petrogenesis of the pan-african el-bula igneous suite, central

Pergamon Journal of African Earth Scrences, Vol. 31, No. 2, PP. 317-336. 2000

o 2000 Elsevw Scxnce Ltd

PII:SO899-5382(00)00101-9 All rights reserved. Prmted I” Great Brltam

08995362100 s- see front matter

Petrogenesis of the Pan-African El-Bula Igneous Suite, central Eastern Desert, Egypt

S.M. EL-SHAZLY” and M.M. EL-SAYED Geology Department, Faculty of Science, Alexandria University, Alexandria, Egypt

ABSTRACT-The Precambrian rocks of the El-Bula area include syn-erogenic intrusive granitoids associated with gneisses, metavolcanics and metagabbro-diorite complexes. The metavolcanics range in composition from basalt to basaltic andesite. They show tholeiite affinity and provide unfractionated, flat rare earth element patterns and have geochemical features suggesting their generation in a back-arc environment. Low and variable Mg# of the basic metavolcanics suggest that they were derived from the upper mantle with subsequent modification by fractional crystallisation. The least evolved basaltic sample could be generated by 25% batch melting of a spine1 lherzolite source followed by 35% fractionation of clinopyroxene, olivine and plagioclase in proportions of 45%, 30% and 25%, respectively. The most evolved basaltic rock could be generated by 55% fractional crystallisation (65% hornblende, 30% plagioclase and 5% titanomagnetite) of the least evolved basaltic sample liquid. The El-Bula syn-erogenic granitoids show a normal igneous evolution from metaluminous tonalites to weakly peraluminous granodiorites. They exhibit geochemical features of l-type talc-alkaline granites emplaced in a volcanic arc environment. The least evolved tonalitic sample could be generated by about 30% batch melting of the most evolved basic metavolcanic sample. The El- Bula tonalites and granodiorites have geochemical and petrological characteristics indicating that they might be genetically related by fractional crystallisation. The chemical variation in the tonalites is dominantly controlled by about 45% fractional crystallisation of hornblende, plagioclase and K-feldspar in proportions of 55%, 44% and I%, respectively. The least evolved granodioritic sample could be produced from the most evolved tonalitic sample by about 20% crystallisation of plagioclase, biotite, hornblende and K-feldspar phases in proportions of 54%, 43%, 2.5% and 0.5%, respectively. The variations in the granodiorites could be modelled by about 30% fractional crystallisation of plagioclase, K-feldspar and biotite (62%, 24% and 14%, respectively). o 2000 Elsevier Science Limited. All rights reserved.

RESUME-Le Precambrien de la region d’El-Bula comprend des granitoides intrusifs anciens syn- orogeniques associes a des gneiss, a des metavolcanites et a un complexe de metagabbro- diorites. Les metavolcanites ont des compositions basaltiques a basaltiques andesitiques. Ces roches possedent une affinita tholeiitique et definissent des spectres de terres rares plats non fraction&s. Leurs caracteristiques gaochimiques suggerent une origine dans un environnement d’arriere-arc. Les Mg# bas et variables des metavolcanites basiques suggerent une source dans le manteau superieur et une evolution par cristallisation fractionnee. L’echantillon basaltique le moins evolue pourrait avoir Btt! engendre par la fusion partielle d’une lherzolite a spinelle suivie par 35% de fractionnement de clinopyroxene, olivine et plagioclase en proportion 45:30:25, respectivement. L’echantillon basaltique le plus Bvolue pourrait avoir Bte engendre par la cristallisation fraction&e (65% hornblende, 30% plagioclase et 5% titanomagnetite) d’un liquide correspondant a l’echantillon le moins Bvolue. Les granitoides syn-orogeniques d’El-Bula montrent une evolution magmatique normale depuis des tonalites metalumineuses jusqu’a des granodiorites faiblement hyperalumineuses. Ils possedent

*Corresponding author [email protected]

Journal of African Earth Sciences 3 17

S. M. EL-SHAZL Y and M. M. EL-SA YED

les caracteres geochimiques des granites calco-alcalins de type I d’arcs insulaires. L’echantillon tonalitique le moins evolue pourrart avoir ete engendre par la fusion partielle (30%) de la metavolcanite la moins evoluee. Ces granito’ides possedent des caracteristiques geochimiques et petrologiques indiquant qu’ils pourraient btre genetiquement lies par un processus de cristallisation fractionnee. Les variations chimiques des tonalites sont principalement controlees par la cristallisation de la hornblende, du plagioclase et du feldspath K dans une proportion 55:44: 1. L’echantillon granodioritique le moins evolue pourrait avoir ete produit a partir de la tonalite la plus Bvoluee par 20% de cristallisation de plagioclase, biotite, hornblende et feldspath K dans une proportion de 54:43:2.5:0.5. Les variations au sern des granodiorites peuvent dtre modelisees par la cristallisation fractionnee de plagioclase, feldspath K et biotite dans une proportion de 62:24:14. o 2000 Elsevier Science Limited. All rights reserved.

(Received 3/l l/98: revised version received 1315199: accepted 2718199)

INTRODUCTION

The Precambrian basement complex of the Nubian Shield is dominated by rocks that have evolved and been cratonised during the Pan-African tectono- thermal event (ca 1100-450 Ma: Gass, 1977). These rock suites include volcano-sedimentary successions, ultramafic/mafic complexes of ophiolitic affinity, gabbro-diorite-tonalite complexes of arc charac- terisations, Dokhan volcanic rocks and granodiorite- granite complexes.

The volcanics in the Precambrian crust of the Egyptian Shield (ES) are classified into two major episodes, namely Early Pan-African (950-750 Ma) and Younger Pan-African (680-550 Ma) volcanic successions (Abdel-Rahman, 1996). The Early Pan- African volcanic rocks are represented by Shadli metavolcanics, which were classified by Stern (I 98 1) into older (OMV) and younger (YMV) metavolcanics. The former is dominated by pillow basalts while the younger metavolcanics are mainly andesites and volcanogenic metasediments. Evidence for a significant age difference between the OMV and YMV units is not clear and it is postulated that the two groups might represent a coeval back-arc basin island- arc pair (Khalil et a/., 1993). The Younger Pan-African voluminous volcanic activity produced the Dokhan volcanics, which are considered the oldest, unmetamorphosed volcanic rocks in the Egyptian Shield (El-Ramly, 1972; Akaad and Noweir, 1980). The abundance of ignimbrites during the Younger Pan- African volcanic activity suggests a subaerial environment for this volcanic episode (Moghazi, 1987; El-Gaby et a/. , 1988).

Granitoid rocks cover about 60,000 km* of the Egyptian Shield (Hassanen et a/., 1996). Two different granitoid groups, namely older and younger granitoids, have been recognised (El-Gaby, 1975; Hashad, 1980; El-Gaby et al., 1988; Hassan and Hashad, 1990). The older granitoids comprise syntectonic diorites, quartz- diorites, tonalites, granodiorites and rarely granites, They were emplaced within a time span from 1000

3 18 Journal of African Earth Snences

to 850 Ma (Hassan and Hashad, 1990). The younger granitoids are made up of late to post-tectonic granites emplaced between 620 and 530 Ma (Hassan and Hashad, 1990). Among the younger granitoids group are some associated with rare metal mineralisation. There is speculation concerning the origin of the younger granitoids in the Nubian Shield including anatexis of older tonalite-granodiorite suites (Gillespie and Dixon, 1983) and assimilation of older crustal components (Bickford et al., 1989; Sultan et a/., 1990) *

The intent of this study is to present new geochemical data for the El-Bula syntectonic granitoids and the associated metavolcanics to assign their petrogenesis and tectonic environment. Repre- sentative samples from the metavolcanics were studied in order to test the probable source for the El- Bula granitoids.

FIELD GEOLOGY

The El-Bula area forms a portion of the Pan-African belt in the central Eastern Desert of Egypt. El-Gaby and Habib (I 982) classified the rock units in the study area (Fig. I) into highly metamorphosed gneisses (the El-Markh Tonalite Gneiss and Abu Furad Gneiss), metavolcanics, a metagabbro-diorite complex (the Safaga metagabbro-diorite complex) and intrusive tonalites-granodiorites.

The highly metamorphosed gneisses are recorded in the northern and western parts of the map area (Fig. 1). Field relations suggest that the El-Markh and Abu Furad Gneisses represent the oldest rock units in the study area. The El-Bula metavolcanics represent a part of the older metavolcanics (OMV) succession described by Stern (I 981). Where contacts between the El-Bula granitoids and the metavolcanics are exposed, intrusive relations consistently indicate that the granitoids were generated after the tectono- thermal event responsible for the emplacement of


0 2km I I

26’ 40

ECBula intrusive granitoids

Metagabbrodiorite complex

El-Markh tonaliie gneiss

/ Fault

26’ 35

Figure 1. A simplified geological map of the El-Bula suite (modified after El-Gaby and Habib, 1982J.

the older metavolcanics. Moreover, metavolcanic enclaves and xenoliths of variable sizes occur within the El-Bula granitic pluton (Takla et a/. , 1997). The metavolcanics form moderately elevated mountainous ridges and show a rough topography with gentle to steep slopes. The metavolcanics range in composition from basalt to basaltic andesite, which commonly exhibit porphyritic textures, and are associated with some interbeds and lenses of metapyroclastics. The mineral assemblages of the metavolcanics indicate regional metamorphism in the greenschist-facies. The contact between the El-Bula intrusive granitoids and the Safaga metagabbro-diorite complex, which is intruded into the metavolcanics, is of an intrusive

nature. Granitic apophyses and veins are commonly observed in the metagabbroid rocks.

The granitoid rocks located southwest of the port Safaga area were divided (El-Gaby and Habib, 1982) into:

il syn-erogenic talc-alkaline older granitoids; ii) late erogenic talc-alkaline younger granitoids; and iii) post-erogenic alkaline to peralkaline younger

granitoids. The syn-erogenic granitoid masses were further

sub-divided according to the nature of their contacts with the enveloping country rocks into three main types, namely autochthonous, parautochthonous and intrusive.



