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Mineralogy and Petrology (2004) 82: 81–104DOI 10.1007/s00710-004-0052-6
Continental back-arc basin originof some ophiolites from the EasternDesert of Egypt
E. S. Farahat1, M. M. El Mahalawi1, G. Hoinkes2,and A. Y. Abdel Aal1
1 Department of Geology, Minia University, EL-Minia, Egypt2 Institute of Mineralogy and Petrology, Karl-Franzens University, Graz, Austria
Received May 2, 2003; revised version accepted June 17, 2004Editorial handling: E. F. Stumpfl
Summary
Geochemical and petrographical data of three ophiolitic pillow metavolcanicoccurrences from the central Eastern Desert of Egypt are presented. The investigatedrocks show a subalkaline, tholeiitic affinity. Chemical data indicate that themetavolcanics have transitional within-plate basalt to island-arc basalt features, whichare characteristics of basalts formed in ensialic back-arc basins. The association of theinvestigated ophiolites with volcanoclastic metasedimentary rocks of marine tocontinental facies is a further confirmation of their ensialic evolution. This suggestion,along with the geochronologic, isotopic and crustal growth rate evidences, revivesinterest in models that involve contribution from a pre-Pan-African continental crust atleast in the southern part of the Egyptian Shield. Mixing between a depleted mantle-derived magma and an enriched crustal melt, a process similar to AFC (assimilation andfractional crystallization), is suggested for the evolution of the investigated rocks. Thisstudy provides evidence for formation of some ophiolites in the Eastern Desert of Egyptin continental (ensialic) back arc basins.
Introduction
The basement complex in the Eastern Desert of Egypt is a part of the Arabian-Nubian Shield (ANS) that extends from Saudi Arabia and Egypt in the north, tothe Sudan, Ethiopia and Uganda-Kenya in the south (Fig. 1a). The ANS consistsessentially of dismembered ophiolite suites, island-arc volcano-sedimentaryassociations, and calc-alkaline granitoids. The occurrence of ophiolites and theirassociation with calc-alkaline rocks of island-arc affinity has led to the conclusion
that the continental crust in this area formed by microplate accretion related tosubduction processes and collision tectonics (El Gaby et al., 1988; Stern, 1981;Kr€ooner et al., 1988, 1992). These processes took place during the Pan-Africanorogenic event between 900 and 614 Ma (Stern and Hedge, 1985; Beyth et al.,1994; Stern, 1994). Large masses of volcanic rocks (Dokhan volcanics) and alka-line granites penetrated this crust during the post-orogenic stage (614–550 Ma).
Metavolcanics with pillow structures were reported as part of ophiolitic succes-sions in several localities in the Eastern Desert (e.g. Dixon, 1981; Nasseef et al.,1980; Ries et al., 1983; Shackleton, 1994). These ophiolites are commonly dismem-bered. In addition, variably sized fragments of ophiolitic rocks are frequent in thewidespread Eastern Desert m�eelange and metasediments. Geochemically, the ophio-litic metavolcanic rocks have strong affinities to marginal basin tholeiites (Stern,1981; Berhe, 1990). Marginal basins may be of fore-arc, inter-arc or back-arcsetting. The majority of the world’s back-arc basins are strictly oceanic, thoughrifting of continental lithosphere also produces back-arc basins (i.e. ensialic back-arc basins). Back-arc basin basalts have been extensively studied during the lastseveral years because back-arc basin magmatism provides valuable constraints onthe geotectonic processes (e.g., Gribble et al., 1998; Shinjo et al., 1999; Xu et al.,2003).
This paper presents detailed geochemical data for three well-preservedpillowed metavolcanic occurrences. The aim is to gain more information on the
Fig. 1. a Map showing the distribution of the metavolcanic rocks in the Eastern Desert ofEgypt (El Ramly, 1972) and sample localities. Insert shows location of the Arabian-NubianShield (ANS). b Geological map of Wadi Beririq area (Abu El Ela, 1985)
82 E. S. Farahat et al.
palaeotectonic environments and petrogenesis of some of the widely scatteredophiolites in the central Eastern Desert, taking into consideration the mobility ofsome elements due to alteration and metamorphism.
Geological setting
The selected three localities (Fig. 1a), Wadi Beririq (Fig. 1b), Gabal Ghadir (Fig. 2)and Wadi Ghadir (Fig. 3), are located in the southern part of the central EasternDesert.
Wadi Beririq
At Wadi Beririq (Fig. 1b), pillowed metavolcanics are part of a dismemberedophiolitic succession that occurs within the Mubarak volcano-sedimentary belt.The other ophiolitic components are serpentinites and metagabbros (Abu El Ela,1985; Akaad et al., 1995).
The metavolcanics (5 km2) are encountered as allochthonous masses ofdeformed pillow lavas (Fig. 4a) and are restricted to the Beririq volcanoclasticmetasedimentary rocks west of the entrance of Wadi Beririq from Wadi Mubarak(Fig. 1b). Diabase dykes associated with these metavolcanics are notably absent.The associated ultramafic rocks are serpentinites with no relict phases, which are inpart altered to talc-carbonate rocks.
Fig. 2. Geological map of the area around Gabal Ghadir (Basta, 1983)
Continental back-arc basin of Egypt 83
The Beririq volcanoclastic metasedimentary rocks occupy most of the investi-gated area. They comprise a thick-layered sequence of volcanoclastic turbiditemetasediments including greywackes, siltstones, mudstones, lithic arenites, conglom-erates, and pelitic schists metamorphosed into the greenschist facies. According toAkaad et al. (1995) these volcanoclastic metasedimentary rocks were derived largelyfrom the Abu Dabbab and in part from the Maiyit metavolcanics, which lie SE andNW of these metasedimentary rocks (outside the mapped area). Both the Maiyit andAbu Dabbab metavolcanics follow a calc-alkaline trend thought to represent thejuvenile and mature stages of an island-arc, respectively. On the other hand, theBeririq volcanoclastic metasedimentary rocks are found to contain subordinaterounded to subrounded pebbles and clasts (3–20 cm) of terrigneous marble and gran-odioritic rocks, indicating a continental origin (Akaad et al., 1995). Moreover, thevolcanoclastics grade towards the NE into a mapable subunit called the Talet Gamra(Abu El Ela, 1985) that is characterized by an appreciable proportion of terrestrialcomponents including metamudstones, metagreywakes and minor metaconglome-rates. These rocks differ from the Beririq volcanoclastics in containing a markedproportion of grains and clasts of terrigneous quartzites, granodiorites and marbles.Exotic clasts include chert and quartz-carbonate rocks. These may indicate derivationfrom the continental side of a presumed back-arc basin (Akaad et al., 1995).
