JournalofAfrican Earth Sciences, Vol. 23, No.3, pp. 363-373, 1996
Pergamon
PIh S0899-5362(97)00007-9
Copyright e 1997 ElsevierScienceLtd
All rights reserved.Printed in GreatBritain
0899-5362/96 $15.00 + 0.00
New geochemical and petrographic data on the
Gabbro-Syenite Suite between Hargeysa and
Berbera-Shiikh (northern Somalia)
J. A. ABDALLA, A. A. SAID and D. VlSONA
Dipartimento di Mineralogia e Petrologia, Universit& di Padova,
Corso Garibaldi 37, 3 5 1 0 0 Padova, Italy
A b s t r a c t - - T h e Somali crystalline basement in northern Somalia contains a Neoproterozoic
igneous suite essentially composed of gabbros with minor syenitic and granitic bodies. This
Gabbro-Syenite Suite (GSS) was emplaced at a relatively high crustal level and is related to
Late Precambrian crustal extension. The basic plutons are composed mostly of gabbro with
petrographic and geochemical characteristics ranging from N-type to T- and P-type MORB.
This compositional variability points to heterogeneous sub-continental mantle sources. The
plutons do not s h o w significant in s i t u differentiation, suggesting that the cumulitic rocks may
be related to cooling processes developed in deeper magma chambers and/or during magma
ascent. Copyright © 1997 Elsevier Science Ltd. All rights reserved
R~sum(~--Dans le socle cristallophyllien de la Somalie du Nord, on observe une suite intrusive
d'&ge n~oprot~rozoique, essentiellement compos~e de gabbros en association avec des syenites
et granites subordonn~s. Cette suite gabbro-sy~nitique (GSS) s'est mise en place darts un
niveau de la croQte relativement ~lev~ ~ la faveur de son extension Iors du Pr~cambrien tardif.
Les plutons basiques sont f o r m , s de gabbros & caract~res p~trographiques et g~ochlmiques
m d i q u a n t une t r a n s i t i o n du t y p e N-MORB aux t y p e s T- et P-MORB. C e t t e var i abi l i t e
compositionelle sugg~re des sources h~t~rog~nes du manteau subcontinental. Les plutons ne
montrent pas de diff~renciation in s i t u significative. La presence de roches & cumulats peut
doric s'expliquer par des processus de refroidissement s'~tant d~velopp~s au sein de chambres
magmatiques p r o f o n d e s et/ou durant r a s c e n s i o n du magma.
Copyright © 1997 Elsevier
Science Ltd. All rights reserved
(Received 30 September 1996: revised version received 13 March 1996)
INTRODUCTION
The crystalline basement in northern Somalia
(NSB for short) outcrops in an east-west
o r i e n t a t e d s t r i p r u n n i n g parallel to the
escarpment of the northern Somali Plateau. It
is a pre Pan-African Proterozoic crust (Warden
and Daniels, 1983; Warden and Horkel, 1984;
Sassi et al., 1989; KrSner and Sassi, 1996)
which, according to Sassi eta/. (1989), consists
of t w o large m e d i u m - to h i g h - g r a d e
polymetamorphic complexes, two greenscistfacies volcano-sedimentary sequences, a low
grade metasedimentary sequence, and t w o
plutonic complexes.
A remarkable feature is represented by
numerous lens-shaped plutons running eastwest (Fig. 1), forming an igneous array about
500 km long. The gabbro bodies frequently
associated with minor syenitic-granitic bodies,
dykes and ring dykes (e.g. Daba Shabeli
Pluton; Mason, 1958; Vison~, 1989)
constituting a Gabbro-Syenite Suite (GSS) that
injected the NSB about 800 Ma ago (Sassi et
al., 1989; KrSner and Sassi, 1996). These
intrusions are particularly concentrated in the
Borama-Hargeysa-Berbera-Shiikh area and the
following Pan-African tectono-metamorphic
reworking (amphibolite-facies) transformed the
gabbro into f o l i a t e d a m p h i b o l i t e s often
containing boudins as remnants of the original
igneous rocks (Daniels et al., 1965; Said,
1987,1988).
The GSS was t e n t a t i v e l y related to an
underplating event (Dal Piaz e t a / . , 1985; Dal
Piaz and Sassi, 1986; Kr6ner and Sassi, 1996)
Journal of African Earth Sciences 363
J. A. ABDALLA eta/.
\\
,,o
,,o
i
iiii
G_
"\'-......SOMALIA/
!
...... ""12
030'
/
Berbera
,.j ll.a
o Borama
t9
~
lN#lliN !.t
j
0 km
L-
.
