dating of late proterozoic ophiolites in egypt and the...

Precambrian Research, 59 ( 1992 ) 15-32 15 Elsevier Science Publishers B.V., Amsterdam

Dating of late Proterozoic ophiolites in Egypt and the Sudan using the single grain zircon evaporation technique

A. Krtiner ~, W. Todt b, I.M. Hussein a'c, M. M a n s o u r d a n d A.A. R a s h w a n °

alnstitut fiir Geowissenschafien, Universitiit Mainz, Postfach 3980, 6500 Mainz, Germany bMax-Planck-lnstitut fiir Chemie, Postfach 3060, 6500 Mainz, Germany

CGeological Research Authority, Red Sea Hills Office, P.O, Box 5 73, Port Sudan, Sudan dEgyptian Geological Survey and Mining Authority, 3 Salah Salem Street, Cairo, Egypt

(Received April 10, 1991; revision accepted February 26, 1992 )

ABSTRACT

Kr/3ner, A., Todt, W., Hussein, I.M., Mansour, M. and Rashwan, A.A., 1992. Dating of late Proterozoic ophiolites in Egypt and the Sudan using the single grain zircon evaporation technique. Precambrian Res., 59:15-32.

Zircons from gabbro and plagiogranite in late Proterozoic ophiolites oflhe Arabian-Nubian Shield ( ANS ) in Egypt and the Sudan, as well as post-ophiolite granites have been dated using the stepwise evaporation method. Zircons from a plagiogranite in the Wadi Ghadir ophiolite, Eastern Desert of Egypt, yielded a mean 2°VPb/2°6pb age of 746 +- 19 Ma, while a gabbro and diorite associated with the Abu Swayel ophiolite nappe ~ 250 km southwest ofWadi Ghadir provided zircon 2°VPb/2°6Pb ages of 729 _+ 17 and 736 + l 1 Ma, respectively. Zircons from layered gabbro along the western margin of the Jabal Gerf ophiolite just north of 22°N were dated at 741 _+ 21 Ma, indistinguishable from the Wadi Ghadir and Abu Swayel results. A plagiogranite sample from the Onib ophiolite in the northern Red Sea Hills, Sudan, contains zircons with a 2°Tpb/2°6pb age of 808 +- 14 Ma, while granites which crosscut layered gabbro in this complex have zircon ages of 713 + 12 and 714+- 5 Ma respectively. The youngest granite at Onib was dated at 646+_ l0 Ma and may reflect a phase of post- accretionary intraplate magmatism.

Our zircon data document ocean crust generation between ~ 810 and ~ 730 Ma ago in the Nubian segment of the ANS. and the age data compare well with ophiolite generation in Saudi Arabia. However, some of our data cast doubt on large- scale correlations of suture belts between the Arabian and Nubian segments of the shield.

Introduction

The discovery of ophiolite assemblages in the late Proterozoic accretionary terrain of the Arabian-Nubian Shield (ANS) in recent years has led to a number of evolutionary models that envisage collision of intraoceanic island arcs and exotic microcontinental fragments after closure of several marginal basins (Bakor et al., 1976; Gass, 1981; A1-Shanti and Gass, 1983;

Correspondence to: Prof. A. Kr6ner, Institut fiir Geowis- senschaften, Universit~it Mainz, Postfach 3980, 6500 Mainz. Germany.

Kr6ner, 1985; Stoeser and Camp, 1985; Pallis- ter et al., 1987 ). In these models the ophiolites are envisaged as remnants of back-arc oceanic crust, an interpretation based largely on the LIL-enriched geochemistry of their pillow basalts and sheeted dyke complexes (Bakor et al., 1976; Kr6ner, 1985; Pallister et al., 1988). They were obducted, rather than subducted, in line with current theory that back-arc basins reach maximum ages of about 40 Ma before being closed. This implies that young, hot, pos- itively buoyant oceanic lithosphere, or at least the upper part of it, is thrust over the neigh- bouring terrains during basin closure, similar to obduction of the Semail ophiolite nappe in

0301-9268/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.

16 a. KRONER EI AL.

Oman (Searle and Stevens, 1984). Alterna- tively, ophiolite obduction was part of a backthrusting process following collapse of the arc systems during collision (Silver et al., 1983; Silver and Reed, 1988 ).

During these collision and obduction processes the oceanic segments became tectonically disrupted so that the original magmatic sequences are now difficult or impossible to reconstruct. Those ophiolite fragments which preserve their internal structures rather well were discovered early [e.g. Jabal al Wask and Jabal Ess, Saudi Arabia, Bakor et al. (1976),

Shanti and Roobol (1979), Wadi Ghadir, Egypt, Elbayoumi ( 1980, 1983), Sol Hamed and Onib, Sudan, Hussein (1977), Fitches et al. (1983), Hussein et al. (1984) ], while those strongly fragmented and further disrupted during post-collisional strike-slip deformation were only recognized in recent years as remnants of oceanic lithosphere (see reviews in Vail, 1985, Kr6ner et al., 1987, and Pallister el al., 1988). Surprisingly, and in line with the obduction hypothesis, all these ophiolite fragments and their associated magmatic and sedimentary assemblages display only low- to me-

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Fig. 1. S i m p l i f i e d m a p o f t h e A r a b i a n - N u b i a n sh ie ld ( R e d Sea c l o s e d ) s h o w i n g o p h i o l i t e o c c u r r e n c e s , s u t u r e zones , ter- r a n e s a n d t h e p r e s u m e d m a r g i n o f t h e a n c i e n t A f r i c a n c r a to n . M o d i f i e d f r o m K r 6 n e r et al. ( 1987 ).