Gabal Ghadir
The ophiolitic rocks cropping out around Gabal Ghadir (Fig. 2) are represented byAlpine-type peridotites, gabbros, diabase, and pillow lavas. These rocks occur as
Fig. 3. Geological map of the investigated area in Wadi Ghadir (El Bayoumi, 1980)
84 E. S. Farahat et al.
Fig. 4. Field photographs, photomicrographs and back-scattered electron images of theinvestigated metavolcanics. Field photographs: a Subvertical, elongated, tightly packedpillows. Wadi Beririq. b An assortment of pillows of different form and size. Thesuccession shows way-up attitude. Wadi Ghadir. Photomicrographs: c Variolitic texturewhere radiating fibres are commonly plagioclase. Wadi Beririq (crossed polars).d Unusually elongated plagioclase crystals encountered near the pillow margins. WadiGhadir (crossed polars). Backscattered-electron images: e Core (C) and rim (R) of aplagioclase phenocryst are preferentially replaced by albitic plagioclase. Gabal Ghadir.f Actinolite (Ac) and hornblende (Hb) porphyroblaste containing pyroxene (CPX) relics.Gabal Ghadir
Continental back-arc basin of Egypt 85
fragments in a metasedimentary matrix that encompasses conglomerates, grey-wackes, mudstones, volcanoclastics and schists (Takla et al., 1982).
The ultramafics are the most abundant ophiolitic component forming the mainmass of Gabal Ghadir (Fig. 2) as well as small blocks frequently incorporated inthe ophiolitic m�eelange. They contain partly serpentinized dunites and peridotites,massive and schistose serpentinites and talc-carbonate rich chromitites. Quartz-carbonate veins cut these in the topmost portion.
Metagabbros occur as irregular small masses, distributed mainly in manyplaces around Gabal Ghadir (Fig. 2) and along Wadi Ghadir. They vary in grainsize from medium- to coarse-grained and are composed of sericitized plagioclaseand clinopyroxene; the latter are partially to completely altered to amphiboles.
Diabases occur as an elongated belt trending NW–SE to the south of GabalGhadir (Fig. 2). They are mostly massive to schistose. Pillow lavas are encounteredas small blocks a few meters in diameter, that crop out in several occurrences. Dueto deformation, they cannot be traced over longer distances.
Metasediments occupy most of the area. They include tuff, greywacke, con-glomerate, mudstone, pelitic and calc-pelitic schist, quartzite and chert.
Structurally, the area exhibits very pronounced NW–SE faults, prominent E–Wfaults and rare N–S and NE–SW faults (Takla et al., 1982).
Wadi Ghadir
According to Kr€ooner et al. (1992), the Wadi Ghadir ophiolite is one of the best-preserved sections through late Proterozoic upper oceanic crust anywhere in theworld. The most complete ophiolite section in Wadi Ghadir is well exposed atWadi El Beda (Fig. 3), where most units are exposed, from the layered gabbrosat the base to the pillow basalts with associated deep-sea metasediments at the top(El Sharkawy and El Bayoumi, 1979; El Bayoumi, 1983). The overall exposure isestimated to be more than 2 km. Due to dismembering during obduction ultramaficrocks are missing from this ophiolite suite. Peridotites and their serpentinizedequivalents occur, however, in variably sized masses (e.g. at G. Lawi; Fig. 3).Granitic rocks occupy the eastern part of the area. Abundant later dykes of maficto felsic compositions cut all rock units.
The gabbro complex includes layered gabbro at the bottom which grades upwardinto coarse-grained gabbro and in turn gives way upwards into microgabbro. Smallersize dismembered masses of massive and coarse-grained gabbros were found in thearea. All types of gabbros are composed of plagioclase and uralitized pyroxene.
Sheeted dykes are exclusively found in Wadi Ghadir ophiolites when comparedwith the other areas in this investigation. They form an intact unit of the successionbetween the underlying metagabbro and the overlaying pillow basalts (Fig. 3). Thedykes vary in thickness from about 50 cm to 2 m. Although the contacts betweenindividual dykes are hardly discernible, asymmetric chilled margins were recog-nized in many cases. Pillow basalts were observed in Wadi El Beda where thepillow lavas are about 200 m thick, the pillows indicating upright position (Fig. 4b).Along the southern bank of Wadi Saudi, a less developed more altered pillowedmass (about 70 m thick) was recognized.
Metasedimentary rocks occupy most of the study area. These rocks are com-posed of rolled and fragmented debris of highly variable sizes in a fine-grained
86 E. S. Farahat et al.
matrix of minute fragments of greywacke intercalated with sheared siltstone andmudstone. The fragments represent a heterogeneous assemblage of native andexotic blocks that were tectonically and depositionally-mixed. Some of the frag-ments are of oceanic origin (ophiolitic), while the others represent a continentalenvironment (El Bayoumi, 1983). These rock fragments are comprised of lime-stone, marble, laminated argillite, greywacke, quartzite, chert, gabbro, muscovitegranite and volcanics (El Sharkawy and El Bayoumi, 1979).
Analytical methods
Qualitative and quantitative mineral analyses as well as backscattered-electron im-ages were carried out at the Institute of Mineralogy and Petrology, Karl-FranzensUniversity of Graz using a JEOL-ISM-6310 scanning electron microscope. Theoperating conditions were 15 kV acceleration voltage and 5 nA probe current. Theanalyses were calibrated using a set of international mineral standards. The ma-trix correction was calculated with the ZAF-correction program. Chemical compo-sitions and structural formulae of selected electron microprobe analyses ofplagioclase, amphibole, and pyroxene are listed in Table 1.
Twenty-five representative samples of the metavolcanics were chemicallyanalyzed for major, trace and rare earth elements (Table 2). The analyses wereperformed at Activation Laboratories, Ontario, Canada using Fusion-inductively-coupled plasma (ICP) and Fusion-inductively-coupled plasma=mass spectrometry(ICP=MS) techniques (ACTLABS Group website http:==www.actlabs.com).
Petrography and mineral chemistry
The investigated metavolcanics generally preserve most of their original porphyritic,glomeroporphyritic, ophitic, and variolitic (quench) textures. Variolitic textures arecommonly encountered near pillow margins or in the midway between the pillowmargins and interior where the fine, radiating fibers, commonly of plagioclase, areenclosed in a glassy groundmass (Fig. 4c). In some cases, plagioclase phenocrystsare extremely elongated due to super-cooling of basaltic magmas (Fig. 4d). Amphi-bole or chlorite-rich alteration products sometimes enclose plagioclase laths possi-bly representing the metamorphic products of original ophitic textures. Subophitictextures have been rarely encountered in the Gabal Ghadir metavolcanics.
Gas vesicles are commonly found in all the investigated metavolcanics. Theaverage vesicular cavity volumes (Wadi Beririq, 12.3%, Gabal Ghadir, 9.6%, andWadi Ghadir, 20.4%) were estimated using petrographic microscopy and pointcounting. The vesicular volume of the studied samples is markedly greater thanthe 5% reported from Mid-ocean ridge basalts (Moore, 1979). In contrast, the voidratio is comparable with those of many back-arc basin metavolcanics; e.g. 9–22%from Josephine ophiolite of back-arc basin origin (Harper, 1982) and 5–50% fromLau Basin (James and Hawkins, 1995).
