doi:10.1144/SP297.11 2008; v. 297; p. 233-247 Geological Society, London, Special Publications

Atman Madi Romain Bousquet, Rachid El Mamoun, Omar Saddiqi, Bruno Goffé, Andreas Möller and

of the Bou-Azzer ophiolite suite (Morocco)Mélanges and ophiolites during the Pan-African orogeny: the case

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Melanges and ophiolites during the Pan-African orogeny:

the case of the Bou-Azzer ophiolite suite (Morocco)

ROMAIN BOUSQUET1, RACHID EL MAMOUN2, OMAR SADDIQI2, BRUNO GOFFE3,

ANDREAS MOLLER1,5 & ATMAN MADI4

1Institut fur Geowissenschaften, Universitat Potsdam, Karl Liebknecht Strasse 24, 14476

Potsdam-Golm, Germany (e-mail: romain@geo.uni-potsdam.de)

2Departement de geologie, Universite Hassan II—Aın Chock, Route d’El Jadida, B.P. 5366

Maarif, Casablanca, Morocco

3Laboratoire de Geologie, UMR 8538, Ecole Normale Superieure Paris, 24 rue Lhomond, 75231

Paris cedex 05, France

4Akka Gold Mining, MANAGEM-ONA, Rabbat, Morocco

5Present address: Department of Geology, Kansas University, 1475 Jayhawk Boulevard,

Lawrence, Kansas 66045-5276, USA

Abstract: Since the discovery of ophiolite sequences, the Bou-Azzer inlier has been considered akey area for understanding the evolution of the northern margin of the West African craton duringthe Pan-African orogeny. For about 20 years, it had been commonly accepted that the Bou-Azzerinlier represents an accretionary melange accreted onto the West African craton under blueschistmetamorphic conditions, similar to the Franciscan Complex and the Sanbagawa facies series. Thiswould imply that a low geothermal gradient was prevalent during the subduction of the Pan-African oceanic plate, and that the ocean was subducted with a high convergence rate. A reinves-tigation of the metamorphic conditions by a thermodynamic approach shows that the ophiolitesequence of Bou-Azzer underwent HT greenschist metamorphic conditions instead of blueschistmetamorphic conditions. We propose that the ophiolites of Bou-Azzer are not similar to theSanbagawa facies series or to the Franciscan Complex, but bear similarities to the Albanianor Cyprus ophiolites, which represent dismembered ophiolite sequences overprinted bygreenschist conditions.

Ophiolites are remnants of oceanic lithosphere thathave been tectonically emplaced onto continents.Well-preserved ophiolite sections consist of (in des-cending stratigraphic order) pillow lavas, a sheeteddyke complex, gabbro, cumulate ultramafic rocks,and tectonized mantle. Ophiolites provide valuableinformation on the nature of sea-floor processes,global heat loss, palaeogeographical reconstructionsof the continents, and the subduction processes.However, the mechanisms of ophiolite accretiononto the continental margins are debatable. Did sub-duction and obduction change through time (van derVelden & Cook 1999) or not (Kusky & Polat 1999)?Did the thermal gradient of subduction and orogenicwedge change through time (Maruyama & Liou1998) or not (O’Brien & Rotzler 2003)?

Since the discovery of the first Precambrianophiolite sequence in the Bou-Azzer inlier(Leblanc 1976), this inlier has played a criticalrole in the understanding of the evolution of thenorthern margin of the West African craton(WAC) during the Pan-African orogeny (Leblanc

& Lancelot 1980; Saquaque et al. 1989; Hefferanet al. 2000). This ophiolite, together with theKhzama ophiolite (Sirwa inlier, Admou & Juteau1998), represents a unique remnant of a Pan-African ocean, although the exact localization ofthe Pan-African suture is in question (see discussionby Ennih & Liegeois 2001; Bouougri 2003). It hasbeen proposed that the Bou-Azzer ophiolites wereburied to depths sufficient to generate lowerblueschist-facies mineral assemblages in a scraped-off melange (Hefferan et al. 2002). Such a mechan-ism of accretion of the Bou-Azzer ophiolite ontothe WAC is similar to that for accretion of theFranciscan melange along the North Americanwest coast. This mechanism implies that a lowgeothermal gradient was prevalent during the sub-duction of the Pan-African oceanic plate or thatthe oceanic plate was subducted at a high conver-gence rate. Can we really document such conditionsfor the ophiolite of Bou-Azzer?

