Review: The 2-Ga Eburnean Orogeny in Gabon and the opening of the Francevillian intracratonic basins: A review

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Review: The 2-Ga Eburnean Orogeny in Gabon and the

opening of the Francevillian intracratonic basins: A

review

Francis Weber, François Gauthier-Lafaye, Hubert Whitechurch, Marc Ulrich,

Abderrazak El Albani

To cite this version:

Francis Weber, François Gauthier-Lafaye, Hubert Whitechurch, Marc Ulrich, Abderrazak El Albani.

Review: The 2-Ga Eburnean Orogeny in Gabon and the opening of the Francevillian

intracra-tonic basins: A review. Comptes Rendus Géoscience, Elsevier Masson, 2016, 348, pp.572 - 586.

�10.1016/j.crte.2016.07.003�. �hal-01402337�

Review

paper

The

2-Ga

Eburnean

Orogeny

in

Gabon

and

the

opening

of

the

Francevillian

intracratonic

basins:

A

review

Francis

Weber

*

,

Franc¸ois

Gauthier-Lafaye

,

Hubert

Whitechurch

,

Marc

Ulrich

,

Abderrazak

El

Albani

a

LHYGES–CNRS,3,rueduDoernel,67000Strasbourg,France b

LHYGES–CNRS,14,rueduPetit-Pre´,67800Hoenheim,France c

IPGS,universite´ deStrasbourg,1,rueBlessig,67084Strasbourgcedex,France d

InstitutIC2MP,UMR7285CNRS-INSU,universite´ dePoitiers,86000Poitiers,France

1. Introduction

LateArcheanandPaleoproterozoictimesareregarded asaperiodoftransitionbetweenArcheanandPhanerozoic tectonicstyles(Cagnardetal.,2011;Cawoodetal.,2006; Condie,1986,2005;Windley,1995;Zhaoetal.,2002).The Archeanstyleincludesaccretionwithkomatiiteplumes, short-term subduction within a hotter mantle and tonalite–trondhjemite–granodiorite intrusions resulting frompartialfusionofthedown-goingslabintotheupper plate(Gapaisetal.,2014;MoyenandHunen,2012).The

EarlyProterozoic(2.5–1.8Ma)orogenicdomainsinmany partoftheworldexhibitafold-thrusttectonicstylewith ophiolitesandHPmetamorphismaseclogitescomparable to Phanerozoic mountain belts, juxtaposed with large igneouslopolithintrusionsknownintheArcheancratons (Condie,2005;Windley,1995).Examplesofsuchmodern orogenyinthePaleoproterozoicaredescribedinFinland (Korja andHeikkinen, 2005),Canada(Cook etal., 1999; Whiteetal.,2002),WesternAustralia(CawoodandTyler, 2004), North China cratonand the Proterozoic orogens surrounding theTanzanian craton (Mo¨ller et al., 1995), Arabian-Nubian Shield(Kro¨ner,1985)and southwestern USA(Condie,1986).

TheEburneanorogenyinwesternAfrica(Gabon,Congo, Cameroon)isconsideredtobeafold-thrustbelt,strictly comparabletothePhanerozoicbeltgovernedbymodern Plate Tectonics(Ledruetal.,1994; Lerouge etal.,2006; Thie´blemontetal.,2009a).Indeed,theOgooue´ complexin A R T I C L E I N F O

Articlehistory: Received28April2016

Acceptedafterrevision10July2016 Availableonline5November2016

HandledbyIsabelleManighetti

Keywords: Proterozoic Eburneanorogeny Francevillian Gabon Platetectonic Intracratonicbasins A B S T R A C T

TheEburneanorogenyinAfricaresultsfromthecontinentalcollisionbetweentheSan Francisco and Congoliancratons. The different stages of this orogeny in Gabon are described fromthe initial rifting of the Archeancontinent withthe crustal melting producingpre-Eburneanmigmatitesat2450Ma,theopeningofanocean.Theeastward subductionoftheoceaniclithosphereproducedeclogitesat2120Maandintrusionof calc-alkaline plutons between 2083 and 2040 Ma. The blocking of the subduction by continentalcollision in thesouthinduced strike-slip motion andthe openingofthe intracratonicFrancevillian pull-apartbasins.ThethrustingoftheunitsoftheOgooue´ complexislinkedtotherapidupliftoftheorogenicroot(slabbreak-off?)andissealedby theintrusionoftheLecoue´ graniteat2009Ma.Ourinterpretationlinksthemain tectono-metamorphicphasesregisteredinthemobilezoneofOgooue´ withthesedimentaryevents inthepull-apartFrancevillianbasins.

ß2016PublishedbyElsevierMassonSASonbehalfofAcade´miedessciences.

* Corresponding author.

E-mailaddresses:weber.francis@neuf.fr(F.Weber),

fgl@unistra.fr(F.Gauthier-Lafaye),

hubert.whitechurch@unistra.fr(H.Whitechurch),

mulrich@unistra.fr(M.Ulrich),

abder.albani@univ-poitiers.fr(A.ElAlbani).

ContentslistsavailableatScienceDirect

Comptes

Rendus

Geoscience

ww w . sci e nc e di r e ct . com

http://dx.doi.org/10.1016/j.crte.2016.07.003

Gabon is composed of thrusted units, extending over 200km,resultingfromthecontinentalcollisionbetween two Archeancratons: theWestCongoliancratontothe eastandtheSanFranciscocratontothewest.

InGabon,thePaleoproterozoicterrainsoutcropintwo distinctareas.Ontheonehand,theOgooue´ Belt,situated between the Ikoy–Ikobe´ andOugoulou–Offoue´ faults, is composedessentiallyofmetasedimentaryunits,overlying migmatitic units,resulting fromlow tohighdegrees of metamorphism, respectively. On the other hand, the FrancevillianintracratonicbasinsofM’Bila,Boue´, France-ville,andOkondja,situatedtotheeastoftheOugoulou– Offoue´ fault, are composedof un- toslightly metamor-phosed sediments(Baud,1954;Choubert,1954;Cosson, 1956,1957,1961;DonotandWeber,1968,1969).A 150-km-long strip of Archean formations, formingthe Lope´ High,separatesthesetwoareas(Fig.1).

TheinterpretationoftheGaboneseEburneanmountain range differs according to the authors. Feybesse et al. (1998) suggest an intracratonic model to explain the Eburnean orogeny, while Thie´blemont et al. (2009a)

developed convincingarguments for acontinental colli-sion model. However, their conceptualmodel does not integrate the history of the intracratonic Francevillian basins.