,~
..
0
Shllkh
50
. I
°Hargheysa [~ ~
o,0,
Io,0,
3
~
9030 '
,so
,0, .!
ii
II
Figure 1. Geological sketch o f the crystalline basement/n the Hargeysa-Berbera area (Northern Somalia). I: non-metamorphic
cover; 2: metamorphic rocks; 3: bodies of the Gabbro-Syenite Belt.
apparently connected to the Neoproterozoic
extensional processes which, in Pan-African
times, affected this branch of the Mozambique
Belt (Almond, 1984). Notably, this important
magmatic activity is representative of mantle
melting, which caused crustal accretion by the
injection of fresh magma.
The intrusive rocks of the gabbro-syenite belt
are petrographically and geochemically poorly
known. The present paper aims to characterize,
in terms of geochemistry and petrology, the
gabbroid rocks of the western portion of the GSS.
The basic rocks show subophitic to ophitic
texture; coronitic textures are sometimes observed
in which olivine has orthopyroxene coronae.
The olivine in the gabbros and troctolites is
frequently enclosed in the clinopyroxene and,
together with green spinel, represents an early
phase of crystallisation. Its composition varies
from FO6o to FOT~ (Table 1). The orthopyroxene
WO
PETROGRAPHY AND MINERAL
COMPOSITION
Gabbro
The GSS is characterized by only a few rock
types, mainly in the Hargeysa-Berbera-Shiikh
area. Here, the belt is mainly composed of
medium-grained blackish rocks: olivine gabbro
and pyroxene gabbro, with minor diorite and
scarce ultrabasites (pyroxenite, peridotite) and
monzonites. Rock types with large (up to 3 cm)
poikilitic pyroxenes are rare. The rock types do
not s h o w sharp c o n t a c t s , e x c e p t for the
monzonites (e.g. Shiikh) which are always formed
as subvertical dykes rich in schistose xenoliths
and xenocrystic cordierite (Primon, 1988).
36,~ Journal of A f r i c a n Earth Sciences
FS
EN
i
FO
'
I
--
r
i
'
I
FA
Figure 2. Pyfoxene quadrilateral graph, The clinopvroxene
compositions plot along the tho/ei/tic trend of the Skaergard
p/uton/c complex.
New geochemical and petrographic data on the Gabbro-Syenite Suite between Hargeysa and Berbera-Shiikh
Table 1. Representative chemical c o m p o s i t i o n s of the main Fe-Mg silicate phases in the mafic
members of the G a b b r o - S y e n i t e Suite
OLIVINE
211-1
210-1
SiO 2
38.69
35.12
TiO 2
0.00
0.18
AI203
Cr203
NiO
FeO
MnO
MgO
CaO
Na20
!K20
Total
Si
Ti
AI
Fe2+
Mn
Mg
Ca
Ni
Total
0.00
0.00
0.18
24.70
0.38
35.79
0.23
0.00
0.00
99.97
1.018
0.000
0.000
0.543
0.008
1.402
0.006
0.004
2.982
0.00
0.00
0.00
34.24
0.59
29.67
0.08
0.00
0.00
99.88
0.976
0.004
0.000
0.796
0.014
1.229
0.002
0.000
3.020
Fo
Fa
Teph
71.77
27.8
0.43
211-2
50.26
0.32
2.96
Si
AI =v
Tet
AI VI
Fe2÷
Fe3+
Mg
Mn
Ti
Cr
Ca
Na
60.28 Wo
39.04 En
0.68 Fs
PYROXENE
210-4
211c7
51.17
50.17
0.32
0.35
3.75
4.85
0.47
nd
20.79
0.58
22.64
1.74
0.22
0.00
99.98
1.862
0.129
1.991
0.000
0.530
0.113
1.249
0.018
0.009
0.014
0.069
0.016
0.50
nd
20.48
0.50
21.38
2.30
0.00
0.00
100.40
1.893
0.107
2.000
0.057
0.616
0.017
1.179
0.016
0.009
0.015
0.091
0.000
0.47
nd
8.27
0.35
14.64
19.36
1.54
0.00
100.00
1.836
O. 164
2.000
0.046
0.059
0.195
0.799
0.011
0.010
0.014
0.759
0.109
3.7
67.6
28.7
4.8
62.5
32.7
47
49.4
3.6
coronae s h o w polygranular t e x t u r e w i t h crystals
having c-axes a p p r o x i m a t e l y perpendicular to
grain boundaries. More c o m p l i c a t e d t e x t u r e s
involve outer s i m p l e c t i c coronae of hornblende
and green spinel.
