Precambrian Research, 14 (1981) 119--133 119 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

AGE AND CHEMICAL CHARACTERISTICS OF SOME PRE-PAN-AFRICAN ROCKS IN THE EGYPTIAN SHIELD

T.H. DIXON*

Geological Research Division, Scripps Institution of Oceanography, La Jolla, CA 92093 (U.S.A.)

(Received September 11, 1979;revision accepted September 1, 1980)

ABSTRACT

Dixon, T.H., 1981. Age and chemical characteristics of some pre-Pan-African rocks in the Egyptian Shield. Precambrian Res., 1 4 : 119--133.

U--Pb ages on zircons from several types of sialic rocks, together with a variety of geo- chemical data on one sample of early plutonic activity within the Egyptian Shield are presented.

The earliest autochthonous granitic rocks in the Egyptian Shield are probably quartz diorites (tonalites) emplaced just prior to the main peak of Pan-African igneous activity (550--650 Ma). The studied example has an age of 711 Ma +- 7 Ma, and is characterized by very low initial Sr ratio (8~Sr/S6Sr = 0.7026) and very low abundances of K20 and re- lated large-ion lithophile trace elements. REE contents are low (less than 20 × chondritic abundances). Abundance patterns show only moderate light rare-earth enrichment (La/Yb = 4.5). These characteristics are inconsistent with any models requiring fractional fusion of pre-existing continental crustal material.

Granitic and arkosic cobbles occur in rare conglomerate beds within volcanoclastic :and greywacke sedimentary sequences. The cobbles show a wide range in ages (1.1--2.3 Ga) and have no obvious source within the Egyptian Shield. It is suggested that they are not indicative of any sialic plutons in the Egyptian Shield which might predate deposition of the enclosing immature sediments. More probably, they were derived from adjacent conti- nental areas such as the Uweinat region to the west, and were deposited in an evolving arc- ocean basin complex.

INTRODUC~ON

The Eastern Desert of Egypt contains a Late Precambrian shield area charac- terized by the widespread occurrence of granitic plutons. Many of these define a uniform group of post-tectonic, K-rich granitic rocks, intruded in the narrow age span of 570--590 Ma (Abdel-Monem and Hurley, 1978; Fullagar and Greenberg, 1978). Their intrusion marks the igneous expression in Egypt of the so-called Pan-African thermo-tectonic event (Kennedy, 1964; Clifford, 1970) represented by granitic activity throughout much of Africa in the age range 550--650 Ma (e.g. Allegre and Caby, 1972; Bertrand et al., 1978; Rogers et al., 1978).

*Present address: Jet Propulsion Lab., Mail Code 183-701, 4800 Oak Grove Drive, Pasadena, CA 91109, U.S.A.

0301-9268/81/0000--0000/$02.50 © 1981 Elsevier Scientific Publishing Company

120

In most parts of southern Africa, this orogeny is ensialic {Holmes, 1951; Hurley, 1972; Shackleton, 1973; KrSner, 1976, 1977). Newly intruded granitic material in these regions probably represents remobilized older con- tinental crust. In northwest and north central Africa, both ensialic and ensim~ atic orogens are recognized. In Algeria, for example, older basement has been reactivated during the Pan-African orogeny in the central Hoggar (Lelubre, 1952; Bertrand, 1974). The western Hoggar's Pharusian belt, on the other hand is recognized as a late Precambrian ensimatic belt, probably representing an intra-oceanic arc complex (Caby, 1970; Caby et al., 1977).

In Egypt, the nature of the Pan-African orogeny, and characteristics of the early crustal complex, are more controversial. Most authors have suggested that the Egyptian orogen is also ensialic, requiring that continental (sialic) material significantly older than Pan-African plutonism should be present (Ball, 1912; Hume, 1934; Clifford, 1968; Akaad and Noweir, 1969; E1 Ramly, 1972; Hashad et al., 1972; Abdel-Monem and Hurley, 1978; Church, 1979). Other authors have suggested that early crust in the Egyptian shield is oceanic in nature, characterized by mafic to intermediate volcanics and thick sequences of immature sediments and volcanoclastic material (Lotfi and Hafez, 1973; Engel et al., 1978, 1980). In this latter view, so-called older basement or "fundamental gneiss" actually represents the highly deformed and metamor- phosed equivalent of metavolcanic and metasedimentary rocks (Hussein et al., 1978). Similar conclusions have been expressed for the Saudi Arabian Shield (e.g. Greenwood et al., 1976; Schmidt et al., 1978).

