Precambrian Research 121 (2003) 157–183
Geochemistry of the mafic rocks of the ophiolitic fold and thrust
belts of southern Ethiopia: constraints on the tectonic regime
during the Neoproterozoic (900–700 Ma)
B. Yibas a,1 , W.U. Reimold a,∗ , C.R. Anhaeusser a , C. Koeberl b
a
School of Geosciences, University of the Witwatersrand, Private Bag 3, Wits 2050, Johannesburg, South Africa
b Institute of Geochemistry, University of Vienna, Althan Street 14, A-1090 Vienna, Austria
Received 24 October 2000; accepted 24 October 2002
Abstract
There are four Neoproterozoic ophiolitic belts in southern Ethiopia—Megado, Kenticha, Moyale-El Kur and Bulbul. The
mafic rocks, which form the bulk of these belts, are dominantly subalkaline, low-K, low-Ti tholeiitic basalts (LOTI). This study,
for the first time, shows occurrences of boninites in the Moyale-El Kur Belt and in the Geleba area, in addition to the already
known occurrence in the Megado Belt. The mafic rocks of the Bulbul Belt consistently exhibit high-Ti, high-K calc-alkaline
basaltic geochemistry, similar to that reported from the Kenticha Belt. Several samples from the Geleba area and Moyale-El Kur
Belt also show high-K, high-Ti calc-alkaline geochemistry.
The REE patterns of the Megado low-Ti tholeiites are similar to arc-tholeiites of the Southern Sandwich Islands, whereas
the Moyale tholeiites show patterns similar to back-arc tholeiites of the Scotia Sea Rise. The most distinctive features in spider
diagrams of the LOTI include the selective enrichment of Sr and Ba, and the relative lack of enrichment for K, P, Zr, Ti, Ce,
Sm ± Y, similar to those of oceanic basalts from supra-subduction zone (SSZ) settings, where boninitic and tholeiitic magma
mixing could occur.
The boninites are dominantly high-Ca boninites and are more akin to tholeiites and boninites from known marginal basins, such
as the Mariana fore-arc basin. The presence of boninites in association with low-Ti tholeiites in the Moyale-El Kur Belt suggests
that this belt also represents an ophiolite sequence formed in a SSZ setting similar to the Megado ophiolite. Concentrations of
most immobile elements (Ti, Nb, P, Ce, Zr, Th, V) in the Megado rocks are lower than in the Moyale rocks. This may imply that
the magnitude of subduction was less during the formation of the Moyale ophiolite than in the case of the Megado ophiolite.
On tectonic discrimination diagrams, the Megado and Geleba mafic rocks and the boninitic rocks of the Moyale-El Kur belt
consistently show fore-arc basin affinity, whereas the Moyale-El Kur tholeiites plot into fore-arc basin and back-arc basin fields.
The Bulbul mafic rocks consistently have calc-alkaline basaltic affinity, suggestive of a continental-arc setting. However, high-K
calc-alkaline basalts are also known from a SSZ setting, such as the New Hebrides.
The geochemical interpretation presented in this study, together with discussion of lithological association and geochronological and structural data, is used to decipher the tectonic evolution of the Neoproterozoic of southern Ethiopia.
© 2002 Elsevier Science B.V. All rights reserved.
Keywords: East African Orogen; Mozambique Belt; Arabian-Nubian Shield; Supra-subduction zone; Boninite
∗
Corresponding author.
E-mail addresses: reimoldw@geosciences.wits.ac.za (W.U. Reimold), bisraty@phd.co.za (B. Yibas).
1 Present address: Pulles-Howard-De Lange Environmental and Water Quality Management, P.O. Box 861, Auckland Park, Johannesburg
2006, South Africa.
0301-9268/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 3 0 1 - 9 2 6 8 ( 0 2 ) 0 0 1 9 7 - 3
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B. Yibas et al. / Precambrian Research 121 (2003) 157–183
1. Introduction
The Precambrian of southern Ethiopia, which
consists of high-grade ortho- and para-gneisses and
migmatites, as well as low-grade ophiolitic belts and
granitoids, occupies an important position between
the Pan-African Mozambique Belt and the ArabianNubian Shield, which, together, form the East African
Orogen (Stern, 1994; Fig. 1a).
The tectonostratigraphic classification of the Precambrian terrane of Ethiopia into Lower, Middle and
Upper Complexes (Kazmin, 1975; Kazmin et al.,
1978) has long been in use, although it was based
mainly on metamorphic grade and deformational differences. This classification suggested a prevalence
of Archaean gneisses in the Precambrian of southern
Ethiopia, but was not based on absolute geochronological data. The validity of this classification has
been challenged recently following U–Pb zircon dating of various granitoids and several amphibolites
(Gichile, 1991; Ayalew et al., 1990; Teklay et al.,
1998; Worku, 1996; Yibas, 2000; Yibas et al., 2002).
Yibas et al. (2002) proposed a new tectonostratigraphic classification, together with a geological map
of the Precambrian of southern Ethiopia, based on new
geochronological and geochemical data, and extensive
field investigation. Two distinct lithotectonic terranes
were recognised: (1) the granite–gneiss terrane, and
(2) the mafic–ultramafic–sedimentary ophiolitic belts.
These terranes are separated by repeatedly reactivated
structural zones (Fig. 1b; see also Yibas, 2000; Yibas
et al., 2000a; also Yibas et al., 2002, who provided a
thorough discussion of the lithotectonic classification
of the study region, along with a detailed geological
map).
The granite–gneiss terrane has been subdivided
into the Burji-Moyale and Adola-Genale sub-terranes,
both of which are further subdivided into complexes
based on the spatial association and inter-relationship
of rocks types, their internal structures, and their
lithostructural similarity (Fig. 1b; Yibas et al., 2002).
The mafic–ultramafic–sedimentary assemblages
have been referred to as “fold and thrust belts” in view
of their deformational styles, and as ophiolites, in as
far as their origin and lithological association (volcanic, gabbroic, and ultramafic sequences) are concerned (Yibas, 2000). Four such belts are recognised
in the Precambrian of southern Ethiopia: (1) Megado,
(2) Kenticha, (3) Bulbul, and (4) Moyale-El Kur
(Fig. 1b).