The El-Bula granitoid pluton represents one of the syn-erogenic intrusive granitoid plutons (Akaad et al., 1973) which are widespread in the basement complex of the Nubian Shield. It is a sub-rounded pluton of about 25 km*. The pluton ranges in composition from tonalite to granodiorite. The field observations show that the granitoid pluton cuts all the rock types present in the study area, suggesting younger age. The El-Bula intrusive contacts are commonly modified by magmatic stopping, where the roofs and marginal parts of the intrusion are characterised by foliation within a narrow zone a few metres width. In addition, spheroidal xenoliths of different dimensions are not uncommon. The enclaves are arranged parallel to the contact and dip away from the pluton (Akaad et al., 1973). The contact is distinctly sharp, highly irregular and dips away towards the country rock envelope. In some places, the contacts show varying degrees of interaction against the sur-rounding metavolcanics and gabbros.

PETROGRAPHY Metavolcanics The basic metavolcanics exhibit massive and porphyritic textures with green to blackish colours. The rocks are composed of plagioclase, hornblende and minor amounts of relict pyroxene of augitic composition. Secondary minerals are mainly epidote, zoisite, sericite, actinolite and chlorite, whereas magnetite and titanite are accessories.

Plagioclase (An 35-55), occasionally altered to sericite and epidote, occurs as euhedral to subhedral tabular phenocrysts and glomerocrysts, as well as small laths in the groundmass. Most of the plagioclase phenocrysts exhibit complex twinning and zoning. The majority of the plagioclase pheno- trysts are corroded by the groundmass components.

Hornblende ranks second in abundance. It is present as large phenocrysts, as well as small, fine- grained laths. The hornblende phenocrysts, commonly showing diagonal and lamellar twins, occur as subhedral prismatic crystals. In some places, they are partially altered to chlorite and actinolite, especially along the grain boundaries. Fine-grained hornblende laths, forming a part of the groundmass, are often altered to chlorite. Augite, partly to com- pletely replaced by fibrous actinolite and/or chlorite, is usually present as irregular cores in the hornblende. Actinolite is the most abundant secondary mineral formed after augite and hornblende. Epidote and zoisite are mainly formed after plagioclase, augite and hornblende, and are present

320 Journal of African Earth Sciences

as disseminated microlites in the groundmass. Opaques are the most common accessories associated with altered ferromagnesian minerals. Titanite is present as rims around titaniferous opaques or as disseminated microlites in the groundmass.

Granitoid assemblage The granitoid rocks of the El-Bula area are generally medium-grained (tonalite) to coarse-grained (granodiorite). They possess equigranular and seriate- porphyritic textures. The granitoids consist mainly of plagioclase, alkali-feldspar, quartz and subordinate biotite. Hornblende occurs only in the tonalitic variety. Sericite, chlorite, epidote and muscovite are secondary phases, while titanite, Fe oxides, apatite and zircon are accessories.

Plagioclase, displaying varying degrees of saus- suritisation, occurs as subhedral to euhedral tabular crystals ranging in composition from An,, to An,, and from An, to An,, in the tonalites and granodiorites, respectively. A few fresh plagioclase crystals show lamellar twinning and some zoning. The crystals of plagioclase commonly exhibit corroded boundaries and in some places are replaced by muscovite. Myrmekitic overgrowth into adjoining potash feldspar is some- times observed.

Potash feldspars (Or,,_,,) are commonly represented by abundant microcline, showing tartan twinning, as well as a few crystals of orthoclase. They vary considerably in abundance from tonalite to granodiorite. Euhedral to subhedral potash feldspar crystals, up to 2.5 cm long, occur in the granodiorite; in contrast, a few, small anhedral, interstitial crystals are observed in the tonalite. Micrographic texture is observed in some samples. Alkali feldspars are in some places present in plagioclases, forming an antiperthitic texture.

Quartz occurs as large crystals and in some places fills the interstices between the larger feldspar crystals. The deformed quartz in the foliated marginal parts is usually elongated parallel to the foliation direction.

Hydrous ferromagnesian phases are represented by both hornblende and biotite in the tonalite and by biotite in the granodiorite. Biotite forms subhedral flakes and shreds. It is generally unzoned showing pleochroism: X (straw yellow) <Y and Z (red-brown). Biotite IS partially altered to chlorite. Zircon and apatite are the main inclusions in the biotite flakes. Hornblende is present in the tonalite as subhedral prisms commonly enclosing numerous plagioclase and apatite crystals. Titanite occurs mainly as subhedral crystals scattered through the El-Bula granitoid rocks.

Petrogenesis of the Pan-African El-Bula igneous Suite, central Eastern Desert, Egypt

GEOCHEMISTRY Analytical technique The present study is based on 62 fresh samples collected from the El-Bula area. Thirty-two samples (five of the tonalites, 19 of the granodiorites and eight of the metabasalts) were selected for major and trace element analyses using a Philips PW-2400 X-ray fluorescence spectrometer (XRF). The USGS international standards have been used for calibration. Six samples from the total analysed samples were chosen for REE (two metavolcanics, two tonalites and two granodiorites) using ICP-MS, after ion-exchange separation. The uncertainties of analyses are l-2% for the major elements and around 10% for the trace elements. The light rare earth elements (LREE) have errors of 5-10% and heavy rare earth elements (HREE) are within - 3%. Microprobe analyses were made on the different minerals forming El-Bula granitic body and metavolcanics. The mineral phases were analysed using a wavelength-dispersive CAMECA electron microprobe. All chemical analyses were carried out in the Mineralogical Institute, Cologne University, Germany.

Whole rock geochemistry Meta volcanics The complete chemical analyses of the granitoid rocks and associated metavolcanics are listed in Tables 1 and 2. The nomenclature of the metavolcanics is based on the total alkali-silica (TAS) classification diagram suggested by le Maitre et al. (I 989). The metavolcanic samples are classified as basalt and basaltic andesite.

The metavolcanics have 46.03-53.25 wt% SiO, with a high Al,O,content (14.21-15.35 wt%) and contain 0.08-0.58 wt% K,O. They show a relatively wide variation in Fe,O,, MgO, V, Ni, Sr and Ba contents (Table 1) and low incompatible trace element abundances, notably Rb (4-I 1 ppm) and Th (I-7 ppm). The metavolcanics have low Nb/Y ratios ( < 1) and can be classified as a sub-alkaline suite (Pearce and Gale, 1977). These metavolcanics plot also in the sub-alkaline field using a Zr versus P,O, diagram (Fig. 2). In order to distinguish between the tholeiitic and talc-alkaline magmas, the metavolcanics are plotted on the Na,O + K,O-FeO*-MgO (AFM) diagram (Fig. 3). The present metabasaltic rocks display a marked trend of Fe-enrichment, suggesting tholeiitic affinity.

Figure 4 shows major and trace element variation diagrams for the El-Bula Suite, using silica as an index of differentiation. The plotted data for the metavolcanics (Fig. 4) generally form pronounced linear to gently curved trends suggestive of the operation of a single stage magmatic process. It is evident that TiO,, MgO, CaO, A&O,, V and Ni show compatible behaviour,

with concentrations falling in more evolved liquids. In contrast, the alkalis (Na,O + K,Ol, Y and Ba behave incompatibly and increase in concentrations in more evolved liquids. The other major and trace elements do not show covariation with SiO,. A chondrite-normalised REE diagram of the metabasaltic rocks shows unfractionated, ftat patterns with - 1 O-20 chondritic abundances and an absence of Eu anomalies (Fig. 5).

Granitoid assemblage Based on the Q-P discriminant diagram (Debon and le Fort, 19831, the El-Bula granitoids are tonalites and granodiorites, which matches the petrographic classification. On the basis of the alumina saturation index [A/CNK = mol. AI,O,/(CaO + Na,O + K,O)l, the

0.5 [ I

MHO 0.4 R - Alkaline 0 H .

./. s 0.3 - ..

s .)/ Subalkaline 4

0 0” 0.2 //I - 0 0 ci”

cp 00 0

0.0 1 I

50 100 150 200 250 Zr pm

Figure 2. PzO, versus Zr relationship for the metavolcanics showing their sub-alkaline affinity (after Winchester and Floyd, 1976).

Fe0 l

A

Caloalkaline

v v v v v v v v v Na,O + K,O MN

Figure 3. An AFM diagram for the El-Bula metavolcanics showing their tholeiitic affinity. The field boundary between the tholeiitic and talc-alkaline fields is after Irvine and Baragar 179711. The fields of older IOMVI and Younger [YMV) metavolcanics are after Khalil (19971.


121

Table

1.