Chemical and isotopic data on intrusive rocks in a region are an important constraint to the nature of pre-existing crust. K-rich granitic rocks in the Egyptian Shield have been the focus of previous investigations (e.g. Rogers et al., 1978). In contrast, K-poor tonalitic rocks, which may represent an earlier pulse of intrusion, have not been investigated in detail. This report describes a well-studied quartz diorite (tonalite) representative of a large area of quartz-dioritic to granodioritic plutons in the Southeastern Desert (El Ramly, 1972; Dixon, 1979; Fig. I). U--Pb geochronologic data on zircons, and various geochemical data including initial 87Sr/8~Sr and rare earth element (REE) data are presented.

In addition, a suite of granitic cobbles from the central Eastern Desert was studied. Such cobbles occur in conglomerate beds and lenses exposed infre- quently within thick sequences of immature sediments, and may be indicative of pre-existing sialic crust in the Egyptian Shield. In addition, they can provide a maximum deposition age for the enclosing sediments.

Prior to this study, it was thought that such cobbles might be derived from a nearby sialic terrane ca. 800--900 Ma old, as suggested by the geochronologic work of Hashad et al. (1972). These authors report an Rb--Sr age of 876 Ma on a granodiorite with relatively high initial STSr/86Sr (= 0.7081). As the subse- quent discussion will show, no evidence was found for such an origin at the particular locale studied.

1 2 1

S A M P L E D A T A

Sample localities and petrographic data are given in Fig. 1 and Table I. Chemical data on the quartz diorite sample 118 are given in Table II.

The granitic cobbles occur in a large conglomerate lens within the Wadi Mobarak metasediment unit (El Ramly, 1972; E1 Ramly and Hermina, 1978). The unit consists mainly of greywackes and volcanoclastic material over- lying a metavolcanic assemblage. The two analyzed specimens represent large (3--5 kg) samples typical of granitic cobbles preserved in the conglomerate. However, granitic cobbles make up less than one-third of preserved clasts, the remainder being volcanic and sedimentary in origin.

2¢

2 8 0

2 7 °

2 6 °

2 5 o

24 o

2 3 °

2 2 ° 32 ° 3 3 ° 34 ° 35 ° 36 o 3? o

Fig . 1. R e g i o n a l g e o l o g i c m a p o f t h e E a s t e r n D e s e r t o f E g y p t w i t h s a m p l e l o c a l i t i e s ( m o d i f i e d f r o m El R a m l y , 1 9 7 2 ) . N o t e d e c r e a s e d a b u n d a n c e o f g r a n i t i c r o c k s s o u t h o f l a t i t u d e 2 6 o 3 0 ' .

122

T A B L E I

Sample locations and modal analyses in vol. % (based on more than 2500 points)

Sample

78-D 78-L 118

Sample type granite cobble granite cobble tonalite p luton Locat ion Wadi Mobarak Wadi Mobarak Wadi ShQt L a t i t u d e (N) 25o19 , 25°19 ' 23 ° 4 5 ' L o n g i t u d e (E) 34 ° 3 5 ' 34 ° 3 5 ' 35 ° 1 2 '

Qua r t z 36.0 25.2 19.3 O r t h o e l a s e 22.5 42 .6 - - Plagioclase 31.8 30.3 56.8 Muscov i t e - - 0 .6 - - Biotite 9.6 1.3 7.1 A m p h i b o l e - - - - 1 4 . 9

P y r o x e n e - - - - 1.1 Magne t i t e - - - - 0.7

T A B L E IIa

Major -e lement chemis t ry of qua r t z dior i te 118 and c ons t i t u en t minera ls

Whole r o c k average Plagioclase Bioti te A m p h i b o l e (range of dup l ica te (Ab61 ) analyses)

