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Emh Sciencw Vol. 22. No. 2, pp. 147-157. 19%
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The petrology of the Abu Zawal gabbroic intrusion, Eastern Desert, Egypt: an
example of an island-arc setting
F. F. ABU EL-ELA
Geology Department, Assiut University, Assiut, Egypt
(Received 19 January 1994: revised version received 8 December 1995)
Abstract - The Abu Zawal gabbroic intrusion consists of three gabbroic zones. Each of these zones has a distinctive mineralogical composition. Plagioclase and altered clinopyroxene are abundant in the lower zone gabbro
(umlitized gabbro). Hornblende and Fe-Ti oxides occur in the middle and upper zone gabbros (hornblende and
feno-gabbros, respectively). Fe-Ti oxides are more abundant in the upper zone gabbro. The composition of the
plagioclase cores ranges from An.ss(Towerzone) to Arus (upper zone). The primary clinopyroxene and cakzicamphibole are augite and magnesio-hornblende, respectively. Application of the hornblende geobarometer indicates a
pressure of crystallization ranging from 2.9 to 3.5 kbar. in addition application of the amphibole-plagioclase geothermometer yieY1d.s
crystalhzation temperatures of about 1050-11CQ“C.Major oxide, trace element and RRE data
are suggestive of an island-arc evolved high alumina basalt as the parent for these gabbros. The differentiationof
the gabbmic zones can be accounted for by low pressme, &sed-system in situ aystaUization under wet conditions.
Resume - L’intmsion gabbrolque de Abu Zawal est compo&e de trois ensembles gabbroiques, presentant
chacun une composition min&&gique distincte. Dans 1’
ensemble inf&ieur abondent le plagioclase et le clinopyrox&neah&n?(gabbro ourahtise), tandis que darts les ensembles moyen et sup6rieur la hornblende et les oxydes de
Fe-Ti sont pr&ents (respectivement gabbros a hornblende et ferrogabbros). C’est dans l’ensemble gabbrdque
superieur que les oxydes de Fe-Ti sont davantage abondants. La composition des noyaux de plagioclases zon&
varie de AXIS(ensemble inferieur) a An10(ensemble sup&ieur). Le clinopyrox&neprimaire et Famphibole calcique
correspondent respectivement 21de l’augite et B de la hornblende magnesienne. L’apphcation du g&obarom&e
hornblende indique une pression de cristallisation comprise entre 2,9 et 3,5 kbar. Par atlleurs, l’utilisation du
g&othermometre amphibol~plagioclase donne des temperatures de cristalhsation d’environ lOt30-1100°C. Les
&ments majeurs et en trace ainsi que les Terms Rams sugg&ent pour ces gabbros un magma parental de type basalte d’art insulaire, &volt16et hyper&mineux. La d&%nciation en ensembles gabbroiques distincts peut s’exphquer par un syst&ne de cristallisationfern& in situ, A base pression et sous conditions hydratees.
INTRODUCIION
Desert of Egypt, are less well-known (El-Gaby et al.,
1988). The mantle derived island-arc gabbro may
have been mapped by some workers as first group
gabbros.
The present paper deals with the Abu Zawal gabbroic intrusion from the point of view of mineral
chemistry, major and trace element bulk analyses and
REE. These are used to unravel the magmatic evolution of the gabbroic intrusion and to infer some constraints on the origin of the parent magma.
The Late Precambrian Pan-African gabbroic rocks
in the Eastern Desert of Egypt occur in two main
groups. The first group was mapped as an epidiorite
complex (El-Ramly and Akaad, 1960) or as metagabbros and diorites (Akaad and Essawy, 1964; ElRamly, 1972) or as older metagabbros (Takla et al.,
1981). This group is nowadays considered to represent a member of an ophiolite sequence (El-Sharkawy
and El-Bayoumy, 1979,: Abu El-Ela, 1990,199l). The
second group was mapped as younger gabbros
(Takla, 197l; El-Ramly, 1972; Basta and Takla, 1974a,
b). These gabbros are post Hammamat (molasse-type
sediments) intrusions, presumably just older than the
post tectonic younger granites.
Although the concept of Precambrian platetectonics is generally accepted, the plutonic equivalents of island-arc andesites and dacites, as well as
mantle derived island-arc gabbros in the Eastern
GEOLOGY
The Abu Zawal gabbroic intrusion has been
mapped as amphibolite (El-Tahir, 1978) or as part of
an island-arc association (Sharara et uZ., 1998). It
forms an elongate body (25 km?) trending northeastsouthwest (Fig. 1). The gabbroic rocks are intruded
by syn-kinematic granodiorites, comparable to the Gl
granites of Hussein et al. (1982), and post-kinematic
147
F. F. ABU ELELA
148
*
,
3;2ioii
,I
26U 5
..................
..................
..................
...................
...................
....................
....................
....................
...................
...................
..................
..................
..................
......
.
.
............
................
................
. .
...
1
II-
.
.
.......
.
Wadi deposlls
( Youngest)
.. . .