Orthopyroxene occurs as single crystals or as
polycrystalline aggregates, frequently with
simplectic o v e r g r o w t h s of brown hornblende and
green spinel. The polycrystalline aggregates of
o r t h o p y r o x e n e s o m e t i m e s contain simplectites
w i t h Fe-Ti oxides, occasionally accompanied by
euhedral green spinel. Orthopyroxene composition
is characterized by Fs13_33, Wo0.9_4.9 and a high
AI203 content (up to 3 . 7 5 % , Table 1).
C l i n o p y r o x e n e f o r m s subhedral to anhedral
grains, g i v i n g t h e t y p i c a l o p h i t i c t e x t u r e . It
occasionally f o r m s large poikilitic (up to 3 cm)
crystals. Schiller m i c r o t e x t u r e is f r e q u e n t and
PARGASITE
210a2
211a2
40.60
41.34
0.74
0.64
210cl
49.18
1.48
6.76
0.30
nd
9.60
0.13
14.22
17.42
0.80
0.00
99.97
1.824
0.183
2.000
0.112
0.252
0.045
0.783
0.004
0.041
0.009
0.690
0.064
Si
AI Iv
AI w
Cr
Fe3+
Ti
Mg
Fe2+
Fe2÷
Mn
Ca
40 Ca
45.4 Na
14.6 K
15.60
nd
nd
11.54
0.35
14.64
10.55
3.37
0.31
97.70
5.934
2.066
8.000
0.623
0.000
0.267
0.081
3.189
0.839
5.000
0.304
0.043
1.652
2.000
0.000
0.955
0.058
1.013
14.75
nd
nd
12.81
0.13
13.00
11.72
2.87
0.49
97.75
6.112
1.888
8.000
0.640
0.000
0.000
0.071
2.864
1.381
5.000
0.203
0.016
1.781
2.000
0.076
0.823
0.092
0.991
b r o w n h o r n b l e n d e c o r o n a e are c o m m o n . Its
c o m p o s i t i o n f o l l o w s t h e e v o l v i n g t r e n d of
tholeiitic p y r o x e n e s (Fig. 2) and s h o w s , like the
o r t h o p y r o x e n e , a high AI203 (up to 6 . 7 % ) and
Cr203 (up to 0 . 5 % ) content.
A m p h i b o l e f o r m s m a i n l y either coronae on
pyroxene (in which case it is in optical continuity
w i t h it), or on Fe-Ti oxides. It occasionally forms
small single anhedral crystals. It is Ti-rich and
the c o m p o s i t i o n ranges f r o m Fe-pargasite to
pargasite (Table 1).
Biotite is a less c o m m o n phase and f o r m s
interstitial laminae w i t h y e l l o w to b r o w n colour.
It has a w i d e range of F e / F e + M g ratios (0.280.62),
which
m a y be d u e t o v a r y i n g
crystallization t e m p e r a t u r e s .
Green spinel is considered to be an earlyformed mineral. It is associated w i t h Fe-Ti oxides
Journal o f African Earth Sciences
365
J. A. ABDALLA et al.
80--
30--
A
70
A
CO
A
.o_
or)
A
0
20-
~r
A
A
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5O
m
4O
10-
30
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0.2
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0.6
8--
'
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02.
0.0
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0.6
0.4-m
ZX
6
~
O4
o_
0
AAA * A
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,,.A. A
I,.-
:~ o.~-
2
**
0 -, . . . .
l
0.0
(
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~
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0.4
02.
o.o-
'
.
*
,
0.0
0.6
[
,d.
zpA A
I
0,2.
~
t
0.4
0.6
20
A
n
A,
A
LL
*
0
I
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02
Z~
[
'
A/"
I
0.4
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02
0.0
0.6
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/
04
0.6
6--
20
a , ,,d~
0
0
10
&
z
ZX
2
A
~
ZX
I
0.0
'
02
I
'
0.4
'
[
0.6
0.0
I
0.2
'
(
z~
A
'
1
04
0.6
0.4--
6--
It
*
4~ , •
AZ~
0Cq
4--
A
•
O~ o~-~
A~
2--
o
'
0.0
I
'
02
MgO/(MgO+FeO+Fe203)
I
0.4
'--
I
0.6
0.0
02
0.4
Figure 3. Variation diagram of major elements versus fractional zndex (MgO/MgO + FeO + Fe20 J. ~ .
