-
Precambrian Research 121 (2003) 157–183
Geochemistry of the mafic rocks of the ophiolitic fold and
thrustbelts of southern Ethiopia: constraints on the tectonic
regime
during the Neoproterozoic (900–700 Ma)
B. Yibasa,1, W.U. Reimolda,∗, C.R. Anhaeussera, C. Koeberlba
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. Themafic
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
alreadyknown occurrence in the Megado Belt. The mafic rocks of the
Bulbul Belt consistently exhibit high-Ti, high-K
calc-alkalinebasaltic geochemistry, similar to that reported from
the Kenticha Belt. Several samples from the Geleba area and
Moyale-El KurBelt 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, whereasthe Moyale
tholeiites show patterns similar to back-arc tholeiites of the
Scotia Sea Rise. The most distinctive features in spiderdiagrams 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 magmamixing could occur.
The boninites are dominantly high-Ca boninites and are more akin
to tholeiites and boninites from known marginal basins, suchas the
Mariana fore-arc basin. The presence of boninites in association
with low-Ti tholeiites in the Moyale-El Kur Belt suggeststhat this
belt also represents an ophiolite sequence formed in a SSZ setting
similar to the Megado ophiolite. Concentrations ofmost immobile
elements (Ti, Nb, P, Ce, Zr, Th, V) in the Megado rocks are lower
than in the Moyale rocks. This may imply thatthe 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 beltconsistently
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-Kcalc-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 geochrono-logical
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:[email protected] (W.U. Reimold),
[email protected] (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: S0301-9268(02)00197-3
-
158 B. Yibas et al. / Precambrian Research 121 (2003)
157–183
1. Introduction
The Precambrian of southern Ethiopia, whichconsists of
high-grade ortho- and para-gneisses andmigmatites, as well as
low-grade ophiolitic belts andgranitoids, occupies an important
position betweenthe Pan-African Mozambique Belt and the
Arabian-Nubian Shield, which, together, form the East AfricanOrogen
(Stern, 1994; Fig. 1a).
The tectonostratigraphic classification of the Pre-cambrian
terrane of Ethiopia into Lower, Middle andUpper Complexes (Kazmin,
1975; Kazmin et al.,1978) has long been in use, although it was
basedmainly on metamorphic grade and deformational dif-ferences.
This classification suggested a prevalenceof Archaean gneisses in
the Precambrian of southernEthiopia, but was not based on absolute
geochrono-logical data. The validity of this classification hasbeen
challenged recently following U–Pb zircon dat-ing 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 tectonostrati-graphic
classification, together with a geological mapof the Precambrian of
southern Ethiopia, based on newgeochronological and geochemical
data, and extensivefield investigation. Two distinct lithotectonic
terraneswere recognised: (1) the granite–gneiss terrane, and(2) the
mafic–ultramafic–sedimentary ophiolitic belts.These terranes are
separated by repeatedly reactivatedstructural zones (Fig. 1b; see
alsoYibas, 2000; Yibaset al., 2000a; alsoYibas et al., 2002, who
provided athorough discussion of the lithotectonic classificationof
the study region, along with a detailed geologicalmap).
The granite–gneiss terrane has been subdividedinto the
Burji-Moyale and Adola-Genale sub-terranes,both of which are
further subdivided into complexesbased on the spatial association
and inter-relationshipof rocks types, their internal structures,
and theirlithostructural similarity (Fig. 1b; Yibas et al.,
2002).
The mafic–ultramafic–sedimentary assemblageshave been referred
to as “fold and thrust belts” in viewof their deformational styles,
and as ophiolites, in asfar as their origin and lithological
association (vol-canic, gabbroic, and ultramafic sequences) are
con-cerned (Yibas, 2000). Four such belts are recognisedin 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–ultra-mafic belts of
Adola has long been recognised(Kazmin, 1975; Berhe, 1990), this has
been disputedby 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 ofamphibolitic and tonalitic
samples from the Gelebaarea, favoured late Proterozoic island-arc
magmatismfor the formation of the Adola mafic
rocks.Yibas(1993)showed, however, that the mafic–ultramafic
as-semblage of Adola is a remnant of a supra-subductionzone (SSZ)
ophiolitic sequence based on the associa-tion of boninites,
island-arc and MORB-type tholei-ites.Woldehaimanot and Behrmann
(1995)andWoldeet al. (1996)also recognised the presence of
boniniticmagmatism and, hence, the SSZ tectonic setting.Worku
(1996), however, argued that the metabasitesof the Megado Belt do
not classify unequivocally aseither SSZ or MORB ophiolites.
Woldehaimanot and Behrmann (1995)comparedthe geochemistry of the
Kenticha metabasites toN-MORB geochemical characteristics and
suggestedthat these rocks represented a back-arc setting;Worku
(1996)interpreted the Kenticha mafic rocks asintra-oceanic
calc-alkaline and tholeiitic-arc basaltsformed in a fore-arc SSZ
setting.
The only published geochemical work for theMoyale-El Kur mafic
rocks is that ofAlene and Barker(1997), who interpreted geochemical
characteristicsas indicative of tholeiitic island-arc and/or
ocean-ridgebasalts, and showed alkalic features that they
thoughtwere not consistent with a definite tectonic setting.
The objective of this work is to compare the geo-chemistry of
the mafic suites from the ophiolitic beltsof southern Ethiopia,
with the aim of inferring thepalaeotectonic setting of these belts
within the con-text of the geodynamic evolution of the East
AfricanOrogen.
2. Descriptions of the mafic rocks
2.1. Megado mafic rocks
The Megado Belt occurs in the western part ofthe 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
afterWorku and Schandelmeier (1996)and Shackleton (1996), showing
theposition of the Precambrian of southern Ethiopia within the
confines of the East African Orogen. (b) Simplified geological map
of thePrecambrian of southern Ethiopia (modified afterYibas, 2000;
Yibas et al., 2002).