This study first reviews the mechanisms ofophiolite accretion, formation of melanges and the

From: ENNIH, N. & LIEGEOIS, J.-P. (eds) The Boundaries of the West African Craton. Geological Society, London,Special Publications, 297, 233–247.DOI: 10.1144/SP297.11 0305-8719/08/$15.00# The Geological Society of London 2008.

evolution of the thermal gradients occurring in sub-duction zones through time, and then reinvestigatesthe metamorphic P–T conditions experienced bythe ophiolite suite of Bou-Azzer, using thermodyn-amic methods, to re-evaluate the mechanisms ofaccretion of the ophiolites onto the WAC.

Geological setting

The Bou-Azzer inlier is a critical element of theAnti-Atlas area (Fig. 1) during the Pan-Africanorogeny for the following reasons: (1) it containsoutcrops of the northern margin of the WAC anda succession of arc and oceanic crustal componentsthat record a progressive history of deformationand metamorphism in the region; (2) it is uncon-formably overlain by only slightly deformedPhanerozoic sedimentary sequences, implyingthat many of the original Neoproterozoic relation-ships survive; (3) it is intruded by post-tectoniccalc-alkaline intrusions. The Bou-Azzer inlier con-sists of a complex association of rock units occur-ring within a series of tectonic blocks surroundedby a latest Neoproterozoic to Early Cambriancover (Leblanc 1981). The tectonic blocks areseparated by oblique slip faults that are parallel tothe main suture zone with the WAC to the south

(Saquaque et al. 1989) and are believed to representdismembered parts of a subduction zone complex.

South of the Bou-Azzer inlier a deformedplatform sequence of quartzites and stromatoliticlimestones, which is overlain by basic lava flowsand a volcano-sedimentary pile (Leblanc 1975),rests upon basement previously interpreted asPalaeoproterozoic gneiss (c. 2 Ga) and as part ofthe WAC. However, D’Lemos et al. (2006)suggested a Neoproterozoic age for the whole base-ment, based on U–Pb dating and Nd isotopic signa-ture of the Tazigzaout gneiss. The passive marginunits occur in close proximity to a variety ofigneous, meta-igneous, and metasedimentaryrocks (Leblanc 1975). These include augen-gneissand muscovite pegmatite and leucogranite,which, on the basis of lithological similarity anddeformation, have been correlated with rocks ofthe nearby Zenaga Massif (Leblanc 1975) andhave been considered to be Eburnean basementof 2 Ga or older age by all previous workers.Northern parts of the Bou-Azzer inlier exposevolcano-sedimentary sequences (e.g. TichibinineFormation) considered to be part of an arc- orforearc-related sequence that is Neoproterozoic inage. Near the platform sequence crop out an intri-cately interleaved sequence of tectonic slicesincluding ophiolitic fragments, metavolcanic rocksand metasediments (Leblanc 1975). This sequence

Fig. 1. Geological sketch map of the Anti-Atlas Proterozoic belts in southern Morocco and the location of thestudy area.

R. BOUSQUET ET AL.234

is dominated by mafic–ultramafic plutonic bodiesthat had traditionally been viewed as parts of anophiolite series (Leblanc 1981), but were laterinterpreted as a melange complex (Saquaque et al.1989; Hefferan et al. 2002), with deformationtaking place at blueschist-facies metamorphicconditions (Hefferan et al. 2002).

The northern sector of the Bou-Azzer inlierconsists of a metasedimentary sequence withsubordinate volcanic units bearing calc-alkalineand island arc tholeiitic geochemical signatures(Naidoo et al. 1991). A pervasive greenschist-faciesfabric, associated with recumbent tight to isoclinalfolds, is variably developed within each of the tec-tonic blocks in the Bou-Azzer inlier, orientedNNE–SSW (Leblanc 1981). Kinematic analysisof these fabric elements in the southern part of theinlier is consistent with dominantly south-vergentsinistral oblique slip movements interpreted torecord thrusting of the Bou-Azzer complex ontothe WAC (Leblanc 1975; Saquaque et al. 1989).A clastic sedimentary succession, termed the Tiddi-line Formation, unconformably overlies the tectonicblocks (Leblanc 1975). The majority of the aboveunits are unconformably overlain by a thick succes-sion of subhorizontal ignimbrites and conglomer-ates termed the Ouarzazate Supergroup.

