and traceelementsanalysesaswellasSr–Ndisotopicdataof mafic-dike rocks were collected from representative sectionsinKahelTabelbala(KT).Thesedataallowusto identify the magma types, investigate their mantle sourcesandpetrogenesis,andunderstandtheevolution oftheMesozoiclithosphericmantlebeneathKT,Ougarta Range.
2. Geologicalsetting
TheOugartaRangecorrespondstoanimposingseriesof reliefsarisingwithintheSaharanplatform(Fig.1a).These reliefs are confined to the Saoura and Daoura basins (Menchikoff, 1930). The KT, which is a major orogenic entity within the Daoura, is aligned along a Ougartian directionfromN1308toN1408.Itisboundedtothenorth bythe‘‘HamadaofMande’’,totheeastbyErgerRaoui,and tothesouthbytheErgChech,andtothewest,bytheIguidi andElAaˆtchaneergs(Fig.1aandb).TheOugartaRange belongingtothenorthernboundaryoftheWestAfrican Craton(Fig.1c)isinvolvedintheLatePan-Africanhistory (Ennihand Lie´geois,2001;Kurekand Preidl,1987). The continentalcollisionbetweenGondwanaandLaurasiain theLateCarboniferouswasoriginallyamajoruprisingand
Fig.1.Mapofthestudiedarea,showing(A)thelocationofOugartaRange(Michard,1976),asimplifiedgeologicalmapofthestudiedarea,KahelTabelbala (B),aWACsketchmapplacingtheOugartarangeintoitsregionalcontext(C)(modifiedafterFabre,1976;EnnihandLie´geois,2001).
overlaps in the northwestern partof the North African platformbyfoldingandinversioninassociatedintraplate areas.TheOugarta Rangewasoriginatedin asubsiding area that developed during the Cambrian along an old oceanic suture bordering the West African Craton. The Central Atlantic Oceanic Opening during the Trias–Lias, andthesimultaneousseparationoftheApulianterraneof northeasternAfrica,followedamajorextension accompa-niedbyvolcanicemissionsthroughouttheNorthAfrican platform(Bensalah etal.,2011; Bertrand,1991;Chabou et al., 2007; Chabou et al., 2010; Marzoli et al., 1999; Meddah et al., 2007; Mekkaoui, 2015; Mekkaoui and Remaci-Be´naouda,2014;Sebaietal.,1991; Veratietal., 2007). A second major extensive Mesozoic stage took placeduringtheCretaceous,followedby theopeningof thesouthequatorialAtlanticOcean,whichgeneratedthe development of a series of aborted rift systems in the northernandcentralAfricanplatform(Gomezetal.,2000; Pique´ etal.,1998).Theriftingofthenorthernpartofthe North Atlantic during the Cretaceous was originally a suddenchangeinthemovementsoftheEuropeanplate. IntrusionsofmaficdikesofMesozoicagealongtheKT structure are widespread, occurring as NW–SE-trending swarms,orientationsdeterminedalsoinMorocco( Bensa-lahetal.,2011;Bertrand,1991;MahmoudiandBertrand, 2007; Verati et al., 2007). These rocks cut across the Neoproterozoic and Cambrian-Ordovician sedimentary stacks (Bouima and Mekkaoui, 2003; Chikhaoui, 1974; ColumbandDonzeau,1974;Fabre,1976,2005;Mekkaoui, 2015;Ne´djari,2007). Twoareasare brieflydescribedin this structure (Mekkaoui, 2015; Mekkaouiand Remaci-Be´naouda, 2014): (i) at Guelb Berrezouk, the volcanic occurrencesalsoshowaremarkablegrowth,spreadingas dikesandsills(FigureislistedinelectronicannexasA1a). TheyarereferredtoastheJebelGuelbBerrezoukgroup (Group1).AtthefootofJebel,a0.5- to1-m-thickdike, visible over 150m, punctuates the Ougartian fault separating the Neoproterozoic from the Cambrian. The topofthisJebeliscrownedbytwosills,from0.5mto0.8m inthickness.Inthereversesideofthispeak,asystemof dikesandsillsoutcropsintheCambriansandstones.The largestdikeisupto5minthickness.Theserockscross-cut theCambrianand, in somelocalities,theOrdovicianas well;(ii)inthesoutheasternpartofthepericlinalclosure, itisaffectedbyanaxialdoleriticdikeaccompanied,from either side, by other satellite dikes (Figure is listed in electronicannexasA1b).Thisaxialdikeisvisibleovera distanceoffewhundredmetersanditswidthcanexceed 150m.Thefieldobservationsreveala medium-to-coarse-grainedcoreunderlined,onbothsides,byblackishborders with a fine-grained texture at the close contact with theCambrian. Thisdike and itssatellites constitutethe periclinalclosuregroup(Group2).
