17
Geol.Mag. 133(1), 1996, pp. 17-31. Copyright© 1996 Cambridge University Press
Pan-African volcanism: petrology and geochemistry of the
Dokhan Volcanic Suite in the northern Nubian shield
ABDEL-FATTAH M. ABDEL-RAHMAN
Department of Geology, Concordia University, 7141 Sherbrooke St. West, Montreal, Quebec H4B 1R6, Canada
(Received 11 November 1994; accepted 13 July 1995)
Abstract - The Late Proterozoic Dokhan volcanic suite (620 Ma) of the northern Nubian shield is the product of Late Pan-African volcanism. The suite covers the entire spectrum from basalt to high-silica rhyolite
and occurs as two units: a dark-coloured unit containing basalt-andesite-dacite, and a light-coloured unit
encompassing dacite-rhyodacite-rhyolite. The latter unit is made up largely of ash flow tuffs and ignimbrites that are locally interstratified with basalt and andesite lava flows. The suite forms a continuum in
composition with a wide range of SiO2 (48-77 wt%), CaO (0.1-8.9 wt%), Sr (81-906 ppm), Zr (85-340
ppm) as well as most other elements, and is moderately enriched in incompatible elements, including rare
earth elements (REE). The suite exhibits fractionated, subparallel REE patterns that are similar overall to
Andean andesites and ignimbrites. Well-defined major and trace element trends and fractionated REE profiles are consistent with a fractionated basalt to rhyolite calc-alkaline magma series. It is a typical calc-alkaline orogenic complex and exhibits mineralogical-geochemical traits of arc-related volcanism. The suite
neither resembles products of extensional nor transitional tectonic regimes as previously thought, but was
produced in a subduction-related tectonic environment. The mafic nature of the least-evolved rocks of the
suite, along with its relatively low initial ^Sr/^Sr ratio (0.7039) are considered to indicate a mantle source.
A mantle-derived basaltic magma fractionated, with amphibole and plagioclase dominating the fractionating assemblage, to produce the more felsic varieties, as suggested by major and trace element fractionation
modelling.
1. Introduction
Two major volcanic episodes have been recognized in the
Precambrian crust of the Nubian shield. Volcanism associated with the first episode produced the 'Shadli
metavolcanic' sequences; these are older (c. 950-750
Ma), metamorphosed, largely mafic in composition. This
was succeeded by a younger Pan-African volcanic cycle
(680-550 Ma) that produced voluminous volcanic
assemblages, referred to as the 'Dokhan volcanic rocks'
(e.g. El-Ramly, 1972; Hashad, 1980). The term 'Dokhan'
describes vari-coloured volcanic rocks with a wide range
of (mafic to felsic) compositions.
The Dokhan volcanic assemblage, with its relatively
large stratigraphic extent, represents a major crustal component in northeastern Egypt (Fig. 1), which occupies the
northern part of the Late Proterozoic Nubian shield. The latter is separated by the Red Sea from its previously contiguous counterpart, the Arabian shield. The Arabian-Nubian
shield consists mainly of metasedimentary rocks and
migmatites, metavolcanic suites, serpentinites and dismembered ophiolite complexes, gabbro-diorite-granodiorite
complexes, Dokhan volcanic sequences and younger
(680-550 Ma) granitic batholiths (El-Ramly, 1972; Fleck et
al. 1976; Gass, 1981; Vail, 1985; Abdel-Rahman & Doig,
1987; Kroner e?a/.1988).
The Pan-African plutonic complexes of eastern Egypt
have been studied in some detail (Brown, 1980; Greenberg,
1981; Gass, 1981, 1982; Stern & Gottfried, 1986; AbdelRahman & Martin, 1987, 1990a; Abdel-Rahman, 1990).
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Products of Early Pan-African volcanism represented by
the 'Shadli' metavolcanic group have been studied by Stern
(1981) and Kroner et al. (1991); Stern subdivided this
metavolcanic group into older metavolcanic rocks (ultramafic rocks, gabbros and pillowed basalts), and younger
metavolcanic rocks (dominantly andesites and volcanogenic metasediments).
However, the Dokhan volcanic sequences were largely
neglected. There is limited geochemical data on Dokhan
volcanic rocks near Safaga and Mount Dokhan (Fig. 1),
but little work has been done on the Dokhan volcanic
rocks within the area under study. The tectonic environment of emplacement of the Dokhan volcanic rocks is
uncertain. The aim of this study is to use petrologicalgeochemical data to interpret the evolution and tectonic
setting of the Dokhan volcanic suite, and evaluate its
origin. This will shed light on Late Proterozoic crustal
development in the region. The study also provides a classic example of the evolution of calc-alkaline magmas via
amphibole and plagioclase fractionation.
2. Regional geology
The 'Dokhan' volcanic sequences are widely distributed
in northeastern Egypt (Fig. 1). The study area, the Mount
El-Kharaza region, represents the largest Dokhan volcanic continuous exposures in eastern Egypt. The area
was previously mapped (scale 1:100 000) by Francis
(1972) and Ghanem (1972). The absence of detailed mapping combined with lack of isotopic age dating, and the
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A.-F.
\
-1
28°-
Phanerozoic
cover
Other shield
rocks
~
Dokhan volcanic
rocks
M. ABDEL-RAHMAN
The investigated Dokhan volcanic rocks range from
green, buff and brown to purple and greyish black. These
rocks include porphyries of andesite, dacite and rhyolite, as
well as tuffs and moderately to highly welded ignimbrites.
The latter are interstratified with porphyritic lava flows. The
pyroclastic (locally ignimbritic) rocks are composed mainly
of lithic and pumice clasts in a tuffaceous groundmass. In
hand specimen, collapsed pumice may appear as dark blebs
of devitrified glass (Ross & Smith, 1961). Lapilli fragments
including quartzite and granite fragments are present. The
repeated eruption of mafic to felsic volcanic materials
locally produced layered structures, forming horizontal to
slightly-tilted sheets, commonly 3-8 m thick. These
Dokhan volcanic rocks overlie gabbro-diorite-tonalite
complexes (Abdel-Rahman, 1990), but are intruded by granodiorite-granite rocks, trondhjemites and dykes (Fig. 2).
3. Geochronological context
24°-
Figure 1. Distribution of the Dokhan volcanic rocks in the
Eastern Desert of Egypt. Note the scarcity of these rocks in the
southern part of Eastern Egypt. MK - Mount El-Kharaza
Dokhan volcanic suite; MD - Mount Dokhan (the type locality);
and F-S, - Fatira-Safaga Dokhan volcanic rocks. The rectangle
outlines the map area shown in Figure 2.
presence of welded ignimbritic textures in these rocks
(possibly previously misidentified as metamorphic textures), may have led to these volcanic rocks being linked
with the much older Shadli metavolcanic group.
However, the absence of dioritic offshoots, and the presence of granitic clasts in these extrusive rocks, in addition
to their younger age (620±16Ma; Abdel-Rahman &
Doig, 1987), clearly indicate that these rocks belong to
the Dokhan group. Equivalents to the Shadli metavolcanic group evidently do not exist in the study area. This
agrees with observations by Ries et al. (1983) and Stern,
Gottfried & Hedge (1984) that older metavolcanic and
metasedimentary units are characteristic of southeastern
Egypt, but are missing northwards.