from Da/ Plaz et al. (1985)," A: metamorphosed igneous rocks; A:/~Tneous rocks•
366 Journal of African Earth Sciences
0.6
MgO/(MgO+FeO+Fe203)
Shiikh pluton, taken
N e w g e o c h e m i c a l a n d p e t r o g r a p h i c data on the Gabbro-Syenite Suite b e t w e e n Hargeysa and Berbera-Shiikh
as exolutions in magnetite, at the magnetiteilmenite interfaces or included in orthopyroxene
together with magnetite and ilmenite (Abdalla,
1989). In the latter case the composition of the
spinel is approximately 9 7 % of AI203 mole
component, with Fe/Fe + M g ratio about 0.5, and
less than 0.3% Cr203. Secondary green spinel
occurs in the s p i n e l - h o r n b l e n d e s i m p l e c t i c
coronae on the orthopyroxene.
M i n o r a c c e s s o r y mineral phases include
a b u n d a n t e u h e d r a l a p a t i t e and s u l p h i d e s
(pyrrhotite, chalcopyrite and pyrite).
Metagabbro and gabbro-amphibolite
The mineralogical and textural effects of the
amphibolite facies metamorphic overprint are
found in the outer parts of the plutons, in shear
zones, and in boudins of foliated amphibolites,
as already reported by Daniels et al. (1965), Dal
Piaz et al. (1985); and Said (1987, 1988). In
the u n f o l i a t e d g a b b r o a m p h i b o l i t e and
metagabbro boudins the replacement of igneous
minerals has taken place, preserving the igneous
t e x t u r e . In these the m e t a m o r p h i c effects
include: serpentinization of olivine; formation of
Mg-cummingtonite from orthopyroxene;
formation of tremolite and green hornblende from
clinopyroxene + plagioclase; and the formation
of epidote and o l i g o c l a s e from m a g m a t i c
plagioclase. These metamorphic transformations
also produced other new minerals such as quartz,
chlorite and sulphides.
GEOCHEMISTRY
The analytical data (major and trace elements)
refer to 12 igneous rocks not affected by
metamorphism and 33 meta-igneous rocks
variously transformed by metamorphism. Fifteen
representative samples are reported in Table 2
(for analytical method see A p p e n d i x ) . The
studied rocks plot in the basalt field according
to the TAS classification (Le Bas et al., 1986)
and, as reported in Table 2, are olivine and/or
nepheline normative. In Fig. 3, unmetamorphosed
igneous rocks from the Shiikh Pluton (Dal Piaz
e t a L , 1985) are plotted as a reference in the
variation diagrams relative to major elements vs.
MgO/(MgO + FeO + Fe203) ratio. In general, the
metamorphosed rocks lie in the same fields as
the unmetamorphosed ones. This suggests that
the metamorphism has not significantly affected
major element compositions. If siliceous rocks
('Syenitic suite", Dal Piaz et aL, 1985) of the
Shiikh pluton are excluded, the analysed rocks
show only a small compositional range of SiO 2
(45.5 to 48 w t % , Fig. 3). This indicates that
only silicate minerals were involved in the
differentiation process shown by the variation
i n d e x . The c u m u l i t i c rocks can be easily
recognized, as they usually plot well outside of
the main trend (Fig. 3). It is particularly true of
plagioclase cumulates, which have the lowest
TiO 2, MnO, FeO and MgO and the highest AI203
and Na20 contents relative to other samples of
similar variation indices (c.f. Shiikh), while
clinopyroxene cumulates are characterized by
the highest CaO and the lowest Na20 contents.
The non-metamorphic rocks generally show low
~:REE (14.6-53.1 ), usually slightly inclined patterns
with (La/Lu)cn of 4-18 and Eu/Sm of 0.37-1.19,
values common in tholeiitic intrusive complexes
(Cullers and Graf, 1984). The geochemical
characteristics of these rocks vary from N-type
MORB (La/Sm)cn = 0.8, Zr/Y = 3.6, Zr/Nb = 14 and
Y / N b = 4 . 6 ) to enriched tholeiites (T- and P-type
MORB: [ L a / S m ] c o = l . 2 - 2 . 9 , Z r / Y = 3 . 2 - 8 . 5 ,
Z r / N b = 9 . 6 - 1 8 and Y / N b = 1 . 1 - 3 . 3 and high
contents of compatible elements; Table 2).
The positive correlation between Ni and Mg (Fig.
5), even for the m e t a g a b b r o s , s u g g e s t
fractionation of olivine - as shown by the existence
of rare cumulitic peridotites (e.g. Daba Shabele
Pluton, northeast of Shiikh; Mason, 1958).