3. Samplingandanalyticaltechniques
Electron microprobe analyses of minerals were obtainedusingCAMECASX100(CNRS-UMR6524,Blaise PascalUniversity,Clermont-Ferrand,France).The accel-eratingvoltagewas15kVandtheelectronbeamcurrent
15nA,withabeamdiameterof1mm.Thecountingtime was10s.
Asuiteof11freshsampleswereselectedformajorand trace element analysis, using X-ray fluorescence (XFR spectrometerPhillipsPW1404)attheUniversityofLyon. Theprecisionwas1–2%formajorelementsand10–15%for traceelements.Sevenofthesesampleswereanalyzedfor rare-earthelements(REE)usinginductivelycoupledplasma mass spectrometry (ICP–MS) (Jobin YvonJY38 P)at the ‘‘E´coledesmines’’,Saint-E´tienne,France.Theanalyseswere realizedonsolidsolutions,afterHFdissolutionto1308C, evaporationofthefluorideswasresumedusingHCl2N.
Sr and Nd isotopes were measured by TIMS at the GET(ObservatoireMidi-Pyre´ne´es,Toulouse-3University, France).100mgofwhole-rockpowderwereweighedina Teflonbeakeranddissolvedina2:1HF/HNO3mixture.Nd
andSrwereextractedfromthematrixusingacombination ofSr-Spec,Thru-specandLn-Specresins.Anequivalentof 500ngSrand150ngNdwererunonaMAT261Finnigan massspectrometer.NBS987andLaJollaisotopicstandards wereregularlyrunduring themeasurements.The stan-dardreproducibilityvaluesare0.51085518(n=50)for LaJollaand0.71025035(n=70)forNBS987.Typicalblanks were30pgforNdand150pgforSr.
4. Results
4.1. Petrography–Mineralogy
Distinct petrographic features were noted for two Groups. The Guelb Berrezouk Group-1 volcanic rocks outcropasbasalticdikes.Theyhaveamainlyporphyritic texture, and contain various amounts of olivine and clinopyroxene phenocrysts (Fig. 2a and b). Plagioclase phenocrystsaregenerallyabsent.Thegroundmassconsists of plagioclase, clinopyroxene, olivine and oxides. In contrast,thepericlinalclosureevent(axialdikes)Group 2iscomprisedofsubvolcanicrocks.Theyaremassiveand aremedium-grained(inthecore)tofinegrained(atthe rim).Theydisplayinter-granularandsub-ophitictextures typicalofdoleriteswitheuhedralplagioclase,interstitial clinopyroxene,andFe–Tioxides(Fig.2candd).Accessory quartzandfeldsparoccurincore-facies.