Plagioclase is the most abundant constituent of the studied metavolcanics.Virtually, all the original plagioclase in the studied rocks of Wadi Beririq and WadiGhadir are albitized. Despite the perfect alteration of the primary calcic plagioclaseto albite, its primary shape and in many cases twinning are well preserved. This ledmany workers in the past to consider this albite to be of igneous origin and to
Continental back-arc basin of Egypt 87
Tab
le1
.Microprobeanalysesofsomeminerals
ofthemetavolcanics
Min
eral
sP
lag
iocl
ase
Am
ph
ibo
leP
yro
xen
e
Lo
cali
tyW
.B
erir
iqG
.G
had
irW
.G
had
irW
.B
erir
iqG
.G
had
irW
.G
had
irG
.G
had
ir
SiO
26
8.2
76
8.9
95
5.5
05
9.1
05
9.6
26
9.1
46
8.1
86
7.4
75
3.9
55
3.1
24
5.4
44
8.0
95
4.1
25
1.3
44
9.2
34
8.3
95
0.3
65
0.0
25
2.2
25
0.9
65
1.2
6
TiO
2–
––
––
––
–0
.00
0.0
00
.22
0.3
20
.00
0.1
30
.37
0.0
00
.00
0.0
00
.82
1.3
01
.15
Al 2
O3
19
.71
18
.76
27
.40
25
.75
26
.45
19
.86
19
.88
19
.44
1.5
10
3.5
08
.16
5.4
22
.65
3.3
61
0.1
37
.54
3.5
95
.06
1.6
63
.27
3.1
7
Fe 2
O3
0.0
00
.00
0.0
00
.00
0.0
00
.00
0.0
00
.00
1.0
60
.00
7.4
54
.98
0.0
03
.76
0.0
03
.69
2.0
90
.06
0.0
00
.00
0.0
0
FeO
0.0
00
.00
01
.89
0.4
81
.29
0.0
00
.18
0.0
01
3.6
41
5.6
31
5.5
41
7.8
71
7.1
21
2.8
81
9.8
11
8.3
01
8.8
12
1.0
90
.00
0.0
80
.36
CaO
0.6
60
.14
10
.00
7.0
74
.38
0.4
31
.20
0.5
30
.23
0.0
00
.34
0.4
10
.00
0.3
20
.27
0.4
50
.40
0.4
11
3.6
41
0.2
21
0.1
3
Mn
O0
0.0
00
.00
0.0
00
.00
0.0
00
.00
0.0
00
.00
14
.38
12
.70
6.8
07
.74
11
.61
12
.57
5.9
77
.28
9.7
78
.89
0.3
10
.28
0.2
6
Mg
O0
0.0
00
.00
0.0
00
.00
0.0
00
.00
0.0
00
.00
13
.40
12
.93
12
.05
12
.12
11
.14
12
.83
10
.21
10
.70
12
.12
12
.35
15
.44
14
.92
15
.34
Na 2
O1
1.6
61
1.8
50
4.0
36
.71
4.7
60
9.6
11
0.4
21
1.7
70
.19
0.3
10
.80
0.5
70
.43
0.3
10
.77
0.7
80
.40
0.6
71
5.1
71
7.9
51
7.6
9
K2O
0.1
30
.00
0.3
60
.57
2.6
90
.00
0.0
00
.00
0.0
00
.00
0.3
00
.24
0.0
00
.08
0.6
00
.25
0.1
90
.34
0.2
70
.43
0.4
2
H2O
––
––
––
––
2.0
82
.06
1.9
71
.99
2.0
32
.05
2.0
12
.00
2.0
00
2.0
1–
––
To
tal
10
0.4
29
9.7
49
9.1
09
9.8
59
9.1
99
9.0
59
9.8
69
9.2
21
00
.44
10
0.2
59
9.0
69
9.7
49
9.1
09
9.6
49
9.3
79
9.3
99
9.7
31
00
.92
99
.52
99
.42
99
.78
Si
2.9
73
.02
02
.56
2.7
22
.73
3.0
97
3.0
12
.965
7.7
88
7.7
29
6.9
06
7.2
59
7.9
88
7.5
24
7.3
44
7.2
65
7.5
44
7.4
51
1.9
66
1.9
06
1.9
08
Ti
––
––
––
––
0.0
00
.00
0.0
25
0.0
36
0.0
00
.015
0.0
41
0.0
00
.00
0.0
00
.023
0.0
37
0.0
32
Al
1.0
10
.968
01
.49
1.3
31
.43
1.0
48
1.0
34
1.0
07
0.2
57
0.5
99
1.4
62
0.9
65
0.4
60
.581
1.7
81
1.3
34
0.6
34
0.8
89
0.0
74
0.1
44
0.1
39
Fe3
þ0
.00
0.0
00
.00
0.0
00
.00
0.0
00
.00
0.0
00
.115
0.0
00
.852
0.5
65
0.0
00
.414
0.0
00
.417
0.2
36
0.0
07
0.0
00
.00
0.0
0
Fe2
þ0
.00
0.0
00
.07
0.0
20
.05
0.0
00
.00
70
.00
1.6
46
1.9
02
1.9
76
2.2
55
2.1
14
1.5
79
2.4
72
2.2
98
2.3
56
2.6
28
0.0
00
.002
0.0
1
Ca
0.0
30
.007
0.5
00
.33
0.2
20
.02
10
.05
70
.025
0.0
28
0.0
00
.043
0.0
53
0.0
00
.04
0.0
34
0.0
58
0.0
51
0.0
52
0.4
29
0.3
20
.31
6
Mn
0.0
00
.00
0.0
00
.00
0.0
00
.00
0.0
00
.00
3.0
93
2.7
54
1.5
41
.742
2.5
53
2.7
45
1.3
27
1.6
29
2.1
81
1.9
73
0.0
10
.009
0.0
08
Mg
0.0
00
.00
0.0
00
.00
0.0
00
.00
0.0
00
.00
2.0
73
2.0
15
1.9
62
1.9
61
.761
2.0
15
1.6
33
1.7
22
1.9
45
1.9
72
0.8
66
0.8
32
0.8
51
Na
0.9
81
.006
0.3
60
.57
0.4
20
.83
50
.89
21
.003
0.0
53
0.0
88
0.2
34
0.1
66
0.1
23
0.0
88
0.2
22
0.2
28
0.1
17
0.1
95
0.6
12
0.7
19
0.7
05
K0
.007
0.0
00
.02
0.0
30
.16
0.0
00
.00
0.0
00
.00
0.0
00
.058
0.0
46
0.0
00
.016
0.1
15
0.0
48
0.0
37
0.0
64
0.0
20
.031
0.0
3
H–
––
––
––
–2
.00
2.0
02
.00
2.0
02
.00
2.0
02
.00
2.0
00
2.0
02
.00
––
–
To
tal
5.0
05
.00
5.0
05
.00
5.0
05
.00
5.0
05
.00
17
.05
17
.09
17
.06
17
.05
17
.00
17
.02
16
.97
17
.00
17
.10
17
.23
4.0
04
.00
4.0
0
Form
ula
eca
lcula
ted
on
the
bas
isof
8(p
lagio
clas
e),
23
(am
phib
ole
),an
d6
(pyro
xen
e)oxygen
s
Tab
le2
.Major,minorandrare
earthelem
entchem
icalanalyses
Lo
cali
tyW
adi
Ber
iriq
Gab
alG
had
ir
An
aly
s.n
o.
12
34
56
78
91
01
11
21
3
Wt.