:I::
Syn-kinemat
El
+ 1 Post-kinematic
L-l
u
= 1
c
i c granites
granites
c
Figure 1. Geological mapof the Abu Zawal gabbroic intrusion.
granites, comparable to the G2 and G3 granites of
Hussein et al. (op. tit). The contacts are sharp and irregular. Swarms of gabbroic xenoliths are enclosed
with@ the granitic intrusions. The xenoliths are angular, possess sharp boundaries and are highly dissected by granitic veinlets. The Abu Zawal gabbroic
rocks are intruded into metavolcanics of island-arc
affinity (Charara et al., op. tit) outside of the southwestern part of the map area. The contacts are mostly
sharp, although the gabbroic intrusion sent tongues
into the metavokanics.
Three zones of gabbroic rocks are disthguished in
the Abu Zawal gabbroic intrusion, a lower zork (LX),
middle zone (MZ) and upper zone (UZ), passing
from the western to the eastern edge of the intrusion
(Fig. 1). The division into zones is based on the distribution of rock types and textures. The LZ is represented by fine- to medium-grained uralitized gabbro,
The petrology of the Abu Zawal gabbroic intrusion
Table 1. Selected plagioclase analyses from the Abu Zawal
54
149
Si02
gabbroic intrusion
1 ?§j
TiOz
A1203
Fee*
MlIO
CaO
Na20
K20
Total
0.05
28.06
0.05
27.60
0.06
0.01
9.74
6.06
0.10
0.01
11.89
4.71
0.10
99.92
99.18
0.03
27.03
0.07
0.01
9.41
6.31
0.19
99.32
0.02
25.89
8.39
6.74
0.20
98.74
FeO”=totdinmasFeO.
Table 2 Selected clinopyroxene
46-
--
42
I
I
W3
analyses from the Abu
Zawal gabbroic intrusion.
Sample NC
SiO2
TiOz
Lower Zone
Middle Zone Upper Zone
(218C)
-210
52.44
52.71
0.12
0.12
0.95
0.65
9.39
9.82
Al203
0.17
0.84
FeO*
8.66
9.44
MIIO
0.34
0.38
14.09
0.40
13.35
0.55
12.99
21.82
0.20
100.15
23.02
0.22
99.89
22.70
MgO
CaO
Na20
Total
14.51
22.08
0.18
99.47
0.22
11.25
0.23
99.77
F~=totalimmasFeO
which is composed mainly of plagioclase and augite.
Augite is partly to completely altered to actinolite.
Hornblende was not found. Primary (igneous) Fe-Ti
oxides are lacking. An adcumulus texture is characteristic of this zone. The average modal composition
(vol.%) is: plagioclase 66.8X, augite and actinolite
31.9% and fine opaques 1.3%. The MZ is represented by medium-gra:med hornblende gabbro. It is
composed essentially of plagioclase, pale-brown
hornblende and altered augite. Small quantities of
primary ibnenite and magnetite occur in the rocks of
this zone. The presence of a considerable amount of
pale-brown hornblende and the presence of some
primary Fe-Ti oxides characterize the MZ gabbro
compared with the LZ gabbro, in which both homblende and primary Fe-Ti oxides are absent. The average modal composition of hornblende gabbro is:
plagioclase 68.2%, pale-brown hornblende 22.5%, altered augite 4.4%, Fe-Ti oxides 3.6%, quartz 0.7% and
apatite 0.6%. An adcumulus texture is also characteristic of this zone. The LJZ is represented by coarse to
pegmatoidal ferro-gabbros. It is composed mainly of
plagioclase, brown hornblende and Fe-Ti oxides. Altered augite and apatite are minor phases. A great
abundance of Fe-Ti o)ddes is characteristic of this
zone and results from the high TiOr content of these
Figure 2. Clhopyroxene dkaimhnt
diagram of le EJas(l%Z) for
the Abu Zawal gabbroic intrusion. O=LZ gabbro; ? =?h-lZ gabbro;
??
=UZ gabbro.
rocks. The Fe-Ti oxides are enclosed in brown homblende and augite relicts and they also form interstitial grains between hornblende and plagioclase.
Green hornblende occurs in these rocks and appears
to have been formed by the recrystallization and replacement of brown hornblende. A mesocumuhis
texture is characteristic of this zone. The average modal composition of the ferro-gabbro is: plagioclase
56.2%, brown hornblende 22.0%, Fe-Ti oxides l&8%,
altered augite 2.1% and apatite 0.9%.
The relation between the LZ gabbro (uralitized gab
bro) and the MZ gabbro (hornblende gabbxo) is transitional, whereas that between the middle and upper zone
gabbros (ferrogabbro) is sharp. No primary igneous
layering was seen in any of the zones. The contacts with
the cormtry rocks are generally sharp with no chilled
margins and no development of magma&es.
MINERALOGY
Compositions of the analysed minerals were determined in polished thin sections with a Jeol Jxa 8600
superprobe and Tracer 5500 ED, using wavelength
dispersive techniques for Na, Cr, Mn and Fe and energy dispersive spectrometry for Mg, Al, Si, K, Ca
and Ti. Operating conditions were 20 kV accelerating
voltage and 10 nA sample current. Matrix corrections
were applied using a ZAF program. The analyses
were carried out at the Department of Geochemistry,
Utrecht University, the Netherlands. Each mineral
analysis represents an average of four points.
Plagioclase is the most important mineral phase in
the Abu Zawal gabbroic intrusion. The average core
compositions of plagioclase for the LZ is Anss. In the
MZ, the average ranges from An474 and in the UZ it
is An40(Table 1).
