Egypt

History - Pan-African plate tectonic geology of the Arabian Nubian Shield

Extracted from: W.R. Church, 1988. Ophiolites, sutures, and micro-plates of the Arabian-Nubian Shield: a critical comment, p. 289-316 in El-Gaby, S., and Greiling, R.O., eds., The Pan-African belt of Northeast Africa and adjacent areas; Tectonic evolution and economic aspects of a Late Proterozoic orogen, Friedr. Vieweg & Sohn, Braunschweig/Wiesbaden   (Jabal Zebara viewed from the North; Zebara is located on the eastern flank of the Hafafit extensional dome)

Google Earth - Arabian-Nubian (Sudan) Shield

(290) INTRODUCTION

        It is now commonplace to represent the Arabian-Nubian Shield (Fig 1) as a cratonized assemblage of oceanic and continental margin arcs located between the Nile craton to the west and some other presently ill-defined craton to the east of the Ar Rayn terrane, the most easterly volcanic terrane within the exposed part of the Arabian Shield (Roobol et al., 1983). Most models of the Shield assume that the accretionary process involved face-on collision of a succession of arcs, and that consequently the collision zone would be marked by a trail of the remnants of the subducted ocean crust. Definition of the five arc terranes (microplates) currently recognised in the Arabian-Nubian Shield (Stoeser and Camp, 1985; Vail, 1985) (Fig. Ib) is based on this assumption, and depends particularly on the supposed presence of ophiolite-decorated suture zones in the Sudan (Embleton et al. 1982), and the long distance correlation of the ophiolites of Bir Umq and Jabal Thurwah (Delfour, 1982), and of Jabal Al Wask and Sol Hamed (Duyverman, 1984; Nassief et al. , 1984). The microplate scheme envisioned by Vail (1985) is similar to that of Stoeser and Camp (1985) but includes the definition of two additional arc terranes; one in the Sinai region, the other the result of the division of the Asir terrane of Stoeser and Camp into two independent microplates. Vail has also raised the status of Camp and Stoeser's microplate distribution map to that of a palinspastic map, implying that the present position of the microplates is essentially that which they occupied at the time of their amalgamation. Although in substantial agreement over the location of suture zones. Vail, and Camp and Stoeser nevertheless disagree concerning the polarity of subduction; Vail, along with Kroner (1985), preferring a subduction direction to the west, and Camp and Stoeser a subduction direction to the east.

        Irrespective of the difficulty of determining subduction polarity, defining plate boundaries in terms of ophiolite trails, while a simple elegant, and apparently rational (291) procedure, may however not be legitimate. Firstly, as is illustrated in Fig 1, defining plate boundaries by joining ophiolite occurrences appears to be an arbitrary procedure; secondly, it is assumed that ophiolites represent intra-arc oceanic crust, which may not be true; and thirdly, available geological data suggests that the evolution of the Saudi-Nubian Shield was more complicated than current microplate models suggest. In the subsequent parts of this paper, following a brief review of the 'ophiolite problem', the status of some of the currently favoured suture zone sites will be examined from the latter two points of view.