Although the ophiolitic nature of the mafic–ultramafic belts of Adola has long been recognised
(Kazmin, 1975; Berhe, 1990), this has been disputed
by some workers until recently (e.g. Worku and Yifa,
1992; Gichile and Fayson, 1993). Gichile and Fayson
(1993), for example, based on a geochemical study of
amphibolitic and tonalitic samples from the Geleba
area, favoured late Proterozoic island-arc magmatism
for the formation of the Adola mafic rocks. Yibas
(1993) showed, however, that the mafic–ultramafic assemblage of Adola is a remnant of a supra-subduction
zone (SSZ) ophiolitic sequence based on the association of boninites, island-arc and MORB-type tholeiites. Woldehaimanot and Behrmann (1995) and Wolde
et al. (1996) also recognised the presence of boninitic
magmatism and, hence, the SSZ tectonic setting.
Worku (1996), however, argued that the metabasites
of the Megado Belt do not classify unequivocally as
either SSZ or MORB ophiolites.
Woldehaimanot and Behrmann (1995) compared
the geochemistry of the Kenticha metabasites to
N-MORB geochemical characteristics and suggested
that these rocks represented a back-arc setting;
Worku (1996) interpreted the Kenticha mafic rocks as
intra-oceanic calc-alkaline and tholeiitic-arc basalts
formed in a fore-arc SSZ setting.
The only published geochemical work for the
Moyale-El Kur mafic rocks is that of Alene and Barker
(1997), who interpreted geochemical characteristics
as indicative of tholeiitic island-arc and/or ocean-ridge
basalts, and showed alkalic features that they thought
were not consistent with a definite tectonic setting.
The objective of this work is to compare the geochemistry of the mafic suites from the ophiolitic belts
of southern Ethiopia, with the aim of inferring the
palaeotectonic setting of these belts within the context of the geodynamic evolution of the East African
Orogen.
2. Descriptions of the mafic rocks
2.1. Megado mafic rocks
The Megado Belt occurs in the western part of
the Adola area as a linear belt sandwiched between
B. Yibas et al. / Precambrian Research 121 (2003) 157–183
159
Fig. 1. (a) Geological map of Northeast Africa, modified after Worku and Schandelmeier (1996) and Shackleton (1996), showing the
position of the Precambrian of southern Ethiopia within the confines of the East African Orogen. (b) Simplified geological map of the
Precambrian of southern Ethiopia (modified after Yibas, 2000; Yibas et al., 2002).
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B. Yibas et al. / Precambrian Research 121 (2003) 157–183
Fig. 1. (Continued ).
two granite–gneiss blocks of the Adola granite–gneiss
complex (Fig. 1b). Mafic and ultramafic rocks,
psammitic-pelitic schists, subordinate graphite-schist
and metagreywacke are the dominant lithologies of
this belt. Amphibole-schists, amphibolites, and meta-
gabbros are the dominant mafic rocks. Amphibolechlorite schists are fine- to medium-grained and range
from amphibole-dominated to amphibole-chlorite
schists. In the southeast of Digati village, deformed
pillow structures are exposed (Yibas, 1993, and
B. Yibas et al. / Precambrian Research 121 (2003) 157–183
references therein). Representative mineral compositions of the basic schists include: (1) actinolite/hornblende (rarely tremolite), albite-oligoclase,
epidote-clinozoisite, chlorite, quartz; (2) actinolitetremolite/actinolite-hornblende, albite-oligoclase, epidote-clinozoisite, Fe-chlorite, rarely Mg-chlorite,
+/− quartz and opaque minerals; and (3) actinolite/hornblende, albite-oligoclase, epidote-clinozoisite,
quartz and chlorite (Yibas, 2000).
Lensoidal outcrops of metagabbroic rocks of higher
topographic relief within the dominant amphiboleschists and amphibolitic bodies are common. They
are medium- to coarse-grained and massive, and, in
places, contain pseudomorphs after plagioclase and
pyroxene that are replaced by metamorphic minerals.
These rocks are commonly exposed in the central
part of the belt and, locally, preserve their plutonic
texture, both in outcrop and at the microscopic scale.
Hornblende, actinolite, albite, epidote, chlorite, almandine garnet, and quartz, in various proportions,
are the main minerals in the metagabbroic rocks.
2.2. Kenticha mafic rocks
The Kenticha Belt (Fig. 1b) mainly comprises ultramafic rocks (serpentinite, talc-tremolite and talcanthophyllite schists), mafic rocks, staurolite- and
sillimanite-bearing biotite-schists, and minor occurrences of Fe–Mn quartzites, marbles and siliceous
metapelites. The Kenticha mafic rocks include amphibolites, epidote-amphibole gneisses and amphiboleschists. Most commonly they occur sandwiched
between ultramafic bodies as discontinuous bands
interlayered with metasediments. The mafic rocks are
composed of actinolite-hornblende, plagioclase, epidote and quartz, with subordinate amounts of apatite
and opaques (most commonly sulphides).
2.3. Bulbul mafic rocks
The Bulbul Belt occurs in the easternmost part of
the Precambrian of southern Ethiopia (Fig. 1b). The
main rock types that constitute the Bulbul Belt are
amphibolites, chlorite-schists, metagabbros and ultramafic rocks. Slices of granitoid gneisses and dioritic
mylonites are tectonically interlayered with the mafic
rocks in the western part of the belt (Yibas, 2000;
Yibas et al., 2002).
161
2.4. Moyale-El Kur mafic rocks
The Moyale-El Kur Belt covers the area around the
town of Moyale close to the Ethio-Kenyan border and
extends southward into Kenya. It is subdivided into
the Moyale and the Jimma-El Kur sub-belts, which
are separated by the Roukka shear zone (Fig. 1b).
The main lithologic types in the Moyale sub-belt
are metabasic rocks, metaultramafics, and minor
metasediments (Walsh, 1972; Tolessa et al., 1991;
Alene and Barker, 1993; Yibas, 2000). The mafic
rocks of the Moyale-El Kur Belt are mainly amphibolites, with local occurrences of amphibole gneisses,
amphibole-chlorite schists and metagabbro. In places,
they occur intercalated with metasediments, such
as graphitic schist and quartz-feldspar schists and
gneisses (Yibas, 2000; Yibas et al., 2002).