Representativeelectron microprobeanalysesof mine-ralsarelistedassupplementarydatainTableA2.InGroup 1,olivinecompositionsvaryfromphenocrysts(Fo90–83)to
microphenocrysts (Fo80–77). Olivine phenocrysts display
normal compositional zoning with Fo90 core to Fo83–78
rim. In the core of the olivine phenocrysts, the CaO contents range between 0.05 and 0.08 wt%. In micro-phenocrystal olivine, the CaOcontentsarehigh (0.25 to 0.45wt%).Clinopyroxenedisplaysadiopsidiccomposition (Wo47.6–51.6En40.8–30.9Fs11.6–17.4)(Fig.3a).Theyarerichin
Al2O3(upto10wt%)andTiO2(2.2to5.6wt%).Theamounts
of Ca+Na (0.93 to 0.96) are typical of alkali basalts according to Leterrieret al. (1982) (Fig.3b).Plagioclase phenocrysts were absent, but in the groundmass they display labradorite to andesine (An68.3–46.5 Ab30–46.7
Or1.7–6.9) compositions. Oxides were ulvospinel (Ti6.5–7.3
Fig.2.RepresentativephotomicrographsofpetrographicfeaturesofKTmaficdikes:aandb:theGuelbBerrezoukGroup1withporphyritictextureand olivineandclinopyroxenephenocrysts;candd:thepericlinalclosureevent(axialdike)Group2withdoleriticcompositionwithmedium-grained(core)to fine-grained(rim)respectively.Ol:olivine,Cpx:clinopyroxene,Pl:plagioclase,Op:opaque.
Fig.3. ClassificationdiagramsofmineralsofKTdikes;a:nomenclaturediagramofMorimoto(1988);b:distributionofclinopyroxenesinthediagramof
In Group 2, olivine was absent, but plagioclase and clinopyroxenewerethepredominantminerals;the chemi-cal compositions differ from the core to the rim axial dike.Attherim,plagioclasedisplayedbytownite–andesine compositions(An71–46.5Ab28.7–51.2.Or0.4–1.3)and
clinopyro-xene crystals are augite (Wo33–41.2En45.5–42.1Fs22.5–16.6).
In the core of the dike, plagioclase is less calcic (labradorite–oligoclase: An59.2–27.3 Ab40–69.5.Or0.7–3.2)
with augite and Ca-poor clinopyroxene (Wo10.2–13.8En65.4–33.8Fs24.4–52.4) (Fig. 3a). Oxides are
ilmenite(Ti0.91–0.98Fe2+0.83–0.92Fe3+0.16–0.04Mn0.08–0.06O3).
4.2. Geochemistry:Majorelements,traceelementsand Sr–Ndisotopes
Major,traceelement,Rb,Sr,SmandNdconcentrations, measuredandage-corrected143Nd/144Nd,87Sr/86Srratios
ofKTdikesarelistedinelectronicannexrespectivelyas TablesA3,A4andA5.
Mostof the analyzedsamples werepetrographically freshandshowlittlealteration.Thelossonignition(LOI) valuesrangefrom1.41to3.77wt%.Thesamplesexhibit significant variations in major and trace elements and Sr–Ndisotopiccompositions.
TheGroup-1dikesshowbasalticcompositionswitha restrictedrangeofSiO2 (43.89–44.72wt%)and potassic–
alkaline affinity with K2O/Na2O>1 and total alkalis
(K2O+Na2O)contentsof3.56–4.26wt%.Inaddition,they
display relatively high MgO (8.58–10.27wt%), Mg# (100Mg/Mg +Fe2+=59–63), TiO
2 (2.48–2.85 wt%), CaO
(9.62–10.80 wt%), Cr (290–396.9ppm), and Ni (209.4-283.8ppm). In terms of CIPW normative compositions, theserocks areundersaturated, withupto1.63 wt%of nephelineand22.58wt%ofolivine.Positivecorrelations ofMgOversusFe2O3t,CaO,NiandCr(Fig.4b,c,gandh)
and negativecorrelations ofMgO versus Al2O3,TiO2,K2O
andSrareobserved(Fig.4a,d,eandf).Theyarelightrare earthelement(LREE)enrichedwithLa/YbNrangingfrom
10.2to15.4andLREEcontentsupto122timeschondrite, withnoobviousEuanomaly(Eu/Eu*=0.91–1.06)(Fig.5a). Inprimitive-mantle(PM)normalizedmulti-element pat-terns,theyareenrichedinlargeionlithophilouselements (LILE),withpositive BaandSranomalies and high-field strengthelements(HFSE)withverydiscretepositiveNb andZranomalies(Fig.5c).
