Bou Azzer ophiolites
Mélanges and ophiolites during the Pan-African orogeny:
the case of the Bou-Azzer ophiolite suite (Morocco)
by Romain Bousquet(1), Rachid El Mamoun(2), Omar Saddiqi(2),
Bruno Goffé(3), Andreas Möller(1), Atman Madi(4)
Citation of this article:
Bousquet R., El Mamoun, R., Saddiqi, O. & Goffé, B., Mélanges and ophiolites: was the Bou-Azzer’s ophiolite suite (Morocco) a Franciscan-type wedge during the Pan-African orogeny? in The Boundaries of the West African Craton Eds Ennih, N. & Liégois, J.-P., Geological Society, London, Special Publications, 297: 233-247
Abstract
Since the discovery of ophiolite sequences, the Bou-Azzer inlier has been named a key area to understand the evolution of the northern margin of the Western African craton during the Pan-African. For about twenty years, it had been commonly accepted that the Bou-Azzer inlier represents an accretionary mélange accreted onto the Western African under blueschist metamorphic conditions, similar to the Franciscan complex and the Sanbagawa facies series. This would imply that a low geothermal gradient was prevalent during the subduction of the Pan-African and that the ocean was subducted with a high convergence rate.
A reinvestigation of the metamorphic conditions by a thermodynamic approach shows that the ophiolite sequence of Bou-Azzer underwent HT-greenschist metamorphic conditions instead of blueschist metamorphic conditions. We propose that the ophiolites of Bou-Azzer are not similar to the Sanbagawa facies series or to the Franciscan complex but bears similarities to the Albanian or Cyprus ophiolites that represent dismembered ophiolite sequences overprinted by greenschist conditions.
Introduction
Ophiolites are remnants of oceanic lithosphere that have been tectonically emplaced onto continents. Well preserved ophiolite sections consist of (in descending stratigraphic order) pillow lava, sheeted dike complex, gabbro, cumulate ultramafics, and tectonized mantle. Ophiolites provide valuable information on the nature of seafloor processes, global heat loss, paleogeographic reconstructions of the continents, and the subduction processes. However, the mechanisms of ophiolite accretion onto the continental margins are debatable. Did subduction and obduction change through time (van der Velden & Cook 1999) or not (Kusky & Polat 1999)? Did the thermal gradient of subduction and orogenic wedge change through time (Maruyama & Liou 1998) or not O′Brien & Rötzler (2003)?
Since the discovery of the first Precambrian ophiolite sequence in the Bou-Azzer inlier (Leblanc 1976), this inlier has played a critical role in the understanding the evolution of the northern margin of the Western African craton (WAC) during the Pan-African (Leblanc & Lancelot 1980; Saquaque et al. 1989; Hefferan et al., 2000). This ophiolite, together with the Khzama ophiolite (Sirwa inlier, Admou & Juteau 1998), represents a unique remnant of a Pan-African ocean, although the exact localization of the Pan-African suture is in question (see discussion in Ennih & Liégeois, 2001; Bouougri, 2003). It has been proposed that the Bou-Azzer ophiolites were buried to depths sufficient to generate lower blueschist facies mineral assemblages in a scraped-off mélange (Hefferan et al. 2002). Such an accretion mechanism of the Bou-Azzer ophiolite onto the WAC is similar to the accretion of the Franciscan mélange along the North American West Coast. This mechanism implies that a low geothermal gradient was prevalent during the subduction of the Pan-African oceanic plate or that the oceanic plate was subducted at a high convergence rate. Can we really document such conditions for the ophiolite of Bou-Azzer?
This study first reviews the mechanisms of ophiolite accretion, formation of mélanges and the evolution of the thermal gradients occurring in subduction zones through time, and then reinvestigates the metamorphic PT conditions experienced by the ophiolite suite of Bou-Azzer, using thermodynamic methods, to re-evaluate the mechanisms of accretion of the ophiolites onto the WAC.