DATING OF LATE PROTEROZOIC OPHIOLITES IN EGYPT AND THE SUDAN 17

dium-grade metamorphic overprinting; high- pressure blueschist assemblages, such as reported from Phanerozoic subduction-accretion complexes, are not known from the ANS.

In the Eastern Desert of Egypt (ED) most of the ophiolite assemblages are part of extensive nappe complexes (Shackleton et al., 1980 ) that were thrust over continental margin type sediments (El Ramly et al., 1983) or arc terrains (Ries et al., 1983 ), and their origin is un- known. In almost all cases internal structures are fiat, and contacts with the surrounding volcano-sedimentary and gabbro-granitoid plutonic suites are frequently defined by shear zones. These ophiolite klippen cover an extensive area in the ED from the Qena-Quseir road in the north to Wadi Allaqi in the south (see area west and northwest of Marsa Alam on Fig. 1), and the largest nappe unit is exposed around Barramiya west of Marsa Alam (Shackleton et al., 1980; see Fig. 1 ). It is unlikely that they all belong to one giant nappe or to one single ophiolite unit originating some- where in the east.

In Saudi Arabia and in the Red Sea Hills (RSH) of the Sudan the ophiolite complexes or their remnants constitute well-defined belts, often associated with strong deformation, and have therefore been interpreted as marking the sites of major suture zones along which the arc terranes collided (Stoesser and Camp, 1985; Kr6ner, 1985; Vail, 1985; Shackleton, 1986; KriSner et al., 1987; Pallister et al., 1987; Stoe- ser and Stacey, 1988 ).

Original continuity of some sutures with ophiolite fragments from Arabia into the ED and RSH has been demonstrated or claimed by several authors (e.g. Stoesser and Camp, 1985; Vail, 1985; Shackleton, 1986; Kr6ner et al., 1987) and makes it likely that several of the terranes identified in the ANS continue from Nubia into Arabia. However, in view of considerable strike-slip deformation along the NW-SE-striking Nadj fault system some of these correlations remain speculative. It is therefore important to establish the absolute

ages of formation and, if possible, provide age constraints on emplacement of the various ophiolite complexes or belts on both sides of the Red Sea. While this has now been achieved for most of the major ophiolites in Arabia through a combination of Sm-Nd whole-rock and mineral work and U-Pb zircon dating (see Pallister et al., 1988, and references therein), so far no reliable ages are available for compa- rable rocks in the Nubian part of the shield.

An extensive program of dating suitable rock types and minerals by various techniques from the Nubian ophiolites is now in progress in Mainz, and these data should not only establish reliable chronologic criteria but should also provide information on the isotopic composi- tion of the mantle source region (s). In this first report we present single zircon 2°Tpb/2°6pb ages for plagiogranites of selected ophiolites in the ED and RSH, obtained using the evaporation technique of Kober (1986,1987). Their ages are crucial for late Proterozoic plate tectonic reconstructions in northeast Africa and Arabia. We also dated a gabbro-diorite suite and felsic dykes crosscutting ophiolitic rocks in Wadi Allaqi, southernmost ED, in an attempt to establish minimum ages for these ophiolites.

Rock types and results

Wadi Ghadir Ophiolite, Egypt

One of the best preserved sections through late Proterozoic upper oceanic crust anywhere in the world occurs in a side stream of Wadi Ghadir, some 40 km southwest ofMarsa Alam (Figs. 1 and 2), southern ED (Elbayoumi, 1980, 1983; KriSner, 1985 ). The sequence consists, from top to bottom, of thin lenses of chert, underlain by pillow basalt, a superbly exposed sheeted dyke complex (see cover photograph, EOS, Am. Geophys. Union, 62, #12, 1982), isotropic gabbro with diffuse vein-like bodies ofplagiogranite (Fig. 3 ) and a sequence of layered gabbro. This succession, about 300-400 m thick, rests in thrust contact on ultramafic

1 8 A. KRONER ET AL

-.-.;.-^ ' ~ . < + + + + , - " :_-_-__ (f~.-

35 ° E

Magmatlc arc association

Legend Undeformed leucocratic granite

Undeformed pink grey granite and

Fohated pink granite

Sheared pink granite

Undifferentialed metavolc~n~cs

~ Ophiolitic melange

Pillow lava

Sheeted dykes

Isotropic & layered gabbro veins & lenses of plagtogran'.t~

FaLllt

Sample locality

.. Opt,ohte 21,o ~ s~quenca • ~5' ,i

Fig. 2. Generalized geological map (from Elbayoumi, 1980) showing location of ophiolite sequence in Wadis, Saudi and El Beida, Eastern Desert of Egypt, and location of plagiogranite sample 87/E3.

Fig. 3. Plagiogranite vein in dark isotropic gabbro of Wadi Ghadir ophiolite in Wadi El Beida.

m61ange including fragments of serpentinite, pyroxenite, peridotite, dunite and harzburgite, all extensively altered and deformed (Elbay- oumi, 1983 ). The sheeted dykes and pillow basalts have trace element patterns resembling those of Phanerozoic back-arc basin and su- prasubduction zone oceanic crust (Pearce et al., 1984; Kr6ner, 1985 ), and the sheeted dykes also display a magnetic polarity reversal (Kr6ner, McWilliams, Layer and Elbayoumi, unpubl, data), similar to those found in mod- em oceanic spreading centres.