-
160 B. Yibas et al. / Precambrian Research 121 (2003)
157–183
Fig. 1. (Continued).
two granite–gneiss blocks of the Adola granite–gneisscomplex
(Fig. 1b). Mafic and ultramafic rocks,psammitic-pelitic schists,
subordinate graphite-schistand metagreywacke are the dominant
lithologies ofthis belt. Amphibole-schists, amphibolites, and
meta-
gabbros are the dominant mafic rocks. Amphibole-chlorite schists
are fine- to medium-grained and rangefrom amphibole-dominated to
amphibole-chloriteschists. In the southeast of Digati village,
deformedpillow structures are exposed (Yibas, 1993, and
-
B. Yibas et al. / Precambrian Research 121 (2003) 157–183
161
references therein). Representative mineral com-positions of the
basic schists include: (1) actino-lite/hornblende (rarely
tremolite), albite-oligoclase,epidote-clinozoisite, chlorite,
quartz; (2) actinolite-tremolite/actinolite-hornblende,
albite-oligoclase, epi-dote-clinozoisite, Fe-chlorite, rarely
Mg-chlorite,+/− quartz and opaque minerals; and (3)
actino-lite/hornblende, albite-oligoclase,
epidote-clinozoisite,quartz and chlorite (Yibas, 2000).
Lensoidal outcrops of metagabbroic rocks of highertopographic
relief within the dominant amphibole-schists and amphibolitic
bodies are common. Theyare medium- to coarse-grained and massive,
and, inplaces, contain pseudomorphs after plagioclase andpyroxene
that are replaced by metamorphic minerals.These rocks are commonly
exposed in the centralpart of the belt and, locally, preserve their
plutonictexture, both in outcrop and at the microscopic
scale.Hornblende, actinolite, albite, epidote, chlorite, al-mandine
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 ul-tramafic rocks
(serpentinite, talc-tremolite and talc-anthophyllite schists),
mafic rocks, staurolite- andsillimanite-bearing biotite-schists,
and minor occur-rences of Fe–Mn quartzites, marbles and
siliceousmetapelites. The Kenticha mafic rocks include
amphi-bolites, epidote-amphibole gneisses and amphibole-schists.
Most commonly they occur sandwichedbetween ultramafic bodies as
discontinuous bandsinterlayered with metasediments. The mafic rocks
arecomposed of actinolite-hornblende, plagioclase, epi-dote and
quartz, with subordinate amounts of apatiteand opaques (most
commonly sulphides).
2.3. Bulbul mafic rocks
The Bulbul Belt occurs in the easternmost part ofthe Precambrian
of southern Ethiopia (Fig. 1b). Themain rock types that constitute
the Bulbul Belt areamphibolites, chlorite-schists, metagabbros and
ultra-mafic rocks. Slices of granitoid gneisses and
dioriticmylonites are tectonically interlayered with the maficrocks
in the western part of the belt (Yibas, 2000;Yibas et al.,
2002).
2.4. Moyale-El Kur mafic rocks
The Moyale-El Kur Belt covers the area around thetown of Moyale
close to the Ethio-Kenyan border andextends southward into Kenya.
It is subdivided intothe Moyale and the Jimma-El Kur sub-belts,
whichare separated by the Roukka shear zone (Fig. 1b).The main
lithologic types in the Moyale sub-beltare metabasic rocks,
metaultramafics, and minormetasediments (Walsh, 1972; Tolessa et
al., 1991;Alene and Barker, 1993; Yibas, 2000). The maficrocks of
the Moyale-El Kur Belt are mainly amphi-bolites, with local
occurrences of amphibole gneisses,amphibole-chlorite schists and
metagabbro. In places,they occur intercalated with metasediments,
suchas graphitic schist and quartz-feldspar schists andgneisses
(Yibas, 2000; Yibas et al., 2002).
In the Moyale sub-belt, the mafic rocks show vari-able texture
and fabric from amphibolite/amphibolegneiss to fine-grained basic
schists, with lenticularbodies of metagabbro, although amphibolite
and am-phibole gneiss represent the largest proportion of
theserocks. In the western part, the amphibolite/amphibolegneiss
units are in tectonic contact with the Moyalegranite–gneiss
complex. In the east, they grade intoamphibolites and basic
schists, often with garnet por-phyroblasts that may be up to a
centimetre in diameter.Low-relief outcrops of amphibolites are most
abun-dant in the eastern part of the Moyale sub-belt, wherethey
occur intercalated with ultramafic rocks. Elon-gated pillow-like
blocks found within the altered basicschists suggest a possible
submarine extrusion.
In the eastern part, massive sulphide-bearing blocksof
amphibolite occur overlying foliated amphibole-schists. Further to
the southeast, amphibole gneissesoccur intercalated with
amphibole-schists. Less fo-liated, granular, gabbroic to dioritic
rocks underlienon-foliated (massive) amphibolite. The
successionshows layering from coarse-grained gabbroic rockto
well-foliated, fine- to medium-grained amphibo-lites (metabasalts).
Migmatitic amphibole gneissesoccur close to the Moyale granodiorite
(Yibas, 2000).Metagabbroic rocks also occur occasionally as
cir-cular to elliptical bodies with intrusive relationshipswith the
surrounding amphibolites of eastern Moyale.
In the northern part of the Jimma-El Kur sub-belt,around 4◦N
latitude, amphibolites and amphibole-schists are intercalated with
varying proportions of
-
162 B. Yibas et al. / Precambrian Research 121 (2003)
157–183
metasediments (quartzites) and subordinate ultra-mafic rocks.
Extensive outcrops of amphibolite andgabbro-amphibolite form a
ridge near the water wellat Jimma village.
The metamorphic signature of the mafic suites in thePrecambrian
of southern Ethiopia, as seen in the min-eral assemblages of these
rocks, indicates metamor-phic grade not exceeding the
greenschist–amphibolitetransition facies ofTurner (1981). Locally,
however,mid-amphibolite facies metamorphism is evidentwhere 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 Depart-ment of
Geology, University of the Witwatersrand,Johannesburg, South Africa
and in the Central Labora-tory of the Ethiopian Institute of
Geological Surveys,Addis Ababa, Ethiopia. Samples were milled
usingchrome-steel discs in a rotary mill. Major and trace el-ements
were analysed by XRF on fused glass discs andpowder pellets at the
University of the Witwatersrand.Precision and accuracy (as
determined by standardand duplicate sample analysis) are similar at
(in wt.%)about 0.4 for SiO2, 0.03 for TiO2, 0.2 for Al2O3 andMgO,
0.1 for Fe2O3, CaO, and K2O, 0.01 for MnOand P2O5, and 0.3 for
Na2O. 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
highconcentrations, respectively). All other trace elementswere
determined by instrumental neutron activationanalysis (INAA) at the
Institute of Geochemistry,University of Vienna, using methods
described byKoeberl et al. (1987)andKoeberl (1993). Represen-tative
major and trace element analyses are listed inTable 1.