150
F. F. ABU ELELA
Table 3. Selectedcalcicamphiboles from the Abu Zawal gabbroic
Table 4. Selected oxide analyses from the Abu Zawal
intrusion.
gabbroic intrusion.
Sample N#
Middle Zone
Upper Zon
-210
Mg-Hb
(n8C)
Mg-Hb
SiO2
TiQ
Fe0
MnO
Ml@
CaO
NazO
KZO
iTotal
45.44
1.78
8.43
15.03
45.64
1.74
8.18
15.34
46.66
0.7l
7.73
15.09
0.25
12.21
0.32
12.39
0.29
12.92
0.22
18.12
0.26
15.82
12.31
0.95
0.82
97.22
11.79
0.93
0.83
97.16
12.17
1.08
0.60
97.25
11.92
0.23
0.05
97.00
12.12
0.39
7.25
7.50
6
Reaction Amphibole!
(218C)
(202A)
Act.
Act. -Hb
52.48
50.92
0.10
0.88
3.59
4.98
10.80
12.73
Si
6.50
0.18
98.28
6.25
5.75
100
A
P
A
.
250
*
0
c
F
G
H
w 00
D
E
I
J
P
8
O<
Figure 3. Composition of cakic amphiboles (after Lake, 1978) in
the Abu ZawaI gabbmic intrusion. Whomblende
of the MZ;
W=hornblende of the UZ; A=reaction amphibole of the LZ;
A=reaction amphibole of the LZ. A=actinoIite; B=actinolitic
hornblende; C=Mg-hornblende;
D=tschermakitic hornblende;
E=tschermakite;F=fernxxtinoIite; G=ferro-actinoIitic hornblende;
H=ferro-hornblende; I=ferro-tschermakitic hornblende; J-ferrotschermakite.
The clinopyroxene is classified as augite according
to the scheme of Poldervaart and Hess (1951). It occurs as irregular crystals and as relicts in uralitic amphiboles. Its low A1203 content (0.65-1.25 wt.%; Table
2) is suggestive of crystallization at low pressure
(Green and Ringwood, 1968). The clinopyroxene plots
in the subalkaline field (Fig. 2) of le Bas (1962).
Calcic amphiboles form pale-green fibrous rims
around the clinopyroxene, but they also occur as sub
hedral pale-brown to brown crystals interstitial between the palgioclase crystals, especially in the
hornblende gabbro and ferro-gabbro. The amphiboles
forming rims around clinopyroxene occupy the actinolite and the actinolitic hornblende fields (Fig. 4) on
the classification diagram of Leake (1978), pointing to
an origin by replacement of clinopyroxene (Nakajima
and Ribbe, 1981). The subhedral amphibole crystals
in the hornblende gabbro and ferro-gabbro occupy
the magnesio-hornblende field on the same classification diagram (Table 3; Fig. 3).
Application of the hornblende geobarometer of
Hammarstrom and Zen (1986) and Hollister et al.
(1987) suggests crystallization at pressures of about
2.9 to 3.5 kbar. This range of crystallization pressure
can be reasonably extrapolated to all of the gabbroic
zones, as is also corroborated by the low AhO3
Magnetite
Ilmenite
(218C,MZ) (210,UZ) (218C,MZ) (210,UZj
0.30
0.20
0.48
0.16
0.03
0.21
45.42
46.48
0.07
0.08
0.18
0.11
0.12
91.51
91.51
0.07
0.08
0.20
92.32
0.09
92.18
50.96
2.01
0.24
0.28
99.47
49.55
2.30
0.18
0.02
98.87
FeO* = toti ironasFe0
content of clinopyroxenes. In addition, application of
the amphibole-plagioclase geothermometer (Bhmdy
and Holland, 1990) yields crystallization temperatures of about 1080 to 1lOO’C for the amphiboleplagioclase pairs in the middle and upper zones.
Among the opaque oxides, ilmenites from the
middle and upper zones are Al, Mg and Cr poor but
Mn rich (~2.0%; Table 4) and the magnetites are Ti,
Al, Mg, Cr and Mn poor (Table 4).
GEOCHEMISTRY
Representative samples of the Abu Zawal gab
broic intrusion have been analysed for major and
trace elements (Table 5). The major elements have
been determined by ICP methods using an ARG
34000 emission spectrometer. SQ, Fe0 and LO1
(Loss on Ignition) were determined using the wetchemical methods of Shapiro (1975). Trace elements
were determined by an automated Philips 1400 XRF
spectrometer. REE were determined by instrumental
neutron activation analysis using the methods of de
Bruin (1983). All analyses were carried out at the
Geochemistry Department, Utrecht University and at
IRI, Delft, the Netherlands.
Whole rock chemistry
Table 5 shows chemical analyses of representative
samples from the Abu Zawal gabbroic intrusion arranged in order of increasing FeO*/MgO and hence
decreasing Mg number ~g#=lOO molar MgO/
(MgO+FeO)]. The uralitized gabbro (LZ) has FeO*/
MgO ranging from 0.56 to 0.85 and Mg# from 77.5 to
69.7. The hornblende gabbro (MZ) has FeO*/MgO
ranging from 1.16 to 1.38 and Mg# from 63.4 to 58.4
and the ferro-gabbro (LIZ) has Fo/MgO
ranging
from 1.97 to 2.90 and Mg# from 51.1 to 42.2. Therefore,
the three gabbroic zones may represent three stages of
fractional crystallization in which the lower, middle
and upper zone gabbros represent the early, middle
X.4?
n.d.
n.d.