A particular behaviour of Sr and Y is shown
by the diagram in Fig. 5. The sample group with
high Sr also has a positive Eu anomaly (e.g.
samples 50 and GN, c.f. Fig. 6) suggesting that
in these rocks plagioclase accumulation was
300 - -
/~
meta-gabbros and gabbro-amphibolite
•
gabbro
A
200
Z
/k
/k ~
/k
100 -
o
0
5
10
15
MgO
Figure 4. Ni versus MgO plot. The positive correlation in the
n o n - m e t a m o r p h o s e d rocks suggests an olivine separation
f r o m the fiquid phase (symbols as in Fig. 3).
Journal of African Earth Sciences 367
J. A. ABDALLA et al.
2000
100--
--
Z~
1000
Z~
n."
Z~
50-
Z~A
A
Z&
A
Z~
A
A
'
40
0
I
160
I
120
80
A
A
'
~
'--
0
I
40
L
8O
I
120
'
[
160
300
A
4OO--
200 ¸
A
&
nn
Z
A
A
2OO---100-
A
A
AA
A
A
0
40
80
L
160
120
4O
'
1
160
1200--
100 --
8O0-
8
I
120
8O
50-
Az~
z ~ A
Z~
A
A
400--4
z~'~
zs
A
A
I
i
I
4O
T
80
1
'
120
0
I
100--
I
'
0
160
A
AZ~
'
I
f
120
4o
160
Z~
30-m
A
20-
A
A
50
AZ~
t',
Z
A
10 -
Z~
A
.Z~A z ~ A
A
'
I
40
'
l
80
'
I
120
'
I
40
I
160
'
I
80
L
I
120
L
I
160
~
l
160
1000-A
~>
A
A
500,,
A
AA
A
A
A
'j~•
I
40
80
120
160
Zr
Figure 5a. Trace elements and REEs v e r s u s Zr diagrams, (symbols as in Fig. 3).
3 6 8 Journal o f A f r i c a n Earth Sciences
40
L
I
80
L
Zr
I
120
N e w g e o c h e m i c a l a n d p e t r o g r a p h i c data on the Gabbro-Syenite Suite b e t w e e n Hargeysa a n d Berbera-Shiikh
A
~
10-
0
'
I
20
'
l
40
'
i
60
'
i
80
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0
L
100
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I
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I
20
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L
60
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L
80
L
100
I
40
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I
60
'
I
80
]
'tO0
I
4O
'
I
60
~
I
80
4o
2--
20
&•
zx
t
I
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••L
I
4O
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I
60
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I
80
~
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l
IO0
I
20
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4--
r'~
A
2-
t,
Z~
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L
60
40
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I
80
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100
L
20
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100
0.4--
4--
t
2
0.2
~k
/k
i
i
•
•
zx
I
2O
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40
I
6O
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I
8O
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O0
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100
k
A
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I
20
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t
L
40
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60
t
'
80
I
100
2--
A
••
.0
>-
A
z~
•A•
k
~ A
z~
l
/k
I
20
'
I
40
'
I
60
'
I
80
~
1
~00
'
Zr
I
20
'
L
4O
'
I
60
'
I
80
'
L
100
Zr
Figure 5b. Trace elements a n d REEs versus Z r diagrams, (symbols as in Fig. 3).
Journal o f African Earth Sciences
369
J. A. A B D A L L A et al.
, , o ~ Z
~ ~
0
~
~
0
~
0
0
0
~
0
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co~o~
oi ,.6 06
,~o
¢0c0 o ~
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370 Journal o f African Earth Sciences
o
~
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ii
N e w g e o c h e m i c a l and p e t r o g r a p h i c data on the Gabbro-Syenite Suite b e t w e e n Hargeysa a n d Berbera-Shiikh
1O0
1
~
50
r
I
gabbro
Shiikh
GN Bixinduulegabbro
0
La
100
Ce
1
Nd
I
Sm
I
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I
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Yb
I
I
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4
101
211
--4F-La
Ce
w3
Nd
Sm
Eu
Gd
Dy
Er
Yb
Lu
Figure 6. C h o n d r i t e - n o r m a l i z e d REE plot. (a) Cumufitic
plagioclase gabbros s h o w p r o n o u n c e d Eu anomalies. (b)
Non cumufitic gabbros have a characteristic flat pattern
like the N-type belt.
more important than is evident from their fabric.
In any case, the positive correlation between
the hygromagmatophile elements Y and Zr (Fig.