SiO 2 60 .35 (0 .33) 58.79 37 .36 46.99 TiO 2 0.61 (0 .01) - - 3 .47 0 .84 A120 ~ 17 .19 (0 .17) 25 .10 14 .70 7.97 F%0~ 1.85 (0 .15) FeO 3 .65 (0 .04) 0 .06 a 16 .82 a 14.62 a MgO 3.65 (0 .01) - - 13 .75 13 .46 MnO 0.09 (0 .01) - - 0 .35 CaO 6.49 (0 .01) 8 .04 - - 11 .54 Na~O 4.01 (0 .02) 7 .10 0 .20 1.00 K 2 0 0 .62 (0 .03) 0 .15 9.66 0 .34

P205 0.15 (0.01) CI 0.02 0.09 0.06 BaO 0.02 0.06 Cr203 0 .04 H 2 0 0 .76 (0 .20) 3 .90 b 2.82 b

ZrO 2 V~03 Nb20 $

Tota l 99 .46 99 .31 100 .0 b 'c 100.0 b-

Py roxene Magnet i te (EnsgFS3sW%)

54 .95 0 .04 0 .08 0 .05 0 .78

51 .33 20 .96 a 46 .18 19 .04

0 .98 1 .45 0 .05

98 .29

0 .27

0 .05 0 .47 0 .05

9 8 .4 4

aTo ta l Fe as FeO; bH~o by d i f fe rence ; Cincludes 0 .07% ZnO; b lank = n o t ana lyzed ; - - = n o t de t ec t ed .

123

TABLE IIb

Rare-earth and other trace element data for quartz diorite 118 and masked standard BCR-1 a

118 BCR-1

this study Flanagan (1973)

La 5.32 24.37 26.0 Ce 11.34 53.17 53.9 Sm 2.42 6.86 6.6 Eu 0.781 1.94 1.94 Tb 0.29 0.66 1.0 Yb 1.21 3.37 3.36 Lu 0.18 0.52 0.55

Rb 7.6 42.3 46.6 Cs 0.38 0.87 0.95 Ba 191 638 675 Hf 1.40 5.35 4.7 Th 0.49 6.09 6.0

Sc 16.2 32.4 33 Cr 41.5 7.9 17.6 Co 22.4 36.3 38 Zn 25 62 120

aDetermined by instrumental neutron activation analyses; methods and errors described in Jacobs et al. (1977).

The quar tz d ior i te sample (118) was loca ted in the Sou theas te rn Desert , where tonal i t ic to granodior i t ic in t rus ions are a b u n d a n t (Fig. I; E1 Ramly , 1972) . Individual p lu tons are charac te r ized b y wide b o rd e r zones con ta in ing n u m e r o u s amphibol i t ic inclusions (Dixon , 1979) . The s tudied sample is f r o m the cent ra l po r t i on o f a large p lu ton , and is typ ica l o f the tonal i t ic in t rusions in the region.

Sample 118 is excep t iona l ly fresh, the on ly observed a l te ra t ion consist ing o f rare veins less t han 2 m m wide where plagioclase has been a l tered to epi- do t e and sericite. In cont ras t , the two granitic cobbles (78-D and 78-L) are highly al tered. The feldspars have part ia l ly a l te red to ep ido te and sericite, while chlor i te and hema t i t e f o rm at the expense of the mafic minerals. Original mineralogical da t a are r e p o r t e d in Table I, a l though the a l te ra t ion p roduc t s make up 10- -15% o f these samples.

ANALYTICAL METHODS

Zi rcon dissolut ion and Pb ex t r ac t i on fo l lowed the m e t h o d s o f Krogh (1973) wi th modi f ica t ions by Chen (1977) . Measured Pb blanks for the to ta l process ranged f r om 0.3 ng to 3 ng. I so topic and age da ta are given in Table III. Errors fo r individual age de te rmina t ions are on the o rde r o f 1% o f the r e p o r t e d age.

t~

TA

BL

E I

II

Res

ults

of

U--

Pb

ana

lyse

s o

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irco

ns

Sam

ple

S

ize,

m

agn

etic

fr

acti

on

a (w

t. i

n ra

g)

Co

nce

ntr

atio

n (

pp

m)

U

118

+2

00

NM

(8.3

) 72

11

8 +

32

5N

M(5

.0)

116

78-D

+

20

0H

P(0

.6)

44

6

78-D

+

20

0N

M(3

.4)