Attheverybeginningofthegeologicalsurveyinthe 1950s, geologists, particularly Cosson (1956), suspected thattheOgooue´ BeltandtheFrancevillianbasinmightbe related. But it is only during the last decade of the twentiethandthebeginningofthetwenty-firstcenturies, thankstotheconstructionofthe‘‘economicroad’’andofa railwaycrossingthecountry,thatgeologicalobservations could be performed in good conditions, despite dense tropical forest environments. New radiometric datings help to better constrain the events affecting these two domains.TheneweditionofthegeologicalmapofGabon at the 1/1,000,000 scale (Thie´blemont et al., 2009b), resulting fromrecent geologicalsurveys,reports allthe radiometric ages that have been performed in the ProterozoicrocksofGabon.Thisdatabankisourreference inthepresentstudy(supplementarymaterial,TableS1).

This paper provides a review of the existing data concerning theEburnean orogeny in Gabon and a new interpretationintegratingtheeventsaffectingtheOgooue´ complexandtheintracratonicbasins.

2. DescriptionoftheProterozoicformationsinGabon 2.1. ProterozoicformationsoftheOgooue´ Belt

TheOgooue´ beltislimitedtothewestbythemainshear zoneoftheIkoy–Ikobe´ faultandtotheeastbytheLope´ High(Fig.1a).Itrepresentsanow-erodedmountainbelt built during the Eburnean orogeny, cartographically covering a surface of 13,000km2 _{corresponding to a}

volumegreaterthantheHP–LT‘‘Schisteslustre´s’’inthe Alps(Thie´blemontetal.,2009a).

This belt is composed of thrusted units of different metasedimentary and meta-volcanic formations with various degrees of metamorphism, from high grade to

very low grade. These units have recorded three main tectonicphases.

ThreemainunitshavebeenrecognizedintheOgooue´ belt.

2.1.1. Thepre-Eburneanmigmatites

Thepre-EburneanmigmatitesareregardedasArchean materialaffectedbypartialfusion.Thesemigmatitesare similartothoseoccurring tothewestoftheIkoy–Ikobe´ fault(Lambare´ne´ migmatites). Theyhavebeen datedat 245070Ma(supplementarymaterial,TableS1;Feybesse et al., 1998; Mayaga-Mikolo, 1996; Thie´blemont et al., 2009a). The migmatitesinside the Ogooue´ complex form thecoreofawideanticlinestructure(Abamie´ dome)oriented roughlynorth–south.Theseformationswere submittedto twometamorphicevents:thefirstoneathightemperature andthesecondathighpressure(eclogiticfacieswithCpx, Opx,Gt,Plat8Kb,8108C)(Feybesseetal.,1998;Thie´blemont etal.,2009a,2009b).TwoU/Pbagesonmonazitehavebeen proposed to the two metamorphic peaks, respectively 220838fortheHTmetamorphismand212243Mafor the HP metamorphism (Mayaga-Mikolo, 1996). These migmatites have been interpreted either as independent slicesof Archean basementmixedtogetherwithslices of metamorphic rocks of the Ogooue´ complex or as the basement of all the tectonic slices of the Ogooue´ belt (Thie´blemontetal.,2009a).Weretainthislatter interpreta-tion(seebelow).

2.1.2. TheOgooue´ complex

TheOgooue´ complexitselfonlyoccursintheeastofthe Ikoy–Ikobe´ fault (Fig. 1). It is composed of series of metasedimentaryformationscontaining tholeiitic meta-volcanicrocks.Theseformationshavebeensubmittedto twometamorphicevents,oneathightemperature(Bt,Ms, Gt, St, Sill; 6.5kbar and 6108C) and the other at high pressure (Ky–Bt–Gr–St, Ilm) at 8 kbar and 5508C. (Feybesse et al., 1998; Thie´blemont et al., 2009a). In agreementwithThie´blemontetal.(2009a),weretainthe Sm/Ndageof212038MaforthepeakofHP metamor-phism(Feybesseetal.,1998)asitisconcordantwiththeage of the HP metamorphism of migmatites. A retrograde metamorphismepisode(amphibolitetogreenschistfacies) is relatedto a fastadiabatic exhumationof theHProcks before the intrusion of the Lecoue´ granite dated at 20097Ma(Thie´blemontetal.,2009a).

2.1.3. TheN’Djole´ Group

TheN’Djole´ Groupoutcropsontheeasternsideofthe Ikoy–Ikobe´ fault,andonlyintwolocations.Thefirstoneis alongtheeasternedgeofthedomain,betweentheOgooue´ complexandtheLope´ High(seriesofOkanda,Figs.1b),and the second one is located northwest of the Ogooue´ complex (series of N’Djole´ s.s.). This last sedimentary seriesismadeofschistosemetasediments,mainlypelites affectedbyaverylow-tolow-grademetamorphism.Basic metavolcanitesarefrequentinthenorthwesternarea.The onlyageconstrainton theN’Djole´ Groupis providedby zirconsfromtheEbeleregion(N’Djole´ series),whichyield anageof211810Ma(Thie´blemontetal.,2009a).These authorsconsiderthatthedepositionofthesedimentsofthe

N’Djole´ Groupissubsequenttothemetamorphicepisodes affectingthe Ogooue´ complex.Nevertheless, theseries of Okandato the east ofthe Ogooue´ complex, composed of quartzites,sandstonesandconglomerates,blackshalesand chertyquartzites,isconsideredtobeatthebaseofthegroup but might be older, taking into account a prograding sedimentationprocessfromeasttowest.

2.2. StructuralrelationshipsbetweentheOgooue´ complex andtheN’Djole´ Group

We observed an important shear zone between the Ogooue´ complex and the Okanda series of the N’Djole´ Group,alongthetrans-Gaboneserailway,atthelatitudeof equator. North of the Mole of Ebele (around Biboulo), Fig.1. (a)SchematicgeologicalmapofGabon.FromThie´blemontetal.(2009a,2009b),modified.2083...agesofEburneanplutons.Thelinesofthegeneral cross-sectionsA–BandC–DthroughtheOgooue´ mobilezoneandtheintracratonicbasinsareshown.(b)Generalcross-sectionsfromtheEburneansystem inGabon,showingtheunitsoftheOgooue´ mobilezoneandtheintracratonicbasinsofBoue´,Franceville,andOkondja.

Thie´blemont et al. (2009a) described garnet bearing paragneissesoftheOgooue´ complexoverlaintectonically byschistsoftheN’Djole´ Group.Thetwostructuralunits areclearlyintectoniccontact.Thispileofunitsthrusted ontotheArcheanmigmatiteshasbeenfoldedina large anticlinorium(domeofAbamie´)andsynclinorium(N’Djole´ basin)withNE–SWaxes.

ThreemainphasesofdeformationsD1toD3(Feybesse et al., 1998; Gauthier-Lafaye,1986; Ledru et al., 1994; Thie´blemont et al., 2009a, 2009b) are described. The intensity of thedeformation depends on the degree of metamorphism affecting the units (Gauthier-Lafaye, 1986).