        THE OPHIOLITE PROBLEM

        It is clear that many ophiolites formed at spreading centers analogous to those of mid ocean ridges. Nevertheless, the origin of many ophiolites as examples of MOR-type oceanic crust has long been questioned, and it has been suggested that they variously represent the roots of primitive island arcs, back- arc marginal basins, and, more recently (Pearce et al. 1984), spreading centres formed above subduction zones at the inception of arc development - and therefore located within the fore-arc parts of the subsequently evolved arc. Basaltic rocks formed at mid-ocean ridges are usually plagioclase-phyric, exhibit Ti02 (wt.) values numerically equivalent to the FeOt/MgO ratio of the rocks, are LREE and LIL depleted, have Ti/Zr ratios of about 100, and only rarely (Bouvet Fracture zone of the Southwest Indian Ridge, Le Roex et al. 1984) exhibit negative Nb-Ta chondrite-factorized anomalies. Cumulates in ophiolites with these characteristics (e.g. Macquarie Island, Vale, 1972; Bay of Islands, Church and Riccio, 1977; the 'Alps', Serri, 1981) characteristically include troctolites, and the crystallisation sequence at low pressures is olivine-plagioclase-clinopyroxene-orthopyroxene. It should be noted however that anomalous low TiO2 basaltic rocks have been found in the vicinity of transform fault zones (0.5 wt.; Bryan, 1979, ARP74 stations 31 and 33), as well as (292) at one site in the Somali Basin (Frey et al. 1980; site 236). Basalts taken from back-arc basins such as the Scotia Sea include MOR, LIL-enriched ocean island, and arc types, as well as mixed MOR/arc rocks exhibiting LREE enrichment and Nb depletion relative to La and Ba-Rb-K (Saunders and Tarney, 1984). In contrast, fore-arc basement rocks (e.g. Marianas, Crawford, 1981) include arc tholeiites and rocks of the boninite series. Cumulate rocks formed in this environment, such as - according to Pearce et al. (1984) - those of the Troodos ophiolite (Desmet, 1977; Robinson et al., 1983; Murton, 1986), crystallise in the sequence olivine (chromite; high Cr)- clinopyroxene-orthopyroxene-plagioclase, whereas boninitic volcanic rocks with extremely low TiO2 values (0.2 wt. percent) and concave upwards REE patterns crystallized in the sequence olivine-orthopyroxene-clinopyroxene-plagioclase. The associa tion of such rocks with a ductile fault zone in the Troodos ophiolite (Arakapas zone; Murton, 1986) and with a spreading centre in the case of the Belts Cove ophiolite of Newfoundland (Coish and Church, 1979; Coish et al., 1982; Church, 1987) might suggest that ophiolites with these characteristics owe their preservation in part to their origin as strike-slip fault slivers detached from the frontal part of arcs as a result of oblique subduction. If such ophiolitic slivers are transported by strike slip movement prior to obduction and arc assembly, they may be less useful in the delineation of arc boundaries than has tended to be assumed, although of course they do indicate the one-time existence of active subduction.

        In the Arabian-Nubian Shield petrographic descriptions and limited chemical data are available for the Al Wask (Bakor et al. 1976), Jebel Ess (Shanti, 1984), Jebel Thurwah (Nasseef et -al. 1984), and Al Amar Idsas (Nawab, 1979; Church, 1980) ophiolites in Saudi Arabia, the Sol Hamed (Hussein, 1981; Fitches et al. 1983) and Wadi Onib (Hussein et al. 1984) ophiolites in the Sudan, and the Fawkhir (Stern, 1979; Dixon, 1979), Wadi Ghadir (El Bayoumi, 1980) and Sabahiya (Basta, 1983) ophiolites in Egypt. Although plagio-phyric lavas occur in the high-Ti (>3 wt percent.) 'within-plate' Wadi Ghadir ophiolite, (293) in no case have troctolitic cumulates been recorded, and for this reason alone the interpretation of any of the above ophiolites as major ocean or back-arc oceanic crust is presently uncertain.

        THE SUTURE PROBLEM

        Historical Perspective

        The earliest mention of ophiolitic rocks in the Arabian-Nubian Shield and their interpretation as fractionated products of deep-sea mafic magmatism can be attributed to Rittman (1958), but the first references to the Arabian-Nubian ophiolites in the context of plate tectonic theory are contained in papers by Garson andShalaby (1974; 1976), Bakor, Gass and Neary (1976), Neary, Gass, and Cavanagh (1976) (Fig. la), and Al Shanti and Mitchell (1976). Garson and Shalaby considered the ophiolites to mark oceanic sutures separating a series of continental margin arcs that developed episodically above a long-lived - since the Archean - westward dipping subduction zone. Bakor et al. and Neary et al., however, following the suggestion of Greenwood et al. (1975) that the Arabian Shield could be considered a cratonized island-arc developed above an easterly dipping subduction zone, proposed that the ophiolites represent oceanic remnants of as many as seven northwest-trending back-arc marginal basins, five of which could be recognized within the Nubian Shield of the Eastern Desert of Egypt. This idea was also taken up by Frisch and Al-Shanti (1977), who described the development of the Arabian Shield in terms of a complex of arcs and back-arc basins which were sequentially closed along slip planes dipping generally to the east, and further developed by Gass (1977) in his proposal that the whole of continental North Africa east of the West African Craton was formed of cratonized oceanic island arcs of Late Proterozoic age.

