Ophiolites and the evolution of tectonic boundaries in the late

Ophiolites and the evolution of tectonic boundaries in the late

Precambrian Research, 27 (1985) 2 7 7 - - 3 0 0 277 Elsevier Science Publishers B.V., A m s t e r d a m -- Printed in The Netherlands OPHIOLITES AN...

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Precambrian Research, 27 (1985) 2 7 7 - - 3 0 0

277

Elsevier Science Publishers B.V., A m s t e r d a m -- Printed in The Netherlands

OPHIOLITES AND THE EVOLUTION OF TECTONIC BOUNDARIES THE LATE PROTEROZOIC ARABIAN--NUBIAN SHIELD OF NORTHEAST

AFRICA

IN

AND ARABIA

ALFRED KRONER

lnstitut fiir Geowissenschaften, Johannes Gutenberg-Universitiit, Postfach 3980, 6500 Mainz 1 (F.R.G.)

ABSTRACT Kr6ner, A., 1985. Ophiolites and the evolution of tectonic boundaries in the late Proterozoic Arabian--Nubian shield of northeast Africa and Arabia. Precambrian Res., 27 : 277--300. The Arabian--Nubian shield is currently regarded as one of the best examples to d e m o n s t r a t e that processes of lateral crustal growth and m o d e r n - t y p e obduction--accretion tectonics have operated since at least late Precambrian times. In Arabia a n u m b e r of Pan-African v o l c a n o - s e d i m e n t a r y / p l u t o n i c belts have been identified that display internal evolutionary patterns suggesting a d e v e l o p m e n t from primitive intraoceanic arcs s o m e 9 0 0 - - 9 5 0 Ma ago to mature, andesite-dominated arcs s o m e 640 Ma ago through processes of ocean-crust o b d u c t i o n , arc collision and magmatic crustal thickening. Several ophiolite-decorated sutures are preserved, but many early tectonic boundaries were obliterated during later overthrusting, faulting and shieldwide granitoid p l u t o n i s m towards the end of Pan-African e v o l u t i o n and stabilization in the earliest Palaeozoic. In southeastern Egypt and in the Red Sea Hills of the Sudan early Pan-African clastic sediments suggest that a passive continental margin was probably separated from several evolving arcs to the east by marginal seas. These arc segments were later thrust over each other, f r o m east to west, during widespread and considerable horizontal shortening and gave rise to spectacular nappe structures and extensive ophiolite m~langes. The apparent lack of well-defined accretionary thrust stacks, high-pressure metam o r p h i c assemblages and widespread ophiolitic m~langes in Arabia indicates that accretion either did not occur along margins with deep ocean trenches but involved b u o y a n t crust, or extensive overthrusting t o o k place through which the forearc segments were overridden and are now concealed. This, together with the recognition of distinct tectonic belts and isolated fragments of possible ancient continental crust and oceanic plateaus, supports the c o n t e n t i o n that Arabia m a y represent a collage of previously i n d e p e n d e n t exotic terranes that accreted by oblique convergence and strike-slip translation during shield evolution. It is suggested that the Arabian shield contains remnants of m i c r o c o n t i n e n t s with pre-Pan African (i.e., > 1000 Ma) crustal history and, perhaps, oceanic plateaus and that its evolution bears similarities with aspects of terrane accretion in the North American Cordillera and in the present western Pacific. The evolution in Egypt and in the Sudan, however, seems characterized by the t r a n s f o r m a t i o n of a passive continental margin into a tectonically active belt along which ophiolites and arc volcanics were thrust over each o t h e r at a p p r o x i m a t e l y the same time when the exotic terranes and arcs of Arabia accreted farther east. Final stabilization of the shield occurred when the evolving Arabian plate " d o c k e d " with Nubia after marginal basin closure and cessation of arc magmatism s o m e 6 0 0 - - 6 4 0 Ma ago. 0301-9268/85/$03.30

© 1985 Elsevier Science Publishers B.V.