InthemetamorphicseriesoflowestgradefromN’Djole´ andOkanda,theoriginalstratificationS0isdeformed(D1) byisoclinalF1folds,overturnedtotheENEandassociated withaschistosityS1bearing mineralparagenesisofthe anchizonalzonetogreenschistfacies.In theeast,inthe

vicinityofthecontactwiththemetamorphicseriesofthe Ogooue´ complex, the N’Djole´ series develop a second schistosity S2 in the axial plane of isoclinal F2 folds, exhibitingmineralparagenesesofthegreenschistfacies.

In the series of the Ogooue´ complex, the major structures are the F2 isoclinal folds overturned to the ENE,whichdevelopaS2schistositydippingtothewest and bearing mineralsof theamphibolite facies. ThisS2 cleavagefullytransposesallpreviousstructures,including the S1 schistosity linked to the HP metamorphic peak observedin placesas relict.TheS2 cleavagecontains a stretchinglineationtrendingENE–WSWtoNW–SEwith kinematiccriteriaindicatinganoveralltoptotheENEto east transportation (Feybesse et al., 1998; Ledru et al., 1989).

Allthesestructuresareaffectedbyathirddeformation markedbyF3foldswithNStoN508axes,responsiblefor the large structures, such as the N’Djole´ and Abamie´ Fig.2.ShematiclithologiccolumnoftheFrancevillianseries.

anticlinoriums.ThisD3phaselocallydevelopsintheseries of Ogooue´ aS3 cleavage oflow-degree metamorphism. This third phase would be linked to late strike-slip displacementsalongtheIkoy–Ikobe´ andOgoulou–Offoue´ major faults. One K/Ar age on a amphibolitic mylonite withintheIkoy–Ikobe´ faultyieldsanageof19707Ma (Corsinietal.,1993).Itcanreasonablybeviewedastheageof thefirstpost-collisionalstrike-slipmotionalongthefault. 2.3. Eburneanplutonicintrusions

Depending on the age, the geochemistry and the location,theEburneanmagmaticintrusionsareclassified intwodistinctgroups(Thie´blemontetal.,2009a): awesterncalc-alkalineprovinceonthewestsideofthe

Ikoy–Ikobe´ fault. They are mainly composed of pink porphyroid granite, granodiorite, tonalite associated with diorite and gabbro. Their age ranges from 208325 to 204012 Ma (supplementary material, TableS1).TheyoungestagesarefoundatLambare´ne´,in the North of the province. These granitoids exhibit a shoshonitic composition (Thie´blemont et al., 2009a) involving themelting oflowercontinental crustand/or metasomatizedmantle(Campbelletal.,2014;Ku¨sterand Harms, 1998; Morrison, 1980; Turner etal., 1996). The alignmentoftheseplutons,theircalc-alkalinenatureand their ages,posterior to the HPmetamorphism dated at 212038Ma(Feybesseetal.,1998),wouldsuggestthat this set of igneous plutons represents intrusive rocks underlyingacontinentalarcaboveasubductionzoneand mighthavebeenemplacedaftertheonsetofthecollisionof thetwocontinentalmargins(Campbelletal.,2014).This continentalarcwouldhavebeendisruptedbytheWest Congolianorogeny(900–700Ma).Therefore,theseplutons are distributed on both sides of the Neoproterozoic synclinalofNyanga(Fig.1a);

an eastern province of hyper-aluminous granite,on the easternsideoftheIkoy–Ikobe´ faultintrudingtheOgooue´ belt.TheseplutonsconsistofgranodioriteintheLecoue´ massif,alongtheeasternsideoftheIkoy–Ikobe´ fault,and ofperaluminousporphyroidleucograniteintrusiveinto the migmatitic Abamie´ dome and paragneisses of Ogooue´.ThesegranitoidsareofS-type,andresultfrom the melting of crustal metasediments (Thie´blemont et al.,2009a).Theyareyoungerthanthegranitoidsof the western province. The granodiorite of Lecoue´ intrudingthemetamorphicrocksandmigmatitesyield U/Pb and Rb/Sr ages ranging from20428 (Feybesse etal.,1998)to198020Ma(Cahen-Vachetteetal.,1988), butthemostrecentdatingsyieldU/Pbagesrangingfrom 19889 to 20097 Ma (Thie´blemont et al., 2009a). Granitesandassociatedpegmatitesintrudingthe migma-titesoftheAbamie´ domeyieldagesrangingfrom195030 Ma (Vachette, 1964) to 19977 Ma (Mayaga-Mikolo, 1996); the Mboumi granite yields a U/Pb age of 19939Ma(Thie´blemontetal.,2009a).Theintrusionof these granitoids is subsequent to the retromorphic exhumation(192035,Mayaga-Mikolo,1996)and thrust-ingofthemetamorphics.Accordingly,itsealsthenappe emplacementoftheOgooue´ belt.

2.4. SedimentaryformationsoftheFrancevillian intracratonicbasins

Thesedimentsaredepositedwithintwomainbasins, namely Franceville and Okondja. To the west of these sedimentary basins, reduced sedimentary series are observeduponthebasementintheBoue´ andN’Bilaareas (Fig.1a).

The Franceville basin is the best known, due tothe occurrence of manganese deposits (Baud, 1954, 1956; Bouladonet al.,1966;Leclercand Weber,1980;Weber, 1993)andofuraniumdeposits(BourrelandPfiffelmann, 1972;Gauthier-Lafaye,1986;Gauthier-LafayeandWeber, 1989,2003),whichinducedimportantgeologicalsurveys and drilling campaigns. Moreover, these basins are certainlyamongthebestpreservedoftheirageonearth. The sediments were deposited at the time of the first oxidationoftheatmosphere(Canfieldetal.,2013).Apart fromthemanganeseanduraniumdeposits,twoimportant discoveriesassuredthereputationofthisbasin.In1972, naturalnuclearfissionreactionzoneswerediscoveredin the Oklouranium deposit (Naudet,1991; Neuillyet al., 1972). In 2010, the oldest macrofossils ever known on earthwerediscoveredbyElAlbanietal.(2010,2014).

Thereferencestratigraphiccolumnofthesedimentary serieshasbeenestablishedintheFrancevillebasin(Bouton etal.,2009a,2009b;DonotandWeber,1968,1969;Weber, 1968).Fiveformationshavebeendescribed,labelledFAto FEfromthebottomtothetop(Fig.2).