The earliest identifiable structure within theinlier occurs only in the gneissic basement. Itconsists of a NW–SE-striking, NE-dipping, uppergreenschist-facies ductile fabric and a mineralstretching lineation that dips shallowly to the NW.A lower-grade, greenschist-facies fabric overprintis the dominant structure within the inlier. Muchof the main schistosity is represented by a compo-site foliation and is associated with the developmentof folds and several generations of subparallelfoliations trending WNW–ESE. In the south, struc-tures associated with the main schistosity are domi-nated by WNW–ESE-striking composite foliationsthat dip steeply to the north with a commoneastward-plunging mineral stretching lineation(Inglis et al. 2005). Greenschist-facies structuresare repeatedly overprinted at successively lowertemperatures by increasingly brittle fault zones,duplexes and cataclastic shear zones with ageneral WNW–ESE orientation (Leblanc 1981).This is consistent with faults and shear zones thatcrosscut both the Tiddiline sedimentary successionand late orogenic intrusions such as the Bleidagranodiorite. A new precise U–Pb age of579.4+ 1.2 Ma for the Bleida granodiorite (Ingliset al. 2004) provides a firm constraint on the lateststages of brittle transcurrent movement in the

Table 1. Representative analyses of Na-amphiboles from Ait-Atman and diabase bulk-rock compositionused for the thermodynamic calculations

Sample: Ait021 Ait023c1 Bulk rock composition

Analysis no.: 2 5 7 9SiO2 51.60 52.74 52.96 51.9 53.41TiO2 0.04 0.11 0 0.04 0.32Al2O3 3.24 5.15 2.59 2.11 14.68Fe2O3 * * * * 3.00FeO 23.31 21.9 22.49 22.11 6.37MnO 0.25 0.35 0 0.03 0.20MgO 7.73 7.47 7.82 8.56 7.24CaO 2.18 2.5 2.83 2.98 7.86Na2O 7.94 5.61 7.03 8.03 4.02K2O 0.00 0.01 0.14 0.01 0.19Total 96.29 95.84 95.89 95.85 97.27

Si 7.739 7.735 7.961 7.873Ti 0.005 0.012 0.000 0.005Al 0.573 0.890 0.459 0.377Fe3þ† 0.931 1.233 0.632 0.535Fe2þ 1.993 1.453 2.196 2.271Mn 0.032 0.043 0.000 0.004Mg 1.728 1.633 1.752 1.936Ca 0.350 0.393 0.456 0.484Na 2.309 1.595 2.049 2.362K 0.000 0.002 0.027 0.002

*Not analysed.†Calculated.It should be noted that the Na-amphibole analyses are comparable with those of Hefferan et al. (2002).

MELANGES AND OPHIOLITES, MOROCCO 235

Bou-Azzer inlier. Block faulting and weak foldingduring the formation of the Atlas Mountains inMesozoic times resulted in the uplift and exhuma-tion of the Bou-Azzer basement inlier.

Subduction and thermal gradients

throughout Earth’s history

Reviews of the formation of the Archaean conti-nents (e.g. Kusky & Polat 1999) show that subduc-tion and collision processes during the Archaean atcrustal scale are not well known, and thus hardlydiscussed. Although many examples of wedgestructures are recognized in active mountain beltsdisplaying an HP–LT (blueschist–eclogite con-ditions) evolution, such as the Central Alps or theApennines in a Tethyan setting, and Japan(Shikoku island), the Franciscan Complex and thesouth of Alaska (Aleutians) in a Pacific setting,this mechanism may have also been active duringArchaean times (see review by Sengor 1999).

However, mechanisms of accretion within thewedge structure are supposed to have changedthrough time (Kusky & Polat 1999; Stern 2004).The thermal regime of orogenic belts through timehas been the subject of many discussions (seeStern 2005). Considering the fact that the oldestultrahigh-pressure (UHP) rocks that have beenfound until now occur only in the Pan-Africanbelt in Mali (Caby 1994), dated at around 620 Ma(Jahn et al. 2001), Maruyama & Liou (1998) pro-posed that no UHP rocks occur in the Archaeanand Proterozoic belts. They assumed that thermalregimes of subduction and collisional processeschanged through time (Fig. 2).

Other workers (Moller et al. 1995, 1998;O’Brien & Rotzler 2003; Reddy et al. 2003),knowing that medium-T eclogites and high-pressuregranulites are known from both old and youngmetamorphic terranes (e.g. c. 45 Ma, NamcheBarwa, Eastern Himalayas; 400–340 Ma, EuropeanVariscides; 620 Ma, African belt, Mali; 1.9 Ga,Snowbird, Saskatchewan (Baldwin et al. 2003,

Fig. 2. Metamorphic facies diagram (after Oberhansli et al. 2004) and subduction gradients occurring at differenttimes.Whereas eclogite and UHP conditions were probably possible at all times during Earth history, blueschist and LTeclogite conditions are documented only since Palaeozoic times.