%
SiO
24
6.0
45
5.2
05
5.8
35
4.1
04
8.8
75
6.0
35
1.5
05
8.1
85
3.8
64
7.7
95
1.8
85
1.8
75
0.4
1T
iO2
3.1
72
.08
1.6
31
.93
2.4
12
.18
2.1
01
.29
1.9
63
.46
2.7
92
.52
2.8
7A
l 2O
31
4.6
81
3.0
11
6.0
81
2.4
71
6.8
11
2.3
21
5.5
91
4.6
41
4.9
01
3.1
11
4.7
81
4.4
31
4.2
3F
e 2O
31
3.5
88
.97
8.4
88
.63
13
.35
9.2
61
0.9
96
.46
9.7
51
4.3
21
1.2
71
2.9
81
3.2
3M
nO
0.2
60
.10
0.1
00
.10
0.1
30
.11
0.1
50
.09
0.1
30
.23
0.1
40
.17
0.2
1M
gO
4.0
55
.15
3.7
14
.59
3.5
25
.69
3.1
63
.42
4.6
65
.03
4.2
03
.16
4.5
7C
aO7
.18
7.6
95
.49
8.3
06
.07
7.6
28
.24
4.3
76
.41
9.0
46
.81
4.6
47
.52
Na 2
O3
.60
4.3
05
.46
3.3
64
.92
3.7
73
.50
4.2
43
.35
3.2
54
.16
3.8
53
.39
K2O
1.7
80
.34
0.7
20
.49
0.6
00
.61
0.2
71
.48
1.4
70
.49
0.7
01
.28
0.7
5P
2O
50
.61
0.4
40
.47
0.3
60
.37
0.4
70
.35
0.4
80
.56
0.3
50
.71
0.6
00
.33
LO
I5
.04
1.9
32
.00
4.2
42
.80
2.5
24
.22
5.6
73
.01
2.3
92
.80
3.9
72
.42
Su
m9
9.9
99
9.2
19
9.9
79
8.5
79
9.8
51
00
.61
00
.11
00
.31
00
.19
9.4
61
00
.49
9.4
79
9.9
3
pp
m
V2
34
15
21
60
15
32
52
15
62
35
10
91
75
33
51
78
20
92
94
Cr
22
02
88
29
12
32
39
02
16
31
58
51
29
67
55
60
66
Co
40
33
28
24
30
29
20
19
28
35
32
26
39
Ni
32
12
81
38
63
12
39
21
17
57
69
22
19
26
20
Ga
22
.31
17
.69
17
.94
17
.34
17
.38
20
.62
20
.28
18
.71
18
.77
19
.45
22
.79
21
.82
3.6
6R
b3
2.6
14
.44
8.4
74
.59
12
.91
8.0
25
.61
34
.79
32
.89
12
.05
19
.32
38
.06
18
.11
Sr
65
34
48
57
74
60
52
45
94
47
76
79
65
01
66
60
62
01
24
7Y
37
.32
20
.56
20
.72
1.2
73
2.3
72
1.1
12
9.0
31
4.5
51
9.6
34
6.4
32
7.5
14
1.3
84
9.8
1Z
r2
38
21
42
07
20
51
67
23
31
53
17
91
72
18
73
44
19
42
41
(continued)
E. S. Farahat et al.: Continental back-arc basin of Egypt 89
Tab
le2
(continued
)
Loca
lity
Wad
iB
erir
iqG
abal
Ghad
ir
An
aly
s.n
o.
12
34
56
78
91
01
11
21
3
Nb
51
.03
12
.14
11
.19
10
.11
19
.16
11
.69
25
.31
7.3
59
.51
5.0
84
0.3
91
0.6
55
.78
Ba
71
6.0
86
.07
13
2.2
91
.51
13
4.7
25
1.0
44
.81
57
6.2
42
5.9
68
.93
26
9.2
17
6.6
14
1.6
Hf
6.3
35
.38
5.2
94
.00
4.3
16
.10
3.9
14
.51
4.2
65
.05
8.3
15
.10
5.9
5T
a3
.01
0.6
80
.65
0.5
31
.20
0.7
01
.54
0.4
80
.55
0.3
72
.48
0.7
10
.43
Th
4.2
02
.13
2.1
81
.79
1.6
12
.28
1.9
74
.47
2.2
60
.91
4.7
93
.68
2.3
6U
0.8
80
.76
0.5
50
.55
0.3
00
.64
0.2
92
.46
0.6
60
.43
1.4
21
.13
0.9
5
pp
m
La
39
.71
22
.00
22
.70
20
.10
15
.82
25
.37
19
.84
24
.23
18
.16
8.9
64
3.4
91
6.4
81
3.5
0C
e8
7.5
65
0.9
45
4.2
14
3.9
63
8.3
35
8.7
84
2.4
35
7.0
84
7.9
92
5.4
09
5.5
64
0.9
63
5.5
5P
r1
0.4
26
.70
7.0
25
.83
5.1
77
.82
5.2
47
.19
6.4
63
.96
12
.00
5.4
75
.10
Nd
41
.39
28
.69
29
.80
26
.44
22
.02
34
.49
22
.16
27
.14
27
.55
20
.89
48
.98
23
.61
24
.63
Sm
9.0
76
.86
7.0
76
.12
5.3
67
.70
5.6
05
.46
6.2
06
.41
9.5
76
.10
7.2
1E
u3
.15
2.2
32
.41
2.3
41
.85
2.6
11
.81
1.6
41
.99
2.3
02
.72
2.3
52
.31
Gd
9.2
76
.92
7.3
35
.41
6.5
38
.13
6.0
35
.28
6.1
68
.29
8.7
57
.72
8.3
9T
b1
.26
0.9
30
.93
0.7
70
.99
1.0
30
.94
0.5
80
.76
1.5
01
.21
1.2
21
.47
Dy
6.9
64
.30
4.3
03
.97
5.6
34
.76
5.4
62
.85
3.7
08
.75
5.6
67
.00
8.9
3H
o1
.38
0.7
40
.77
0.7
31
.18
0.8
01
.05
0.5
30
.69
1.8
21
.01
1.4
81
.78
Er
3.5
71
.85
1.8
61
.79
3.3
92
.03
2.9
31
.28
1.7
05
.36
2.5
34
.08
4.9
6T
m0
.51
0.2
40
.22
0.2
20
.49
0.2
50
.45
0.1
70
.22
0.8
30
.36
0.6
10
.81
Yb
3.0
91
.35
1.1
61
.36
2.8
01
.35
2.6
40
.94
1.2
54
.65
1.9
23
.30
4.7
7L
u0
.46
0.1
60
.14
0.1
80
.43
0.1
50
.37
0.1
20
.17
0.6
80
.25
0.5
20
.67
Eu=E
u�
1.0
50
.99
1.0
21
.24
0.9
61
.01
0.9
50
.93
0.9
90
.97
0.9
11
.05
0.9
1(L
a=Y
b) n
9.2
21
1.6
81
4.0
11
0.5
94
.06
13
.52
5.3
91
8.5
91
0.4
41
.38
16
.21
3.5
82
.03
(continued)
90 E. S. Farahat et al.
Tab
le2
(continued
)
Lo
cali
tyG
abal
Gh
adir
Wad
iG
had
ir
An
aly
s.n
o.
14
15
16
17
18
19
20
21
22
23
24
25
Wt.