7) show firstly that fractional crystallisation is
the main differentation process responsible for
the o b s e r v e d c h e m i c a l v a r i a t i o n s w i t h i n
individual plutons (c.f. arrows in Fig. 7); and
secondly that different degrees of partial melting
of the same source can explain the chemical
differences between the plutons (Treuil and
Joron, 1975).
DISCUSSION
A petrogenetic model for the GSS genesis
requires consideration of the following field and
laboratory data. Field data show:
i) the clear prevalence of gabbroid rock types
with layering and lack of appreciable quantities
of cumulitic rocks;
ii) the lack of i n t e r m e d i a t e c o m p o s i t i o n s
b e t w e e n g a b b r o and s y e n i t e or g r a n i t e ;
monzonite is an interaction product between
gabbro and metasediments, as suggested by its
xenolithic and xenocrystic content;
iii) the intrusive character of granite and syenite
into the gabbro plutons, but also the occasional
mingling between these and gabbro (see also
Gatto et aL, 1985; Vison&, 1989);
iv) the high-level emplacements, as indicated
by the ring dyke structures in the Daba Shabeli
pluton (Mason, 1 9 5 8 ) and by x e n o c r y s t i c
cordierite in the Shiikh monzonites (Primon, 1988).
These data point to the absence of any largescale differentiation in the shallower magmatic
chambers, however several processes in deeper
chambers brought the basic magma into contact
w i t h an acidic one ( c , f . V i s o n & , 1 9 8 9 ) .
Geochemical data show:
i) relatively small compositional variations in
the mafic rocks, in which the composition ranges
from w i t h i n - p l a t e tholeiites to transitionalenriched tholeiites;
ii) low REE values and variable patterns from
flat to HREE depleted;
iii) widespread positive Eu anomalies;
iv) relative enrichment in Eu, Rb, Th and K,
and a high Zr/Y value;
v) a positive correlation b e t w e e n hygromagmatophile elements (e.g. Y and Zr).
The REE data suggest that the basic magma
may have originated through partial melting of
unmodified or only slightly modified mantle (c.f.
le Roex e t a l . , 1985). Fractionation processes
which determined plagioclase accumulation are
indicated by widespread Eu anomalies and
relative enrichment in AI203, but these must have
originated in deeper chambers or during magma
ascent (e.g. by removal of OI _+Px sustained even
the relative scarcity of Nd and the Sr abundance;
c.f. Zindler e t a L , 1981). The enrichment in some
LILE (Rb, Th and K) cannot be attributed to an
interaction with continental crust: assimilation
is confined to the rare m o n z o n i t e s rich in
schistose xenoliths, while the other lithotypes
have constantly low 87Sr/86Sr values (0.70360 . 7 0 3 8 at 690 Ma, Sassi et al., 1989) that
calculated for 800 Ma (Kr6ner and Sassi, 1996)
are typical of the mantle source (Faure, 1977).
Finally,
the correlation
between
the
hygromagmatophile Zr and Y and the REEs
indicate t w o or more different basic magmas
not joined by fractional processes. It may be a
feature related to a source inhomogeneity or a
different degree of source melting.
As regards the metamorphic history of these
basic rocks, the mobilization of elements is
related mostly to the shear zones rather then
only recrystallization, as already observed in
Journal of African Earth Sciences 371
J. A. ABDALLA et aL
3(~
Shiikhgabbro
•
Bixinduulegabbro
involved the Somali National University. The
s u p p o r t of the " C e n t r o di Studio per la
Geodinamica Alpina" of CNR (Padova), is also
acknowledged. The authors are grateful to R.
Carampin (CNR Padova) for m i c r o p r o b e
laboratory assistance. A first draft of the
manuscript benefited from the comments of
G. Bellieni and E. M. Piccirillo. Finally, R.
Shackleton and two anonymous reviewers are
thanked for their comments, suggestions and
review of the final paper.
metagabbro and gabbro-amphibolite
-
>-
j
A ~'n
20
ix
I
0
z~
p
40
7 7
I
80
I
120
I
i
REFERENCES
160
Zr
10
-
~
'
I
40
~
A
'
~~
J
80
'
[
120
'
I
160
7
~
Zr
Figure 7. Y versus Zr and Zr/Y versus Zr plots. Sample from
different masses (e.g. Shiikh and Bixinduule) plot on different
trends o f fractional crystallisation (f.c.), which apparently
are generated by partial melting (p.m.) o f the same source.
