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Arabian Journal of Geosciences ISSN 1866-7511 Arab J GeosciDOI 10.1007/s12517-011-0506-1

Geochemical characterization of zonedzircon from Wadi Abu Rusheid psammiticgneiss, south Eastern Desert, Egypt

Sayed A. Omar, Hassan A. A. Shahin &Masoud S. Masoud

1 23

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ORIGINAL PAPER

Geochemical characterization of zoned zircon from WadiAbu Rusheid psammitic gneiss, south Eastern Desert, Egypt

Sayed A. Omar & Hassan A. A. Shahin &

Masoud S. Masoud

Received: 28 May 2011 /Accepted: 13 December 2011# Saudi Society for Geosciences 2011

Abstract A unique zircon was studied in the gneiss sam-ples collected from the Wadi Abu Rusheid psammitic gneissusing electron scanning microscope and electron probemicroanalyses. This zircon can be categorized into twotypes according to the texture and trace element content:(l) magmatic zircon slightly enriched in HfO2 with ordinaryzone. (2) Overgrowths of zircon occur as two species, thefirst species being highly enriched in HfO2 with irregularzoning. The second species is highly enriched in HfO2

forming a rim around the second species with a very sharpthinner boundary. The first type shows a distinct oscillatoryinternal zoning pattern without change in shape of this zoneand has conspicuous inclusion-free zircon overgrowths withdistinct poor concentrations in Y, Hf, Th, U, Nb, and Ta inboth rim and core. The second type shows two species, thefirst one displays distinct irregular interval zoning and irreg-ular overgrowth with abrupt change in composition of thesezones with distinct enrichment in Y, Hf, Th, U, Nb, and Tain the rim relative to the core. The second species is forminga rim around the first species also with distinct enrichmentin Y, Hf, Th, U, Nb, and Ta content. These indicate that twoevents (crystallization environment) have played an impor-tant role in the formation of this zircon and largely reflectdifferences in whole-rock trace element contents betweenthe successive generations of this zircon. The first event isbelieved to be of magmatic origin giving rise to normalcomposition of magmatic zircon. The second event showsan intense successive process of metasomatic activity during theformation of the Abu Rusheid radioactive gneiss. Electronmicroprobe analysis indicates that oscillatory zoned zircon

shows poor content of Y, Hf, Th, U, Nb, Ta, and rare earthelements (REE) in the rim and core, while overgrowths ofzircon are slightly enriched by these elements. Also, theseanalyses indicate that the Abu Rusheid psammitic gneiss hasbeen significantly enriched by the thoritemineral (Th content upto 54.72% ThO2) and columbite-bearing minerals (Nb contentup to 64.74%Nb2O5, Ta content up to 9.32% Ta2O5). The poorcontent of REE in overgrowths of zircon indicates mobilizationof REE during the metamorphism processes of gneiss.

Keywords Zoned zircon . Psammitic gneiss . Egypt

Introduction

Zircon is a chemically inert mineral that can survive theprocesses of weathering, transportation, diagenesis, metamor-phism, and even crustal melting (Luo et al. 2009). Magmaticzircon incorporates minor and trace amounts of geochemicallyimportant lithophile elements such as Sc, Y, Ti, Hf, Th, U, Nb,Ta, V, P, and the rare earth elements (REE) (Hinton and Upton1991). Zircon contains trace amounts of uranium and thorium(from 10 ppm up to 1 wt.%) and can be dated using severalmodern analytical techniques, e.g., isotopic and fission-trackgeochronology, and for geochemical tracer studies (Sawka1988; Bea 1996; O'Hara et al. 2001). Because of this, zirconcan contain a rich and varied record of geological processes.Hafnium (Hf) is a particularly important minor element inzircon, because its isotopic composition is a sensitive tracerof crustal and mantle processes (e.g., Taylor and McLennan1985; Vervoort and Blichert-Toft 1999).