3.2. General geochemical characteristics androck
classification
Determining the original chemistry of a metamor-phosed rock is
rendered possible with the combineduse of both major and trace
elements. The low-grademafic suites of southern Ethiopia show no
signif-
Fig. 2. Plot of the Precambrian metabasic rocks of
southernEthiopia to investigate the possible effect of alteration
on primarychemical compositions (afterHughes, 1973).
icant alteration, although silica veins are commonand clearly
defined alteration zones may be asso-ciated with shearing. Samples
of the various basicrocks were collected away from alteration
zones,which may be associated with gold mineralisation(e.g. in the
Moyale area and in the Mi-essa Ridgesouth of Digati village in the
Megado Belt). Thegeochemical data (Table 1) were screened to
deter-mine whether alteration due to metamorphism and/orweathering
might have affected the original chem-istry of the rocks. Plotting
of the alkali elements in a(K2O+Na2O) versus (K2O/(K2O+Na2O)×100)
di-agram (Fig. 2), together with the concentration rangesof the
major element oxides (Table 1), indicates thatchemical
heterogeneity due to alteration is minimalfor most of the sample
groups. However, some datafalling outside of the igneous field
(Fig. 2) werediscarded.
Although the range of SiO2 content in the maficrocks under
discussion is rather narrow to see distinctvariation trends in
variation diagrams, most majorelements plotted against SiO2 (not
shown here) re-veal systematic negative correlation (e.g. TiO2,
MgO,MnO, CaO, Fe2O3T), which can be explained by dif-ferentiation.
Likewise, the trace elements V, Cr, Coand Ni show negative
correlation, whereas Sr, Rb, Zr,
-
B.
Yib
as
et
al./P
reca
mb
rian
Re
sea
rch1
21
(20
03
)1
57
–1
83
163
Table 1Representative major, minor and trace elements of the
mafic rocks of the Precambrian of southern Ethiopia: major element
data (in wt.%) by XRF
-
164B
.Y
iba
se
ta
l./Pre
cam
bria
nR
ese
arch
12
1(2
00
3)
15
7–
18
3
Table 1 (Continued)
-
B.
Yib
as
et
al./P
reca
mb
rian
Re
sea
rch1
21
(20
03
)1
57
–1
83
165
-
166B
.Y
iba
se
ta
l./Pre
cam
bria
nR
ese
arch
12
1(2
00
3)
15
7–
18
3
Table 1 (Continued)
-
B.
Yib
as
et
al./P
reca
mb
rian
Re
sea
rch1
21
(20
03
)1
57
–1
83
167
LOI, loss on ignition at 1100◦C; all Fe as Fe2O3; trace elements
(in ppm); V, Cu, Y, and Nb by XRF: Ni, Zr, and Sr: INA and XRF; all
other data by INAA; (∗) indicatesbelow detection limit; nd, not
determined.
-
168 B. Yibas et al. / Precambrian Research 121 (2003)
157–183
Nb, Y and the REEs show positive correlation withSiO2. Most of
the mobile elements such as Rb and Srshow a wider scatter.
With the exception of the Bulbul rocks and a fewsamples from the
Moyale-El Kur belt, which straddlethe alkaline and subalkaline
boundaries on the alka-line versus subalkaline discrimination
diagrams, mostsamples exhibit subalkaline affinity. These
samplescan further be subdivided into low-K, low-Ti tholei-itic
basalts (LOTI), and calc-alkaline andesitic basaltsand andesites on
AFM and FeO/MgO versus SiO2diagrams (not shown here). Most of the
calc-alkalinesamples show boninitic affinity in that they havehigh
SiO2 (>50 wt.%), high MgO (>7 wt.%), andlow-TiO2 (see
alsoFigs. 3 and 4). Yibas (1993),Woldehaimanot and Behrmann (1995),
and Woldeet al. (1996)recorded the presence of boninites in
theMegado Belt. However, the presence of boninites inthe Moyale-El
Kur Belt and amongst the Geleba maficrocks has not been reported to
date (compareGichile,1991; Gichile and Fayson, 1993; Alene and
Barker,1997).
On variation diagrams of elements plotted againstMgO (Fig. 3a),
SiO2, TiO2 and P2O5 show strongnegative correlation with MgO,
whereas CaO, totalFeO, and MnO show positive correlation within
eachmafic suite. Na2O, K2O and Al2O3 show slight nega-tive
correlation with MgO for most samples. Zr, Nb,Y, V and Sr show
negative correlation, whereas el-ements such as Cr and Ni show
positive correlation(Fig. 3a). Plots of major elements against MgO
revealhow much of the original chemistry of the metamor-phosed
mafic rocks is affected. This is because MgOis an important
component of the solid phases in equi-librium with mafic melts and
shows a great deal ofvariation, either as a consequence of the
breakdownof magnesian phases during partial melting, or be-cause of
their removal during fractional crystallisation(Rollinson, 1993).
TiO2 and P2O5 and, to a lesser ex-tent, MnO, K2O and SiO2 show
positive correlationwith Zr, whereas MgO and CaO show negative
cor-relation (Fig. 3b). This negative correlation with Zris
consistent with early crystallising minerals such asolivine,
pyroxene and Ca-plagioclase forming duringmagma differentiation.