%
SiO
27
4.0
16
2.1
46
1.5
45
4.1
45
4.8
45
3.0
15
1.8
75
5.6
95
5.4
75
5.4
15
3.9
24
9.1
9T
iO2
0.4
1.0
20
.74
3.1
72
.66
1.8
43
.08
2.3
61
.62
3.0
42
.11
2.6
Al 2
O3
13
.29
15
.31
5.4
91
2.9
81
6.1
91
4.3
91
3.0
91
6.4
61
6.3
61
2.0
71
4.1
81
4.3
6F
e 2O
32
.71
9.1
44
.79
.78
8.9
21
5.0
79
.68
.38
8.4
41
0.7
61
1.7
11
3.7
4M
nO
0.0
20
.05
0.1
20
.18
0.1
60
.28
0.1
90
.13
0.1
0.1
70
.17
0.2
4M
gO
0.3
21
.48
1.1
42
.28
1.8
12
.14
2.2
41
.73
3.7
2.2
12
.56
3.8
8C
aO0
.43
1.3
24
.45
7.5
55
.34
6.1
49
.54
5.1
65
.51
7.2
95
.17
7.0
9N
a 2O
5.8
75
.04
5.7
54
.46
.78
4.5
84
.57
.06
5.4
23
.96
3.8
74
.88
K2O
2.0
12
.51
1.0
80
.26
0.2
40
.94
0.2
80
.20
.68
0.2
40
.34
0.1
5P
2O
50
.06
0.4
30
.38
0.2
60
.52
0.7
80
.28
0.5
70
.46
0.2
90
.65
0.5
8L
OI
1.0
82
.35
4.9
75
.18
1.7
50
.94
4.9
81
.54
2.2
34
.12
4.7
81
.81
Su
m1
00
.21
00
.81
00
.41
00
.29
9.2
11
00
.19
9.6
59
9.2
89
9.9
99
9.5
69
9.4
69
8.5
2
pp
m
V1
5.0
12
1.7
91
5.0
23
57
12
7.6
16
.31
35
41
34
.51
60
.33
18
.51
05
.21
90
.9C
r2
02
02
03
02
75
52
02
3.1
33
.08
20
20
59
.75
Co
2.4
63
10
.43
.32
22
5.2
92
0.0
81
2.2
22
7.0
42
1.1
92
3.8
62
9.0
72
1.2
62
8.5
Ni
15
15
15
16
19
.57
22
.41
53
9.9
93
4.4
81
53
5.5
32
6.3
7G
a2
3.7
92
4.2
19
.95
14
.72
20
.18
26
.67
16
.17
21
.71
21
.86
16
.86
23
.18
22
.47
Rb
26
.88
23
.07
29
.59
3.8
19
3.2
85
42
.77
4.1
45
3.6
51
19
.74
.50
68
.93
72
Sr
70
.32
23
6.5
49
72
71
.52
18
.32
06
.72
68
.82
25
.66
51
18
5.5
20
1.7
17
4.6
Y8
1.5
84
9.5
72
6.5
23
95
4.5
67
0.6
44
05
6.8
31
9.1
73
5.6
34
7.6
15
1.2
4Z
r4
58
.56
09
.62
44
.71
70
33
0.8
22
71
70
36
2.5
19
41
60
20
11
78
(continued)
Continental back-arc basin of Egypt 91
Tab
le2
(continued
)
Lo
cali
tyG
abal
Gh
adir
Wad
iG
had
ir
An
aly
s.n
o.
14
15
16
17
18
19
20
21
22
23
24
25
Nb
16
.35
45
.11
10
.84
4.1
51
0.3
56
.77
94
.39
11
1.2
38
.09
13
.47
6.7
27
3.8
55
Ba
396.9
710.3
272.2
70.0
786.4
5137
60.5
391.1
392.3
42.5
9176.3
34.1
Hf
12
.61
2.1
86
.23
73
.51
69
.05
25
.85
64
.51
9.5
97
4.6
36
4.0
68
5.2
99
4.7
08
Ta
1.0
72
2.5
70
.73
50
.27
60
.70
40
.42
50
.30
50
.70
90
.58
30
.30
60
.49
0.3
28
Th
5.1
17
.82
75
.38
41
.65
26
.31
2.2
26
1.7
81
6.8
99
3.3
87
1.9
06
3.4
43
1.0
01
U1
.73
82
.82
82
.40
70
.59
11
.64
70
.88
10
.46
62
.02
21
.48
80
.80
11
.17
20
.44
1
pp
m
La
29
.84
59
.04
32
.23
10
.34
28
.49
15
.94
11
.14
28
.89
25
.29
11
.31
19
.23
10
.48
Ce
72.5
7122
70.1
825.2
768.3
341.2
530.4
568.1
654.8
028.7
845.1
828.8
2P
r9
.72
15
.34
8.9
43
.76
8.8
66
.45
4.2
59
.12
6.9
93
.93
6.2
24
.52
Nd
39
.28
61
.07
36
.46
18
.29
38
.84
33
.83
19
.54
39
.65
34
.10
19
.91
30
.32
26
.92
Sm
9.8
31
2.3
57
.33
5.1
61
0.2
21
0.1
05
.63
10
.02
6.9
65
.68
7.9
28
.46
Eu
1.7
63
.56
2.0
01
.98
2.2
63
.77
2.1
52
.23
2.0
41
.99
2.4
42
.77
Gd
12
.30
10
.52
6.7
85
.56
10
.71
12
.68
6.8
81
1.1
06
.37
6.4
99
.14
9.9
3T
b2
.06
1.5
40
.93
0.9
81
.77
2.2
51
.14
1.8
40
.80
1.0
41
.42
1.6
4D
y1
2.7
28
.43
4.6
55
.87
10
.37
12
.43
6.7
41
0.2
63
.99
6.3
98
.17
9.8
9H
o2
.82
1.7
10
.92
1.3
02
.03
2.5
31
.45
2.0
20
.65
1.3
11
.60
1.8
4E
r8
.45
4.6
42
.56
3.6
65
.44
7.1
24
.00
5.6
21
.77
3.6
84
.76
5.5
1T
m1
.34
0.6
50
.41
0.5
20
.89
1.0
60
.60
0.9
10
.26
0.5
40
.67
0.8
5Y
b7
.75
3.9
62
.26
3.2
35
.20
6.1
43
.65
5.0
51
.57
3.5
24
.01
5.0
9L
u1
.24
0.5
70
.33
0.4
70
.71
0.9
00
.55
0.7
30
.17
0.4
90
.56
0.6
9
Eu=E
u�
0.4
90
.95
0.8
71
.13
0.6
61
.02
1.0
50
.65
0.9
41
.00
0.8
80
.93
(La=
Yb
) n2
.76
10
.71
10
.22
2.3
03
.93
1.8
62
.19
4.1
01
1.5
52
.31
3.4
41
.48
92 E. S. Farahat et al.
suppose that these metabasalts with Na-rich plagioclase (spilites) were formedfrom primary ‘‘spilitic’’ magma. In most recent studies, petrographic quantificationof plagioclase alteration proves to be very difficult (e.g. Gillis and Thompson,1993).
In the studied metavolcanics, backscattered-electron images exactly character-ize the alteration products of plagioclase (Fig. 4e). Microprobe analyses of thesesecondary feldspars confirm that they are nearly pure albite (Table 1). In contrast,plagioclase in Gabal Ghadir varies in composition and relict cores of the primaryfeldspar (An36–58) are commonly preserved.
