Ophiolites and El Graara - Attribution of Data and Ideas.

The most important early contributions to our knowledge of the El Graara inlier were those of Gilbert Choubert and Anne Faure-Muret, whose map of the inlier published in 1962 remains an invaluable source of information on the distribution of the main stratigraphic and structural sequences at El Graara.
The El Graara ophiolitic rocks were described by Choubert (1963) as greenstones and migmatites, and as late as 1972 (Choubert and Faure-Muret, 1972, p 20) he rejected the interpretation of the El Graara ultramafic-mafic rocks as ophiolites in the sense of Brunn and Routhier's 'epanchements sous-marins'. On the other hand, Routhier (1963), Bouladon et Destombes (1963) and Leblanc (1972) considered the ophiolites to be large scale submarine mafic effusive units representing initial mafic magmatism during the development of an eugeosynclinal trough as per the model of Aubouin (1961; 1965). Leblanc (1972, p. 131) specifically rejected the model for ophiolite development as oceanic crust proposed by Reinhardt (1965) for the Oman ophiolite, and Leblanc's statement (Leblanc, 1972, p. 139) that the Bou Azzer ophiolite was situated 'sur la suture ophiolitique qui bordait de facon presque continue Ie craton ouest-africain' must be read in this context. The term 'suture' in this case marks the localization of the initial mafic magmatism of Aubouin’s eugeosynclinal model, not the suture of oceanic closure between colliding continents.
In 1972 Karig (1972) published his classic paper on Remnant Arcs in the Pacific, formed as a result of the collisional elimination of small back-arc basins formed by extension at the rear of arc systems.
Nevertheless, in attempting to link the geology of the Anti-Atlas to that of the Hoggar of Algeria, Caby and Leblanc (1973) suggest that the greenstones decorating the margins of the West-African Craton were of two types. The first included mafic complexes demonstrably injecting rocks of the West-African Craton, and represented a phase ofcrustal basification. Within the Pharusian geosynclinal trough of the Hoggar equivalent rocks include large amounts of quart diorite and lesser serpentinite. The second type is represented by an effusive series of basalts, spilites and keratophyres associated with the andesitic volcanogenic greywackes of the "Serie Verte". The andesitic source rocks overlie basified crust whereas the basalt/spilite/keratophyre are held to represent the upper units of an ophiolite similar to that found at Bou Azzer in the Anti-Adas. The ophiolites represent therefore the effusive component of the magmatism leading to crustal basification and oceanization of a zone extending from the Anti-Atlas to the Hoggar region. While this model obviously bears similarities to plate tectonic processes, it is nevertheless essentially autochthonist
Elsewhere, the early speculations ofGass (1968) and Davies (1968) on the oceanic nature of the Troodos and Papuan ophiolites, respectively, and Reinhardt's recognition and attempt to explain the Oman sheeted diabase unit in terms of a mid-ocean ridge sea-floor spreading model, had led to the finding of sheeted diabase units in Troodos (Vine and Moores, 1969) and during the summer of 1969 the sheeted diabase units of both the external and internal zone ophiolites of Newfoundland (Church and Stevens 1970; 1971). During an excursion to the El Graara inlier organised by Choubert in 1973 an apparent sheeted diabase unit was recognised at Ait Ahmane and the following year the finding were reported by Church and Young (1974) in support of the view that oceanic mid-ocean material was present in the Pan-African system of the Anti-Atlas. At that time Miyashiro (1973) put forward his controversial suggestion that the Troodos ophiolite represented an island arc rather than oceanic crust formed at a mid-ocean ridge, but this was largely rejected by most other Troodos workers. The problem was partly resolved by Rocci et al. (1975) who on the basis of their study of Alpine ophiolites, including Troodos, proposed a division of ophiolites into low-Ti (Troodos) and high-Ti types (true oceanic), whereas in the Appalachians, Church (1976) and Church and Riccio (1977) recognised a spectrum of types, including the Type I and type II ophiolites of Rocci et al., on the basis of the variation in the order of crystallization of the ultramafic-gabbro cumulates and of the relative variation of Ti with respect to FeO/MgO. Church and Coish (1976) pointed out on the basis of the Appalachian data that irrespective of the nature of the chemistry of the Troodos ophiolites, they nevertheless must have formed at a spreading centre.