Majo

r an

d tra

ce

elem

ent

conc

entra

tions

fo

r th

e ro

ck

asso

ciatio

n in

the

Wad

i El

-Bula

ar

ea,

Cent

ral

Easte

rn

Dese

rt,

Egyp

t

Samp

le no

. 10

5 11

1 10

9 10

7 10

1 11

2 11

0 97

18

8 15

0 16

6 16

5 19

2 16

9 17

2 18

3 Ba

sic

met

avolc

anics

To

nalite

s Gr

anod

iorit

es

Rock

typ

e Si

02

46.0

3 46

.98

48.1

1 49

.03

50.1

6 50

.77

52.4

1 53

.25

61.1

4 61

.27

63.3

6 63

.40

66.7

6 71

.01

71.3

6 71

.77

TiOz

TiOz

1.46

1.65

AW3

AW3

15.2

9 15

.18

Fe20

3*

Fe20

3*

12.1

0 12

.61

MnO

MnO

0.20

0.21

MN

MN

8.49

7.43

CaO

CaO

10.6

2 10

.29

Na,O

Na

,O

2.12

2.23

W

0.35

0.31

p205

0.1

5 0.1

6 LO

I 3.2

0 2.9

9 Su

m 10

0.01

10

0.04

1.69

15.0

2

13.1

4 0.2

3 6.1

8 9.9

8 2.5

8 0.2

5 0.1

7 1.9

9 99

.34

1.85

1.75

1.62

1.55

1.49

0.69

0.89

0.34

0.37

14.8

7 15

.35

15.1

2 14

.91

14.7

2 15

.21

15.9

6 13

.19

13.6

0 13

.54

13.5

8 13

.10

11.7

5 11

.29

5.67

5.44

7.17

7.39

0.25

0.22

0.21

0.21

0.20

0.09

0.09

0.13

0.13

5.13

4.56

4.42

4.41

5.41

4.36

3.41

4 70

3.9

6 9.6

2 8.2

6 8.1

5 8.1

9 8.4

9 5.7

1 5.3

2 6.0

4 6.5

3 2.9

1 3.2

3 3.4

5 3.3

1 3.1

1 3.4

3 3.9

8 2.5

6 2.7

9 0.0

8 0.4

3 0.4

3 0.4

3 0.4

5 1.4

0 1.4

9 0.9

5 0.6

4 0.1

9 0.1

6 0.1

8 0.2

1 0.2

4 0.1

5 0.2

4 0.0

8 0.0

7 2.6

1 2.3

7 2.2

6 2.1

5 1.4

0 2.0

5 1.7

0 1.4

0 1 .

oo

100.

08

100.

07

99.7

1 99

.53

100.

05

99.9

0 99

.79

99.9

2 99

.88

0.55

0.32

15.6

1 14

.87

3.50

2.42

0.06

0.07

1.61

0.84

2.55

2.58

4.29

4.54

2.52

2.34

0.17

0.10

2.14

0.66

99.7

6 99

.75

0.28

0.26

14.6

7 14

.71

2.06

1.98

0.06

0.06

0.70

0.66

2.54

2.28

4.56

4.50

2.19

2.16

0.09

0.09

1.11

1.18

99.6

2 99

.65

MM

37.7

0 33

.69

28.8

6 24

.63

22.4

5 22

.54

24.4

5 29

.24

AlCN

K 0.6

6 0.6

7 0.6

6 0.6

7 0.7

4 0

72

0.72

0.70

v 28

7 30

1 34

0 38

2 36

6 32

9 25

1 22

6 Cr

34

9 30

5 29

2 60

13

6 12

8 12

9 18

0 co

41

38

36

34

35

33

30

28

Ni

20

0 17

4 13

5 30

58

61

53

73

cu

69

75

84

92

72

66

57

52

Zn

84

10

5 12

5 15

7 11

2 10

6 96

88

Ga

16

16

17

18

18

18

17

18

Rb

11

9

7 4

8 9

10

10

Sr

186

202

219

237

220

224

234

234

Y 28

31

35

38

37

40

51

54

Zr

87

91

102

109

110

135

196

212

Nb

2 2

2 3

3 4

6 9

cs 5

7 8

9 0

5 16

23

Ba

86

11

8 12

5 36

14

8 16

0 19

2 20

3 SC

35

34

35

36

35

34

32

31

Th

3

4 4

5 1

2 5

7 U

1 1

2 2

2 2

3 3

Pb

3 2

1 1

2 2

4 4

NblY

W

Y Sr

lZr

ZrlN

b Rb

lZr

TilZr

K/

Zr

KIP

0.07

0.06

6.64

6.52

2.14

2.22

43.5

0 45

.50

0.13

0.10

0.06

6.26

2.15

51.0

0 0.0

7

0.08

6.24

2.17

36.3

3

100.

61

108.

70

33.4

0 28

.28

4.44

3.69

0.04

99.3

3 10

1.75

20

.35

6.09

2.80

0.80

0.08

0.10

0.12

0.17

5.95

5.60

4.59

4.33

2.00

1.66

1.19

1.10

36.6

7 33

.75

32.6

7 23

.56

0.07

0.07

0.05

0.05

95.3

8 71

.94

47.4

1 42

.13

32.4

5 26

.44

18.2

1 17

.62

5.11

4.54

3.90

3.57

39.8

7 35

.09

0.87

0.90

110

111

185

106

24

19

102

71

40

23

62

61

14

16

35

33

421

533

19

22

118

200

7 IO

21

12

34

5 41

5 19

13

1

2 0

0 5

4

0.37

0.45

22.1

6 24

.23

3.57

2.67

16.8

6 20

.00

0.30

0.17

35.0

6 26

.68

98.4

9 61

.84

17.7

5 11

.81

36.1

1 0.8

1 173

254 20

92

58

52

11

23

21

6 16

94 4 2

272 28

4 0 2

0.25

13.5

0 2.3

0 23

.50

0.24

21.6

8 83

.90

22.5

9

31.6

1 28

.40

0.79

1.08

193

53

195

63

22

8 74

31

59

13

59

62

12

16

16

62

24

6 43

5 13

17

78

19

0 2

11

9 7

230

662

27

6 3

5 2

0 4

12

0.15

0.65

18.9

2 25

.59

3.15

2.29

39.0

0 17

.27

0.21

0.33

28.4

4 17

.35

23.0

4 1

.Ol 28

57 9 21

14

46

18

47

291 18

12

4 11 7

491 6 9 0 17

0.61

16.1

7 2.3

5 11

.27

0.38

15.4

7

22.6

6 22

.33

1.01

1.06

22

19

39

48

0 4

12

18

23

11

42

41

17

17

43

35

292

300

17

19

126

116

10

12

21

12

467

442

6 8

9 10

2

0 17

16

0.59

17.1

8 2.3

2 12

.60

0.34

13.3

2

0.63

15.7

9 2.5

9 9.6

7 0.3

0 13

.44

154.

58

45.6

5 68

.11

110.

10

156.

65

144.

28

17.3

9 28

.20

44.5

1 46

.29

lTh/U

3.0

0 4.0

0 2.0

0 2.5

0 0.5

0 1.0

0 1.6

7 2.

331

Tabl

e1

.con

tinue

d.

Sam

ple

no.

178

154

175

177

189

184

185

173

163

182

171

151

186

164

160

153

Rock

typ

e Gr

anod

lont

es

Si02

71

.81

71.8

7 72

.36

72.3

9 72

.40

72.4

8 72

.86

72.9

6 73

.00

73.0

1 73

.02

73.0

9 73

.56

TiO,

0.26

0.

37

0.28

0.

28

0.23

AW3

14.7

2 14

.35

14.2

4 14

.47

14.5

9

Fe10

3*

2.00

2.

57

2.08

2.

19

1.81

MnO

0.06

0.

09

0.06

0.

07

0.06

MsO

0.63

1.

00

0.69

0.

69

0.62

Ca

O 2.

25

2.38

2.

48

2.43

2.

47

NazO

4.

66

4.39

4.

47

4.61

4.

44

K20

2.04

1.

27

1.93

1.

76

2.29

p2°5

0.

09

0.09

0.

09

0.09

0.

07

LOI

1.15

1.

19

1.14

0.

87

0.74

Su

m 99

.67

99.5

7 99

.82

99.8

5 99

.72

0.24

0.

23

0.19

0.

33

0.25

0.

24

0.25

0.

22

14.4

9 13

.95

14.2

7 13

.41

13.9

4 14

.23

14.4

7 14

.08

1.87

1.

86

1.48

2.

64

1.88

1.

77

1.58

1.

85

0.06

0.

06

0.06

0.

09

0.05

0.

05

0.04

0.

06

0.62

0.

57

0.48

0.

68

0.64

0.

59

0.64

0.

59

2.45

2.

27

1.96

1.

70

2.48

2.

29

1.77

2.

28

4.45

4.

48

4.44

4.

56

4.36

4.

45

4.93

4.

30

2.05

1.

75

2.62

2.

05

2.02

2.

20

1.99

2.

24

0.07

0.

08

0.06

0.

08

0.08

0.

08

0.09

0.

07

0.96

1.

18

0.76

0.

79

1.24

0.

59

0.89

0.

55

99.7

4 99

.29

99.2

8 99

.33

99.9

5 99

.51

99.7

4 99

.80

73.5

7 74

.26

0.31

0.

31

13.3

4 13

.11

2.47

2.

55

0.09

0.

06

0.69

0.

69

1.91

1.

90

4.75

4.

85

1.55

0.

94

0.08

0.

07

0.87

1.

11

99.6

3 99

.85

75.4

4

0.36

12.6

2

1.98

0.

05

0.70

1.

25

5.04

1.26

0.07

0.88

99

.65

Ms#

21.3

6 25

.12

AICN

K 1.

05

1.11

V Cl

co

Ni

cu

Zn

Ga

Rb

Sr

Y zr Nb

CS

Ba

SC

Th

19

37

54

53

3 6

22

29

16

4 38

62

17

14

34

31

30

8 31

3 20

11

13

1 12

6 11

8

26

11

448

357

5 10

8

13

0 2

13

10

U Pb

NblY

Sr

lY

SrlZ

r Zr

lNb

RblZ

r Ti

lZr

f. I I Kl

Zr

2 K/

P w ,_

ThlU

0.55

15

.40

2.35

11

.91

0.26

11

.90

129.

27

43.1

2

0.73

28

.45

2.48

15

.75

0.25

22.2

4 21

.37

1.02

1.

04

19

22

37

50

1 3

20

15

16

18

38

43

16

16

33

34

303

291

19

18

126

141

12

13

20

0 37

6 33

4 5

7 7

8 0

0 15

11

0.63

0.

72

15.9

5 16

.17

2.40

2.

06

10.5

0 10

.85

0.26

0.

24

22.8

0 1.

02 18

31

2 7 6

37

17

45

279 20

116 11

8

434

7

11 3

13

0.55

13

.95

2.41

10

.55

0.39

17

.60

13.3

2 11

.90

11.8

9

22.2

3 1.