The plagiogranite veins are best exposed near the confluence of Wadis Saudi and El Beda (Fig. 2 ) and emanate from diffuse, leucocratic zones within the isotropic gabbro. They are reddish to grey in colour, consist of a medium- grained mosaic of plagioclase, quartz, minor hornblende and biotite. The veins display extensive low-temperature alteration with pro- found sericitization of plagioclase and chlori- tization of hornblende. Accessory minerals include magnetite, ilmenite, apatite and zircon. The latter crystals are between 20 and 80 #m in size, have perfectly preserved bipyramidal morphology and are clear, transparent to yellowish in colour. No magmatic zoning, no inclusions and no overgrowth were recognized under the microscope. We rule out that these


TABLE 1

Major and trace element analyses for dated samples of plagiogranite and layered gabbro from ophiolite complexes in Egypt and Sudan, and post-ophiolite granites

Oxide 87/E3 87/E7 87/E8 GG260 SD241 SDI06 SDI07 plagiogr, gabbro diorite gabbro plagiogr, granite granite

Major elements, wt% SiO2 68.99 48.47 50.45 48.32 77.41 69.46 66.68 TiO2 0.48 2.19 1.20 1.25 0.03 0.13 0.25 AIzO3 14.93 17.63 17.46 16.07 12.55 14.81 16.33 FeO 1.83 7.87 6.03 7.00 0.08 0.93 1.77 Fe203 2.25 3.97 2.10 2.00 0.23 0.38 0.78 MnO 0.07 0.18 0.19 0.18 0.01 0.01 0.04 MgO 0.75 5.24 2.75 8.99 1.16 0.55 1.1 I CaO 1.95 10.18 6.48 10.73 2.74 3.51 4.52 Na20 7.43 3.13 4.77 3.21 4.73 5.38 4.10 K20 0.08 0.20 0.82 0.16 0.05 0.57 0.92 P205 0.18 0.06 0.48 0.22 0.02 0.05 0.50 CO2 <0.01 0.05 0.02 <0.01 0.55 0.10 0.06 H20 + 0.88 1.10 3.33 0.86 0.91 3.10 2.04 H20- 0.10 0.05 0.02 0.17 0.02 0.12 0.17

Total 99.9 100.3 100.3 99.17 99.49 99.10 99.45

Trace elements in ppm Ba 31 58 157 59 Ga 24 22 21 - Rb < 1 1 18.5 0.5 < 1 8 12 Sr 177 539 385 473 163 259 343 Nb 4 3.5 4.5 2.4 <3 4 7 Zr 846 26.5 94 29 38 53 81 Y 80 10 25 13 2 10 6 V 26.5 418 101 - 11 26 38 Co 8 43.5 16.5 - <4 <4 <5 Cr 9.5 78 13 - < 3 7 25 Ni 4 33.5 7.5 - < 2 < 3 4 Cu 48 34 15.5 - <2 <2 <2 Zn 33 85 120 - < 2 2 37 La 3 <2 8 6 Ce 9 6 18 18 Nd 7 5 13 11 Sm <2 <9 <13 <11 Pr <1 <2 <3 2

Major element analyses by wet chemistry and XRF. Trace element analyses by XRF on duplicate powder pellets using techniques as described in Laskowski and Krtiner ( 1985 ).

are hydrothermal zircons formed during low- temperature alteration. A chemical analysis of the dated sample 87/E3 is given in Table 1 and indicates (Coleman, 1977; Pearce et al., 1984 ) that this rock is a plagiogranite or ocean ridge granite (ORG).

Four grains from sample 87/E3 were evaporated individually at temperatures between

1470 and 1520°C, and the resulting 2°7pb /

2°rPb ratios and ages are presented in Table 2 and in the histogram of Fig. 4. All four grains yielded similar age spectra which, combined, constitute a normal data distribution with a mean 2°7pb/2°6pb age of 746 + 19 Ma (Fig. 4 ). Since stepwise temperature increases during evaporation did not produce statistically sig-

20 A, KRONER ET &L.

TABLE 2

Isotopic data from single grain evaporation

Sample Grain Mass Evapora- Mean 2°7pb/ No. scans ~ tion temp. 2°6pbratio 2

( °C) and SE 3

87/E3 1 59 1480 0.06412 _+46 87/E3 2 42 1470 0.06419-+60 87/E3 3 50 1470 0.06417 _+ 74 87/E3 4 48 1520 0.06404_+ 70 87/E3 1-4 199 0.06413 _+ 59

87/E7 1 78 1480 0.06363-+63 87/E7 2 56 1490 0.06366_+42 87/E7 3 52 1470 0.06355_+ 34 87/E7 1-3 186 0.06363_+ 52

2O~pb/zO6pb

age and SE 3

745 _+ 15 748 _+ 20 747 _+ 24 743_+23 746_+19

729-+ 21 730± 14 727 ± 11 729 _+ 17

76/E8 1 104 1500 0.06383 _+ 34 736 _+ 11

8O

60 t _':2

t ~ , 3

ck 40 t

"o

D

z 20

600 I

Age in Ma 650 700 750

I I I 8O0

I

Mean age: 746_+19 Ma

i n G r a i n 1 59 ratios

inGrain 2 42 ratios

[ ]Gram 3 50 ratios

r"-] Grain 4 48 ratios

GG 260 1 60 1510 0.06398_+61 741 -+20 GG 260 2 69 1500 0.06405-+ 68 743 _+ 22 G G 2 6 0 3 29 1490 0.06386_+51 737_+17 G G 2 6 0 4 22 1530 0.06403_+74 742_+24 G G 2 6 0 1-4 180 0.06399_+64 741 +_21

SD 241 1 77 1510 0.06605-+ 46 808___ 14 SD241 2 57 1540 0.06602_+33 807+11 SD241 3 54 1520 0.06605_+41 808_+ 13 SD 241 4 58 1550 0.06606-+ 55 808-+ 17 SD 241 I-4 246 0.06604_+44 808+_ 14

0 0600 0.0625 00650 (207pb/206pb i.