Leblanc (1975) in a major study of the El Graara inlier accepted the evidence of the sheeted diabases as indicating that the Bou Azzer ophiolite had formed at an oceanic spreading ridge, but nevertheless continued to believe, quoting Miyashiro's (1973) conclusions concerning the arc origin of Troodos, that the quartz-diorites and keratophyres of the ophiolite belt were a penecontemporaneous and consanguinous part of the ophiolite. He also continued to believe (see also Leblanc and Billaud, 1978) that the ophiolite, although telescoped during Pan-African orogenesis, was located essentially in its original position of formation marginal to the Ebumean continent.
In 1978 the presence of boninites as basement beneath the trench-slope break of the Mariana arc was discovered by Meijer et al. and Dietrich et al.
A limited study of the dikes at the Oumarou section of the Bou Azzer ophiolite (Church 1980) showed that the dikes varied in character from low-Ti clinopyroxene-phyric to MORB to plagioclase-phyric calc-alkaline, whereas the low Ni relative to Cr abundance's in the altered gabbro screens may indicate that they were originally orthopyroxene bearing. Church also attempted to draw an analogy between the geology of the El Graara inlier and that of the western margin of the Appalachians, elaborating the view that ophiolite emplacement in Phanerozoic systems such as the Appalachians usually took place relatively early during ocean closure along a slip plane dipping oceanward, such that the present ophiolites must have been separated from the Ebumean continental margin by a significant stretch of oceanic crust. The arc component now recognised in such ophiolites would of course be related to the same phase of closure and the same slip plane.
The REE analysis ofBodinier et al. (1984) clearly showed that the effusive rocks could be divided into three main sets: a LREE depleted tholeiitic set which included the diabases of the ophiolite section at Oumarou and one sample from the volcanics north of the Tiddiline belt, a slightly LREE enriched set of two samples from within the main Graara belt (also sampled by Bloomer (Naidoo et al. 1991) in the Douissat region); and a moderately LREE enriched set of spilites from the Ousdrat and Skouraz regions with calc-alkaline arc affinities. One of the spilites from the Skouraz region showed a composition very similar to one of the plagioclase-phyric diabase dikes of the Oumarou section, claimed by Church to have calc-alkaline affinities. The microgabbro from the Oumarou ophiolite section is compositionaly similar to the diabase textured dikes analyzed by Church (1980) and has MORB characteristics, whereas the two diabase samples analyzed by Bodinier et al. were similar to the low-Ti clinopyroxene-phyric dike analyzed by Church. With respect to the presence of high-Ti basaltic rocks within the ophiolite belt it should be noted that the association of low Ti and high Ti basalts is a feature of the internal zone ophiolites of the Appalachians both in Quebec and Newfoundland. Bodinier also recorded the presence of only subordinate orthopyroxene in the layered olivine- plagioclase-clinopyroxene ultramafic-gabbro cumulates and plagioclase-clinopyroxene-orthopyroxene in the layered gabbros. The REE patterns of the gabbros all exhibited the positive Eu anomalies reflecting plagioclase accumulation, but whereas the less fractionated samples showed LREE depletion, the more fractionated gabbro sample showed a slightly concave upwards pattern. The keratophyres from the Ousdrat arc terrain also exhibit slightly concave patterns with marked negative Eu anomalies, feature reminiscent of boninites. Most important, the data indicated that the ophiolites and the quartz-diorites spatially associated with them were not consanguineous. Bodinier et al. concluded that the Bou Azzer ophiolite was formed in a marginal basin just behind a still active island arc. At the same time Moores et al. (1984) suggested an origin for the Troodos ophiolite also as a back-arc basin, whereas Pearce et al. (1984) introduced the term suprasubduction zone ophiolite for those ophiolites characterized by low-Ti abundances or boninitic characteristics, suggesting that they form above a subduction zone in the very early stages of arc development.