A unique type of zircon occurs in the radioactive psam-mitic gneiss of the Wadi Abu Rusheid area which is locatedwithin the basin of Wadi Al Gemal in the south EasternDesert, at a distance of about 70 km southwest of Marsa

S. A. Omar :H. A. A. Shahin (*) :M. S. MasoudNuclear Materials Authority,P. O. Box 530, El-Maadi Cairo, Egypte-mail: hassanshahin03@Yahoo.com

Arab J GeosciDOI 10.1007/s12517-011-0506-1

Author's personal copy

Fig. 1 Location map and Google image for the Abu Rusheid area

Fig. 2 Geologic map for theWadi Abu Rusheid area, afterDawood (2010)

Arab J Geosci

Author's personal copy

Alam, along the Red Sea coast at latitude 24°35′ to 24°40′and longitude 34°40′ to 34°50′. (Fig. 1).

Geomorphologically, the area is characterized by rough,steep slopes and rugged mountains dissected by severalwadis including Wadi Nugrus, Wadi Abu Rusheid, andWadi Sikait. The area was studied geologically, minera-logically, and radiometrically by numerous authors (Ball1912; Akaad and El-Ramly 1960; Basta and Zaki 1961;Hassan 1964; El-Shazly and Hassan 1972; Hassan 1973;El-Gemmizi 1979; Hassan et al. 1983; Hassan and El-Gemmizi 1985; Hilmy et al. 1990; Mohamed and Hassanen1997; Raslan 2008; Assran 2009; Ibrahim et al. 2007, 2010a, b,and Dawood 2010).

This paper focuses on textures and trace element contentsof a unique zoned zircon from the Wadi Abu Rusheidpsammitic gneiss.

Geologic setting

The Abu Rusheid area is a part of the Arabian-NubianShield and can be considered a key domain in that shield,beside its very complex structures. This area lies in the WadiAl Gemal basin and hosts a wealth of sources of uraniumand other nuclear elements. In addition, this area is consid-ered as the southeastern extension of the Migif–Hafafitmetamorphic complex (El Ramly and Akaad 1960; Hassan1964; El-Shazly and Hassan 1972; Hassan and Hashad1990), which is highly tectonized and shows the featuresof the multiple types of mineralization and alteration pro-cesses in long time spans. The Migif–Hafafit metamorphiccomplex represents one of three major domal structures inthe Eastern Desert of Egypt (Fowler and Kalioubi 2002 andAbd El-Naby et al. 2008). The geology of the Hafafit area

Fig. 3 Photomicrograph showing zircon crystals surrounded by a regular rim. Under crossed polars

Arab J Geosci

Author's personal copy

Tab

le1

Microprob

eanalyses

(weigh

tpercent)of

zircon

sfrom

theWadiAbu

Rusheid

gneiss

S.No.