Pyroxene is recorded as the least-altered primary mineral from the metavolca-nics of Gabal Ghadir (Fig. 4f), while the mineral is completely altered at WadiBeririq and Wadi Ghadir. The composition of pyroxene (Table 1) is rather uniformand plots in the augite field of the wollastonite–enstatite–ferrosilite diagram.Clinopyroxene of Gabal Ghadir has a range of low to high Ti-content (0.28–1.44 wt.%), characteristic of mixed island-arc to MORB environments (Beccaluvaet al., 1989).
Amphiboles are common secondary minerals in most of the investigated rocks.Using backscattered-electron images and microprobe analysis, amphibole in WadiBeririq has been identified as actinolite (Table 1). In Gabal Ghadir and WadiGhadir metavolcanics, actinolite and hornblende coexist forming well-developedepitaxial intergrowths (Fig. 4f). The two amphiboles also form porphyroblasts with‘‘patchwork’’ intergrowths. Microprobe analyses reveal an abrupt compositionalbreak between the two mineral phases (Table 1).
The cores of some pillows in Wadi Beririq and Gabal Ghadir mainly consist ofepidote as phenocrysts and groundmass with subordinate quartz and plagioclase.This rock, called ‘‘epidosite’’, is commonly found associated with the old ophio-lites of back-arc basins, and is scarcely encountered in recent mid-ocean ridgeenvironments (Harper et al., 1988). Metavolcanics of Gabal Ghadir are occasion-ally accompanied by less basic, albite–chlorite bearing non-pillowed rocks knownas keratophyres. The mineral assemblage of these rocks is mostly secondary inorigin. Like in the spilites, the mineral assemblage of keratophyres may result fromthe hydrothermal ocean-floor metamorphism.
The investigated rocks vary in their secondary mineral assemblages, andconsequently in their metamorphic facies. Two distinct metamorphic facieshave been distinguished based on what appears to be equilibrium mineralassemblages:
A) Greenschist facies assemblages at Wadi Beririq (albiteþ actinoliteþ chloriteþepidoteþcalciteþ titaniteþquartzþbiotiteþmuscoviteþmagnetiteþ sulfides).
B) Greenschist-amphibolite transitional facies at Gabal Ghadir (albiteþoligoclase þ hornblendeþ actinolite þ chloriteþ epidoteþ calcite þ titanite þquartzþ biotiteþ ilmeniteþ titanomagnetiteþ sulfides) and at Wadi Ghadir(albite þ actinolite þ hornblende þ chlorite þ epidote þ calcite þ titaniteþquartzþ biotiteþ ilmeniteþ sulfides).
The greenschist-amphibolite transitional facies is usually characterized by thecoexistence of actinolite and hornblende and=or albite and more calcic plagioclase(Miyashiro, 1994).
Continental back-arc basin of Egypt 93
Geochemistry
Alteration effects, classification and magma type
If whole-rock geochemical data of altered or metamorphosed rocks are used, pos-sible modifications caused by alteration and metamorphism must be considered.This is especially important in the investigated rocks which have been affected byboth hydrothermal ocean-floor, and regional metamorphism, of greenschist togreenschist-amphibolite transitional facies. Many studies have shown that the con-centrations of REE and high field-strength elements (HFSE) in igneous rocks arenot generally affected during alteration (e.g. Pearce and Norry, 1979; Bienvenu
Fig. 5. a MORB-normalized spidergrams of the investigated metavolcanics. The order ofelements and MORB data are from Pearce (1982). b REE distribution in the investigatedmetavolcanics. Chondrite data from Sun and McDonough (1989)
94 E. S. Farahat et al.
et al., 1990; Farahat, 2001). Figure 5a and b depict the MORB-normalized traceelement diagrams and REE patterns for the investigated metavolcanics. Indeed, theREE and HFSE (e.g. Zr, Nb, Y, Hf, Ti, and P) contents of the investigated rocksshow coherent, sub-parallel concentration patterns. In general, the investigatedmetavolcanics still preserve the signature of some of their original trace elementcomposition. However, some mobile elements such as Ba, Rb, Sr, and K showvariable concentrations (Fig. 5a), implying that their original concentrations werelikely changed by alteration and metamorphism. Consequently, the geochemicalinterpretation is based mainly on the less mobile to immobile elements, while themore mobile elements have been avoided or used with caution.
When plotted on a Zr=TiO2 versus Nb=Y diagram (Fig. 6a) the investigatedrocks occupy mainly the subalkaline basalt and andesite basalt fields. However, thekeratophyres of Gabal Ghadir fall in the rhyodacite, dacite and trachyandesitefields. Using a Nb=Y versus Zr=P2O5 diagram, (Fig. 6b) almost all samples fallin the tholeiitic field.
Palaeotectonic environments
Interpreting the tectonic setting of Proterozoic rocks using trace element geochem-istry inevitably encounters many difficulties. Apart from the effects of alterationand metamorphism on mobile elements, as well as fractionation and crustal con-tamination on trace element patterns, the applicability of many discriminationdiagrams based on Phanerozoic rocks is also controversial. Pharaoh and Pearce(1984) discussed this problem and pointed out that field boundaries defined bymodern volcanic rocks must be considered merely as a reference framework. Theyalso indicated that during partial melting, crystal fractionation or during crustalgrowth over the past 2.0 Ga, insignificant changes occur in the inter-element ratiosfor many immobile elements such as Zr, Ti, Nb, and Y. Following this logic, thereis little reason to believe that the discrimination boundaries in each diagram differmuch for Proterozoic rocks (Zhao, 1994).
Fig. 6. a Zr=TiO2 vs. Nb=Y (Winchester and Floyd, 1977) and b Nb=Y vs. Zr=P2O5
(Winchester and Floyd, 1976) diagrams. Symbols as in Fig. 5
Continental back-arc basin of Egypt 95
In the TiO2–Zr diagram (Fig. 7a) the majority of Wadi Beririq, Gabal Ghadirand Wadi Ghadir data fall in the ‘‘within-plate’’ field with some overlap betweenMORB and within-plate basalt. Therefore, it is important to confirm if these rockshave a true within-plate affinity or if they are in fact subduction related rocks. Theplate margin environment of the investigated rocks is shown using the ternary Th–Hf=3–Ta diagram (Wood et al., 1979) where most of the data plot in the destructiveplate margin field (Fig. 7b). Moreover, using the Th=Yb versus Nb=Yb diagram(Pearce and Peate, 1995) most of the Wadi Ghadir and Gabal Ghadir samples plotin the overlap area between oceanic and continental arc basalts (Fig. 7c), similar tocontinental back-arc basin basalts (Shinjo et al., 1999). However, Wadi Beririq
Fig. 7. a TiO2 vs. Zr (Pearce, 1982), b Hf=3–Th–Ta (Wood et al., 1979) and c Th=Yb vs.Nb=Yb (Pearce and Peate, 1995) tectonomagmatic discrimination diagrams. Normal andenriched MORB values are after Sun and McDonough (1989). MORB mid-oceanic ridgebasalt; WPB within-plate basalt; VAB volcanic-arc basalt. Symbols as in Fig. 5
96 E. S. Farahat et al.
rocks fall in the continental arc and enriched MORB fields that may refer to theirback-arc basin affinity.