2.4.1. TheFrancevillianA(FA)

TheFrancevillianA(FA)ismainlycomposedofdetrital sediments,quartz–arenitemoreorlessconglomeratic.The topoftheformationcontainsuraniumdeposits,withthe famousOkloandMounanadeposits.Thethicknessofthe FAformationreaches1000mintheFrancevillebasin.In theOkondjabasin, thethickness mayalsobeof several hundred meters. In the Boue´ and M’Bila areas, the FA formationisthinnerthaninthemainbasins,reaching a few tens of meters. Most of the measurements of the currentstructuresshowageneralflowdirectionfromwest toeastduringthesedimentationofthisdetritalformation. In the central part of the Franceville basin, the FA formationevolvesfromfluvialdepositsatthebottomto deltaic and coastal deposits, containing beach and shoreline sand bars facies at the top (Deynoux and Duringer,1992;Gauthier-Lafaye,1986).

2.4.2. TheFrancevillianB(FB)

TheFrancevillianB(FB)ischaracterizedbyan impor-tant subsidence.AtOklo,inthebasinofFranceville,the transitionbetweentheFAandFBformationshasbeenwell observed. The sediments start with black silty pelites typicalofatransgressivemarinesedimentationfollowing theshorelinesandstonesofthetopof theFA formation (Deynoux and Duringer, 1992). But very soon appear furrowing debris-flows containing dolomitic olistoliths (blocksofseveralhundredcubicmeters)withfragmentsof chertsandturbidites,revealingagravitycollapse,probably

linkedtotectonicactivity.Theseresultinimportantand abrupt variations in thickness of this formation, which makescorrelationsbetweenboreholesdifficult.Thetotal thicknessofthisformationmayreachupto1000metersin thecentreofthebasin.

Depositionofblackshalesfollowsthemainturbidites deposits. It has occurred in a confined and restricted environmentresultingintheformationofdolomitesthat becomemoreandmoremanganiferousatthetopofthe formation.Thesemanganeserichcarbonatelayersarethe protoreoftheMndepositofMoanda.Sm–Ndagesonsmall fractions of clay minerals (<0.2

m

Fine-grainedsiltstoneslayersintheblackshalesmay contain splintery quartz, rhyolitic quartz exhibiting embayments and well-preserved feldspars and biotites thatarelikelyfromvolcanicorigin(Boutonetal.,2009b). The chemistryof thesesedimentaryrocks suggeststhat theyderivefromtheerosionofacontinentalislandarcon anactivecontinentalmargin(Thie´blemontetal.,2014).

Beforethedepositionofthestromatoliticcherts(FC), appearsaunit(FB2a)ofverydifferentcompositioncalled ‘‘Poubara sandstones’’. The Poubara sandstone is com-posed of 1- to 5-m-thick layers of fine-grained and isogranular sandstones with little internal structures if we exclude thedish structures andsome angular lithic fragmentsatthebottomofthelayers.Thesesandstones are interpreted as turbidites that were deposited at a shallowbathymetrywithahighenergy(ElAlbanietal., 2010).TheyimmediatelyprecedetheemergenceoftheFC and contain typical sedimentary features indicating a decrease in the water column before and after the depositionofthesandstones(WeberandGauthier-Lafaye, 2013).

IntheOkondjabasin,theFBformationissimilartothe FBoftheFrancevillebasin.Themaindifferencebetween these two basins is the presence of basanite, either in massiveflowsorin pillow-lavasand sills,interstratified with the sediments of the Ambinda anticline, in the northeasternpartoftheOkondjabasin.Thevolcanismis relatedtotheemplacementoftheannularhypo-volcanic alkaline N’Goutou complex (Fig. 1). The plutonic rocks includenephelinicsyenitesaswellasamphibolegranite. Thisstructureisassociatedwithawholesetofvolcanic andsubvolcanicrocks,adjacenttotheintrusivecomplex: syenites, microsyenites and microgranites, pegmatites, trachytes, rhyodacites dykes, sills or lava flows. The N’Goutou complex yields a U–Pb age on zircon of 202755Ma(MoussavouandEdouMinko,2006). 2.4.3. TheFrancevillianC(FC)

The Francevillian C (FC) consists of dolomite, often evaporitic(Pre´atetal.,2011),andofstromatoliticcherts (Bertrand-Sarfati and Potin, 1994; Bouton et al., 2009a, 2009b) interlayered with black shales. This formation corresponds to a very shallow environment almost emerged,linkedtoaquasi-emersionafterthedeposition oftheFB.IntheFrancevilleandOkondjabasins,thetopof theFCformationcontainslayersofpyroclasticswith well-characterized splinters of glass (Weber, 1968). The FC

formationoverliesthepeliticFBformationinthecentral partsofthesebasins.Incontrast,inBoue´ andM’Bilaareas andonthehighsseparatingtheFrancevilleandOkondja basins,theFCformationrestsdirectlyontheFAsandstones andevenupontheArcheanbasement.Intheseareas,the FCformationisacondensedseries,equivalenttothewhole FBandFC.

2.4.4. TheFrancevillianD(FD)

TheFrancevillianD(FD)initsbasalpart(FD1)consists ofblackshaleswithhighorganicmattercontent,devoidof detritalelementsandinterlayeredwithpyroclastics.The pyroclastic materials are vitroclastic volcanic ashes containingisolatedsplinters,madeofeasilyrecognizable devitrifiedglass.Theycorrespondtoaerialfalloutsfroman explosivecalc-alkaline,andesitictoryodaciticvolcanism. Thecalc-alkalinechemistryofthesevolcanicrocksdiffers muchfromthealkalineoneassociatedwiththeN’Goutou complex,reflectingadifferentgeodynamiccontext( Thie´-blemontetal.,2014).

Horie et al. (2005) attribute a zircon U–Pb age of 20836 Ma to a FD ignimbritic flow. Actually, this ignimbritic flow contains inherited zircons with ages between2825Maand2000Ma.AreappraisaloftheU–Pb datasetshowsthattheageof20836Macorrespondstoa mixingoftwopopulationsofzirconwithages,of2136–2068 Ma and2042–2003 Ma,respectively. Theyoungest ageof 200316 Ma obtainedon a subconcordant zircon could representthetimeofemplacementofthesepyroclasticflows. Alternating sequences of dominant sandstones and pelites reston the lowerblack shales and pyroclastites (FD2).Thesesandstonesareepiclasticwithmostoftheir elementsofvolcanicorigin,withstrongacidiccharacter. Thedepositionofthesesandstonesseemstobeturbiditic. Itissuggestedthattheycomefromtheerosionofvolcanic structures.

2.4.5. TheFrancevillianE(FE)

TheFrancevillianE(FE)ismainlyrepresentedbya 400-m-thickunitofsandstonesoftheMountN’Gouadi(Bouton etal.,2009a)wheretheyformseveralcuestasandacliffin thesynclineoftheLoulaRiver,northwardofOkondja.The unitcontainstwomegasequencesofsandstones.Theyare coarsesandstones,arkosic,withconglomeraticlayersand frequentcrossed beddings.Mostof thesesediments are devoid of reworked volcanic elements (Bouton et al., 2009a), except rare occurrences of ignimbritic pebbles, nearLastoursvilleandM’Bila(Weber,1968).