0.36
0.35
19.16
18.25
1.50
1.48
3.46
3.62
0.10
0.10
8.53
8.11
11.70
11.39
2.81
2.%
0.20
0.77
0.05
0.02
0.73
0.90
99.96
100.44
77.48
76.00
its (in ppm)
2
13
735
655
84
132
44
41
104
44
31
36
204
210
84
113
167
72
7
7
10
11
51 3R
i_.-..
n.d. = not de&t&.
Total
Mg#
Trace elem
Rb
Sr
Ba
Zn
Cll
co
Ni
V
Cr
Y
zr
Nb
P205
L.O.I.
TioZ
212
202A
n.d.
28
830
137
52
59
35
178
100
67
7
7
n.d.
6
7l4
93
46
30
33
176
91
54
6
5
n.d
16
684
122
45
5!
36
83
102
51
7
4
Lower Zone (uralitized gabbros)
215
2098
206C
5717
52.82
51.42
i-___
0.30
0.29
0.32
17.19
18.13
18.60
1.52
1.32
1.59
4.06
4.62
4.06
0.11
0.12
0.10
8.79
8.80
7.96
9.82
11.79
10.00
3.15
2.68
3.51
1.16
0.49
0.75
0.02
0.02
0.02
1.16
0.92
0.96
100.10
100.60
100.04
75.70
74.18
73.65
3
25
779
252
55
75
31
86
120
14
12
17
0.63
18.37
2.19
3.44
0.10
6.54
8.91
3.52
1.33
0.08
1.30
100.06
70.64
5%6&i
i-s__
221
n.d
1
816
124
52
92
35
109
112
24
9
8
0.45
18.64
2.14
4.07
0.11
7.03
10.60
3.54
0.23
0.07
0.60
100.62
69.73
5114
i_.__
203
n.d.
35
781
174
79
43
34
58
156
25
19
22
0.98
16.96
3.14
4.28
0.13
6.12
9.65
3.68
1.25
0.15
1.47
99.18
63.41
51.19
3
21
522
234
81
6
23
49
158
37
18
24
0.92
17.46
2.43
3.70
0.15
4.74
7.11
5.12
1.29
0.15
1.57
100.49
61.64
55.94
3
16
771
407
75
53
38
55
214
14
14
22
1.48
17.12
2.37
5.75
0.14
5.73
8.84
3.74
0.90
0.18
0.89
100.14
58.44
53.MI
Middle Zone (hornblende gabbros)
219
230B
218
Table 5. Major (wt%) and trace element @pm) analyses of the Abu Zawal gabbroic intrusion.
2
17
591
634
62
165
69
109
636
11
14
26
4.00
14.82
6.94
6.80
0.13
6.61
10.09
2.70
0.81
0.32
i.15
100.10
51.13
2
31
473
288
174
74
48
64
645
7
19
31
3.55
14.61
6.04
7.72
0.25
5.7l
7.83
3.59
1.43
0.37
1.37
99.47
46.74
0.34
3
7
723
508
90
27
64
13
469
5
18
24
3.33
14.50
9.00
6.27
0.17
5.23
9.64
3.16
0.52
0.45
0.41
100.34
43.60
47.66
3
16
695
571
105
124
64
47
616
5
15
21
3.77
14.75
10.26
7.40
0.26
5.74
9.41
2.79
0.70
0.47
0.65
100.55
42.23
4435
Upper Zone (ferro-gabbros)
250
220
210
45.93
208
152
F. F. ABU EL-ELA
0.4
0.5
0.2
0.0
16
0.3
12
0
4
%
0
20
16
.rn
12
????
No70
6
4
2
0
56
8
52
6
10
46
0
44
6
40
4
0.5
1
2
FeO*/
3
MgO
Figure 4. Variation diagram for major elements illustrating the main trends exhibted by rock samples of the Abu Zawal gabbroic intrusion. Symbols as for Fig. 2.
and late-stages of crystallization, respectively.
Variation diagrams for the major and trace elemenk plotted versus FeO*/MgO as a differentiation
index are shown in Figs 4 and 5. Five significant geochemical points are demonstrated by these variation
diagrams:
i) Ti@ increases with increasing FeO*/MgO. This
behaviour is also followed by FeO*, MnO, Co and V;
ii) strontium decreases gently with FeO*/MgO.
This trend is followed by Al203 and CaO;
iii) yttrium displays a bell-shaped trend against
FeO*/MgO, where some of the UZ gabbros have
lower values than those of the MZ gabbros. This behaviour is also followed by SiOr, NazO and Zr;
chromium
decreases
with
increasing
iv)
FeO*/MgO for all gabbroic zones. This trend is also
followed by MgO and Ni; and
v) barium increases with increasing FeO*/MgO for
all gabbros. This behaviour is also followed by PzOs.
The noted decrease in Cr and Ni contents from the LZ
gabbro (167 ppm Cr, 204 ppm Ni) to the UZ gabbro (5
ppm Cr, 13 ppm Ni) is consistent with the fractionation of spine1 and clinopyroxene.
Titanium and V abundances correlate with the
modal abundances of Fe-Ti oxides. Strontium contenk reflect the modal abundance of plagioclase.