28

9

78-D

+

20

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(2.4

) 4

29

78

-D

+3

25

NM

(3.2

) 47

5

78-L

+

10

0 N

M(2

.0)

98

4

78-L

+

20

0H

P(0

.4)

768

78-L

+

20

0N

M(3

.0)

911

78-L

+

32

5N

M(3

.1)

15

42

Mea

sure

d P

b is

oto

pe

rati

os

(ato

mic

)

207

20

6

0.0

71

03

81

0

.06

67

17

0.0

84

76

7

0.0

72

26

1

0.0

72

42

9

0.0

70

96

0

Pb

(rad

iog

enic

) 2

06

2

04

8.5

17

84

13

.8

38

38

69

86

3

41

31

74

60

2

79

7

65

33

81

20

8

20

6

132

27

22

12

4 8

03

11

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46

0

158

21

87

Cal

cula

ted

Pb

iso

top

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s

20

6*

2

07

*

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23

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65

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16

61

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30

32

0.2

95

80

0

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97

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1.2

31

2

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68

49

0

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62

71

0

.12

33

7

1.1

53

9

0.0

67

83

3

0.2

76

71

0

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1.1

43

4

0.0

67

39

9

0.2

73

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0

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01

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06

8

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66

79

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0.0

89

86

2

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96

76

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31

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1.5

57

8

0.0

84

88

6

0.1

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0

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1.9

41

9

0.0

93

43

1

0.0

89

29

4

0.0

92

76

0

.12

66

3

1.4

61

4

0.0

83

70

2

0.0

81

91

9

0.1

05

58

0

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21

1

1.0

63

7

0.0

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55

7

NB

S 9

83

(av

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f --

--

2

61

1

0.0

71

24

5

0.0

13

74

2

3 an

alys

es)

-+ 4

3

+-0

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090

+-0

.000

080

Co

mm

on

Pb

(mea

sure

d

lab.

bla

nk

by

J.

Gil

lesp

ie)

18.9

2 0

.81

06

51

1

.99

64

1

--

Ap

par

ent

age

b (M

a)

20

6

207

207

23

8

23

5

20

6

71

0.5

7

11

7

12

71

1 71

1 7

10

787

81

5

88

4

75

0

77

9

86

3

74

8

77

4

85

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757

831

80

6

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13

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1

09

6

14

97

76

9 9

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1

28

6

627

73

6

10

83

b 2

31

10

aN

M =

no

n m

agn

etic

, M

= m

agn

etic

at

1.8

Am

ps,

2 °

side

slo

pe,

HP

= s

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ally

han

d p

ick

ed f

ract

ion

of

clea

r, e

uh

edra

l gr

ains

; ~

U =

1.5

51

25

•

10-

y-~,

k~

sU =

9.8

48

5

• 10

"~°

y-~,

23

8/2

35

=

13

7.8

8 (

Jaff

ey e

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., 1

97

1);

* =

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.

125

For discordant samples (78-D, 78-L), reported errors are based on the devia- tion of the data points about the best fit (least-squares) line on the concordia diagram (Figs. 2 and 3). Maximum and minimum intercept ages were determined using all pairs of zircon fractions except the two central points.

A whole-rock sample and apatite separated from the quartz diorite 118 were analyzed for Rb and Sr contents and Sr isotope composit ion, to deter- mine the initial STSr/86Sr ratio. Dissolution and element separation procedures are described elsewhere (Dixon, 1979). Data are given in Table IV. Listed error represents the standard deviation of repeated isotopic measurements, except for the mean initial ratio, where the range of the two determinations is given.

TABLE IV

Sr isotope composition, and abundances of Rb and Sr in quartz diorite 118

Sample *'Sr/*~Sr Rb(ppm) Sr(ppm) (*~Sr/*~Sr)ao

Whole rock 0.70365 12.8 304.6 0.70246 ±0.00015

Apatite 0.70280 0.22 216.8 0.70277 ±0.00012

mean= 0.70262 +0.00015

aInitial ratio calculated for an age of 711 Ma and STRb = 1.419 • 10 -~ y'~ (Davis et al., 1977).

