,NCL”LvNG
ISOTOPE GEOSCIENCE
ELSEVIER
Chemical Geology
123 (1995) 29-51
The Gabal Gerf complex: A Precambrian N-MORB ophiolite
in the Nubian Shield, NE Africa
M. Zimmer a*b,A. KrGner
b** , K.P . Jochum a, T. Reischmann
aMax-Planck-institutfiir Chemie, Postfach 3060, D-55020 Mainz, Germany
b Institutfiir Geowissenschaften,
b, W. Todt a
Universitiit Mainz, D-55099 Mainz, Germany
Received 26 January 1994; accepted after revision 7 February 1995
Abstract
We report geochemical and isotopic data for tectonically dismembered units of the Cabal Gerf mafic-ultramafic complex, the
largest Neoproterozoic (Pan-African) ophiolite in the Arabian-Nubian Shield and located near the Red Sea in the border region
between Egypt and the Sudan. The complex consists of basaltic pillow lavas, sheeted dykes, isotropic and layered gabbros and
an ultramafic melange, all in tectonic contact along thrust sheets. Major- and trace-element data, including REE, for the pillow
lavas and sheeted dykes are indistinguishable from modem high-Ti N-MORB. Chemical variations in the various rock types can
be ascribed to fractionation and accumulation involving olivine, clinopyroxene and plagioclase. A comparison with chemical
data from ophiolites of the Arabian-Nubian
Shield and elsewhere in the world shows the Cabal Gerf complex to be the only
Precambrian ophiolite with N-MORB chemistry, and we suggest that its basalts and sheeted dykes originally formed in a major
ocean basin.
Sm and Nd isotope analyses combined with published zircon data suggest an age of -750 Ma for the time of igneous
crystallization of the Gabal Gerf complex. Ed,, initial values vary between + 6.5 and + 8.8, some of the highest yet reported for
Neoproterozoic mantle-derived rocks. Pb isotopic data for the basalts and sheeted dykes are similar to modem N-MORB, while
the gabbros are more akin to island arc and back-arc basin rocks. We ascribe their elevated Z07Pb/2MPb ratios to mixing of a
small amount of pelagic sediment with the magma source of the gabbros during subduction and subsequent melt generation
above a subduction zone. The pillow basal&, sheeted dykes and gabbros were brought together by tectonic stacking during the
abduction process when collision of island arc complexes with the active margin of the African continent occurred during an
accretion event - 600-700 Ma ago.
1. Introduction
1983,1985; Kroneret al., 1987). Ophiolite occurrences
in the ANS have been known for over 15 years (Bakor
The Arabian-Nubian
Shield (ANS) consists of the
Precambrian basement in Saudi Arabia, also known as
the Arabian Shield, as well as crystalline rocks and their
cover in the Eastern Desert of Egypt, the Red Sea Hills
of the Sudan as well as northern and western Ethiopia,
collectively known as the Nubian Shield (NS) (Vail,
et al., 1976; Carson and Shalaby, 1976) and are of
Neoproterozoic age (Pallister et al., 1988). Together
with volcanic and plutonic rocks of island arc affinity
they document processes of lateral crustal growth in
northeast Africa and Arabia, involving island arc and
ocean crust formation, subsequent ocean closure, amalgamation of the arc complexes and accretion to the
ancient African continent (Gass, 1981; Camp, 1984;
* Corresponding
author.
JNAI
0009-2541/95/$09.50
0 1995 Elsevier Science B.V. All rights reserved
.SSDIOOO9-2541(95)00018-6
30
M. Zirnrner et al. /Chemical
Geology 123 (1995) 29-51
j 380
Qirseir
LEGEND
Phonerozoic
cover
sediments
late -to post- tectonic
and ring complexes
Early to syntectonic
(gabbro to granite
\ldfu
Gneisses,
Marble
a
Aswo"
granites
plutonic
suite
1
migmatites
and
associated
schist
Volcano-sedimentary
sequences
structural
trend lines
]
Ophlolitic
,Fig.
with
assemblage
2
22
Fig. 1. Simplified geological map of a part of the Nubian Shield (modified after KrBner et al., 1987)
Krijner et al., 1987). During this process, the oceanic
crust of marginal and interarc basins was abducted,
tectonically fragmented and is now preserved in many
ophiolite complexes. These are aligned in discrete
M. Zimmer et al. /Chemical Geology I23 (1995) 29-51
suture zones as in Arabia (Pallister et al., 1988)) Sudan
(Kroner et al., 1987) and Ethiopia (Berhe, 1990) and
may separate individual arc terranes (Camp, 1984;
Krijner et al., 1987)) or they occur in large nappe complexes as in the Eastern Desert of Egypt (Shackleton
et al., 1980; Kroner et al., 1987). Reliable age data
place these ophiolites in the age range N 850-740 Ma
(Claesson et al., 1984; Pallister et al., 1988; Kroner et
al., 1992).
The largest of these tectonically
dismembered
ophiolite complexes occurs near the Red Sea around
Gabal Gerf between lat. 22” and 23”N and long. 34”30’
and 35”30’E (Fig. 1) . It consists of a tectonically lower
unit of serpentinized ultramafic melange and an upper
unit of layered and isotropic gabbro, sheeted dykes and
massive, rarely pillowed basalt (Krbner et al., 1987;
Zimmer, 1989).
Several geochemical studies have characterized the
ophiolites of the ANS as enriched in incompatible elements (e.g., Bakor et al., 1976; Price, 1984; Kroner,
1985; Pallister et al., 1988), and they have therefore
been regarded as supra-subduction
zone ophiolites
whose chemistry was strongly influenced by subduction processes generating the island arc complexes and
probably also responsible for intra-arc rifting and the
formation of marginal basins. This is supported by geochronological data for rocks of the ophiolite association
(zircon ages for gabbro and plagiogranite,
Sm-Nd
whole-rock ages for gabbro-basalt association) which
show close similarity with ages for the island arc associations (Claesson et al., 1984; Pallister et al., 1988;
Kroner et al., 1992).
Although several geochemical and isotopic studies
of the various ophiolite complexes in the ANS are now
available (e.g., Price, 1984; Pallister et al., 1988;
Abdel-Rahman et al., 1990; Kroner et al., 1992; Abdelthe purpose
of the present
Rahman,
1993))
investigation was to combine several isotopic systems
(Sr, Nd and Pb) in the analysis of both whole-rock
samples and mineral separates from the Gabal Gerf
Ophiolite (GGO) and to interpret the isotopic data in
conjunction
with major- and trace-element
analyses
and petrogenetic considerations. Our chemical and isotopic data on surprisingly fresh rocks of this complex
characterize the different ophiolite lithologies, constrain their age of crystallization and provide information on the tectonic setting of the complex and mantle
chemistry in the Neoproterozoic.
31
1.1. Geology, petrography and previous
geochronology
Mafic-ultramafic complexes in Egypt and the Sudan
were first mapped as fault-bounded intrusive bodies by
Soviet geologists (e.g., Ivanov and Hussein, 1972),
but were reinterpreted
as tectonically
fragmented
ophiolites by Garson and Shalaby ( 1976). The GGO
is the largest mafic-ultramafic
complex in the entire
ANS. It consists of the Gabal Gerf nappe, named after
Gabal Gerf, a prominent mountain peak (Fig. 2)) and
is composed of serpentinized ultramafic melange with
fragments of gabbro and basalt reaching dimensions of
several tens of metres. At its southwestern margin, a
section consisting of layered and isotropic gabbros with
chromite lenses is in tectonic contact with the melange
(Fig. 2). Gabal Harga Zarga and environs, SSW of
Gabal Gerf, consists of massive and fractured basaltic
rocks which include both lavas and dykes that are difficult to differentiate in the field. The Heiani Complex,
still farther SW, constitutes the southern margin of the
nappe complex and consists of pillowed lava and
sheeted dykes, locally well preserved and underlain by
layered gabbro and serpentinized or carbonated ultramafic rocks. Contacts between the different parts of the
ophiolite nappe complex are not exposed, but the tectonically lowest ophiolite nappes, mostly serpentinite
melanges, locally have tectonic contacts with volcanielastic and turbiditic metasediments of unknown origin.
Tectonic transport of the ophiolite nappes was from NE
to SW (Kroner et al., 1987; Stern et al., 1991).
Thirty samples of pillow lavas and sheeted dykes,
thirty samples of gabbros and four samples of ultramafics or ultramafic cumulates were collected during
visits to Egypt in 1985 and 1987. Primary structures
and textures are still preserved in some rocks of the
GGO, in spite of greenschist-facies
metamorphism. In
the ultramafic rocks, primary minerals are typically
replaced by serpentine, calcite, actinolite, epidote and
quartz, but remnants of primary olivine and orthopyroxene were found in a few samples. Most of the gabbro
samples investigated come from a layered section and
represent cumulates exhibiting various degrees of alteration. When fresh, they consist of olivine (01)) orthopyroxene (opx), clinopyroxene
(cpx), plagioclase
(plag) and opaque minerals. Alteration products are
actinolite, chlorite and epidote. The pillow lavas and
sheeted dykes from Gabal Harga Zarga and Gabal Hei-
32
M. Zirnrner et al. /Chemical
El
Geology
123 (1995) 29-51
Wadi deposits
It/
Unfoliated
J granites
young
lxxx4
Granodiorites
-1I.
I
Metagabbro-diorite
complex
I.
(__- Gneisses
\
Thrust fault
283
Sample number
Fig. 2. Simplified geological map of the region around and west of Gabal Gerf, showing localities of samples analyzed for geochemistry
isotopes. Based on unpublished 1: 100.000 Geological Mup, Egyptian Geological Survey and Mining Authority. Cairo.
ani are extensively
altered and now largely contain a
greenschist-facies
metamorphic
mineral
assemblage
with green amphibole/actinolite,
chlorite and altered
and
plagioclase being the main constituents.