The presence in the Anti-Atlas of rocks belonging to the low-Ti ophiolite group was confirmed only following the description of ultra-low Ti mafic volcanic rocks associated with the Khzama ophiolite in the Siroua region by El Boukhari, A., Chabane, A., and Rocci, G., in a 1986 unpublished manuscript entitled 'Upper Proterozic Ophiolite in Siroua Massif (Anti-Atlas, Morocco): A Paleo-Transform Fracture Zone' (submitted to the Canadian Journal of Earth Sciences but regrettably never published). El Boukhari et al recognised the geochemical affinity of these rocks to the low-Ti rocks of the Troodos ophiolite, thereby confirming the tectonic similarity of the Siroua-El Graara ophiolites to the low Ti internal zone ophiolites of the Appalachians proposed by Church, but preferred to interpret their environment of formation as being an oceanic transform fault. At the same time Oshin and Crockett (1986) suggested that the very similar Thetford low-Ti boninitic ophiolite of the Appalachian system formed in a back-arc basin. In reviewing and recommending the El Boukhari et al. manuscript for publication Church (1986) suggested to the authors that they consider the possibility that the Khazama and Bou Azzer formed in a spreading centre within the frontal part of a primitive oceanic arc adjacent to an obliquely subducting oceanic plate, rather than having formed in an oceanic transform fracture zone. This proposal was subsequently published (Church 1986) in a discussion of the origin of the Thetford ophiolite of the Appalachian internal zone. The existence of the El Boukhari manuscript and the interpretation of the Khzama rocks as boninitic was reported to Hefferan in a letter (June 7th 1990). Whether the petrographic and chemical characteristics of the low-Ti ophiolites can be reconciled in terms of an oblique subduction model remains to be evaluated.
Saquaque et al. proposed that the pre-PII-II rocks of the El Graara inlier are composed of a southerly accretionary prism (Skouraz, Menissi, Bleida), a northern fore-arc (Ousdrat), and a central uplifted sub-forearc basement (Bou Azzer, Oumarou ophiolite). The arc formed above a north dipping subduction zone and the quartz-diorites of the ophiolite belt were emplaced into the fore-arc as a result of ridge subduction.
In discussing the Saquaque et al. model for the development of the Bou Azzer ophiolite Church (1991) argued that neither the arc sequences or the quartz-diorites of the El Graara ophiolite belt need to have been formed above the same subduction zone that produced the ophiolitic rocks, nor need they have been formed in a fore-arc environment. They could have formed as a result of south directed subduction prior to or following obduction of the ophiolite -which must be true of the Bou Azzer quartz-diorite now dated by Mrini et al. (1989) as being as old as 728+50 Ma and which transects both ophiolitic and Ebumean basement. Church further argued that the age and paleogeographic relationship of the ophiolites and Ousdrat - Skouraz -Menissi arc rocks was indeterminate: the calc-alkali arc rocks occur both south and north of the ophiolite belt and either represent an arc formed directly on a basement of primitive SSZ ophiolite, or an arc that has been thrust over the latter.
Naidoo et al (1991) analyzed a comprehensive suite of sample from most of the pre-Tiddiline volcanic units of the El Graara inlier. The results largely replicated those of Leblanc (1975) and Bodinier et al. (1984), but emphasize the presence of an important component of high-Ti within-plate volcanic rocks in the Douissat area of the ophiolite belt, of evolved arc rocks in the southeastern Skouraz and west Menissi tracts, and of more primitive arc rocks in the western part of the Menissi tract. Of the diabase dikes analyzed by Naidoo et al. from the Oumaru ophiolite section, samples lklcF3Bc and !K37cF2c closely resemble the high Zr, low Cr - Ni calc-alkaline samples 29-5 and 29-7, respectively, of Church (1980). These rocks were not represented in the Bodinier et al. data set. Sample lK20DYlb resembles the low-Ti sample 29-2, and samples DC8Dlc and 1-11 resemble the diabase textured dikes 29-3 and 29-6 of Church. Naidoo et al. also report the existence of two diabase dikes with elevated Ti02 values (2.27,3.24) perhaps representing fractionated MORB. Several of the calc-alkaline dikes cut the serpentinite as well as occurring in the diabase unit, thus confirming Church's surmise in this respect and indicating that the Ousdrat-Skouraz-Menissi arc rocks were laid down directly over the ophiolitic rocks.