04 17

90

2 36

14

39

17

36

292 21

118 12

9 43

8 3 8 0 12

0.57

13

.90

2.47

9.

83

0.31

12

.19

20.9

0 21

.86

1.04

1.

04

16

18

44

65

2 0

13

25

9 6

41

34

16

17

37

47

261

263

18

16

114

98

12

13

2 0

317

510

5 3

10

10

3 0

13

18

0.67

0.

81

14.5

0 16

.44

2.29

2.

68

9.50

7.

54

0.32

0.

48

12.1

0 11

.62

18.1

8 22

.69

22.3

3 25

.89

21.5

7 1.

05

1.01

1.

03

1.07

1.

03

17

18

21

14

20

149

48

32

77

55

4 4

5 3

5 61

17

9

27

21

11

3 3

17

12

42

37

35

33

36

13

16

16

15

17

34

40

48

40

42

159

275

280

297

271

43

22

19

8 17

14

5 12

7 11

3 12

5 11

5 11

11

11

8

10

19

17

5 5

9 64

6 29

1 47

4 56

5 48

8 11

5

4 2

4 11

9

11

11

9 0

0 0

0 0

9 14

13

12

14

0.26

0.

50

0.58

1

.oo

0.59

3.

70

12.5

0 14

.74

37.1

3 15

.94

1.10

2.

17

2.48

2.

38

2.36

13

.18

11.5

5 10

.27

15.6

3 11

.50

0.23

0.

31

0.42

0.

32

0.37

13

.64

11.8

0 12

.73

11.9

9 11

.47

83.6

7 12

7.15

10

3 62

16

3.88

14

4.22

12

7.43

22

1.93

11

7.36

13

2.04

16

1.62

13

2.16

16

1.69

26.8

4 40

.79

37.2

0 62

.23

55.7

1 41

.61

83.0

7 48

.75

48.0

3 52

.31

42.0

6 tj0

.87

19.4

1 18

.92

1.03

1.

05

22

21

153

140

3 6

57

52

15

10

47

21

13

12

24

22

181

210

42

41

151

146

11

10

8 25

58

3 37

0 5

7 11

12

2

3 8

11

0.26

0.

24

4.31

5.

12

1.20

1.

44

13.7

3 14

.60

0.16

0.

15

12.3

1 12

.73

85.2

1 53

.45

36.8

6 25

.55

23.3

6 1.

06 19

10

6 5 43

17

14

9 27

16

0 29

172 10

3 44

1 6 12

2 9

0.34

5.

52

0.93

17

.20

0.16

12

.55

60.8

1 34

.24

c.1

G

LOI:

Loss

on

ig

nitio

n;

Fe,O

,: to

tal

Fe

as

ferri

c; M

g#:

lOOx

(MgO

/MgO

+FeO

*);

A/CN

K:

AI,O

,/(Na

,O+K

,O+C

aO)

mol

ar.

S. M. EL-SHAZL Y and M. M. EL -SA YED

Table 2. Rare earth element concentrations for the rock association in the Wadi El-Bula area, central Eastern Desert, Egypt

ISample no. I 105 971 188 1921 169 1531 Rock type Basic metavolcanrcs Tonahtes 1 Granodlorltes La 3.32 5.991 13.50 16.501 16.10 17.90

10.50 8.21 2.72 0.93 3.07 3.18 0.69 1.83 0.24 1.34 0.18

36.21

28.10 15.60

3.09 0.64 2.97 2.49 0.45 0.84 0.27 1.05

0.15 69.15

tonalites are of metaluminous character (A/CNK < I), except for one sample which has a peraluminous affinity (Table 1). On the other hand, the granodiorites have a slightly peraluminous affinity (A/CNK< 1.1). Generally, A/CNK values are < 1.25 in the El-Bula tonalite-granodiorite association characterising calc- alkaline l-type granitoids (White and Chappell, 1977). Also, the El-Bula granitoids have mineralogical and geochemical characteristics of talc-alkaline l-type granites as defined by Chappell and White (1974). These diagnostic characteristics include: presence of hornblende, biotite, magnetite and titanite phases, relatively high Na,O (normally> 3.2 wt%), talc-alkaline affinity, metaluminous to slightly peraluminous composition and < 1% normative corundum.

Most of the major and trace elements of the tonalitic samples show no clear covariations when plotted against SiO, (Fig. 4). However, MgO, CaO and Ni exhibit a slightly negative correlation with SiO, (Fig. 4). Differences in the proportion of modal amphibole among tonalite samples are indicated by rather variable TiO, (0.34-0.89 wt%) and CaO (2.55-6.53 wt%) contents. The granodiorites, on the other hand, show a very narrow range in both major and trace elements (Fig. 4). However, AI,O,, CaO and Sr show slightly negative correlations with SiO, (Fig. 4).

Chondrite-normalised REE patterns of the El-Bula tonalites and granodiorites are shown in Fig. 6. There is uniformity in the REE patterns for the analysed El-Bula tonalites and granodiorites. The El-Bula granitoids are characterised by fractionated LREE [(La/Sm)” = 3.21, on average, for the tonalites and 3.55, on average, for the granodiorites], relatively flat HREE [(Gd/Lu), = 1.87, on average, for the tonalites and 0.98, on average, for the granodioritesl and an absent to very low negative Eu anomaly. LREE abundances are almost constant from the tonalites to granodiorites; whereas HREE

324 Journal of A frtcan Earth Sciences

34.00 16.20

3.15 0.71 2.34 2.55 0.45 1.47 0.26 148 0.21

7892

contents increase slightly from the tonalites to the granodiorites (Table 2).

DISCUSSION Tectonic environment The metavolcanics have major and trace element compositions similar to magma generated in island- arc and mid-ocean ridge settings. Trace elements, unlike many major elements, are often immobile during weathering and hydrothermal alteration. Therefore, the immobile trace elements can be used to constrain the tectonic setting in which lavas, and especially low-silica lavas, were erupted. The low Nb/Y ratios and Zr contents of the metavolcanic samples are reflected in the Nb-Zr-Y tectonic setting diagram of Meschede (19861, in which the metavolcanics plot in the field of volcanic-arc and mid-ocean ridge basalts (MORB) (Fig. 7). On the Ti versus V diagram (Shervais, 19821, it is clearly shown that the metavolcanic samples plot in the MORB and back-arc field (Fig. 8). Normal-MORB (N-MORB) have a Zr/Nb ratio of - 30, while enriched-MORB (P-MORB) have a Zr/Nb ratio of around 10 (Jones et a/., 1993). The El-Bula metavolcanics have Zr/Nb ratios (38, on average) similar to N-MORB. Moreover, the metavolcanics have Sr/ Zr ratios around 2 (Table I) resembling MORB (Hawkins and Melchior, 1985). A more detailed analysis can be made by use of multi-element diagrams, in which a series of elements are normalised against some suitable standard, In this case primitive mantle. Primitive mantle normalised plots can reveal tectonic regimes for both mafic and felsic igneous rocks, although interpretation of felsic rock trace element patterns can be complicated by the removal of mineral phases during partial melting or via crystal fractionation (Whalen et al., 1996). On the primitive mantle-normalised multi-element diagram (Fig. 9), the


2.0 - 0

0 1.6 -go o.

TiO, wt% 4 0

1.2 -

0.8 A - A

0.4 - A

0.0

18

16

14

12

10

AI,O, wt%

A

-4% o* A

X

16 -

eoQ% Fe,O, wt% 12 4

00

a - A

a 4 -

0

10

8 -O MgO wt%

0

6- 0 0 4

4- Be AA A

A

2 -

O-

a , Na,O+K,O wt% A

6 - A xx

A

4- 4% A

d 2 -

0

12

10 -**o.

a- ape*

6 -

4 -

2 -

CaO wt%

45 55 65 75

250, I

NI pm

AA

0 0 mm 300 -*a

A

200 - 0 AA

BO % loo - A X

0 A 0 &,

Y pm

800. Ba pm A

wo-

400 - A

A

200 - sao %

00 0

0 O 45 55 65 75

Figure 4. Major and trace element variation diagrams for the El-Bula granitoids and associated metavolcanics. 0: metavolcanics; A : tonalites; X : granodiorites.

Journal of African Earth Sciences 325


1’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ 1

La Ce Nd Sm Eu Gd Dy Ho Er Tm Yb Lu

Figure 5. A chondrite-normalised REE diagram for the metavolcanics of the El-Bula Suite. Normalisatlon values are after Haskin et al. 119681.

100 t I

-A- GD-169

-X-GD-153

La Ce Nd Sm Eu Gd Dy Ho Er Tm Yb Lu 1

Figure 6. A chondrite-normalised REE diagram for the El- Bula tonalites and granodiorites. Normalisatlon values are after Haskin et al. 119681.

LILE (large ion lithophile elements) (Rb, Ba and K) for the metavolcanics are markedly enriched above the level of primitive mantle, as well as the level of the immobile HFSE (high field strength elements). This enrichment is a typical feature of subduction-related magmas (Pearce, 1982, 1983). Also, the negative Nb anomaly, except for sample BA-97, is another feature of subduction-related magmatism for the El- Bula metavolcanics. Hence, the El-Bula metavolcanics are characterised by both arc-like and MORB-like affinities, suggesting a back-arc tectonic environment (Shervais, 1982).

Granitoids are classified according to their tectonic setting into ocean ridge granites (ORG), volcanic-arc granites (VAG), within-plate granites (WPG) and collision granites (COLG), which are further subdivided into syn-collision and post-collision granites (Pearce et al., 1984). Projected on Nb-Y and Rb-(Y + Nb) discriminant diagrams (Fig. IOa, b), the El-Bula granitoids fall into the VAG + Syn-COLG field of the

Zr14 Y 1 Figure 7. A Zr-Y-N6 diagram for the El-Bula metavolcanics. Al: within-plate alkali basalts; All: within-plate alkali basalts and within-plate tholeiites; B: E-type MORB; C: within-plate tholeiites and volcanic-arc basalts; D: N-type MORB and volcanic-arc basalts (after Meschede, 19861.