278 INTRODUCTION The recognition of true ophiolites or their dismembered fragments together with the identification of chemically distinct island-arc volcanic and plutonic complexes in the Arabian shield, the Eastern Desert of Egypt and the Red Sea Hills of the Sudan has led to general agreement that this part of the continental crust developed through a process of horizontal crustal accretion during the Pan-African period (Kr6ner, 1 9 8 4 ) ~ 9 5 0 Ma to ~600 Ma ago (Garson and Shalaby, 1976; Greenwood et al., 1976; Shimron. 1980; Fleck et al., 1980; Shackleton et al., 1980; Gass, 1981; Vail, 1983; Roobol et al., 1983}. A wealth of isotopic data shows that this accretion process was virtually completed in the Arabian shield by about 640 Ma ago. and the shield acted as a single crustal unit thereafter (Stoeser et al., 1984), while the data for Egypt indicate that accretion continued up to about 600 Ma ago (Stern, 1981; Stern and Hedge, 1984). Although virtually all investigators agree that the above accretion was brought about by plate tectonic processes and thus provides compelling evidence for the operation of modern-type global tectonics m the lat0 Precambrian, there is still considerable disagreement on the detailed or overall mechanism of this crustal growth process. For example, Greenwood et al. (1976), Gass (1981), Roobol et al. (1983) and others (see papers m A1-Shanti, in press) suggest early ensimatic (i.e., intra-oceanic) evolution and accretion by arc suturing and ophiolite obduction in Arabia, a view largely based on geochemical data and isotopic systematics (e.g., Duyverman et al., 1982). Some of these models infer the existence of a wide ocean, between ~ 9 5 0 Ma and ~ 6 4 0 Ma ago, whose destruction by subduction gave rise to the arcs and their collision (A1-Shanti and Gass, 1983), while, others suggest opening and closure of back-arc basins {e.g., Claesson et al., 1984}. There are opposing views, however, on subduction polarities m these models. The recognition of older (i.e., pre-Pan African, > 1000 Ma) crustal components in Arabia, either as discrete rocks or through isotopic systematics (Stacey and Stoeser, 1983; Calvez et al., 1983; Claesson et al., 1984), has revived interest in models that rely on repeated rifting of an Archaean to mid-Proterozoic craton with the formation of small ocean basins and their closure (e.g., Delfour, 1981). This scenario has also been advocated for the evolution of the shield in Egypt (Garson and Shalaby, 1976; Stern, 1981). Based on the similarity of evolved (i.e., continent-derived) Pb isotopes between Aswan, Egypt and the southern Arabian Peninsula, Stacey and Stoeser (1983) speculated that the two areas might have been part of a single crustal unit prior to about 1000 Ma ago. These authors also suspected widespread older sialic basement under the eastern Arabian shield from the occurrence of tin-rich peraluminous granites. Based on the recognition of well defined tectonic provinces in the southern Arabian shield that display distinctly separate isotopic, geochemical

279 and age characteristics ( G r e e n w o o d et al., 1982), KrSner et al. (1982) and Krbner (1983) suggested that the evolution of the Arabian--Nubian shield was neither entirely ensialic nor ensimatic but may reflect an assemblage of accreted terranes consisting of juvenile arcs, oceanic plateaus and microcontinents that were swept together in a similar manner as now recognized in the Cordillera of western North America (Coney et al., 1981), in the present West Pacific (e.g., Ben-Avraham et al., 1981) and in the Indonesian archipelago (Hamilton, 1979). Stoeser et al. (1984) also followed this c o n c e p t and have recently interpreted the Afif domain of Arabia as an allochthonous terrane. R e y m e r and Schubert (1983) also d o u b t e d the entirely juvenile nature of the Pan-African crust in NE Africa and Arabia. They showed that Phanerozoic addition rates to the continental crust through arc magmatism are in the range 20--40 km ~ km -' Ma-', while the growth rate over the lifetime of juvenile accretion in the Arabian--Nubian shield would be more than 300 km ' k m -' Ma - ', a figure clearly unlikely, even considering that the global heat flow was slightly higher in the late Precambrian than it is today. Although we can confidently rule out an ensialic evolution of the Arabian--Nubian shield (i.e., assuming c o n t i n u o u s older basement under the Pan-African cover, Kemp et al., 1980) there is growing evidence for lateral accretion by other than purely magmatic processes as dem onst rat ed above, and future models should take these limitations into consideration. ACCRETION IN ARABIA

Although many contacts between tectonic provinces and most ophiolitedecorated suture zones are steep, these structural discontinuities have either been modified during late-tectonic strike-slip faulting or they represent the upper, steep parts of listric thrust zones that flatten at middle to lower crustal levels (e.g., Gettings, 1984}. A1-Shanti and Gass (1983} reported rare occurrences of ophiolitic m~langes from the easternmost shield and suggest that these units represent thrust sheets extending below the adjacent turbidite sediments ont o which the original ophiolites were thrust westwards. High-pressure, low-temperature metamorphic assemblages typical of accretionary wedges in m ode r n forearc or trench regions have not been f o un d in the Arabian shield and suggest that such rocks were either consumed during subduction-erosion or " d e c r e t i o n " (Dewey and Windley, 1981) or that large-scale shallow overthrusting has concealed these parts of the original arc or terrane margins. It is also possible that the accreted crust was b u o y a n t and new arcs or crustal segments did n o t collide "heado n " but by oblique, gentle " d o c k i n g " . Flat obduct i on of so many fragments of oceanic crust further suggests that deep trenches did not develop and that the net positive b u o y a n c y of oceanic and arc crust favoured overthrusting and interstacking during collision of the accreting terranes. The