The remaining oxides show awide scatter and poorly defined trends
that suggestpossible 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 maficsuites of
southern Ethiopia can be summarised as anincrease in Ti, Zr, P, Y,
V, Zn with decreasing MgO,Ni, Cr, V, and Sr (Fig. 3). Fractional
crystallisationof mineral phases such as clinopyroxene,
plagioclase,olivine and possibly, orthopyroxene, could explainthese
chemical characteristics. Low-P2O5, low-Zr andan increase of Ti
with increasing Zr further corroboratethe dominance of tholeiitic
basaltic rocks. Ti, Zr, Nb,Y and REEs are among the elements
considered rela-tively immobile and they are, thus, commonly used
toinvestigate types of protolithic magma, degree of
dif-ferentiation and possible tectonic setting (e.g.Pearceand
Norry, 1979; Winchester and Floyd, 1977).
3.3. REE geochemistry and spider diagrams
The possibility that REE patterns might be affectedby sea-floor
alteration and low-grade metamorphismhas been discussed byPearce
and Cann (1973), Pearce(1975), Frey (1983), Frey and Green
(1974)andFreyet al. (1978). Plotting of the REE abundances
againstZr is one of the most reliable tests to detect possi-ble
alteration and effects of metamorphism on theREE geochemistry (Fig.
4). These plots show thatthe REE abundances for these sample suites
showa systematic increase with increase of Zr concentra-tions,
which suggests that the overall REE patternshave not changed
significantly by alteration andmetamorphism.
As tholeiitic and boninitic rocks are recognised inthe mafic
suites of southern Ethiopia, the REE andspider diagram patterns of
these two rock suites havebeen treated separately to determine if
these plots alsosupport this classification (Fig. 5).
3.3.1. Tholeiitic rocks
3.3.1.1. Megado tholeiites.The Megado tholeiiticrocks display
two distinct REE patterns (Fig. 5aand c), both of which show
overall REE concen-trations that are higher than those of the
boniniticseries. Group 1 tholeiites show strong LREE deple-tion
((La/Sm)N = 0.4–0.7) compared to averagetholeiitic MORB and N-MORB
rocks and flat HREEpatterns ((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 inwt.%, trace element data in ppm.
-
170 B. Yibas et al. / Precambrian Research 121 (2003)
157–183
Fig. 3. (Continued).
-
B. Yibas et al. / Precambrian Research 121 (2003) 157–183
171
Fig. 4. Zr vs. rare earth elements (data in ppm) in the
Precambrian metabasic rocks of southern Ethiopia. Symbols as inFig.
3a.
-
172 B. Yibas et al. / Precambrian Research 121 (2003)
157–183
Fig. 5. Chondrite-normalised REE patterns (normalisation factors
fromNakamura, 1974) and spider diagram plots (normalisation data
fromPearce, 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 forN-MORB and tholeiitic MORB (fromNakamura, 1974)
are included for comparison.
-
B. Yibas et al. / Precambrian Research 121 (2003) 157–183
173
(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 con-centrations also increase with increasing
differenti-ation. The overall patterns of the Megado
tholeiitesresemble the island-arc tholeiites (IATs) of the
SouthSandwich Islands (Hawkesworth et al., 1977). Twogabbroic
samples show a marked positive Eu anomalyand lower total REE values
than the amphibolites.The relatively stronger positive Eu anomaly
observedin the metagabbroic samples could be ascribed to
acombination of the degree of Eu fractionation andthe amount of
cumulus plagioclase in the magma.The behaviour of Eu in the
tholeiites of SouthSandwich also varies from negative to positive
inthe high- and low-total REE samples, respectively,which is
attributed to a protracted fractionation his-tory (low-pressure
fractionation of plagioclase andolivine) (Hawkesworth et al.,
1977). These authorsfurther argued that the source region is
enrichedto some degree in K and Rb, relative to abyssaltholeiites,
and with a significant component of sub-ducted oceanic crust. The
overall REE pattern of theMegado Group 1 tholeiites ((La/Yb)N =
0.24–0.38)is also similar to patterns of evolved tholeiites fromNew
Caledonia, which are associated with boninites(Cameron, 1989).
However, the Megado Group 1tholeiites and the Moyale tholeiites
have lower REEvalues. In addition to the similarity in the REE
pat-terns, these rocks also have very low abundances ofTi, P, and
Zr and somewhat higher Cr and Ni values,similar to lavas from Guam
(Cameron, 1989, andreferences therein). REE patterns similar to
those ofthe Megado tholeiite-2 have also been reported fromthe
mafic rocks of the Fawakhir ophiolite (El-Sayedet al., 1999) and
the Abu Zawal gabbroic intrusionof the central Eastern Desert of
Egypt (Abu El-Ela,1996).
Although they have many features in common, thespider diagrams
for the samples from the Megadotholeiites can also be subdivided
into two groups(Fig. 5b and d). Group 1 tholeiites lack the
troughsat Y when compared with Group 2 tholeiites, whichalso show
higher Cr concentrations than the Group 1tholeiites.
3.3.1.2. Moyale tholeiites.The Moyale tholeiiticrocks display
two distinct groups of REE patterns
(Fig. 5e). The amphibolites and amphibole-chloriteschists have
relatively higher overall REE and slightlyenriched LREE patterns,
and flat to depleted HREEpatterns ((Tb/Yb)N = 0.88–2.00). The REE
pat-terns for the metagabbros from the eastern part ofthe Moyale-El
Kur Belt are characterised by strongpositive Eu anomalies, with
lower REE abundancesthan those of the amphibolites and
amphibole-chloriteschists. They also show a slight LREE
enrichmentand HREE depletion.
The REE patterns of the Moyale tholeiites are moreakin to
back-arc tholeiites such as those occurring inthe Scotia Sea Rise
and the Mariana Basin (Hart et al.,1972; Hawkesworth et al., 1977).
The spider diagrampatterns (Fig. 5f) for the Moyale tholeiites also
dif-ferentiate the metagabbros from the amphibolites andschists
(metabasalts).
The most distinctive features exhibited by the spiderdiagrams of
the LOTI are the selective enrichmentsof certain elements (Sr, Ba)
and the relative lack ofenrichment of others (P, Zr, Ti, Ce, Sm±
Y). Thesepatterns and the HFS element variations exhibited bythe
tholeiitic rocks are characteristic of a SSZ setting,where
boninitic and tholeiitic magma mixing couldoccur (Pearce et al.,
1984).