SiO

2Al 2O3

TiO

2MgO

CaO

P2O5

ThO

2UO2

ZrO

2PbO

Y2O3

La 2O3

Ce 2O3

Nd 2O3

Gd 2O3

Yb 2O3

HfO

2S

Ta 2O5

Nb 2O5

Pr 2O3

SmO

Total

114-6-1

31.077

00.01

20.00

80.01

30

00

68.452

0.08

10.09

40

00.18

0.16

20.114

0.86

10

00

0.00

40

101.05

7

114-6-3

30.437

00.12

20.00

20.09

20

00.07

167

.109

0.03

51.39

60

00.27

70.23

0.29

91.00

60

00

00.12

101.19

6

114-1-1

31.95

0.01

50.07

30

0.00

90

00.18

765

.768

00.16

70

00.10

60

0.01

61.37

90.00

60

00.03

40

99.709

GE7-9-3

28.503

0.08

50.02

60.03

0.119

00.011

0.46

463

.287

00.62

70

00

00.74

32.74

40.00

90

00.10

40

96.751

GE7-9-3

29.176

0.13

0.04

90

0.08

00

0.44

862

.834

00.32

20.00

90

00

0.59

52.50

90.05

10

00

0.08

296

.284

GE7-9-3

29.788

0.22

10.06

50.01

80.12

90

0.19

80.28

264

.245

00.51

30

00.18

80.02

40.75

32.40

20.03

10

00

098

.858

GE7-9-3

29.61

0.08

60.06

20.00

40.07

90

00.20

564

.868

0.10

60.19

90

00.05

20

0.43

32.94

70.03

40

00

098

.686

GE7-9-4

15.138

0.29

0.06

10.01

71.19

21.78

150

.393

4.60

46.48

20.02

95.73

10

00

0.14

51.55

0.16

30.03

60

0.29

80

0.09

88

GE7-9-5

16.251

00.05

10.04

81.17

12.02

251

.443

4.72

66.35

0.15

56.18

90

00.07

01.45

10.06

10.05

90

0.59

70.05

30

90.697

GE7-9-6

14.245

0.17

20.03

10.03

60.94

1.57

443

.69

3.411

4.28

60.19

34.61

70

00

00.77

10.24

30.07

30

0.59

90

0.01

474

.896

GE7-9-7

16.43

0.23

10.10

90.04

1.20

51.94

153

.622

3.66

15.01

70.02

15.09

90

00

01.30

10.03

50.05

90

0.55

30.04

20.24

689

.613

GE7-9-8

15.417

0.23

50.10

30.05

71.19

31.82

154

.663

4.03

44.75

60.35

25.15

90

00.08

80.10

41.22

30.34

30.08

40

0.61

20.05

20.12

990

.425

GE7-9-9

12.099

0.08

50.01

0.05

50.9

1.34

448

.495

2.93

43.58

70.20

63.87

60

00.12

10

1.47

0.13

60.04

60

0.45

20.08

40

75.9

GE7-9-10

16.973

0.24

50.00

90.03

61.211

1.84

853

.154

2.78

5.26

70.18

66.37

0.05

00

0.48

41.54

40

0.06

40

0.27

20.04

10.03

290

.566

GE7-9-11

16.984

0.22

40.07

50.03

61.16

81.99

652

.151

3.85

56.17

30

5.88

30

00.07

20.12

91.60

30.27

80.05

70

0.89

30.09

70

91.674

GE7-9-12

15.891

0.17

30.07

70.05

21.18

81.95

254

.721

4.00

36.41

90.22

35.85

50.00

90

0.09

80.31

91.06

70.38

0.04

90

0.39

20

0.08

492

.953

GE7-9-13

27.838

0.28

90.03

30

0.27

80

0.01

40.45

261

.713

0.08

1.03

40

00.20

30

1.27

52.49

50.00

40

00.05

095

.758

GE7-9-14

29.721

00.01

20

00

0.04

90

57.397

0.02

60.06

70

00.08

70

0.90

910

.992

0.03

40

00

0.06

199

.356

GE7-11-9

30.522

0.06

30.03

60.01

70.06

40

00.17

163

.006

0.09

0.112

00

00

0.29

85.45

60.01

70

00

0.14

399

.994

GE7-11-

1029

.691

0.08

80.00

70.00

30.115

00.111

0.23

362

.578

0.07

40.62

90

00.10

20.03

70.66

43.44

10.03

70

0.13

20

097

.94

GE7-13

-125

.914

0.57

30.04

70.05

0.43

20

0.15

50.65

357

.198

0.03

33.04

70

00

0.29

51.58

32.17

20.04

70

0.00

20

092

.201

GE7-13

-229

.282

0.41

50.00

20.01

50.37

90

0.02

10.33

861

.28

01.37

90.01

00

00.84

52.33

50

00

0.08

10.05

796

.437

GE7-13

-330

.469

0.03

0.04

10.00

90.07

30

0.13

30.33

464

.734

00.38

60

00

0.07

90.69

42.96

30.02

50

00.07

20

100.04

2

GE7-12

-130

.193

0.07

60.00

80.00

70.04

30

0.07

20.10

563

.412