Geological setting
The Bou-Azzer inlier is a critical element of the Anti-Atlas area (Fig. 1) during the Pan-African orogeny because (a) it contains outcrops of the northern margin of the WAC and a succession of arc and oceanic crustal components that record a progressive history of deformation and metamorphism in the region; (b) it is unconformably overlain by only slightly deformed Phanerozoic sedimentary sequences, implying that many of the original Neoproterozoic relationships survive; (c) it is intruded by post-tectonic calc-alkaline intrusions. The Bou-Azzer inlier consists of a complex association of rock units occurring within a series of tectonic blocks surrounded by a latest Neoproterozoic to Early Cambrian cover (Leblanc 1981). The tectonic blocks are separated by oblique slip faults that are parallel to the main suture zone with the WAC to the south (Saquaque et al. 1989) and are believed to represent dismembered parts of a subduction zone complex.
 Figure 1: Geological sketch map of the Anti-Atlas Proterozoic belts in southern Morocco and the location of the studied area. |
South of the Bou-Azzer inlier a deformed platform sequence of quartzites and stromatolitic limestones, which is overlain by basic lava flows and a volcano-sedimentary pile (Leblanc 1975), rests upon basement previously interpreted as Paleoproterozoic gneiss (≈2 Ga) and as part of the WAC. However, D′Lemos et al. (2006) suggested a Neoproterozoic age for the whole basement, based on U-Pb dating and Nd isotopic signature of the Tazigzaout gneiss. The passive margin units occur in close proximity to a variety of igneous, meta-igneous, and metasedimentary rocks (Leblanc 1975). These include augen-gneiss and muscovite pegmatite and leucogranite, which, on the basis of lithological similarity and deformation, have been correlated with rocks of the nearby Zenaga Massif (Leblanc 1975) and have been considered to be Eburnean basement of 2 Ga or older age by all previous workers. Northern parts of the Bou-Azzer inlier expose volcano-sedimentary sequences (e.g., Tichibinine Formation) considered to be part of an arc/forearc related sequence that is Neoproterozoic in age. Near the platform sequence crop out an intricately interleaved sequence of tectonic slices including ophiolitic fragments, metavolcanic rocks, and metasediments (Leblanc 1975). This sequence is dominated by mafic-ultramafic plutonic bodies that had traditionally been viewed as parts of an ophiolite serie (Leblanc 1981), but later on interpreted as a mélange complex (Saquaque et al. 1989; Hefferan et al. 2002), with deformation taking place at blueschist facies metamorphic conditions (Hefferan et al. 2002). The northern sector of the Bou-Azzer inlier consists of a metasedimentary sequence with subordinate volcanic units bearing calc-alkaline and island arc tholeiitic geochemical signatures (Naidoo et al. 1991). A pervasive greenschist facies fabric, associated with recumbent tight to isoclinal folds, is variably developed within each of the tectonic blocks in the Bou-Azzer inlier, orientated NNE (Leblanc 1981). Kinematic analysis of these fabric elements in the southern part of the inlier is consistent with dominantly south-vergent sinistral oblique slip movements interpreted to record thrusting of the Bou-Azzer complex onto the WAC (Leblanc 1975; Saquaque et al. 1989). A clastic sedimentary succession, termed the Tiddiline Formation, unconformably overlies the tectonic blocks (Leblanc 1975). The majority of the above units are unconformably overlain by a thick succession of subhorizontal ignimbrites and conglomerates termed the Ouarzazate Supergroup. The earliest identifiable structure within the inlier occurs only in the gneissic basement. It consists of a NW-striking, NE-dipping, upper greenschist facies ductile fabric and a mineral stretching lineation that dips shallowly to the NW. A lower-grade, greenschist facies fabric overprint is the dominant structure within the inlier. Much of the main schistosity is represented by a composite foliation and is associated with the development of folds and several generations of sub-parallel foliations trending WNW. In the south, structures associated with the main schistosity are dominated by WNW-striking composite foliations that dip steeply to the north with a common eastward-plunging mineral stretching lineation (Inglis et al. 2005). Greenschist facies structures are repeatedly overprinted at successively lower temperatures by increasingly brittle fault zones, duplexes, and cataclastic shear zones with a general WNW orientation (Leblanc, 1981). This is consistent with faults and shear zones that crosscut both the Tiddiline sedimentary succession and late orogenic intrusions such as the Bleida granodiorite. A new precise U-Pb age of 579.4±1.2 Ma for the Bleida granodiorite (Inglis et al. 2004) provides a firm constraint on the latest stages of brittle transcurrent movement in the Bou-Azzer inlier. Block faulting and weak folding during the formation of the Atlas Mountains in Mesozoic times resulted in the uplift and exhumation of the Bou-Azzer basement inlier.