3.3.2. Calc-alkaline rocksMinor occurrences of calc-alkaline
lavas have
been recognised amongst these mafic suites, withthe exception of
the Megado suite. The REE pat-terns of these rocks are strongly
LREE-enriched andHREE-depleted (Fig. 5g).
The calc-alkaline rocks also show overall highertrace element
abundances compared to the Moyaletholeiites (Fig. 5h) and display
strongly differentiatedpatterns when compared to the tholeiitic
rocks. Thesepatterns are similar to those of the high-K
calc-alkalineSSZ basalts from the New Hebrides (Pearce et
al.,1984).
3.3.3. BoninitesThe Megado boninites display flat patterns or
may
have slight LREE enrichment and flat HREE patterns,with
variability in Eu behaviour (Fig. 6a). The sam-ples with strongly
positive Eu anomalies are boniniticmetagabbros.
The boninites from Geleba and Moyale displayshallow 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
fromNakamura, 1974) and spider diagrams (normalisation data
fromPearce, 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-oceanicridge 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-arcbasalt, 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 boninitesfrom the Izu-Bonin
forearc site 786 (Murton et al.,1992) and boninites from New
Caledonia (Cameron,1989). LREE enrichment in boninites could
resultfrom metasomatism of their harzburgitic sources by anLREE-
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 spi-der diagrams
for boninitic rocks are variable (Fig. 6band d), which could be due
to tholeiitic and boniniticmagma mixing. Such a possibility has
been reported,at least for the Megado rocks, byWolde 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 meta-morphosed
and deformed terrane is difficult.
Thevolcano–sedimentary–ultramafic belts of southernEthiopia
suffered metamorphism up to greenschist–amphibolite transition
facies (Beraki et al., 1989;Yibas, 1993; Worku, 1996). This
undoubtedly affectedthe chemistry of, in particular, the LIL
elements (suchas K, Ba, Rb and Sr), which are highly mobile dur-ing
metamorphism. Therefore, the interpretation ofthe tectonic setting
for the region should, to a largeextent, depend on elements of high
ionic potential(Ti, Zr, Cr and Y), as these elements are
effectivelyimmobile during metamorphism (Cann, 1970).
A large portion of the samples of metabasic rocksplot, on
Ti–Zr–Y and Ti–Zr diagrams, into the fieldwhere calc-alkaline (CAB)
IAT and mid-oceanic ridgebasalts (MORB) overlap (Fig. 7a and b).
However, afew samples from the Moyale-El Kur Belt fall into theMORB
field in the Ti–Zr diagram. The mafic rocks ofthe Bulbul Belt fall
into the calc-alkaline basaltic field.
In order to differentiate samples of MORB affin-ity from those
of volcanic-arc affinity, the Ti–Cr dia-gram afterPearce (1975)may
be useful (Pharaoh andPearce, 1984). Whereas a large part of the
Moyale-ElKur 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.% silicacan be
subdivided on the basis of their MnO, TiO2 and
P2O5 concentrations into MORB, ocean-island tholei-ites (OIT),
ocean-island alkali basalts (OIA), IAT andcalc-alkali basalts (CAB)
(Mullen, 1983). The boninitefield occupies the MnO-rich sector of
the CAB field.The majority of the samples from the study area
fallinto the IAT and CAB fields but lie close to the MnOapex. A few
samples from the Moyale-El Kur Beltshow MORB affinity. The Bulbul
mafic rocks strad-dle the boundary between the CAB and OIA
fields(Fig. 7d).
On the Ti–V discrimination diagram (Shervais,1982), the Megado
tholeiites and associated boninitesfall into the fields of oceanic
island (OIB) and back-arcbasalts (BAB), but plot at the lower left
corner of thediagram due to their very low Ti and V values.
Theboninites of the Moyale-El Kur Belt and most Gelebasamples fall
into the VAT field due to their higher Vvalues relative to the
Megado rocks (Fig. 8e). Fur-thermore, the Moyale tholeiites are
discriminated inthis diagram as VAT and BAB-MORB rocks.
The Cr–Y diagram (Pearce, 1982) is also impor-tant for the
discrimination of IAT from MORB rocks(e.g.Pearce and Gale, 1977;
Garcia, 1978; Bloxhamand Lewis, 1972). A Cr–Y plot discriminates
effec-tively between MORB and volcanic-arc basalts. Inaddition,
this diagram has been used to discriminatebetween different
marginal basin rocks.Pearce et al.(1984)tested its usefulness in
discriminating betweentrue oceanic floor basalts and those of
marginal basinorigin (i.e. fore-arc and back-arc basins). Most
sam-ples from back-arc basins fall into the MORB field,
-
B. Yibas et al. / Precambrian Research 121 (2003) 157–183
177
Fig. 8. Geodynamic evolution of the Precambrian of southern
Ethiopia during the East African Orogeny (900–500 Ma).
-
178 B. Yibas et al. / Precambrian Research 121 (2003)
157–183
whereas those from fore-arc basins (FAB), such as theMariana
FAB, plot into or to the left of the IAT field.Moreover, those
samples which fall into the FAB fieldexhibit boninitic
characteristics, and the geochemicalcharacteristic of these basalts
represent the best ana-logues for SSZ ophiolites. Over 90% of the
data frommafic rocks of southern Ethiopia fall into the IATfield,
where FAB basalts overlap (Fig. 7f). Only a fewsamples from the
Moyale-El Kur Belt fall into theBAB-MORB field.
Based on these tectonic discrimination diagrams,the Megado and
Geleba mafic rocks and the boniniticrocks of Moyale-Jimma-El Kur
consistently showIAT-FAB affinity, unlike the Moyale-El Kur
tholeiitesthat can be broadly classified into IAT-FAB and back-arc
tholeiites. The Bulbul mafic rocks consistentlyshow calc-alkaline
basaltic affinity suggestive of acontinental-arc setting. However,
high-K calc-alkalinebasalts are also known from a SSZ setting such
asthat 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 beltsof Adola has
long been recognised (Kazmin, 1975;Berhe, 1990) but has been
disputed by some work-ers. This study has shown that subalkaline,
LOTItholeiitic basalts are the most dominant mafic rocksin the
low-grade belts of the Precambrian of southernEthiopia. These
tholeiites have low P2O5 and Zr val-ues and positive correlation
between Ti and Zr. Thisstudy shows the occurrence of boninites also
in theMoyale-El Kur Belt and in the Geleba area, in addi-tion to
their occurrence in the northern and centralparts of the Megado
Belt.