Nb-Y diagram and into the VAG field of the Rb-(Y + Nb) diagram. On the primitive mantle-normalised multi- element diagram (Fig. 91, the tonalites and granodiorites are characterised by enrichment of LILE (e.g. K, Rb and Ba ) relative to HFSE (e.g. Nb, Zr, Y and Yb) with negative Nb anomalies, suggesting a subduction- generated magma environment (Pearce et al., 1984). Therefore, the El-Bula granitoids can be regarded as belonging to the volcanic-arc granites.

Evolution of the El-Bula magma suite The relative ages of the El-Bula metavolcanics and syn-erogenic granitoids are unknown. However, the Shadli metavolcanics, which resemble the El-Bula metavolcanics, yield an age of 842 f 22 Ma (whole rock Rb/Sr system) (El-Shazly et al., 1973). On the other hand, the syn-erogenic El-Mia Granodiorite, south of the El-Bula granitoids, and the syn-erogenic Mons Claudianus Granodiorite, just north of the El-Bula granitoids, yield ages of 674 f 13 Ma and 693 f 2 1 Ma, respectively (Stern and Hedge, 1985). The large age difference between the older metavolcanics and the syn-erogenic granitoids pre-elude the presence of a genetic link between the two rock units through fractional crystallisation processes. Several lines of geochemical evidence suggest that the El-Bula metavolcanics and granitoids can not be related by fractional crystallisation. A group of magmas of a uniform source, related by fractional crystallisation, exhibit changes in the concentrations of two specific, strongly incompatible, trace elements in successive liquids, while the ratio of those elements remains

326 Journal of African Earth Soences

Petrogenesis of the Pan-African El-B&a Igneous Suite, central Eastern Desert, Egypt

600 , I

500

400

300

200

100

0

MORE C Back-arc basin bat&s

0 5 10 15 20

Till000 1 Figure 8. A V versus Ti diagram (after Shervais, 1982) for the metavolcanics, showing their plot in the MORE? and back-arc field.

loo0 ~ 00

10

*e-105 3 2 1

I

-O- BA-97 Y +TO-166 . -G-TO-192

, , ,o, , , , , , , ,

+GD-169 +GD-153

0.1’ ’ ’ ’

Rb Ba K Nb La Ca Nd Sr Zr Sm Eu Gd DY Y Yb Ti SC

Figure 9. A primitive mantle-normalised multi-element diagram for the El-Bula tonalites, granodiorites and associated metavolcanics. Normalisation values are after Hofmann (1988).

a)

100

10

1

b) 1000

100

10

1

1 10 100 1000 1 10 100 Y bpm)

1000 Y + Nb

Figure 10. El-Bula granitoids plotted on the Nb-Y (a) and Rb-(Nb+ Y) (b) discrimination diagrams of Pearce et al. (1984). Syn-COLG: syn-collision granites; VAG: volcanic-arc granites; WPG: within-plate granites; ORG: ocean ridge granites. Symbols are the same as in Fig. 4.


S.M. EL-SHAZLYandM.M. EL-SAYED

0.8 I I

01 I /

10 20 30 40 Mg# I

Figure 11. A CaO/AI,O, versus Mg# binary diagram for the El-Bula metavolcanics and granitoids. The diagram shows two independant fractionation paths for the metavolcanics and the granitoids. Symbols are the same as in Fig. 4.

constant. The large difference in the incompatible element ratios between the El-Bula metavolcanics and granitoids (Table 1) suggests there is no relation through fractionation. A diagram of CaO/AI,O, versus Mg# for the El-Bula metavolcanics and granitoids (Fig. 1 1) shows two possible fractionation paths for these rocks. There is no evidence that the metavolcanics have produced the granitoid rocks through fractionation. It is more likely that the metavolcanics and granitoids have been evolved by fractional crystallisation from two separate parental liquids.

The metavolcanics are used in this study to investigate the composition and variability in the primitive member of the El-Bula Suite. Rocks representing melts in equilibrium with mantle olivine (primitive magmas) without fractionation after their separation from their mantle source should have Mg# [l OOMg/(Mg + Fe)] in the range 68-75 (Roeder and Emslie, 19701, with high Ni and Cr contents (> 250 ppm Ni and > 1000 ppm Cr: Perfit et al., 1980; Wilson, 1989). According to the low Ni (100 ppm, on average) and Cr (200 ppm, on average) contents with low and variable Mg# (22-381, none of the present metavolcanic samples represent primary mantle- derived melts. Thus, the absence of true primitive lava suggests that the examined metavolcanics are not a direct partial melt of mantle source. Because the metavolcanics are compositionally evolved, it is necessary to examine whether crystal fractionation processes following magma generation can account for some of the observed chemical variatrons. The metavolcanics show evidence for extensive crystal fractionation of ferromagnesian minerals, indicated by the low Mg# and the low concentration of compatible transition metals (e.g. Ni and Cr), in spite of the low SiO, contents (Table 1). The behaviour of most major and trace elements is qualitatively

consistent with the hypothesis that the metavolcanic samples are related via fractionation. On Harker diagrams (Fig. 41, the steep fall of MgO and Ni with increasing differentiation indicates significant fractionation of hornblende and pyroxene. CaO decreases with increasing silica (Fig. 4) as anticipated from pyroxene and plagioclase removal. Moreover, A&O, and Cr display compatible trends (Fig. 4) for the metavolcanics to suggest fractionation of plagioclase and pyroxene, respectively. TiO,, Fe,O, and V are moderately compatible in titanomagnetite, so the crystallisation of this phase is likely to be the most important control on their variation. TiO,, Fe,O, and V define convex arrays when plotted against SiO, (Fig. 41, which suggests that these elements were relatively less compatible during the earlier stages of magma evolution compared with the later stages. The variation in the Ti/Zr ratio (Table 1) within the range of the metavolcanrc samples, decreasing in more evolved samples, reflects significant titanomagnetite fractionation in the late stage of evolution. This could reflect the change in the solid assemblage removed during fractionation history of the metavolcanics from pyroxene, hornblende and plagioclase during the early stages to pyroxene, hornblende, plagioclase and titanomagnetite during the later stages.

The sub-linear to linear relationship and the consistent relative degrees of fractionation between the tonalite and granodiorite on the Harker variation diagram (Fig. 4) and on CaO/AI,O, versus Mg# (Fig. 11) may be interpreted as implying a co-magmatic origin for the El-Bula granitoids. The linear trends can be interpreted in terms of either fractional crystallisation or restite unmixing. According to the restite unmixing hypothesis (White and Chappell, 1977; Chappell et a/. , 1987), straight line variations can best be explained by progressive separation of residuum and melt. Those authors (op. cit.) suggested that mafic enclaves, calcic cores to complex zoned and twinned plagioclase crystals with pyroxene inclusions and pyroxene cores of hornblende phenocrysts, represent restite. All these features, except mafic xenoliths in the marginal parts of the pluton, are not present in the El-Bula granitoids. Hence, it appears unlikely that the restite-unmixing model is responsible for the chemical variation within the El-Bula granitoids. It seems reasonable to consider an alternative fractional crystallisation process as an explanation for the compositional variations in the El-Bula tonalite- granodiorite association. The variation of Mg# in both tonalites and granodiorites of the El-Bula Suite (Table 1; Fig. 11) suggests that fractional crystallisation played a major role in the evolution of these rocks. The El-Bula tonalrtes and granodiorites have nearly constant Rb/Zr ratios (0.25 and 0.30, on average,



respectively) suggesting that these two rock types could have been related by fractional crystallisation (Allegre and Minster, 1978). In addition, fractional crystallisation can account for the compositional variation within each granitic type (tonalites and granodiorites).

The decrease of CaO and Sr, accompanied by a decrease in A&O, with increasing SiO, in the granodiorites (Fig. 4), suggests plagioclase fractionation. Moreover, the decrease of SrN values of the tonalite- granodiorite association with increasing SiO, (Table 1) is also attributed to plagioclase fractionation. A regular MgO, TiO, and Y decrease (Fig. 4) in the tonalites can be explained by hornblende and/or biotite fractionation (the main ferromagnesian phases), as the Fe-Ti oxides are very minor in these rocks. On the Rb/Sr versus Sr and Ba diagram (Fig. 12a, b), the chemical variations in the tonalites and granodiorites can be attributed to the fractional crystallisation of mainly plagioclase, K-feldspar and biotite in the melt with a minor role of the hornblende fractionation.

Chemical evidence implies that, although fractionation processes can account for much of the intergroup (metavolcanics and granitoids) chemical variations, crustal contamination is likely to be a minor factor in trace element variation of the El-Bula Suite. A number of geochemical parameters can be used to evaluate the role of contamination in the genesis of El-Bula Suite.

The K/P ratio for non-contaminated basalts ranges from 0 to 7 (Hart et al., 1989). The K/P ratio of the metavolcanics ranges from 2.8 to 5.11, except for sample no. 107 which has K/P= 80. This ratio points to uncontaminated basalts. Most Th/U ratios from the El-Bula metavolcanics are normally < 2.2 and fall within the uncontaminated basalts (Defant et al., 1991). However, there are some samples that have a high Th/U ratio (Table I), reflecting involvement of limited crustal material. The elevated LILE/HFSE ratios (K/Zr and Rb/Zr) in the granitoids and the presence of negative Nb anomalies in the spider diagram of the metavolcanics and granitoids (Fig. 9), except for sample no. 97, reflect some contribution of crustal material in their petrogenesis because the LILE are enriched with respect to Nb in the continental crust (Taylor and McLennan, 1985).

From the foregoing discussion, it is obvious that crustal contamination was minor and not significant enough to obscure the geochemical signature of the mantle sources during the genesis of the El-Bula Suite.