        In contrast Kazmin et al. (1978) explained the development of the Arabian-Nubian Shield in terms of the opening and closing (294) of an intracontinental Proto-Red Sea basin with dimensions approximately defined by the present distribution of greenstones in the Arabian-Nubian Shield, whereas Shackleton (1979), in linking the Egyptian and Saudi Arabian suture zones of Bakor et al. to ophiolite occurrences in southern Sudan, Ethiopia, and Kenya (Fig. la), suggested that oceanic crust was periodically extended by the southward propagation of spreading ridges of oceanic crust in the north into continental crust in the south; there was therefore a transition from crust formed by arc accretion in the north to crust characterised by Himalayan-type collision in the south. Some support for these views was provided by Dixon’s discovery of Archean-age zircons in quartzite clasts in a conglomerate located within the ophiolite zone of the southern Eastern Desert of Egypt (Dixon, 1979).

        In the Eastern Desert of Egypt mention of ultramafic rocks as obducted slabs of mantle and oceanic material was made by Abdel-KhaIek (1979), and the back-arc hypothesis was reinforced by the discovery of typical ophiolite sequences at Wadi Ghadir (El-Sharkawy and El-Bayoumi, 1979) and Bir Fawkhir. The representation of the Eastern Desert ophiolites as marking a series of intra-arc suture zones was however questioned by Church (1979; 1980; 1983), who argued that the ophiolitic material of the Eastern Desert occurred in association with exogeosynclinal deposits, and its primary distribution was therefore lithostratigraphically controlled; that is, the ophiolitic material represents olistostromal debris derived from an 'internal'oceanic source undergoing east towards west obduction in a manner similar to that invoked in the case of the emplacement of the early Ordovician flysch and associated ophiolite sheets of the western margin of the Appalachian system. On this basis Church concluded that the ophiolitic zones of the Eastern Desert did not mark the location of in situ marginal basin oceanic crust, but rather, that the present distribution of the ophiolitic 'geosynclinal sequence' was controlled by secondary deformation structures. In terms of modern plate systems analogy was drawn with the Pacific region (295) between Australia and New Zealand. Stern (1979) also proposed that the Arabian-Nubian orogen originated as an intra- continental rift, and Engel et al. (1980) drew attention to similarities between the Arabian-Nubian Shield and the development of Archean systems.

        The nature of the ophiolite-bearing melange of the Eastern Desert was described by El Sharkawi and El Bayoumi (1979) and by Shackleton et al. (1980), and petrographic details of the ophiolite at Fawkhir were given by Nasseef et al. (1980). El Bayoumi (1980) interpreteted the melange as having originated in a trench environment, and the tectonic history of the Eastern Desert in terms of a rift ocean basin which was closed by consumption of oceanic crust along a westerly dipping subduction zone. In the Arabian shield, arc development as a result of westerly subuction was also favoured by Nawab (1979), Schmidt et al. (1979), and Hadley and Schmidt (1980), whereas Gass (1979) considered the Shield to have developed above one or more easterly-inclined subduction zones. An exogeosynclinal arc-obduction model for the development of the ophiolite- bearing terrane of the Eastern Desert was espoused by Ries et al. (1983), who, however, suggested that ophiolitic melange material was deposited on both continental and oceanic crust close to the interface between an arc and a continental margin about to enter into collision with one another. The melange received debris from the arc, by westward sliding of oceanic material into a trench from thrusts in the fore-arc prism, as well as from the continent, and was tectonically imbricated with slabs of serpentinite during a thrusting event - coeval with the extrusion of the Dokhan volcanics - that brought the melange' terrane over shelf sediments of the subducting continental margin.