Subduction and thermal gradients throughout Earth′s history
 Figure 2: Geological sketch map of the Anti-Atlas Proterozoic belts in southern Morocco and the location of the studied area. |
Reviews about the formation of the Archean continents (e.g. Kusky & Polat 1999) show that subduction and collision processes during the Archean at crustal scale are not well known, and thus hardly discussed. While many examples of wedge-structures are recognised in active mountain belts displaying an HP-LT (Blueschist-eclogite conditions) evolution, such as the Central Alps or the Apennines in a Tethys setting, and Japan (Shikoku island), the Franciscan Complex or the South of Alaska (Aleutians) in a Pacific setting, this mechanism may have also been active during Archean times (see review by Sengör 1999). However, mechanisms of accretion within the wedge-structure are supposed to have changed through time (Kusky & Polat 1999; Stern 2004). The thermal regime of orogenic belts through time is subject of many discussions (see Stern 2005). Considering the fact that the oldest Ultra-High-Pressure (UHP) rocks that have been found until now occur only in the Pan-African belt in Mali (Caby 1994), dated at around 620 Ma (Jahn et al. 2001), Maruyama & Liou (1998) proposed that no UHP rocks are occurring in the Archean and Proterozoic belts. They assumed that thermal regimes of subduction and collisional processes changed through time (Fig. 2). Other authors (Möller et al., 1995, 1998; Reddy et al, 2003; O′Brien & Rötzler, 2003) knowing that medium-T eclogites and high-pressure granulites are known from both old and young metamorphic terranes (e.g. ca. 45 Ma, Namche Barwa, E Himalaya; 400-340 Ma, European Variscides; 620 Ma, African belt, Mali; 1.9 Ga, Snowbird, Saskatchewan (Baldwin et al. 2003, 2004, 2.0 Ga in Tanzania) and that many new occurrences of UHP rocks are found as relics in HP-granulites terranes, argue that thermal and tectonic processes in the lithosphere have not changed significantly since at least the end of the Archean, and that HP conditions could have existed during Archean times. Despite apparent contradictions between these two views, discussion about secular changes in the P-T regimes of subduction processes will not constrain geothermal gradients at any time, because the possible temperature range of UHP minerals is very large (see Chopin, 2003). In fact, UHP-eclogite conditions may have been present even when the Earth was much hotter (Fig. 2). The large temperature range for UHP minerals is supported by recent experiments showing that partial melting of hydrous basalt under eclogite facies conditions produces granitoid liquids with major- and trace- element compositions equivalent to Archean TTG (Rapp et al. 2003). Geotherms along subduction zones (referred to below as subduction gradients) in the lithosphere are hence better constrained by occurrences of blueschists in older time. Blueschists require unusually cold upper mantle geotherms, only found in recent subduction zones (van Keken et al. 2002).
Metamorphic conditions of the Bou-Azzer Ophiolites
Petrology
 Figure 3: Mineral chemistry of sodic amphiboles in a Mg/(Fe2++Mg) vs. Al/(Fe3++Al) diagram regardless of the parageneses and the chemical compositions of the rocks. We note a clear difference betwenn the chemistry of Na-amphiboles from blueschist terranes and those growing under greenschist facies conditions. The chemistry of Na-amphiboles from the Bou-Azzer ophiolite is compatible with greenschist metamorphic conditions. |
Hefferan et al. (2002) described and mapped occurrences of sodic amphiboles in the ophiolite suite of Bou-Azzer. These sodic amphiboles occur in a mineral assemblage together with garnet (of grossular-rich composition), epidote, albite, chlorite and quartz. Sodic amphiboles occur also in mafic rocks (diabase and gabbro) always at the contact with the Tiddiline formation mainly composed of greywackes and sandstones (see Fig. 3 of Hefferan et al. 2002).