280

involvement of oceanic plateaus in the accretion process, as suggested below, would also tend to inhibit subduction into deep trenches since such terranes are more b u o y a n t than old oceanic crust (Ben Avraham and Cooper, 1981) and thus become involved in upper crustal thrust tectonics. Stoeser et al. (1984) and Gettings (1984) pointed out that comparatively deep crustal levels are exposed in several linear belts of the southen~ Arabian shield where high-T gneiss--migmatite complexes reach the surface and may be interpreted as the elevated root zones of Pan-African arc segments. The associated supracrustal rocks display evidence for considerable horizontal shortening such as in the recently defined Nabitah mobile belt (Stoeser et al., 1984; see Fig. 1), and it is suggested that inter- and overthrusting of the accreting terranes produced crustal thickening in the collision zones. Seismic refraction profiling in the southern shield reveals several slightly NE-inclined low velocity layers at depths between 10 km and 25 km (Prodehl, 1984), similar to those found in the Alps (Hsfi, 1979), and I suggest that these layers represent low density shear zones and/or supracrustal assemblages that record the crustal thickening mechanism resulting from interstacking (Fig. 2). For example, the linear gneiss domains of the Nabitah belt are underlain by a low-velocity zone, and these rocks may owe their exposure to crustal thickening and subsequent erosion after westwardoverthrusting of the entire Afif terrane over the Hijaz-Aziz terrane farther west. A further thrust may have thickened the crust in the Ranyah terrane and led to exposure of high-grade gneisses south of Ranyah (Ramsay et al., 1979; see figs. 1, 2 and 4 and also fig. 19 of Gettings, 1984). The discovery of distinct continental lead isotope characteristics and reliable zircon ages of 1.6--2.0 Ga from granitoids in the Afif domain (Stacey and Hedge, 1983) suggest it to be an allochthonous terrane (Stoeser et al., 1984), now tectonically sandwiched between more juvenile crustal blocks and perhaps originating from a continental microplate that became caught and fragmented in the Arabian accretion process. The presence of clearly continent-derived K-rich sediments in the A1-Lith terrane (KrSner and Basahel, 1984) and other blocks of the southern shield (Ramsay et al., 1979; Jackson and Ramsay, 1980), while such rocks are apparently absent in other provinces of Arabia, also suggests some of these blocks to be of " e x o t i c " origin. Furthermore, Claesson et al. (1984} reported Sm--Nd isochron ages of 743 -+ 24 Ma and 782 -+ 38 Ma, respectively, for the Jabal al Wask and Jabal Ess ophiolites in the northwestern Arabian shield with strongly positive eNd values of +6.6 to +7.6. These data not only set m a x i m u m time limits for ocean closure and ophiolite obduction but also suggest that magmatic rocks with eNd values significantly less than the above as reported by Duyverman et al. (1982) may signify contamination by older continental crust. The possible occurrence of oceanic plateaus in the collage of accreted segments of Arabia has not been investigated to date, largely because the rock associations of m o d e m plateaus and their geochemistry are poorly

281

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Fig. 1. Simplified sketch map of the southern Arabian shield showing distribution of medium- to high-grade gneissic plutonic rocks and migmatites (in black, after Ramsay et al., 1979; Johnson, 1983; Stoeser et al., 1984) and the outline of the Nabitah belt in which the tectonized plutonic rocks occur together with strongly deformed and metamorphosed Hulayfah Group supracrustal assemblages (Stoeser et al., 1984). Heavy broken line follows seismic refraction line and shows shot points as in Fig. 2 (from Prodehl, 1984). Thrust boundaries are suggested terrane margins, and Nabitah belt is interpreted as upthrust root zone of an arc complex (see also Gettings, 1984).

282 $sw

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Fig. 2. Model of the lithosphere beneath the southern Arabian shield as interpreted from a seismic refraction line shown in Fig. 1 (from Prodehl, 1984). Lines of equal velocity in the upper diagram are plotted with interval of 0.2 km s-l. Velocity inversions are indicated by dotted fields. The lower part shows velocity--depth functions for each individual profile (from Prodehl, 1984). Heavy broken lines are suggested fossil terrane boundaries along which thrusting has taken place during terrane accretion. Note two pronounced low-velocity layers under the present Hijaz-Asir and Najd provinces

known (Nur and Ben-Avraham, 1983) and there are thus no generally agreed criteria to distinguish such terranes from normal oceanic crust or evolving island arcs. However, because of their thickened crust, the plateaus have light roots (Ben-Avraham and Cooper, 1981} and, apart from tholeiitic or transitional basalts, contain more evolved rock types such as are found in primitive arcs. Thus the plateaus behave like continents during accretion (Dickinson, 1978) and contribute to crustal thickening. It appears particularly significant that oceanic plateaus also exist inside modern marginal basins (Ben-Avraham and Cooper, 1981), and since such basins have been postulated in the Arabian--Nubian evolution it seems logical to suspect the involvement of allochthonous plateaus there. Reischmann et al. {1984) reported thick pillowed tholeiitic basalt from the 820--850 Ma old Baish Group in the Al-Lith area southeast of Jiddah. These rocks have elevated REE patterns (Fig. 3) and trace element characteristics similar to lavas from the Galapagos Plateau and were interpreted by these authors as possible ocean plateau volcanics, overthrust, from the east, by an island arc terrane. In summary, the mechanism of crustal accretion in Arabia during the Pan-African episode is still little understood, and simple juvenile arc collision models are insufficient to account for the features discussed above and would require unrealistic growth rates. Instead, oceanic arcs, oceanic plateaus and continental fragments were probably involved in the growth of the shield, and such scenarios make it impossible to establish shield-