        Following abandonment of the view that the ultramafic rocks of the Eastern Desert delineated in situ zones of back-arc ocean collision, Embleton et al. (1984) defined a new set of northeast trending suture zones in the Sudan, whereas in Saudi Arabia Delfour (1982) depicted the Jabal Thurwah ophiolite as (298) lying in the same northeast trending zone as the Bir Umq ophiolite. The northeast trending sets of sutures on either side of the Red Sea were then correlated by Duyverman (1984) (Fig. 1e) and Nassief et al. (1984) (Fig. 1d); that is, the original northeast trend suggested for the ophiolite-decorated zones was abandoned in favour of a northwest trend. Following extensive mapping in Sinai, Shimron (1984) interpreted the geology of the Wadi Kid area in terms of northwards (westwards) subduction of oceanic crust beneath Proterozoic continental crust and the resulting development of accretionary prisms marginal to an Andean type margin -although the existence of ophiolites in the Wadi Kid and surrounding areas of the northern Eastern Desert was denied by Stern et al. (1985). El Bayoumi and Greiling (1984) and El Ramly et al. (1984) also considered the Nubian Shield to have formed above a west dipping subduction zone following the development of an island arc and a marginal basin arranged such that the latter separated the arc from the Nile craton. Following closure of various oceanic systems to the east, rocks of the marginal basin and the arc were thrust over the continental margin. In this model the ophiolitic material of the melange unit(s) forms the basal units of a series of imbricate slices and are therefore considered to have been tectonically incorporated into the melange rather than formed as olistostromal units. Stacey and Hedge (1984) confirmed the presence of continental crust beneath the eastern part of the Arabian-Nubian Shield - proving that 'accretion' and 'intracratonic' are not mutually exclusive concepts - and, adopting the suture distribution of Nasseef et al. (1984), Stoeser and Camp (1985) have attempted to describe the development of the Arabian Shield in terms of the amalgamation of three ensimatic island arc terranes and two microplates with continental affinities. Vail (1985) has now extended this concept into the Red Sea Hills region of the Sudan. Stoeser and Camp also revert to the view of Greenwood et al. (1976) in considering closure of the main ocean basin to (300) the northwest to have involved subduction towards the southeast. Kroner (1985) and Vail (1985) on the other hand invoke a pattern of subduction closure opposite to that adopted by Stoeser and Camp.

        The Eastern Desert

        The ‘ultramafic rocks as sutures' point of view has previously been debated (Church, 1979, 1981; Nassief and Gass, 1977) with respect to the serpentinites of the Eastern Desert of Egypt, and it is now generally conceded that in the latter instance, other than Sinai (Vail, 1985), the ultramafic rocks are indeed allochthonous. Work since that time in the Marsa Alam - Gebel Zabara - Hafafit and Meatiq regions of the Eastern Desert (El Bayoumi, 1980, 1984; Basta, 1983; Sturchio et al. 1984; Habib et al. 1985) has also shown that the ophiolite/arc debris-bearing terranes (including low-Ti high MgO 'boninitic' units) constitute a set of east-facing thrust sheets, down through wich there appears to be an incremental increase in strain, style of deformation, and degree of metamorphism (Church, 1983). The geology of the higher sheets is further complicated by the structural imbrication of apparently younger volcanic and volcano-sedimentary units. In the southern part of the Egyptian Desert, continental margin sediments, autochthonous or parallochthonous relative to Nubian basement rocks, are possibly represented by marbles beneath the schists and overlying ultramafic sheets and associated olistostromal mudstones of the Abu Swayel region (e.g. at Um Krush in the Abu Swayel region; AS, Fig. 3). In this respect it is conceivable that the whole of the Eastern Desert is a composite allochthonous sheet, the leading edge of which is located along the line of the Nile, with the Nuba Mountains (El Ageed and El Rabaa, 1981) and Ingessana (Kabesh, 1961) ophiolites of the Sudan to the south occupying an external position analogous to that of the Bay of Islands ophiolite of the Appalachian system.