As index minerals of blueschist facies for mafic rocks, Na-Amphiboles have been very important for the characterization of metamorphic conditions. While the chemical composition of Na-Amphiboles characterizing blueschist facies are generally Al3+-rich (XAl ≥ 0.6) with varying Mg2+-content, ranging from Glaucophane to ferro-Glaucophane composition (Fig. 3, e.g. Oberhänsli 1986; Evans 1990; Bousquet et al. 1998; Katzir et al. 2000), the composition of Na-Amphiboles characterizing the greenschist facies are Al3+-poor (XAl ≤ 0.3) with varying Mg2+-content, ranging from Crossite to Riebeckite and Mg-riebeckite compositions (Fig. 3, Oberhänsli 1986; Evans 1990; Frey et al. 1991).
Mineral chemistry of Na-amphiboles from Bou-Azzer ranges between Crossite and Mg-Riebeckite composition (Figure 3), the typical composition for greenschist facies conditions.
PT estimates
Although first order analyses of chemical composition of Na-Amphiboles are indicating greenschist facies conditions, we will quantify P-T conditions of diabase samples from the Bou-Azzer ophiolite by thermodynamic methods. Hefferan et al. (2002) suggest greenschist to lower blueschist facies (≈7 kbar, ≈350°C) based on the contents of Na vs. AlIV (Brown, 1977). New developments in thermodynamic studies however, show that mineral compositions are not only controlled by pressure and temperature conditions but also by whole rock chemistry (de Capitani & Brown, 1987; Powell et al., 1998; Connolly & Petrini, 2002; Karpov et al., 2002).
Methods
The method used to determine P-T conditions is based on the notion of “bulk rock equilibrium” that computes stable assemblages, including mode and composition of solution phases, for specific chemical rock compositions using the program DOMINO (De Capitani 1994). The independent variables are any combination of temperature, pressure, activity of a particular phase or compositional vectors. In order to include highly non-ideal solution models for minerals with potential miscibility gaps, stable mineral assemblages are computed using a Gibbs free energy minimization (De Capitani & Brown 1987). In such equilibrium phase diagrams, all phases are considered for each point assuming complete thermodynamic equilibrium for the whole rock. In this case each field represents the predicted stability-field of a particular assemblage. However, the interpretation of the diagrams is limited by the degree of equilibrium reached in rocks at each step of the metamorphic evolution. In none of the studied samples has any relic of the prograde, “burial” evolution been observed, thus we assume rocks were fully equilibrated at the pressure peak of their history. In this case we can only use the “bulk rock equilibrium” method to constrain the peak pressure. The updated JAN92.RGB thermodynamic database of Berman (1988) was used for all calculations, supplemented with the following thermodynamic data: the Mg-chloritoid data of B. Patrick (listed in Goffé & Bousquet 1997), the Fe-chloritoid data of Vidal et al. (1994), the chlorite data of Hunziker (2003), and the alumino-celadonite data from Massonne & Szpurka (1997). Thermodynamic data for Riebeckite were not available in the used database, but we use Cp- and Volume- functions of Holland & Powell (1998) for Na-amphiboles and experimental data from Holland (1988) for glaucophane, from Hoffman (1972) for ferroglaucophane 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.
 Figure 4: Stability field of mineral assemblage composed of Na-Amph + garnet + epidote + albite + chlorite + quartz for diabase from the Bou-Azzer ophiolitic suite (a). Computed isopleths of Riebeckite (b), of Ferro-Glaucophane (c) and of Glaucophane (d) in the stability field of mineral assemblage Na-Amph + garnet + epidote + albite + chlorite + quartz. All diagrams are computed using the THERIAK-DOMINO software (cf. http://titan.minpet.unibas.ch/minpet/theriak/theruser.html). It is a program collection written by C. de Capitani (De Capitani, 1994) to calculate and plot thermodynamic functions, equilibrium assemblages and rock-specific equilibrium assemblage diagrams (elsewhere also called pseudo-sections). Based on its approach to equilibrium by means of Gibbs free energy minimization (c.f. de Capitani & Brown, 1987) rather than solving complex and large equation systems, the Theriak-Domino software calculates and plots without user intervention that might be a source of errors. |
Results
The equilibrium phase diagram for Bou-Azzer ophiolites shows that the stability field of the assemblage Na-amphibole, garnet, epidote, albite, chlorite and quartz in a diabase composition is well constrained in pressure between 5 and 9 kbar for temperatures varying between 300 and 600°C (Fig. 4a). In this stability field, the composition of different minerals, specificially Na-amphibole, varies according to P and T. While Riebeckite and Ferro-Glaucophane components show opposite chemical evolution trends mainly controlled by temperature, with increase of the Riebeckite (Fig. 4b) and decrease of the Ferro-Glaucophane component (Fig. 4c) towards higher temperature conditions, the Glaucophane component is controlled by pressure as well as temperature (Fig. 4d).