Fig. 4. Histogram showing distribution of radiogenic lead isotope ratios derived from evaporation of f ou r z i rcon grains from plagiogranite sample 87/E3, Wadi Ghadir ophiolite, Eastern Desert o f Egypt. Mean age is given with standard error.

SD106 1 60 1520 0.06313_+ 16 714_+ 5

S D I 0 7 1 61 1520 0.06313_+23 713 + _ 7 SD107 2 36 1550 0 .06315+49 713_+ 16 SD 107 1-2 97 0,06314_+34 713_+ 12

O N 3 1 40 1510 0 .06119+30 646_+10 O N 3 2 31 1520 0 .06117+25 645_+ 9 O N 3 1-2 71 0.06118_+28 646_+10

Number of 2°Tpb/2°6pb ratios evaluated for age assessment. 2Observed mean ratio corrected for non-radiogenic Pb where necessary. Errors based on uncertainties in counting statistics. Data combined for several grains, e.g. I -4 in 87/3, are based on statistical assessment of all individual ratios. 3SE = standard error.

nificant changes in the measured 2°7pb/z°6pb ratios, we conclude from this that Pb-loss, if any, occurred in recent times and that the mean age of 746 _+ 19 Ma reflects the time of plagiogranite crystallization.

This date compares well with the age range of 740 + 11 to 780 + 11 established for zircons from gabbro and plagiogranite of the Jabal al Wask and Jabal Ess ophiolite complexes in the

Yanbu suture of the northern Arabian shield (Pallister et al., 1987; see Fig. 13 ) and suggest that all these oceanic fragments are of approx- imately the same age. However, if the Red Sea is closed, the Yanbu suture does not link up with the Wadi Ghadir and Barramiya ophiolite nappes of the ED (Fig. 1 ), and it is therefore unlikely that these oceanic fragments are all part of the same back-arc basin. The ages reported by Stern and Hedge (1985) suggest that formation of volcanic and plutonic arc complexes in the southern ED took place from ~ 770 to ~ 720 Ma ago so that formation of the Wadi Ghadir oceanic crust is virtually contemporaneous with this arc formation event. We conclude from this and geochemical work on Wadi Ghadir basalts and sheeted dykes (high SiO2, strong incompatible element and Fe-enrichment, Kr6ner, 1985) as well as me- tavolcanics north of Wadi Ghadir (Zimmer, 1985 ) that the Ghadir ophiolite reflects back-


Fig. 5. Geological map of part of the Abu Swayel area (simplified and modified from E1 Shazli et al., 1975 ) showing major rock units and sample localities of the gabbro and diorite dated in this study.

arc basin crust that originated from intra-arc splitting during a time when significant vol- umes of arc terrain were already present in the region that was to become the ANS. Basin closure and ophiolite obduction must therefore have occurred after ~ 745 Ma ago and was contemporaneous with production of further arc material through continuous subduction of oceanic lithosphere and arc accretion.

Abu SwayeL Egypt

The ophiolitic klippe east of the Abu Swayel copper prospect (Figs. 1 and 5) consists of a strongly sheared serpentinite body whose original rock types are difficult to reconstruct. The serpentinite is intruded by isotropic gabbro (Table 1, Fig. 5, sample 87/E7), in places showing faint magmatic layering, and grading imperceptibly into massive and unfoliated

diorite (Table 1, sample 87/E8) towards the south. This gabbro-diorite complex is also in- trusive into a sequence of well foliated elastic metasediments (Fig. 5) with elongated granite-gneiss pebbles and interlayered metavol- canics that belong to a continental margin assemblage as revealed by the presence of Archaean detrital zircons and Archaean to early Proterozoic Nd crustal residence ages (Wust et al., 1987a). It is therefore doubtful whether the above gabbro-diorite complex is, in fact, part of the ophiolite assemblage since diorites were never identified in other ocean crust successions of the ED and RSH, and in- trusive contacts with continent-derived metasediments are also unusual, except for very narrow ocean basin such as, for instance, the Gulf of California (Kelts, 1981 ). Gabbro- diorite bodies are, however, part of the volu- minous plutonic, arc-related association that is

22 A. KRONER ET AL.

co o

AD

,,Q O.,.

o ('4

'5

c'~

E Z

80

60

40

20

650 I

Age in Ma 70O 750 80O

I I I

Mean age: 729_+17 Ma

850 I

900

~ Grain 1, 78 ratios

~ Grain 2, 56 ratios

W Grain 3, 52 ratios

30

~.0

?0

Agein Ma 700 750

I I

Mean age: 736t l l Ma

800

0.060 0.062 0.064 0 066 0.068 0.062 0.064 0.066 (207pb/206pb)' (207pb/206pbl"

(a) (b)

Fig. 6. Histograms showing distribution of radiogenic lead isotope ratios derived from evaporation of zircons from Abu Swayel gabbro-diorite complex, Eastern Desert of Egypt. (a) Spectrum for 3 grains of gabbro sample 87/E7, integrated from 186 ratios. (b) Spectrum for one grain of diorite sample 87/E8, integrated from 194 ratios. Mean ages are given with standard errors.

widespread in the southern and central ED and that is mapped as Metagabbro-Diorite Com- plex on Geological Survey maps (EI-Ramly, 1972; Geological Map of Egypt, 1981 ).