        (302) The isotope studies of Harris et al. (1984) and Ries et al. (1985) indicate that metasediments (Rahaba Group) of the Bayuda Desert and southern Eastern Desert of Egypt (Wadi Haimur in the Abu Swayel region), while intruded by mantle derived igneous rocks and metamorphosed at c. 900 Ma., include the erosion products of older early-middle Proterozoic continental crust. Furthermore, in the northern part of the Eastern Desert of Egypt Abdal Rhaman (1986) has determined an age of 881 ± 44 for a tonalite suite at Ras Gharib. The high Ri value of these rocks suggests the existence of older crust at depth beneath this part of the Eastern Desert. On the other hand, the very low R- values of some rocks (e.g. Abu Swayel, Stern, 1979; Bayda and Jizi, Kemp et al. 1982; Um Aud diorite, unpublished data) and the Nd/Nd data of Bokhari and Kramers (1981), Claesson et al. (1984) and Harris et al. (1984) indicate that the mantle beneath at least parts of the Arabian-Nubian Shield is anomalously LIL depleted, and in this respect the problem of the presence or absence of thinned old continental crust beneath the Eastern Desert of Egypt and the Red Sea Hills of the Sudan is therefore not entirely resolved. It is also puzzling that the Abu Hamed Quartz ite unit of the Bayuda Desert, although dominated by seemingly shallow water carbonates and quartzites and therefore similar to the Wadi Haimur sediments of the Abu Swayel region, does not have a strong older crustal isotopic signature comparable to the Wadi Haimur and Rahaba rocks. Nevertheless, the available isotopic data are compatible with the view that the Eastern Desert is in part composed of continental margin slope and rise deposits, including thick marbles at Abu Swayel, and pelitic and quartzitic rocks, intercalated with ophiolitic melange units, at Hafafit, Wadi Miya (chiastolite and calc-siliceous rocks), and Meatiq; intrusive arc (?) rocks emplaced at about 900 Ma; and a thrust cover of ophiolitic rocks unconformably overlain by arc volcanics (Abu Swayel 768 ± 30, Neferdeib 712 ± 60, El Koro 800 ± 80, Kadaweib 723 ± 6; Figs. 2 and 3) with an age uncertainty range of 700-800 Ma (Fig. 2), all intruded by tonalite at 711 ± 6 Ma (Dixon, 1979). Other than in the northern Eastern Desert, the isotopic data indicate that 'old' (303) basement rocks if present are of considerably reduced thickness beneath the Eastern Desert. According to Sturchio et al. (1984) thrusting continued until at least 614 Ma. It is uncertain therefore whether the Abu Swayel-Naferdeib volcanics were thrust with the ophiolitic rocks or were formed after ophiolite emplacement. The ophiolites were however emplaced prior to 680 Ma, the age of diorite (Sturchio et al. 1984) intrusive into deformed ophiolitic material of the Hafafit area, and probably prior, to 711 Ma (Dixon, 1981).

        A further problem concerning the source of material in the melange sheet relates to the origin of pebbles of older Precambrian gneiss and granitoid material, and of quartzites and arkoses containing Archean age detrital zircons (Dixon, 1979). Since the melange is almost certainly derived from the east the presence of this material implies either that foundered continental crust, now considerably thinned and basified, and its cover of slope and rise sediments extended well to the west of the present Red Sea, or that a continental microplate was at one time interspersed with the arcs of the Arabian Shield, or that at one time the Afif zone was much closer to the Eastern Desert of Egypt. Clearly, further field and isotopic studies are required to resolve these problems. The Yanbu Suture The Yanbu suture is defined by Camp (1984) in terms of the Karig model of accretionary prism development, with the Farri Group representing the accretionary prism and the Al Ays Group the overlying fore-arc basin. Camp also suggests that the Yanbu suture zone was the site of origin of the ophiolitic melange of the Eastern Desert of Egypt, thereby implying that formation of the melange post-dates the initial phase of development of the Al Ays fore-arc basin, and is therefore likely older than 725 ± 12 Ma, the age of the Jabal Salajah tonalite. However, while the geology of the Al Ays area is indeed complex, it is (304) not certain that available descriptions and age relationships are compatible with the accretionary prism model.