A few samples
preserve phenocrysts
of plag and cpx. The lavas have
a fine-grained intersertal texture while the dykes exhibit
M. Zimmer et al. /Chemical
a typical ophitic to subophitic texture and are coarser
grained than the lavas. Further details on the petrography are reported by Zimmer ( 1989).
Kriiner et al. (1992) have analyzed single zircons
from a coarse leucogabbro
unit within the layered
sequence (sample locality near GG 263, see Fig. 2),
using the evaporation method developed by Kober
(1987). They obtained a mean 207Pb/206Pb age of
74 1 + 42 Ma (2a error) for four grains and interpreted
this as the crystallization age of the gabbro. This age is
identical to a 207Pb/206Pb age of 745 f 23 Ma for zircons from plagiogranite of the Wadi Ghadir ophiolite
farther N in the Eastern Desert of Egypt (Kroner et al.,
1992) and to a Sm-Nd whole-rock age of 743 + 24 Ma
for the Jabal Al Wask ophiolite in Saudi Arabia (Claesson et al., 1984).
2. Analytical procedures
Major- and trace-element analyses cover rock types
from all three lithologic units of the GGO, namely
pillow lavas, sheeted dykes and gabbros, and were performed on 64 samples selected on the basis of freshness.
Samples were first cleaned with distilled water and then
crushed and ground with a corundum mill to < 0.063
mm. Major elements were determined by DC plasma
emission spectroscopy ( BeckmaneU SpectraSpan VI)
using the method of Feigenson and Carr ( 1985). The
precision is between - 0.3 and - 6.1%, depending on
concentration. Trace elements were measured by either
X-ray fluorescence
spectroscopy
(XRF) or sparksource mass spectrometry (SSMS). The XRF measurements were performed on duplicate powder pellets
using a Siemen?
SRS 200 spectrometer and employing data reduction as detailed in Laskowski and Krijner
( 1984). Precision and accuracy are better than - 5%.
Twenty-four samples covering each ophiolite rock type
were further analyzed for rare-earth elements (REE),
Hf, Th and U on an AEIm MS702R spark-source mass
spectrometer using a multi-element
isotope dilution
technique (Jochum et al., 1988). Precision and accuracy for most elements are between -5 and - lo%,
depending on concentration.
Thirty-six samples were selected for isotopic analyses on the basis of variation in major- and trace-element composition.
Mineral separates were obtained
magnetically and purified by hand-picking. 130 mg of
Geology 123 (1995) 29-51
33
clean plagioclase and 65 mg of clean clinopyroxene
were hand-picked for isotopic analysis.
For Sr and Nd isotopic measurements, 50-100 mg
of basaltic rock powder and 1000 mg of gabbroic powder were spiked and dissolved in HF and HNO, in
Teflona bombs. For Pb isotopes, crushed splits were
cleaned in 6 N HCl and then dissolved in HF. The
separation method is described in Manhes et al. ( 1978)
and White and Patchett ( 1984). Sr, Nd and Pb isotopes
were measured on a Finnigan@ MAT 261 thermal ionization mass spectrometer using a 5-cup multicollector
configuration in static mode (Todt et al., 1983; White
and Patchett, 1984). Typical blanks were 180 pg for
Sr, 70 pg for Nd, 25 pg for Sm and 200 pg for Pb.
Fractionation
corrections
were made using ‘?+I
88Sr = 0.1194 and ‘46Nd/ ‘@Nd = 0.7219. The initial
‘43Nd/‘44Nd ratios of a rock of age t are expressed in
e-units using the following chondritic values: i4’Sm/
and
l”Nd = 0.1966
‘43Nd/‘44Nd
for
present
day = 0.5 12638 (DePaolo, 1976). Regression analyses
followed York ( 1969), modified by Titterington and
Halliday ( 1979)) and errors on ages and initial ratios
are 2a.
3. Geochemistry
3.1. Major and trace elements
Thirty samples of lava and dyke material from Gabal
Gerf, Heiani and Harga Zarga display basaltic compositions with SiO, around 50 wt% and Mg# [ = Mg/
(Mg + Fe’+) ] from 43 to 66 (Tables l-3). The range
in MgO is from 4.8 to 10.9 wt%, while TiO, varies
between 0.82 and 2.04 wt%. If all rocks represented by
these samples are cogenetic, then fractionation must
have played a significant role in their formation. Variations in Cr (93-565 ppm) and Ni (42-185 ppm)
contents suggest that pyroxene, olivine and possibly
chromite are the fractionating minerals (see Fig. 13).
Tables l-3 also demonstrate that the lavas and dykes
have a similar chemistry, suggesting a cogenetic origin
for both.
Table 1 also shows the analyses for 30 gabbros and
gabbro cumulates as well as five ultramafic rocks. The
gabbros have Mg# from 47 to 84. SiOz concentrations
range from 37 to 53 wt%, MgO varies from 4.9 to 10
wt%. TiO, contents from 0.07 to 6.82 wt% depend on
M. Zimmer et ul. / Chemicul Geology 123 (I 995) 29-51
34
3.2. Rare-earth elements (REE)
the cumulus Ti phase. Cr, Ni and Sr concentration vary
sympathetically
with silica and magnesia. Additional
trace elements for 10 basalts, and 14 gabbros and gabbro cumulates are shown in Tables 4 and 5.
In order to demonstrate the variation in major and
trace elements as a result of fractionation, A&O, and
Sr concentrations are plotted against Zr contents in Fig.
3 (left). A&O, and Sr contents are dependent on the
amount of plagioclase in the gabbro cumulates, which
is evident from Fig. 3. The gabbros have low Zr abundances, typical for cumulates, and high A&O3 and Sr
contents, depending on the amount of plagioclase.
The variation of TiOz. and Y is independent of plagioclase fractionation, and the TiO,-Zr and Y-Zr diagrams (Fig. 3, right) highlight the chemical correlation
between the volcanic and gabbroic rocks of the Gerf
complex. High TiO, contents in a few gabbro samples
can be explained by a cumulate Ti phase such as ilmenite.
Table
REE data (Tables 4 and 5) are presented graphically
in Figs. 4 and 5. Chondrite normalization is based on
Evensen et al. ( 1978). The nearly parallel patterns for
Heiani basalts (Fig. 4, bottom) are characterized by
light REE (LREE) depletion relative to middle REE
(MREE) and heavy REE (HREE). Some samples
exhibit a slight negative Eu anomaly. Harga Zarga
basalts (Fig. 4, top) have almost flat REE patterns with
less pronounced LREE depletion and absolute abundances of N 10-30X chondrite. There is no apparent
difference between pillow lavas and sheeted dykes.
REE distribution patterns for the gabbros and one
ultramafic rock (Fig. 5) clearly separate these rock
types into several groups, independent of geographic
location. The REE patterns are influenced by varying
amounts of plag and cpx, while 01 does not change the
REE distribution.
Plag-rich gabbros have LREEenriched patterns with a distinct positive Eu anomaly
(Fig. 5, top). It is obvious that this distribution is com-
I
Major-element
(in wt’%) and trace-element (in ppm) compositions for ophiolitic rocks from the Gabal Gerf Complex
GG 12
GG I3
GG251
GG3l
GG 35
GG 36
GG 68
GG 74
GG 17
GG 79
GG 80
GG 81
GG 82
3
P
a
”
”
S
”
”
S
g
S
g
S
45.53
51.09
40. IO
52.00
51.42
46.83
41.09
29.95
48.17
44.45
44.49
46.68
TiO>
I .29
2.03
2.5 I
0.07
0.08
0.07
0.03
0.02
0.51
3.64
0.69
0.30
I .69
Al@?
Fe0
18.80
17.32
15.02
0.69
1.37
19.81
16.55
18.83
22.02
24.08
20.59
7.41
6.85
11.44
6.90
5.84
2.77
4.82
3.13
6.43
7.68
7.43
4.13
4.96
FqO,
2.64
1.90
2.41
3.09
2.22
1.19
2.21
2.88
I .6X
2.42
I .74
I.16
2.52
MgO
cao
7.48
4.81
10.90
27.31
23.55
9.85
34.78
38.24
9.60
6.48
7.86
5.55
6.25
12.13
7.51
II.20
1.65
9.83
14.06
0.03
0.20
14.60
9.98
8.05
9.97
Na,O
2.17
4.38
2.95
0.09
0.13
I .92
0.06
0.05
1.79
2.63
3.39
3.32
2.86
K20
0.12
0.45
0.3 I
0.06
0.06
0.05
0.06
0.06
0.13
0.44
0.26
0.20
0.18
LO1
2.66
2.08
7.39
4.85
3.15
16.46
25.13
0.89
98.92
99.25
99.35
99.70
99.54
99.66
100.35
99.18
4 15
-99.54
1.16
99.00
2.5
99.05
3.25
Total
I .36
-98.93
99.45
Mg#
58
50
59
83
84
82
90
92
54
61
66
61
b.d.1.
II
87
b.d.1.
b.d.1.
I71
SiO,
Rb
Sr
Nb
zr
289
1.2
74
22
I50
b.d.1.
7.4
b.d.1.
b.d.1.
--
b.d.1.
3.7
0.7
241
b.d.1.
b.d.1.
0.11”
4.5
b.d.1.
0.16
415
b.d.1.
b.d.1.
b.d.1.
0.6
0.97”
b.d.1.
b.d.1.
b.d.1.
7.0
0.07”
b.d.1.
b.d.1.
b.d.1.