The average of the hygromagmatophile element
abundances in the three gabbroic zones have been
normalized to N-type MORB concentrations (Pearce,
1984) and plotted in Fig. .6. This figure demonstrates
that there is a relative enrichment in large ion lithophiIe (LIL) elemenk (Rb, Sr, Ba and K) over the other
incompatible elements (Nb, P, Zr, Ti and Y) in all
gabbroic zones. These hyg-romagmatophile element
patterns (Fig. 6) are very distinct from those of modem alkali basalk (which are enriched in Nb) and mid
oceanic ridge basalk (Wood et al., 1981; Tamey et al.,
1980), but are comparable to those of talc-alkaline basalt (Wood et aZ., 1981). Depletion in Nb and other
high field strength elements (HFSE) (P, Zr, Ti and Y),
especially in the LZ gabbros (uralitized gabbro) relative to LIL elements, is a characteristic feature of all
subduction-related magma (Saunders et d., 1980).
This has been attributed to:
i) partitioning of HFSE into residual Ti phases
(e.g. ihnenite and sphene) which are stabilized during
hydrous partial melting conditions; and
ii) the transportation of the LIL elements into the
source regions of the talc-alkaline magmas as a result
of dehydration of the downgoing slab (Saunders et
al., 1980).
In addition, the low Zr/Y (0.57-1.86) observed among
the gabbroic zones support an oceanic-arc setting
rather than a continental-arc setting (Pearce, 1984).
Chondrite-Rormalized REE patterns for representative gabbroic samples from the lower, middle and
The petrology of the Abu Zawal gabbroic intrusion
OLower
153
zone
0 Middle
zone
20
-
rfh
??
??
??
0
10
0
-
25
-
Zr
so
41
15
-
ppm
_
5_
200
??
0
w
SO”
-
Y
0
A#
??
0
0
0
Ba
0
0
1
2
FeO*/
3
MgO
900
-
700
-
0
O8 ? ?o
3
4
2
1
FeO*/
3
big0
Figume5. Variation diagram for trace elements illustrating the main trends exhibted by
rock samples of the Abu Zawal gabbroic intrusim.
upper zone gabbros (Table 6) are plotted in Fig. 7.
REE become steadily enriched from the LZ to the
UZ gabbros. The gabbros have moderately fractionated REE patterns with (La/Yb)N from 2.27 to
5.75 and (Ce/Yb)N
from 2.14 to 4.00, passing from
the LZ gabbro (uralitized gabbro) to the UZ gabbro
(ferro-gabbro). This is due to marked light rare
earth elements (LREE) enrichment with increasing
differentiation (i.e. with increasing FeO*/ MgO
from 0.56 to 2.90). The heavy rare earth elements
(HREE) show a smooth and flat trend with
(Tb/Yb)N from 1.20 to 1.95. This indicates that the
generation of magma was not accompanied by
significant HREE fractionation and that the parent
magma was generated in the spine1 stability field
rather than the garnet stability field (cJ Weaver
and Tarney, 1981; Gill, 1974). The positive Eu
anomaly in the LZ gablbro is due to preferential Eu
incorporation by the first accumulating plagioclase. The variations in the scale of the Eu anomaly
in the middle and upper zone gabbros are ascribed
to a combination of the degree of Eu fractionation
in the magma and the amount of cumulus plagioclase present in each sample.
ESTIMATION OF THE PARENT MAGMA
COMPOSITION
A common problem in studying mafic layered intrusions is the estimation of the parent magma composition in as much as the bulk chemical composition
of rock samples is unlikely to match that of the parental magma because of the occurrence of cumulus
processes (Irvine, 1979).
The chilled margin method used to obtain the parent magma composition (Wager and Brown, 1968)
cannot be applied to the Abu Zawal gabbroic intrusion because extensive interaction processes have
been operative along all contacts. However, the parental magma composition can be estimated by the
weight summation method (Ragland and Butler,
1972; Klewin, 1990; Tommasini and Poli, 1992). This
method has been applied to the Abu Zawal gabbroic
intrusion summing the average chemical composition
of each gabbroic zone, weighted according to its outcrop surface. The calculated major element composition of the parent magma is reported in Table 7. This
composition is similar to the average high alumina
basalt in island-arc settings (see Table 7).
F. F. ABU EL-ELA
154
1
0 Lower
10
0
Middle
zone
zone
m Upper zone
Table 6. REE abundances (ppm) for representative
samples from the Abu Zawal gabbroic intrusion.
Sample No Lower Zone Middle Zone Upper Zone
(202A)
(218C)
(210)
La
2.46
9.31
11.22
Ce
6.02
18.29
20.37
Sm1.15
2.98
3.812
Eu
0.576
1.22
1.365
Tb
0.198
0.395
0.583
Yb
0.73
1.555
1.318
Lll
0.129
0.219
0.211 -I
L
estimation of the crystallization temperature on the
amphibole - plagioclase pairs in the middle and the
upper zone gabbros ranges from 1080 to 1100°C and
indicates a water content in the magma of about 4-5
wt.% and a liquidus temperature of about llOO1150°C (Baker and Eggler, 1983, Fig. 3). The water
content in the magma is close to the water-saturated
curve for basalts at 3-4 kbar (Holloway and Bumham, 1972; Hughes, 1982). Therefore, the crystallization of the gabbroic zones took place under wet
conditions.