The Farri Group: an accretionarv prism

Whereas Camp (1984) refers to the Farri Group as being formed of highly deformed ophiolite-bearing accretionary prism deposits, Kemp et al. (1982) describe the group as being composed of thick, welded keratophyric tuff (locally peralkalic) with an age of 742 ± 6 Ma (Calvez, 1983, oral communication quoted by Claesson et al., 1984), mixed sedimentary and volcanic rocks, andesites and epiclastic breccias, turbiditic sediments, and pillowed basalt. Furthermore, the presence of two-pyroxene gabbros, volcanics with low TiO2 relative to P2O5, and absence of a sheeted diabase unit in the Jabal Al Wask ophiolite (Bakor et al., 1976) suggests that the Al Wask complex may represent part of an arc rather than ocean crust separating adjacent arcs.

 The Al Wask ophiolite occurs in association with the upper basalt unit of the Farri Group and has a radiometric age (Sm/Nd, 743 ± 24 Ma, Claesson et al., 1984), identical to that of the underlying felsic volcanic rocks of the Farri Group. The age of the ophiolite and Farri Group would therefore appear to be well constrained at c. 730-750. However, zircons from the Haja plagiogranite supposedly associated with the ophiolite have an age of 780 ± 20 Ma (Calvez, 1983, oral communication quoted by Claesson et al., 1984). Therefore, either the Haja granophyre and Al Wask ophiolite have an 'overlap' age of 760-770 Ma (Fig. 2) and are older than the Farri Group, or the granophyre represents remobilized material of the 796 ± 23 Ma old Jar tonalite suite melted by the intrusion of Al Wask mafic liquids. In either case, rocks mapped by Kemp et al. (1982) as Farri but cut by the 796 ± 23 Ma old Jar tonalite (Kemp et al. 1982) likely belong to a volcanic unit older than the Farri Group. These rocks, along with the gneissic Al Hinu Formation and the gneissic rocks of the domes to the north of Al Wask could be equivalents of the lower thrust units exposed in the (305) Hafafit (El Ramly et al., 1984) and Urn Samuiki (Shukri and Mansour, 1980) domes of the Eastern Desert (Fig. 3).

 Since the Farri-Al Wask rocks are unconformably overlain by Al Ays sediments (which are cut by the 725 Ma old Salajah tonalite), they could well represent the older (750-725 Ma) part of a southeasterly migrating arc (Al Ays Group) presently located between pre-800 Ma rocks to the northwest (older Farri) and 805 Ma (Hedge pers. comm. to Stoeser and Camp, 1984) and 945 ± 45 Ma (Al Shanti et al. 1984) intrusive-volcanic complexes (Birak and Rabigh rocks) to the southeast.

The root zone of the Eastern Desert and Jabal Ess ophiolites

The well preserved Jabal Ess ophiolite described by Shanti and Roobol (1979) and Shanti (1984) is characterised by the enigmatic presence of both low-Ti, 'normal' Ti (TiO2 FeOt/MgO), and 'within-plate' basaltic liquids. It therefore represents an example of an ophiolite complex in which interlayered basaltic and dike sequences exhibit both oceanic and arc characteristics. In terms of its low degree of metamorphism and deformation and its association with melange, turbidites, and arc volcanics, the Jebel Ess ophiolite is comparable to the adjacent (after closure of the Red Sea) uppermost ophiolite - melange unit of the Wadi Ghadir - Marsa Alam - Hafafit nappe pile of the Eastern Desert of Egypt. In as much as it is the most easterly ophiolite in this zone, it may therefore represent the highest structural unit of the nappe. In the southern part of the Eastern Desert relatively undeformed volcanic rocks (Abu Swayel volcanics, 768 ± 31; Stern, 1979) overlie intensely deformed allochthonous sheets of ultramafic rock and associated distal mudstones and minor olistostromal units that have been thrust over garnet-amphibolite grade layered metasedimentary and metavolcanic schists and gneisses. The age of the Abu Swayel volcanics relative to the Jebel Ess ophiolite and Jar tonalite is not certain due to the large ± values associated with the isotopic ages of these rocks (Fig. 2), but an age of 782 or older for the Jabal Ess ophiolite (Claesson et al. 1984) is compatible (306) with an emplacement age for the Eastern Desert ophiolites of 760 to 780 Ma, slightly later than the intrusion of the Jar tonalite suite in the Al Wask area, and the Birak und Dhukhr tonalite suites to the southeast. Arc plutonic and volcanic rocks equivalent in age to the Jar tonalite could therefore be the source of the abundant felsic plutonic and volcanic debris in the Eastern Desert melange; which would imply that the surface outcrop of the boundary separating the melange from its arc source lies east of the Jabal Ess ophiolite and west of the Iqwaq granodiorite (821 ± 40 Ma). However, if the gneiss domes of the Al Wask region are analogous to those of Hafafit and Um Samuiki of the Eastern Desert, it is conceivable that the Jar tonalite suite, if not also the Farri and Al Ays groups, has been thrust to the west of the location of the root zone of the suture. Locating the suture may therefore be an intractable problem.