R. BOUSQUET ET AL.236

2004); 2.0 Ga in Tanzania) and that many newoccurrences of UHP rocks are found as relicts inHP granulite terranes, argued that thermal andtectonic processes in the lithosphere have notchanged significantly since at least the end of theArchaean, and that HP conditions could haveexisted during Archaean times. Despite apparentcontradictions between these two views, discussionabout secular changes in the P–T regimes of subduc-tion processes will not constrain geothermal gradi-ents at any time, because the possible temperaturerange of UHP minerals is very large (see Chopin2003). In fact, UHP eclogite conditions may havebeen present even when the Earth was much hotter(Fig. 2). The large temperature range for UHP min-erals is supported by recent experiments showingthat partial melting of hydrous basalt under eclogite-facies conditions produces granitoid liquids withmajor- and trace-element compositions equivalentto those of Archaean tonalite–trondhjemite–granodiorite (TTG) (Rapp et al. 2003). Geothermsalong subduction zones (referred to below assubduction gradients) in the lithosphere are hencebetter constrained by occurrences of blueschists inearlier times. Blueschists require unusually cold

upper mantle geotherms, found only in recentsubduction zones (van Keken et al. 2002).

Metamorphic conditions of the

Bou-Azzer ophiolites

Petrology

Hefferan et al. (2002) described and mapped occur-rences of sodic amphiboles in the ophiolite suiteof Bou-Azzer. These sodic amphiboles occur in amineral assemblage together with garnet (ofgrossular-rich composition), epidote, albite, chloriteand quartz. Sodic amphiboles occur also in maficrocks (diabase and gabbro) always at the contactwith the Tiddiline Formation, which is mainly com-posed of greywackes and sandstones (see Hefferanet al. 2002, fig. 3).

As index minerals of blueschist facies formafic rocks, Na-amphiboles have been very import-ant for the characterization of metamorphicconditions. Whereas the chemical compositions ofNa-amphiboles characterizing blueschist faciesare generally Al3þ-rich (XAl � 0.6) with varying

1.0

0.8

0.6

0.4

0.2

0.0

1.0 0.8 0.6 0.4 0.2 0.0

0.0 0.2 0.4 0.6 0.8 1.0

Ferroglaucophane Riebeckite

Mg-RiebeckiteGlaucophane0.0

0.2

0.4

0.6

0.8

1.0

Crossite

XAl3+ = Al3+/(Al3+ + Fe3+)

XFe3+ = Fe3+/(Al3+ + Fe3+)

XM

g2

+ =

Mg

2+/(

Mg

2+

+ F

e2+)

XF

e 2+ =

Fe

2+/(M

g2+

+ F

e2+)

Blueschist

terranes

Greenschist

environments

Bou Azzer

ophiolites

Fig. 3. Mineral chemistry of sodic amphiboles in a Mg/(Fe2þþMg) v. Al/(Fe3þþAl) diagram regardless of theparageneses and the chemical compositions of the rocks. We note a clear difference between the chemistry ofNa-amphiboles from blueschist terranes and those growing under greenschist-facies conditions. The chemistry ofNa-amphiboles from the Bou-Azzer ophiolite is compatible with greenschist metamorphic conditions.

MELANGES AND OPHIOLITES, MOROCCO 237

Mg2þ content, ranging from glaucophane to ferro-glaucophane composition (Fig. 3; e.g. Oberhansli1986; Evans 1990; Bousquet et al. 1998; Katziret al. 2000), the compositions of Na-amphibolescharacterizing the greenschist facies are Al3þ-poor(XAl � 0.3) with varying Mg2þ content, rangingfrom crossite to riebeckite and Mg-riebeckitecompositions (Fig. 3; Oberhansli 1986; Evans1990; Frey et al. 1991). Mineral chemistry ofNa-amphiboles from Bou-Azzer ranges betweencrossite and Mg-riebeckite composition (Fig. 3), thetypical composition for greenschist-facies conditions.

P–T estimates

Although first-order analyses of chemical compo-sition of Na-amphiboles indicate greenschist-faciesconditions, we will quantify P–T conditions ofdiabase samples from the Bou-Azzer ophiolite bythermodynamic methods. Hefferan et al. (2002)suggested greenschist to lower blueschist facies(c. 7 kbar, c. 350 8C) based on the contents of Nav. AlIV (Brown 1977). New developments in ther-modynamic studies, however, show that mineralcompositions are controlled not only by pressureand temperature conditions but also by whole-rockchemistry (de Capitani & Brown 1987; Powell et al.1998; Connolly & Petrini 2002; Karpovet al. 2002).