Schumann (1966) indicated a maximum estimated
thickness of 1200 m for the Dokhan volcanic rocks in eastern Egypt. Basta, Kamel & Awadallah (1979) and Basta,
Kotb & Awadallah (1980) reported on some petrographic
and petrochemical features of these rocks in the type locality 'Mount Dokhan', 150 km southeast of the study area
(Fig. 1). Ressetar & Monrad (1983) studied the geochemistry of some Dokhan volcanic rocks in the Fatira-Safaga
area (about 300 km southeast of the study area).
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Abdel-Rahman & Doig (1987) obtained a 7-point whole
rock Rb-Sr age of 620+16 Ma, with an initial "Sr/^Sr
ratio of 0.7039 ±0.0004 for the Dokhan volcanic rocks
investigated. Rb-Sr ages of 654 to 665 Ma were obtained
(El-Shazly et al. 1973) on Dokhan volcanic rocks from the
central Eastern Desert of Egypt. Stern & Hedge (1985)
obtained an Rb-Sr age of 592 ± 13 Ma for Dokhan volcanic
rocks from Mount Dokhan. A Rb-Sr age of 602 ± 13 Ma
was also obtained by R. J. Stern (unpub. Ph.D. thesis, Univ.
California, San Diego, 1979) for some Dokhan
andesite-rhyolite rocks near Qena-Safaga road. Ressetar &
Monrad (1983) reported preliminary Rb-Sr data that suggest that eruption of Dokhan volcanic rocks took place as
early as 620 Ma in the Fatira-Safaga area. Similar Rb-Sr
ages (633-548 Ma; Segev, 1987) were obtained on orogenic, calc-alkaline, andesite-rhyolite rocks from Sinai,
such as the Kid rhyodacite and the Rutig andesite-rhyolite
(609±12Ma and 587±9Ma, respectively; M. Bielski,
unpub. Ph.D. thesis, Hebrew Univ., Jerusalem, 1982). In
Saudi Arabia, Fleck et al. (1980) obtained Rb-Sr ages of
620 ±90 Ma on andesites (Juqjuq Formation), 612 Ma on
rhyolitic ignimbrite and 593 ± 53 Ma on dacite (Halaban
Formation). Darbyshire et al. (1983) reported ages of
573 ± 23 Ma (Hummah volcanic rocks; andesite-dacite),
608 ± 9 Ma (Arfan volcanic rocks; basalt-andesite) and
616±13Ma (Jahhad volcanic rocks; basalt-rhyolite),
whereas Duyverman, Harris & Hawkesworth (1982)
reported an age of 625 ± 16 Ma for Nuqrah volcanic rocks,
and Aldrich et al. (1978) obtained an age of 633 ± 15 Ma for
Al-Madenah rhyolite (Murdama suite), all from Saudi
Arabia. In Sudan, ages of numerous syn-orogenic to late
orogenic arc-related volcanic rocks (similar to the Dokhan
volcanic rocks) and coeval plutonic complexes ranging
from 660 to 550 Ma are reported by Vail (1990).
4. Petrography
Several petrographic units have been recognized within the
Dokhan volcanic suite. Basalt- and andesite porphyry are
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Pan-African volcanism, Nubian shield
3 3° '00'
10 km
Phanerozoic sedimentary
Gabbro-diorite-tonalite
cover
Dokhan volcanic rocks
Granodiorite-adamelliteleucogranite
Dyke
^
s*
Fault
+
+ M
+ +
Trondhjemite
+ + +
Figure 2. Geological map of igneous complexes in Mount El-Kharaza region, northeastern Egypt.
composed of phenocrysts (10-40%), mainly of plagioclase,
amphibole and minor augite (partially altered to chlorite and
epidote), with magnetite microphenocrysts, in a microcrystalline groundmass. Plagioclase forms subhedral zoned
grains showing glomeroporphyritic texture. Dacite porphyry
is made up of quartz, feldspars, chloritized amphibole and
biotite phenocrysts (15-20%) in a devitrified groundmass.
Opaque magnetite occurs mostly as microphenocrysts. The
groundmass consists of a dense mat of anhedral quartz and
feldspars, occasionally in a microspherulitic array, probably
representing devitrified glass. Rhyolite porphyry consists of
quartz, alkali feldspar and plagioclase phenocrysts, with
some magnetite and biotite microphenocrysts, in a microcrystalline felsitic groundmass (about 70% of the rock),
occasionally showing micrographic or spherulitic textures.
Accessory zircon, apatite and iron oxides are present.
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Ignimbrite is characterized by the predominance of
vesiculated juvenile material (pumice and shards), showing features indicating a pyroclastic flow origin. The ignimbritic flow-unit includes these petrographic varieties:
(a) Fine-grained crystal-lithic welded tuff is a moderately to highly welded, fine-grained rock showing
eutaxitic texture and composed of feldspar phenocrysts,
devitrified pumice and glass shards (Fig. 3), with some
iron oxides, in a tuffaceous to devitrified groundmass.
Fine-grained, compacted and devitrified lapilli, commonly showing flow banding, are present. These lapilli
are moulded against sharp corners of the phenocrysts in
the firmly welded specimens (Fig. 3). Highly flattened,
stretched and devitrified pumice clasts account for a small
amount (20-25 %) of the rock. In some samples, slightly
coarser, lenticular pumice clasts show the filamentous
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A.-F.
M. A B D E L - R A H M A N
Table I. Major element whole-rock compositions (in wt %) and trace elements (in ppm) of the Dokhan volcanic suite
Sample
no.