2.5. StructurationoftheFrancevillianbasins

Gauthier-Lafaye(1986)showsthatduringFB deposi-tion,these sedimentarybasins werestructuredas pull-apartbasinsbetweenlargenorth–southsenestral strike-slipfaults.TheNW–SEfaults,limitingthebasins,display vertical offsets up to 400–500m controlling the FB deposition with various thicknesses. A late strike-slip movementregisteredbythesefaultsinducedwide anti-clinesandsynclines,particularlyimportantintheOkondja basin. Important flexures with a north–south axis, observedintheFA formationoftheOkloregion,would

suggestthattheNSfaultsmighthaveanormalcomponent duringFAdeposition.

2.6. Eburneanminedoredeposits

ThetopoftheFAformationcontainsthefamousOklo and Mounanauranium deposits.The uranium hasbeen transported by oxidizing fluids and reduced by hydro-carbons(asphaltite), which migrated fromthe FBblack

shales trappedto theFA sandstonesinfaulted contacts with the FB formations (Gauthier-Lafaye, 1986). The uranium mineralization is 205030 Ma old (Devillers etal.,1975;Gancarz,1978),andthenuclearfissionoperated 195040Maago(Holliger,1992;Naudet,1991;Ruffenach, 1979;Ruffenachetal.,1976).

AtthetopoftheFBformation,theconfined environ-mentresultsintheformationofdolomitesthatbecome manganeserichand formtheprotoreofthemanganese

Fig.3.ConceptualmodelfortheevolutionoftheEburneanorogenyinthemobilezoneofOgooue´ andconsequencesinsedimentationandmagmatic activityintheFrancevillianbasins.A.Theinitialriftingstage:theextensionoftheArcheanbasement(overamantleplume?)mightbeattheoriginofthe pre-Eburneanmigmatitesbymeltingofthecontinentalcrustalat2450Ma.Theyconstitutethebasementoftherift.TheHighofLope´ isconsideredtobethe easternshoulderoftherift.ItserosiongavetheFAfluvialFrancevilliansandstonestotheeastandtheOkandaquartzitesatthebottomoftheN’Djole´ Group tothewest.B.Duringtheopeningoftheocean,theOkandamarinesedimentsaredepositedontheeasternmargin.Astheoceanbecomeswider,thermal subsidenceoftheriftshouldersleadtoamarinetransgressiontotheeast,withthedepositionoflittoraltomarinesedimentsatthetopoftheFAformation.

deposit of Moanda, later concentrated in the lateritic formation overlyingthecarbonates (Leclercand Weber, 1980;Weber,1968,1997).

The primarygolddepositsare linkedtotheArchean greenstone belts. Eburnean post-orogenic hydrothermal remobilizationtransportedand concentratedgoldinthe EburneanthrustingcontactoftheOgooue´ belt.TheE´te´ke´ golddepositintheOugoulou–Offoue´ inversefaultyielded agesrangingfrom2100to1950Ma(Feybesseetal.,1998; Prianetal.,1991).

3. Interpretations:towardsanewgeodynamicscenario ThetypeofmetamorphismintheOgooue´ complexand thechemistryofthegranitoidsoftheplutonicbeltledthe authors of the geological map of Gabon to view the Eburnean orogeny as a continent-continent collision between theSanFranciscoand WestCongoliancratons, aftertheresorptionbysubductionofanoceanseparating twocontinentalmargins(Thie´blemontetal.,2009a).The Ikoy–Ikobe´ faultwouldthenbeamajortectonicstructure, viewed as a suture between these two continental lithospheric blocks, along which part of the Ogooue´ complex would have subducted, and subsequently ex-humedandthrustedontothewesternCongocraton.

Thesubductionprocessshouldberesponsibleforthe majorEburneanHPmetamorphicepisodeandthe intru-sionofcalc-alkalineplutonsintheeasternSanFrancisco craton.Thepeakofmetamorphism(700–5508,8–10Kb) suggests a burialdepth of about25 to 30km withlow thermalgradient,consistentwithasubductionregime.In this model, the initial series (Ogooue´ complex) was depositedon theoceanicfloor,tectonicallyslicedinthe accretionary prismand enteredpartiallythesubduction channel.Thelow-grademetasedimentaryseriesofN’Djole´ are supposed to have beendeposited at the topof the accretionary prism that developed above the oceanic subduction.

Theorogenycouldbedividedintosixmainsteps: breakupofaninitialsingleArcheancontinent,followed

by;

oceanicopening;

closureofthisoceanbysubductionoftheeasternplate under thewestern one and opening of the pull-apart Francevillianbasins;

collisionofthecontinentalmarginsofthetwoArchean blocks and cessation of the sedimentation in the Francevillianbasin;

slabbreak-offandthrustingoftheOgooue´ complex; post-collisionreadjustments.

3.1. Initialriftingstage

The first post–Archean episode is the formation of migmatitesat2450Ma,whichtookplacelongafterthe LateArcheanorogenyat2750Ma(Fig.3A).Feybesseetal. (1998)considerthatthefusioneventcouldhaveoccurred duringapre-Eburneanorogeny.However,notraceofsuch

anorogenyhasbeenrevealedforthatperiodintheregion. Thus, we must envisage that this partial fusion of the continental crust has occurred during an ‘‘anorogenic’’ event. Anextension of the Archean basement (abovea lithosphericdelaminationoramantleplume?)followedby crustal upwelling might be at the origin of the crustal melting (Thompson and Connoly, 1995; Whitney et al., 2007; Rey et al., 2009). The north–south alignment of migmatitesoneachsideoftheIkoy–Ikobe´ fault,observed uptoCameroun,couldrepresentthebasementoftherift zone.

Thenorth–south-orientedArcheanLope´ Highdivides thenon-metamorphic FrancevillianFA formation tothe eastfromthelow-degreemetamorphicOkanda metase-dimentaryseriesatthebaseoftheN’Djole´ Grouptothe west.

The eastwards-directed sedimentary currents mea-suredinthesandstonesoftheFAformationsuggestthat the north–south High of Lope´ was an elevated horst structure during FA sedimentation. The north–south flexures observed in the FA formation could testify to the activity of normal faults during the deposition of detritalsediments.TheHighofLope´ mightcorrespondto theeasternshoulderoftheriftingzone.Theerosionofthis elevatedhorststructureproducedthegravel materialof thefluvialsandstonesatthebaseoftheFAformationand the Okanda series that spreads on each side of the shouldersoftherift.ThedepositionofFAsandstones,like the basal sandstones of the group of N’Djole´ (Okanda quarzites), should begin after the migmatization at 2450Ma.