Methods. The method used to determine P–Tconditions is based on the notion of ‘bulk-rockequilibrium’, which computes stable assemblages,including mode and composition of solutionphases, for specific chemical rock compositionsusing the program DOMINO (de Capitani 1994).The independent variables are any combinationof temperature, pressure, activity of a particularphase or compositional vectors. To include highlynon-ideal solution models for minerals with poten-tial miscibility gaps, stable mineral assemblages arecomputed using a Gibbs’ free energy minimization(de Capitani & Brown 1987). In such equilibriumphase diagrams, all phases are considered for eachpoint assuming complete thermodynamic equili-brium for the whole rock. In this case, each fieldrepresents the predicted stability field of a particularassemblage. However, the interpretation of the dia-grams is limited by the degree of equilibriumreached in rocks at each step of the metamorphicevolution. In none of the studied samples has anyrelic of the prograde, ‘burial’ evolution beenobserved; thus we assume that the rocks werefully equilibrated at the pressure peak of theirhistory. In this case, we can use the ‘bulk-rock equi-librium’ method only to constrain the peak pressure.The updated JAN92.RGB thermodynamic databaseof Berman (1988) was used for all calculations,

supplemented with the following thermodynamicdata: the Mg-chloritoid data of B. Patrick (listedby Goffe & Bousquet 1997), the Fe-chloritoiddata of Vidal et al. (1994), the chlorite data ofHunziker (2003), and the alumino-celadonite dataof Massonne & Szpurka (1997). Thermodynamicdata for riebeckite were not available in the data-base used, but we use Cp- and Volume-functionsof Holland & Powell (1998) for Na-amphibolesand experimental data from Holland (1988) forglaucophane, from Hoffmann (1972) for ferroglau-cophane, and from Ernst (1962) for riebeckite.The solution models for phengite from Parra et al.

(2002) and for chlorite from Hunziker (2003)have been used.

Results. The equilibrium phase diagram for Bou-Azzer ophiolites shows that the stability field ofthe assemblage Na-amphibole, garnet, epidote,albite, chlorite and quartz in a diabase compositionis well constrained in pressure between 5 and 9 kbarfor temperatures varying between 300 and 600 8C(Fig. 4a). In this stability field, the composition ofdifferent minerals, specificially Na-amphibole,varies according to P and T. Whereas riebeckiteand ferro-glaucophane components show oppositechemical evolution trends mainly controlled bytemperature, with increase in riebeckite (Fig. 4b)and decrease in ferro-glaucophane (Fig. 4c)towards higher temperature conditions, the glauco-phane component is controlled by pressure as wellas temperature (Fig. 4d).

Based on the isopleths of different Na-amphibole end-members within the stability fieldof the Na-amphibole, garnet, epidote, albite, chlor-ite and quartz association, we can better constrainmetamorphic conditions experienced by the dia-bases of Bou-Azzer. For the mineral assemblagedescribed the composition of Na-amphibolesvaries from pure glaucophane compositions atlower temperatures (below 400 8C) to riebeckitecomposition at higher temperatures conditionsbetween 500 and 600 8C (Fig. 5). At intermediatetemperature conditions, the composition is pressuredependent (Fig. 5). The sodic amphibole compo-sitions found in Bou-Azzer rock samples arestable between 4 and 6 kbar for temperaturesbetween 420 and 600 8C.

As mentioned above, Hefferan et al. (2002)suggest greenschist to lower blueschist facies(c. 7 kbar, c. 350 8C) based on the content of Nav. content of AlIV (Brown 1977). However, accord-ing to many facies definitions (Yardley 1989; Spear1993; Oberhansli et al. 2004) pressures between 4and 6 kbar for temperatures between 420 and600 8C are typical for low-temperature to high-temperature (epidote–) greenschist conditions.

R. BOUSQUET ET AL.238

Melanges and ophiolites

Tectonic melanges are one of the hallmarks of con-vergent margins, yet understanding their genesisand relationships of specific structures to platekinematic parameters has proven elusive becauseof the complex and seemingly chaotic nature ofthese units. Many field workers regard melangesas too deformed to yield useful information, andsimply map the distribution of melange type rocks

without further investigation. Other workersmap clasts and matrix types, search for fossils ormetamorphic index minerals in the melange, andassess the origin and original nature of the highlydisturbed rocks. Analysis of deformational fabricsin tectonic melange may also yield informationabout the kinematics of past plate interactions(e.g. Le Pichon et al. 1988; Cowan & Brandon1994; Kusky et al. 1997).