In the Moyale sub-belt, the mafic rocks show variable texture and fabric from amphibolite/amphibole
gneiss to fine-grained basic schists, with lenticular
bodies of metagabbro, although amphibolite and amphibole gneiss represent the largest proportion of these
rocks. In the western part, the amphibolite/amphibole
gneiss units are in tectonic contact with the Moyale
granite–gneiss complex. In the east, they grade into
amphibolites and basic schists, often with garnet porphyroblasts that may be up to a centimetre in diameter.
Low-relief outcrops of amphibolites are most abundant in the eastern part of the Moyale sub-belt, where
they occur intercalated with ultramafic rocks. Elongated pillow-like blocks found within the altered basic
schists suggest a possible submarine extrusion.
In the eastern part, massive sulphide-bearing blocks
of amphibolite occur overlying foliated amphiboleschists. Further to the southeast, amphibole gneisses
occur intercalated with amphibole-schists. Less foliated, granular, gabbroic to dioritic rocks underlie
non-foliated (massive) amphibolite. The succession
shows layering from coarse-grained gabbroic rock
to well-foliated, fine- to medium-grained amphibolites (metabasalts). Migmatitic amphibole gneisses
occur close to the Moyale granodiorite (Yibas, 2000).
Metagabbroic rocks also occur occasionally as circular to elliptical bodies with intrusive relationships
with the surrounding amphibolites of eastern Moyale.
In the northern part of the Jimma-El Kur sub-belt,
around 4◦ N latitude, amphibolites and amphiboleschists are intercalated with varying proportions of
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metasediments (quartzites) and subordinate ultramafic rocks. Extensive outcrops of amphibolite and
gabbro-amphibolite form a ridge near the water well
at Jimma village.
The metamorphic signature of the mafic suites in the
Precambrian of southern Ethiopia, as seen in the mineral assemblages of these rocks, indicates metamorphic grade not exceeding the greenschist–amphibolite
transition facies of Turner (1981). Locally, however,
mid-amphibolite facies metamorphism is evident
where garnet porphyroblasts are developed (Yibas,
1993; Yibas, 2000, and references therein).
3. Geochemistry
3.1. Sample preparation and chemical analyses
Sample preparation was carried out at the Department of Geology, University of the Witwatersrand,
Johannesburg, South Africa and in the Central Laboratory of the Ethiopian Institute of Geological Surveys,
Addis Ababa, Ethiopia. Samples were milled using
chrome-steel discs in a rotary mill. Major and trace elements were analysed by XRF on fused glass discs and
powder pellets at the University of the Witwatersrand.
Precision and accuracy (as determined by standard
and duplicate sample analysis) are similar at (in wt.%)
about 0.4 for SiO2 , 0.03 for TiO2 , 0.2 for Al2 O3 and
MgO, 0.1 for Fe2 O3 , CaO, and K2 O, 0.01 for MnO
and P2 O5 , and 0.3 for Na2 O. The concentrations of V,
Cu, Y, and Nb were also determined by XRF analysis
(accuracy 0.5–1, 0.5–1, 1–1.5 ppm for the low to high
concentrations, respectively). All other trace elements
were determined by instrumental neutron activation
analysis (INAA) at the Institute of Geochemistry,
University of Vienna, using methods described by
Koeberl et al. (1987) and Koeberl (1993). Representative major and trace element analyses are listed in
Table 1.
3.2. General geochemical characteristics and
rock classification
Determining the original chemistry of a metamorphosed rock is rendered possible with the combined
use of both major and trace elements. The low-grade
mafic suites of southern Ethiopia show no signif-
Fig. 2. Plot of the Precambrian metabasic rocks of southern
Ethiopia to investigate the possible effect of alteration on primary
chemical compositions (after Hughes, 1973).
icant alteration, although silica veins are common
and clearly defined alteration zones may be associated with shearing. Samples of the various basic
rocks were collected away from alteration zones,
which may be associated with gold mineralisation
(e.g. in the Moyale area and in the Mi-essa Ridge
south of Digati village in the Megado Belt). The
geochemical data (Table 1) were screened to determine whether alteration due to metamorphism and/or
weathering might have affected the original chemistry of the rocks. Plotting of the alkali elements in a
(K2 O + Na2 O) versus (K2 O/(K2 O + Na2 O) × 100) diagram (Fig. 2), together with the concentration ranges
of the major element oxides (Table 1), indicates that
chemical heterogeneity due to alteration is minimal
for most of the sample groups. However, some data
falling outside of the igneous field (Fig. 2) were
discarded.
Although the range of SiO2 content in the mafic
rocks under discussion is rather narrow to see distinct
variation trends in variation diagrams, most major
elements plotted against SiO2 (not shown here) reveal systematic negative correlation (e.g. TiO2 , MgO,
MnO, CaO, Fe2 O3 T), which can be explained by differentiation. Likewise, the trace elements V, Cr, Co
and Ni show negative correlation, whereas Sr, Rb, Zr,
Table 1
Representative major, minor and trace elements of the mafic rocks of the Precambrian of southern Ethiopia: major element data (in wt.%) by XRF
B. Yibas et al. / Precambrian Research 121 (2003) 157–183
163
164
B. Yibas et al. / Precambrian Research 121 (2003) 157–183
Table 1 (Continued )
B. Yibas et al. / Precambrian Research 121 (2003) 157–183
165
166
B. Yibas et al. / Precambrian Research 121 (2003) 157–183
Table 1 (Continued )
B. Yibas et al. / Precambrian Research 121 (2003) 157–183
LOI, loss on ignition at 1100 ◦ C; all Fe as Fe2 O3 ; trace elements (in ppm); V, Cu, Y, and Nb by XRF: Ni, Zr, and Sr: INA and XRF; all other data by INAA; (∗ ) indicates
below detection limit; nd, not determined.