The initial isotopic ratios were all corrected to 2048Ma(Group1)and1835Ma(Group2)basedon Rb–Sr whole-rocks isochrons (Mekkaoui, 2015; Mekkaoui andRemaci-Be´naouda,2014).
Group-1 maficdikes display significantvariations in
87Sr/86Sr
i(0,7037to0,7069)andrelativelyhomogeneous
(
e
Nd)204Mavalues(+2.99to+2.60);theyareconsistentwithasourcemainlycomposedofdepletedmantle.
Incontrast,theGroup-2dikesaremafictointermediate in composition (SiO2 50.91–56.17 wt%) and exhibit a
continental tholeiitic affinity with low TiO2 (0.81–1.14
wt%),totalFe2O3(8.20–10.04wt%),P2O5(0.09–0.16wt%),
Zr(upto73.4ppm),andNb(upto15.4ppm)similartothe caseoflow-Titholeiites(Bellienietal.,1990;Cox,1983;De Minetal., 2003;Fodor,1987; Fodoretal., 1990;Merle etal.,2011),andhighabundancesinAl2O3(14.26–17.07
wt%) and CaO(7.80–10.65 wt%).They alsocontain low MgO(4.86–6.66wt%),Mg#(50–62),Cr(22–149ppm)and Ni(61.6–105.1ppm).IntermofCIPWnormative compo-sitions,themostbasicrockshaveamountsofquartz(from 1.55to3.48wt%),andhypersthene(20.12to23.04wt%). PositivecorrelationsofMgOversusAl2O3,CaO,K2O,Sr,Ni
andCr(Fig.4a,c,e,f,gandh),andnegativecorrelations betweenMgOversusFe2O3,andTiO2areobserved(Fig.4b
and d). All the analyzed samples show a moderate enrichment in LREEwith La/YbNvalues of 4.9–5.5, and
LREEcontentsupto40timesthoseofchondritewithno significantEuanomalies(Eu/Eu*=0.99–1.12)(Fig.5b).In thePM-normalizedtraceelementdiagram(Fig.5d),these doleritesalsodisplayamoderateenrichmentinLILEwith distinctive positive Ba, Pb and Sr anomalies and the absenceof anegativeNb anomaly.Theyhaverelatively homogeneous(
e
Nd)183Mavalues(+0.19to–0.08)comparedwith the significant variations in 87Sr/86Sr
i (0.7044–
0.7083),oftenobservedincontinentaltholeiites(Bertrand, 1991; Lightfootand Hawkesworth, 1988; Philpotts and Asher,1993;Veratietal.,2007).
5. Discussion
Thechemicalvariationsofbasalticlavasincontinental settingsaregenerallycontrolled bymantle temperature, source composition, degree of partial melting and pro-cessessuchascrystalfractionationand crustal contami-nation (McKenzie and Bickle, 1988). While Cox (1983), Hergtetal.(1991),TurnerandHawkesworth(1995)favor amodelwithdifferentmantlesourcesforthetwomagma types,Fodor(1987) andArndtet al.(1993) supportthe consequenceofdistinctmeltingconditionsfora homoge-neousmantlesourcecoupledwithcrustalcontamination. 5.1. Fractionalcrystallization
AccordingtoDesmursetal.(2002)andRe´villonetal. (2002),thecriteriaofprimarymantlemeltswithMgO>8% compositions,andolivinewithFo91–88,themostprimitive
Group-1 rocks seem likely tohave resultedfrom a low degreeofmagmafractionation,aswitnessedbyhighMgO levels (10.27–9.40ppm), Mg# (63–61), and compatible elements likeNiandCr, withcontentsup284ppm and 397ppm,respectively.Likewise,olivinephenocrystsfrom the studied rocks are confined to the range Fo90–83
(Table A2). In the core of the olivine phenocrysts, the CaO contents range between 0.05 and 0.08 wt%, in a fashionsimilartothoseofmantleolivine,whichgenerally display<0.1 wt% CaO (Ma et al., 2011; Thompson and Gibson, 2000). In microphenocrystal olivine, the CaO contents are high (0.25 to 0.45 wt%). These olivine phenocrystcompositionssuggestthattheywereprobably intratelluricmineralsthatcrystallizedearlierfromamore primitive magma and were subsequently incorporated intoamoreevolvedmagma,ratherthanbeingxenocrysts of lithospheric mantle origin.These Group-1 dikesmay have experiencedfractionationofolivineand clinopyro-xenefromtheirparentalmagma.Thisissupportedbythe positivecorrelationbetweenMgOandFe2O3,CaO,Niand
presence of olivine and clinopyroxene as thedominant phenocrysts in thesebasaltic rocks. No significant frac-tionationofplagioclaseinthemagmaaswitnessedbythe negativecorrelationbetweenMgO,Al2O3andSr(Fig.4a
and f) and the absence of plagioclase phenocrysts and
of any negative Eu anomaly. The negative correlation between MgO and TiO2 (Fig. 4d) contents indicates
insignificantfractionationofFe–Tioxides.