 Figure 5: Computed Na-amphibole chemistry within the stability field of Na-Amph + garnet + epidote + albite + chlorite + quartz for a basaltic composition. Each vertical line represents a composition range of Na-amphibole at each point in the stability field. The chemistry of Na-amphiboles occurring in the Bou-Azzer inlier are documenting HT-greenschist metamorphic conditions at around 420-600°C at 4-6 kbar. |
Based on the isopleths of different Na-amphibole end-members within the stability field of the Na-amphibole, garnet, epidote, albite, chlorite and quartz association, we can better constrain metamorphic conditions experienced by the diabases of Bou-Azzer. For the described mineral assemblage the composition of Na-amphiboles varies from pure Glaucophane compositions at lower temperatures (below 400°C) to Riebeckite composition at higher temperatures conditions between 500 and 600°C (Fig. 5). At intermediate temperature conditions, the composition is pressure dependent (Fig. 5). The sodic amphibole compositions found in Bou-Azzer rock samples are stable between 4 and 6 kbar for temperatures between 420 and 600°C.
As mentined above, Hefferan et al (2002) suggest greenschist to lower blueschist facies (≈7 kbar, ≈350°C) based on the content of Na vs. content AlIV (Brown, 1977). But according to many facies definitions (Yardley, 1989; Spear, 1993; Oberhänsli et al., 2004) pressures between 4 and 6 kbar for temperatures between 420 and 600°C are typical for low-temperature to high-temperature (epidote-) greenschist conditions.
Mélanges and ophiolites
Tectonic mélanges are one of the hallmarks of convergent margins, yet understanding their genesis and relationships of specific structures to plate kinematic parameters has proven elusive because of the complex and seemingly chaotic nature of these units. Many field workers regard mélanges as too deformed to yield useful information, and simply map the distribution of mélange type rocks without further investigation. Other workers map clasts and matrix types, search for fossils or metamorphic index minerals in the mélange, and assess the origin and original nature of the highly disturbed rocks.
Analysis of deformational fabrics in tectonic mélange may also yield information about the kinematics of past plate interactions (e.g. Le Pichon et al. 1988; Cowan & Brandon 1994; Kusky et al. 1997)
Mélange is a special type of breccia containing local or exotic competent blocks embedded in a less competent matrix in regions where high level incompletely consolidated and unlithified sediments have been disturbed by imbricate faulting or gravitational gliding (Greenly 1891; Ramsay & Hubert 1987). A mélange is defined on the basis of the following criteria. i) a mélange must be a mapable unit (typically at 1:25000 scale). ii) It includes blocks of many sizes and diverse lithologies, some of which are "exotic" - i. e. not derived from immediately adjacent units. iii) It has a matrix of fine-grained material - typically shale, slate, or serpentinite, with a tectonic fabric. iv) The matrix supports the blocks that are not in contact with each other.
Ophiolites are commonly associated to underlying continental margin units by mélange.
The mélanges can be subdivided into two general types (Fig. 6): (1) mélanges that are sandwiched between ophiolites and underlying continental margin units; (2) other mélanges that form large exposures and commonly include basic volcanic rocks, cherts and serpentinite, as blocks and broken formations, they can be classified as accretionary complexes. Intact ophiolites are not found regionally overlying such mélanges, although ophiolitic mélange is widespread (Gansser 1974). The mélanges as a whole may be of tectonic, sedimentary, or composite origin in different examples. Those of mainly sedimentary origin commonly correspond to the olistostromes (with olistoliths) of the classical literature.