0.14

0.10

40

00.05

50

0.30

24.64

60

00

00.00

499

.165

GE7-12

-130

.14

0.14

70.00

50

0.09

50

0.111

0.12

63.135

0.14

0.411

00

0.04

50.02

0.51

95.18

90.04

60

00.00

90.04

410

0.17

5

GE7-12

-131

.098

0.02

50.02

60

0.03

90

0.05

40.08

463

.723

00

00

0.08

60.10

30

5.56

80.03

70

00

0.05

110

0.89

2

GE7-12

-128

.769

0.17

40.08

50.02

40.25

70

00.22

763

.055

0.02

80.92

60

00.06

50.01

80.92

62.18

40.00

50

0.06

40.00

20.14

796

.957

GE7-12

-130

.083

00.07

30

0.111

00.07

30.40

563

.819

00.42

20

00.06

80

0.54

2.14

30.02

80

0.13

90.01

80.01

797

.938

GE7-12

-129

.838

00

0.02

40.15

50

0.04

90.39

63.175

0.04

70.46

30

00.12

40.23

10.95

2.71

50

00

0.00

20

98.161

GE7-12

-130

.614

0.02

80.04

30.01

20

00.01

80.14

261

.103

00

00

00.16

0.50

68.26

40.02

80

00.07

20

100.99

1

GE7-12

-230

.721

00

0.00

60.01

20

00.16

163

.989

00

00

00.11

0.29

35.35

40

00

0.07

70.04

910

0.77

3

Arab J Geosci

Author's personal copy

has been described as the Hafafit and Nugrus unitsseparated by Nugrus Thrust and undeformed leucogranites(Abd El-Naby and Frisch 2006).

The Hafafit unit (western unit) consists of Hafafitdomes which include from core to rim granite gneiss oftonalitic and trondhjemitic composition, banded am-phibolite which is overthrusted by ultramafic rocks,alternating bands of biotite and hornblende gneiss,and the psammitic gneisses at the rim of the domalstructure. The Nugrus unit (eastern unit) is composedmainly of low-grade mica schists and metavolcanicsand related volcaniclastics. The Hafafit and Nugrus unitshave been intruded by undeformed leucogranites, especiallyalong thrust zones.

The main rock units encountered in the Abu Rusheid areaare represented by metasediments, ophiolitic melange,amphibolites, metagabbros, hornblende gneisses, psammiticgneisses, undeformed leucogranites, and pink granites(Fig. 2). This area is located between two major thrusts tothe NE. The metasediments are represented mainly by NE–SW elongated separated masses of pale gray highly foliatedmica schist locally thrusted over the psammitic gneisses.The ophiolitic melange comprises a metamorphosed sed-imentary matrix enclosing amphibolite sheets, serpentin-ite and gabbroic masses, as well as quartzitic bands.Amphibolites and metagabbros are probably related tothe calc-alkaline metagabbros associated with Hafafit gneisses(El Ramly et al. 1993). The hornblende gneisses are formed ofNW elongated mass, gray–green in color, coarse to mediumgrained, and highly foliated.

Abu Rusheid gneisses were studied by many authors.They identify it as psammitic gneisses (Hassan 1973; AbdEl-Monem and Hurley 1979; Hilmy et al. 1990; Abd El-Naby and Frisch 2006 and Dawood 2010). Some authorsdescribed these rocks as gneissic granites (Ibrahim et al.2000; Raslan 2008) and cataclastic granites (Ibrahim et al.2007). The effects of the hydrothermal solutions on the AbuRusheid gneisses were reported by El-Shazly and Hassan(1972), Hassan (1973), Abd El-Monem and Hurley(1979), and Ibrahim et al. (2000). The gneisses exhibitextensive alteration, including silicification, sericitization,kaolinitization, chloritization, hematitization, and limonitiza-tion (Abd El-Naby et al. 2008). These alterations are closelyassociated with the shear zones indicating that some ofthese fractures have worked as channel ways for meta-somatizing fluids.