27a
37a
40a
41a
43a
71a
77a
62a
69a
85a
86a
90a
SiO,
TiO,
A1 2 6 3
K26
P,O,
LOI
75.80
0.09
14.10
1.11
0.19
0.00
0.38
3.33
3.40
0.03
1.53
69.52
0.43
15.31
2.06
0.24
0.11
1.58
4.57
4.54
0.08
1.91
73.18
0.34
14.16
1.82
0.31
0.08
0.32
2.00
6.19
0.06
1.74
76.32
0.12
12.50
1.57
0.07
0.02
0.13
3.32
4.90
0.03
0.78
68.26
0.33
16.01
3.19
0.15
0.27
1.34
6.84
1.56
0.06
2.22
76.94
0.14
12.36
1.42
0.13
0.02
0.45
3.41
3.91
0.03
1.08
67.39
0.74
15.07
4.52
1.11
0.12
2.71
4.98
2.56
0.22
0.92
71.35
0.24
14.75
2.06
0.13
0.06
0.79
3.34
6.65
0.04
1.07
69.57
0.49
15.37
2.53
0.25
0.03
0.94
4.34
4.99
0.09
1.72
74.16
0.20
13.65
1.49
0.16
0.06
0.65
4.69
4.12
0.05
0.39
69.19
0.50
16.22
2.31
0.28
0.06
0.87
5.93
3.31
0.08
0.84
67.56
0.58
15.73
3.78
0.67
0.08
1.46
6.12
2.03
0.15
1.60
Total
99.96
100.36
100.21
99.76
100.23
99.88
100.34
100.48
100.32
99.61
99.58
99.76
50
445
194
113
99
72
41
111
1241
243
139
268
273
FeO
MgO
MnO
CaO
Na,0
Rb
Sr
Ba
Zr
Hf
Y
Nb
U
Th
87
95
137
116
58
77
102
148
89
81
148
87
193
33.1
12.3
12.9
13.1
295
244
200
336
21.3
12.0
23.7
13.2
20.1
13.8
4.8
9.2
4.4
8.7
89a
LOI
67.77
0.47
16.62
2.71
0.63
0.10
1.81
5.17
4.14
0.11
0.86
Total
100.38
Rb
Sr
Ba
107
355
163
25.9
13.0
11.1
23.2
16.6
10.5
13.4
27b
78b
84b
66.60
0.89
16.02
4.84
0.30
0.07
1.66
4.31
4.53
0.27
0.50
48.62
0.99
13.13
8.61
8.82
0.15
7.41
2.53
0.86
0.26
9.27
56.12
1.01
17.66
7.93
3.36
0.13
7.06
3.38
1.14
0.31
1.76
99.99
100.65
197
207
6.3
762
341
292
30.0
13.1
4.8
6.5
16.5
85b
82b
70b
90b
56.92
0.71
18.86
6.25
3.24
0.08
5.06
4.16
2.38
0.20
2.34
53.51
0.81
18.17
8.04
3.75
0.18
8.86
4.14
1.99
0.27
0.65
63.99
0.72
15.69
5.56
1.57
0.06
4.39
4.67
1.67
0.18
1.58
47.94
1.91
16.13
11.79
5.44
0.19
7.34
2.84
2.03
0.53
4.33
62.23
0.86
15.67
6.37
2.06
0.12
4.78
4.23
2.19
0.24
1.01
57.74
1.23
15.32
8.60
2.43
0.13
4.04
4.90
1.85
0.33
3.23
55.25
0.72
17.48
7.26
4.07
0.11
6.30
4.89
1.52
0.18
1.92
58.13
0.62
17.70
6.55
2.66
0.08
5.34
5.03
1.40
0.15
2.20
99.87
100.22
100.37
100.07
100.48
99.76
99.80
99.70
99.86
29.3
7.8
6.1
17.3
14.1
20.0
12.1
6.9
7.9
13.5
11.2
8.1
7.9
234
5.8
27.1
10.0
12.6
6.5
19.3
12.8
184
27.9
12.7
10.0
5.8
Sample
no.
SiO 2
TiO 2
A1 2 O 3
FeO
MgO
MnO
CaO
Na,0
K26
P,O5
Zr
Hf
Y
Nb
U
Th
125a
109
368
328
531
59
722
829
32
906
875
33
392
708
46
577
50
379
34
405
754
243
117
121
163
162
162
144
201
168
3.5
20.5
1051
310
7.0
20.6
13.3
22.5
13.8
4.2
6.3
7.4
11.4
106b
3.7
3.5
3.5
16.6
16.1
15.6
20.8
9.1
8.4
8.3
3.2
8.8
8.2
2.7
9.6
11.0
10.5
11.0
13.1
1.4
2.1
11.0
3.4
22.6
10.8
23.1
10.8
22.6
10.6
9.0
1.3
7.9
3.1
7.5
2.2
225b
229b
50
407
38
430
85
89
11.9
11.2
6.5
9.1
2.0
11.6
6.5
2.2
Total iron is given as FeO. LOI - loss on ignition. Sample numbers followed by the letter 'a' represent dacite-rhyolite (light-coloured) rocks, whereas
those followed by the letter 'b' represent basalt-andesite-dacite (dark-coloured) rocks. Ba and Hf were analysed by NAA technique.
nature of fiamme with their characteristic tails; devitrification products are quartz and feldspars.
(b) Coarse-grained pumice-rich tuff is composed of
large, coarse-grained, light grey filamentous pumice
clasts and shards (70 % of the rock), with some feldspar
phenocrysts in a tuffaceous, brownish groundmass.
Scattered grains of magnetite and oxidized mafic minerals are present. The poorly sorted nature and the predomi-
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nance of relatively large clasts of (devitrified) pumice are
remarkable features in this rock.
(c) Thin-bedded graded tuff is a greenish grey, banded,
clast-free tuffaceous rock. It is made up of very
fine-grained, vitreous-ashy materials, with tiny particles
of quartz and feldspars, showing vari-coloured banding
and laminations due to gradational changes in grain size.
This rock is interpreted as an air-fall tuff, or the fine dust
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Pan-African volcanism, Nubian shield
exhibit a compositional gap. It forms roughly linear
arrays with relatively smooth variations showing progressive decrease in Ca, Fe, Mg and Al (Fig. 4a), and progressive increase in alkalies (Fig. 5a), with gradual
increase in silica. Most rocks plot in the 'subalkaline'
field of Irvine & Baragar (1971; Fig. 5a), indicating that
they are not alkaline in nature. Furthermore, the rocks
define a typical calc-alkaline trend on the standard AFM
diagram (Fig. 5b), forming a continuous calc-alkaline
magmatic series.
6.b. Trace elements
Figure 3. A photomicrograph of crystal-lithic welded tuff showing
angular feldspar phenocrysts, lithic clasts, stretched glass shards
and a pumice clast. Plane-polarized light, width of view 3.3 mm.
elutriated from the top of the pyroclastic flow during
flowage. Sparks, Self & Walker (1973) pointed out that
the billowing cloud seen rising above nuees ardentes is
thought to be the source of material for such a layer.
5. Analytical techniques
5.a. Major elements
Concentrations of the major elements were determined
by X-ray fluorescence spectrometry (Philips PW1400) on
fused discs (operating conditions: Rh radiation, 40 kV, 70
mA). Loss on ignition (LOI) was determined by heating
powdered samples for 50 minutes at 1000 °C.
5.b. Trace elements
Concentrations of Rb, Sr, Zr, Y, Nb, U and Th were determined on pressed pellets by X-ray fluorescence (operating conditions: Rh radiation, 70 kV, 40 mA). The
analytical precision, as calculated from 20 replicate
analyses of one sample, is better than 1 % for most major
elements and better than 5 % for most trace elements.
5.c. Rare earth elements (REE)
The neutron-activation technique of Barnes & Gorton
(1984) was used to determine concentrations of Ba, Cr,
Sc, Co, Hf and eight rare-earth elements (REE; La, Ce,
Nd, Sm, Eu, Tb, Yb and Lu). A precision of better than
5 % was achieved for most elements. Normalization values are those of the Leedy chondrite (Masuda, Nakamura
6 Tanaka, 1973) divided by 1.2 (Taylor & Gorton, 1977).
6. Geochemistry
6.a. Major elements
The Dokhan volcanic rocks cover a wide range of composition and exhibit geochemical trends spanning the entire
range from basalt to high-silica rhyolite (48-77 wt%
SiO • Table 1, Fig. 4a). The suite does not appear to
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The Dokhan volcanic rocks are also characterized by a wide
range of trace elements; Sr varies from 81 to 906 ppm, Rb
from 21 to 193 ppm, Ba from 708 to 1241 ppm, Zr from 85
to 340 ppm, Nb from 6.5 to 16.6 ppm and Y from 11 to 33
ppm (Tables 1, 2), typical of calc-alkaline orogenic suites
(Gill, 1981). Hf does not show much variation (3.5-7 ppm)
as it is usually much more depleted than Zr in most calcalkaline rocks. The compatible elements Cr, Sc and Co vary
from 12 to 91,3.6 to 24 and 1 to 22 ppm, respectively.