No Albite

No

Ga

rne

t

No Na-AmphiboleNo Na-Amphibole

Riebeckite

end-member

300 400 500 600

Temperature [°C]

2

4

6

8

10

Pre

ss

ure

[k

ba

r]

Th

er

i a- D

mn

o

k o

i

No Albite

No

Ga

rne

t

No Na-AmphiboleNo Na-Amphibole

Fe-Glaucophane

end-member

300 400 500 600

Temperature [°C]

2

4

6

8

2

4

6

8

10

Pre

ss

ure

[k

ba

r]

Th

er

i a- D

mn

o

k o

i No Albite

No

Ga

rne

t

No Na-AmphiboleNo Na-Amphibole

Glaucophane

end-member

300 400 500 600

Temperature [°C]

2

4

6

8

10P

res

su

re [

kb

ar]

Th

er

i a- D

mn

o

k o

i

Alb-Gt-Chl

Ep-Gln

Alb-Gt-Chl

Ep-Gln

Alb-Gt-Chl

Ep-Gln

0.1 0.2

0.2

0.2

0.1

0.3

0.3

0.3

0.4

0.4

0.40.5

0.5

0.6

0.60.7

No Albite

No

Garn

et

No

Na-AmphiboleNo

Na-Amphibole

300 400 500 600

Temperature [°C]

10P

res

su

re [

kb

ar]

Th

er

i a- D

mn

o

k o

i

(b)

(d)

(a)

(c)

Alb-Gt-Chl

Ep-Gln

0.5

Stability field

Fig. 4. Stability field of mineral assemblage composed of Na-Amphþ garnetþ epidoteþ albiteþ chloriteþ quartzfor diabase from the Bou-Azzer ophiolitic suite (a). Computed isopleths of riebeckite (b), ferro-glaucophane(c) and glaucophane (d) in the stability field of the mineral assemblage Na-Amphþ garnetþ epidoteþ albiteþchloriteþ quartz. All diagrams are computed using the THERIAK-DOMINO software (http://titan.minpet.unibas.ch/minpet/theriak/theruser.html). This is a program collection by de Capitani (1994) to calculate and plot thermodynamicfunctions, equilibrium assemblages and rock-specific equilibrium assemblage diagrams (elsewhere also calledpseudo-sections). Based on its approach to equilibrium by means of Gibbs’ free energy minimization (see de Capitani& Brown 1987) rather than solving complex and large equation systems, the THERIAK-DOMINO software calculatesand plots without user intervention that might be a source of errors.

MELANGES AND OPHIOLITES, MOROCCO 239

Melange is a special type of breccia containinglocal or exotic competent blocks embedded in aless competent matrix in regions where high-levelincompletely consolidated and unlithified sedi-ments have been disturbed by imbricate faultingor gravitational gliding (Greenly 1891; Ramsay &Hubert 1987). A melange is defined on the basisof the following criteria: (1) a melange must be amappable unit (typically at 1:25 000 scale); (2) itincludes blocks of many sizes and diverse litho-logies, some of which are ‘exotic’ (i.e. not derivedfrom immediately adjacent units); (3) it has amatrix of fine-grained material, typically shale,slate, or serpentinite, with a tectonic fabric; (4)the matrix supports the blocks, which are not incontact with each other.

Ophiolites are commonly associated withunderlying continental margin units by melange.Melanges can be subdivided into two generaltypes (Fig. 6): (1) melanges that are sandwichedbetween ophiolites and underlying continentalmargin units; (2) melanges that form large

exposures and commonly include basic volcanicrocks, cherts and serpentinite, as blocks andbroken formations. The latter can be classified asaccretionary complexes. Intact ophiolites are notfound regionally overlying such melanges, althoughophiolitic melange is widespread (Gansser 1974).The melanges as a whole may be of tectonic,sedimentary or composite origin in differentexamples. Those of mainly sedimentary origin com-monly correspond to the olistostromes (with olisto-liths) of the classical literature.

Melanges beneath ophiolites

Melanges are commonly found between overridingophiolites and underlying continental marginunits (Fig. 6a). Melanges and broken formationswere traditionally seen as deformed thrust sheetsof ‘volcanic–sedimentary’ successions in whichthe present complexity was the result of a pervasivefaulting of otherwise coherent units duringemplacement (Oman, Eastern Mediterranean). In

Fig. 5. Computed Na-amphibole chemistry within the stability field of Na-Amphþ garnetþ epidoteþ albiteþchloriteþ quartz for a basaltic composition. Each bold vertical line represents a composition range of Na-amphibole ateach point in the stability field. The chemistry of Na-amphiboles occurring in the Bou-Azzer inlier documents HTgreenschist metamorphic conditions at 4 and 6 kbar for temperatures between 420 and 600 8C.