Mass balance implications The petrogenetic modelling of the El-Bula metavolcanics, using mantle composition as a source, is mainly based on the immobile elements such as REE contents, Ti, Ni, Y and Zr. The Low (La/Yb), for the

metavolcanics (Fig. 5) reflects that garnet was not residual in the source, suggesting generation by partial melting of an anhydrous spine1 lherzolite source (Takahashi and Kushiro, 1983). The melting models used are those of Nicholls et al. (1980), who dealt with possible mantle sources for tholeiitic and calc- alkaline magmas, with REE concentrations twice chondritic abundances for the mantle source (also assumed by Haskin et al., 1968; Thompson et al., 1984). For melting models, the batch melting equation of Shaw (1970) was applied.

The REE contents of the least evolved basaltic sample (no. 105) can be modelled by 25% batch melting of a spine1 lherzolite source (Ol,,, Opx,,, Cpx,,, Sp, modal%) followed by subsequent 35% fractional crystallisation of clinopyroxene, olivine and plagioclase in proportions of 45%, 30% and 25%, respectively. The calculated melt has REE contents closely similar to the observed REE contents of sample no. 105 (Table 3). The most evolved basaltic sample (no. 97) could be generated by 55% fractional crystallisation of the least evolved sample liquid (no.

a) l t

X X

L

so.1 : X

P’ag 3 xl Bi

r A A A

A 0.01

b) 1

Hb

1 B d

100 Sr pm

1 ooc

100 Ba pm

1ooc

Figure 12. Binary variation diagrams of Rb/Sr versus Sr (a) and Ba lb) for the El-B&a granitoids. The vectors represent crystallisation of mineral phases. The partition coefficients used are from Hanson (1978, 19801. Symbols are the same as in Fig. 4.


S.M. EL-SHAZL Y and M. M. EL-SA YED

Table 3. Results of REE geochemical batch melting modelling for the generation of the least evolved basic metavolcanics (sample no. 105)

La

Calculated liquid generated by 25% batch melting of spine1 lherzolite (Mj)

2.38

35% fractional crystallisation of Ml melt Observed daughter (45% Cpx, 30% 01 and 25% Plag) (sample no. 105)

(calculated daughter) 3.57 3.32

Ce 7.02 10.54 10.50

Nd 4.51 6.70 8.21

Sm 1.61 2.37 2.72

Eu 0.56 0.73 0.93

Gd 2.31 3.29 3.07

Ho 0.45 0.60 0.69

Tm 0.23 0.33 0.24

Yb 1.26 1.84 1.34

IL” 0.23 0.34 0.18

The pattltlon coefficients used are taken from different sources.

Table 4. Results of trace element geochemical fractional crystallisation modelling for the generation of the most evolved basic metavolcanics (sample no. 97)

Source Calculated melt derived by 55% Observed daughter (sample no. 105) fractionation (65% Hb, 30% Plag (sample no. 97)

and 5% mag) from the source La 3.32 6.43 5.99 Ce 10.50 18.54 17.30 Nd 8.21 13.43 10.70 Sm 2.72 3.57 3.06 Eu 0.93 1.05 1.09 DY 3.18 4.61 4.92 Ho 0.69 0.82 1.09 Er 1.83 2.65 3.07 Tm 0.24 0.51 0.47 Yb 1.34 1.91 2.65 Lu 0.18 0.27 0.35 Ni 200 122 73 Sr 186 241 234 Zr 87 138 212 Ba 86 165 203 K20% 0.35 0.58 0.45 Ti02% 1.46 1.26 1.49

The partition coeffrclents used are taken from different sources

105) mainly of hornblende, plagioclase and titano- finity (El-Shazly, inpress). It is worth mentioning that magnetite in the respective proportions: 65%, 30% fractional crystallisation and/or partial melting models, and 5%. The modelled melt and the observed rock using the Safaga metagabbro-diorite complex as a sample (daughter) are similar (Table 4). source, could not produce the El-Bula tonalitic liquid.

The geochemical characteristics of the Safaga metagabbro-diorite complex suggest melting of a lithospherical mantle source, which may be hydrated and enriched by subduction-related processes. The rocks have been injected in an island-arc environment and display a transitional tholeiitic to talc-alkaline af-

The compositional gap between the studied metavolcanics and tonalites is not compatible with fractional crystallisation, but probably can be attributed to other petrogenetic processes. The presence of mafic xenoliths hosted in the tonalite may reflect the role played by basic source (metavolcanics)



to generate the tonalitic melt. To test the contribution of batch melting for the generation of the tonalites, the non-modal batch melting equation of Shaw (I 970) was applied using the most evolved basaltic sample (no. 97) as the source. The results (Table 5) indicate a close match between calculated trace element values in the melt (F=30%) and those in the least silica tonalite sample (no. 188).

The fractional crystallisation hypothesis for the generation of the tonalites and granodiorites can be tested using the conventional least-squares mixing calculations devised by le Maitre (1981). The model has utilised whole rock major element abundances and microprobe analyses of the observed mineral phases. The results of the calculations are given in Table 6. The calculations for the evolution of the tonalites have used the most basic sample (no. 188) as the parent composition and the most evolved tonalitic sample (no. 192) as the daughter. A reasonable model can be achieved by about 46% crystallisation of hornblende (55%), plagioclase (44%)and K-feldspar (I %I, and with 54% residual liquid (daughter) (Table 6). The calculations show also that the least evolved granodioritic sample (no. 169) could be produced from the most evolved tonalitic sample (no. 192) from - 20% crystallisation of plagioclase, biotite, hornblende and K-feldspar phases in proportions of 54%, 43%, 2.6% and 0.4%, respectively (Table 6). The most evolved granodioritic sample (no. 153) could be produced from 28.4% fractionation of the least evolved granodioritic sample (no. 169). The fractionating phases are plagioclase, K-feldspar and biotite in proportions of 61%, 24% and 15%, respectively (Table 6). All the sums of the squares of the residuals of the three models are < 1.5 (1.2,0.23 and 1.28, respectively), which indicates a good fit (Luhr and Carmichael, 1980).

The genetic relationship between the tonalites and granodiorites by fractional crystallisation is further supported by using trace element modelling, applying

the results of major element models (the same fractionating phases and proportions). The calculations and results are given in Table 7. The calculated daughter has similar element contents to those of the observed daughter (sample no. 169; Table 7). For the tonalites evolution, *the calculated daughter contents are similar to the observed daughter (sample no. 1921, but the calculated daughter has slightly lower HREE, Y, Sr, Ba and Zr (Table 7). The low contents of HREE, Y and Zr may be due to the higher amount of hornblende fractionation, while the low contents of Sr and Ba reflect the higher amount of plagioclase fractionation. The absence of negative Eu anomalies in the studied granitoids assemblage may be due to low oxygen fugacity during its formation. Regarding the granodiorites evolution, the calculated granodioritic daughter has similar trace element contents to those of the observed daughter (sample no. 153; Table 7). Thus, the geochemical variations of the El-Bula tonalites can be modelled by plagioclase+ hornblende + K-feldspar fractionation while the granodiorites can be modelled by plagioclase + biotite + K- feldspar fractionation. The genetic relationship between the El-Bula granitoids and associated basic metavolcanics is summarised in Fig. 13.

CONCLUSION The Pan-African basement rocks cropping out in the El-Bula Suite are metamorphosed gneisses, metavolcanics, metagabbro-diorite complex and syn- erogenic intrusive older granitoids. The El-Bula granitoid rocks range in composition from tonalite to granodiorite. The contact between the granitoids and sur- rounding gneisses, metavolcanics and metagabbros is distinctly sharp and, in some places, the intrusive granitoids show varying degrees of interaction with the associated rocks.

The metavolcanics exhibit tholeiitic affinity and have relatively unfractionated REE patterns. They have a

Table 5. Results of trace element geochemical non-modal partial melting modelling for different degrees of melting (F)

Source Measured melt Calculated melt at: sample no. 97 (sample no. 188) F=25% F = 30% F=35% F=40%

co 28 24 17.45 17.22 17.00 16.78 SC 31 19 23.93 23.31 22.72 22.16 v 226 110 166.12 164.50 162.91 161.34 Rb 10 35 38.55 32.41 27.96 24.58 Sr 234 421 392.90 395.94 399.03 402.16 La 5.99 13.50 20.41 17.74 15.69 14.06 K20% 0.45 1.40 1.53 1.34 1.19 1.07

The partitlon coefficients used are taken from different sources.