A plot of SiO2 vs. selected trace elements (Fig. 4b)
shows a gradual general decrease in Sr and gradual
increase in Rb, Zr and Nb with increasing silica. Note that,
in the case of Rb and Zr, as well as the alkalies (cf. Fig.
5a), convex trends are obtained; the trends show gradual
increases in these elements with increasing silica up to a
maximum value of about 70wt% SiO,, after which a
sharp decrease with increasing SiO2 occurs (between 70
and 77 wt% SiO,). This feature, also observed in other
fractionating calc-alkaline suites, has been documented by
several authors (Dietrich, 1968; Watson, 1979; Miller &
Mittlefehldt, 1984). Watson (1979) demonstrated experimentally that zircon saturation in a magma shows pronounced dependence upon the (Na2O + FC,O/A12O3) ratio
of the melt; the low solubility of zircon in melts with low
alkali/aluminium ratio result in an early crystallization of
zircon, causing Zr depletion in the more evolved products
of crystallization (high-silica rhyolites). This is consistent
with the striking similarity in the variation of both Zr and
alkalies vs. silica obtained in Figures 4b and 5a.
When highly incompatible elements are plotted against
each other, such as Zr vs. Nb (Fig. 6a), a linear trend (with
some scatter) is obtained, showing gradual increase in Zr
with increasing Nb. Rb also shows a similar trend. The
Nb/Y vs. Zr/TiO2 diagram (Fig. 6b; Winchester & Floyd,
1977) shows the classification of this volcanic suite; it
contains basalt, andesite, dacite, rhyodacite and rhyolite.
It should be noted that these rock types exhibit relatively
similar Nb/Y ratios. The Zr/TiO2 ratios increase gradually from basalt to rhyolite (Fig. 6b), as Zr is more incompatible than Ti.
The relative concentrations of trace elements are shown
in the form of a primitive mantle-normalized diagram (Fig.
7a), based on the normalization factors of Sun &
McDonough (1989). In general, all rocks show moderate
concentrations of most large ion lithophile elements, high
IP address: 137.222.24.34
22
A.-F.
10 -
0
ID
0
8
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i
L
•
N
200
D
a •
j
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6
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4
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2
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I
i
i
n
i
i
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120
40
D
i
160
Oo O
o
D
D
. •
D
D _
nlP
D
1000
a
800
600
.
0
400
D *
20
12
I
80
8
16
i
a
10
0
<
a
i
200
a
-
o
D
D
100
a
cP
a
• D
0
U.
oo
300
6
2
0)
I
0 0°oOo
m
0
o
o£Ln
,
, ° cP.SP
2 -
01 4
0
o o
0
|
Z --
Dacite-rhyolite 0
D°
4 -
(b)
20
Basalt-andesite o
D
a
6 •
_
10 -
0
(a)
a
8
M. ABDEL-RAHMAN
- •
a
ada
30
QD
O°o
200
OO
•
0
50
ooo
60
,OO°,QP
70
80
SIOo.
CD
i
50
60
70
80
SiO 2
Figure 4. Variations of SiO., vs. (a) other major elements, and (b) selected trace elements, within the Dokhan volcanic suite.
field strength elements and light rare earth elements. The
dacite-rhyolite rocks show pronounced Nb, Sr and Ti negative anomalies, whereas the less-evolved basalt-andesite
rocks exhibit Th and Nb negative anomalies (Fig. 7a). These
patterns are typical of modern arc-related calc-alkaline orogenic suites, as the rhyolite-dacite ash-flow tuffs and coeval
andesite-rhyolite lavas of the Indian Peak volcanicfieldin
Nevada and Utah (Best, Christiansen & Blank, 1989).
REE patterns of the less-evolved rocks (53.5-64 wt%
SiO2; Fig. 7b) are similar overall to those of arc-related
basalts (50-53 wt% SiO2) of the southern Andes
36-41°S (Lopez-Escobar, 1984), but with relatively
higher REE values. The REE profiles of the dacite-rhyolite rocks (Fig. 7c) resemble those of the Andean ignimbrites of northern Chile and northwestern Argentina
(Francis et al. 1989), and are thus typical of light-rareearth-enriched calc-alkaline volcanic rocks.
6.c. The rare earth elements (REE)
Rock varieties of the Dokhan volcanic suite show a general increase in REE contents toward the more-evolved
rocks (Table 2), and exhibit subparallel REE chondritenormalized patterns (Fig. 7b, c). All samples show light
REE enrichment over heavy REE, and display a range of
negative Eu-anomalies; the more-evolved lithologies
exhibiting larger anomalies compared to the less-evolved
ones (Fig. 7b, c). This could be due to low-pressure fractionation of plagioclase that led to depletion in Sr and Eu
in the more-evolved rocks. Higher oxidation conditions
in more-evolved melts could also cause negative Euanomalies.
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7. Discussion
7.a. Bimodality
The Dokhan volcanic rocks described here form a continuum in composition from basalt and andesite to dacite and
rhyolite, with no apparent compositional gap (cf. Figs 4a,
b, 5b). Abdel-Rahman & Martin (1989) showed also that
other Dokhan volcanic rocks from the Safaga-Fatira area
which were studied by Ressetar & Monrad (1983) cover a
similar wide range in composition. Thus, the basis for the
inference that Dokhan volcanic rocks are generally
'bimodal suites' (Stern, Gottfried & Hedge, 1984; Stern &
Gottfried, 1986; Stern, Sellers & Gottfried, 1988) is not
IP address: 137.222.24.34
23
Pan-African volcanism, Nubian shield
20
18
16
14
Table 2. Rare earth and other trace element compositions (in ppm) of
the Dokhan volcanic suite
(a)
D
O
Biull-andeiite
Dacite-rhyolite
12
10
»
6
®
Po
o°
(9 C
s/
DD
]
4
Suba Ikalln
2
/
35 40 45 50 55 60 65 70 75 80 85
Sample
no.
62a
90a
La
Ce
Nd
Sm
Eu
Tb
Yb
Lu
46.49
86.49
34.47
6.46
0.79
1.00
2.83
0.44
27.01
53.55
24.48
6.09
1.23
0.77
2.77
0.40
Cr
Sc
Co
16.10
3.56
1.04
16.50
11.20
2.39
84b
85b
82b
31.72
71.06
24.73
5.12
1.28
0.71
1.95
0.37
24.97
51.02
20.97
4.69
1.31
0.70
1.47
0.30
21.58
44.53
20.43
4.50
1.20
0.46
1.57
0.26
16.96
41.29
18.62
4.39
1.31
0.80
2.21
0.39
17.64
38.70
19.96
4.92
1.15
0.64
1.79
0.32
19.90
4.35
3.02
31.80
12.60
22.33
90.70
16.30
22.34
24.20
16.90
9.17
12.10
24.32
20.75
125a
106b
SiO 2
The rare earth elements as well as Cr, Sc and Co were analysed by the
NAA technique. Sample numbers followed by the letter 'a' represent
dacite-rhyolite (light-coloured) rocks, whereas those followed by the
letter 'b' represent basalt-andesite-dacite (dark-coloured) rocks.