Zircons were separated from the gabbro close to where it intrudes the serpentinite (sample 87/E7 ). They are small (20-60/tin ), euhedral and clear to yellow-brown in colour. Three grains evaporated at temperatures between 1470 and 1490 ° C yielded uniform 2°7pb/2°6pb ratios that combine to a mean 2°7pb/2°6pb age of 729_+ 17 Ma (Table 1; Fig. 6). One zircon from a diorite sample (87/E8) collected about 1 km south of gabbro 87/E7 gave a mean 2°Tpb/2°6pb age of 736 _ 11 Ma (Table 1; Fig. 6 ); this age is indistinguishable, within the error, from the gabbro age. We suggest the time

of formation of the gabbro-diorite complex is best approximated by a pooled age of 732 + 15 Ma. This is somewhat younger than the ages for ophiolites in the western Arabian shield but remarkably close to that established for granitoid plutons in the southern ED (e.g. Stem and Hedge, 1985 ). It is, however, younger than the silicic arc volcanics south ofAbu Swayel dated at 768 +_ 31 Ma by Stern and Hedge ( 1985 ).

We interpret the zircon age for the Abu Swayel gabbro-diorite complex as reflecting the time of igneous emplacement and suggest that this complex intruded the already deformed metasediments and the already sheared serpentinite. This implies that our age of 732 _+ 15 Ma sets a lower age limit for ophiolite obduction and nappe emplacement in this part of the ED.

DATING OF LATE PROTEROZOIC OPHIOLITES IN EGYPT AND THE SUDAN 2 3

(2. r~

"6

t'~

E Z

60

40-

20-

Age in Ma

700 750 800

Mean age: 770-2-_9 Ma

Age in Ma

3000 3010 3020 3030 6 0 j i i i

1

40 Mean age: 3017+3 Ma

' t !

~ ~ i l I I I [ i I I

0.062 0.064 0.066 0.222 0.224 0.226 (207pb/2O6pb)* (2OTpb/2O6pb)*

(a) (b)

Fig. 7. Histograms showing distr ibution of radiogenic lead isotope ratios derived from evaporation of zircons from dykes cutting Wadi Al laqi ophiolite, samples AI 17-20, Wadi Allaqi, Egypt. (a) Spectrum for simultaneous evaporation of 5 small grains, integrated from 142 ratios, (b) Spectrum for one xenocrystic grain from dyke sample AI 21, integrated from 68 ratios. Mean ages are given with standard errors.

Wadi Allaqi ophiolite remnant, Egypt

A small ophiolitic nappe fragment in tectonic contact with metagreywacke and about 100 m in size occurs in the lowermost part of Wadi Allaqi where it enters Lake Nasser (Fig. 1 ). Field relationships with most other rock units are concealed by an extensive sand cover, and the exposed rocks consist of strongly car- bonated and tectonically disrupted serpentinite with near-horizontal foliation, very similar in appearance to other, but larger, serpentinite fragments farther E in Wadi Allaqi that are associated with layered gabbros and are clearly of ophiolite affinity. The serpentinite and metagreywacke below are locally intruded by 10- 50 cm wide intermediate to felsic dykes that crosscut the foliation and thus suggest that they were emplaced after the now serpentinized ultramafic rocks had been emplaced tectonically over the metagreywacke.

Most zircons in the dyke samples were clear, euhedral and needle-like but rare and very

small (20-60 #m). Five needle-like identical grains concentrated from three samples (AI 17- 19) had to be evaporated together in order to yield a stable and lasting ion beam in the mass spectrometer. The high-temperature spectrum integrated from 142 2°7pb/2°6pb ratios yielded an age of 770+_9 Ma (Table 2; Fig. 7a) which we interpret to reflect the time of dyke emplacement. One slightly larger grain (70 gm) with well rounded pyramidal ends and red- brown colour was evaporated separately and provided a much higher age of 3017 +_ 3 Ma (Fig. 7b). Clearly, this grain is a xenocryst, probably derived from pre-Pan-African base- ment through which the dykes were emplaced. If this is correct, this grain provides evidence for the existence of Archaean crust in the lower Wadi Allaqi region, probably representing the ancient African continental margin (Fig. 1). Wust et al. (1987b) have also reported a detrital zircon age of 2.39 Ga from metagreywacke collected about one km west the above ophiol- ire fragment.

24 A. KRONER ET AL.

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/ Fault

• Sample tocahl,¢

Fig. 8. Generalized geological map.of the western part of the Gabal Gerf Complex (based on unpubl, geological map 1:100.000 of the Gabal Gerf area, southeastern Desert, Egyptian Geological Survey and Mining Authority, Cairo), showing location of layered gabbro sample GG 260.

In principle, the 2°Tpb/2°6pb age of 770_+ 9 Ma based on simultaneous evaporation of five zircons could reflect a mixture between zircons that grew during dyke emplacement and older xenocrysts, but we exclude this interpretation since the five grains evaporated together were morphologically distinct from the older grain in that they were clear and had sharp ter- minations, whereas the xenocryst was brown and had rounded ends.