Themainfeatures ofGroup-2dikesindicatethatthe most mafic dolerites do not represent primary mantle
melts, as judged from their low MgO (<7 wt%), Mg# (62), Ni and Cr contents (105 and 149ppm respectively).Thissuggeststhattheymayhaveundergone significantamountsoffractionalcrystallizationfromtheir parentalbasalticmagma.Thesamplefeaturesare consis-tentwithextensiveplagioclaseandclinopyroxenecrystals supported by positive correlations between MgO and Al2O3,CaO,K2O,SrandCr(Fig.4a,c,e,fandh).Likewise,a
positiveSranomalyand adistinctlackof anegativeEu anomalyargueagainstsignificantplagioclasefractionation (BogaardandWo¨rner,2003).
5.2. Crustalcontamination
Since mafic dikes are emplaced into a continental environment,thereisgeneralagreementthatthesemafic mantle-derived magmas experienced some degree of crustal contamination during ascent and/or residence withintheir high-levelcrustalmagma chambers(Mohr, 1987). Furthermore, the ratios of trace elements in the contrastingmantleandcrustalrocksmayhelptoevaluate thepossibleinfluenceofcontamination. Thecorrelation between Sr–Nd isotopic data and some trace element ratios provide evidence of the involvement of various
mantleandcrustalcomponentsinthepetrogenesisofthe KT rocks. For example, the incompatible trace element ratios, forCe/Pband Ba/Nbinthecontinentalcrust,are approximately3.9and57respectively(RudnickandGao, 2003).
Inthe-Group1dikes,thegeochemicalcharacteristics, includingnodepletioninNb,lowtomoderateSrisotopic composition (0,7037 to0,7069) and relativelyhigh
e
Ndt(+2.99 to +2.60) do not support significant crustal contaminationduringmagmatransportandemplacement. In addition,thesame incompatibletraceelement ratios inthemostmaficrocks(Ce/Pb12andBa/Nb9)are different from those for continental crust, approaching thetraceelementconcentrationsoftypicalmantle-derived rocks likeOIB (Ce/Pb 25 and Ba/Nb 7.3) (Sun and McDonough,1989).
In contrast, Group-2 dikes with large positive Pb anomalies,theirelongatetrenddisplacedfromthemantle array at relatively constant
e
Nd (0.19 to–0.08) in thedirection ofincreasing Sri (0.7044to 0.7083), and their
reducedCe/Pbratios(4.37–3.32)andelevatedBa/Nbratios (20.98–118.23)indicateaccordinglytheinvolvementofa crustal component in the petrogenesis of the basaltic magmas.
Fig.5.Chondrite-normalizedrareearthelementpatterns(aandb)andprimitive-mantle-normalizeddiagrams(candd)fortheKTmaficdikes.The normalizedvaluesarefromBoynton(1985)andSunandMcDonough(1989),respectively.
5.3. Mantlesourcesignatures
Finally,themajorelementtrends,theREEpatternsand mostconclusivelytheinitialSrandNdisotope composi-tionsoftheGroup1andGroup2maficdikesuiteintheKT, preclude derivation by crystallization from a common parentalmagma.Theevidenceabovesuggeststhatthese twodikegroupscouldrepresentmeltsofadistinctmantle sourcedbyvariouspetrogeneticprocesses.