167
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B. Yibas et al. / Precambrian Research 121 (2003) 157–183
Nb, Y and the REEs show positive correlation with
SiO2 . Most of the mobile elements such as Rb and Sr
show a wider scatter.
With the exception of the Bulbul rocks and a few
samples from the Moyale-El Kur belt, which straddle
the alkaline and subalkaline boundaries on the alkaline versus subalkaline discrimination diagrams, most
samples exhibit subalkaline affinity. These samples
can further be subdivided into low-K, low-Ti tholeiitic basalts (LOTI), and calc-alkaline andesitic basalts
and andesites on AFM and FeO/MgO versus SiO2
diagrams (not shown here). Most of the calc-alkaline
samples show boninitic affinity in that they have
high SiO2 (>50 wt.%), high MgO (>7 wt.%), and
low-TiO2 (see also Figs. 3 and 4). Yibas (1993),
Woldehaimanot and Behrmann (1995), and Wolde
et al. (1996) recorded the presence of boninites in the
Megado Belt. However, the presence of boninites in
the Moyale-El Kur Belt and amongst the Geleba mafic
rocks has not been reported to date (compare Gichile,
1991; Gichile and Fayson, 1993; Alene and Barker,
1997).
On variation diagrams of elements plotted against
MgO (Fig. 3a), SiO2 , TiO2 and P2 O5 show strong
negative correlation with MgO, whereas CaO, total
FeO, and MnO show positive correlation within each
mafic suite. Na2 O, K2 O and Al2 O3 show slight negative correlation with MgO for most samples. Zr, Nb,
Y, V and Sr show negative correlation, whereas elements such as Cr and Ni show positive correlation
(Fig. 3a). Plots of major elements against MgO reveal
how much of the original chemistry of the metamorphosed mafic rocks is affected. This is because MgO
is an important component of the solid phases in equilibrium with mafic melts and shows a great deal of
variation, either as a consequence of the breakdown
of magnesian phases during partial melting, or because of their removal during fractional crystallisation
(Rollinson, 1993). TiO2 and P2 O5 and, to a lesser extent, MnO, K2 O and SiO2 show positive correlation
with Zr, whereas MgO and CaO show negative correlation (Fig. 3b). This negative correlation with Zr
is consistent with early crystallising minerals such as
olivine, pyroxene and Ca-plagioclase forming during
magma differentiation. The remaining oxides show a
wide scatter and poorly defined trends that suggest
possible secondary effects. Among the trace elements,
Y, Sr, Nb, Hf, Rb and Ba show positive correlation,
and Cr, Ni and Co show negative correlation, with Zr
(Fig. 3b).
The chemical characteristics observed for the mafic
suites of southern Ethiopia can be summarised as an
increase in Ti, Zr, P, Y, V, Zn with decreasing MgO,
Ni, Cr, V, and Sr (Fig. 3). Fractional crystallisation
of mineral phases such as clinopyroxene, plagioclase,
olivine and possibly, orthopyroxene, could explain
these chemical characteristics. Low-P2 O5 , low-Zr and
an increase of Ti with increasing Zr further corroborate
the dominance of tholeiitic basaltic rocks. Ti, Zr, Nb,
Y and REEs are among the elements considered relatively immobile and they are, thus, commonly used to
investigate types of protolithic magma, degree of differentiation and possible tectonic setting (e.g. Pearce
and Norry, 1979; Winchester and Floyd, 1977).
3.3. REE geochemistry and spider diagrams
The possibility that REE patterns might be affected
by sea-floor alteration and low-grade metamorphism
has been discussed by Pearce and Cann (1973), Pearce
(1975), Frey (1983), Frey and Green (1974) and Frey
et al. (1978). Plotting of the REE abundances against
Zr is one of the most reliable tests to detect possible alteration and effects of metamorphism on the
REE geochemistry (Fig. 4). These plots show that
the REE abundances for these sample suites show
a systematic increase with increase of Zr concentrations, which suggests that the overall REE patterns
have not changed significantly by alteration and
metamorphism.
As tholeiitic and boninitic rocks are recognised in
the mafic suites of southern Ethiopia, the REE and
spider diagram patterns of these two rock suites have
been treated separately to determine if these plots also
support this classification (Fig. 5).
3.3.1. Tholeiitic rocks
3.3.1.1. Megado tholeiites. The Megado tholeiitic
rocks display two distinct REE patterns (Fig. 5a
and c), both of which show overall REE concentrations that are higher than those of the boninitic
series. Group 1 tholeiites show strong LREE depletion ((La/Sm)N = 0.4–0.7) compared to average
tholeiitic MORB and N-MORB rocks and flat HREE
patterns ((Tb/Yb)N = 0.76–1.04) (Fig. 5a). Group 2
B. Yibas et al. / Precambrian Research 121 (2003) 157–183
169
Fig. 3. (a) MgO vs. major and trace element abundances of the Precambrian mafic rocks of southern Ethiopia; trace element data in ppm;
(b) Zr vs. major and trace elements for the Precambrian metabasic rocks of southern Ethiopia. Symbols as in (a). Major element data in
wt.%, trace element data in ppm.
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B. Yibas et al. / Precambrian Research 121 (2003) 157–183
Fig. 3. (Continued ).
B. Yibas et al. / Precambrian Research 121 (2003) 157–183
Fig. 4. Zr vs. rare earth elements (data in ppm) in the Precambrian metabasic rocks of southern Ethiopia. Symbols as in Fig. 3a.
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B. Yibas et al. / Precambrian Research 121 (2003) 157–183
Fig. 5. Chondrite-normalised REE patterns (normalisation factors from Nakamura, 1974) and spider diagram plots (normalisation data from
Pearce, 1983) for tholeiitic rocks of the Precambrian of southern Ethiopia: (a) and (c) Megado tholeiite-1, (b) and (d) Megado tholeiite-2,
(e) and (f) Moyale tholeiites, (g) and (h) calc-alkaline basalts: GC = Geleba, Bu = Bulbul, ME = Moyale-El Kur. In (a), patterns for
N-MORB and tholeiitic MORB (from Nakamura, 1974) are included for comparison.