R. BOUSQUET ET AL.240

addition, melanges play an important role in ophio-lite emplacement in many areas such as Albania,the Himalayas (e.g. Corfield et al. 1999; Robertson2000; Hock et al. 2002). Facies analysis, andgeochemical and structural studies show thatmost of the accreted melanges range in settingfrom the distal continental rise to open oceanic.In summary, melanges beneath ophiolites areproduced by obduction of large ophiolites ontocontinental margins. They record the obductionprocesses of oceanic crust and deep-sea sediments(Robertson 2004). Such melanges are formedduring the thrusting of the ophiolites onto thecontinental margin.

Accretionary complexes

The second type of melange is very widespreadbut is not found directly beneath large overridingophiolites (Fig. 6b). Such melanges are typicallycomplex and often affected by multiple defor-mation events such that any genetic distinctionbetween sedimentary and tectonic origins is diffi-cult and unreliable. The main impetus for therecognition of melanges as recording subductionof oceanic crust came instead from studies of theFranciscan Complex. This melange includesophiolitic material (e.g. serpentinite), but is notoverlain by any ophiolite, as is the case in

many accretionary complexes (e.g. Alps, Bousquetet al. 2002; Bousquet 2008; Sulawesi, Parkinson1996; Olympic Mountains, Brandon & Vance1992). Blocks in the Franciscan Complex includeblueschist metamorphic rocks with glaucophaneand lawsonite, which indicate HP–LT metamorph-ism. All the blocks, like ultramafic rocks, areembedded in a fine-grained matrix composedmainly of black shale. No consensus on the tec-tonic or sedimentary origin of the FranciscanComplex exists yet; some researchers have inferreda mainly tectonic origin (e.g. Cloos 1984), whereasothers have envisaged a mainly sedimentary origin(e.g. Cowan 1978).

Accretionary complexes similar to the FranciscanComplex occur widely along the Tethys belt fromthe Alps (Bousquet et al. 2002) to New Caledonia(Potel et al. 2006) including the Eastern Mediterra-nean region (Jolivet et al. 1998; Robertson 2004),Turkey (Okay et al. 1996), Iran (Gansser 1974)and Sulawesi (Parkinson 1996). All examples, dis-playing HP–LT metamorphism, are regionallyassociated with subduction, but are not directlyoverlain by ophiolites.

The various melanges, rather than the ophiolitesthemselves, are important indicators of the formerexistence of oceanic areas. The main reason isthat most of the ophiolites preserve unusual tectonicsettings (e.g. suprasubduction-zone genesis),

Fig. 6. Schematic models of the emplacement of ophiolites in subduction processes. (a) Obducted ophiolites are oftenassociated with melanges at their base ((1) Hock et al. 2002; (2) Collins & Robertson 1997; (3) Ferriere et al. 1988;(4) Searle & Malpas 1980). This type of melange is a consequence of the thrusting of the ophiolitic sequenceover a platform. The matrix is composed either of serpentinite or of sediments. (b) Accretionary complexes displayultramafic and/or mafic rocks embedded in a sedimentary matrix, but are not overlain by an ophiolitic sequence. Oftensuch mafic rocks were deeply buried at blueschist metamorphic conditions ((5) Cloos 1986; (6) Parkinson 1996;(7) Bousquet 2007; (8) Bousquet et al. 2002; (9) Schwartz et al. 2000; (10) Brandon & Calderwood 1990).

MELANGES AND OPHIOLITES, MOROCCO 241

whereas most ‘normal’ mid-ocean ridge basalt(MORB) was subducted. When such subductiontakes place only fragments of oceanic crustand sediments are preserved, including serpen-tinite, volcanic seamounts and related pelagicsediments.

The Bou-Azzer ophiolite: melange beneath

ophiolites or accretionary complex?

Saquaque et al. (1989) and Hefferan et al. (2002)described the Bou-Azzer inlier as resulting froman accretionary complex formed during the

Fig. 7. Geological map of the western part of the Bou-Azzer inlier (modified after Leblanc 1975). Cross-sections 1and 2 are shown in figure 8.

Fig. 8. Geological cross-sections across the Bou-Azzer inlier (locations are shown in fig. 7; modified after Leblanc1975; Saquaque et al. 1989). Structural relations between the units clearly show that the melange sequence islocated structurally above the basement and below the ophiolitic sequence.