We conclude from these two zircon ages that the Wadi Allaqi ophiolite is older than 770 _+ 9 Ma and was probably tectonically emplaced into its present position prior to this age. The discovery of a very old xenocrystic zircon probably implies that ophiolite obduction occurred onto the ancient continental margin and not within the z, ccreted arc complexes farther east. It is possible that the Wadi Allaqi and Abu Swayel ophiolite remnants were part of the same nappe complex.

Gabal Gerf ophiolite This body constitutes the largest Pan-Afri-

can ophiolite complex of the entire ANS and is exposed in the area just north of 22°N (Fig. 1 ). The largest portion of the complex is a giant ultramafic mrlange consisting of highly serpentinized ultramafic rocks as well as gabbroic and basaltic rock fragments which were tectonically emplaced over a sequence of grey- wackes and shales (Krrner et al., 1987 ). In the western portion, an exceptionally fresh sequence of layered gabbros with lenses of chromite is preserved from which sample GG 260 was collected (Fig. 8 ). The layered gabbro has typical ophiolitic affinities with flat REE- patterns and pronounced positive Eu-anoma- lies, suggesting plagioclase as a cumulate phase (Zimmer, 1989, see also Table 1 ).

We were able to separate a few zircon crystals between 60 and 100 #m in size from a ~ 30 kg sample of coarse-grained gabbro sample GG

DATING OF LATE PROTEROZOIC OPHIOLITES IN EGYPT AND THE SUDAN 2 5

5O0

100tl

80-1

g

~60 £o o

n t ~ o cq

"64O

E 3-

20

Age in Ma 600 700 800 900 1000

I | I I I 100

Mean age: 741+_21 Ma

I Grain 1, 60 ratios


[ ]G ra i n 3, 29 ratios

r ~ Grain 4, 22 ratios

80.

60

650 Age in Ma

700 750 800

Mean age: 808+14 Ma



[ ]Gra in 3, 54 ratios

[ ]Gra in 4, 58 ratios

850 900

0.060 0.065 0.070 0.075 0.0625 0.0650 0.0675 0 0700 (207pb/206pb)* (207pb/206pb).

Fig. 9. Histograms showing distribution of radiogenic lead isotope ratios derived from evaporation of zircons from ophiolitic complexes in the Red Sea Hills. (a) Spectrum for 4 grains from layered gabbro sample GG 260, Jabal Gerfcomplex, integrated from 246 ratios. (b) Spectrum for 4 grains from plagiogranite sample SD 241, Wadi Onib complex, integrated from 246 ratios. Mean ages are given with standard errors.

260. The grains are clear, transparent, without inclusions and have well preserved bypyrami- dal surfaces. The four grains evaporated separately at temperatures between 1490 and 1530°C have uniform 2°Tpb/2°6pb ratios that constitute a perfect normal distribution and combine to a mean age of 741 + 21 Ma (Table 2; and Fig. 9a). Z immer (1989) has deter- mined the S m - N d isotopic composit ion in several gabbros from the same suite where our sample comes from and obtained a S m - N d whole-rock isochron age of 721 + 37 Ma, thus supporting the zircon result.

The Gabal Gerf age is identical to the age established for the Wadi Ghadir plagiogranite some 200 km to the north and supports our

suggestion for virtual contemporaneity of ocean crust formation in the central and southern ED and in northwestern Arabia. However, the geochemical signatures for the Wadi Gha- dir basalts and sheeted dykes suggest a back- arc basin setting (Kr6ner, 1985) whereas the Gabal Gerf basalts and sheeted dykes apparently formed in a "normal" ocean basin since their major and trace element chemistry is identical to modern MORB (typical depletion in light REE and no enrichment in incompatible elements, Z immer et al. 1987; Zimmer, 1989). We therefore propose that the two ophiolite complexes were derived from different oceanic domains. However, our structural data are insufficient to present a coherent ev-

26 .~,. KRONER ET AL.

: 0 . L k ] : : . t . : . + . . . . o e n °

)< x × @~

N \ vp' \ I

x . .- L jh '/

',V ~" , , 1 ~ , ~ mlln~"'~.. " ." ~T . %v --"'JP'a~/'/J,),,X t \.~ / (~, ', ~ ~ I

o , k . . , o t I

Post-tectonic gabbro

Batholithic granite

Gabbro-dlor i te

Stromatol i thic h rnestone

Calc-alkal ine metavolcan~cs

Clastic metasediments

L VOlCanIC arc I associat ion ]

Pi l low lava I

Sheeted dykes i- Opmoi~te

Layered gabbro & plag~ogran~te [ sequence

Ultramaflc rocks

Thrust contact

Sample locality

Fig. 10. Simplified geological map.of the Wadi Onib-Wadi Sudi area Red Sea Hills, Sudan, showing major ophiolite units and location of dated samples (from Hussein, 1992 ).

olutionary model for the southern ED and the adjacent Midyan terrane in northwest Arabia.