The extension of the Yanbu suture into the Sudan

 Nasseef et al. (1984; Fig. 1d) and later Vail (1985; Fig. 1b) extended the Yanbu suture into the northern Red Sea Hills of Sudan to link up with the supposed Sol Hamed - Wadi Onib suture. However, as discussed above, the Al Wask complex may not represent oceanic crust. Furthermore, given the clearly allochthonous nature of the ophiolites in southern Egypt (e.g. Urn Krush in the Abu Swayel area. Fig. 3) it is more likely that the Sol Hamid ophiolite belt constitutes the leading edge of a major northwesterly directed thrust than the actual zone of closure of an oceanic basin. Embleton et al. (1984) have proposed that a second more southerly suture be recognised to include the Nakasib belt of the Sudan and the Jabal Thurwah ophiolite of Saudi Arabia. It should be noted however that serpentinites occur southwest of Muhammed Qol in the area between the Sol Hamid and Nakasir belts - and others may remain to be discovered. It is therefore not inconceivable that ultramafic material underlies more area between Sol Hamid and the region to the south than just the supposed suture zones. Furthermore, the presence of wehrlite and Iherzolite cumulates in the Sol Hamed ophiolite (Fitches et al., 1983) as (307) well as perhaps also the Wadi Onib ophiolite, and the absence of troctolitic cumulates in any of these ophiolites, suggests that rather than representing intra-arc oceanic crust they may well be strike-slip fault slices of primitive suprasubduction zone fore-arc crust.

The Bir Umq - Jabal Thurwah Suture

 The Jebel Thurwah ophiolite (Nasseef et al., 1984) is petrologically (presence of orthopyroxene-bearing cumulates) and chemically (low Ti) unlike 'normal' oceanic crust. Rather, as suggested by Nasseef et al. (1984), it is more similar to oceanic crust developed within the fore-arc segment of a primitive arc (Troodos, Cyprus; Betts Cove, Appalachians). The ophiolite can be interpreted as an allochthonous sheet emplaced onto the Samran Formation from either the northwest or southeast from beyond the present outcrop of the Samran, or from beneath the Samran. Nor is it proven that it is in continuity with either the Bir Umq ophiolite or with the widely spaced and poorly developed ultramafic rocks in the vicinity of Port Sudan in the Sudan. Even in terms of Camp's attempt (Camp, 1984) to model the At Taif metamorphics, Baish volcanics, and An Nimas plutonics in terms of a frontal arc, back arc basin, and remnant arc system, it is apparent that the trend of the marginal basin and remnant arc is north-south, the purported northeast trend of the suture being evident only in terms of a supposedly primary elongation of the much younger Fatimah volcanic basin, the northeast trend of faults cutting sediments and volcanics of the supposed arc - trench gap, and the supposed recognition of an upper slope discontinuity. The status of the Jebel Thurwah ophiolite as marking a suture zone is therefore highly uncertain. It is of interest to note that the boundary that does separate zones containing rocks with an -age greater than 900 Ma from terranes of less than 900 Ma within the Asir terrane is not marked in Saudi Arabia by a suture. (308)