MORB-normalized spidergrams provide an effective means for comparing Pro-terozoic rocks with modern samples from different tectonic environments. Basaltsformed in subduction-related tectonic environments (such as continental margins,island-arcs and initial back-arc or intra-arc basins) exhibit patterns characterized byvariable enrichment in LILE (Sr, K, Rb, Ba and Th) and=or LREE and depletion inHFSE (Zr, Hf, Ti and Nb). This is in contrast to within-plate basalts, which typi-cally display a ‘‘humped’’ pattern where most elements except those compatiblewith garnet lherzolite (e.g. Y and Yb) are enriched relative to N-MORB (Pearce,1982). The investigated metavolcanics (Fig. 5a) exhibit enrichment in LILE andnegative Nb and Ta anomalies, in agreement with most of the above-discusseddiscrimination diagrams. This is a characteristic feature of island-arc tholeiites.The enrichment in elements from Sr to Ti is a feature characteristic of within-platetholeiites. Both features are comparable with the transitional basalt patterns iden-tified by Pearce (1982) from transitional basalts found in or adjacent to continentalareas during the initial stages of back-arc spreading or back-arc rifting.
The notable opposite anomalies of Nb (negative) and Th (positive) that are exhib-ited in the MORB-normalized patterns of the investigated rocks are common featuresof rocks in a mantle wedge formed above a subduction zone (Saunders et al., 1980).
Compared to island-arc basalt patterns, the investigated metavolcanics showgreater overall incompatible element enrichment and smaller negative Nb anomaly,which implies immature arc characteristics. These MORB-normalized patterns(Fig. 5a) are characteristic of basalts formed in continental (ensialic) back-arcbasins (Saunders and Tarney, 1984). This indicates back-arc basins had not openedfar enough to form significant oceanic crust or tap large volumes of depletedmantle. Since most ensialic back-arc basin volcanics indicate a subaqueous origin,seawater likely covered the basin (Condie, 1986). The association of the ophiolitesunder investigation with volcanoclastic metasediments of marine to continentalfacies is a further confirmation of their ensialic evolution.
The REE patterns of the metavolcanics have also been used to interpret theirtectonic setting. The REE patterns of the investigated rocks (Fig. 5b) are LREE-enriched similar to those of subduction related igneous rocks (Lan=Ybn ¼ 1.38–18.59, Table 2). These REE patterns may represent an intermediate positionbetween typical island-arc tholeiites and within-plate basalts (Tarney et al., 1982),an interpretation which is consistent with the observed MORB-normalized patterns.
Petrogenesis
As discussed above, rock=MORB- and REE- patterns of the investigated metavol-canics represent an intermediate position between island-arc and within-platebasalts. This may suggest that they have been derived by binary mixing betweena depleted mantle component and an evolved crustal component, accompanied bycrystal fractionation at different stages. The main differences between arc volcanicpatterns and those of the investigated metavolcanics are the higher degrees ofLREE- and LILE-enrichment and the smaller negative Nb anomaly of the latter,implying relatively higher degrees of crustal contamination.
Continental back-arc basin of Egypt 97
The alternative explanation of the observed MORB-normalized and REE-patterns (Fig. 5a and b) is that mixing between depleted mantle wedge materialand enriched terrigneous sediments took place by way of subduction and mantlewedge metasomatism. However, as the subducted terrigneous sediments are volume-trically too insignificant to produce large variations in major and trace element com-positions (Li et al., 1997), mixing between a depleted mantle-derived magma and anenriched crustal melt at crustal levels, similar to the AFC (assimilation and fractionalcrystallization) process of De Paolo (1981), is more applicable for the origin of theinvestigated rocks. Most of the samples from Wadi Ghadir and Gabal Ghadir define acoherent trend, with a nearly balanced Th=Nb ratio (Fig. 8), which may be attributedto the combined effects of crustal assimilation and fractional crystallization (i.e.AFC). Additional geochemical indicators pointing to such contamination are the highZr and Hf contents (Table 2). Furthermore, the investigated metavolcanic samplesplot above the enriched end of the MORB array in the Th=Yb versus Nb=Yb diagram(Fig. 7c) and thus seem to provide strong support for the involvement of subconti-nental lithosphere in their petrogenesis (Pearce and Peate, 1995).
With the exception of negative Nb and Ta anomalies, the MORB-normalizedpatterns of the investigated rocks (Fig. 5a) are more akin to that of within-platebasalts. Pearce (1983) suggested that the mantle source of such magmas was theenriched subcontinental lithosphere (as opposed to the depleted asthenosphere inthe case of island-arc basalt) to which mobile elements (Sr, K, Rb, Ba, and to alesser extent Ce and Sm) have been added by subduction zone fluids.
The REE patterns of the investigated metavolcanics (Fig. 5b) are sub-parallel,with substantial light- to heavy-REE fractionation. This may indicate that themetavolcanics from each of these areas could be related by low-pressure fractiona-tion of phases that incorporate or fractionate the HREE (such as olivine, garnet or
Fig. 8. Th=Y vs. Nb=Y diagram (Wilson et al., 1997) for the studied metavolcanics. Theopen arrow represents the AFC (assimilation and fractional crystallization) trend. The filledarrow represents the effect of subduction (fluid) enrichment of the mantle source. MORBmid-ocean ridge basalt; OIB oceanic-island basalt. Symbols as in Fig. 5
98 E. S. Farahat et al.
amphibole), although variable degrees of partial melting of the source may beinvolved. The tendency towards lower HREE concentrations in some samples ofWadi Beririq, suggests that these lavas may have incorporated basalts melted atgreater depths, leaving some residual garnet (Tarney et al., 1982).
The absence of a well pronounced negative Eu anomaly in the REE patterns inalmost all of the investigated rocks suggests that the conditions were oxidizing atthis stage of the magmatic evolution, rather than a lack of significant plagioclasefractionation (in the light of the observed plagioclase phenocrysts). Under theseconditions, most Eu is converted into the trivalent state that does not enter intoplagioclase (Hess, 1989). The formation of Fe–Ti oxides in the basalts is consistentwith increasing fO2, during melt evolution. This condition typically occurs withinthe vicinity of a subduction zone (Saunders et al., 1980).
Implications for Proterozoic crustal evolution
Plate tectonics versus ensialic rift models
Although the concept of plate tectonics has been successfully applied to the Pro-terozoic ANS, different interpretations have been adopted in the study of the evo-lution of this shield (e.g. El Bayoumi, 1983; Stern et al., 1991). These authors drewattention to the similarity between some of the metavolcanic rocks in the Egyptianshield and those of modern ensialic rift zones and proposed an ensialic-riftingmodel. The enrichment of Wadi Ghadir pillow basalts in the immobile elementsled El Bayoumi (1983) to consider this as being within-plate characteristics and tosuggest a plume origin of the basaltic magma.
Based on the trace element characteristics of both the mafic and felsic membersof the bimodal Shadli metavolcanics, Stern et al. (1991) rejected the hypothesisthat these basalts were formed in an island-arc setting. Instead, they proposed thatthese rocks were erupted in a magmatic rift environment not associated with aconvergent margin, i.e. similar to the Rio Grand Rift or Afar Triangle.