FeO*
(b)
D Basalt-andesite
o Dacite-rhyolite
Na2O+K2O
MgO
Figure 5. (a) SiO2 vs. Na,0 + K.,0 variation diagram; the alkaline and subalkalinefieldsare after Irvine & Baragar(1971). (b)
A-F-M diagram showing the calc-alkaline nature of the
Dokhan volcanic suite.
readily apparent. It should be noted that, in these three
studies, the latter authors used data from unrelated rock
types as dykes, 'Younger' granites, and volcanic rocks of
different ages and localities (80 to 100 km apart), and considered all to be products of the same tectonic environment. Thus, the apparent bimodality obtained by those
authors was in part the result of using unrelated stratigraphic units and may also be an artifact of sampling.
7.b. Tectonic environment
7.b.l. Previous models
With three different tectonic models previously proposed
by various authors for its emplacement (extensional, transitional and arc-related settings), the tectonic environment of the Dokhan volcanic suite is still controversial.
In the opinion of Stern & Gottfried (1986), the
'Dokhan' suite in the Dokhan-Qattar area is dominantly
mafic in character, defines a calc-alkaline trend on the
AFM diagram and is comparable to typical orogenic
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andesites, but with moderate enrichments in incompatible
elements. However, they considered the suite to be
bimodal in nature and have used this, along with its moderate enrichment in some incompatible elements to
regard these volcanic rocks to be products of 'strong
crustal extension' (or anorogenic). Ressetar & Monrad
(1983) found the Dokhan volcanic rocks of the
Safaga-Fatira area to exhibit abundances of most major
and trace elements similar to subduction-related calcalkaline volcanic rocks in orogenic belts, but with slightly
higher contents of alkalies, Ti, Zr and Nb than typical
island arc or continental margin volcanic suites. These
authors found that the analyses plot near the boundary
between suites considered to be subduction-related and
those considered typical of late-orogenic or extensional
environments (their figs 5, 6). They interpreted these
rocks to have been formed in an environment transitional
between compressive and extensional tectonic settings. It
should be noted, however, that slight enrichment in some
incompatible elements within arc-related volcanic rocks
is not uncommon.
As pointed out by Saunders & Tarney (1979), dehydration of the downgoing oceanic slab enriches the overlying
mantle wedge, and subsequently magmas generated from
it, in large-ion-lithophile (LIL) elements. Higher Zr contents which may occasionally reach within-plate magma
values (see, e.g. Gokten & Floyd, 1987) are a characteristic feature of calc-alkaline volcanic rocks formed in a
continental arc (margin) environment (Pearce, 1983;
Capan & Floyd, 1985). Gill (1981) and Ringwood (1990)
demonstrated that the behaviour of trace elements, particularly REE and HFS elements, in subduction zone environments, will influence the trace-element signature of
arc-related volcanic rocks. Recent results of Hickmott,
Sorensen & Rogers (1992) showed that heavy REEs are
selectively transported during metasomatism in a subduction complex, whereas Philippot & Silverstone (1991)
showed that both Ti and Zr may be strongly concentrated
in aqueous fluids at subduction zone P-Tconditions.
IP address: 137.222.24.34
24
A.-F.
7.c. Nature of analogous volcanic and coeval plutonic rocks
(a)
- • Basalt-andesite
o Dacite-rhyolite
O
15
Q10
Z
©° o
-
•
s^
5 1
1
i
1
200 -
i
1
0
§
o
40 i
i
100
i
180
1
1
260
8°
I
o
o
1
340
(b)
O
Biialt-udesite
O
Dacite-rhyolite
h .1
L
N
.01
Andesite/Basalt
SubAlkaline Basalt
.001
.01
.1
Alk-Bas
Bsn/Nph"
10
Figure 6. (a) Nb and Rb vs. Zr variation diagram, (b) Log
(Nb/Y) vs. log (Zr/TiO,) diagram showing the classification of
the Dokhan volcanic rocks; fields are after Winchester & Floyd
(1977).
7.b.2. Suggested tectonic model
The Dokhan volcanic suite is calc-alkaline in nature (cf.
Fig. 5), and its trace element and REE patterns resemble
those typical of modern, arc-related, orogenic suites (Fig.
7). Data points of basalt-andesite rocks plot, on geochemical-tectonic discriminant diagrams (such as Fig.
8a), in the field of calc-alkaline basalts (CAB) of Pearce
& Cann (1973). The dacite-rhyolite rocks plot (Fig. 8b)
in the volcanic arc field of Pearce, Harris & Tindle (1984)
and in the late orogenic and syn-collision fields (Fig. 8c)
of Batchelor & Bowden (1985). None of the data points
plot in the 'anorogenic' field. Thus, the Dokhan volcanic
suite is interpreted to be the product of subduction-related
volcanism produced during late stages of the Pan-African
orogeny.
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M. ABDEL-RAHMAN
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In Sinai, Segev (1987) showed that the 633-548 Ma rhyodacitic rocks belong to the orogenic stage. In Saudi
Arabia, similar volcanic suites including the Hummah
suite (573 ±23 Ma; Darbyshire et al. 1983), Shammar
suite (572-557 Ma; Hadley, 1973) and Murdama suite
(633±15Ma;Aldriche?a/. 1978; Roobol et al. 1983) are
calc-alkaline, orogenic rocks. Schmidt & Brown (1982)
and Stuckless et al. (1983) showed that the 620-560 Ma
magmatism in northwestern Arabia predominantly
involved calc-alkaline I-type melts. In Sudan, numerous
synorogenic to late orogenic volcanic rocks (equivalent to
the Dokhan volcanic rocks) and coeval plutonic complexes (660-550 Ma) are interpreted to be the result of
arc-magmatism (e.g. Vail, 1990, and references therein).
Thus, analogous volcanic suites in Sinai, Sudan and
Saudi Arabia also have been interpreted as products of
destructive plate margin magmatism. Also, coeval granodioritic batholiths covering large areas in the
Arabian-Nubian shield were emplaced during the Late
Pan-African magmatic episode (~650-550 Ma); these
occur as volumetrically significant, I-type, calc-alkaline,
Andean-type complexes (Neary, Gass & Cavanagh, 1976;
Brown, 1980; Gass, 1982; Abdel-Rahman & Martin,
1987; Vail, 1990).