0.1
Sr
K
Rb Ba Nb
P
Zr
Ti
Y
Figure 6. Spidergrams of the averages of the Abu Zawal gabbroic
zones. Nommlization data after Pearce (1984).
DIFFERENTIATION OF THE GABBROIC ZONES
A model of low-pressure, closed-system in situ
crystallization is proposed for the differentiation of
the Abu Zawal gabbroic intrusion. The mafic magma
was emplaced probably in a single, relatively rapid
injection and crystallization commenced throughout.
The following order of crystallization is proposed for the gabbroic zones on the basis of petrographical and mineral chemistry data. Plagioclase
(Ar@+clinopyroxene nucleated at the begining of
crystallization. Then, plagioclase grading from Anss
to &+calcic
amphibole (brown hornblende)+Fe-Ti
oxides followed in the crystallization sequence and
were successively joined by the crystallization of
quartz and apatite.
The hydrous phases crystallized in response to
an increase in Hz0 activity due to the early crystallization of anhydrous phases, as has been documented in experimental studies (Baker and Eggler,
1983). On the basis of microscopic and mineral
chemistry studies, magnesio-hornblende crystallized directly from the evolving liquid, whereas
actinolite and actinolitic hornblende formed by reaction between the liquid and clinopyroxene. The
DISCUSSION AND CONCLUSION
The Abu Zawal gabbroic intrusion shows features
which in part reveal the emplacement mechanism:
the absence of a chilled margin and grain-sized
graded layering (Irvine, 1982) indicate that the gabbro was probably not emplaced in a completely molten state. The absence of magmatitic structures indicates that the gabbro crystallized in situ and was not
emplaced as a crystal mush nor were significant portions being crystallized as new magma was still being
injected. Thus, the Abu Zawal cumulates probably
began to crystallize in the crust at pressures which
were not significantly higher than those where final
solidification took place (cfi Sutcliffe et al., 1989). This
conclusion is confirmed by the hornblende geoba,rometer, which gives a crystallization pressure of
about 2.9-3.5 kbar, and the low A1203 content of the
clinopyroxene suggests crystallization at relatively
low pressure.
The Abu Zawal gabbroic intrusion consists of
three gabbroic zones (LZ, MZ and UZ). These three
zones may represent three stages of fractional crystallization which can be demonstrated by the evolutional geochemistry. For example, the PROScontent is
as low as 0.02-0.08 wt.% in the LZ gabbro (early-stage
gabbro) but during fractional crystallization PZOS
probably became highly concentrated in the residual
liquid. The very low P205 content in the LZ gabbro
suggests that this intercumulus liquid was driven out
155
The petrology of the Abu Zawal gabbroic intrusion
La Cc
Sm Eu
Tb
Yb Lu
Figure 7. Chondrite-mmnaked
(after Evensen et al., 1978) REE patterns of the Abu Zawal gabbroic intrusion, Symbols as for Fig. 6.
Table 7. Estimated parental magma composition according to the weight summation
method and comparsion of the estimated parent magma with high-alumina basahs.
1.z
N
P
7
56.:30%
0.38
Ti02
18.33
AI203
1.68
Fez03
3.90
Fe0
0.10
MnO
7.97
MgO
10.61
CaO
3.17
Na20
0.70
IGO
0.04
E205
0.94
L.O.I.
N=number
of samples;
as FeO; l=average
MZ
3
32.50%
1.13
17.18
2.65
4.58
0.14
5.53
8.53
4.24
1.15
0.16
1.31
P=percentage
uz
4
11.20% Estimated
3.66
14.67
8.06
7.05
0.20
5.82
9.24
3.06
0.87
0.40
0.90
0.99
17.54
2.71
4.48
0.13
6.94
9.77
3.50
0.86
0.11
1.06
0.92
18.98
9.79*
0.19
5.77
10.69
3.36
0.99
0.22
-
0.73
17.30
3.40
5.54
0.22
5.50
8.94
3.10
0.90
0.20
1.01
18.10
9.45*
0.21
4.47
8.93
3.49
0.75
0.23
of the outcrop surface of each zone, A=average composition;
0.71
19.70
7.75
0.13
5.66
10.00
2.51
0.52
0.97
??
=total iron
of Aleutian high-ahunina basalt (Marsh, 1976; Brophy, 1984); 2=New Georgia (Solomon Is.)
high-alumina basalt (Brown and khairer,
1967); 3,4=high-ahnnb~a
basalt (cf. Crawford et al., 1987, Table 1,
sample No. 1,2).
by post-accumulation crystal growth (i.e. this gabbro
may have adcumulus properties as defined by Wager
and Brown, 1968). Then, during the middle- and latestage fractionation (middle and upper zone gabbros),
the E205 content of the gabbros was increased by being fixed in crystaking apatite. The upper zone gab
bros have between 0.32 and 0.47% I’&,. In addition,
the TiO2 content of the LZ gabbro is low, ranging
from 0.29 to 0.63 wt.% (Table 5), which suggests that
during this early stage of fractionation TiOz was also
highly concentrated in the liquid. Then, during the
crystallization of the MZ gabbro, the TiOr content of
the gabbros increased due to the crystallization of FeTi oxides. In the UZ gabbro, TiOz is 4.00 wt.%. This
156
F. F. ABU ELELA
probably means that most of the TiOz remained in the
liquid in the early-stage of fractionation and most of
it entered cumulates in the late-stage of fractionation
(UZ gabbro).