3.2. Openingoftheoceanandcollapseofthemargins The existence of an ocean has been suggested by

Thie´blemontetal.(2009a,2009b)basedontheoccurrence oftholeiiticbasicrockssubsequentlyaffectedbytheHP metamorphicrocks,associatedwithoceanicsedimentsin the Ogooue´ complex (Fig. 3B). No trace of ophiolite complexhasbeenobservedin theregion.Itistherefore difficulttoassessthewidthoftheoceanseparatingthetwo continental blocks. Nevertheless, its size should be sufficient to allow the cooling and subduction of the oceanic lithosphere. Anyway, a long period of time is available to create this oceanic lithosphere, between 2450Ma,theageofthemigmatitesat thebeginningof therifting,and212038Ma,theageoftheHP metamor-phism, linked to the subduction (at least 230Ma), ifwe excludealongperiodofriftingwiththemantledenudationof amaximumof100Ma,assuggestedbyCheninetal.(2015)

fortheopeningoftheAtlanticocean.

Theorientationoftheaccretionzoneshouldbealmost paralleltothenorth–south-trendingcontinentalmargins, likeinmodernoceanicsituations.Theeast–westN’Koland the N’Tem major faults (Fig. 1) could represent the remaining traces of transform faults that shifted the accretionzone.

Astheocean becomeswider,themantleheatsource moves away from the marginsto themid-ocean ridge. Thermal subsidence of the rift shoulders and isostatic readjustments lead to a marine transgression, at least

observedontheWestCongolianmargintotheeastofthe ocean,withthedepositionoflittoralmarinesedimentsat the top of the FA formation and the marine N’Djole´ sedimentaryseries.

3.3. Subductionandopeningofpull-apartbasins

The best witnesses of this subduction are the calc-alkaline plutons, intruded in the upper western plate (Fig. 4). These plutons display ages ranging from

204012to208025Ma(supplementarymaterial,Table S1).

During the subduction process, the oceanic plate subducts under the western continental plate. The sediments deposited on the oceanic crust form an accretionary prism. The initial series (Ogooue´ complex) was deposited onto the oceanic floor and tectonically slicedintheaccretionaryprismandpartiallyenteringthe subductionchannel.Theyunderwentaseriesof metamor-phicevents, welldescribedby Feybesseet al.(1998) at Fig.4.ConceptualmodelfortheevolutionoftheEburneanorogenyinthemobilezoneofOgooue´ andconsequencesinsedimentationandmagmatic activityintheFrancevillianbasins(continued).A.Theoceaniclithospheresubductsunderthewesterncontinentalplateat2200to2120Ma;the calc-alkalineplutonsintrudedintheupperwesternplateandthesedimentsattheoriginoftheOgooue´ complexaredepositedintheaccretionaryprism.B.The subductionoftheeasternmarginoftheocean(2120to2040Ma)blockedthesubductiontothesouthandinducedstrike-slipmovementsalongthemajor faultinheritedfromtheriftingstage.Thesestrike-slipmovementsallowtheopeningofpull-apartFrancevilleandOkondjabasinsfilledbyturbiditesand shalesoftheFBformation.TheN’GoutoualkalinecomplexintrudestheOkondja.IntheOgooue´ complex,themetasediments,volcanicrocksandmigmatites aretransformedinHPmetamorphicrocksandexhumed.TheN’Djole´ seriesaredepositedontothemetamorphicunits.

2120Ma.ThepeakoftheHPmetamorphism(700–5508,8– 10kbar)correspondstoaburialdepthofabout25to30km withalowthermalgradient,consistentwithasubduction regime. The age of the peak of metamorphism (212038Ma)agreeswell,withintheerrorbars,withthe

oldest age of the calc-alkaline plutons (208025 Ma), linkingthesetwoeventstothesameprocessofsubduction. Younger ages yielded by the calc-alkaline plutons located in the North (Lambare´ne´ region) indicate that the onset of the collision between the two continental Fig.5. ConceptualmodelfortheevolutionoftheEburneanorogenyinthemobilezoneofOgooue´ andconsequencesinsedimentationandmagmaticactivity intheFrancevillianbasins(end).A.Thecontinentalcollision,stopofthesubduction(2040to2000Ma).ThefastriseoftheOgooue´ Complexmightbetheresultof aslabbreak-off.Themetamorphicandunmetamorphosedunitsarethrustedtowardthenortheast.Ahotasthenosphericflowlinkedtotheslabbreak-offleadsto partialfusionwithinthemiddletolowercrust,generatingtheLecoue´ granite,whichsealsthetectonicunitsoftheOgooue´ complex.Thesubsidenceofthe Francevillianbasinsceasestobeactiveandthebasinsarefilled.TheFCformationcorrespondstoaquasi-emersionofthebasin.Thealkalinemagmaticactivityin theOkondjabasinceases.AftertheemersionoftheFC,theimportanttransgressionFDcoversthewholeregion,afterageneralcollapseoftheforelandofthe Ogooue´ complex.Thecalc-alkalinemagmatismobservedatthebaseoftheFDformation(FD1)resultsfrompartialfusionalongdeeplithosphericfaults.B.Atthe endoftheEburneanorogeny,thepost-collisionaladjustments(rotationofblocks)occurring1950Maagocauseimportantdeformationsinthebasins(mainlyin theOkondjabasin)andareresponsiblefortheoverallemersion.ThefinalerosionofthereliefsproducestheFEsandstones.

marginswouldhavetakenplaceinthesouthandendedto thenorth,implyinganobliquesubduction.Thishypothesis isalsosupportedbytheoccurrenceofanangleofabout308 betweentheLope´ High,whichmarkstheedgeoftheold rift,andthefaultoftheIkoy–Ikobe´ fault,whichmarksthe suturebetweenthetwoplates(Fig.5).

TheHPmetamorphismaffectingthemigmatitesofthe Abamie´ dome suggests that the southern part of the marginoftheWestCongoliancratonhasbeeninvolvedin the process of subduction, while in the north, the subductionoftheoceaniclithosphereisstillactiveatthat time.We hypothesize that thecollision ofthesouthern part of the continental margin has induced a general sinistral rotational movement of the cratonic block, probably along the N’Kol fault (Fig. 6). The normal displacements along the north–south faults, during the rifting stage, have shifted during the collision of the southern partof theWest Congolian cratontosinistral strike-slipmovements.Thesestrike-slipmovementshave allowedtheopeningofFrancevilleandOkondjabasins,as pull-apartbasins resultingfromthestrike-slip displace-ments along the north–south and NW–SE conjugated faultsasproposedbyGauthier-Lafaye(1986)(Fig.6).The Okondjabasin hasundergone probably more extension thantheFrancevillebasin,allowingthemeltingofalkaline

magmaoftheN’Goutoucomplexinthemantlebeneath thisbasin.Thiseventoccurred202755Maago,afterthe peak of HP metamorphism of the Ogooue´ complex at 212038Maandbeforetheexhumationofthe metamor-phic rocks dated at 20097 Ma by the retrograde metamorphicreactions.