Chrondrite values after Taylor and McLennan (1985).
other situations (e.g. Vocke e t a / . , 1987). The
mobilization of REEs following the schemes
described by Helmann eta/. (1979) is clear, and
the trend simulating those of magmatic rocks
may be related to crystallization of epidote, a
mineral phase which simulates the REE trend of
basic rocks (Nystr6m, 1984).
As regards the neighbouring basement, the
GSS can be correlated with the "dense swarm
of northeast-trending mafic and felsic dykes"
(the mafic-felsic dyke swarm [MFDS]; Ba-Bttat
et al., 1989) of Yemen. This hypothesis
strengthens the similarities between the Yemen
and north Somalia basements as pointed out by
Warden and Honkel (1984) and by Ba-Btatt et
al. (1989), and the resultant overall picture is of
generalized e x t e n s i o n a l processes in the
southeastern part of the Arabian-Nubian Shield
at the end of the Neoproterozoic.
ACKNOWLEDGEMENTS
The Italian National Council for Research (CNR)
and the Ministry for University and Research
(MURST) supported the survey activity, which
372 Joumalof African Earth Sciences
Abdalla, J. A. 1989. Platinoidi ed il Ioro s i g n i f i c a t o
m e t a l l o g e n i c o nel c o m p l e s s o di 8arkasan (Somalia
Settentrionale). Ph.D. Dissertation 150p. University of
Padova, Italy.
Almond, D. C. 1984. The concepts of "Pan-African Episode"
and "Mozambique Belt" in relation to the geology of East
and North-East Africa. Earth Sciences King Abdulaziz
University Jeddah 6, 71-87.
Ba-Bttat, M. A., Windley, B. F., AI-Mishwt, A. T. and
Almond, D. C. 1989. Geology of the Lowder-Mudiah area,
Yemen. In: Geology and mineral resources o f Somalia.
(Edited by Abbate, E., Sagri, M. and Sassi, F. P.). pp143
152. Relazioni e M o n o g r a f i e 1 1 3 A / 1 9 9 3 I s t i t u t o
Agronomico Oltremare, Firenze.
Cullers, R. L. and Graf, J. L. 1984. Rare earth elements in igneous
rocks of the continental crust: intermediate and silicic rocks ore petrogenesis. 510p. In: Rare Earth Element Geochemistry
(Edited by Henderson, P.). Elsevier, Amsterdam.
Dal Piaz, G. V. and Sassi, F. P. 1986. The crystalline
basements of Somalia: a r e v i e w . M e m o r i e Societa
Geologica Italiana 31, 351-361.
Dal Piaz, G. V., Rigatti, G., Galli, L., Massiga, B., Piccardo,
G. B., Gosso, G., Spalla, M. I. and Paccerillo, A. 1985. II
gabbro di Shiikh (Somalia settentrionale) dati geologici,
petrografici, geochimici e geocronologici preliminari.
Quaderni Geologia Somalia 8, 151-186.
Dal Piaz, G. V., Martin, S., Rigatti, G. and Venturelli, G.
1987. The greenschist pillow lavas of Tagga Lo' Aneba,
Maydh area, northern Somalia. In: Abstracts International
Meeting Geology Somalia GEOSOM "87. Somali National
University. Mogadishu, Somalia.
Daniels, J. L., Skiba, A. J. and Sutton, J. 1965. The
deformation of some banded gabbros in the northern
Somalia fold belt. Quarterly Journal Geological Society
London 121, 111-142.
Faure, G. 1977. Principles o f isotope geology, 464p. J.
Wiley and Sons, New York.
Gatto, G. O., Vison& D. and Said, A. A. 1985. Note di
rilevamento sul basamento cristallino della Somalia
settentrionale (distretto di Berbera and Burao). Quaderni
Geologia Somalia 8, 243-255.
Govindaraju, K. and Mevelle, G. 1987. Fully automated
dissolution and separation methods for inductively coupled
plasma atomic emission s p e c t r o m e t r y rock analysis.
Application to the determination of rare earth elements.
Journal Analytical Atomic Spectrometry 2, 615-621.
Hellmann, P. L., Smith, R. E. and Mevelle, G. 1979. The
mobility of Rare Earth Elements: evidence and implications
from the selected terrains affected by burial metamorphism.
Contributions Mineralogy Petrology 71, 23-44.
Kr6ner, A. and Sassi, F. P. 1996. Evolution of the northern
Somali basement: new constraints from zircon ages.
Journal African Earth Sciences 22, 1-15.
New geochemical and petrographic data on the Gabbro-Syenite Suite between Hargeysa and Berbera-Shiikh
Le Bas, M. J., Le Maitre, R. W., Streckeisen, A. and
Zanettin, B. 1986. A chemical classification of volcanic
rocks based on the Total Alkali-Silica diagram. Journal
Petrology 27, 745-750.