92
21
15
26
33
51
V
302
275
350
II9
80
66
29
I08
86
3,092
2.473
1,101
2,515
2,540
I53
2,215
2,273
33
37
CI
I81
301
205
Ni
64
82
154
CU
39
29
39
Zn
I18
85
0.2
588
1.3
Y
CO
69
128
4.6
I25
50
II8
5.4
576
2.5”
36
13”
402
2.8
883
0.3”
4.4”
0.8”
2.4”
800
0.96
30
4.2
213
26
47.92
Il.32
0.6
602
I.0
24”
7.0
209
39
50
31
30
78
294
13
155
68
50
503
355
66
52
88
b.d.1.
b.d.1.
I8
b.d.1.
b.d.1.
34
45
II
b.d.1.
b.d.1.
24
b.d.1.
b.d.1.
b.d.1.
62
65
6.2
39
30
48
M. Zimmer et al. /Chemical
Geology 123 (I 995) 29-51
parable to REE patterns dominated by plagioclase.
Gabbros with less plag and a higher proportion of cpx
have LREE-depleted
patterns with no Eu anomaly
(sample GG 83, Fig. 5, bottom). The pattern of sample
GG 83 is similar to that of clinopyroxene. Sample GG
36 shows an enrichment in La and Ce, which can be
explained by low-temperature alteration. Most gabbros
have REE patterns with a slight LREE enrichment and
a small positive Eu anomaly (Fig. 5, middle). These
patterns result from about equal proportions of cpx and
plag in the samples as also indicated by petrographic
observations.
35
ridge basalts (N-MORB) or back-arc basin (BAB)
magmas (see Figs. 6-8) (Sun and Nesbitt, 1982; Saunders, 1984; Saunders and Tarney, 1984).
Variations in the abundances of some trace elements,
in particular elements with small partition coefficients
(D-values < 0.01) , such as Ti, Zr, Hf, Nb and the REE
have been used as indicators of magma genesis and the
tectonic setting in which oceanic lavas are erupted
(e.g., Pearce and Cann, 1973; Pearce and Norry, 1979).
These elements have the advantage of being only little
affected by alteration processes. In general, the transition from basic to intermediate character is marked by
the appearance of Fe-Ti-oxides
as a cumulus phase
which causes a sudden decrease in the Ti/Zr ratios of
the residual magma. In Fig. 6 GGO basalts are plotted
in a TiO, vs. Zr diagram of Pearce and Norry ( 1979).
Only one basalt from the melange plots outside the field
for MORB, while all other samples follow the MORB
trend. In a Cr vs. Y diagram (Fig. 7), useful for discriminating
between MORB and island arc basalt
(Pearce, 1980), all Gerf samples are clearly identified
as MORB . Ti /V ratios between 17,000 and 45,000 also
4. Tectonic setting deduced from chemical
composition
In terms of major- and trace-element compositions,
the Heiani and Harga Zarga basalts are classified as
high-Ti basalts (Serri, 1981), and the trace elements
such as high field strength elements (HFSE) and REE
also suggest similarities with normal-type mid-ocean
Table 1 (continued)
GG 83
GG84
GG 85
GG 86
GG 87
GG 88
GG 89
GG90
GG 91
GG 92
GG 93
GG 250
GG 25 I
g
g
S
g
g
g
g
g
g
g
g
g
u
SiOx
47.73
46.82
44.99
47.26
45.16
47.00
50.76
51.31
46.12
48.59
36.83
49.05
TiO,
0.84
0.65
I .72
0.77
0.49
0.53
0.35
0.56
0.43
0.31
2.61
1.22
1.97
AhO,
Fe0
20.00
19.95
18.63
19.64
20.57
22.77
18.31
18.84
21.17
23.72
17.52
15.76
7.61
6.49
5.36
8.02
5.71
6.36
6.23
4.49
4.51
5.14
4.62
15.31
8.45
7.10
Fe@,
0.79
1.68
2.01
I .74
2.13
1.80
1.22
1.33
1.10
0.76
3.03
1.15
I .21
MS0
cao
7.96
8.86
8.28
8.51
8.80
7.48
9.04
7.34
9.25
5.76
8.84
6.85
17.16
10.95
10.64
9.51
8.91
7.84
8.62
10.38
11.81
11.75
9.49
7.80
Na,O
2.81
2.81
3.01
2.90
3.34
3.14
2.66
2.73
2.72
3.32
2.11
1.83
1.41
KzG
LO1
0.17
0.48
0.47
0.58
0.29
0.19
0.21
0.14
0.17
0.27
0.25
0.12
0.18
2.15
2.46
1.54
2.09
0.93
2.00
99.28
99.30
99.5 I
99.50
99.85
99.30
0.62
-98.41
2.93
99.1
2.65
-99.49
4.00
99.40
3.35
99.37
3.70
Total
I .30
--99.04
98.38
%#
66
70
60
68
64
63
74
70
73
66
47
56
79
Rb
Sr
Nb
zr
Y
V
0.3”
151
0.6
12
11”
372
14
12
14
465
404
585
_---
2.1”
623
0.32
671
1.7
548
b.d.1.
b.d.1.
b.d.1.
0.3’
0.2”
b.d.1.
36
32
15
4.4”
5.7”
10
4.8
0.8”
14
16
142
94
49
147
63
236
291
73
91
71
CO
46
42
187
51
Cr
326
395
65
Ni
89
43
CU
10
36
17
21
Ztl
b.d.1.
55
88
61
Ill
6.4
63
0.6”
140
8.4
0.16”
576
0.9’
14”
8.5”
0.25”
b.d.1.
7.4”
25
8.4”
4.9
1.1
386
b.d.1.
7.3
18
87
374
653
124
103
315
36
98
54
378
26
55
55
74
67
17
22
26
47
44
b.d.1.
34
487
6.3
b.d.1.
14
131
267
_
31
53
2.5
73
2
53
29
826
319
b.d.1.
110
9.78
336
b.d.1.
36
128
54
5.5
721
78
42
98
9.2
0.3”
575
3.36
49.03
13
b.d.1.
9.4
13
_
36
Table
SiO,
TiOz
Al201
Fe0
Fez07
M. Zimrner
I
Geology 123 (I 995) 29-51
(continued)
GG253
GG258
GG263
GG265
GG266
GG 267
GG268
GG270
GG271
GG279
GG280
GG283
g
g
g
g
g
g
g
g
g
g
g
g
40.84
4.37
17.83
10.44
3.35
8.77
7.07
3.14
0.15
2.59
43.33
4.43
18.88
8.97
2.71
7.54
7.92
3.40
0.19
1.71
44.4 1
3.96
15.22
7.98
4.16
8.26
10.83
2.49
0.17
1.06
51.61
0.3 1
18.41
4.55
1.23
9.01
10.99
2.45
0.09
0.61
48.80
0.22
17.06
3.36
1.23
7.01
18.62
2.12
0.2 1
1.18
43.15
0.14
27.71
1.21
1.21
4.87
15.78
2.93
0.07
2.88
46.87
0.25
17.77
2.06
1.28
9.15
17.94
1.61
0.07
2.26
48.57
1.01
14.04
8.86
1.66
8.22
12.12
2.83
0.07
1.36
46.92
1.45
16.40
7.56
1.60
10.00
9.71
2.50
0.14
2.29
98.65
38.65
6.82
16.30
10.02
7.75
8.84
6.42
2.78
0.16
0.71
-98.45
98.55
99.08
98.54
99.26
99.81
99.95
99.26
98.74
98.57
65
48
54
54
56
74
74
79
84
59
66
MgO
cao
Nag0
K20
LO1
52.62
0.22
19.37
5.77
1.12
5.33
10.47
3.47
0.17
0.7
48.32
1.25
16.83
7.00
2.00
8.99
10.73
3.21
0.16
0.25
Total
99.24
Mg#
58
Rb
Sr
Nb
Zr
Y
V
co
Cr
Ni
CU
Zn
et ul. /Chemical
b.d.1.
577
b.d.1.
8
9.6
196
_
0.5”
323
2.4”
29”
13”
185
_
b.d.1.
479
4.7
28
b.d.1.
702
_
b.d.1.
497
3.8
21
b.d.1.
493
_
b.d.1.
536
3.7
27
2.3
399
_
b.d.1.
423
3.2
31
11
534
_
0.25”
520
0.4”
9.7a
5.9”
100
_
3
88
b.d.1.
6.7
5.5
174
_
145
21
II
_
263
102
37
_
237
94
47
_
145
93
81
_
122
74
33
_
294
88
57
_
346
70
16
_
429
164
264
_
Mg# = 100 X Mg/ (Mg + Fe’+ ). p = pillow lava; sd = sheeted dyke; a = amphibolite;
limit; - = not determined. All major oxides without Pz05.
“Trace elements by SSMS (other trace elements by XRF)
suggest a MORB or BAB origin (Shervais, 1982).
Although the application of the above diagrams to late
Proterozoic rocks may be questionable, the discrimination is in agreement with all other chemical data, in
particular the REE (see Fig. 4).
Immobile trace-element abundances for pillow lavas
and sheeted dykes, normalized to primitive mantle
(Hofmann,
1988), are plotted in Fig. 8 in order of
decreasing degree of incompatibility.
Harga Zarga
basalts (Fig. 8, top) are compatible with the N-MORB
distribution pattern. Note the absence of a Nb depletion.
Heiani basalts (Fig. 8, bottom) have lower abundances
of trace elements and are distinctly depleted in the very
incompatible
elements. However, both rock types,
although from different localities, are very similar to
N-MORB and very different from supra-subduction
zone lavas, in particular there is no Nb depletion.
b.d.1.
80
b.d.1.
3.4
b.d.1.
25
_
60
183
49
_
b.d.1.
57
b.d.1.
3.4
6.2
187
_
b.d.1.
84
1.8
53
26
357
3,110
192
58
_
261
85
3.1
_
25
256
b.d.1.
91
24
226
52
700
248
58
77
g = gabbro; u = ultramafic rock. b.d.1. = below detection
5. Results of isotopic analyses
Isotopic data for Sr, Nd, Pb as well as .+,,-values are
given in Tables 6 and 7. An age of 750 Ma has been
assumed for the ophiolite in the calculation of initial
E,,-values, and this is in line with the published zircon
age (Kroner et al., 1992) and our Sm-Nd ages reported
below.