3.4. Continentalcollisionandclosureoftheintracratonic basins

Afterthesubductionandcollisionoftheeasternmargin intheSouth,thetwomarginshavecollidedintheareaof N’Djole´ to the north, and the subduction ceased after 2040 Ma,which is theageof theyounger calc-alkaline plutonsintheLambare´ne´ area(Fig.5A).

ThesubsidenceoftheFrancevillianbasinsceasedtobe active,theturbiditicsedimentationstoppedandthebasins werefilledby blackshalesand manganiferousdolomite layers, deposited in a quiet environment, the depth of whichwasprogressivelyreduced.Thealkalinevolcanism in the Okondja basin also ceased to be active. The 202735Maage,markingtheendoftheplutonicactivity of N’Goutou,is almost contemporaneouswithin the error bars(orjustfollow)withtheageoftheyoungestLambare´ne´ calc-alkaline pluton of 204012 Ma. This age could Fig.6.Schematicdiagramofthevariousdomainsduringthefirststageofthecollisionbetweenthemargins:(1)thesubductionremainsoceanicinthe northernpartoftheocean;(2)tothesouth,themarginoftheeasterncontinentisinsubductionbeneaththewesternone;thiscollisionresultsina rotationalconstraintinducingstrike-slipfaultsandtheopeningofthepull-apartFrancevillianbasins;(3)theBoue´ basin;(4)theFrancevillebasin;(5)the Okondjabasin;(6)alkalineplutonofN’Goutou;(7)calc-alkalineplutonsintrudingtheSanFranciscocraton;(8)themarginisshiftedahundredkilometres tothewestbytheN’Kolfault.

correspond to the end of the opening process of the Francevillianbasins.

In the meantime, the deformations affecting the Franceville basin create hydrocarbon traps (anticlines and faults) that will be mineralized with uranium for some of them at 205030 Ma (Devillers et al., 1975;

Gancarz,1978).

3.5. Slabbreak-offandemersionoftheintracratonicbasins For theauthors of thegeological map (Thie´blemont etal.,2009a,2009b),theplutonslocatedeastoftheIkoy– Ikobe´ faultweresupposed tohave beenformedon the westernsideofthesuture,abovethesubduction,andthen driftedduringtheoverthrustingof theOgooue´ complex (Fig.4A).However,thishypothesisisnotcompatiblewith the fact that these plutons have different chemical compositionsthanthewesternones.Wethereforesuggest adifferentscenariofortheirformation.

The rapid uplift of the Ogooue´ Complex with its migmaticsubstratumrecordedintheformofapervasive retrogrademetamorphismofitsunitscouldbetheresultof a slab break-off. The orogenic root rose by isostatic readjustment as often invoked in modern orogens (Westphalet al., 2003). Thisrapid upliftof theOgooue´ complex induced substantial transfer of asthenospheric materialfromtheeasternblocktowardthesuture,atthe rootoftheOgooue´ complex.Itwouldberesponsibleforthe meltingofthecontinentalcrustandtheintrusionofthe Le´coue´ graniteinthesuture(2000Ma).Thethrustingto thenortheastofthemetamorphicunits,asindicatedbythe D2overturnedfoldsandassociatedstretchinglineations,is contemporaneous with this isostatic readjustment. The thrustingtotheeastoftheN’Djole´ unitsovertheOgooue´ complexandofthislatterovertheautochthonformations ofOkandacouldmarktheendoftheprocess.Probablyat thesametime,thewesternpartofthebasinsofBooue´ was detachedfromitssubstratum(Ledruetal.,1989).

Meanwhile,theadiabaticriseoftheOgooue´ complex locallyledtopartialfusionwithinthemiddleandlower crust.Thisgaverisetointrusionsofscatteredsmallplutons in the Ogooue´ complex and Abamie´ dome (M’Boumi granite)around2000Ma.

TheonsetoftheuplifttotheeastoftheIkoy–Ikobe´ fault wouldhaveinducedthereworkingoftheFAsandstones and thedeposition, intheFrancevillebasin, oftheFB2a (Poubara sandstones) and FCformations, theFC corres-pondingtoaquasi-emersionofthebasins.Inthecentral part of the basins, the deposition of the FB2a and FC formationsmayhavetakenplacewithinaveryshorttime, contrarytothecondensedseriesoftheFClocatedonthe edgesofthebasinsofFrancevilleandOkondja.

After the quasi-emersion of the FC formation, the importantFDtransgressioncoversthewholeregion.

TheareaoccupiedbytheOgooue´ complexbeingabout 13,000km2_{, thevolume excavated by isostasy is of the}

orderof150,000to200,000km3_{,consideringathicknessof}

theaccretionaryprismthat overcomes12 to15km. We proposethatthevolumeofasthenospheretransferredto thewestatthebaseoftheOgooue´ complextocompensate its exhumation induced a general subsidence of the

foreland to the east (the Francevillian basins). This subsidencewasamplifiedbytheoverloadcausedbythe thrust of the Ogooue´ complex and the buckling of the continentallithosphereoftheeasternblock.The depres-sioncreatedintheforelandwouldhaveallowed theFD transgression,whichcoveredwholetheFrancevillianbasin andthebasemententirely.

TheFDcalc-alkalinemagmatismoccurringatthebase oftheFDdepositsat2003Maisdifficulttointerpret.This volcanicepisodeisposteriortothesuturebetweenthetwo continental blocks as thecalc-alkaline pyroclastites are observed at the base of the FD. No trace of a new subduction (HPmetamorphism) wasobserved afterthe intrusionsoftheN’Goutoucomplex(202755Ma)atthe endofFBandFC.Oneefficientprocesstocausethemeltingof thesemagmascouldhavebeenlinkedtothereactivationof faultsduringtheseisostaticreadjustments.Thecirculations of water, particularly along the large north–south faults, couldinducepartialmeltinguptothelowercontinentalcrust andevenatthetopofthemantle(Eyaletal.,2010). 3.6. Hydrothermalismandoredepositimplications

The reactivation of the faults during the isostatic readjustmentscould havebeen at theorigin of the so-called ‘‘post-orogenic hydrothermal circulation’’ around 2000–1950Ma. These hydrothermal waters werelikely responsibleforthesilicificationoftheFCformation.The silicificationaffectedmainlythestromatolites,butalsothe firstlevelsofcineritesatthetopofthecherts(Madjouma River;Weber,1968)andtoalesserextenttheevaporitic dolomites.