That the Abu Zawal gabbroic intrusion may have
crystallized from an island-arc high alumina basaltic
magma, which was derived from a mantle source, is
suggested by the following observations:
i) The gabbroic rocks have very low abundances
of incompatible elements (K, Rb, Ba, Nb, I’, Zr, Ti and
Y) and the abundances of these elements increase
with increasing differentiation. LIL elements (Sr, K,
Rb and Ba) have higher abundances relative to high
field strength (I-IFS) elements (Ti, P, Zr and Nb). In
addition, the low concentration of Cr and Ni are
characteristic of an island-arc basalt parentage.
ii) LILE/LREE enrichment, in combination with
negative Nb anomalies, are characteristic features of
basaltic rocks from recent destructive plate margins
(Pearce, 1984; Hohn, 1985).
iii) The estimated parent magma composition for
the Abu Zawal gabbroic intrusion is equivalent to an
evolved high alumina basalt (Table 7).
iv) The crystallization sequence in the Abu Zawal
gabbroic intrusion is similar to that in experimental
high alumina basaltic systems crystallized under wet
conditions (Green and Ringwood, 1968; Brophy and
Marsh, 1986).
Acknowledgements
A scholarship from the Dutch Government and the
Institute of Earth Sciences, State University of Utrecht,
is gratefully acknowledged. Discussion with Dr. J. P. P.
Huysmans was very helpful. Prof. Dr. El-Gaby is
thanked for his reading and criticism of the original
draft. Comments by Dr. D. Hughes and another reviewer have greatly improved the manuscript.
REFERENCES
Abu El-Ela, F. F. 1990. Supra-subduction zone
ophiolite of Qift-Quseir road, Eastern Desert,
Egypt. Bulletin Faculty Science Assiut University,
Egypt 19(2-F), 51-70.
Abu El-Ela, F. F. 1991. Ophiolitic melange around
Gebel Atud, Eastern Desert, Egypt. Bulletin Faculty
Science Assiut University, Egypt 20(1-F), 215-230.
Akaad, M. K. and Essawy, M. 1964. The metagabbrodiorite complex N.E. of Gebel Atud, Eastern Desert
and the term “epidiorite”. Bdetin Science Technology
Assiu t Uqiversity, Egypt 7,83-108.
Baker, D. R. and Eggler, D. H. 1983. Fractionation
paths of Atka (Aleutians) high alumina basalk:
constraints from phase relations. Journal Volcanology.
Geothermal Research 18,387-404.
Bask, E. Z. and Takla, M. A. 1974a. Distribution of
trace elements in the gabbroic rocks of Egypt. Chemie Erde 30(3), 282-299.
Bask, E. Z. and Takla, M. A. 1974b. Distribution of
opaque minerals and the origin of the gabbroic
rocks of Egypt. Bulletin FacuZty Science Cairo University 47,347-364.
Blundy, J. D. and Holland, T. J. B. 1990. Calcic amphibole equilibria and a new amphibole-plagioclase
geothermometer. Contributions Mineralogy Petrology
104,208~224.
Brophy, J. G. and Marsh, B. D. 1986. On the origin of
high-alumina arc basalt and the mechanics of melt
extraction. Journal Petrology 27,763-789.
De Bruin, M. 1983. Instrumental neutron activation
analysis - a routine method. Ph. D. dissertation 270p.
Delft University, the Netherlands.
El-Gaby, S., List, F. K. and Tehrani, R. 1988. Geology,
evolution and metallogenesis of the Pan-African
Belt in Egypt. In The Pan-African Belt of Northeast
Africa and Adjacent Areas (Edited by El-Gaby, S. and
Greiling, R. 0.) ~~17-68. Friedrich Vieweg and
Sohn, Braunschweig, Wiesbaden.
El-Ramly, M. F. 1972. A new geological map for the
basement rocks in the Eastern and South-Western Desert of Egypt Annals Geological Survey Egypt 2,1-18.
El-Ramly, M. F. and Akaad, M. K. l%O. The basement complex in the Central Eastern Desert between Lat. 24”30’ and 25’40’ N. Geological Survey,
Cairo, Egypt Paper 8,35p.
El-Sharkawy, M. A. and El-Bayoumi, R. M. 1979. The
ophiolite of Wadi Ghadir, Eastern Desert, Egypt.
Annals Geological Survey Egypt 9,125-135.
El-Tahir, M. A. 1978. Relation between geology and
radioactivity of some basement rocks to the north of
Qena-Safaga asphalitic road, Eastern Desert, Egypt.
M. SC. Thesis 154~. Al-mar
University, Cairo,
Egypt*
Evensen, N. M., Hamilton, P. J. and O’Nions, R. K.
1978. Rare-earth abundance in chondritic meteorites. Geochemica Cosmochemica Acta 42,1199-1212.
Gill, J. B. 1974. Role of under&r&t oceanic crust in
the genesis of a Fijian talc-alkaline suite. Contibutions Mineralogy Petrology 43,150-166.
Green, T. H. and Ringwood, A. F. 1968. Genesis of the
talc-alkaline igneous rock suite. Contributions Mineralogy Petrology 18,105-162.