5.1. Sr isotopes
*‘Sr/*%r ratios for the basaltic rocks range from
0.7021 to 0.7032, while those in the gabbros are
0.7024-0.7029
(Table 3). The Rb/Sr ratio is low in
all rocks, independent of the grade of alteration. The
majority of basalts have Sr concentrations of - 70 ppm,
while plagioclase accumulation in the gabbros resulted
in Sr contents of -400-600
ppm (Table 1). In spite
of very low Rb/Sr ratios ( <O.OOlXLO3), there may
M. Zimmeret al. /Chemical Geology123 (1995) 29-51
31
Table 2
Major- and trace-element
compositions
for rocks from Jabal Harga Zarga
GG9
GG 10
GG 69
GG 70
GG71
GG 72
GG 73
GG 94
GG181
GG182
GG183
GG184
GG185
P
P
P
P
sd
sd
P
P
P
P
P
P
P
SiOz
48.73
49.86
48.82
49.74
45.74
46.79
45.86
51.48
49.40
50.95
51.19
51.14
TiO,
1.31
0.82
0.96
0.95
1.55
1.61
1.05
0.92
2.04
1.91
1.17
1.98
2.01
Al&
Fe0
15.46
14.74
18.21
17.29
18.00
17.95
16.65
14.19
12.81
12.82
14.26
12.83
13.02
11.90
48.79
8.46
7.76
7.74
7.16
7.73
6.69
8.18
6.87
11.00
10.87
8.53
11.31
Fe,O,
1.50
1.02
0.97
1.33
1.45
2.25
1.97
1.73
3.02
2.24
1.50
1.73
1.57
MgO
CaO
8.15
9.27
6.97
6.75
9.59
8.49
8.82
9.29
5.80
5.95
6.44
5.87
5.94
11.59
11.16
12.14
13.43
9.72
9.95
13.58
10.65
10.26
9.42
10.29
9.45
10.98
NazO
2.60
2.26
1.65
0.81
2.67
2.93
1.21
1.85
2.09
2.29
2.44
2.13
1.99
K?O
0.13
0.12
0.12
0.07
0.16
0.17
0.11
0.06
0.19
0.16
0.13
0.14
0.17
LO1
2.09
1.89
1.36
1.43
2.29
2.13
1.42
1.93
2.04
2.06
1oo.o2-
98.20
98.94
98.96
98.90
98.96
98.80
98.54
98.65
2.86
--98.81
1.91
Total
I .55
--98.59
98.49
98.43
66
59
44
65
63
61
66
43
45
54
45
44
b.d.1.
b.d.1.
b.d.1.
b.d.1.
b.d.1.
b.d.1.
b.d.1.
85
60
62
75
48
56
48
Mg#
Rb
Sr
Nb
Zr
Y
V
60
0.97”
92
6.5”
93
27”
314
3.2
46
4.2
60
1.8
57
21
25
26
250
280
270
0.7”
245
1.7”
133”
32”
209
3.2”
268
1.8”
136”
32”
229
0.68”
76
3.0”
65”
3.0
56
b.d.1.
123
2.4
118
b.d.1.
66
23
23
58
56
33
306
273
499
487
356
1.5”
44
2.3”
124”
63”
478
1.3
90
2.0
120
57
505
co
44
36
38
36
43
40
36
37
40
43
42
39
38
Cr
378
565
437
399
555
427
359
320
192
184
245
192
198
Ni
122
161
104
103
185
152
98
106
76
69
66
63
58
Co
26
22
71
46
43
25
116
94
46
81
83
43
40
Zn
90
80
76
72
78
82
101
72
140
127
92
134
139
Explanations
as in Table 1.
have been small changes in 87Sr/86Sr through time due
to radiogenic growth. Furthermore, the relatively small
range encompassed by the gabbros (0.7024-0.7029)
relative to the basalts (0.7025XL7032)
suggests that
the basalts were more affected by alteration. The variability in measured ratios is therefore considered to be
partly due to radiogenic growth and/or alteration but
may partly also represent primary source variations.
In contrast to the relatively small Sr isotopic variation in the fresh gabbros, the variation in the basalts is
significant. Pillow lavas have the highest 87Sr/86Sr
ratios because of stronger alteration (Table 6). Altogether, the 87Sr/86Sr ratios of Gabal Gerf rocks are low
and are comparable to modern N-MORB ( O’Nions et
al., 1977; Hofmann and Hart, 1978; Ito et al., 1987).
Island arc basalts, on average, have higher values
(Hawkesworth et al., 1977; White and Patchett, 1984)
than N-MORB and Gabal Gerfrocks. Back-arc basalts
have 87Sr/8”Sr ratios intermediate between N-MORB
and island arc basalts (Hawkins, 1976; Hawkesworth
et al., 1977; Volpe et al., 1987, 1988) and are also
comparable to Gabal Gerf rocks.
5.2. Nd isotopes and geochronology
We analyzed whole-rock samples from the best preserved gabbros as well as mineral separates from the
freshest sample in our collection (GG 258) in order to
establish the crystallization age of the Gerf ophiolite
(Table 6). Whole-rock data for all 15 gabbro samples
from Gabal Gerf display considerable scatter in the
‘47Sm/144Nd vs. 143Nd/144Nd diagram (MSWD =
11.5) but can be fitted to a regression line with an age
of 720 k 9 Ma (Fig. 9a). Clinopyroxene
and plagioclase mineral separates, together with the whole-rock
analysis of gabbro sample GG 258, define a linear array
in the isochron diagram (MSWD = 1.8) corresponding
to an age of 770 f 52 Ma (Fig. 9b). Eighteen samples
38
M. Zimmer et al. /Chemical
Geology 123 (1995) 29-51
Table 3
Major- and trace-element compositions
for rocks from Gabal Heiani
GG 170
GG 171
GG 172
GG 173
GG 174
GG 175
GGl76
GG 177
GG 178
GG 179
GG180
GG95
GG 281
GG 282
sd
sd
sd
sd
sd
sd
sd
sd
P
P
P
sd
sd
P
48.66
49.00
48.89
49.39
48.17
49.33
47.61
48.54
48.12
48.15
50.00
48.41
49.47
TiO,
0.84
0.88
1.35
I .53
1.83
1.44
1.86
1.75
0.92
1.03
0.87
0.86
1.48
0.90
Al&‘,
Fe0
15.99
15.72
14.98
13.58
13.33
13.74
13.15
12.96
16.62
15.92
16.11
15.20
13.65
15.12
7.43
7.39
9.80
10.20
11.53
9.81
12.44
11.47
7.85
8.97
6.93
7.66
10.38
8.49
FqO,
2.07
I .95
I .41
I .57
2.52
1.76
2.19
2.19
2.03
2.05
2.60
2.51
1.89
2.16
M@
cao
7.63
7.76
7.24
6.43
6.66
7.53
6.78
6.46
1.47
7.59
6.42
8.19
7.41
8.55
II.07
11.10
10.13
10.81
9.28
9.86
9.11
9.80
II.00
9.88
11.72
11.78
10.19
9.90
SO,
49.39
Nag0
2.55
2.51
2.46
2.44
2.28
2.70
2.28
2.31
2.25
2.30
1.88
2.14
2.75
2.37
K,O
LO1
0.10
0.11
0.16
0.10
0.12
0.13
0.15
0.22
0.14
0.09
0.08
0.06
0.10
0.15
2.60
2.63
2.18
2.58
2.75
2.38
2.81
2.80
2.58
2.78
2.39
2.12
0.98
1.81
Total
98.94
99.05
98.66
98.63
98.47
98.68
98.38
98.50
98.98
98.76
99.00
98.93
98.30
98.84
Mg#
59
60
54
50
46
54
46
46
58
56
55
60
52
59
Rb
b.d.1.
b.d.1.
b.d.1.
b.d.1.
b.d.1.
b.d.1.
b.d.1.
b.d.1.
b.d.1.
b.d.1.
Sr
92
89
70
74
66
59
69
66
Nb
2
1.6
0.98”
75
1.4”
I.7
2.1
b.d.1.
I.6
Zr
48
SO
82”
88
109
81
112
Y
25
23
38”
41
51
40
54
V
277
217
412
465
397
487
394
1.5"
60
1.9"
118
49"
462
0.52
63
0.64
1.3
0.6
0.23
73
0.7
73
2.1
96
<I
48
48
47"
45"
92
29"
29
27"
21"
38
24
436
349
321
357
303
306
42
CO
41
41
42
41
44
43
46
43
43
47
40
41
45
47
Cr
289
328
276
183
109
214
93
II8
255
179
234
235
171
364
Ni
96
98
95
71
42
77
49
55
90
68
61
92
76
122
CU
62
14
70
11
40
32
18
41
51
76
65
91
62
75
ZIl
79
75
110
118
130
119
142
121
87
101
84
85
Explanationsasin Table I.
of basalt (pillow lavas, sheeted dykes) from Cabal
Harga Zarga and Heiani and one amphibolite from
Gabal Gerf display even larger scatter than the wholerock gabbro samples (MSWD = 36) and define a linear
array corresponding
to an age of 834 + 19 Ma (Fig.
SC). Since these samples come from different localities
we consider this linear array as least geologically meaningful. The Heiani basalts alone do not display sufficient spread in the isochron diagram (MSWD = 90) to
permit calculation of a meaningful linear array (see
Fig. SC). However, the Harga Zarga basalts are more
variable in their Sm-Nd isotopic composition
and
define a regression line (MSWD = 11.9) corresponding to an age of 758 f 34 Ma (Fig. SC). All these ages,
except that calculated for all basaltic rocks (834 f 19
Ma) are identical within the relatively large errors, and
the fact that the mineral and most whole-rock data
define linear arrays suggests that these approximate the
igneous crystallization age of the complex.