Mélanges beneath ophiolites
Mélanges are commonly found between overriding ophiolites and underlying continental margin units (Fig. 6a). Mélanges and broken formations were traditionally seen as deformed thrust sheets of “volcanic-sedimentary′′ successions in which the present complexity was the result of a pervasive faulting of otherwise coherent units during emplacement (Oman, Eastern Mediterranean). In addition, mélanges play an important role in ophiolite emplacement in many areas such as Albania, Himalaya (e.g., Corfield et al. 1999; Robertson, 2000, Höck et al. 2002). Facies analysis, geochemical and structural studies show that most of the accreted mélanges range in settings from the distal continental rise to open-oceanic. In summary, mélanges beneath ophiolites are produced by obduction of large ophiolites onto continental margins. They record the obduction processes of oceanic crust and deep-sea sediments (Robertson, 2004). Such mélanges are formed during the thrusting of the ophiolites onto the continental margin.
 Figure 6: Schematic models of the emplacement of ophiolites in subducting processes. a) Obducted ophiolites are often associated with mélanges at their base [(1) Höck et al. 2002, (2) Collins & Robertson, 1997, (3) Ferrière et al. 1988, (4) Searle & Malpas 1980]. This type of mélange is a consequence of the thrusting of the ophiolitic sequence over a platform. The matrix is composed either of serpentinite or of sediments. b) Accretionary complex are displaying ultramafic and/or mafic rocks embedded in a sedimentary matrix, but accretionary complex are not overlain by any ophiolitic sequence. Often such 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]. |
Accretionary complexes
The second type of mélange occurs very widespread but is not found directly beneath large overriding ophiolites (Fig. 6b). Such mélange are typically complex and often affected by multiple deformation events such that any genetic distinction between sedimentary vs. tectonic origins is difficult and unreliable. The main impetus for the recognition of mélanges as recording subduction of oceanic crust came instead from studies of the Franciscan Complex. This mélange includes ophiolitic material (e.g., serpentinite), but is not overlain by any ophiolite, as it is the case in many accretionary complexes (e.g., Alps, Bousquet et al. 2002, Bousquet, 2007; Sulawesi, Parkinson 1996; Olympic Mountain, Brandon & Vance 1992). Blocks in the Franciscan include blueschist metamorphic rocks with glaucophane and lawsonite - that indicate HP-LT metamorphism. All blocks, like ultramafic rocks, are embedded in a fine-grained matrix composed mainly of black shale. No consensus on the tectonic vs. sedimentary origin of the Franciscan Complex, exists yet; some infer a mainly tectonic origin (e.g.,; Cloos 1984), whereas others envisage a mainly sedimentary origin, (e.g., Cowan 1978).
Accretionary complexes similar to the Franciscan Complex occur widely along the Tethys belt from the Alps (Bousquet et al. 2002) to New Caledonia (Potel et al. 2006) including the Eastern Mediterranean region (Jolivet et al. 1998; Robertson, 2004), Turkey (Okay et al. 1996), Iran (Gansser 1974) or Sulawesi (Parkinson, 1996). All examples, displaying HP-LT metamorphism, are regionally associated with subduction, but are not directly overlain by ophiolites.
The various mélanges, rather than the ophiolites themselves, are important indicators of the former existence of oceanic areas. The main reason is that most of the ophiolites preserve unusual tectonic settings (e.g., supra-subduction zone genesis), whereas most “normal′′ MORB was subducted. When such subduction takes place only fragments of oceanic crust and sediments are preserved including serpentinite, volcanic seamounts, and related pelagic sediments.
The Bou-Azzer ophiolite: mélange beneath ophiolites or accretionary complex?
 Figure 7: Geological map of the western part of the Bou-Azzer inlier (modified after Leblanc, 1975). |
Saquaque et al. (1989) and Hefferan et al. (2002) described the Bou-Azzer inlier as resulting from an accretionary complex formed during the Pan-African subduction and not representing a coherent sample of Precambrian oceanic lithosphere obducted onto the Eburnean margin (Leblanc 1976). In this model the whole Bou-Azzer inlier is considered to be an accretionary complex formed by a huge mélange juxtaposed to the ophiolite (Fig. 7). However, coherent petrographic and stratigraphic relationships can be traced along strike, in some cases for kilometers (Leblanc & Billaud 1978; Church 1991; Leblanc & Moussine-Pouchkine 1994). The major part of the inlier is composed of different ophiolite sequences including ultramafic rocks juxtaposed each other along faults and shear zones (Fig. 8), indicating early top-to-the-south movements (Leblanc, 1975).