Leucogranites are pink granites and minor intrusions offelsite and aplite. The younger granites are locally devel-oped along thrust faults and contain numerous enclavesfrom older rocks particularly mica schist. Generally, theleucogranites are medium to course grained, composedmainly of quartz, K-feldspar, plagioclase, garnet, biotite,and muscovite. T

able

2Microprob

eanalyses

(weigh

tpercent)of

Nb-Tafrom

theWadiAbu

Rusheid

gneiss

S.No.

SiO

2Al 2O3

TiO

2MgO

CaO

P2O5

ThO

2UO2

ZrO

2PbO

Y2O3

La 2O3

Ce 2O3

Nd 2O3

Gd 2O3

Yb 2O3

HfO

2S

Ta 2O5

Nb 2O5

Pr 2O3

SmO

Total

GE7-10

-10

00.42

40.02

70.011

00.00

80

00.07

90.12

40.05

40

00

00

0.03

47.25

969

.344

00.119

77.482

GE7-10

-10

00.54

80

0.03

50

00

00.114

0.18

50.01

20

0.03

00.27

70.19

20.011

8.33

968

.885

00.09

78.717

GE7-10

-113

.106

0.24

93.75

20.12

82.16

0.09

81.32

642

.082

0.00

80.59

90

00

00.05

90.16

60

0.00

70

12.503

00.03

976

.28

GE7-10

-10.21

0.04

90.52

50

0.05

90.01

80.05

40.01

20

0.04

90.19

30

00

0.13

10.06

10.03

90.01

35.61

662

.719

00

69.748

GE7-10

-10

00.53

90.00

60.03

90.00

30

00

0.46

10.18

90

00

00.117

00.01

37.28

671

.383

00.23

880

.274

GE7-10

-20

00.43

00.011

0.011

00

00.35

60.16

20

00

0.11

0.115

0.17

30.00

27.19

271

.104

00.07

879

.744

GE7-11-1

00

0.41

30

0.00

50

0.00

70

00.21

30.04

50

00.03

60

0.31

90.17

30.00

17.02

371

.676

0.01

40.00

179

.927

GE7-11-2

0.04

60.02

30.46

40

00.01

60.04

90.05

20

0.49

50.19

10

00

00.02

70

0.01

36.88

170

.608

00

78.866

GE7-11-3

00.011

0.45

80

0.03

50.00

10

0.07

10

0.21

20.20

60

00.12

50

0.02

60.115

0.05

7.69

369

.727

00

78.73

GE7-11-4

00

0.40

30

0.01

90

00

00.09

80.10

20

00

00.05

0.13

50.00

17.04

969

.541

00

77.399

GE7-11-5

00

0.41

70.01

90.02

30.02

30

0.01

40

0.41

70.09

80

00.06

40

0.01

30.14

40

7.62

270

.812

00.115

79.781

GE7-11-6

00

0.51

50

0.02

70

00

00.40

40.03

40

00.02

60

00.115

08.24

968

.922

00

78.292

GE7-11-7

00

0.52

0.00

30

00.07

40

00.27

30.14

30.01

40

0.14

40.04

70

0.04

80.01

28.20

770

.185

00

79.67

GE7-11-8

00.00

10.76

10

0.03

30.02

10

0.22

90

0.24

0.21

40

00

0.10

20

00

6.51

669

.509

00.01

777

.645

GE7-11-9

00.011

0.39

30

0.02

60

0.04

0.00

60

0.28

20.13

00

00

0.35

50.29

70

7.02

970

.064

0.03

20

78.665

Arab J Geosci

Author's personal copy

Zircon occurrence

The topmost 50 m of the psammitic gneiss is radioactive andhosts a great number of economic minerals of uranium, suchas Th, Nb, Ta, Zr, and Hf (Hassan 1973; Bugrov et al. 1973;

Krs et al. 1973; Hassan et al. 1983; and El-Gemmizi 1984).Among these minerals, a unique type of zircon with abnor-mally high content of hafnium was recorded which may beused as a fingerprint of geological history and a good source ofhafnium in this area. El-Gemmizi (1984) called it mud zircon.