DISCUSSION

U--Pb ages

The cobble samples yield highly discordant ages, and consequently have poorly defined upper intercept ages. Figures 2 and 3 are concordia diagrams for the granitic cobbles 78-D and 78-L. The upper intercept age for 78-D is 1120.230_90 Ma. Using the episodic Pb loss model (Wetherill, 1956) the lower intercept age is 580 ÷6s_90 Ma, which is readily related to the widespread Pan- African thermo-tectonic event, and consequent metamorphism of the sedi- ments.

Sample 78-L has a significantly higher U content than 78-D, and is more discordant. Figure 3 shows two possible age interpretations for this sample. Using all fractions, upper and lower intercept ages are 1860 ÷300 Ma and - 1 7 0

on ÷ 120 ~v _ 90 Ma. Using only the three most concordant fractions, the ages are ÷ 36 Ma. These latter ages are considered more valid 2060 _ ÷ 10040 Ma and 575 _ Is

since the most discordant fraction is susceptible to recent disturbances, such as groundwater leaching (e,g. Pidgeon, 1978).

126

I t I 1 2 0 0 / I 1200 "

0.20 U - P b C O N C O R D I A ~ -

7 8 - D

0.16 9 0 9 / /

0.12

0.08 I I I I 0.8 1.2 1.6 2.0 2 4

2o7pb / ?s5 U

Fig. 2. Concordia plot for four zircon fractions from granite cobble 78-D.

04 i i i i i i

U-Pb C O N C O R D I A ~ / / / ~

o.s 7 8 -L ,~oo / - " ~ ~/:~.>" ~ Y "

..02 ~ o~.~/ ~ 9oo / ~ - - 5 ~

/ ~ " 700 / / . ~ " <~°" /

OV I 1 I I I l 2 ;~07pb / 2354U 5 6

Fig. 3. Concordia plot for four zircon fractions from granite cobble 78-L. The most dis- cordant zircon fraction plots below a straight line chord defined by the three more con- cordant fractions. Using only the three most concordant fractions yields upper and lower intercept ages of 2060 Ma and 575 Ma. For n = 4, these ages are 1860 Ma and 490 Ma. Dashed line marked C.D. is the continuous diffusion lead loss curve for a sample 1900 Ma old (Wasserburg, 1963).

The granite cobble 78-D suggests an upper limit o f 1120 Ma for the age o f the enclosing metasediments . Rb- -Sr age data on metavolcanics essentially co-eval with similar metased iment assemblages in the central Eastern Desert imply depos i t ion o f m u c h of this volcanically derived material be tween 600 Ma and 700 Ma ago (Stern, 1979).

127

Granitic plutons in the Egyptian Shield analogous in age and type to the analyzed cobbles 78-D and 78-L have not been reported. The great range in age exhibited by this sialic material, coupled with the lack of obvious analogues in the Egyptian Shield itself, is most consistent with derivation from a variety of continental areas adjacent to the evolving Egyptian arc-ocean basin. One likely source is the early Proterozoic--Archean crust to the west, represented by exposures in Libya and southwestern Egypt in the Uweinat area. Rb--Sr studies on gneisses and migmatites in this region have yielded ages between 1800 Ma and 2600 Ma (Klerkx and Deutsch, 1977). The presence of quartzite clasts in some Egyptian metasedimentary units, and their absence in Saudi Arabia, is also consistent with the presence of an older continental mass (the "Nile craton") to the west (Naseef and Gass, 1979). One exposure of such quartzite clasts in the Southeastern Desert contains zircons of Archean provenance (Dixon, 1979).

The quartz diorite pluton (118) yields a concordant date of 711 + 7 Ma. Concordant dates are only rarely reported for Precambrian samples. In this case, the exceptionally fresh nature of the rock, coupled with the very low U content of the zircons (less than 120 ppm) explains the Pb retention. The low U content observed in these zircons presumably reflects a low U abundance in the total rock, consistent with low abundances of K20, Rb, and other in- compatible trace elements (Table II).