B. Yibas et al. / Precambrian Research 121 (2003) 157–183
(Fig. 5c) shows low overall REE fractionation ((La/
Yb)N = 0.98–1.75), modest positive Eu anomaly,
and more or less flat HREE patterns. The REE concentrations also increase with increasing differentiation. The overall patterns of the Megado tholeiites
resemble the island-arc tholeiites (IATs) of the South
Sandwich Islands (Hawkesworth et al., 1977). Two
gabbroic samples show a marked positive Eu anomaly
and lower total REE values than the amphibolites.
The relatively stronger positive Eu anomaly observed
in the metagabbroic samples could be ascribed to a
combination of the degree of Eu fractionation and
the amount of cumulus plagioclase in the magma.
The behaviour of Eu in the tholeiites of South
Sandwich also varies from negative to positive in
the high- and low-total REE samples, respectively,
which is attributed to a protracted fractionation history (low-pressure fractionation of plagioclase and
olivine) (Hawkesworth et al., 1977). These authors
further argued that the source region is enriched
to some degree in K and Rb, relative to abyssal
tholeiites, and with a significant component of subducted oceanic crust. The overall REE pattern of the
Megado Group 1 tholeiites ((La/Yb)N = 0.24–0.38)
is also similar to patterns of evolved tholeiites from
New Caledonia, which are associated with boninites
(Cameron, 1989). However, the Megado Group 1
tholeiites and the Moyale tholeiites have lower REE
values. In addition to the similarity in the REE patterns, these rocks also have very low abundances of
Ti, P, and Zr and somewhat higher Cr and Ni values,
similar to lavas from Guam (Cameron, 1989, and
references therein). REE patterns similar to those of
the Megado tholeiite-2 have also been reported from
the mafic rocks of the Fawakhir ophiolite (El-Sayed
et al., 1999) and the Abu Zawal gabbroic intrusion
of the central Eastern Desert of Egypt (Abu El-Ela,
1996).
Although they have many features in common, the
spider diagrams for the samples from the Megado
tholeiites can also be subdivided into two groups
(Fig. 5b and d). Group 1 tholeiites lack the troughs
at Y when compared with Group 2 tholeiites, which
also show higher Cr concentrations than the Group 1
tholeiites.
3.3.1.2. Moyale tholeiites. The Moyale tholeiitic
rocks display two distinct groups of REE patterns
173
(Fig. 5e). The amphibolites and amphibole-chlorite
schists have relatively higher overall REE and slightly
enriched LREE patterns, and flat to depleted HREE
patterns ((Tb/Yb)N = 0.88–2.00). The REE patterns for the metagabbros from the eastern part of
the Moyale-El Kur Belt are characterised by strong
positive Eu anomalies, with lower REE abundances
than those of the amphibolites and amphibole-chlorite
schists. They also show a slight LREE enrichment
and HREE depletion.
The REE patterns of the Moyale tholeiites are more
akin to back-arc tholeiites such as those occurring in
the Scotia Sea Rise and the Mariana Basin (Hart et al.,
1972; Hawkesworth et al., 1977). The spider diagram
patterns (Fig. 5f) for the Moyale tholeiites also differentiate the metagabbros from the amphibolites and
schists (metabasalts).
The most distinctive features exhibited by the spider
diagrams of the LOTI are the selective enrichments
of certain elements (Sr, Ba) and the relative lack of
enrichment of others (P, Zr, Ti, Ce, Sm ± Y). These
patterns and the HFS element variations exhibited by
the tholeiitic rocks are characteristic of a SSZ setting,
where boninitic and tholeiitic magma mixing could
occur (Pearce et al., 1984).
3.3.2. Calc-alkaline rocks
Minor occurrences of calc-alkaline lavas have
been recognised amongst these mafic suites, with
the exception of the Megado suite. The REE patterns of these rocks are strongly LREE-enriched and
HREE-depleted (Fig. 5g).
The calc-alkaline rocks also show overall higher
trace element abundances compared to the Moyale
tholeiites (Fig. 5h) and display strongly differentiated
patterns when compared to the tholeiitic rocks. These
patterns are similar to those of the high-K calc-alkaline
SSZ basalts from the New Hebrides (Pearce et al.,
1984).
3.3.3. Boninites
The Megado boninites display flat patterns or may
have slight LREE enrichment and flat HREE patterns,
with variability in Eu behaviour (Fig. 6a). The samples with strongly positive Eu anomalies are boninitic
metagabbros.
The boninites from Geleba and Moyale display
shallow dish-shaped patterns due to slight L-REE
174
B. Yibas et al. / Precambrian Research 121 (2003) 157–183
Fig. 6. Chondrite-normalised REE patterns (normalisation factors from Nakamura, 1974) and spider diagrams (normalisation data from
Pearce, 1983) for the boninitic rocks of the Precambrian of southern Ethiopia: (a) and (b) Megado boninites, (c) and (d) Moyale boninites,
(e) and (f) Geleba boninites.
B. Yibas et al. / Precambrian Research 121 (2003) 157–183
175
Fig. 7. Tectonic discrimination diagrams for the Precambrian metabasic rocks of southern Ethiopia. Diagrams after: (a) and (c)
Pearce and Cann (1973); (b) Pearce (1975); (d) Shervais (1982); (e) Pearce et al. (1984); (f) Mullen (1983). MORB = mid-oceanic
ridge basalt, CFB = continental-floor basalt, WPB = within plate basalt, IAT = island-arc tholeiite, CAB = calc-alkali basalt,
LKT = low-potassium basalt, OFB = ocean-floor basalt, OIT = ocean-island tholeiite, OIA = ocean-island alkali basalt, BAB = back-arc
basalt, OIB = oceanic-island basalt, VAT = volcanic-arc tholeiite.
enrichment of otherwise flat middle-REE and H-REE
(Fig. 6c and e), similar to the patterns of the boninites
from the Izu-Bonin forearc site 786 (Murton et al.,
1992) and boninites from New Caledonia (Cameron,
1989). LREE enrichment in boninites could result
from metasomatism of their harzburgitic sources by an
LREE- and Zr-enriched fluid (Sun and Nesbitt, 1977;
Jenner, 1981; Hickey and Frey, 1981; Cameron et al.,
1983; Nelson et al., 1984; Hickey-Vargas, 1989).