283 wide lithostratigraphic units for Arabia for the period prior to about 640 Ma ago. The final assemblage of all terranes into one crustal unit involved considerable horizontal shortening and crustal thickening and resulted in a heterogenously structured crust as revealed by seismic refraction profiling. Wadi Sadiyah 1,4eta-Basalts, Wadi Lith Area, Saudi Arabia

100 E113 C" 0 0 X~

I

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Fig. 3. REE patterns for tholeiitic basalt from Wadi Sadiyah, A1-Lith area, southern Arabian shield (from Reischmann et al., 1984).

ACCRETION IN THE EASTERN DESERT OF EGYPT Major progress in the understanding of the Pan-African tectonic evolution of the Eastern Desert was made with the identification of widespread ophiolitic [email protected] and the recognition that the long-standing "stratigraphy" can no longer be maintained in view of tectonic contacts between lithostratigraphic units (Shackleton et al., 1980). Furthermore, rocks previously interpreted as pre-Pan-African basement in view of their high metamorphic grade and poly-deformation were shown to be no older than about 800 Ma, and almost all magmatically derived suites have isotopic systematics implying a juvenile (i.e., mantle-derived) origin (Engel et al., 1980; Stem, 1981; Stern and Hedge, 1984). In contrast to the situation in Arabia, however, there is abundant evidence that most of the clastic components in the so-called Geosynclinal Metasediments are derived from an older sialic basement, presumably in the west of the present Eastern Desert (Abdel-Monem and Hurley, 1979; Dixon, 1981), and detrital zircon ages between 1120 Ma and possibly 2060 Ma suggest that this basement may be the so-called Nile Craton (Rocci, 1965) whose existence is indicated by Archaean granulites at the Oasis Oweinat, ~ 1000 km west of the present Red Sea coast (Klerkx and Deutsch, 1977). Stacey and Stoeser (1983) and Gillespie and Dixon {1983) found high isotopic lead ratios in whole-rock and K-feldspar of the Aswan Granite

284 that indicate an Archaean or early Proterozoic crustal source, and this result makes it likely that the Nile Craton extended at least as far east in southern Egypt as the River Nile. El Rarely et al. (1984) investigated a traverse from the Red Sea to the Hafafit culmination of granitoid gneiss domes and interpreted a sequenc~ of alternating psammitic and pelitic gneisses as original shallow-water sediments that were deposited on a stable continental margin (presumably the Nile Craton). A preliminary Nd model age of ~ 800 Ma for a metavolcanic clast in the Hafafit psammitic gneiss (Harris, Greiling and KrSner, unpubl. data) indicates that pas,+ive margin evolution was established at about this time in southern Egypt, and it is likely that the clastic sediments at Meatiq in the central Eastern Desert are of similar age and derivation. The lack of widespread early and juvenile Pan-African tonalite--trondhjemite--diorite complexes of the age range 700--900 Ma as found in Arabia (Fleck et al., 1980; Marzouki et al., 1982) strongly suggests that subduction and accretion processes were already under way in the east while a passive margin developed farther west. The two environments were probably separated by an oceanic domain since African continental detritus did not reach the evolving early Arabian arc terranes (Roobol et al., 1983}. Stern (1981) concluded from the geochemistry of pillowed ophiolitic basalts of his Older Metavolcanics in the Eastern Desert of Egypt and their association with clastic sediments that continental crust was nearby during ocean development and suggested a back arc basin setting. This model is attractive in that it suggests to dynamically relate the contrasting evolution m Arabia and Egypt between - 700 Ma and ~ 900 Ma ago that I visualize as follows (Fig. 4): ~-----

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Fig. 4. Hypothetical composite N W - - S E cross-sections across southern Eastern Desert of Egypt (Nubian Domain) and southern Arabian shield (Arabian Domain) showing suggested evolution at about 750 M a ago (top) and at about 640 M a ago (bottom). Based on Stoeser et al. (1984), Gettings (1984) and El Rarely et al. (1984). Black signature in lower section denotes ophiolites and/or ophiolitic m~lange.

285

Subduction-related magmatism in the oceanic domain in the east created the first Pan-African arcs of Arabia, and the westward-directed subduction process might have been the prime cause for back-arc related rifting and passive margin evolution farther west in Egypt. This scenario also allows for the possibility of small continental fragments to be rifted off the African margin that may subsequently have become incorporated in the collage of terranes in Arabia (c.f. Garson and Shalaby, 1976). The evolving marginal basins represent a tensional setting while compressive deformation in Arabia resulted from arc and terrane collision and ocean consumption {Fig. 4, upper section)• Natland (1984) showed that normal oceanic crust of m o d e m environments does not contain basaltic lavas with SiO2 in excess of 53%, and Alabaster et al. (1982) suggested that oceanic tholeiites intermediate between MORB and island arc basalts were probably generated in marginal basins. b