S. M. EL -SHAZL Y and M. M. EL-SA YED

Table 6. Results of fractionation modelling for the El-Bula granitoids based on the method of le Maitre (1981)

Fractionation of tonalites (from sample no. 188 to sample no. 192) Fractionating phases Observed Observed Calculated

SiOz

Plagioclase K-feldspar Hornblende daughter

61.39 64.55 47.99 66.76 parent

61.14 parent

60.9:

TiOz 0.00 0.00 0.68 0.55 0.69 0.4'

AW3 23.30 17.56 6.15 15.61 15.21 14.8t

Fe203* 0.00 0.00 15.37 3.50 5.67 5.7(

MnO 0.00 0.00 0.39 0.06 0.09 0.1: MN 0.00 0.00 12.98 1.61 4.36 4.14 CaO 5.77 0.02 11.08 2.55 5.71 5.31 NazO 8.47 0.48 0.93 4.29 3.43 4.2'

K20 0.12 16.09 0.47 2.52 1.40 1.51

Parent (sample no. 188) = 54.03% daughter (sample no. 192) + 20.3% Plag. +0.48% K-feld. + 25.19% Hb

Fractionatrng phases (45.97%): 44.16% plagioclase, 1.04% K-feldspar and 54.8% hornblende

R2=1.2

Fractionation of tonalites (sample no. 192) to granodiorites (sample no. 169) Fractionating phases Observed Observed Calculated

Si02

Plagioclase K-feldspar Biotite Hornblende daughter parent parent

62.54 65.91 35.82 47.99 71.01 66.76 66.8:

Ti02 0.00 0.00 2.81 0.68 0.32 0.55 0.5(

A1203 22.41 17.28 14.63 6.15 14.87 15.61 15.6:

Fe203 * 0.00 0.00 19.56 15.37 2.42 3.50 3.7: MnO 0.00 0.00 0.94 0.39 0.07 0.06 O.ld MN 0.00 0.00 10.56 12.98 0.84 1.61 1.6( CaO 4.79 0.00 0.02 11.08 2.58 2.55 2.61

Na20 9.07 0.43 0.11 0.93 4.54 4.29 4.6:

K20 0.23 16.43 9.20 0.47 2.34 2.52 2.5(

Parent (sample no. 192) =79.63% daughter (sample no. 169) + 10.99% Plag. +0.09% K-feld. + 8.76% 81. +0.53% Hb

Fractronatrng phases (20.37%): 53.95% plagioclase, 0.45% K-feldspar, 43% biotite and 2.6% hornblende

R2 =0.23

Fractionation of granodiorites (from sample no. 169 to sample no. 153) Fractionating phases Observed Observed Calculated

SiO,

Plagioclase K-feldspar

62.54 65.91

Biotite

35.82 daughter

75.44 parent

71.01 parent

70.91 TiO, 0.00

A1203 22.41 Fe203 * 0.00 MnO 0.00 MN 0.00 CaO 4.79 Na20 9.07

0.00 2.81 0.36 0.32 0.3‘ 17.28 14.63 12.62 14.87 14.7:

0.00 19.56 1.98 2.42 2.2: 0.00 0.94 0.05 0.07 0.0' 0.00 10.56 0.70 0.84 0.91 0.00 0.02 1.25 2.58 1.7: 0.43 0.11 5.04 4.54 5.2:

K20 0.23 16.43 9.20 1.26 2.34 2.41

Parent (sample no. 169) =71.6% daughter (sample no. 153) + 17.49% Plag. +6.78% K-feld. +4.13% Hb

Fractionating phases (28.4%): 61.59% plagioclase, 23.87% K-feldspar and 14.54% biotrte

R2 = 1.28


Tabl

e 7.

Res

ults

of

trace

el

emen

t ge

oche

mic

al

mod

ellin

g fo

r th

e ev

olut

ion

of t

he E

l-Bul

a gr

anito

ids

I Fr

actio

natio

n of

tonali

tes

45%

FC

(4

4.16

%

Plag

. +

1.04

%

K-fe

ld.

+54.

8%

Hb)

Obse

rved

Ob

serv

ed

Calcu

late

d

Pare

nt

daug

hter

da

ught

er

La

Sam

ple

no.

188

Sam

ple

no.

192

13.5

0 16

.50

16.4

0 Ce

Nd

Sm

Eu

Gd

Dv

Er

Yb

Lu

Y Rb

Sr

Ba

Nb

28.1

0 35

.60

28.8

9 15

.60

17.8

0 11

.33

3.09

3.

45

1.62

0.

64

0.63

0.

35

2.97

2.

61

0.97

2.

49

2.25

0.

66

0.84

1.

15

0.24

1.

05

1 .o

o 0.

50

0.15

0.

14

0.09

19

.00

17.0

0 9.

27

35.0

0 62

.00

62.1

3 42

1 .O

O

435.

00

276.

50

345.

00

662.

00

529.

88

7.00

11

.oo

4.

52

Zr 11

8.00

19

0.00

74

.96

The

parti

tion

coef

ficie

nts

used

ar

e ta

ken

from

differ

ent

Frac

tiona

tion

of ton

alites

to

gran

odior

ites

Frac

tiona

tion

of gr

anod

iorite

s 20

%

FC

(43%

8i

. +5

3.95

%

Plag

. +0

.45%

K-

feld

. +2

.6%

Hb

) 30

%

FC

(61.

59%

Pl

ag.

+23.

87%

K-

feld

. +

14.5

4%

8i.I

Obse

rved

Ob

serv

ed

Calcu

late

d Ob

serv

ed

Obse

rved

Ca

lcula

ted

Pare

nt

daug

hter

da

ught

er

Pare

nt

daug

hter

da

ught

er

Sam

ple

no.

192

Sam

ple

no.

169

Sam

ple

no.

169

Sam

ple

no.

153

La

16.5

0 16

.10

18.2

5 La

16

.10

17.9

0 20

.27

Ce

35.6

0 34

.00

39.2

2 C

e 34

.00

42.0

0 43

.33

Nd

17.8

0 16

.20

19.4

6 Nd

16

.20

18.9

0 20

.86

Sm

3.45

3.

15

3.72

Sm

3.

15

3.57

4.

10

Eu

0.63

0.

71

0.57

Eu

0.

71

0.68

0.

60

Gd

2.61

2.

34

2.71

Gd

2.

34

3.31

3.

00

Dv

2.25

2.

55

2.50

Dy

2.

55

3.84

3.

52

Er

1.15

1.

47

1.28

Er

1.

47

2.50

1.

96

‘Yb

1 .oo

1.

48

1.10

Yb

1.48

2.

59

2.03

Lu

0.

14

0.21

0.

16

LU

0.21

0.

37

0.29

Y

17.0

0 18

.00

17.6

6 Y

18.0

0 29

.00

23.0

8 Rb

62

.00

47.0

0 56

.69

Rb

47.0

0 27

.00

54.2

1 Sr

43

5.00

29

1 .O

O

335.

99

Sr

29 1

.oo

160.

00

132.

79

Ba

662.

00

491.

00

493.

41

Ba

491

.oo

441

.OO

32

5.32

Nb

11

.00

1 1

.oo

10.7

3 Nb

11

.00

10.0

0 11

.11

Zr 19

0.00

12

4.00

19

7.80

Zr

124.

00

172.

00

157.

47

Jrces

.

S. M. EL -SHAZL Y and M. M. EL-SA YED

30% fractional crystallisation (62% Plag., 24% K-feld., 14% Bi.)

*I

t 20% fractional crystallisation

(54% Plag., 43% Bi., 2.5% Hb., 0.5% K-feld.)

61


(55% Hb., 44% Plag., 1% K-feld.)

I

Least evolved tonalitic sample (sample no. 188)

t 30% batch melting


(65% Hb., 30% Plag., 5% Mag.)

I I / Least evolved

basaltic sample (sample no. 105)

+ 25% batch melting followed by 35%fractional

crystallisation (45% Cpx, 30% Ol., 25% Plag.) I

Spine1 lherzolite (q,, CPX,,, CPX,,, Sp,)

composition that is comparable with that of both MORB-like and arc-like affinity and might represent a back-arc setting. The metavolcanics do not represent a primitive magma, but they were generated from upper mantle by the melting of spine1 lherzolite followed by the fractional crystallisation of pyroxene, olivine, plagioclase, hornblende and titanomagnetite.

The syn-erogenic talc-alkaline tonalite-granodiorite assemblage exhibits a metaluminous (tonalite) to slightly peraluminous (granodiorite) character, and the geochemical characteristics resemble the l-type granites generated in a volcanic-arc environment. The El-Bula granitoids have fractionated LREE and unfractionated HREE. The least evolved tonalitic rocks could be generated by - 30% batch melting of the metavolcanics. The studied tonalites and granodiorites represent a cogenetic sequence related by fractional crystallisation.

ACKNOWLEDGEMENTS

The authors would like to thank Prof. E. Seidel, who kindly made available facilities to carry out the chemical and microprobe analyses at the Mineral- ogical Institute of Kijlne University, Germany. They are also indebted to Dr M.A. Hassanen and Dr A.M. Moghazi for their help during the fieldwork. The writers are grateful to the reviewer for his con- structive remarks. Editorial handling - K. Walton and J. Walden

REFERENCES Abdel-Rahman, A.M., 1996. Pan-African volcanrsm: petrology

and geochemrstry of the Dokhan volcanic surte in the northern Nubian Shreld. Geologrcal Magazine 133 (I), 17-31.

Akaad, M.K., El-Gaby, S., Habib, M.E., 1973. The Barud gnersses and the origin of grey granrte. Bulletin Faculty Science, Assrut University 2 (l), 55-69.

Akaad, M.K., Nowetr, A.M., 1980. Geology and lrthostratrgraphy of the Arabtan Desert orogenrc belt of Egypt between Lat. 25”35’ and 26”30’. In: Coaray, P.G., Tahoun, S.A. (Eds.), Evolutron and Mmeralizatron of the Arabian Nubran Shreld. Institute Applied Geology Jeddah Bulletm 3, Pergamon Press 4, pp. 127-134.

Allegre, C.J., Mmster, J.F., 1978. Quantrtatrve models of trace-element behavrour in magmatic processes. Earth Planetary Scrence Letters 38, 1-25.

Brckford, M.E., Sultan, M., Arvidson, R.E., El-Kalrouby, B., 1989. Evrdence for Involvement of pre-Late Proterozoic crust In the Nubran Shield, Egypt: Common Pb data from K-feldspars. Geologrcal Socretv American Abstracts Programs 6 (21), A24.

Chappell, B.W., White, A.J.R., 1974. Two contrasting granrte types. Pacrfrc Geology 8, 173-174.

Chappell, B.W., Whrte, A.J.R., Wyborn, D., 1987. The Importance of residual source material (restate) In granite petrogenesrs. Journal Petrology 28, 1 11 l-l 138.

Figure 13. Schematic diagram showing the genetic relationship of the El-&la Suite.



Debon, F., le Fort, P., 1983. A chemrcal-mineralogical classificatron of common plutonic rocks and associatrons. Transactions Royal Society Edrnburgh Earth Science 73, 135-149.