 Figure 8: Geological cross-sections across the Bou-Azzer inlier (modified after Leblanc, 1975 and Saquaque et al. 1989). Structural relations between the different units clearly show that the mélange sequence is located structurally above the basement and below the ophiolitic sequence. |
The mélange is limited to the southern part of the inlier at the contact with the basement (Fig. 8). The matrix of the mélange varies from serpentinite to strongly deformed volcano-sedimentary rocks. Blocks mainly consist of ophiolitic fragments, metagraywackes and quartzites. Occurrences of metabasaltic breccias within the mélange testify to the tectonic origin of the blocks. The mélange forms slices interspersed between the ophiolite sequence in the North and a basement in the South.
This geometry, already described by Saquaque et al. (1989), suggests that the mélange forms the base of the ophiolitic rocks. Thus we interpret the whole sequence as an ophiolite obducted southwards onto the WAC platform with its underlying mélange (Fig. 9).
After the obduction of the ophiolite, we note accretion of terranes, such as island arcs, to the ophiolitic complex. During terrane assembly, the ophiolites were dismembered and thickened allowing HT-greenschist metamorphic conditions (Fig. 9).
 Figure 9: Model for the emplacement of the Bou-Azzer ophiolitic suite. 750-700 Ma: North-dipping subduction of the Pan-African ocean. 680-660 Ma: Formation of mélange and HT-greenschist facies metamorphic overprint during obduction and terrane assembly. 650-640 Ma: syn-collisional magmatism crosscutting earlier tectonic structures. A northern continent is assumed based on the absence of Pan-African age oceanic crust north of Bou-Azzer. |
During collision between northern continent and the WAC, diorite to quartz diorite syn-tectonic plutons and diabase dikes intruded the different accreted terranes. Radiometric dates on various granodiorites of Bou-Azzer have yielded ages between 650 and 640 Ma (Inglis et al. 2004, 2005). All units were subsequently deformed and metamorphosed under lower greenschist facies conditions, indicated by growth of chlorite within shear zones in granodiorite. The entire inlier appears to be a complex ensemble of diverse igneous, metamorphic, and sedimentary rock units that have been accreted, juxtaposed and deformed at different times during the closure of the Pan-African ocean (Fig. 9) and not assembled at the same time in an accretionary complex.
Several studies (Saquaque et al. 1989; Hefferan et al. 2000, 2002; Ennih & Liégeois 2001) clearly show that the Bou-Azzer ophiolitic suite is the remnants of a fore-arc assemblage that evolved above a north-dipping subduction zone. Despite this, the dipping-orientation of the subduction in the Moroccan Anti-Atlas during the Pan-African orogen is still discussed controversially (Gasquet et al. 2005). However, the present geometry of the ophiolite suite (Soulaimani et al. 2006), the accretion sequence with a mélange at its base at the contact with the WAC basement in the south, the early top-to-the-south sense of shear combined with the calc-alkaline volcanism in the north (Saghro massif, Saquaque et al. 1992) clearly allow us to argue for a north-dipping subduction.
The present study shows that i) the ophiolite suite of Bou-Azzer is not part of an accretionary complex, but is an obducted ophiolite with a mélange at its base. The ophiolite sequence was dismembered and delaminated during its accretion onto the WAC margin. ii) no evidence for HP-LT (blueschist facies) metamorphic conditions can be found in these rocks. Metamorphic conditions experienced by the ophiolites are limited to HT-greenschist facies conditions (5- 6 kbar, 500-550°C) that are typical conditions for collision or obduction settings.
We therefore consider the ophiolites of Bou-Azzer not to be similar to the HP-ophiolites of the Franciscan complex of medium-pressure ophiolites of Sanbagawa, but rather similar to the Albanian or Cyprus ophiolites that are dismembered ophiolite sequences overprinted by greenschist facies conditions.
Acknowledgments
RB thanks R. Caby for fruitful discussions and for his encouragements to write this paper. Funding by the French -Moroccan project MA n°13 and the University of Basel are greatly appreciated. This paper benefited from constructive comments of K. Hefferan, J.C. Schumacher and D. Marquer. N. Ennih is thanked for his helpful editorial work