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Fig. 5. Major element discrimination diagrams (Natland, 1984) showing distribution of pillow lavas and sheeted dykes from Pan-African ophiolites in Egypt and the Sudan (anhydrous compositions). + Pillow lavas, Wadi Ghadir, Egypt (El Bayoumi, 1980), • Sheeted dykes, Wadi Ghadir, Egypt (El Bayoumi, 1980), x Older Metavoleanies, Egypt (Stern, 1981), * Pillow laves, Wadi Onib, Sudan (Hussein, unpub, data), e Sheeted dykes, Wadi Sudi, Sudan (Hussein, unpubl, data)• (a) SiO 2 versus TiO 2. Delineated fields are: Troodos Massif, Cyprus, Lower Pillow Laves and sheeted dykes (full line, based on Pantazis, 1980); Oman ophiolite, "Geotimes laves" and dykes (long dashed line, based on Alabaster et al., 1982); Sarmiento ophiolite, Chile, lavas, dykes and sills (short dashed line, based on Saunders et al., 1979); Scotia Sea and Bransfield Strait, lavas (dotted line, based on Natland, 1984)• Hachured line gives lower TiO~ contents for scattered ocean crust and back-are basin glass analyses and represents more than 99% of all published data (from Natland, 1984). (b) SiO~ versus Fe~O3*. Delineated fields are: Troodos Massif, Cyprus, and Oman ophiolite (full line, sources as in Fig. 5a); Samiento ophiolite (dashed line, souree as in Fig. 5a); hachured line as in Fig. 5a but for Fe203* (after Natland, 1984).

286 Available data for the ophiolitic lavas and sheeted dykes of the Eastern Desert and the Sudan are plotted in Fig. 5 and reveal high SiO2 contents, similar to back-arc basin rocks such as the "Geotimes Lavas" and sheeted dykes of Oman (Alabaster et al., 1982), the Sarmiento ophiolite of southern Chile (Saunders et al., 1979} and the Zambales Range ophiolite, Philippine Islands (Hawkins and Evans, 1983), but also similar to the lower pillow lavas and dykes in the Troodos Massif of Cyprus (Pantazis, 1980) that are probably of forearc or intra-arc origin (Miyashiro, 1973). Figure 5a also shows that the Egyptian rocks almost exclusively belong to the high-Ti ophiolite variety (Serri, 1981), and the Wadi Ghadir sheeted dykes in particular display strong F e ~ n r i c h m e n t as also shown by some rocks in the Sat miento complex (Fig. 5b}. The Egyptian ophiolites were tectonically emplaced and thrust westwards over the continental margin during marginal basin closure between ~ 700 Ma and 800 Ma ago, and this is consistent with the onset of calc-alkaline vol. canism in the southern Eastern Desert during this period. Stern and Hedge (1984) reported a Rb--Sr whole-rock isochron age of 768 _~ 31 Ma for rhyodacites at Abu Swayel SE of Aswan and a :°~'Pb/2°6Pb minimum age of 709 Ma for a tonalite at Lat. 2 3 ~ , while preliminary data for the t a l c alkaline Shadli volcanics from the type locality at Um Samiuki suggest a Rb--Sr whole-rock isochron age of ~ 715 Ma (KrSner, Stern and Greiling, unpubl, data). These rocks were largely emplaced after the ophiolites had been tectonically dismembered and thrust onto the continental margin. and in the northernmost Sudan calc-alkaline volcanic rocks with a Rb--Sr isochron age of 712 -+ 58 Ma, that are equivalent to the Shadli volcanics farther N in Egypt, are found unconformable on a tectonized ophiolite complex (Fitches et al., 1983}. Considerable horizontal shortening must have taken place during marginal basin closure as shown by the extensive m61ange units found throughout the Eastern Desert (Ries et al., 1983) and by nappe and thrust structures now identified at m a n y places in and below the ophiolite m61anges (Basta et al., 1983; E1 Ramly et al., 1984). Some of the most complete ophiolite sections such as in Wadi Ghadir and in Wadi Mubarak constitute well preserved, large blocks within the m~langes (Ries et al., 1983; El Bayoumi, 1980; E1 Bayoumi and Hassanein, 1983) and exhibit spectacular examples of preserved oceanic crust such as cherts, pillow lavas, sheeted dykes {Fig. 6), isotropic gabbro and layered gabbro (Fig. 7), in that order from top to bottom. The lower, ultramafic units of the ophiolites were most severely tectonized during thrust emplacement, and their occurrence as chaotic fragments in the m~lange (Fig. 8) does not permit reconstruction of the original igneous stratigraphy. A schematic model of the suggested evolution in the southern Eastern Desert of Egypt, based on work in the Hafafit-Wadi Ghadir area (El Ramly et al., 1984), is shown on the left side (Nubian Domain) of the sections in Fig. 4 and implies considerable crustal thickening during thrusting and

287

Fig. 6. Sheeted dyke complex in the Late Precambrian ophiolite of Wadi Ghadir, Eastern Desert of Egypt. Top: Dyke complex as exposed in Wadi El Beida, consisting of SiOcrich tholeiitic basalt as shown in Fig. 5. Bottom: Dyke with chilled margin intruding isotropic gabbro in Wadi El Beida.