Defant, M.J., Clark, L.F., Stewart, R.H., Drummond, M.S., de Boer, J.Z., Maury, R.C., Jackson, T.E., Restreps, J.F., 1991. Andesites and dacrte genesis via contrasting processes: the geology and geochemistry of El Valle volcano, Panama. Contributions Mineralogy Petrology 106, 309-324.

El-Gaby, S., 1975. Petrochemistry and geochemistry of some granites from Egypt. Neues Jahrbuch Mineralogre Abhandlungen 124, 147-189.

El-Gaby, S., Habib, M.E., 1982. Geology of the area southwest of Port Safaga, with special emphasis on the granitic rocks, Eastern Desert, Egypt. Annals Geological Survey Egypt XII, 47-71.

El-Gaby, S., List, F.K., Tehrani, R., 1988. Geology, evolution and metallogenesis of the Pan-African Belt In Egypt. In: El-Gaby, S., Greilling R.D. (Eds.), The Pan- African Belt of The North East Africa and Adjacent Area. Vreweg Verlag, Wiesbaden, pp. 17-68.

El-Ramly, M.F., 1972. A new geological map for the basement rocks in Eastern Desert and southwestern Desert Egypt. Annals Geological Survey Egypt 2, 1-18.

El-Shazly, S.M., in press. Geochemical evidence for AFC processes in the petrogenesis of the Safaga intrusive complex, Eastern Desert, Egypt. Journal African Earth Sciences.

El-Shazly, E.M., Hashad, A.H., Sayyah, T.A., Bassyuni, F.A., 1973. Geochronology of Abu Swayel area, South Eastern Desert, Egypt. Egyptian Journal Geology 17, l-l 8.

Gass, I.G., 1977. Evolution of the Pan-African crystallrne basement in north-east Africa and Arabta. Journal Geological Society London 134, 129-l 38.

Gillespie, J.G., Dixon, T.H., 1983. Lead isotope systematics of some igneous rocks from the Egyptran Shield. Precambrian Research 20, 63-77.

Hanson, G.N., 1978. The application of trace elements to the petrogenesrs of igneous rocks of granitic composition. Earth Planetary Science Letters 38, 26-43.

Hanson, G.N., 1980. Rare earth elements in petrogenetrc studies of igneous systems. Annual Revrew Earth Planetary Sciences 8, 371-406.

Hart, W.K., Woldegabrrel, G., Walter, R.C., Mertzman, S.A., 1989. Basaltic volcanism in Ethiopia: constraints on continental rifting and mantle Interactions. Journal Geophysical Research 94, 7731-7748.

Hashad, A.H., 1980. Present status of geochronologrcal data on the Egyptran basement complex. Institute Applred Geology, Jeddah Bulletin 3, 31-46.

Haskin, L., Haskin, M.A., Frey, F.A., Wrldeman, T.R., 1968. Relative and absolute terrestrial abundances of the rare earths. In: Ahrens, L.H. (Ed.), Origin and Distribution of the Elements. Pergamon Press, New York, pp. 889-912.

Hassan, M.A., Hashad, A.H., 1990. Precambrian of Egypt. In: Said, R. (Ed.), The Geology of Egypt. Balkema, Rotterdam, pp. 201-245.

Hassanen, M.A., El-Nisr, S.A., Mohamed, F.H., 1996. Geochemistry and petrogenesis of Pan-African l-type granrtoids at Gabal lgla Ahmer, Eastern Desert, Egypt. Journal African Earth Sciences 22 (11, 29-42.

Hawkins, J.W., Melchior, J.T., 1985. Petrology of Mariana Trough and Lau Basin basalts. Journal Geophysical Research 90 (13). 11,431-l 1,468.

Hofmann, A.W., 1988. Chemical differentiation of the Earth: the relationship between mantle, continental crust, and oceanic crust. Earth Planetary Science Letters 90, 297-314.

Irvine, T.N., Baragar, R.A., 1971. A guide to the chemical classrfrcation of the common volcanic rocks. Canadian Journal Earth Sciences 8, 523-548.

Jones, G., Sano, H., Valsami-Jones, E., 1993. Nature and tectonic setting of accreted basalts from the Mino terrane, central Japan. Journal Geological Society London 150, 1167-1181.

Khalil, E., 1997. Ocean-arc to continental margin volcanics sequences of the Pan-African Precambrian belt, Egypt: geochemrcal approach. Egyptian Journal Geology 41 (2B). 477-528.

Khalrl, K.A., Sadek, M.F., Benjamin, N.Y., Hawkins, M.P., 1993. Geochemistry of the Zerdun-El-Miyah volcanics, Central Eastern Desert, Egypt. Annals Geological Survey XIX, 79-96.

Le Maitre, R.W., 1981. GENMIX-A generalized petrological mixing model program. Computers Geoscience 7, 229- 247.

Le Maitre, R.W., Bateman, P., Dudek, A., Keller, J., Lameyer, J., LeBas, M.J., Sabine, P.A., Schmid, R., Sorensen, H., Streckeisen, A., Woolley, A.R., Zanettin, B., 1989. A Classification of Igneous Rocks and Glossary of Terms: Recommendatrons of the International Union of Geologtcal Sciences Subcommission on the Systematics of Igneous Rocks. Blackwell Scientific Publications, Oxford, 193p.

Luhr, J.F., Carmichael, I.S.E., 1980. The Colima volcanic complex, Mexico. Contrrbutrons Mineralogy Petrology 71, 343-372.

Meschede, M., 1986. A method of discriminating between different types of mid-ocean ridge basalts and continental tholeiites with the Nb-Zr-Y diagram. Chemical Geology 56, 207-216.

Moghazi, A.M., 1987. Petrochemical and geochemical studies of Wadi Ball Dokhan volcanics, Eastern Desert, Egypt. M.Sc. thesis, Alexandria University, Egypt, 194p.

Nicholls, I.A., Whitford, D.J., Harris, K.L., Taylor, S.R., 1980. Variation in the geochemrstry of mantle sources

for tholeritrc and talc-alkaline mafic magmas, western Sunda volcanic arc, Indonesia. Chemical Geology 30, 177-199.

Pearce, J.A., 1982. Trace element characteristics of lavas from destructive plate boundaries. In: Thorpe, R.S. (Ed.), Andesites, Orogenrc Andesrtes and Related rocks. Wiley and Sons, Chichester, pp. 525-548.

Pearce, J.A., 1983. The role of sub-continental lithosphere In magma genesis at destructive plate margms. In: Hawkesworth, C.J., Norry, H.J. (Eds.), Continental Basalt and Mantle Xenolrths. Nantwrch, Shiva, pp. 230-249.

Pearce, J.A., Gale, G.H., 1977. ldentifrcatron of ore- deposition environments from trace-element geochemistry of associated igneous host rocks. Journal Geological Society London 7, 14-24.

Pearce, J.A., Harris, N.B., Tindle, A.G., 1984. Trace element discrimination diagrams for the tectonic interpretation of granitic rocks. Journal Petrology 25, 956-983.

Perfit, M.R., Gust, D.A., Bence, E., Arculus, R.J., Taylor, S.R., 1980. Chemical characteristics of island arc basalts: implrcation for mantle source. Chemical Geology 30, 256-277.

Roeder, P.L., Emslie, R.F., 1970. Olivine-liqutd equilibrium. Contributions Mineralogy Petrology 29, 275-289.

Shaw, S.M., 1970. Trace element fractionation during anatexis. Geochemica Cosmochemica Acta 34, 237-243.

Shervars, J.W., 1982. Ti-V plots and the petrogenesis of modern and ophiolitrc lavas. Earth Planetary Science Letters 59, 101-118.



Stern, R.J., 1981. Petrogenesis and tectonic setting of Late Precambrran ensimatic volcanic rocks, Central Eastern Desert of Egypt. Precambrran Research 16, 195-230.

Stern, R.J., Hedge, C.E., 1985. Geochronologic and isotopic constramts on Late Precambrian crustal evolution in the Eastern Desert of Egypt. American Journal Science 285, 97-l 27.

Sultan, M., Chamberlam, K.R., Bowring, S.A., Arvrdson, R.E., Abuzied, H., El-Kalrouby, B., 1990. Geochronologrc and isotopic evrdence for involvement of pre-PanAfrIcan crust rn the Nubran Shield, Egypt. Geology 18, 761- 764.

Takahashi, E., Kushiro, I., 1983. Meltrng of a dry pendotrte at high pressures and basalt magma genesrs. American Mmeralogists 68, 859-879.

Takla, M.A., Khalaf, I.M., Helmers, H., Al-Boghdady, A.A., 1997. The shteld rocks of Wad1 Safaga area, Central Eastern Desert, Egypt. In: Third Conference on Geo-chemrstry, Alexandria Umversrty 1, pp. 17-31,

Taylor, S.R., McLennan, S.M., 1985. The continental crust: Its composition and evolution. Blackwell Scientific, Oxford, 312p.

Thompson, R.N., Mornson, M.A., Hendry, G.L., Parry, S.J., 1984. An assessment of the relatrve roles of crust and mantle in magma genesis: an elemental approach. Philoso- phical Transactions Royal Society London A310, 549-590.

Whalen, J.B., Jenner, G.A., Longstaffe, F.J., Robert, F., Ganepy, C., 1996. Geochemrcal and isotoprc (0, Nd, Pb and Sr) constrarnts on A-type granrte petrogenesrs based on the Topsails igneous suite, Newfoundland Appalachians. Journal Petrology 37 (69), 1463-1489.

White, A.J.R., Chappell, B.W., 1977. Ultrametamorphism and granitord genesrs. Tectonophysrcs 43, 7-22.

Wrlson, M., 1989. Igneous Petrogenesrs. Unwon Hyman, London, 457~.

Winchester, J.A., Floyd, P.A., 1976. Geochemical magma type drscrrmrnatron: applrcatron to altered and metamorphosed basrc igneous rocks. Earth Planetary Science Letters 28, 459-469.


petrogenesis of the pan-african el-bula igneous suite, central

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