Rift-related volcanic (and plutonic) complexes do
occur within the Pan-African belt, but these are younger
(mostly less than 550 Ma), post-Pan-African minor
suites, locally emplaced within an otherwise calc-alkaline, Pan-African orogenic assemblage of batholithic
dimensions. The bimodal volcanic-subvolcanic suite
(550-540 Ma) that crops out on the eastern flank of Wadi
Araba Rift Valley, southwest Jordan (Jarrar, Wachendorf
& Saffarini, 1992), is a good example of rift-related, postPan-African anorogenic volcanic rocks.
Unlike orogenic volcanism, products of anorogenic
volcanism typically erupt along ring fractures, forming
relatively small circular or elongated tabular structures
and are alkaline to peralkaline in composition (see, e.g.
Abdel-Rahman & Martin, 1990/?; Abdel-Rahman &
Miller, 1993). In view of the lack of clear evidence for
'crustal extension', or 'transitional' tectonic environment
advanced by previous authors, it is suggested that the
slight enrichment in some incompatible elements (Na, K,
Ti, Zr, Nb) observed in some Dokhan volcanic rocks from
the Qattar-Dokhan and Safaga-Fatira areas may reflect
enrichment above subduction zones, and do not reflect
anorogenic emplacement. Thus, a subduction-related tectonic setting for the emplacement of the investigated suite
is indicated by the petrological-geochemical features
documented herein.
7.d. Petrogenesis of the Dokhan volcanic suite
l.d.l. Fractional crystallization vs. other processes
Rocks of the Dokhan volcanic suite cover the entire spectrum from basalt to rhyolite. The smooth variations in
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Pan-African volcanism, Nubian shield
25
Basalt-Andesite
Dacite-Rhyolite
I «•
a:
o
O
o
1 -
i
1
:
1
(b)
Basalt-Andesite
I ROCK /CHONDRI1
UJ
IAB (S. Andes)
i
i
i
i
La
Co
Pr
Nd
i
i
i
i
I
1
1
1
1
1 1
Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Dacite-Rhyolite
(c)
Basalt-Andesite
La
Co
Pr
Nd
I
I I
Pm Sm Eu Gd Tb Dy Ho Er TmYb Lu
Figure 7. (a) Primitive mantle-normalized trace element plot
showing comparison of element distributions for the lessevolved basalt-andesite rocks (patterns with a square symbol),
superimposed on a dotted envelope representing the moreevolved dacite-rhyolite rocks of the Dokhan volcanic suite, (b)
Chondrite-normalized plot showing four rare-earth patterns for
less-evolved Dokhan volcanic rocks (basalt-andesite; 53.5-64
wt% SiO2), and an envelope (in diagonal lines) representing
REE patterns for typical arc-related basalts from Laguna del
Maule, Antuco and Cordon Cenizos volcanic complexes of the
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major and trace elements with well-defined geochemical
trends (Figs 4,6), and the general increase in the contents
of REE accompanied by larger negative Eu-anomalies in
the more evolved rocks, may suggest that the different
lithologies of the suite are co-magmatic, that is, derived
from a single magma body, and were possibly evolved by
fractional crystallization.
There is no field, isotopic or any other evidence to suggest processes such as magma mixing or crustal assimilation for the evolution of this suite. Furthermore, the
Sr-isotope data indicate that all analysed samples fall on a
linear array, defining a exceptionally coherent trend with
a positive correlation between 87Sr/86Sr and Rb/Sr ratios
(Fig. 9). The 87Sr/86Sr ratios occupy a wide range from
0.7053 to 0.7436 (Abdel-Rahman & Doig, 1987), reflecting the wide compositional range of the suite, and further
support the concept that its various rock types were
derived from the same source.
The mafic nature of the less-evolved members of the
suite, along with its low initial 87Sr/86Sr ratio (0.7039),
which is considered lower than contemporary values in a
source region containing ancient sialic basement, preclude an origin by melting of older (e.g. Archean) crustal
material. Moreover, there is no evidence to suggest the
presence of Archean crust at least in the northern Nubian
shield (see, e.g. Kroner et al. 1988).
Several genetic models have been proposed for the origin of calc-alkaline magmas. The partial melting of
water-bearing upper-mantle and lower-crust rocks provides a general working hypothesis for calc-alkaline
magma genesis. Subducted oceanic crust, modified by
exchange with seawater, is found to be an important
source material for such magma (Gill, 1981; DeVore,
1983). But partial melting of such subducted oceanic
crust (at depth 100-200 km) would produce magmas of
andesitic-dacitic compositions (Green & Ringwood,
1968; Marsh & Carmichael, 1974; Green, 1980).
Judging from the mafic nature of the magma of the
Dokhan volcanic suite (calc-alkaline basalt; 48wt%
SiO2) and the low initial Sr-isotopic ratio, partial melting
of a mantle material (e.g. hydrated lherzolite) is a more
likely source. The relative enrichment of incompatible
elements including light-rare-earth elements (cf. Tables
1, 2; Fig. 7), is considered to manifest incorporation of
slab-derived incompatible components (via a fluid phase)
into a relatively undepleted mantle wedge above a subduction zone. Partial melting of hydrated mantle rocks
southern Andes 3 6 ^ 1 °S, taken from Lopez-Escobar (1984). (c)
Chondrite-normalized plot showing three REE patterns for the
more-evolved Dokhan volcanic rocks (dacite-rhyolite;
67.6-71.4 wt% SiO2), and an envelope (in diagonal lines) representing the less-evolved Dokhan volcanic rocks. These patterns are superimposed on an envelope (defined by thick lines)
representing REE profiles of typical Andean ignimbrites of
northern Chile and northwestern Argentina, taken from Francis
etal. (1989); see text for details.
IP address: 137.222.24.34
26
A.-F.
M. ABDEL-RAHMAN
Ti/100
D Basalt-andesite
- O Dacite-rhyolite
(a)
o
L
D Basalt-andesite
(/}
is
a
0.730
-
i
0.720
_
o
0
w
»
o°
0.710
DO
i
0.0
i
0.4
I
I
I
0.6
I
1.2
I
I
1-6
Rb/Sr
Sr/2
Zr
1000
D Basalt-andesite
; o Dacite-rhyolite
Figure 9. Rb/Sr vs. "Sr/^Sr variation diagram for the Dokhan
volcanic suite; data points exhibit an exceptionally coherent trend.
would produce calc-alkaline basaltic magma, from which
this basalt-rhyolite series could have been produced, possibly by fractional crystallization.
(b)
7.d.2. Fractionation modelling: major elements
100
*
2000
0)
2
+
1-Mantle Fractionates
2—Pre-plate Collision
3-Post-collision Uplift
4—Late-orogenic
5-Anorogenic
6-Syn—collision
7—Post-orogenic
(c)
O Dacite-rhyolite
Q 1000
(0
II
W
o:
3000
1000
2000
R 1 =4Si-1 1 (Na+K)-2(Fe+Ti)
Figure 8. (a) Ti-Zr-Sr triangular diagram showing that the
Dokhan basalt-andesite rocks (squares) plot in the field of calcalkaline basalt (CAB) of Pearce & Cann (1973). The other two
fields are island-arc basalt (IAB) and ocean floor basalt (OFB).