288

Fig. 7. Gabbroic complex of the Wadi Ghadir ophiolite, Egypt. Top" Layered gabbro in Wadi Sudi. Bottom: Isotropic gabbro (rosette-gabbro of El Bayoumi, 1980) in Wadi El Beida.

289

Fig. 8. Ultramafic m~lange in Wadi Ghadir, southern Eastern Desert of Egypt. Note large blocks set in a matrix of serpentinite. The largest block near the top of the photograph is about 5 m across. nappe emp lacem e nt t hat may have triggered gravitative uplift and d o m e f o r m a t i o n at Hafafit. Stern and Hedge (1984) observed a general progression of igneous activity in the Eastern Desert from south to north with time, with the bulk of the Pan-African rocks in the south generated prior to 650 Ma ago while major crust-forming accretionary episodes in the nort h c o n t i n u e d until ~ 600 Ma ago. These authors also show that, after this time, intraplate magmatism signifies the Eastern Desert crust to have acted as a single tectonic unit tha t was probably part of the Arabian--Nubian shield. In comparison to Arabia this age pattern suggests that terrane accretion and stabilization of the late Pan-African Arabian plate was virtually com pl et e by a b o u t 640 Ma ago (Stoeser et al., 1984} while subduction-related accretion still occurred some 40 Ma later to the west in Egypt, perhaps marking the final " d o c k i n g " event when Arabia was finally welded to the African continent. OPHIOLITES AND ACCRETION IN THE RED SEA HILLS OF THE SUDAN The Sudan section of the Arabian--Nubian shield is m uch less know n than the regions discussed so far, and there is only fragmentary reliable

290 geochronologic information. Vail (1976, 1979, 1 9 8 3 ) s u m m a r i z e d most data and separated a dominantly volcano-sedimentary---plutonic low-grade assemblage of late Precambrian to Early Palaeozoic (i.e., Pan-African) age west of the River Nile (his greenschist assemblage) from a terrane of amphibolite to granulite grade gneisses and migmatites that he considered to be of much older age and part of an ancient craton. Stern et al. (1985) showed, however, that the granulites at Sabaloka N of Khartoum have a metamorphic age of ~ 710 Ma and cannot be derived from mid-Proterozoic or older sialic crust, while Harris et al. (1984) reported maximum Nd model ages and eNd values from the Bayuda Desert still farther N that lead to similar conclusions. The Precambrian basement rocks along the River Nile north of Khartoum are therefore part of the Pan-African domain, and the ancient craton must be sought farther west (see Fig. 9). Ahmed (1979) and Vail (1979) included most of the low grade metavolcanic---metasedimentary sequences of the Red Sea Hills in the Nafirdeib Series or Group and equate it with the Shadli volcanics of southern Egypt. A higher-grade and apparently older sequence of schists and amphibolites was separated as the Kashebib Series b u t the exact relationship with the Nafirdeib is not known. A number of mafic--ultramafic complexes are known from the Red Sea Hills (Fig. 9), and detailed work has established the Sol Hamed complex NW of Halaib as the first u n d o u b t e d Pan-African ophiolite (Hussein, 1977; Fitches et al., 1983) in the Sudan. Work is currently in progress on a number of other complexes in the Red Sea Hills that are all tectonically disrupted but show clear affinities to original ophiolitic sequences. These include the localities of Jabal Tohado and Wadi Amur while the largest and most complete ophiolite complex occurs between Wadis Onib and Sudi (Fig. 9) (Hussein et al., 1984). The Onib ophiolite is the only complex in the Nubian shield where the entire ocean crust sequence from pillow lavas to pyroxenites and peridotites at the b o t t o m is preserved. The complex is found in tectonic contact with the Nafirdeib Group rocks that are apparently younger than the ophiolites since, locally, conglomeratic or pebbly Nafirdeib metasediments contain rare ultramafic clasts. A Rb--Sr whole-rock age of 712 -+ 58 Ma was reported for the Nafirdeib volcanics at Sol Hamed (Fitches et al., 1983), and this age confirms the Nafirdeib--Shadli correlation and thus shows for the northeastern Sudan as for southern Egypt that ophiolite generation and obduction occurred prior to ~ 712 Ma ago. The Onib ophiolite sequence is now partly dismembered as a result of deformation, possibly related to the obductive mechanism of emplacement, and the structural style is characterized by generally SE-verging tight folds. The pillow lavas and cherts are only exposed in a small area in the upper Onib river and are in faulted contact with the ultramafic rocks. The lavas are tholeiitic basalts, and their major element geochemistry is compatible with an ocean-floor or back-arc basin origin. The associated sheeted dykes constitute a mappable unit in Wadi Sudi where they intrude isotropic and