R. BOUSQUET ET AL.242

Pan-African subduction and not representing acoherent sample of Precambrian oceanic litho-sphere obducted onto the Eburnean margin(Leblanc 1976). In this model the whole Bou-Azzerinlier is considered to be an accretionary complexformed by a huge melange juxtaposed to the

ophiolite (Fig. 7). However, coherent petrographicand stratigraphic relationships can be traced alongstrike, in some cases for kilometres (Leblanc &Billaud 1978; Church 1991; Leblanc & Moussine-Pouchkine 1994). The major part of the inlier iscomposed of different ophiolite sequences

Fig. 9. Model for the emplacement of the Bou-Azzer ophiolitic suite. 750–700 Ma, north-dipping subduction of thePan-African ocean; 680–660 Ma, formation of melange and HT greenschist facies metamorphic overprint duringobduction and terrane assembly; 650–640 Ma, syncollisional magmatism crosscutting earlier tectonic structures.A northern continent is assumed based on the absence of Pan-African age oceanic crust north of Bou-Azzer.

MELANGES AND OPHIOLITES, MOROCCO 243

including ultramafic rocks that are juxtaposed alongfaults and shear zones (Fig. 8), indicating earlytop-to-the-south movements (Leblanc 1975).

The melange is limited to the southern partof the inlier at the contact with the basement(Fig. 8). The matrix of the melange varies fromserpentinite to strongly deformed volcano-sedimentary rocks. Blocks mainly consist of ophio-litic fragments, metagreywackes and quartzites.Occurrences of metabasaltic breccias within themelange testify to the tectonic origin of theblocks. The melange forms slices interspersedbetween the ophiolite sequence in the north and abasement in the south. This geometry, alreadydescribed by Saquaque et al. (1989), suggests thatthe melange forms the base of the ophioliticrocks. Thus we interpret the whole sequence asan ophiolite obducted southwards onto the WACplatform with its underlying melange (Fig. 9).

After the obduction of the ophiolite, we noteaccretion of terranes, such as island arcs, to theophiolitic complex. During terrane assembly, theophiolites were dismembered and thickened,allowing HT greenschist metamorphic conditions(Fig. 9). During collision between the northerncontinent and the WAC, diorite to quartz dioritesyntectonic plutons and diabase dykes intrudedthe different accreted terranes. Radiometric dateson various granodiorites of Bou-Azzer haveyielded ages between 650 and 640 Ma (Ingliset al. 2004, 2005). All units were subsequentlydeformed and metamorphosed under lowergreenschist-facies conditions, indicated by thegrowth of chlorite within shear zones in grano-diorite. The entire inlier appears to be a complexensemble of diverse igneous, metamorphic andsedimentary rock units that have been accreted,juxtaposed and deformed at different timesduring the closure of the Pan-African ocean(Fig. 9) and not assembled at the same time inan accretionary complex.

Several studies (Saquaque et al. 1989; Hefferanet al. 2000, 2002; Ennih & Liegeois 2001) clearlyshow that the Bou-Azzer ophiolitic suite is theremnants of a fore-arc assemblage that evolvedabove a north-dipping subduction zone. Despitethis, the dip orientation of the subduction in theMoroccan Anti-Atlas during the Pan-Africanorogen is still controversial (Gasquet et al.

2005). However, the present geometry of theophiolite suite (Soulaimani et al. 2006), the accre-tion sequence with a melange at its base at thecontact with the WAC basement in the south,and the early top-to-the-south sense of shearcombined with the calc-alkaline volcanism in thenorth (Saghro massif, Saquaque et al. 1992)clearly allow us to argue for a north-dippingsubduction.

The present study shows that the ophiolite suite ofBou-Azzer is not part of an accretionary complex,but is an obducted ophiolite with a melange at itsbase. The ophiolite sequence was dismembered anddelaminated during its accretion onto the WACmargin. No evidence for HP–LT (blueschist-facies)metamorphic conditions can be found in these rocks.Metamorphic conditions experienced by the ophio-lites are limited to HT greenschist-facies conditions(5–6 kbar, 500–550 8C), which are typical conditionsfor collision or obduction settings. We therefore con-sider the ophiolites of Bou-Azzer not to be similarto the HP ophiolites of the Franciscan Complexor medium-pressure ophiolites of Sanbagawa, butrather similar to the Albanian or Cyprus ophiolites,which are dismembered ophiolite sequences over-printed by greenschist-facies conditions.

R.B. thanks R. Caby for fruitful discussions and for hisencouragement to write this paper. Funding by theFrench–Moroccan project MA No. 13 and the Universityof Basel are greatly appreciated. This paper benefited fromconstructive comments of K. Hefferan, J. C. Schumacherand D. Marquer. N. Ennih is thanked for his helpfuleditorial work.

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