Hammarstrom, J. M. and Zen, E. 1986. Aluminum in
hornblende: an empirical igneous geobarometer.
American Mineralogist 7l, 1297-1313.
Hollister, L. S., Grisson, G. C., Peters, E. K., Stowell,
H. H. and Sisson, V. B. 1987. Confirmation of the
empirical correlation of Al in hornblende with pressure of solidification of talc-alkaline plutons. American Mineralogist 72,23X239.
Holloway, J. R. and Bumham, C. W. 1972. Melting
relations of basalt with equilibrium water pressure
less than total pressure. IournaZPetrology 13,1-29.
The petrology of the Abu Zawal gabbroic intrusion
Hobn, P. E. 1985. The geochemical fingerprints of different tectonomagmatic environments using hygromagmatophile elements of tholeiitic basalts and
basaltic andesites. ChetnicuZ
GeoZogy
51,303323.
Hughes, C. J. 1982. Igneous Petrology. 551~. EIsevier,
Amsterdam.
Hussein, A. A., AIi, M. M. and El-RamIy, M. F. 1982.
A proposed new classlication of granites of Egypt.
]oumaZVolcanology Geothermal Research 14,187-198.
Irvine, T. N. 1979. Rocks whose composition is determined by crystal accumulation and sorting. In
Theevolution of igneous rocks (Edited by Yoder, H. S.)
245-306~~. Princeton University Press, Princeton,
New Jersey.
Irvine, T. N. 1982. Ter:minology for layered intrusions. ]oumaZPetroZogy 23,127-162.
KIewin, K. W. 1990. Petrology of the Proterozoic Potato River layered intrusion, Northern Wisconsin,
USA. ZoumaZPetrology ,31,1115-1139.
Leake, B. E. 1978. Nomenclature of amphiboles.
American Mineralogist 63,1023-1052.
Le Bas, M. J. 1962. The :role of ahuninum in igneous
chnopyroxenes with relation to their parentage.
American Journal Science 260,267-288.
Nakajima, Y. and Ribbe, P. H. 1981. Texture and
structural interpretation of the alteration of pyroxene to other biopyroboles. Contributions Mineralogy
Petrology 78,230239.
Pearce, J. A. 1984. Role of the sub-continental Iithosphere in magma genesis at active continental
margins. In Continenbzl basalts and mantle xenoliths
(Edited by Hawkesworth, C. J. and Norry, N. J.)
~~230-249. Shiva Publishing, Nantwich.
Poldervaart, A. and Hess, H. H. 1951. Pyroxenes in
the crystaBization of basaltic magma. Journal GeoZogy 59,472489.
Ragland, P. R. and Butler, J. R. 1972. Crystallization of
the West Farrington pluton, North Carolina, USA.
Journal Petrology 13,38:1-404.
Saunders, A. D., Tamey, J. and Weaver, S. D. 1980.
Transverse geochemical variations across the Antarctic Peninsula: implications for the genesis of talc
157
aIkaIine magma. Earth Planetary Science Letters 46,
344-360.
Shapiro, L. 1975. Rapid analysis of silicate, carbonate
and phosphate rocks (revised edition). American
GeoZogicaZSurvey BuZZetin1401.
Sharara, N. A., Abu El-Ela, F. F., El-Nady, 0. M. and
Sohman, M. F. 1990. Geology and geochemistry of
the island arc association of the area around GabaI
El Urf, Eastern Desert, Egypt. BuZZetinFacuZty Science Assiut LZniversity,Egypt 19(1-F), 97-122.
Sutchffe, R. H., Sweeny, J. M. and Edgar, A. D. 1989.
The Lac des lies complex, Ontario: Petrology and
platinum-group elements mineralization in an Archean mafic intrusion. Canadian JoumaZ Earth Science
26,1408-1427.
TakIa, M. A. 1971. On mineralogical and geochemical
studies of some basic and associating ultrabasic igneous rocks, Eastern Desert, Egypt. Ph. D. dissertution 347~. Cairo University, Egypt.
TakIa, M. A., Basta, E. and Fawzi, E. 1981. Characterization of the older and younger gabbros of
Egypt. DeZtaJournal Science 5,279-314.
Tamey, J., Wood, D. A., Saunders, A. D., Cann, J. R.
and Varte, J. 1980. Nature of mantle heterogeneity
in the North Atlantic: evidence from deep sea
drilling. PhiZosphicaZTransactions Royal Society London A297,179-202.
Tommasini, S. and PoIi, G. 1992. Petrology of the Late
Carboniferous Punta Falcone gabbroic complex,
north Sardinia, Italy. Contributions MineraZogy Petrology 110,16-32.
Wager, L. R. and Brown, G. M. 1968. Layered igneous
rocks. 588~. W. H. Freeman and Company.
Weaver, B. L. and Tarney, J. 1981. The scourie dyke
suite: Petrogenesis and geochemical nature of the
Proterozoic sub-continental mantle. Contributions
Mineralogy PetroZogy78,175188.
Wood, D. A., Tamey, J. and Weaver, B. L. 1981. Trace
element variations in Atlantic ocean basahs and
Proterozoic dykes from northwest Scotland: their
bearing upon the nature and geochemical evolution
of the upper mantle. Tectonophysics 75,91-112.