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292 layered gabbro. These rocks are distinctly low in TiO2 and Fe20~* and high in SiO2 in comparison to the lavas and also to the ophiolites in Egypt and thus display strong affinities to modern back-arc basin crust (Fig. 5). Since the dykes show no secondary quartz due to post-emplacement alteration, their SiO2 enrichment and corresponding TiO2 depletion must be largely primary and is thus unlike modern MORB (Natland, 1984). Alabaster et al. (1980) described sheeted dykes and sills with similar composition from the Oman back-arc ophiolite, and there are also high-Si/low-Ti/low-Fe sheeted dykes in the intra-arc Cyprus ophiolite (Pantazis, 1980). The Onib gabbros can be subdivided into an upper, thin isotropic um~ and a lower part, several kilometres thick and spectacularly layered at scales from a few millimetres to tens of metres (Fig. 10). The gross igneous stratification allows the recognition of way-up criteria and also exhibits folding that was probably generated during igneous evolution and spreading of the Onib oceanic crust (Fig. 11). Towards the b o t t o m the layered gabbro,~ are intercalated with pyroxenite and dunite and display well presmwed cumulus textures. The ultramafic section of the Onib complex is at least 2 km thick as presently exposed and follows conformably under the layered gabbro unit while its base is in tectonic contact with strongly sheared Nafirdeib volcanosedimentary rocks. It contains lens-shaped, discontinuous layers of dunite

Fig. 10. Layered gabbro from Wadi Onib, northern Red Sea Hills, Sudan, showing well developed igneous stratification.

293

Fig. 11. High-temperature, early flow folding in layered gabbro, Wadi Onib, northern Red Sea Hills, Sudan, interpreted to result from ocean crust spreading.

with well preserved chromite layers (Fig. 12) that alternate with clinopyroxenite and, in the upper part, with gabbro. This sequence is remarkably similar to the cumulate section of the Cretaceous Antalya ophiolite in Turkey (Juteau and Whitechurch, 1980), and individual rock types contain a well preserved primary igneous mineralogy. The lowermost part of the complex is variably serpentinized but harzburgite and lherzolite varieties can still be recognized in spite of silicification and/or carbonatization. At one locality in Wadi Sudi a peculiar breccia occurs within the ultramafic zone and is in tectonic contact with the neighbouring rocks. Blocks of fresh clinopyroxenite ranging in size from several centimetres to several metres are set in a matrix of very coarse-grained but altered clinopyroxenite (Fig. 13). This may be interpreted as the deep crustal part of a leaky oceanic fracture zone where pyroxenite magma consolidated and was subsequently broken up into fragments during transform motion while newly intruding pyroxenite mush " h e a l e d " the fracture. Karson et al. (1983) described a similar situation from the Bay of Islands ophiolite complex in Newfoundland. The recognition of clastic rocks with rare ophiolitic fragments in a strongly sheared N--S oriented belt west of the Onib complex suggests ophiolite obduction from east to west, similar to the situation in southern Egypt.

294

Fig. 12. Dunite with cumulate layers of chromite in ultramafic section of oph|ohte complex in Wadi Sudi, northern Red Sea Hills, Sudan.

Ultramafic m~langes are rare at Onib but may be concealed under extensive R ecen t sand cover. Emb leto n et al. (1984) proposed that there may be three separate crustal entities in the Red Sea Hills of the Sudan, separated by ophiolite belts, and that these blocks should reflect temporally different and distinct cycles o f magmatic activity similar to the situation in Arabia. However, presently available age data do not support this model. The Shadli and Nafirdeib volcanics lie on either side of the Onib-Sol Hamed ophiolite belt (Fig. 9) and have identical ages of 715 Ma. Vail et al. {1984) reported a Rb--Sr wholerock isochron age of 723 -+ 4 Ma for Nafirdeib volcanics some 150 km NW o f Port Sudan while preliminary Rb--Sr data for Nafirdeib rocks south o f Port Sudan indicate an age of ~ 715 Ma (Reischmann, Stern and KrSner, unpubl, data). Thus surprisingly, and in contrast to the p r o n o u n c e d age range in Arabia, the Red Sea Hills volcanic episode appears to have been limited to a short time and may suggest that arc volcanism was virtually synchronous from southeastern Egypt through the entire Red Sea Hills o f the Sudan (Fig. 9). Geochemical data so far indicate that the Nafirdeib volcanic rocks range from basalt to rhyolite in com pos i t i on and are predom i nant l y calc-alkaline in character (Fitches et al., 1983; Klemenic and Poole, 1984; Reischmann

295 and KrSner, 1984). A m a g m a t i c arc e n v i r o n m e n t is the m o s t likely setting, and available 87Sr/86Sr initial ratios lie in t h e n a r r o w range 0 . 7 0 2 - - 0 . 7 0 3 , identical to t h o s e r e p o r t e d f o r A r a b i a n r o c k s a n d suggesting a m a n t l e source with little or no crustal c o n t a m i n a t i o n . It is t h e r e f o r e likely t h a t t h e Red Sea Hills r e p r e s e n t a collage o f closely s p a c e d c o n t e m p o r a n e o u s islandarcs t h a t a c c r e t e d b y marginal basin closure to t h e w e s t of the evolving A r a b i a n plate.