The mafic rocks of the Bulbul Belt consistently ex-hibit
high-Ti, high-K, calc-alkaline basaltic compo-sition, in contrast
to the mafic rocks of the Megadoand Moyale-Jimma El Kur belts. A
few samples fromthe Geleba and Moyale-Jimma-El Kur Belt also
showhigh-K, high-Ti, calc-alkaline geochemistry. Similarhigh-K,
high-Ti, calc-alkaline basalts have also beenreported from the
Kenticha Belt (Worku, 1996).
The tholeiitic rocks from the Megado and Moyale-El Kur belts
also have low-P2O5 and -Zr values and
a positive correlation between Ti and Zr. The Megadotholeiites
show REE patterns similar to arc-tholeiitesof the Southern Sandwich
islands, whereas the Moyaletholeiites show patterns similar to
back-arc tholeiitesof the Scotia Sea Rise (Hawkesworth et al.,
1977).The most distinctive features exhibited in the spiderdiagrams
of the LOTI tholeiites include the selec-tive 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
thoseof oceanic basalts from SSZ settings (Pearce et al.,1984).
The boninites of southern Ethiopia occur togetherwith IATs and
are geochemically more akin to tholei-ites and boninites from known
marginal basins, suchas the Mariana fore-arc basin (Pearce et al.,
1984).According to the boninite classification ofCrawfordet al.
(1989), the Megado boninites are dominantlyhigh-Ca boninites (SiO2
< 56 wt.%; CaO/Al2O3 >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% ofthe Moyale boninites exhibit
similarities to high-Caboninites and the rest is characterised by
high CaOcontents (>12.5 wt.%), FeOT between 7 and 9
wt.%,CaO/Al2O3 > 0.7, and total alkali element contents
-
B. Yibas et al. / Precambrian Research 121 (2003) 157–183
179
to the Megado and Moyale mafic suites. The spiderdiagrams of
these calc-alkaline rocks show similaritywith those of high-K
calc-alkaline SSZ basalts formthe New Hebrides (Pearce et al.,
1984).
4.2. The geodynamic evolution of the Precambrianof southern
Ethiopia
Tibetan-style continent–continent collision be-tween West
Gondwana (Archaean Tanzanian Craton)and East Gondwana is thought to
have been re-sponsible for the formation of the Mozambique
Belt(Burke and Sengör, 1986; Shackleton, 1986; Keyet al., 1989;
Berhe, 1990). Given the position of thePrecambrian geology of
southern Ethiopia betweenthe low-metamorphic grade Arabian-Nubian
Shieldand the high-metamorphic grade Mozambique Belt(which together
form the East African Orogen,Stern,1993, 1994), an understanding of
the geodynamicevolution of this region will help to better
under-stand the evolution of the East African Orogen as awhole.
Based on U–Pb single zircon SHRIMP and laserprobe40Ar–39Ar ages
and field studies (Yibas, 2000;Yibas et al., 2000b), as well as
earlier geochronologi-cal data,Yibas et al. (2002)classified the
granitoidsof the East African Orogen in southern Ethiopia intoseven
generations (Table 2). Based on their geochem-ical characteristics,
these granitoids are classified intovolcanic-arc and within-plate
granitoids, with domi-nance of volcanic-arc granitoids (Yibas,
2000; Yibaset al., 2000c). Combination of the geochronologicaland
geochemical criteria allowed these authors to fur-ther suggest that
the granitoids of southern Ethiopiashow alternation of within-plate
and volcanic-arcgranitic magmatism, suggesting repeated
compres-sional and extensional tectonic regimes during
thedevelopment of the East African Orogen between 900and 500 Ma
ago.
The geochronological order in which the differentophiolites were
formed in southern Ethiopia has be-come clearer as a result of
recent geochronologicalwork. The 700 Ma U–Pb zircon age obtained
for theamphibolitic rocks from the Moyale ophiolitic fold andthrust
belt byTeklay et al. (1998)has been interpretedas an approximation
of their formation age. The for-mation age of the Megado ophiolite
is approximately789±36 Ma (Sm–Nd whole-rock isochron age for
the
Megado metavolcanics,Worku, 1996). The 876±5 Maage (U–Pb zircon
SHRIMP age,Yibas, 2000; Yibaset al., 2000b, 2002) obtained for the
Bulbul dioriticmylonite gneiss from the Bulbul Belt implies the
pres-ence of an early- or pre-Pan African ocean in southernEthiopia
(Yibas, 2000).
Based on the integration of geochronological, geo-chemical (both
granitic and mafic rocks), and struc-tural data (Yibas, 2000, and
references therein), thefollowing evolutionary stages are envisaged
for thetectonic evolution of the Neoproterozoic of southernEthiopia
(Fig. 8).
Stage 1 (from
-
180B
.Y
iba
se
ta
l./Pre
cam
bria
nR
ese
arch
12
1(2
00
3)
15
7–
18
3Table 2A possible geodynamic evolutionary scheme for the
Precambrian of southern Ethiopia during the East African Orogeny,
based on geochronology, geochemistry and deformationof granitoids
(Yibas, 2000; Yibas et al., 2002)
Granitic phases (ages) Dated granites Age (Ma) Associated
deformation(possible tectonic scenario)
Zircons U–Pb 40Ar–39Ar laser
Gt7 (550–500 Ma) Metoarbasebat granite 526± 5� 506 ± 4 (Bt);511
± 3 (Hb)
Post-orogenic cooling and transcurrentfaulting
Berguda charnockitic granite 528± 8.4 (rim)�;538 ± 3 (core)�
Lega Dima granite 550± 18�Robele granite 554± 23ε
Gt6 (550–600 Ma) Wadera foliated granite 576± 5� Sinistral
transpressional (shear zones) dueto oblique-collision (docking)
Wadera megacrystic diorite gneiss 579± 5�Digati dioritic gneiss
570± 5� 502 ± 4 (Bt)
Gt5 (700–600 Ma) Burjiji granitic massif ∼602� Subduction
(closure of the Moyalemarginal basin)
Meleka foliated granodiorite 610± 9� 512 ± 4 (Bt);515 ± 4
(Mu)
Gariboro granite ∼646�Moyale granodiorite 666± 5�
Gt4 (720–700 Ma) Finchaa biotite-foliated granite 708± 5ε
Transition from compressional deformationto an extensional regime
prior to theformation of the Moyale basin
Yabello charnockitic granite–gneiss 716± 5�
Gt3 (770–720 Ma) Alghe granite–gneiss 722± 2� Subduction-related
granitic magmatismassociated with the closure of the
Megadobasin
Sagan basic charnockite 725�
Zembaba granite–gneiss 756± 6�Sebeto tonalite gneiss 765± 3ε
Gt2 (>770 Ma) Melka Guba megacrysticgranodiorite gneiss
778 ± 23� Extensional granitic magmatism associatedwith the
opening of the Megado marginalbasin
Gt1 (>880 Ma) Bulbul diorite mylonite 876± 7� 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 zirconevaporation
method). Hb, hornblende; Bt, biotite; Mu, muscovite.