The patterns of most of the HFS elements in the spider diagrams for boninitic rocks are variable (Fig. 6b
and d), which could be due to tholeiitic and boninitic
magma mixing. Such a possibility has been reported,
at least for the Megado rocks, by Wolde et al. (1996).
176
B. Yibas et al. / Precambrian Research 121 (2003) 157–183
Fig. 7. (Continued ).
3.4. Tectonic setting
The interpretation of the tectonic setting of a metamorphosed and deformed terrane is difficult. The
volcano–sedimentary–ultramafic belts of southern
Ethiopia suffered metamorphism up to greenschist–
amphibolite transition facies (Beraki et al., 1989;
Yibas, 1993; Worku, 1996). This undoubtedly affected
the chemistry of, in particular, the LIL elements (such
as K, Ba, Rb and Sr), which are highly mobile during metamorphism. Therefore, the interpretation of
the tectonic setting for the region should, to a large
extent, depend on elements of high ionic potential
(Ti, Zr, Cr and Y), as these elements are effectively
immobile during metamorphism (Cann, 1970).
A large portion of the samples of metabasic rocks
plot, on Ti–Zr–Y and Ti–Zr diagrams, into the field
where calc-alkaline (CAB) IAT and mid-oceanic ridge
basalts (MORB) overlap (Fig. 7a and b). However, a
few samples from the Moyale-El Kur Belt fall into the
MORB field in the Ti–Zr diagram. The mafic rocks of
the Bulbul Belt fall into the calc-alkaline basaltic field.
In order to differentiate samples of MORB affinity from those of volcanic-arc affinity, the Ti–Cr diagram after Pearce (1975) may be useful (Pharaoh and
Pearce, 1984). Whereas a large part of the Moyale-El
Kur samples plot into the ocean-floor basalt (OFB)
field, the remaining samples plot into the IAT field
(Fig. 7c).
Basalts and basaltic andesites of 45–54 wt.% silica
can be subdivided on the basis of their MnO, TiO2 and
P2 O5 concentrations into MORB, ocean-island tholeiites (OIT), ocean-island alkali basalts (OIA), IAT and
calc-alkali basalts (CAB) (Mullen, 1983). The boninite
field occupies the MnO-rich sector of the CAB field.
The majority of the samples from the study area fall
into the IAT and CAB fields but lie close to the MnO
apex. A few samples from the Moyale-El Kur Belt
show MORB affinity. The Bulbul mafic rocks straddle the boundary between the CAB and OIA fields
(Fig. 7d).
On the Ti–V discrimination diagram (Shervais,
1982), the Megado tholeiites and associated boninites
fall into the fields of oceanic island (OIB) and back-arc
basalts (BAB), but plot at the lower left corner of the
diagram due to their very low Ti and V values. The
boninites of the Moyale-El Kur Belt and most Geleba
samples fall into the VAT field due to their higher V
values relative to the Megado rocks (Fig. 8e). Furthermore, the Moyale tholeiites are discriminated in
this diagram as VAT and BAB-MORB rocks.
The Cr–Y diagram (Pearce, 1982) is also important for the discrimination of IAT from MORB rocks
(e.g. Pearce and Gale, 1977; Garcia, 1978; Bloxham
and Lewis, 1972). A Cr–Y plot discriminates effectively between MORB and volcanic-arc basalts. In
addition, this diagram has been used to discriminate
between different marginal basin rocks. Pearce et al.
(1984) tested its usefulness in discriminating between
true oceanic floor basalts and those of marginal basin
origin (i.e. fore-arc and back-arc basins). Most samples from back-arc basins fall into the MORB field,
B. Yibas et al. / Precambrian Research 121 (2003) 157–183
Fig. 8. Geodynamic evolution of the Precambrian of southern Ethiopia during the East African Orogeny (900–500 Ma).
177
178
B. Yibas et al. / Precambrian Research 121 (2003) 157–183
whereas those from fore-arc basins (FAB), such as the
Mariana FAB, plot into or to the left of the IAT field.
Moreover, those samples which fall into the FAB field
exhibit boninitic characteristics, and the geochemical
characteristic of these basalts represent the best analogues for SSZ ophiolites. Over 90% of the data from
mafic rocks of southern Ethiopia fall into the IAT
field, where FAB basalts overlap (Fig. 7f). Only a few
samples from the Moyale-El Kur Belt fall into the
BAB-MORB field.
Based on these tectonic discrimination diagrams,
the Megado and Geleba mafic rocks and the boninitic
rocks of Moyale-Jimma-El Kur consistently show
IAT-FAB affinity, unlike the Moyale-El Kur tholeiites
that can be broadly classified into IAT-FAB and backarc tholeiites. The Bulbul mafic rocks consistently
show calc-alkaline basaltic affinity suggestive of a
continental-arc setting. However, high-K calc-alkaline
basalts are also known from a SSZ setting such as
that of the New Hebrides (Pearce et al., 1984).
4. Discussion and conclusions
4.1. Geochemistry and palaeotectonic setting
The ophiolitic nature of the mafic–ultramafic belts
of Adola has long been recognised (Kazmin, 1975;
Berhe, 1990) but has been disputed by some workers. This study has shown that subalkaline, LOTI
tholeiitic basalts are the most dominant mafic rocks
in the low-grade belts of the Precambrian of southern
Ethiopia. These tholeiites have low P2 O5 and Zr values and positive correlation between Ti and Zr. This
study shows the occurrence of boninites also in the
Moyale-El Kur Belt and in the Geleba area, in addition to their occurrence in the northern and central
parts of the Megado Belt.
The mafic rocks of the Bulbul Belt consistently exhibit high-Ti, high-K, calc-alkaline basaltic composition, in contrast to the mafic rocks of the Megado
and Moyale-Jimma El Kur belts. A few samples from
the Geleba and Moyale-Jimma-El Kur Belt also show
high-K, high-Ti, calc-alkaline geochemistry. Similar
high-K, high-Ti, calc-alkaline basalts have also been
reported from the Kenticha Belt (Worku, 1996).