Onib ophiolite, Sudan

This complex occurs partly along the eastern margin of a linear belt of highly sheared rocks in the northern RSH, known as the Hamisana shear zone (Stern et al., 1989) and links up with the Sol Hamed ophiolite farther northeast along a terrane boundary (Fig. 1 ) that may be a suture zone (Krtiner et al., 1987). Its main components were described by Hussein et al. (1984) and KrOner et al. ( 1987 ), and the geochemistry and petrology of this well preserved complex is the subject of a detailed study (Hussein, 1992 ). The pillow lavas and poorly developed sheeted dykes have marginal basin geochemical signatures (Kr/Sner, 1985; Hus- sein, 1992). The layered gabbro and exceptionally thick cumulate ultramafic sequences are well preserved in a section south of Wadi Onib along Wadi Sudi (Fig. 10); this section also includes fresh pyroxenites, peridotites and dunites with layered chromite lenses. Numer- ous small lenses of pinkish to yellow-brown

plagiogranite occur throughout the isotropic gabbro section (Fig. 11) and contain fresh, zoned plagioclase together with quartz, minor hornblende, biotite and accessory minerals. The ophiolite complex is intruded by yellowish to gray granite that, in the field, is some- times difficult to distinguish from the plagiogranite. The generally undeformed granites form large, circular bodies and are also found outside the ophiolite complex where they cut volcano-sedimentary rocks of the Gebeit Ter- rane (Fig. 10). These granites were likely intruded after ophiolite emplacement. We have therefore dated zircons from a plagiogranite body as well as from these undeformed granites to constrain the time of tectonic transport of the ophiolite. Plagiogranite sample SD 24 l is unusually rich in silica and also has low con- tents of Nb and Y (Table 1 ), resembling those in volcanic arc granites. Alternatively, this fea- ture may reflect the back-arc origin for the Onib ophiolite.

Zircons were separated from sample SD 24 l collected 5 km northwest of Sudi Well in Wadi Sudi (Fig. 10). The zircons are rare and small ( 30-80/ tm), clear transparent to light yellow


Fig. 11. Plagiogranite dyke cutting isotropic gabbro in Wadi Onib, Onib ophiolite complex, Red Sea Hills, Sudan.

in colour and have perfectly euhedral shapes with bipyramidal varieties predominating. Four grains were evaporated at temperatures between 1510 and 1550°C and, as in the pre- vious cases, define uniform 2°7pb/2°6pb ratios that combine to a mean age of 808 _+ 14 Ma (Table 2, Fig. 9b).

This age is significantly older than the age established for the ophiolites in southern Egypt and in the Yanbu suture of northwestern Ara- bia and raises doubts on a direct continuation of this suture into the Onib-Sol Hamed belt as suggested by Stoeser and Camp (1985), Vail ( 1985 ) and Kr6ner et al. ( 1987 ). It is difficult to assess this problem in detail in view of the lack of age data for the Sol Hamed ophiolite, and it may well be possible that the Onib-Sol Hamed belt contains remnants of oceanic crust generated over a time period of about 60 Ma.

In any case, the Onib ophiolite was apparently derived from one of the earliest oceanic spreading events in the ANS and is only pre- dated by the Thurwah ophiolite of the Bir Umq suture (gabbro zircon age of 870+11 Ma) northeast of Jeddah and the Tuluhah ophiolite in the Nabitah suture (two plagiogranite zircon ages of 823 +_ l 1 and 847 + 14 Ma) in the eastern Arabian shield (Pallister et al., 1988 ). The Onib ophiolite is significantly younger than the oldest known island arc volcanic and plutonic rocks of the ANS, dated between 850 and ~ 950 Ma (Asir terrane of Arabia, Stoeser and Camp, 1985; Haya terrane in the RSH of the Sudan, Kr/Sner et al., 1991 ) and also younger than the 832 +_ 26 Ma Gebeit metavol- canics that occur southeast of the Onib complex (Reischmann et al., 1985).

A circular granite body crosscutting the Onib ophiolite west of Sudi Well (Fig. 10) has also been dated by zircon evaporation. Chemical analyses of the two samples from which zircons were extracted are presented in Table 1, and these show characteristics of volcanic arc granites. The isotopic results are shown in Ta- ble 2 and Fig. 12. The zircons are euhedral and clear to yellowish in colour. Two grains from sample SD 107 yielded a mean 2°7pb/2°6pb age of 713 + 12 Ma (Fig. 12b) while one zircon from sample SD 106 produced an identical, but more precise, age of 714 + 5 Ma (Fig. 12a). Field relationships indicate this granite post- dates tectonic emplacement of the ophiolite, and it is not surprising that our 2°Tpb/2°6pb zircon ages are identical, within error, with the Rb-Sr whole-rock isochron age of 712 + 58 Ma reported by Fitches et al. (1983) for arc volcanics unconformably overlying the Sol Hamed ophiolite farther northeast. We suggest that ophiolite obduction and terrane suturing was complete prior to ~ 713 Ma ago.

A still younger, post-tectonic granite from Wadi Onib (ON 3), cutting layered gabbro of the ophiolite, is similar in appearance and setting to that ment ioned above. It is still younger at 646 + 10 Ma as shown by the 2°7pb/E°6pb

2 8 A. K R O N E R ET AL

Age in Ma

650 700 750 800 550 60 [ ~ 60 /

J Mean age: 714_+5 Ma 4

.

4 0 - 4 0 4 t.o o

2 r, o

"6~ 20" 20 t -

E i z m

Age in Ma Age ~n Ma

600 650 700 750 625 650 675 t i t i 30 i ~ ~ .a.__

Mean age: 713,12 Ma i i n g r a i n 1. 40 ratios

[ ] G r a i n 2, 31 rabos

20- n G r a i n 1. 61 ratios

[ ] G r a i n 2, 36 ratios

10-

0.062 0.064 0.066 0.060 0.062 0.lj64 0.060 0.061 (2OTpb/206pb)" (2o7pb/2O6pb)* (2OTpb/2O6pb) •

(a) (b~ Ic~

0 062

Fig. 12. Histograms showing distribution of radiogenic lead isotope ratios derived from evaporation of zircons from leucogranites cutting the ophiolite assemblage of the Wadi Onib Complex, Red Sea Hills, Sudan. (a) Spectrum for one grain from sample SD 106, integrated from 67 ratios. (b) Spectrum for 2 grains from sample SD 107, integrated from 97 ratios. (c) Spectrum for 2 grains from sample ON 3, integrated from 71 ratios. Mean ages are given with standard errors.

data for two idiomorphic zircon grains presented in Table 2 and Fig. 12c. Granitoids of this age are known from the northeastern RSH (Neary et al., 1976; Stern et al., 1989 ) and large areas of the ANS (Jackson et al., 1984; Stoe- ser, 1986) and are considered to be the result of post-accretionary intraplate magmatism.