-
B. Yibas et al. / Precambrian Research 121 (2003) 157–183
181
and charnockite formation. The presence of arc-grani-toids west
of the Megado ophiolite belt (Sebeto andAlghe granites, with U–Pb
zircon ages of 760 and722 Ma, respectively;Yibas, 2000; Yibas et
al., 2000b,2002) approximates the time of the closure of theMegado
oceanic basin.
Stage 4 (>700–660 Ma): Opening of the Moyalemarginal basin in
a fore-arc SSZ setting, which be-gan in a manner described for the
formation of theMegado basin. This was followed by subduction ofthe
Moyale basin at about 660 Ma (age of the Moyalearc-granodiorite,
SHRIMP U–Pb age,Yibas, 2000;Yibas et al., 2002).
Stage 5a (660–550 Ma): Continuation of subduc-tion-related
magmatism (e.g. Gariboro and Burjijigranitic massif, Meleka
granodiorite, Wadera mega-crystic diorite gneiss, Digati
granodiorite;Worku,1996) and transpressive deformation (Yibas,
2000).This period marks a period of oblique continent-arccollision
(docking) and accretion.
Stage 5b (550–500 Ma): Emplacement of late-to post-tectonic and
post-orogenic granitoids (e.g.Berguda charnockitic granitoid and
non-deformedgranitoids, such as the Robele, Lega Dima
andMetoarbasebat granites, all having similar zirconages, seeYibas
et al., 2002, and references therein),accompanied by thrusting and
transcurrent faultingand shearing associated with uplift and final
coolingat the end of the East African Orogeny (Yibas, 2000).
It can also be concluded that this scenario isconsistent with
the geological evolution of theArabo-Nubian Shield, in general, as,
for example, dis-cussed byKröner (1984), Kröner et al. (1991,
1992),Rieschmann et al. (1984), Stern (1993), Shackleton(1996),
andAbdel Salam and Stern (1996).
Acknowledgements
This paper resulted from the Ph.D. project by B.Yibas, which
benefited from generous financial sup-port from Anglo American
Prospecting Services,Johannesburg. Richard Holdsworth and his group
atthe Anglo American Research Laboratory (AARL),Johannesburg, are
thanked for ICP-MS trace-elementanalyses. Thorough reviews by S.
Bloomer and M.Teklay greatly improved an earlier version of
themanuscript.
References
Abdel Salam, M.G., Stern, R.J., 1996. Sutures and shear zones
inthe Arabo-Nubian Shield. J. Afr. Earth Sci. 23, 289–310.
Abu El-Ela, F.F., 1996. The petrology of the Abu Zawal
gabbroicintrusion, eastern desert Egypt: an example of an
island-arcsetting. J. Afr. Earth Sci. 22, 147–157.
Alene, M., Barker, A.J., 1993. Tectonometamorphic evolution
ofthe Moyale region, southern Ethiopia. Precambrian Res.
62,271–283.
Alene, M., Barker, A.J., 1997. Geochemistry of meta-igneousrocks
from southern Ethiopia: a new insight into Neoproterozoictectonics
of northeast Africa. J. Afr. Earth Sci. 24, 351–370.
Ayalew, T., Bell, K., Moore, J.M., Parrish, R.R., 1990. U–Pb
andRb–Sr geochronology of the western Ethiopian Shield. Geol.Soc.
Am. Bull. 102, 1309–1316.
Beraki, W.H., Bonavia, F.F., Getachew, T., Schmerold,
R.,Tarekegn, T., 1989. The Adola fold and thrust belt,
southernEthiopia: a re-examination with implications for
Pan-Africanevolution. Geol. Magn. 126, 647–657.
Berhe, S.M., 1990. Ophiolites in northeast and east
Africa:implications for Proterozoic crustal growth. J. Geol. Soc.
Lond.147, 41–57.
Bloxham, W., Lewis, A.D., 1972. Ti, Zr, and Cr in some
Britishpillow lavas and their petrogenetic affinities. Nature
(Phys. Sci.)237, 134–136.
Burke, K.C., Sengör, A.M.C., 1986. Tectonic escape in
theevolution of the continental crust. In: Baranzangi, M., Brown,L.
(Eds.), Reflection Seismology: The Continental Crust. Am.Geophys.
Union Geodyn. Ser. 14, 41–53.
Cameron, W.E., 1989. Contrasting boninites-tholeiite
associationsfrom New Caledonia. In: Crawford, A.J. (Ed.),
Boninites.Unwin Hyman, London, pp. 314–338.
Cameron, W.E., McColluch, M.T., Walker, D.E., 1983.
Boninitepetrogenesis: chemical and Nd–Sr isotopic constraints.
EarthPlanet. Sci. Lett. 65, 75–89.
Cann, J.R., 1970. Rb, Sr, Y, Zr, and Nb in some
ocean-floorbasaltic rocks. Earth Planet. Sci. Lett. 10, 7–11.
Chater, A.M., 1971. The geology of the Megado region of
southernEthiopia. Ph.D. thesis. University of Leeds, England,
UK,343 pp.