Tonalitic rocks of similar composition in southern Saudi Arabia are some- what older, with ages ranging from ca. 750 Ma to 900 Ma (Cooper et al., 1979; Fleck et al., 1980}. A plot of available Rb--Sr and U--Pb geochronologic data for tonalitic to granodioritic intrusives and (consanguineous?) calc-alkaline vol- canics in the Egyptian and Saudi Arabian Shields suggest a general younging trend to the north for this class of rocks (Fig. 4). Both areas, however, are in- truded by ubiquitous K-rich granitic rocks between 570 Ma and 640 Ma (Rogers et al., 1978; Fleck et al., 1980). These show no obvious age trend, though available data in Egypt imply a more restricted range (570--590 Ma, Fullagar and Greenberg, 1978).

Chemical characteristics of the quartz diorite, and implications for crustal evolution in Egypt

The lack of potassium feldspar, and a high ratio of amphibole to biotite in the quartz diorite sample are characteristic of the mineralogy of many of the granitic intrusives in the Southeastern Desert (Dixon, 1979). The sampled pluton has low abundances of K20, Rb, Ba and REE, and a low initial ratio Of 8~Sr/a6Sr (Tables II and IV), implying derivation from a source depleted in large ion lithophile elements.

Figure 5 shows chondrite-normalized abundances of REE in sample 118 and a modern suite of intra-oceanic island arc volcanics. Relative to recent arc lavas of similar major-element composition, 118 shows somewhat lower abundances of light rare earths, and heavy rare earth depletion.

128

• i00x AQ ANo RE E NCE I [ ] ....E.ozo,c c o y . . /

• ~ ] PRECAMBRIAN SHIEL

25 ° - . 612-I 671-I ~ Y ~ " " . - "" ' . . " " l i fo • " • . ..

2 0 ~ .~

~ . ~ / ~ 8 , s - 4 • , 6 , - 4

~ Y / . . \ 890-4 ~ 9~2-4 /.:1

" " " " l " l : . / ) . l ~ , e 3 - 3 ~ \ / i l " " l " 7 1 8 3 8 1 1 - 3 " . ~. : j . . \ ~ -~~ o,,-~( ./ 3 0 ° 3 5 °

Fig. 41 Distribution of representative U--Pb and Rb--Sr age data on tonalitic to granodioritic intrusives and calc-alkaline volcanics in northeast Afro-Arabia. The Egyptian and Arabian Shields are shown prior to opening of the Red Sea. Ages are in units of ±0 ~ y, followed by a reference number. Approximate sample locality is indicated by reference number or arrow. Sources of data: (1) Stern, 1979; (2) this study; (3) Cooper et al., 1979; (4) Fleck et al. 1980.

5 0 ~ ~

o b- e~ Z

LU "''~'\\ o. - 118 <C ~ RANGE OF INTRA- CO OCEANIC ARC

BASALTIC ANDESITE AND ANDESITE

1 I I I I I La Ce Sm Eu Tb Yb Lu

Fig. 5. Chondrite-normalized (Haskin et al., 1968) abundances of rare earth elements (REE) in quartz diorite sample 118 and comparative abundance patterns for modern intra- oceanic arc lavas (Dixon and Batiza, 1979). Tb value for 118 is corrected on basis of BCR-1 data (Table IIb).

129

The initial ratio of S~Sr/S6Sr for sample 118 is as low or lower than m o d e m mid-ocean ridge tholeiitic basalts of unequivocal mantle derivation. Taken to- gether, the chemical and isotopic data for this quartz diorite preclude derivation from, or interaction with, pre-existing sialic (continental) crust.

Low initial ratios of STSr/a6Sr are characteristic of most igneous rocks in Egyptian and Saudi Arabian shields (Rogers et al., 1978; Dixon, 1979; Stern, 1979; Fleck et al., 1980). A histogram of available data (Fig. 6) has a mode at 0.7028. Such non-radiogenic values are a severe constraint on the amount of pre-existing sialic crust which could have acted as protolith to the granitic intrusions. The data are consistent with field evidence indicating that the Pan- African orogeny in much of the Egyptian--Arabian shield was ensimatic in nature, i.e. post-orogenic, K-rich granitic rocks intruded an oceanic arc--basin complex (Engel et al., 1980).

The age range of syn- and post-orogenic intrusive rocks can be used to infer the time scale for conversion of oceanic crust to continental crust. Excluding for the moment published data on rare samples with high initial STSr/86Sr, (e.g. Fig. 6), the presently known age range of plutonic rocks intrusive into the arc oceanic complex in Egypt is approximately 140 Ma (710--570 Ma).