Kriiner et al. (1992) have determined an equally
imprecise mean 207Pb/206Pb age of 741 k42 Ma for
zircons from the same locality as sample GG 258, and
the pooled mean age of all available data is - 750 Ma.
We used this age for the calculation of +,-values listed
in Table 6. ??
Nd initial values for an age of 750 Ma in
the basaltic rocks vary between + 6.5 and + 8.8 and
are the same for pillow lavas and sheeted dykes in
contrast to their different Sr isotopic composition. The
+,-values for the gabbros are, on average, lower than
those for the basalts and range from + 6.3 to +7.9
(Table 6). These values clearly demonstrate that the
Gerf rocks are derived from a depleted mantle source.
The relatively high +,-values for the basalts and dykes
are taken to approximate the value for the depleted
mantle at -750 Ma (O’Nions et al., 1977; Ito et al.,
1987). The somewhat lower values for the gabbros may
reflect a subduction component, and this is more fully
discussed in the following section.
M. Zimmer et al. /Chemical Geology I23 (1995) 29-51
39
Table 4
Trace and rare-earth elements (in
ppm), determined
by SSMS, for selected samples of gabbro (g ) and ultmmafic rock ( u) from Gabal Gerf
Complex
GG 35
GG36
GG77
GG 79
GG 80
GG 81
GG82
GG 83
GG 87
GG 88
GG90
GG91
GG258
GG268
U
ii?
g
g
g
g
g
g
g
g
g
g
g
g
cs
b.d.1.
Ba
3.2
La
0.024
0.179
1.14
1.67
0.8
2.7
2.42
0.61
0.91
1.01
1.86
1.3
3.04
1.0
Ce
0.089
0.236
2.74
4.13
1.56
6.81
6.28
2.60
1.8
1.95
4.83
3.32
8.55
8.39
Pr
0.012
0.02
0.49
0.64
0.2
0.93
1.03
0.35
0.2
0.24
0.8
0.54
1.44
0.34
Nd
0.075
0.136
2.53
3.2
0.86
3.86
5.29
2.1
0.8
0.97
4.44
2.71
7.22
1.63
Sm
0.042
0.054
0.87
0.98
0.16
0.91
1.09
0.87
0.22
0.19
1.57
0.97
1.69
0.45
ELI
0.032
0.07 1
0.78
0.63
0.39
0.65
0.73
0.43
0.42
0.4
0.82
0.57
0.91
0.38
Gd
0.051
0.072
1.18
1.03
0.21
1.01
1.08
1.08
0.3
0.22
1.6
1.16
1.83
0.55
Tb
0.022
b.d.1.
0.2
0.17
0.6 1
0.14
0.17
0.21
0.06
0.039
0.26
0.19
0.29
0.099
DY
0.093
b.d.1.
1.29
1.13
0.21
0.88
1.21
1.58
0.25
0.24
1.79
1.16
1.81
0.74
Ho
0.025
0.033
0.27
0.23
0.025
0.17
0.24
0.35
0.04
0.03
0.38
0.23
0.39
0.14
Er
0.07
0.115
0.79
0.64
0.072
0.5 1
0.65
1.06
0.12
0.081
1.12
0.73
1.15
0.36
b.d.1.
13
0.04
31
0.79
82
0.3
95
0.83
49
0.087
91
0.06
23
0.13
53
0.073
35
0.04
76
0.16
33
0.06
59
0.03
21
Yb
0.065
0.082
0.5 1
0.39
0.056
0.33
0.8
0.97
0.073
0.073
0.86
0.55
1.25
0.3
Lu
0.014
0.017
0.09
0.08
0.016
0.05
0.12
0.14
0.013
0.015
0.15
0.092
0.19
0.053
Hf
0.063
0.057
0.54
0.65
0.12
0.85
0.8
0.44
0.133
0.16
0.84
0.5
0.9
0.275
Pb
0.3
0.059
0.21
0.075
0.53
0.7
0.31
0.26
0.46
0.14
0.23
0.22
0.5
0.311
Th
0.015
b.d.1.
0.03 1
0.024
b.d.1.
0.196
0.057
0.01
0.022
0.02
0.016
b.d.1.
0.064
0.02
U
0.014
b.d.1.
0.016
0.025
b.d.1.
0.075
0.034
b.d.1
0.012
0.011
0.01
b.d.1.
0.033
b.d.1.
5.3. Pb isotopes
We analyzed Pb isotopes in selected samples of
lavas, sheeted dykes, gabbros and ultramafic rocks, and
the data are presented in Table 7. Pb, U and Th concentrations were determined by SSMS and are listed in
Tables 4 and 5. Pb concentrations vary between 0.1 and
0.8 ppm for the basalts and 0.03-0.7 ppm for the gabbros. U contents are < 0.01 to 0.1 in basalts and < 0.01
and 0.07 ppm in gabbros. Th concentrations are higher
in basalts (0.04-0.4 ppm) and vary between <O.Ol
and 0.2 ppm in the gabbros.
Due to the different p-values of the samples, measured Pb isotopic ratios must be corrected for in situ
decay. The precision of the Pb, U and Th measurements
by SSMS is not sufficient, however, for such a correction, and a two-stage model (Stacey and Kramers,
1975) was therefore adopted, assuming an age of 750
Ma. p-values were computed to fit the Pb isotopic ratios
to a 750-Ma isochron. Measured Pb isotopic ratios,
corrected ratios and p-values are listed in Table 7.
For samples of pillow lava, ps-values (for explanation, see Table 7) vary between 7 and 12, probably due
to post-crystallization
alteration. The lower /_~3-values
for the sheeted dykes, between 3 and 4, reflect their
lower degree of alteration. This is consistent with the
interpretation of the Sr isotopic data where pillow lavas
have also been interpreted to be more strongly altered
than sheeted dykes [for detailed discussion on alteration, see Zimmer ( 1989) 1. The p3-values for gabbros
( - 1.8) are within the same range as measured for the
Tertiary Xigaze ophiolite in Tibet (Gopel et al., 1984).
The Pb isotopic data are plotted in the 206Pb/2@‘Pb
vs. 207Pb/204Pb evolution diagram of Fig. 10. Measured
values are shown as individual triangular symbols
while age-corrected values cluster together and are
shown as elliptical fields. Gabbros and ultramafic rocks
tend to have higher 207Pb/204Pb ratios than the basalts
for similar 206Pb/204Pb. The Pb data for the basaltic
rocks (lavas and dykes) are comparable to modern NMORB (Fig. 10) as also suggested by the Nd isotopic
data, while the gabbros are more akin to island arc and
BAB rocks which generally have higher 207Pb/204Pb
ratios. These higher 207Pb/204Pb ratios and, possibly,
the somewhat lower +,-values,
may be explained by
mixing of a small amount of pelagic sediment ( - 1%)
to the magma source of the gabbros (Fig. 10). We
envisage such mixing to have occurred during subduction of oceanic crust when part of the sedimentary cover
of the oceanic basalts is carried down into the mantle
40
M. Zitnrner et al. /Chemical
Geology 123 (1995) 29-51
Table 5
Trace and rare-earth elements (in ppm), determined by SSMS, for selected samples of pillow lava (p) and sheeted dyke (sd) from Harga Zarga
and Heiani, Gabal Gerf Ophiolite Complex
Heiani
Harga Zarga
GG9
P
CS
Ba
La
Ce
Pr
Nd
Sm
ELI
Gd
Tb
DY
Ho
Er
Yb
LU
Hf
Pb
Th
U
Mg#
0.12
17
4.91
Il.7
2.11
11.0
3.67
1.2
5.24
0.74
4.72
1.17
3.74
2.69
0.464
2.35
0.49
0.43
0.12
60
GG71
sd
GG 72
sd
0.23
31
4.59
13.9
2.s2
13.4
3.86
1.35
4.95
0.77
5.01
1.19
3.4
2.63
0.399
2.97
0.84
0.133
0.054
0.46
40
5.02
15.4
2.7
14.2
4.13
65
63
0
GG 184
P
P
0.026
9.9
2.93
8.51
I .33
7.28
2.42
0.81
2.89
0.51
3.27
0.82
2.5
2.43
0.369
I .76
0.49
0.18
0.055
1.47
4.42
0.75
4.75
1.11
3.2
2.7
0.4 1
3.07
0.72
0.203
0.078
50
GG 73
0.029
13
2.71
10.7
2.08
11.6
3.9
1.04
4.05
0.76
5.5
1.46
4.3
6.2
0.86
3.48
0.26
0.095
0.063
45
61
IS0
loo
Zr
200
@pm)
Fig. 3. Al,O, and Sr (left) and TiO, and Y (right) concentrations
GG 95
sd
GG 172
sd
GG 177
sd
GG 178
GG 180
P
P
0.029
4.36
0.87
3.03
0.5.5
3.42
1.S
0.57
2.36
0.41
2.81
0.75
2.2
2.04
0.33
1.14
0.3
0.038
_
0.06
17
2.0
6.72
1.32
7.9
3.3
1.1
5.04
0.84
5.4
1.47
4.51
3.5
0.62
2.4
0.39
0.05 1
_
0.19
14
2.61
8.92
1.65
10.0
4.3
1.3
5.9
0.99
6.4
1.69
5.23
4.5
0.64
3.3
0.57
0.075
0.01
0.028
18
0.95
3.56
0.74
4.35
2.18
0.69
2.72
0.56
3.81
0.94
3.13
2.43
0.46
I .26
0.097
0.04
0.02
54
46
58
60
250
100
Zr
0.018
9.58
1.01
3.86
0.74
4.33
I .93
0.69
2.15
0.43
3.19
0.74
2.57
2.3
0.39
1.26
0.14
0.034
0.012
55
150
(ppm)
of Gabal Gerf ophiolitic rocks in comparison
to Zr as fractionation
index.