Le Roex, A. P., Dick, H. J. B., Reid, A. M., Frey, F. A. and
Erlank, A. J. 1985. Petrology and geochemistry of basalts
from the American-Antarctic Ridge, Southern Ocean:
Implication for the westward influence of Bouvet mantle
plume. Contributions Mineralogy Petrology 90, 367-380.
Mason, J. 1958. A preliminary report on the Daba Shabeli
ring complex, Burao district. Somaliland Protectorate.
Geological Survey u n p u b l i s h e d report JEM/8, 17p.
Hargeysa, London.
NystrSm, J. O. 1984. Rare earth element mobility in
vescicular lava during l o w - g r a d e m e t a m o r p h i s m .
Contributions Mineralogy Petrology 88, 328-331.
Primon, S, 1988. Studio petrografico degli inclusi del Gabbro
di Shiikh Somalia settentrionale. Ump. Dissertation 154p.
University of Padova, Padova, Italy.
Said, A. A. 1987. Preliminary data on the amphibolization
of the layered gabbros NW of Sheik (Northern Somalia).
In: Abstracts International Meeting Geology Somalia
GEOSOM "87. p . 1 1 7 . Somali National University.
Mogadishu, Somalia.
Said, A. A. 1988. Trasformazione gabbro-anfibolite: un
esempio dal basamento della Somalia settentrionale. Ph.
D. dissertation 137p. University of Padova. Italy.
Sassi, F. P., Vison~, D., Ferrara, G., Gatto, G. O., Ibrahim,
H, A., Said, A. A. and Tonarini, S. 1989. The crystalline
basement of northern Somalia: lithostratigraphy and the
sequence of events. In: Geology and mineral resources
of Somalia. (Edited by Abbate, E., Sagri, M. and Sassi,
F. P.). pp3-40. Relazioni e Monografie 113A/1993 Istituto
Agronomico Oltremare, Firenze.
Taylor, S. R. and McLennan, S. M. 1985. The continental
crust. Its composition and evolution. 331p., Blackwell,
Oxford.
Treuil, M. and Joron, J. L. 1975. Utilization des ~16ments
hygromagmatophilespour la simplification de la modelisation
quantitative des processus magmatiques, exemples de I'Afar
et de la dorsale m~dio-atlantique. Rendiconti Societa
Ititaliana Mineralogia Petrografia XXXl, 125-174.
Vison~, D. 1989. The Daba Shabeli gabbro-syenite complex:
an element of the Gabbro Belt in the northern Somali
basement. In: Geology and mineral resources of Somalia.
(Edited by Abbate, E., Sagri, M. and Sassi, F. P.). pp5982. Relazioni e M o n o g r a f i e 1 1 3 A / 1 9 9 3 I s t i t u t o
Agronomico Oltremare, Firenze.
Vocke, R. D., Hanson, G. N. and Grunenfelder, M. 1987. Rare
earth element mobility in the Roffna Gneiss, Switzerland.
Contributions Mineralogy Petrology 95, 145-154.
Warden, A. J. and Daniels, J. L. 1983. Evolution of the
Precambrian of Northern Somalia: Earth Sciences King
Abdulaziz University Jeddah 6, 145-164.
Warden, A. J. and Horkel, A. D. 1984. The geological
evolution of the NE-Branch of the Mozambique Belt (Kenia,
Somalia, Etiopia). Mitteilungen (]sterreichen Geologischen
Gesellschaft 77, 161-184.
Zindler, A., Hart, S. R., Brooks, C. 1981. The Shabogamo
intrusive suite, Labrador: Sr and Nd isotopic evidence for
contaminated mafic magmas in the Proterozoic. Earth
Planetary Science Letters 54, 21 7-235.
APPENDIX
Whole
rock
major
and trace
elements
were
d e t e r m i n e d b y I C P - M S a t t h e CRPG l a b o r a t o r y
(Govindaraju and Mevelle, 1987). Chemical
analyses of minerals were performed using a
CAMECA/CAMEBAX
m i c r o p r o b e o p e r a t i n g at 15
kV with a 15 nA sample current.
Natural
standards (provided by CAMECA) were used.
A n a l y s e s o f m a j o r e l e m e n t s are e s t i m a t e d t o be
accurate
to within
1% relative.
The PAP
program,
provided
by CAMECA,
was used to
convert X-ray counts into oxide wt%.
Journalof African Earth Sciences 373