(b) Plot of Rb vs. (Y + Nb); Fields VAG, volcanic arc granite;
Syn-COLG - syn-collision granite; WPG - within-plate granite;
and ORG - oceanic ridge granite, are after Pearce, Harris &
Tindle (1984). Circles represent dacitic-rhyolite rocks and
squares generally represent andesitic rocks and plotted here only
for reference, (c) R1 [4Si - 11 (Na + K) - 2(Fe + Ti)] vs. R2 [6Ca
+ 2Mg + Al] diagram showing that the Dokhan dacitic-rhyolitic
rocks are mostly late-orogenic; none of the samples plot in the
anorogenic field (fields are after Batchelor & Bowden, 1985).
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Modelling of closed-system fractional crystallization by
least-squares methods (Wright & Doherty, 1970) can be
used to approximate the amounts and proportions of fractionating phases (Perry et al. 1990). Due to changes in the
phenocryst assemblages and in the compositions of solidsolution minerals within this compositionally broad
basalt-rhyolite series, modelling in three stages using representative compositions of fractionating phases from three
parental compositions is more appropriate. Parental compositions were chosen at similar intervals of SiO2 contents
along the 'liquid line of descent', starting from the least silica-rich basalt (sample 70b), to the most silica-rich rhyolite
(71a). The composition of the mineral phases used are the
actual microprobe data of observed phenocrysts.
Fractionation models (Tables 3a, b, c) show that the
most evolved (daughter) compositions can be produced
from the least-evolved (parent) magmas. Thus, in stage 1,
andesite (daughter, 106b) was produced from a basaltic
parent (70b) by fractional crystallization (with a 54.2 %
residual liquid) of 26.4% amphibole, 13.4% bytownite
and 6.1 % magnetite (normalized to 100; 57.5 %, 29.2 %,
13.3%, respectively). In stage 2, dacite (daughter, 90a)
was produced from the andesitic parent by fractionation
(with a 63.8% residual liquid) of 11.2% amphibole,
19.9% andesine, and 5.1% magnetite (normalized to
100; 30.9 %, 55 %, 14.1 %, respectively). In stage 3, highsilica rhyolite (daughter, 71a) was produced from the
dacitic parent by fractionation (with a 51.4% residual
liquid) of 2.2% biotite, 43.8% oligoclase and 2.6%
magnetite (normalized to 100; 4.5%, 90.1%, 5.3%,
respectively). The low values of the sum of the squares of
residuals (ZR2: 0.87, 0.20 and 0.26, for stages 1 to 3,
respectively) indicate a reasonable fit (Tables 3a, b, c).
Several authors (e.g. Green & Ringwood, 1968; Gill,
1981; Anderson, 1980; Kokelaar, 1986) have demonstrated the role of amphibole fractionation to produce
IP address: 137.222.24.34
Pan-African volcanism, Nubian shield
27
Table 3a. Results of crystal fractionation least-squares modelling from basalt to andesite
Fractionating phases
amphibole
42.34
1.37
14.72
9.21
14.83
0.05
12.92
0.96
1.11
SiO 2
TiO 2
A12O3
FeO
MgO
MnO
CaO
Na,0
K,6
plagioclase
49.73
29.52
0.41
0.09
16.30
3.56
0.66
magnetite
0.68
11.41
2.32
77.96
3.00
_
—
-
Observed
daughter
(no. 106b)
Observed
Dtircnt
(no. 70b)
Calculated
parent
60.00
1.28
15.92
8.94
2.52
0.14
4.20
5.09
1.92
50.14
2.00
16.87
12.33
5.70
0.20
7.68
2.97
2.12
50.15
1.90
16.62
12.35
5.65
0.22
7.98
3.58
1.54
Weighted
residual
-0.01
0.10
0.25
-0.02
0.05
-0.02
-O.30
-0.61
0.58
The daughter (sample no. 106b) and the parent (no. 70b) are recalculated to 100% and the total iron is given as FeO. The compositions of amphibole,
plagioclase and magnetite are the actual electron microprobe analyses of phenocryst phases. Residual liquid (no. 106b): 54.2%. Fractionating phases:
45.8 % (amphibole 26.4 %, bytownite 13.4 %, magnetite 6.1 %). Sum of the squares of residuals (ER 2 ) is 0.87.
Table 3b. Results of crystal fractionation least-squares modelling from andesite to dacite
Fractionating phases
amphibole
SiO 2
TiO 2
A1,O3
FeO
MgO
MnO
CaO
Na,O
K.6
53.13
0.94
2.60
14.20
14.92
0.18
11.98
0.08
0.11
plagioclase
56.92
27.12
0.23
9.05
5.57
0.82
magnetite
0.78
5.34
0.26
88.53
-
Observed
daughter
(no. 90a)
Observed
parent
(no. 106b)
Calculated
parent
68.93
0.59
16.05
3.86
0.68
0.08
1.50
6.24
2.07
60.00
1.28
15.92
8.94
2.52
0.14
4.20
5.09
1.92
60.00
1.07
15.82
8.95
2.40
0.39
4.33
5.27
1.77
Weighted
residual
0.00
0.21
0.10
-0.01
0.12
-0.25
-0.13
-0.18
0.15
The daughter (sample no. 90a) and the parent (no. 106b) are recalculated to 100 % and the total iron is given as FeO. The compositions of amphibole,
plagioclase and magnetite are the actual electron microprobe analyses of phenocryst phases. Residual liquid (no. 90a): 63.8 %. Fractionating phases:
36.2 % (amphibole 11.2 %, andesine 19.9 %, magnetite 5.1 %). Sum of the squares of residuals ( I R 2 ) is 0.20.
Table 3c. Results of crystal fractionation least-squares modelling from dacite to high-silica rhyolite
Fractionating phases
biotite
SiO 2
TiO 2
A12O3
FeO
MgO
MnO
CaO
Nafi
K,0
36.41
2.97
12.25
20.49
11.96
0.12
0.33
9.28
plagioclase
65.61
21.50
0.12
0.16
2.36
10.47
0.14
magnetite
0.51
2.14
0.32
90.71
-
Observed
daughter
(no. 71a)
Observed
Durcnt
(no. 90a)
Calculated
parent
77.89
0.14
12.51
1.44
0.13
0.02
0.46
3.45
3.96
68.93
0.59
16.05
3.86
0.68
0.08
1.50
6.24
2.07
68.91
0.32
16.06
3.87
0.53
0.13
1.37
6.40
2.41
Weighted
residual
0.02
0.27
-0.01
-0.01
0.15
-0.05
0.13
-0.16
-0.34
The daughter (sample no. 71a) and the parent (no. 90a) are recalculated to 100% and the total iron is given as FeO. The compositions of biotite,
plagioclase and magnetite are the actual electron microprobe analyses of phenocryst phases. Residual liquid (no. 71a): 51.4%. Fractionating phases:
48.6 % (biotite 2.2 %, oligoclase 43.8 %, magnetite 2.6 %). Sum of the squares of residuals (SR 2 ) is 0.26.
andesite from a basaltic magma. Cawthorn & O'Hara
(1976) have shown that in mafic and intermediate calcalkaline magmas, amphibole is an important nearliquidus phase over the range PHlO = 2-\Q kb. More
recently, Romick, Kay & Kay (1992) studied calc-alkaline andesite and dacite tephra from the central Aleutian
http://journals.cambridge.org
Downloaded: 20 Mar 2015
arc (Alaska) and concluded that trace element modelling
supports an open system origin for the andesitic lavas
involving amphibole fractionation, whether or not amphibole is a modal mineral. In view of the results of fractionation modelling, it is clear that amphibole dominates the
early fractionating mineral assemblage from basalt to
IP address: 137.222.24.34
28
A.-F.
andesite. Plagioclase minerals were the dominant fractionating phases from andesite to dacite to high-silica
rhyolite. This is consistent with the abundance of plagioclase as a phenocryst phase in these rocks, and with the
progressive decrease in Sr with increasing SiO r The continuous Fe depletion (Fig. 4a) and suppression of relative
Fe enrichment that produced the calc-alkaline trend (Fig.