M. Zimmer et al. /Chemical Geology 123 (I 99.5) 29-51
1
La CePr
’
Nd
’
1
1
1
’
’
’
r
41
’
SmEuGdTbDyHoErYbLu
Fig. 4. REE concentrations for samples of pillow lavas (solid svmMS) and sheeted dykes (open s.vmbofs) from Harga Zarga (top)
and Gabal Heiani (bottom), normalized to C 1 chondrites.
La Ce Pr Nd
and is admixed
duction zone.
to magmas
generated
above the sub-
6. Comparison
ophiolites
with isotopic data of other
SmEuGd’Tb Dy HoEr YbLu
Fig. 5. REE concentrations
for samples of Gabal Gerf ophiolite
gabbros and one ultmmafic rock (GG 35, solid s.wbol) , normalized
to Cl chondrites. Dominant cumulate minerals are shown in bold.
Simplified PEE distribution patterns for plagioclase (top right) and
clinopyroxene (bottom right) are shown for comparison.
1
10 L
Sr and Nd isotopic compositions for rocks of the
GGO Complex and other ophiolite complexes are
shown in Fig. 11. It is obvious that the Gerf samples
show the lowest R7Sr/86Sr ratios while those of virtually
all other complexes are variably elevated, probably due
to seawater alteration. It is surprising that the Gerf rocks
are so little isotopically altered in spite of their antiquity.
The initial ??
,,-values for rocks of the GGO Complex
are compared with those of other ophiolites in Fig. 12.
The evolution line for the depleted mantle, assuming a
constant rate of depletion over the age of the Earth
(Goldstein et al., 1984) is also plotted in Fig. 12. There
is some scatter in the data which may either be ascribed
to primary isotopic inhomogeneities
in the mantle
sources or to isotopic differences reflecting differences
1000
Zr (pi@
Fig. 6. Discrimination diagram inferring tectonic setting for mafic
volcanic rocks on the basis of TiO, and Zr concentrations
(Pearce
and Norry, 1979). MORB = mid-ocean ridge basalt; IA = island arc
basalt; WPB= within-plate basalt.
42
M. Zimmer et al. /Chemical
Y Wm)
Fig. 7. Tectonic setting for malic volcanic rocks on the basis of Cr
and Y concentration diagrams (Pearce, 1980). For abbreviations,
see Fig. 6.
in MORB-type and marginal basin type source magmas. Low EN,,-values in supra-subduction
zone ophiolites may be due to magma contamination
with
continent-derived
sedimentary material during the subduction process. The Gerfrocks have initial +,-values
comparable to N-MORB -750 Ma ago, and if any
sedimentary material has contaminated their magma
source, as postulated from elevated 207Pb/204Pb ratios
for the gabbros, such contamination
has not visibly
affected the Nd and Sr isotopic systems nor the majorand trace-element compositions.
The basaltic rocks of the GGO have higher eNd(,)
than the gabbroic rocks. In accordance with the Pb
isotopes this implies that the gabbros originated from
a less depleted source than the pillow basalts and
sheeted dykes from Heiani and Harga Zarga. Our preferred interpretation is an admixture of a sedimentary
component into the gabbro source that may have
occurred in a subduction zone environment.
Geology 123 (1995) 29-51
favour mixing of isotopically different magmas. The
best explanation for the observed major- and traceelement variations within the individual rock groups,
therefore, are fractionation processes.
Cumulate successions in the gabbro section of the
GGO are up to 50 m in thickness and show that fractionation has played an important role in the formation
of the ophiolite sequence. There is a larger variation
within the gabbroic rocks than in the basalts for some
elements
which correspond
to cumulate
mineral
phases. Thin-section
examination
reveals cumulate
textures and shows that 01, cpx, plag and Fe-Ti-oxides
are possible fractionating minerals. The gabbroic rocks
show different proportions of these minerals which
points to varying degrees of fractionation and accumulation of these phases.
Low Mg# of 47-59 as well as low Cr and Ni contents
for some gabbroic rocks (e.g., samples GG 79,93,263,
265, 266) show that these do not represent primitive
magmas. Such low values are largely caused by Fe
enrichment relative to Mg. This enrichment is due to
the cumulation
of magnetite-ilmenite
which also
increased the Ti concentration significantly, e.g. up to
7. Petrogenesis
Possible reasons for linear variations of major and
trace elements within the basaltic and gabbroic groups
in correlation diagrams (Fig. 3) are partial melting,
mixing and crystal fractionation. Different degrees of
partial melting could explain the variation in incompatible trace elements, but not variations in compatible
elements. Low “Sr/“%r ratios (0.7021-0.7032)
and
relatively constant eNd-values ( + 6 to + 8.5) do not
c1I 8I 8I 1t I 818 ’f ”
Th“NbLaCep,Nd
SmZrHfTi
E:dTRYyHoErYbL”
0.11’
Fig. 8. Variation of incompatible trace-element concentrations, normalized to primitive mantle (PRIMA) values (Hofmann. 1988).
Pillow lavas are .&id symbols, sheeted dykes are opera symbols.
Shadedjield for N-MORB based on unpublished data of K.P. Jochum
(1993).
hf. Zimmer et al. /Chemical Geology I23 (1995) 29-51
43
Table 6
Sr and Nd isotopic data for rocks from the Gabal Gerf Ophiolite Complex
Sample
Rock
No.
type
s7Sr/s6Sr
Sm
Nd
(ppm)
(ppm)
??
rWr,
Harga Zarga:
GG9
P
0.702676f0.000013
2.97
9.64
0.1863
0.512923f0.000018
+6.6
GG 10
P
0.702763
+ 0.00003 1
1.84
5.50
0.2028
0.5 13030 f 0.000023
+7.1
GG 69
P
0.703000
f 0.000035
2.32
7.07
0.1986
0.513043*0.000014
+ 7.7
GG 70
0.703212f0.000051
2.15
5.83
0.2234
0.513198~0.000014
+8.4
GG71
P
sd
0.702370
f 0.000023
3.86
12.37
0.1888
0.5 12996 f 0.000026
+7.8
GG 72
sd
0.702595
f 0.000034
4.19
14.22
0.1782
0.512957 f 0.000015
+ 8.0
GG 73
P
0.702532
+ 0.000025
2.25
6.25
0.2180
0.513118~0.000018
+ 7.3
GG 94
P
0.702739
+ 0.000020
2.08
5.83
0.2162
0.513114~0.000016
+7.4
GG 183
P
0.702919
f 0.000013
2.31
5.93
0.2359
0.513243~0.000019
+8.1
GG 184
P
0.703165
+0.000015
4.36
10.50
0.2510
0.513277+0.000014
+7.3
GG 95
sd
0.702103
f 0.000029
1.87
4.50
0.25 11
0.513354~0.000016
+ 8.8
GG 172
sd
0.702335
If: 0.000015
2.88
7.17
0.2424
0.513386~0.000013
+8.7
GG 177
sd
0.702604
f 0.000042
3.97
10.28
0.2334
0.513194~0.000016
+7.3
GG 178
P
0.703091*0.000015
1.83
4.18
0.2652
0.513378f0.000018
+7.9
GG 180
0.702739
1.63
3.75
0.2635
0.513330*0.000017
+7.1
GG 281
P
sd
0.702327~0.000013
2.96
7.78
0.2364
0.513138f0.000016
+6.5
GG 282
P
0.703234
1.85
4.43
0.2528
0.513312~0.000014
+7.8
a
Heiani:
f 0.000014
f 0.000014
Gabal Gerf
GG 257
0.702516f0.000015
7.16
7.78
0.1632
0.512816f0.000014
+6.7
GG 35
urn
0.702782
f 0.000025
0.042
0.075
0.3367
0.5 13559 + 0.000024
+4.5
GG 36
g
0.702482
+ 0.000012
0.054
0.105
0.3082
0.513550f0.000013
+7.1
GG 77
g
0.702645
*0.000012
0.90
2.80
0.1949
0.5 12983 f 0.000008
+6.9
GG 79
g
0.702921*
1.17
3.60
0.1941
0.5 12956 f 0.000008
+6.5
GG 80
g
0.702932
* 0.000020
0.20
1.14
0.1061
0.512565 jI 0.000017
+ 7.3
GG 81
g
0.702569
* 0.000012
0.85
3.75
0.1375
0.5 1273 1 f 0.000007
+7.5
GG 82
g
0.702505
f 0.000011
1.32
4.13
0.1933
0.512960f0.000011
+6.6
GG 83
g
0.702617~0.000012
0.9 1
2.14
0.2575
0.513294f0.000011
+7.0
GG 87
g
0.702623
f 0.000011
0.19
1.04
0.1102
0.512586~0.000015
+ 7.3
GG 88
g
0.702474
f 0.000011
0.18
0.98
0.1130
0.512629+0.000017
+ 7.9
GG 90
g
0.702571
f0.000013
1.15
3.44
0.2016
0.512987~0.000015
+ 6.4
GG91
g
0.702492
f 0.000013
0.85
2.62
0.1959
0.512959f0.000014
+ 6.4
GG 93
g
0.702627
f 0.000046
0.17
0.85
0.1219
0.512675 & 0.000046
+ 7.9
GG 263
g
0.702549f0.000012
2.40
11.07
0.1308
0.512646 * 0.000022
+ 6.5
GG 268
g
0.702513f0.000016
0.89
2.87
0.1874
0.512965 k 0.000016
+ 7.3
GG 283
g
0.702577+0.000015
3.12
8.90
0.2121
0.513063 +0.000019
+ 6.8
GG 258
g
0.702408
1.82
5.52
0.1989
0.513002 & 0.000011
+ 6.9
GG 258
PX
5.18
12.59
0.2488
0.513243It:0.000021
+ 6.8
GG 258
Pl
0.17
0.77
0.1303
0.5 12633 + 0.000040
+6.3
0.000020
rt 0.000014
Errors reported for isotopic ratios are 2a,.