Conclusions

2°7pb/2°6pb zircon ages for plagiogranites, gabbros and dykes from ophiolites and neigh- bouring plutonic rocks in the southern Eastern Desert and the northern Red Sea Hills document ocean crust generation between ~ 810 and ~ 740 Ma. This crust was obducted onto the adjacent arc or continental margin terranes during ocean closure and subsequent suturing prior to ~ 715 Ma ago after which magmatism in the ANS was largely of intraplate type. In the case of the Wadi Allaqi ophiolite fragment tectonic emplacement is inferred to have occurred prior to ~ 770 Ma ago. The age data compare well with dates established for the

formation of ophiolites in Saudi Arabia (Fig. 9) and suggest that arc generation and marginal basin formation took place almost con- tinuously over a period from about 870 to 730 Ma ago although no ophiolite contemporaneous with the earliest arc formation period has so far been documented.

Considerable age differences between the Onib ophiolite and fragments of obducted oceanic crust in the Yanbu suture of northwestern Arabia (Fig. 13 ) cast doubt on pre- vious large-scale correlations of suture belts between the Arabian and Nubian segments of the shield (Stoeser and Camp, 1985; Vail, 1985 ). These data also imply that such sutures probably contain remnants of oceanic basins with age differences of as much as 60 Ma and therefore reflecting different tectonic events.

There is no apparent systematic age distribution in the ophiolites of the ANS. Some of the youngest ophiolites such as Wadi Ghadir and Jabal Gerf occur in the western part of the accretionary domain where obduction onto an ancient continental margin is likely, while the oldest ophiolites of the ANS occur in the east-


"~-~" I~, Maior transcurrent ~-~ '~ ..:../f/Structural. grain, ~: and minor strike slip " ~ , ..:i.:" regional structural

shear zones trend

'il ~ /'L- Major abduction and ~ ~'~! ~'~ ~\~ x,t-- napp ........ t direction

© I"i! ~' irTutuh,Qh ~ \~~ 23-8~7-1&MQI ~\~ ~ Ophiol,te b,tts

¢)!Asw "N

o ?;,

,~,~ 1770+-9 MG

f}' -11 ~ X

" I" I I , . ~ 7 ~ 2 t 2 7 Mo I ~-x~,lF

TERRANE, \ -,.

I N ,"j3 ~ /:~[;I: II[, I i I ~ . , . . : . l l . P o r t S u d m n /l~..~./ \ \

" / )1 ~ ~ ' .,'.,.. . . . . . :.':::"...' \ \ \

. } -- ~ ~ } ~ ) II IlllI ; . II . * y A ~ ~ ? A , " #/ ':::,, ~ U TERRANE .."~

~:~ :" ..0 1~. • ,.

. . . . . . . . . . t / / ; / 0 TERRANE \ . //~1/ 1__

Fig. 13. Simplified map of the Arabian-Nubian shield as in Fig. 1 showing location and ages for ophiolite occurrences dated in this paper and in Pallister et al. ( 1988 ).

ern Arabian shield (Fig. 13 ). This may suggest that marginal basin formation occurred in various regions and at different times during arc generation and was triggered by magma generation over subduction zones. Most ophiol- ires may therefore have resulted from intra-arc spreading and were probably abducted as buoyant, relatively hot oceanic lithosphere during basin collapse and closure. Such an upper crustal environment did not permit the formation of high-pressure metamorphic assemblages, and movement along shallow-dip- ping d6collements made it possible that the upper portions of some ophiolites remained

internally intact during the process of abduction.

Lastly, the possible presence of many intra- arc, forearc and back-arc basins in the large island arc terrain that was to become the ANS makes it doubtful that present ophiolite occurrences can all be lined up to mark a few ancient suture zones. We believe that present knowl- edge on the age and tectonic history of many ophiolites in the ANS is incomplete and that further reconstruction of terrane boundaries and the accretionary history of the shield on the basis of ophiolite occurrences must await more detailed field work and precise dating.

30 A. KRONER E T A L

Acknowledgements

This study was initiated by IGCP Project 164 (Pan-African crustal evolution in the Arabian- Nubian shield) and No. 215 (Proterozoic fold belts) and is part of a cooperative project between the Mainz Consortium (University of Mainz and Max-Planck-Institut ffir Chemie), the Egyptian Geological Survey and Mining Authority (EGSMA) and the Geological Re- search Authority of the Sudan (GRAS). Fund- ing for field work was through grants from the Deutsche Forschungsgemeinschaft (DFG), the Volkswagen Foundation and the German Ministries of Science and Technology (BMfT) and Economic Cooperation (BMZ). We are particularly indebted to R.M.A. Elbayoumi for discussions on regional geology, and we thank EGSMA and the Port Sudan Office of GRAS for logistic support.

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