5b) indicate the continuous removal of magnetite which,
with amphibole, is also responsible for the depletion of Ti
(cf. Table 1). The relatively well-defined geochemical
trends obtained for the Dokhan volcanic suite, along with
the progressive increase in Eu-anomalies toward moreevolved compositions, combined with results of the geochemical modelling, support the idea that a fractional
crystallization process has played a major role during the
evolution of the Dokhan volcanic magma series.
Table 4. Partition coefficients used in the trace-element fractional
crystallization modelling (after Arth, 1976, and Gill, 1981)
La
Ce
Nd
Sm
Eu
Yb
Lu
Nb
Rb
Sr
Plagioclase
Magnetite
0.40
0.510
1.20
2.0
1.70
2.0
1.7
1.3
0.05
0.23
0.35
0.24
0.17
0.13
2.11
0.077
0.062
0.025
0.041
4.4
0.098
0.11
0.13
0.15
0.1
0.17
0.19
0.90
0.0
0.0
Table 5. Results of trace element fractional crystallization models at
70 % fractionation
7.d.3. Trace element modelling
La
Ce
Nd
Sm
Eu
Yb
Lu
Nb
Rb
Sr
where C is the initial concentration of the trace element
in the source, F is the fraction of liquid remaining, and Ds
is the bulk distribution coefficients of the fractionating
minerals. Modelling was carried out in one stage using
available trace element data. The calculations were done
using trace element values (Cj) of the least fractionated
sample for which rare earth data are available (85b; 53.5
wt% SiO,). The percentage of fractionating phases used
are those obtained from the least-squares fractionation
modelling (Table 3b); amphibole 12.1, plagioclase 22.5
and magnetite 4.4 (normalized to 100: 31 % amphibole,
58 % plagioclase, 11 % magnetite). The distribution coefficients (KDs) used are taken from Arth (1976) and Gill
(1981) and are listed in Table 4.
The results (Table 5), indicate a close match between
calculated REE and other trace element values in the differentiated liquid at 70 % crystallization and those in the
most fractionated sample measured (62a; 71.4 wt % SiO,).
It should be noted that calculated trace element abundances
are considered acceptable if model values are within 20 %
of measured values (Graham & Hackett, 1987; Patterson &
Graham, 1988). However, the calculated Rb values differ
significantly from those measured. This is due, in part, to
the wide range of KDs of some elements (some may differ
from actual KD), changes in the phenocryst assemblage
and changes in the compositions of solid-solution minerals
during the course of such fractional crystallization process.
8. Summary and conclusions
(1) The volcanic history of the Pan-African belt in eastern
Egypt encompasses two major magmatic episodes, an ear-
http://journals.cambridge.org
Amphibole
Source
(no. 85b)
To further constrain this fractional crystallization hypothesis, trace element modelling using the equation of
Arth (1976) was applied. This equation was solved for CL
(the concentration of the trace element in the differentiated liquid:
Downloaded: 20 Mar 2015
M. ABDEL-RAHMAN
21.58
44.53
20.43
4.5
1.2
1.57
0.26
8.8
32.2
906.0
Fractionated
(no. 62a)
46.49
86.49
34.47
6.46
0.79
2.83
0.44
13.1
194.0
110.5
Fractionated
liquid
(calculated)
48.34
102.75
38.04
6.36
0.52
2.29
0.43
15.26
102.53
150.44
Her episode (950-750 Ma) produced the Shadli metavolcanic assemblages, and a younger episode (680-550 Ma)
produced the Dokhan volcanic rocks.
(2) The Dokhan volcanic suite (620 Ma) includes
basalt, andesite, dacite (dark-coloured rocks), and dacite,
rhyodacite, rhyolite ignimbrite (light-coloured rocks).
(3) Geochemically, the suite forms a continuum in
composition with a wide range of SiO2 (48-77 wt %; Fig.
4a), and all lithologies are considered co-magmatic. The
lack of a compositional gap precludes its identification as
a bimodal suite. The rocks are clearly orogenic calc-alkaline in nature (Fig. 5).
(4) Rb and Nb correlate positively and linearly with Zr
(Fig. 6a). The sum of REE increases from basaltic to rhyolitic compositions, and the patterns are subparallel but
with increasing Eu-anomalies toward rhyolites (Fig. 7b,
c). Trace element patterns generally resemble those of
typical Andean orogenic andesites and ignimbrites.
(5) All lithologies exhibit geochemical traits of arcrelated volcanism (Fig. 8). The suite resembles products
of neither extensional nor transitional tectonic regimes as
previously thought, but was produced in a subductionrelated tectonic environment.
(6) The low initial '"Sr/^Sr ratio (0.7039) of the suite
precludes a crustal ensialic origin. The rocks are considered to be derived from a mantle source produced above a
subducted slab of lithosphere. Melting of such a hydrated
mantle wedge produced a calc-alkaline basaltic magma.
The latter fractionated (11-26% amphibole, 13-44%
IP address: 137.222.24.34
Pan-African volcanism, Nubian shield
29
plagioclase, 3 - 6 % magnetite; normalized to 100:
3 1 - 5 8 % , 2 9 - 9 0 % , 5-14%, respectively) to produce a
continuous basalt to high-silica rhyolite calc-alkaline
series, as suggested by well-defined geochemical trends
and documented by major and trace element fractionation
modelling.
Further investigations are warranted in view of the significance of the Dokhan volcanic sequences that occur on
a regional scale in northeastern Egypt, Sinai, Sudan and
Saudi Arabia, and the potential for increased insight into
Late Proterozoic crustal development in the northern
Arabian-Nubian shield.
Acknowledgements. I thank Dr R. F. Martin for valuable comments and for reading an earlier version of this work. I have
benefitted from discussions with Drs D. Francis, A. Hynes, A.
Lalonde and S. Nadeau. I am grateful to many geologists at the
Geological Survey of Egypt, particularly Dr A. Hussein and M.
Said for their help with logistics which facilitated field work. I
thank A. Ahmadali and G. Piori who facilitated the acquisition
of chemical analysis. This study was funded through a Natural
Sciences and Engineering Research Council of Canada
(NSERC) research grant to the author. Careful reviews by two
anonymous referees and by the editors substantially improved
the manuscript and are greatly appreciated.
results for international
Newsletter^, 17-23.
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Geostandards
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