(b)

(a)

1O

2O

1O Beam No.

Fig. 4 a Back-scattered electron image showing traverse section in magmatic zircon. b Bar diagram showing the electron microprobe analyses forthe same crystal

1O

1O Beam No.

2O

3O

4O

5O

6O

7O

(a)

(b)

Fig. 5 a BSE image showing traverse section in overgrowths of zircon without rim. b Bar diagram showing the electron microprobe analyses forthe same crystal

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Zircon textural features

Two types of zircon were distinguished from the studiedsamples, magmatic zircon and overgrowths of zircon. Thesezircons are prismatic grains with bipyramidal terminationsintergrown, mostly are subhedral to euhedral crystals asso-ciated with quartz, plagioclase, and biotite.

Magmatic zircon occurs as euhedral prismatic crystals upto 200 μm in size, mostly as subhedral to euhedral crystals.This zircon shows an oscillatory internal zoning patternwithout change in composition in these zones and has con-spicuously inclusion-free zircon overgrowths. Overgrowthsof zircon occur as pale yellow to pale brown prismaticgrains with bipyramidal terminations intergrown, mostly

(b)

(a)

6O

1O

2O1

O Beam No.

3O

4O

5O

7O

8O

9O

Fig. 6 a BSE image showing traverse section in overgrowths of zircon with rim. b Bar diagram showing the electron microprobe analyses for thesame crystal

1O 2

O

1OPeam No.

3O

(a)

(b)

Fig. 7 a BSE image showing traverse section in overgrowths of zircon with rim. b Bar diagram showing the electron microprobe analyses for thesame crystal

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subhedral to euhedral crystals reaching up >400 μm in thelongest dimension. The crystals are composed of a core,which is forming themain body of the crystal, and a rim whichforms an outside cover of the core. The core is irregularlyzoned, with irregular and gradation boundaries between colorsof blue, red, and brown. The rim is much thinner than thezoned core and surrounds it with a very sharp contact; its coloris very light blue to deeper blue resembling the intensity of thedeep blue of the core. Most of the crystals of this zircon arecomplex, forming more than two twins; others are irregular inshape and have an irregular boundary and contain many

inclusions of the co-existing minerals (Fig. 3). From theseobservations, we conclude that the core and the rim wereformed during two phases of metasomatism, probably withdifferent intensity, duration, and chemical reactions.

Zircon chemistry

The zircon from the Abu Rusheid area was analyzed using aCAMECA SX-100 electron-probe microanalyzer methodwith four wavelength-dispersive spectrometers at the Centre

(a)

(b)

9O

1O

2O

3O

4O

5O

6O

1O Beam No.

7O

8O

12O 10

O

11O

Fig. 8 a EDX and BSE image showing the semi-quantitative chemical composition of thorite. b Bar diagram showing the electron microprobeanalyses for the same crystal

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de Recherches Petrographiques et Geochimiques, Nancy,France. Analyses were performed using an acceleratingpotential of 15 to 20 kv and a beam current of 10 nA(Tables 1 and 2). From all zircons studied using scan-ning electron microscope (SEM) with a back-scattered elec-tron (BSE) imaging and electron probe microanalyses

(EPMA), two patterns were detectable: (l) zircon withdistinct poor concentrations in Hf, Th, U, Nb, and Ta inboth rim and core and (2) zircon with abrupt change incomposition of these zones and distinct enrichment inHf, Th, U, Nb, and Ta in the rim relative to the core(Figs. 4, 5, 6, 7, and 8).

(a)

(b)

1O

2O

3O

4O

5O

6O

1O Beam No.

Fig. 9 a EDX and BSE image showing the semi-quantitative chemical composition of columbite. b Bar diagram showing the electron microprobeanalyses for the same mineral

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Niobium and tantalum multiple-oxide minerals such ascolumbite associated with zircon in the uppermost divisionof psammitic gneiss. Columbite (Fe, Mn)Nb2O5 occurs inminute crystals disseminated in this gneiss particularly inthe parts that were affected by the metasomatism process.Columbite was studied using SEM with a BSE imaging andEPMA (Fig. 9a and b). Metamict zircon and columbitecould be also considered as additional sources for uranium(Finch and Murakami 1999; El-Kammar et al. 2001).