The Nabitah suture - Nabitah mobile belt

 Interpretation of the ultramafic-mafic complexes of the Nabitah zone as ophiolites is not universally accepted (Caby, 1984; Agar, 1985). Although a plate boundary may well therefore lie somewhere within the Nabitah zone, it is not necessarily demarcated by an 'ophiolite'-decorated fault zone. According to Stacey and Agar (1985) the suture is occupied by the syntectonic Subay igneous suite, in which case strike-slip juxtaposition of the adjacent arc terranes remains a possibility. It might also be noted in this respect that Caby (1984) concluded that continental accretion of the Arabian Shield was essentially vertical and suggested that the absence of evidence for large scale horizontal movements precluded involvement of the Shield in any major collisional event. Furthermore, Stoeser et al. (1984) state that the synorogenic emplacement of 650 to 690 Ma old granodiorite plutons in areas marginal to the supposed Nabitah suture zone reflects major westward-directed compressional orogeny related to a continental collisional event possibly located somewhere to the east of the exposed southeastern Arabian Shield.

The Al Amar suture

 A reasonable case can be made for situating the Al Amar ophiolitic rocks (Al Shanti and Mitchell, 1976; Nawab, 1979; Jackson et al. 1980) in proximity to a suture. However, irrespective of the number of geologic problems concerning correlation and age (Al Shanti et al., 1984; Calvez et al. 1985) that require clarification, there is considerable uncertainty about the oceanic nature of the ophiolites (Le Metour et al. 1982), which, as in the case of the Thurwah ophiolite, do not appear to represent 'normal' oceanic crust (Church, 1980a). Since the Al Amar ophiolites are likely allochthonous, perhaps emplaced in association with the deposition of flysch representing the products of the erosion of the overthrusting plate (Church, 1980b), their present location does not determine the suture zone nor the nature of movement on the suture. The metallogenic zonation - copper to the east, silver-lead-zinc to the west, might imply subduction (309) from east to west leading to the development of a Sea of Japan type back-arc basin as suggested by Nawab (1979), but since copper and gold zones also occur to the west of the polymetallic zone, subduction could equally well have been towards the east.

CONCLUSIONS

 Although some ultramafic-mafic complexes of the Arabian-Nubian shield likely formed in a spreading centre environment, whether fore-arc, back arc or continental margin rift (Tasman-sea- type), their present locations do not delineate with certainty the original sites of the ocean basins. The practice of linking widely separated ultramafic-mafic complexes to define suture zones cannot therefore be accepted without question. Furthermore, given that the width of the Arabian-Nubian shield from the Nile to the Nabitah boundary zone is of the same order of distance as that across the strike-slip amalgamated collage of the Canadian Cordillera, the possibility must be allowed not only that the microplates of the Nubian Arabian shield have been laterally rafted into position, but that the plate boundaries may have been considerably modified as a result of collision. Any assessment of crustal growth rates in the Arabian-Nubian shield should take this point into account.

Notwithstanding the considerable uncertainty concerning the arc amalgamation process in the Shield, geochronological data obtained by Stacey and Hedge (1985) and Stacey and Agar (1985) now indicate that the Arabian Shield - as might be guessed from the distribution of zinc-lead mineral showings - is bordered to the east by older continental crust. The isotopic studies ofHarris et al. (1984), Ries et al. (1985), and Abdel Rahman (1986) suggest that the western margin of the Nubian Shield is formed of an about 900 Ma continental margin volcanic arc over which the Egyptian-Sudanese ophiolite terrain has been thrust from the east. The results of recent studies in southern East Africa (Maboko et al., 1985) also suggest that the granulite (310) belts of western Tanzania were formed 715 Ma ago and may have been thrust (Sacchi et al., 1984), along with 'ophiolite'-type rocks, to the southwest over a basement of 1100 Ma continental margin arc rocks (Ri .7027) and its cover of 900-1000 Ma continental-derived sediments (Ri .7091-.713). If the Hijaz ocean was continuous along the length of east Africa during the late Proterozoic, it would appear therefore that either considerable overthrusting has caused the loss from view of the southern arc equivalents of the Arabian-Nubian Shield or the southerly arc elements have migrated laterally northwards and '\ presently form part of the Arabian-Nubian amalgamated terrane. In this context, a more comprehensive examination of the potential role of strike-slip movements in the assembly of the Arabian-Nubian arcs may prove to be more profitable than is perhaps currently thought.

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