6-

4

tl c

2 :7

3

~ - ~ Post-Orogenic Granite-Adamellite

Syn-Orogenic Tonelite-Granodiorite

B Volcanics, mainly C a l c - A l k a l l n e

0702 0,} '03 0.704 - 0 7 0 6 - 0.710

{ 8?Sr /S6 Sr ) t=°

Fig. 6. Histogram of initial 'TSr/'6Sr ratios for selected igneous rocks in Egypt and Saudi Arabia. Rb--Sr analyses selected when N ~ 4 and reported error for the initial ratio < +_ 0.001. Rock types are indicated in the Figure except for 2a. Egyptian data f rom:(1) Stern, 1979; (2a) plagioclase mineral'separate from a layered mafic--ultramafic intrusion in the South- eastern Desert (Dixon, 1979), (2b) this study; (3) Fullagar and Greenberg, 1978; (5) E1 Shazley et al., 1973. Saudi Arabian data; (4) Fleck et al., 1980.

130

During this time span, unfractionated, arc-type plutonic material evolved to highly fractionated continental crust. Since the oldest reported ages of similar tonalitic rocks in southern Saudi Arabia are some 150 Ma older (Fleck et al.~ 1980; Fig. 4) the conversion of oceanic crust to continental crust through much of the region probably occurred in some 300 Ma or less. Data from a limited area in the central Eastern Desert of Egypt (Fig. 4) suggests that this major change occurred in less than 50 Ma {Stern, 1979).

Pb isotope data on whole-rock and feldspar mineral samples from a suite of Egyptian granitic rocks are similar to the Sr isotope data in their non- radiogenic, mantle-like values (Gillespie and Dixon, 1980). An important exception is the Aswan granite, which crops out in the westernmost portion of the Eastern Desert (Fig. 1). In addition to its considerably more radiogenic Pb isotope ratios, this pluton is also characterized by high initial STSr/86Sr (0.7094 + 0.0009 Hashad et al., 1972). Possibly, this region marks the eastern- most extent of continental crust older than the Egypt ian-Arabian arc-oceanic complex. Significantly older sialic basement has been documented at Gebel Uweinat in the Southwestern Desert of Egypt near the Libyan border (Klerkx and Deutsch, 1977).

CONCLUSIONS

(1) Cobbles of sialic material derived from a continental terrane occur in- frequently in conglomerate units within thick sequences of Late Precambrian immature metasediments in the Egyptian Shield. Rather than indicating the presence of pre-existing sialic crust within the Egyptian complex itself, their lithologic composit ion and age variation suggest derivation from adjacent con- tinental areas to the west and/or south. The indicated maximum age for the enclosing metasediments is approximately 1120 Ma.

(2) Initial plutonic activity in the Egyptian Shield is characterized by (a) low abundances of K20 and related LIL elements, and a low initial ratio of 87Sr/86SI; (b) relative proximity in time (711 Ma) to the main pulse of Pan-African granitic activity (570--590 Ma).

The chemical and isotopic characteristics of the studied sample preclude a sialic protolith. The isotopic characteristics of most intrusive rocks in the region are consistent with the view that Pan-African orogeny in Egypt was ensimatic in nature. The Egyptian Shield probably evolved from oceanic to continental crust in less than 200 Ma.

ACKNOWLEDGEMENTS

This s tudy was made possible by the assistance of many people. A.E.J. Engel gave encouragement throughout the work. E.M. E1 Shazly and W.H. Kanes provided logistical support for the field work. Ahmed Abdullah lent his time and talents in the field. M. Somerville assisted with the zircon separations. G.R. Tilton provided laboratory facilities for the isotopic studies. J.G.

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Gillespie helped at many stages. J. Chen and J. Wright gave technical aid to the geochronological work. A. Chodos assisted with the microprobe analysis, and R. Batiza provided the REE data. The author has benefited from discus- sions with R.J. Stem, C.E. Hedge and W.R. Church. Two anonymous reviewers greatly improved an earlier version of this manuscript. This work was sup- ported in part by NSF Grant EAR77-19373 and a scholarship from the Nation- al Research Council of Canada.

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