Conclusion

Textural and geochemical characteristics of zircon hosted atthe Wadi Abu Rusheid gneiss indicate two types according tothe texture and trace element content: (l) magmatic zirconslightly enriched in HfO2 with ordinary zoning and (2) over-growths of zircon that show two species, the first one beinghighly enriched in HfO2 with irregular zoning. The secondspecies is highly enriched also in HfO2 forming a thinner rimaround the second species with a very sharp boundary. Theprevious two types indicate two events reflecting largely theenvironments in which they were formed.

The first type seems to be ordinary zircon which occurswidely as an accessory mineral in many igneous rocks andhas suffered transportation and deposition and a detritus insedimentary rocks, which was later metamorphosed to givethe psammitic gneiss of Haffafit metamorphic succession.This zircon is characterized by distinct oscillatory internalzoning without change in composition of these zones andhas conspicuously inclusion-free zircon overgrowths withdistinct poor concentrations in Y, Ti, Hf, Th, U, Nb, and Tain both rim and core. This zircon is believed to be of magmaticorigin giving rise to normal composition of magmatic zircon.

The second type is the most abundant and forms the maincomponent of this zircon and indicates a long, intense, andvariable stage of metasomatic activity during the formationof the mineralization in the Abu Rusheid radioactive gneiss.This activity is probably associated with the Pan Africanevent with a high degree of Zr development in a hydrothermalenvironment with distinct enriched zircon in Y, Ti, Hf, Th, U,Nb, and Ta. The metasomatic effect during this event isbelieved to be prevailing during most of the Pan Africanorogeny (Hassan 1973; Hassan and Hashad 1990). This meta-somatic activity is giving rise to development overgrowthzircon which occurs as two species.

The large difference in Hf content between the magmaticzircon of the first type on the one hand and the overgrowthsof zircon of the second type on the other hand may alsosupport the previous two events which reflect different fluidsources for each. The previous two events are also compat-ible with the isochron age of the Wadi Abu Rusheid gneiss,where Abd el-Monem and Hurley (1979) determined an age

of 1,770±40 Ma for the detrital zircon from the psammiticgneiss of Wadi Abu Rusheid and Wadi Sikait. This agerepresents the age of old sediments deposited before thepsammitic gneiss formed and reflects that this zircon is olderin age than the psammitic gneiss and may be derived fromthe weathering of peri-existing felsic rocks. Therefore, thisage is believed to be corresponding to the first event (mag-matic zircon). Hashad et al. (1972) obtained an Rb–Sr age of590±20 Ma for mica separated from the white granite, andHashad et al. (1981) obtained an Rb–Sr age of 600 Ma formica separated from the psammitic gneiss. These two agesof Hashad et al. (1972, 1981) are representing successiveevents of strong metamorphism processes that took place onthe psammitic gneiss of the Abu Rusheid area, where thewhite granite intruded the psammitic gneiss causing strongmetasomatism. This metasomatism enriched the psammiticgneiss by radioactive rare earth elements and radioactiveminerals. Therefore, these ages are believed to becorresponding to the second event (overgrowths of zircon)in the Abu Rusheid psammitic gneiss. These overgrowths ofzircons of the Abu Rusheid psammitic gneiss are also com-patible with Gulson and Krogh (1975) who determined thatzircon overgrowths are related to post-tectonic intrusions inigneous and high-grade metamorphic rocks.

Acknowledgments Thanks are due to the Centre de RecherchesPetrographiques et Geochimiques, Nancy, France, for use of theirelectron microprobe. Many thanks go to Prof. Dr. Mamdouh Hassan,Nuclear Materials Authority (NMA), Egypt, for revising the manuscriptand his constructive remarks.

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