Fig. 13. Angular clasts of clinopyroxenite set in a matrix of very coarse clinopyroxenite. The large clast is about 2 m wide. The rock is interpreted as part of a leaky transform fault zone. Upper par" of Wadi Sudi, northern Red Sea Hills, Sudan.

CONCLUSIONS T h e age z o n a t i o n a n d r o c k associations f r o m E g y p t - - S u d a n to A r a b i a clearly d e m o n s t r a t e t h a t simple arc a c c r e t i o n , f r o m east to west, o n t o the African c o n t i n e n t did n o t o c c u r during crustal g r o w t h o f t h e A r a b i a n - N u b i a n shield. T h e oldest P a n - A f r i c a n t e c t o n o - m a g m a t i c e l e m e n t s o c c u r in s o u t h e r n A r a b i a a n d n o t in Nubia, so t h a t the t w o s e g m e n t s are likely to have evolved i n d e p e n d e n t l y b e t w e e n c. 9 5 0 a n d 6 2 0 Ma ago. S o u t h e r n A r a b i a is likely to have g r o w n b y collision o f oceanic arcs, oceanic p l a t e a u s , c o n t i n e n t a l f r a g m e n t s and closure o f m a r g i n a l seas, in a setting similar to t h a t p r e s e n t l y o b s e r v e d in the I n d o n e s i a n a r c h i p e l a g o ( H a m i l t o n , 1979).

296 Collision tectonics produced extensive horizontal shortening in overthrust zones and caused crustal thickening. These thrust zones are often, but not always, marked by dismembered ophiolites, m~langes and linear fold belts in which high-temperature gneisses and migmatites are now exposed that may constitute the roots of the arc systems. Passive margin development, clearly recognized in southern Egypt but not yet in the Sudan, probably occurred contemporaneously with terrane accretion in southern Arabia, and marginal basin closure between the growing Arabian plate and eastern Nubia, now converted into an active continental margin, began some 750 Ma ago. A likely correlation of the OnibSol Hamed ophiolite belt of the northeastern Sudan with the Jabal DherwahBir Umq ophiolite belt of western Arabia (A1-Shanti and Hakim, 1981) suggests that at least some of the volcanic terranes in this part of Arabia may represent arcs that extended into present Nubia and, at around 720 Ma ago, evolved and accreted in a marginal sea environment (Claesson et al., 1984) behind the earlier accreted collage in southern Arabia that, by c. 700 Ma ago, had already grown into a sizeable plate. Subduction-related arc volcanism came to an end when the southern Arabian plate finally " d o c k e d " with Nubia, first in the south at about 640 Ma ago, then gradually farther north where this activity ceased some 40 Ma later in the northern Eastern Desert of Egypt. Extensive strike-slip faulting such as observed in the Najd system of Arabia did not occur in the Nubian segment of the shield, perhaps suggesting that, by analogy with present tectonics in east Asia, the still "soft", newly-formed and heterogeneous Arabian lithosphere reacted to compressional tectonics from the east. The Nubian Pan-African assemblage, however, now tectonically emplaced on the continental margin of an old craton with thick roots, remained "shielded" from this final Pan-African event. ACKNOWLEDGEMENTS This study resulted from fieldwork initiated by IGCP Project No. 164 in Saudi Arabia, Egypt and the Sudan, and I appreciate funding by the German Research Council (DFG Kr 590/11-1 and 11-2), the German Ministries of Science and Technology (BMFT) and Economic Cooperation (BMZ) and by King Abdulaziz University in Jeddah, Saudi Arabia. Logistic support was also kindly provided by the Geological Survey of Egypt, the Geological and Mineral Resources Department of the Sudan and by the Faculty of Earth Sciences, University of Jeddah. The ideas expressed in this paper have evolved from collaborative efforts with many colleagues and I particularly thank St. Diirr, R. Greiling and T. Reischmann (Mainz), I. Hussein (Port Sudan/Mainz), M.F. E1 Rarely, R.M.A. E1 Bayoumi and A.A. Rashwan (Cairo), A. A1-Shanti, A. Basahel, N.J. Jackson and C.R. Ramsay (Jeddah), R.J. Stern and M. Halpern (Dallas) and R.J. Fleck (Menlo Park). N.J. Jackson and R.M. Shackleton reviewed

297 t h e m a n u s c r i p t a n d s u g g e s t e d s u b s t a n t i a l i m p r o v e m e n t s . C. P r o d e h l a n d M. G e t t i n g s k i n d l y p r o v i d e d p r e p r i n t s o f i m p o r t a n t p a p e r s c o n c e r n e d w i t h the i n t e r p r e t a t i o n of seismic a n d gravity data of t h e A r a b i a n shield. T h i s is a c o n t r i b u t i o n t o I G C P P r o j e c t 1 6 4 a n d t o t h e I n t e r n a t i o n a l Lithosphere Programme.

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