Ensialic rift models have been proposed for the crustal evolution of manyProterozoic mobile zones in the northern Hemisphere (e.g. Condie, 1982; Park,1985). However, the present study, demonstrates that the metavolcanics studiedshow a clear subduction signature in addition to the absence of positive Nb anoma-lies that characterize plume- or rift-related basalts (Holm, 1985).
Geochronological data
The Wadi Ghadir ophiolite complex is the only one dated in the central Eastern
Desert. A zircon 207Pb=206Pb age of 746� 19 Ma was reported for this ophiolitesequence by Kr€ooner et al. (1992). Other ages reported from the Eastern Desert byStern and Hedge (1985) suggest that the main tectonic stage occurred earlier in thesouth than in the north. They also suggested that the formation of volcanic andplutonic arc complexes in the south Eastern Desert took place between �770 to�720 Ma.
The formation of Wadi Ghadir oceanic crust is virtually contemporaneous withthis arc forming event and identical to that recorded for Gabal Gerf ophiolites,
Continental back-arc basin of Egypt 99
some 200 km to the south. Based on these ages Kr€ooner et al. (1992) suggestedsimultaneous ocean crust formation in the central and southern Eastern Desert.However, our geochemical study of Wadi Ghadir ophiolites suggests a continentalback-arc basin setting, whereas the major and trace element chemistry of GabalGerf basalts and sheeted dykes is more identical to modern MORB (typical deple-tion in LREE and no enrichment in incompatible elements, Zimmer, 1989). There-fore, it is unlikely that these two ophiolites are part of the same back-arc basin.
Tectonic interpretations
Two primary models are proposed for the structural evolution of the Pan-Africanbelt in the ANS. The first model proposes that one or more island-arcs accreted toform a passive continental margin (Stern, 1994). The second model favors accre-tion of an ensimatic island-arc onto an attenuated and reactivated older continentalmargin (El Gaby et al., 1988). The main difference between these two models forthe Eastern Desert of Egypt is the controversy concerning the presence or absenceof a pre-Pan-African continental crust underneath overthrust ophiolite=island-arcassemblages. The latter model envisages an older continental crust cropping out inswells or mantled gneisses domes, such as Meatiq and Hafafite (El Gaby et al.,1988). Based on isotopic age and geochemical data, Stern and Hedge (1985), andKr€ooner et al. (1994) suggested that these basement domes formed during the Pan-African magmatic evolution of island-arcs without the involvement of older con-tinental crust.
Regardless of the debate concerning the origin and genesis of the basementdomes, two lines of evidence indicate the presence of a pre-Pan-African continen-tal crust. The first comes from geochronological and isotopic lines of evidence (e.g.Dixon, 1981; Harris et al., 1984); the second concerns the significance of crustalgrowth rates (e.g. Reymer and Schubert, 1986; Dixon and Golombek, 1988).
Harris et al. (1984) and El Gaby et al. (1988) suggested that the Archean toEarly Proterozoic nucleus of Gabal Uweinate in the southwestern corner of Egyptis fringed by mid Proterozoic continental crust, which extends eastward deep intothe Eastern Desert. El Gaby et al. (1988) concluded that the presence of old con-tinental crust underneath the ophiolites and volcano-sedimentary association in theEastern Desert (i.e. Cordilleran structure) can be indirectly inferred from the pres-ence of molasse sediments which usually characterize continental margin orogenicbelts.
Though some of the isotopic data was doubted by Kr€ooner et al. (1988), theynevertheless substantiated the incorporation of some older continental crust atleast in parts of the Eastern Desert. Kr€ooner et al. (1988) concluded that thepre-Pan-African continental margin was either highly irregular, with promontoriesand embayments, or suspected the old crustal components below the EasternDesert represent small crustal blocks or microcontinents that were accreted ontothe African margin.
Many studies have demonstrated that the apparent Late Precambrian growthrates for continental crust in the ANS are implausibly high for simple arc–arcaccretion models (e.g. Dixon and Golombek, 1988). Reymer and Schubert (1986)suggested that the increase in crustal accretion rates could either be related to hot
100 E. S. Farahat et al.
spot volcanism and underplating in addition to arc accretion, or that large amountsof pre-existing basement material has gone undetected.
Compilations of radiometric data in conjunction with the continental affinity ofsome granitic rocks of the Aswan area are consistent with a proximal older con-tinental crust near the western boundary of the exposed Egyptian Shield (Dixonand Golombek, 1988). Stein and Goldstein (1996) have ascribed the high growthrate of the ANS to the formation of anomalously thick oceanic lithosphere (i.e.oceanic plateau) by a plume head, modified by subduction at its margin. Accord-ing to Kerr et al. (2000) the salient features of oceanic plateaus are as follows:thick sequence (�5 km) of basalts; the occurrence of high-MgO lavas (picrites andkomatiites); chemically homogeneous basalts with relatively flat chondrite-nor-malized REE patterns; pillowed lavas; low abundance of volcanoclastic deposits;and a lack of sheeted dyke complexes. However, the geological and geochemicalcriteria of the investigated ophiolite suites do not meet well with those of oceanicplateaus.
The suggested continental (ensialic) back-arc basin origin of the investigatedophiolitic metavolcanics, besides the geochronological, isotopic and crustal growthrate evidences, clearly revive interest in models that involve contribution from apre-Pan-African crust, at least in the southern part of the central Eastern Desert.
Conclusions
Although the investigated rocks have undergone metamorphism of greenschist(Wadi Beririq) to greenschist-amphibolite transitional (Gabal Ghadir and WadiGhadir) facies, the original textures and some of their primary minerals (plagio-clase and clinopyroxene) are generally preserved. Whole-rock chemical data dis-play a subalkaline, tholeiitic affinity.
On most of the discrimination diagrams, MORB-normalized spidergrams, andREE patterns, the investigated rocks have transitional within-plate basalt to island-arc basalt features. Such features are characteristic of basalts formed in continentalback-arc basins. The association of the investigated ophiolites with volcanoclasticmetasediments of marine to continental provenance is a further confirmation oftheir ensialic evolution. This suggestion along with the discussed geochronologi-cal, isotopic and crustal growth rate evidences support the models that involvecontribution from a pre-Pan-African crust at least in the southern part of the centralEastern Desert.
Mixing between a depleted mantle-derived magma and an enriched crustal meltat crustal levels, somewhat similar to an AFC (assimilation and fractional crystal-lization) process, is suggested for the origin of Wadi Beririq, Wadi Ghadir andGabal Ghadir metavolcanics.
Acknowledgments
The major part of this work has been carried out at the Institute of Mineralogy and Petrology,Karl-Franzens University, Graz, Austria. The Egyptian Ministry of higher education andscientific research funded the whole-rock chemical analyses. We are grateful to W. Greenfor his thorough linguistic review. Two anonymous referees reviewed and substantially
Continental back-arc basin of Egypt 101
improved the manuscript and are greatly appreciated. This work is part of the first author’sPh.D. Thesis.
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Authors’ addresses: E. S. Farahat (corresponding author; e-mail: [email protected]),M. M. El Mahalawi, and A. Y. Abdel Aal, Department of Geology, Minia University, EL-Minia 61519, Egypt; G. Hoinkes, Institute of Mineralogy and Petrology, Karl-FranzensUniversity, A-8010 Graz, Austria
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