The Eburnean gold deposits are associated with the importantnorth–southfaultsnetworkasinthedepositsof Ete´ke´ and N’Dambi, which are located along the large Ougoulou-Offoue´ and N’Dambi strike-slip faults. Gold depositionoccursinquartzveins,viewedassecond-order tectonicstructuresrelatedtothelargefaults.Ithasbeen reworkedfromaprimary sourceassociated with green-stonebelts(Thie´blemontetal.,2009a).

ThesourcesofuraniumaretheArcheangranitesand theFA sandstones of theFranceville basin. Ithas been concentratedinthepetroleumtrapsatthetopoftheFA formation.Uraniumpercolatedthroughvarious tectonic structuresbutthemostimportantdeposits,namelyOklo andMounanaarelocatedalongalargenorth–southfault. Here, the hydrothermal circulations along these faults mightexplaintheamountofwaternecessarytotransport uraniumtowardthedeposits(Gauthier-Lafaye,1986)and themarineoriginofthebrine,identifiedinfluidinclusions associatedwithmineralization(Mathieuetal.,2000). 3.7. EndoftheEburneanorogeny

Ageneralisostatic equilibrationgeneratedan adjust-ment oftherespective positions of thetwocontinental blocks by a dextral rotation of the western continent (Fig.5B).Thismotion triggeredthesinistralshearofthe Ikoy–Ikobe´ and Offoue´ faults together withthe dextral shearofN’Kolfault.TheK/Arageof19707Magivenby

theIkoy–Ikobe´ faultdatestheselatepost-collisional strike-slipmovements.TheD3foldingphaseintheOgooue´ complex andtheN’Djole´ Groupmightbecontemporarywiththese rotations.ThismarkedalsotheendoftheFDtransgression, theincreaseinthecompressivestressesintheFranceville andOkondjabasinsresultinginlargefoldedstructuresinthe OkondjabasinandthefaultedmonoclinesintheFranceville basin. At the same time, the diagenetic fluids of the FrancevillebasinremobilizeduraniumintheOklodeposit, resultinginlocalenrichmentswherenaturalnuclearfission reactions started, precisely at the same date, i.e. 195050Ma(Devillersetal.,1991;Gauthier-Lafayeetal., 1989).Finally,theFEformationcorrespondstotheerosionof themountainchainofOgooue´.

4. Conclusions

TheEburneanorogenyinGabonseemstoobeytothe rules of modern Plate Tectonic, in contrast to Archean orogeniesgovernedbytectonicprocessesinahotterEarth.

Thie´blemontetal.(2009a,2009b),inthemostrecent editionofthegeologicalmapofGabon,interprettheHP metamorphism of the Ogooue´ complex as a marker of subduction followed by continental collision. The calc-alkalinenatureofthegraniticplutonsintrudingArchean migmatitessupportstheexistenceofasubductionzone. ThemajorNNW–SSEtransversefaultofIkoy–Ikobe´ would bethesuturebetweenthetwoArchean blocks:theSan FranciscocratontothewestandtheCongoliancratonto theeast.

Threeassumptionspresentedintheabovesectionslead toanewconceptualmodel,whichintegratesthehistoryof theOgooue´ belt withtheevolution of theFrancevillian intracratonicbasins:

the2450Maorthogneissesandmigmatites,oneachside of the Ikoy–Ikobe´ tectonic structure, result from the partialmeltingoftheArcheanbasementduringaperiod ofrifting.Thesemigmatitescouldthereforeberegarded astherelictsofthemarginsofthetwocontinents.The fluvialsandstonesoftheFAformationatthebaseofthe FrancevillianseriesandintheOkandaseries,atthebase oftheN’Djole´ Group,resultfromtheerosionoftherift shoulders. The collapse of therift shouldersafter the continental breakup and the creation of an oceanic lithosphere(atleastduring230Ma)allowtheFAmarine transgression. Then, the oceanic lithosphere of the eastern plate subducts beneath the western one to generate the HP metamorphic units of the Ogooue´ complexat2120Ma(ageofthepeakofmetamorphism); theagesofcalc-alkalineplutonsintrudingthewestern platespreadfrom2080Maforthesouthernintrusionsto 2040 Ma for thenorthern ones. This characteristic is indicativeofanobliquesubduction.Thesouthernpartof the continental margins collided first. This collision blocked the subduction and induced compressional stresses in the eastern craton, resulting in a sinistral strike-slip movement along the north–south faults inheritedfromtheriftingphase.Thesestrike-slipfaults openedthepull-apartFrancevillianbasins,allowingthe

depositionoftheFBmarinesediments,intrudedbythe alkalineN’Goutoucomplexat2027MaintheOkondja basin.Thecollisionbetweenthetwomarginspropagated towards the north. The widespread emersion of the entireeasternareawiththedepositionofstromatolithic chertsandevaporiticdolomiteoftheFCformationwas theresultofthiscollision;

theoceaniclithospherebroke-offfromthemarginand dived into the mantle. The consequence of this slab break-offwasafastisostaticreadjustmenttotheeastof the Ikoy–Ikobe´ sutureand theexhumation of the HP metamorphicunitsoftheOgooue´ complex.Theseunits were thrusted to the NNE. This rapid uplift of the orogenicrootinducesmeltingandintrusionofgranitic plutonsscatteredintheOgooue´ complex.The substan-tialtransferofasthenosphericmaterialfromtheeastern blocktoward thesuturewould beresponsibleforthe collapseof theeasternblockandtheFDtransgression togetherwiththemeltingofthecrustandintrusionof theLecoue´ graniteinthesuture(2000Ma).Moreover, hydrated melting along major lithospheric faults be-neaththeFrancevillianbasinswouldhaveproducedthe K-calc-alkalinevolcanismobservedatthebaseoftheFD formation.

Finally,thepost-collisionaladjustmentsoccurringlater, 1950Maago,induceimportantdeformationsinthebasins, mainlyintheOkondjabasinandareresponsibleforthe overallemersion.Thefinalerosionofthereliefsproduces theFEsandstones.

Acknowledgements

The authors wish to pay tribute to all those who contributedtothegeologyofGaboninacountrywherethe workingconditionsareparticularlydifficultforgeologists. WethankallparticipantsintheSYSMINprogramwithout whom this workwould not have been possible, Pascal Bouton who introduced us to this program, and Denis Thie´blemont, the coordinator who kindly read and commentedourfirstdraft.ThanksalsotoOlabodeBankole andananonymousreaderwhokindlyreadthemanuscript toimprovetheEnglish.Finally,wethankPierreBarbeyand JackyFerrie`re,whoacceptedtoreviewthefirstdraft,and IsabelleManighettiforeditorialcommentsthathelpedus toimprovethemanuscript.

AppendixA. Supplementarydata

Supplementarydataassociated withthisarticlecanbe found,intheonlineversion, athttp://dx.doi.org/10.1016/j. crte.2016.07.003.

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