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  • Precambrian Research, 27 (1985) 277--300 277 Elsevier Science Publishers B.V., Amste rdam -- Printed in The Netherlands

    O P H I O L I T E S A N D T H E E V O L U T I O N O F T E C T O N I C B O U N D A R I E S I N

    T H E L A T E P R O T E R O Z O I C A R A B I A N - - N U B I A N S H I E L D O F

    N O R T H E A S T A F R I C A A N D A R A B I A

    A L F R E D K R O N E R

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

    A B S T R A C T

    Kr6ner, A., 1985. Ophiol i tes and the evolut ion of tectonic boundaries in the late Pro- terozoic Arabian--Nubian shield of northeast Africa and Arabia. Precambrian Res., 27 : 277--300.

    The Arabian--Nubian shield is current ly regarded as one of the best examples to demonst ra te that processes of lateral crustal growth and modern- type obduct ion--accre- t ion tectonics have opera ted since at least late Precambrian times.

    In Arabia a number of Pan-African volcano-sedimentary /p lu tonic belts have been identif ied that display internal evolut ionary pat terns suggesting a deve lopment f rom primitive intraoceanic arcs some 900--950 Ma ago to mature , andesi te-dominated arcs some 640 Ma ago through processes of ocean-crust obduc t ion , arc collision and magmatic crustal thickening. Several ophiol i te-decorated sutures are preserved, but many early tectonic boundaries were obl i terated during later overthrusting, faulting and shield- wide granitoid p lu tonism towards the end of Pan-African evolu t ion 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 cont inenta l margin was probably separated f rom several evolving arcs to the east by marginal seas. These arc segments were later thrust over each other , f rom east to west, during widespread and considerable hor izonta l shortening and gave rise to spectacular nappe structures and extensive ophiol i te m~langes.

    The apparent lack of well-defined accret ionary thrust stacks, high-pressure meta- morphic assemblages and widespread ophiol i t ic m~langes in Arabia indicates that accre- t ion ei ther did not occur along margins with deep ocean t renches but involved buoyan t crust, or extensive overthrust ing took place through which the forearc segments were overr idden and are now concealed. This, together with the recogni t ion of dist inct tectonic belts and isolated fragments of possible ancient cont inenta l crust and oceanic plateaus, supports the con ten t ion that Arabia may represent a collage of previously independent exot ic terranes that accreted by obl ique convergence and strike-slip translat ion during shield evolut ion.

    It is suggested that the Arabian shield contains remnants of mic rocont inen t s with pre-Pan African (i.e., > 1000 Ma) crustal history and, perhaps, oceanic plateaus and that its evolut ion bears similarities with aspects of terrane accret ion in the North Amer- ican Cordillera and in the present western Pacific. The evolut ion in Egypt and in the Sudan, however , seems character ized by the t ransformat ion of a passive cont inenta l margin into a tectonical ly active belt along which ophiol i tes and arc volcanics were thrust over each o ther at approximate ly the same t ime when the exot ic terranes and arcs of Arabia accreted farther east. Final stabil ization 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 magmat ism some 600--640 Ma ago.

    0301-9268 /85 /$03 .30 © 1985 Elsevier Science Publishers B.V.

  • 278

    INTRODUCTION

    The recognition of true ophiolites or their dismembered fragments to- gether 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 ~950 Ma and ~640 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 com- ponents 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 be- tween 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 wide- spread older sialic basement under the eastern Arabian shield from the oc- currence of tin-rich peraluminous granites.

    Based on the recognition of well defined tectonic provinces in the south- ern Arabian shield that display distinctly separate isotopic, geochemical

  • 279

    and age characteristics (Greenwood 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 as- semblage 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 concept and have recently interpreted the Afif domain of Arabia as an al lochthonous terrane.

    Reymer and Schubert (1983) also doubted the entirely juvenile nature of the Pan-African crust in NE Africa and Arabia. They showed that Phan- erozoic addition rates to the continental crust through arc magmatism are in the range 20--40 km ~ km -' Ma- ' , while the growth rate over the life- time 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 confident ly rule out an ensialic evolution of the Arabi- an--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 demonstrated above, and future models should take these limitations into consideration.

    ACCRETION IN ARABIA

    Although many contacts between tectonic provinces and most ophiolite- decorated suture zones are steep, these structural discontinuities have either been modified during late-tectonic strike-slip faulting or they repre- sent 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} re- ported rare occurrences of ophiolitic m~langes from the easternmost shield and suggest that these units represent thrust sheets extending below the adjacent turbidite sediments onto which the original ophiolites were thrust westwards. High-pressure, low-temperature metamorphic assemblages typical of accretionary wedges in modern forearc or trench regions have not been found in the Arabian shield and suggest that such rocks were either con- sumed during subduction-erosion or "dec re t ion" (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 buoyant and new arcs or crustal segments did not collide "head- on" but by oblique, g

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