Error in ‘47Sm/‘44Nd
is estimated to be f 0.2%.
.,,,,-values
??
were calculated for an age of 750 Ma.
Mean value for 24 measurements of NBS 987 standard is 87Sr/86Sr = 0.7 10225 f 0.000024. Mean value for 21 measurements of La Jolla standard
is 14’Nd/ lUNd = 0.5 11843 f 0.000022.
p = pillow Java; sd = sheeted dyke; a = amphibolite; urn = ultramafic; g = gabbro; px = pyroxene;
pl= plagioclase.
6.82 wt% TiOz in sample GG 263 with a corresponding
Mg# of 48. Negative Eu anomalies in some basalts
and positive Eu anomalies in most of the gabbros exemplify the importance of plagioclase as a fractionating
44
M. Zimrner et al. /Chemical
Table 7
Pb isotopic data for rocks of the Gabal Gerf Ophiolite Complex
Sample
Rock ‘OsPb/““Pb
‘oTPb,‘“Pb
-“*Pb
‘@‘Pb
type
GG9
GG73
GG71
GG 72
GGlO
GG69
GG70
GG94
GG 95
GG 172
GG 177
GG68
GG77
GG82
GG88
GG90
GG91
p
p
sd
sd
p
p
p
p
sd
sd
sd
g
g
g
g
g
g
M
T
M
T
M
18.793
18.279
17.511
17.615
18.092
18.123
18.305
18.368
17.640
17.569
17.698
18.304
17.601
17.633
17.444
17.464
17.474
17.312
17.045
17.110
17.123
17.228
17.259
17.071
17.134
17.147
17.075
17.204
17.440
17.379
17.411
17.411
17.242
17.252
15.534
15.477
15.445
15.451
15.498
15.469
15.478
15.503
15.420
15.434
15.445
15.566
15.551
15.508
15.504
15.493
15.507
15.439
15.398
15.419
15.419
15.443
15.414
15.399
15.424
15.388
15.403
15.414
15.511
15.537
15.494
15.494
15.479
15.493
38.053
37.818
37.083
37.194
37.645
37.709
37.395
37.897
37.010
37.024
37.195
37.053
37.506
37.407
37.243
37.229
37.273
I&
/+
12.00
10.00
3.25
3.98
7.00
7.00
10.00
10.00
4.00
4.00
4.00
7.00
1.80
1.80
1.80
1.80
1.80
9.40
9.10
9.18
9.19
9.32
9.29
9.10
9.20
9.16
9.13
9.25
9.57
9.62
9.56
9.56
9.40
9.40
Geology 123 (1995) 29-51
rather than fractionation is the major process controlling the gabbro composition.
The composition of the parental magmas and the
magma sources from which the GGO rocks were
derived are less constrained. The Pb and Nd isotopes
suggest different sources for the gabbros and the
Gabal Gerf
gabbros
Errors arc f 5 - I 0m4, p = 23*U/‘@‘Pb. M = measured values, corrected for fractionation; T = in situ decay corrected values for an age
of 750 Ma. /*, = first stage of Stacey and Kramers ( 1975); p2 =
calculated value for second stage of Stacey and Kramers ( 1975);
/.+=present-day
w-values calculated for an age of 750 Ma after
Stacey and Kramers ( 1975). Measured values for standard NBS 982
‘“‘Pb/‘““Pb=
36.5224
k 0.0018,
2”7Pb/“‘4Pb
=
are:
17.0856 k 0.0009, Z”“Pb/‘04Pb= 36.6198 +0.0018. p= pillow lava;
sd = sheeted dyke; g = gabbro.
mineral. Variations
in Cr and Ni contents vs. MgO
show that 01 and cpx are also fractionating minerals
(Fig. 13). Fractionation calculations after Wright and
Doherty (1970) suggest that variations in major elements can be explained by fractionation of different
amounts of these minerals. Fractionation
within the
Heiani basalt sequence can be calculated using the most
primitive sample GG 95 (Mg# 60) and the most differentiated sample GG I74 (Mg# 46). The resulting
degree of fractionation is 54% with a mineral assemblage of 12% 01, 33% cpx and 54% pl. Fractionation
calculations for the Harga Zarga basalts yielded similar
results. A differentiated basalt (GG 181, Mg# 43) can
be modelled from a more primitive basalt (GG 9, Mg#
60) by removal of 44% of a mineral assemblage composed of 19% 01, 22% cpx and 59% pl. Variations in
the composition of the gabbroic rocks depend on the
role of the cumulus minerals. Because a positive Eu
anomaly is present in almost all samples, cumulation
Minerals from
gabbro GG 258
0512x
? ?Harga
05124
Fig. 9. Sm-Nd evolution diagrams
Ophiolite Complex. Errors are 2a,
0
Zarga
Heiani
for rocks of the Gabal Gerf
and are smaller than size of
SJVthOlS
a.
b.
C.
Linear array of gabbro whole-rock analyses, MSWD = 11.5,
Ebb = 6.9 5 0.2.
Mineral isochron
for sample GG 258, MSWD= 1.7,
ENdCI,=6.8 * 1.4.
Plot for whole-rock analyses of all basaltic rocks and one
amphibolite. Only the Harga Zarga samples display sufficient
spread for calculation
of a meaningful
regression
line
(MSWD=11.9),~~~(~,=7.6~0.9.
M. Zimmr et al. /Chemical Geology I23 (1995) 29-51
16.0
7.1. Comparison
45
with chemical data of other
ophiolites
15.8
2
15.6
w
%
5
15.4
15.2
15.0 I /
16
17
18
19
I
20
206Pb / 204Pb
Fig. 10. ‘“7Pb/204P~‘0”Pb/‘~Pb
diagram for Gabal Gerf wholerock analyses. Measured values for basalts arr shown as circles,
gabbros are shown as triangles. Measured data are corrected for in
situ decay and plot in fields on a 750-Ma isochron. Recent pelagic
sediments are taken as equivalent to Enriched Mantle I1 (.&WI) of
Zindler and Hart ( 1986). The two-stage geochron begins at 3.7 Ga,
following Stacey and Kramers (1975).
basalts. Furthermore,
the basal& indicate different
parental magmas by their trace-element characteristics.
The Heiani basalts, in comparison to the Harga Zarga
basalts, have more strongly depleted LREE patterns
and generally lower REE abundances at lower Mg#.
Therefore, they cannot be related to the Harga Zarga
basalts by simple fractionation but must have originated from a different parental magma.
This is further constrained by the higher concentration of other incompatible elements such as Zr, Nb, Pb,
Th and U in the Harga Zarga basalts although their
higher Mg#, as well as the higher Cr and Ni concentrations, suggest that they are the more primitive magmas. The two groups of basalt, therefore, must have
formed from different parental magmas, probably even
from different magma sources, the source of the Heiani
being the most depleted.
In summary, major-element,
trace-element and isotopic characteristics
suggest three different parental
magmas and probably of three different magma sources
for the gabbros, the Heiani basalts and the Harga Zarga
basalts although all of them suggest a depleted mantle
reservoir. The variation within the two distinct basalt
sequences can be modelled by crystal fractionation,
whereas the composition of the gabbros is largely controlled by crystal accumulation.
The major- and trace-element chemistry of GGO
basalts displays clear N-MORB affinities. To our
knowledge, this is the only ophiolite complex in the
ANS, and worldwide in the Precambrian, showing such
N-MORB characteristics. A possible setting for the
origin of these rocks is a major ocean basin. The Pb
and Nd isotopic signatures of the gabbros, however,
appear to indicate the influence of a subducted component which may suggest an arc or back-arc environment.
.Serri ( 198 1) used the TiO, concentration and the
FeO/ (Fe0 +MgO) ratio to distinguish between Tirich MORB/BAB
ophiolites and Ti-poor island arc
(IA) ophiolites.
Fig. 14a is a TiOz vs. FeO/
(Fe0 + MgO) plot showing ophiolites from different
regions and of different ages, while Fig. 14b only shows
Pan-African ophiolites of the ANS. Most ophiolites are
classified as Ti-rich. Samples of the Khan Taishir
ophiolite, Mongolia, (Zonenshain and Kuzmin, 1978)
plot in the Ti-poor field of IA-type ophiolites. Some
ophiolites which have an island arc affinity (e.g., Sol
Hamed) scatter into the Ti-poor field (Fig. 14a). Most
ANS ophiolites also plot in the Ti-rich field, while some
samples scatter into the Ti-poor field. The GGO is the
only Pan-African ophiolite whose rocks clearly plot
only in the Ti-rich field. The scatter of data in Fig. 14
is not primarily a result of alteration because most
ophiolites originate in back-arc basins in the vicinity of
island arcs, and their rocks should therefore carry geochemical characteristics of magma types from both
environments (Moores, 1982).
In Fig. 15 ophiolite data are plotted in the TiO,-Zr
diagram of Pearce and Norry ( 1979) and demonstrate
that rocks from only a few ophiolites plot exactly in the
field for N-MORB such as the East Taiwan ophiolite
(Jahn, 1986) and the Dras ophiolite, India (Radharkrishna et al., 1984, 1987). Most data points for the
early Proterozoic Jormua ophiolite complex also lie in
the MORB field (Kontinen,
1987). Most ophiolite
basalts display a wide range in TiOz and Zr concentrations which cannot be used to reliably determine the
tectonic setting (Fig. 15a). Rocks from BAB ophiolites
have variable TiOz and Zr, depending on the island arc
component in their source magmas. The same is true
for ANS ophiolites (Fig. 15b) where only the GGO
M. Zimrner et al. /Chemical
46
Geology 123 (1995) 29-51
+14
Gabal Gerf
0 Harga Zargn
_ 0 Heiani
+ Gabbro
+I2
+lO
. 0 Troodos
+8
;
H Trinity
A Samail
1 * Bay of Islands
1 ?? Ballantrae
+6
+4
seawater
0
+2
._
.
c
Y
0.702
0.704
0.706
I
.