The tholeiitic rocks from the Megado and MoyaleEl Kur belts also have low-P2 O5 and -Zr values and
a positive correlation between Ti and Zr. The Megado
tholeiites show REE patterns similar to arc-tholeiites
of the Southern Sandwich islands, whereas the Moyale
tholeiites show patterns similar to back-arc tholeiites
of the Scotia Sea Rise (Hawkesworth et al., 1977).
The most distinctive features exhibited in the spider
diagrams of the LOTI tholeiites include the selective enrichment of certain elements (Sr, Ba, Ce, Sm)
and the relative lack of enrichment of others (K, P,
Zr, Ti, ±Y). These patterns are very similar to those
of oceanic basalts from SSZ settings (Pearce et al.,
1984).
The boninites of southern Ethiopia occur together
with IATs and are geochemically more akin to tholeiites and boninites from known marginal basins, such
as the Mariana fore-arc basin (Pearce et al., 1984).
According to the boninite classification of Crawford
et al. (1989), the Megado boninites are dominantly
high-Ca boninites (SiO2 < 56 wt.%; CaO/Al2 O3 >
0.75 wt.%; total alkalis < 2 wt.%; CaO > 9 wt.%;
and FeO > 7 wt.%). The Megado rocks, however,
are characterised by very low-Zr values (Yibas,
1993; Wolde et al., 1996, this study). Over 50% of
the Moyale boninites exhibit similarities to high-Ca
boninites and the rest is characterised by high CaO
contents (>12.5 wt.%), FeOT between 7 and 9 wt.%,
CaO/Al2 O3 > 0.7, and total alkali element contents
<1.5 wt.%.
The mafic rocks, when plotted on tectonic discrimination diagrams, can be classified into Moyale-1 and
Moyale-2 suites. Moyale-1 (dominantly boninitic)
shows IAT geochemistry similar to modern fore-arc
basin basalts, whereas the Moyale-2 suite (dominantly tholeiitic) plots into the MORB-BAB field.
The presence of boninites in association with low-Ti
tholeiites in the Moyale-El Kur Belt suggests that the
Moyale-El Kur Belt represents an ophiolite sequence
formed in a SSZ setting similar to the Megado ophiolite. The concentrations of most of the immobile
elements (Ti, Nb, P, Ce, Zr, Th and V) in the Megado
rocks are lower when compared to the Moyale rocks.
This might imply that the magnitude of subduction
during the formation of the Moyale ophiolite was less
than that of the Megado ophiolite case.
The absence of boninitic rocks and their consistent
calc-alkaline basaltic characteristics on various discrimination diagrams suggest that the Bulbul mafic
rocks represent a mature island-arc setting, in contrast
B. Yibas et al. / Precambrian Research 121 (2003) 157–183
to the Megado and Moyale mafic suites. The spider
diagrams of these calc-alkaline rocks show similarity
with those of high-K calc-alkaline SSZ basalts form
the New Hebrides (Pearce et al., 1984).
4.2. The geodynamic evolution of the Precambrian
of southern Ethiopia
Tibetan-style continent–continent collision between West Gondwana (Archaean Tanzanian Craton)
and East Gondwana is thought to have been responsible for the formation of the Mozambique Belt
(Burke and Sengör, 1986; Shackleton, 1986; Key
et al., 1989; Berhe, 1990). Given the position of the
Precambrian geology of southern Ethiopia between
the low-metamorphic grade Arabian-Nubian Shield
and the high-metamorphic grade Mozambique Belt
(which together form the East African Orogen, Stern,
1993, 1994), an understanding of the geodynamic
evolution of this region will help to better understand the evolution of the East African Orogen as a
whole.
Based on U–Pb single zircon SHRIMP and laser
probe 40 Ar–39 Ar ages and field studies (Yibas, 2000;
Yibas et al., 2000b), as well as earlier geochronological data, Yibas et al. (2002) classified the granitoids
of the East African Orogen in southern Ethiopia into
seven generations (Table 2). Based on their geochemical characteristics, these granitoids are classified into
volcanic-arc and within-plate granitoids, with dominance of volcanic-arc granitoids (Yibas, 2000; Yibas
et al., 2000c). Combination of the geochronological
and geochemical criteria allowed these authors to further suggest that the granitoids of southern Ethiopia
show alternation of within-plate and volcanic-arc
granitic magmatism, suggesting repeated compressional and extensional tectonic regimes during the
development of the East African Orogen between 900
and 500 Ma ago.
The geochronological order in which the different
ophiolites were formed in southern Ethiopia has become clearer as a result of recent geochronological
work. The 700 Ma U–Pb zircon age obtained for the
amphibolitic rocks from the Moyale ophiolitic fold and
thrust belt by Teklay et al. (1998) has been interpreted
as an approximation of their formation age. The formation age of the Megado ophiolite is approximately
789 ± 36 Ma (Sm–Nd whole-rock isochron age for the
179
Megado metavolcanics, Worku, 1996). The 876±5 Ma
age (U–Pb zircon SHRIMP age, Yibas, 2000; Yibas
et al., 2000b, 2002) obtained for the Bulbul dioritic
mylonite gneiss from the Bulbul Belt implies the presence of an early- or pre-Pan African ocean in southern
Ethiopia (Yibas, 2000).
Based on the integration of geochronological, geochemical (both granitic and mafic rocks), and structural data (Yibas, 2000, and references therein), the
following evolutionary stages are envisaged for the
tectonic evolution of the Neoproterozoic of southern
Ethiopia (Fig. 8).
Stage 1 (from <1050 to »880 Ma): Opening of the
Bulbul (Kenticha?) ocean and deposition of deep-sea
sediments, which must have occurred between about
1050 ± 40 and 880 Ma, the oldest recognised metamorphic ages (Rb–Sr, Chater, 1971) and the oldest
subduction-related magmatism (Bulbul diorite, Yibas,
2000; Yibas et al., 2000b, 2002).