In general, low La/Ybratios reflect a melting regime dominatedbyrelativelylargemeltfractions,and/orwith spinelasthepredominantresidualphase(theyindicatea thinlithosphere),whereashighLa/Ybratioscorrespondto smallermeltfractionsand/orgarnetcontrol(theysuggest athicklithosphere).
In the alkaline Group-1 basalts, the relatively high La/YbNratios (10.24–15.42)suggest thattheymayhave
formedbyrelativelylowdegreesofmeltingofa mantle sourceinthegarnetstabilityfield.
In contrast, in tholeiitic Group-2 dolerites, the REE chondritenormalizedvaluesarehomogeneousandpoorly fractionated (La/YbN=4.86–5.48); hence the parental
magmasmayhavebeenderivedfrompartialmantlemelts withinthespinelstabilityfield.
Furthermore,theheterogeneouscompositionofthese LateTriasandEarlyJurassicmaficdikesareconfirmedby the most radiogenic compositions. This heterogeneity allows one to support the existence of two Groups: (i) an isotopically depleted reservoir (originated most likely fromanOIB-likemantle)with87Sr/86Sr
i(0,7037)
and(
e
Nd)204Mavalues(3);and(ii)agroup,closetotheBSE isotopicvalues, with87Sr/86Sr
i(0.7044–0.7083) and
(
e
Nd)183Mavalues(0),thencontaminated.Theisotopicsignature of Group-2 dikes demonstrates the close compositionalsimilaritywiththelow-Titholeiiticbasalts of the Central Atlantic Magmatic Province (CAMP) (Bertrand, 1991; Chabou et al., 2010; Deckart et al., 2005;DeMinetal.,2003;Marzolietal.,1999;Mekkaoui, 2015;Merleetal.,2011;Sebaietal.,1991;Veratietal., 2005,2007).
InKT,lithosphericriftingwasprobablyinvolvedduring twospecificmafic-igneousepisodes.Couldthis continen-talriftinghavebeeninitiatedbyaplumeheadimpacting beneath the lithosphere? Magmatism may have been inducedeitherbytheimpactoftheplumewiththebaseof thecontinentallithosphere(Courtillotetal.,1999;Wilson, 1997) or by heat incubation under a thick continental lithosphere and/oredge-drivenconvectiongenerated by thickness contrasts of different lithospheric domains (Colticeetal.,2007;DeMinetal.,2003;Merleetal.,2011). Moremantlesourcecharacteristics,asmantle compo-nents can be inferred from systematic element ratios. UsingbasalticcompositionsfromtheIcelandplume,Fitton etal.(1997)suggestthatadeepdepletedmantlesource can be distinguished from the shallow normal MORB (N-MORB)sourceinplume-relatedbasaltsusingZr/Yand Nb/Yratios.InFig.6,the
D
Nblineseparatingplumefrom non-plume basaltic sources seems to provide good discrimination.TheKTgroupsdefineapopulationabove theD
Nb line inthemantleplumefield, fortheoceanic island(Alkaline Group1)andtheoceanicplateaubasalt(TholeiiticGroup2)fieldsdescribedbyCondie(2005).This is consistent with their geochemical composition, and theisotopicradiogenicisotopicdiversityofthetwoGroups presented in this paper. It confirms that their basaltic magmas could have resulted from a heterogeneous mantle-plume source. Therefore, we cannot exclude an evolution of mantle-plume source and various partial meltingratesfromgarnet-bearingperidotitestabilityfield todepletedsuperficialspinelbearingperidotitestability fieldand contaminationinthecontinentalcrust.Sucha mantleplumecouldhavesuppliedsourcecomponentsto themelting zonebeneaththeKT.Moreover,it iswidely acceptedthatplume-relatedmelts(i.e.OIB)donotshow anynegativeNborpositivePbanomalies(Weaver,1991); this is well demonstrated, especially by the chemical characteristicsoftheGroup-1dikes.