Urals
@
East Taiwan 0
Vourinos
0
Wadi Onib
@
.
0.708
0.710
87si-/86s1.
Fig. 1I E~~(,,-~’Sr/‘“Sr diagram to compare rocks from Gabal Gerf and other ophiolites of different ages and localities. Gabal Gerf, Egypt: this
work; Troodos, Cyprus: McCulloch and Cameron (1983), Rautenschlein et al. (1985); Trinity, California, U.S.A.: Brouxel and Lapierre
( 1988); Samail, Oman: McCulloch et al. ( 1981); Bay of Islands, Canada: Jacobsen and Wasserburg (1979); Ballentrae, Scotland, U.K.:
Thirlwall and Bluck (1984); Urals, Russia: Edwards and Wasserburg (1985); East Taiwan ophiolite: Jahn (1986); Vourinos, Greece: Noiret
et al. ( 1981); Wadi Onib, Sudan: Th. Reischmann (pers. commun., 1993). Fields after Faure ( 1986). Ellipse in top left-hand comer is field of
MORB with isotopic ratios as measured at present. Ellipse below is field of MORB corrected for an age of 750 Ma. Horizontalarrowindicates
effect of alteration due to seawater.
rocks (Fig. 6) can be classified as N-MORB in view
of their tight clustering in the MORB field. The others
have variable trace-element concentrations depending
on their island arc component.
The problem of comparing ophiolites of different
ages and different geological settings is in the small
amount and uncertain quality of the available chemical
data. Fig. 15 highlights the uncertainties in geochemical discrimination of ophiolite rocks. Some well-studied ophiolites apparently had a complex history of
formation. One of these is the 95Ma-old
Samail
ophiolite in Oman, which is regarded by many to be a
fragment of an intra-arc basin formed above a shortlived subduction zone (Pearce et al., 1981) During
formation of the complex, the tectonic environment of
magmatism changed from a spreading axis to seamounts and, finally, to a submarine graben (Alabaster
et al., 1982). Magmagenesis was dominated in all three
environments by a high degree of melting of depleted
mantle, and the melt was modified by an input of water
and large-ion lithophile elements (LILE) from subducted oceanic crust (Alabaster et al., 1982).
Such a relatively precise, although debated (see
Nicolas, 1989), reconstruction of oceanic crust formation for the Samail ophiolite is not possible for the
GGO because of its tectonically disrupted character
(see Fig. 2). The original magmatic stratigraphy was
destroyed during the process of abduction and probably
led to superimposition of previously unrelated rocks.
,
”
I
0
100
200
300
400
As
500
600
700
800
900
ION
(MaI
Fig. 12. Comparison of initial E.,-values for different ophiolites
(literature see Fig. 1 I ). Depleted mantle line after Goldstein et al.
(1984).
M. Zimmer et al. /Chemical
4
Geology I23 (1995) 29-51
41
small amounts of subducted sedimentary material contaminated the source of the gabbro complex. Scenarios
where ocean basins with N-MORB and back-arc basin/
island arc complexes occur next to each other can be
found in the present southwest Pacific. We suggest that
collision of an island arc system with the African continent resulted in abduction of ocean floor and BAB
crust as well as island arc rocks and oceanic and/or
fore-arc sediments which are now found in the Gabal
Gerf area where they are overridden by the ophiolite
complex (Kroner et al., 1987).
2
700
Ek 600 sooIt, 400300200100-
0’
0
‘a
“8
2
4
1”
6
g
8
a
IO
a
a
”
12
14
.
MgO (weight %)
Fig. 13. Variation in Ni and Cr contents in relation to MgO concentration. Fractionation trends of olivine and clinopyroxene are shown
Circles = basalts; open squrtres = sheeted dykes;
for comparison.
solid triungles = gabbros; solid square = ultramafic rock. Percentages are degrees of fractionation.
2
Ba,len*rae
*
Khan Tsishn
8
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Ear Taiwan 0.
0
Dras
‘@
LaTenla
0
Jann”a
8. Model for the evolution of the Gabal Gerf
Ophiolite (GGO)
The mechanism of spreading in fore-arc and backarc environments
is not clearly under-stood. Moberly
( 1972) suggested that sinking of the downgoing lithospheric plate at a greater angle than in a normal Benioff
zone would give rise to a compensatory mantle upflow
which could initiate magma production and back-arc
spreading. In a classic area of back-arc spreading, the
Lau Basin in the southwest Pacific, Gill (1976) proposed that the basalts are transitional between ocean
floor and island arc tholeiites.
Any model for the origin and evolution of the GGO
has to explain the N-MORB chemistry of its basalts
and sheeted dykes and the subduction component in
the gabbros, shown by Pb isotopes and, to some extent,
by the +,-values.
A possible environment is an ocean
basin made up of oceanic crust of N-MORB composition, of which the pillow basalts and sheeted dykes of
Heiani and Harga Zarga are remnants. The gabbros, in
contrast, were probably generated in a BAB where
Bou Arzer
A Therford Mines
.
Wad, Cbadn
.
Wadi Omb
9
SolHamed
A
0
S&err
Ingessena
0.9
FeO/(FeO+MgO)
Fig. 14. Classification
diagram for ophiolitic rocks after Serri
( 198 I ). Data for ophiolites are from following sources: Troodos,
Cyprus: Cameron ( 1985), Rautenschlein et al. (1985); Bay of
Islands, Canada: Suen and Frey ( 1979); Samail, Oman: Smewing
(1981), Alabaster et al. ( 1982); Sarmiento, California, U.S.A.:
Saunders et al. (1979); Josephine, California,
U.S.A.: Harper
( 1984); Trinity, California, U.S.A.: Brouxel and Lapierre (1988);
Bou Azzer, Morocco: Bodinier et al. ( 1984); Thetford Mines, Quebec, Canada: Oshin and Crocket ( 1986); Ballentrae, Scotland, U.K.:
Thirlwall and Bluck (1984); Khan Taishir, Mongolia: Zonenshain
and Kuzmin (1978); East Taiwan ophiolite: Jahn (1986); Dras,
India: Radharkrishna et al. ( 1984, 1987) ; La Tetila, Colombia: Spadea et al. ( 1987); Jormua, Finland: Kontinen ( 1987); Wadi Ghadir,
Egypt: Zimmer (1985); Wadi Onib, Egypt: Price (1984); Sol
Hamed, Egypt: Price ( 1984); Sekerr, Kenya: Price ( 1984); Ingessena, Sudan: Price ( 1984); Jabal Al Wask, Saudi Arabia: Baker et
al. ( 1976); Nuba, Saudi Arabia: Hirdes and Brinkmann ( 1985).
M. Zimrner et al. /Chemical
48
10
a
-
0
Troodos
0
Bm of Islands
A Wad, Ghadir
= Wad, Onib
??
Sol Hamed
A Sekerr
1
0
lllgessena
??
Jabal Al Wask
Geology 123 (1995) 29-51
(4) We suggest that the Gabal Gerf Ophiolite is a
composite complex in which sheeted dykes and pillow
lavas are remnants of ocean floor of N-MORB type and
became tectonically interstacked and abducted together
with the BAB-type gabbros in a subduction zone environment.
Possible contamination
of the magma
source(s) is not evident from major- and trace-element
compositions but from elevated 207Pb/204Pb ratios and
less depleted Nd isotopic systematics in the gabbros
that indicate the subduction zone influence.
(5) Gabal Gerf is the only late Precambrian ophiolite
known so far to represent unambiguous N-MORB characteristics, and its REE and Nd isotopic data clearly
indicate that mantle depletion in incompatible elements
was already very significant - 750 Ma ago.
Acknowledgements
10
100
Zr
1000
(wn)
Fig. IS. Discrimination diagram as in Fig. 6 showingdatafor
ophiolites other than Gabal Gerf.
a.
Ophiolites outside the ANS.
b.
Ophiolites of the ANS.
For listing of literature see Fig. 14.
selected
9. Conclusions
( 1) There is a significant difference in major- and
trace-element
chemistry between Heiani and Harga
Zarga basalts. Harga Zarga basalts are relatively
enriched in incompatible elements relative to Heiani
basalts. This is exemplified by the REE, where the
patterns for Heiani basalts show significant LREE
depletion whereas Harga Zarga basalts tend to have
almost flat REE patterns. Both patterns are, however,
comparable to N-MORB.
(2) REE patterns of the gabbroic rocks are controlled by the major cumulate mineral phase or phases
and are characterized by pronounced positive ELIanomalies.
(3) Cumulation played an important role in the genesis of the gabbroic part of the ophiolite complex. This
is shown in the field by a thick layered gabbro sequence,
while the chemistry is in line with the cumulation of
01, cpx, pl and magnetite-ilmenite.
Basalts within the
two groups, however, can be connected by fractionation of pl, cpx and 01.
This investigation was funded by the Deutsche Forschungsgemeinschaft
(DFG), the Max-Planck-Institut
fur Chemie and the German Ministry of Research and
Technology (BMFT) via the International Office, Forschungszentrum Jiilich. M.Z. thanks M. Seufert, I. Raczek, H. Feldmann, N. Laskowski, K. Lehnert and B.
Schulz-Dobrick for assistance during analytical work.
Logistic support in the field was provided by the Egyptian Geological
Survey
and Mining
Authority
(EGSMA) , and we are indebted to AA. Rashwan and
M. Mansour for providing unpublished maps and for
participating in our field investigation. Comments of
N.T. Arndt, J.S. Pallister and R.J. Stern improved the
manuscript.
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