Stage 2 (from 880 to 760 Ma):
(a) Subduction of the Bulbul (Kenticha?) active
spreading center at ∼880 Ma. To the east, the
ridge was subducted under continental crust,
which led to the generation of the Bulbul–Alge
arc complex (Yibas, 2000). The Kenticha ophiolite is interpreted as probably early or pre-Pan
African in age (Worku, 1996). The existence of
a pre-876 ± 5 Ma ocean in southern Ethiopia
is consistent with the argument that the oldest
Pan-African igneous rocks have been erupted at
about 880 Ma (Rieschmann et al., 1984; Stern,
1993) or 840–870 Ma (Kröner et al., 1991, 1992)
in an intra-oceanic-arc setting.
(b) Initiation of rifting for the formation of the
Megado marginal basin. This must have occurred
simultaneously with the subduction of the Bulbul (Kenticha?) ocean basin to the east and took
place above a westerly dipping subduction zone.
Here, the western spreading arm of the Kenticha
oceanic crust was subducted leading to the generation of boninites beneath a fore-arc at Megado at
about 789 ± 36 Ma (Sm–Nd whole rock isochron
age for the metavolcanic rocks of the Megado
ophiolite, Worku, 1996).
Stage 3 (760–710 Ma): Closure of the Megado
marginal (fore-arc) basin and abduction of the Megado
ophiolite, followed by arc and collisional magmatism
180
Table 2
A possible geodynamic evolutionary scheme for the Precambrian of southern Ethiopia during the East African Orogeny, based on geochronology, geochemistry and deformation
of granitoids (Yibas, 2000; Yibas et al., 2002)
Granitic phases (ages)
Gt7 (550–500 Ma)
Dated granites
Metoarbasebat granite
Berguda charnockitic granite
Gt5 (700–600 Ma)
Gt4 (720–700 Ma)
Gt3 (770–720 Ma)
Zircons U–Pb
526 ± 5␤
506 ± 4 (Bt);
511 ± 3 (Hb)
±
±
±
±
8.4 (rim)␣ ;
3 (core)␣
18␥
23ε
Lega Dima granite
Robele granite
528
538
550
554
Wadera foliated granite
576 ± 5␤
Wadera megacrystic diorite gneiss
Digati dioritic gneiss
579 ± 5␤
570 ± 5␤
Burjiji granitic massif
∼602␥
Meleka foliated granodiorite
610 ± 9␤
Gariboro granite
Moyale granodiorite
∼646␥
666 ± 5␤
Finchaa biotite-foliated granite
708 ± 5ε
Yabello charnockitic granite–gneiss
716 ± 5␶
Alghe granite–gneiss
722 ± 2␶
Sagan basic charnockite
Zembaba granite–gneiss
Sebeto tonalite gneiss
725␶
756 ±
765 ±
Gt2 (>770 Ma)
Melka Guba megacrystic
granodiorite gneiss
Gt1 (>880 Ma)
Bulbul diorite mylonite
laser
40 Ar–39 Ar
Associated deformation
(possible tectonic scenario)
Post-orogenic cooling and transcurrent
faulting
Sinistral transpressional (shear zones) due
to oblique-collision (docking)
502 ± 4 (Bt)
Subduction (closure of the Moyale
marginal basin)
512 ± 4 (Bt);
515 ± 4 (Mu)
Transition from compressional deformation
to an extensional regime prior to the
formation of the Moyale basin
Subduction-related granitic magmatism
associated with the closure of the Megado
basin
6␶
3ε
778 ± 23␤
876 ± 7␤
Extensional granitic magmatism associated
with the opening of the Megado marginal
basin
495 ± 5 (Bt)
Subduction-related magmatism
␤ and , Yibas et al. (2002); ␣, Gichile (1991); ␥, Worku (1996); ε, Genzebu et al. (1994); ␶, Teklay et al. (1998) (␤, SHRIMP, U–Pb; all others, U–Pb single zircon
evaporation method). Hb, hornblende; Bt, biotite; Mu, muscovite.
B. Yibas et al. / Precambrian Research 121 (2003) 157–183
Gt6 (550–600 Ma)
Age (Ma)
B. Yibas et al. / Precambrian Research 121 (2003) 157–183
and charnockite formation. The presence of arc-granitoids west of the Megado ophiolite belt (Sebeto and
Alghe granites, with U–Pb zircon ages of 760 and
722 Ma, respectively; Yibas, 2000; Yibas et al., 2000b,
2002) approximates the time of the closure of the
Megado oceanic basin.
Stage 4 (>700–660 Ma): Opening of the Moyale
marginal basin in a fore-arc SSZ setting, which began in a manner described for the formation of the
Megado basin. This was followed by subduction of
the Moyale basin at about 660 Ma (age of the Moyale
arc-granodiorite, SHRIMP U–Pb age, Yibas, 2000;
Yibas et al., 2002).
Stage 5a (660–550 Ma): Continuation of subduction-related magmatism (e.g. Gariboro and Burjiji
granitic massif, Meleka granodiorite, Wadera megacrystic diorite gneiss, Digati granodiorite; Worku,
1996) and transpressive deformation (Yibas, 2000).
This period marks a period of oblique continent-arc
collision (docking) and accretion.
Stage 5b (550–500 Ma): Emplacement of lateto post-tectonic and post-orogenic granitoids (e.g.
Berguda charnockitic granitoid and non-deformed
granitoids, such as the Robele, Lega Dima and
Metoarbasebat granites, all having similar zircon
ages, see Yibas et al., 2002, and references therein),
accompanied by thrusting and transcurrent faulting
and shearing associated with uplift and final cooling
at the end of the East African Orogeny (Yibas, 2000).
It can also be concluded that this scenario is
consistent with the geological evolution of the
Arabo-Nubian Shield, in general, as, for example, discussed by Kröner (1984), Kröner et al. (1991, 1992),
Rieschmann et al. (1984), Stern (1993), Shackleton
(1996), and Abdel Salam and Stern (1996).
Acknowledgements
This paper resulted from the Ph.D. project by B.
Yibas, which benefited from generous financial support from Anglo American Prospecting Services,
Johannesburg. Richard Holdsworth and his group at
the Anglo American Research Laboratory (AARL),
Johannesburg, are thanked for ICP-MS trace-element
analyses. Thorough reviews by S. Bloomer and M.
Teklay greatly improved an earlier version of the
manuscript.
181
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