6. Conclusions
DuringMesozoictimes,suchas theLateTriassicand EarlyJurassic,twoepisodesofmaficmagmatismoccurred sporadicallyin theKT,OugartaRange.Thepetrographic evidence,aswellaschemistrymineral,geochemicaland Sr–Ndisotopiccompositions,havedeterminedclearlytwo groups of mafic dikes that cannot be simply explained bycrystallizationfromacommonparentalmagma.They mayhavebeenderivedfromtwodifferentmantlesources, by various melting conditions, and were subjected to distinctdifferentiationandcontaminationprocesses.
An alkaline potassic Group 1 of basaltic dikes with olivine (Fo90–83), diopside, plagioclase and ulvospinel
displays relatively high MgO, Mg#, TiO2, Cr, and Ni
contents indicative ofa derivation frommantle-derived meltswithminorolivineandclinopyroxenefractionation. The geochemical and isotopic data on these rocks, includingrelativelyhighLREEwithLa/YbN15,suggest
no obvious Eu anomaly, but discrete positive Nb and Zr anomalies, withlow 87Sr/86Sri ratios ( 0.7037) and
relativelyhigh
e
Ndi( +3), indicating theexistenceofadepleted reservoir (OIB-like), and preclude a significant contributionof thecrustalcomponent.We proposethat
Fig.6.SamplesofKTmaficdikesplottedinalog–logdiagramofNb/Y versus Zr/Y after Fitton et al. (1997). DEP: deep depleted mantle, PM:primitivemantle,DM:depletedmantle,EN:enrichedcomponent, UC:uppercontinentalcrust,EM1andEM2:enrichedmantlesources, HIMU:highm(U/Pb).
theparental magmas were derived by a low degreeof partialmeltingofanasthenosphericmantlesourcewithin thegarnetstabilityfield.
Tholeiitic Group-2 dikes contain plagioclase, augite, pigeonite,and ilmenite-bearingdolerites,and their geo-chemistrydemonstrates low MgO,Mg#,Crand Ni con-tents,suggestingthattheymayhaveundergonesignificant crystalfractionationofplagioclaseandclinopyroxene.The geochemical and isotopic data in the most primitive compositions of Group-2samplesdisplay a low enrich-ment in LREE, with La/YbN 5 and no obvious Eu
anomalies, but display distinctly positive Ba, Srand Pb contents,anddemonstratetheabsenceofanynegativeNb anomaly. The large positive Pb anomalies, moderate
87Sr/86Sr
i(0.7044),relativelylow
e
Ndt(0), andtheirreduced Ce/Pb, and elevated Ba/Nb ratios, support an evolved mantle-derived magma with significant crustal contamination during transport and emplacement. The parentalmagma mayhave been produced by thelarge degreeofpartialmeltingofaperidotitewithinthespinel stabilityfield. Theisotopicsignaturewouldsuggest also a mantle source (BSE-like), then contaminated for this group,withclosecompositionalsimilaritywithinthelarge igneousprovinceoflow-TitholeiiticbasaltsintheCentral AtlanticMagmaticProvince(CAMP).
Itisthereforepertinenttonotethatlithosphericrifting wasinvolvedduringtheperiodofKTbasicmagmatism.It wasmostlikelyinitiatedbyarisingheterogeneousmantle plume impacting the fractured lithosphere below. This plume-head couldhave supplied source componentsto the melting zone beneath Daoura, part of the Ougarta Range.
Acknowledgements
WearegratefultoJ.-Y.CottinandD.Gasquetfortheir constructivereviews, andtoM.Chaussidonfor efficient editorialadvicesandpreciouscomments.WethankalsoP. BowdenwhoimprovedtheEnglishstyle.Thisresearchis supported by the French-Algerian cooperation program 14MDU926.
AppendixA. Supplementarydata
Supplementaryfigureanddataassociatedwiththisarticle canbefound intheonlineversionathttp://dx.doi.org/10. 1016/j.crte.2017.06.003.
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