Crawford, A.J., Falloon, T.J., David, H.G., 1989.
Classification,petrogenesis and tectonic setting of boninites. In:
Crawford,A.J. (Ed.), Boninites. Unwin Hyman, London, pp. 2–44.
El-Sayed, M.M., Furnes, H., Mohamed, F.H., 1999.
Geochemicalconstraints on the tectonomagmatic evolution of the
latePrecambrian Fawkhir ophiolite, central eastern desert, Egypt.J.
Afr. Earth Sci. 29, 515–533.
Frey, F.A., 1983. Rare earth element abundances in uppermantle
rocks. In: Henderson, P. (Ed.), Rare Earth ElementGeochemistry.
Elsevier, Amsterdam, pp. 256–259.
Frey, F.A., Green, D.H., 1974. The mineralogy, geochemistry
andorigin of lherzolite inclusions in Victorian basanites.
Geochim.Cosmochim. Acta 38, 1023–1059.
Frey, F.A., Green, D.H., Roy, S.D., 1978. Integrated models
ofbasalt petrogenesis: a study of quartz tholeiites to
olivinemelilites from southeastern Australia utilising geochemical
andexperimental petrological data. J. Petrol. 19, 463–513.
-
182 B. Yibas et al. / Precambrian Research 121 (2003)
157–183
Garcia, M.O., 1978. Criteria for the identification of
ancientvolcanic areas. Earth Planet. Sci. Lett. 14, 147–165.
Genzebu, W., Hassen, N., Yemane, T., 1994. Geology of the
AgereMariam area, vol. 8. Ethiopian Inst. Geol. Surv., Addis
Ababa,Memoir, 23 pp.
Gichile, S., 1991. Structure, metamorphism and tectonic setting
of agneissic terrane, the Sagan-Aflata area, southern Ethiopia.
M.Sc.thesis (unpublished). University of Ottawa, Canada, 224
pp.
Gichile, S., Fayson, W.K., 1993. An inference of the
tectonicsetting of the Adola Belt of southern Ethiopia from
thegeochemistry of magmatic rocks. J. Afr. Earth Sci. 16,
235–246.
Hart, S.R., Glassley, W.E., Karig, D.E., 1972. Basalts and
sea-floorspreading behind the Mariana island arc. Earth Planet.
Sci. Lett.15, 345–361.
Hawkesworth, C.J., O’Nions, R.K., Pankhurst, R.J., Hamilton,
P.J.,Evensen, N.M., 1977. A geochemical study of island-arc
andback-arc tholeiites from Scotia Sea. Earth Planet. Sci. Lett.
36,253–262.
Hickey-Vargas, R., 1989. Boninites and tholeiites from DSDPSite
458, Mariana fore-arc. In: Crawford, A.J. (Ed.), Boninites.Unwin
Hyman, London, pp. 340–354.
Hickey, R.L., Frey, F.A., 1981. Rare earth element
geochemistryof Mariana fore-arc volcanics: Deep Sea Drilling
Project Site458 and Hole 459B. Init. Rep. DSDP Leg 60, 735–742.
Hughes, C.J., 1973. Spilites, keratophyres, and the
igneousspectrum. Geol. Magn. 109, 513–527.
Jenner, G.A., 1981. Geochemistry of high-Mg andesites from
CapeVogel, Papua, New Guinea. Chem. Geol. 33, 307–332.
Kazmin, V., 1975. The Precambrian of Ethiopia and some aspectsof
the Mozambique Belt. Bull. Geophys. Obs., Addis AbabaUniv. 15,
27–45.
Kazmin, V., Alemu, S., Tilahun, B., 1978. The Ethiopian
basement:stratigraphy and possible manner of evolution. Geol.
Rdsch.67, 531–546.
Key, R.M., Charsley, T.J., Hackman, B.D., Wilkinson,
A.F.,Rundle, C.C., 1989. Superimposed upper Proterozoic
collision-controlled orogenesis in the Mozambique Belt of
Kenya.Precambrian Res. 44, 197–225.
Koeberl, C., 1993. Instrumental neutron activation analysis
ofgeochemical and cosmochemical samples: a fast and provenmethod
for sample analysis. J. Radioanal. Nucl. Chem. 168,47–60.
Koeberl, C., Kluger, F., Kiesl, W., 1987. Rare earth
elementdeterminations at ultra-trace abundance levels in
geologicmaterials. J. Radioanal. Nucl. Chem. 112, 482–487.
Kröner, A., 1984. Late Precambrian plate tectonics and orogeny:a
need to redefine the term Pan-African. In: Klerkx, J., Michot,J.
(Eds.), African Geology. Musée R. de l’Afrique Centrale,Tervuren,
pp. 23–28.
Kröner, A., Linnebacher, P., Stern, R.J., Reischmann, T.,
Manton,W., Hussien, I.M., 1991. Evolution of Pan-African
island-arcassemblages in the southern Red Sea Hills, Sudan, and
insouthwestern Arabia, as exemplified by geochemistry
andgeochronology. In: Stern, R.J., Van Schmus, W.R. (Eds.),Crustal
Evolution in the Late Proterozoic. Precambrian Res.53, 99–118.
Kröner, A., Pallister, J.S., Fleck, R.J., 1992. Age of initial
oceanicmagmatism in the late Proterozoic Arabian Shield. Geology
20,803–806.
Mullen, E.D., 1983. MnO/TiO2/P2O5—a minor element discrimi-nant
for basaltic rocks of oceanic environments and implicationfor
petrogenesis. Earth Planet. Sci. Lett. 62, 53–62.
Murton, B.J., Peate, D.W., Arculus, J.R., Pearce, J.A., Van
derLaan, S., 1992. Trace element geochemistry of the volcanicrocks
from site 786: the Izu-Bonin forearc. In: Freyer, P., Pearce,J.A.,
Stockking, L.B. (Eds.), Proceedings of the Ocean DrillingProgram,
Scientific Results, vol. 125, pp. 211–235.
Nakamura, N., 1974. Determination of REE, Ba, Fe, Mg, Na, and
Kin carbonaceous and ordinary chondrite. Geochim. Cosmochim.Acta
38, 757–775.
Nelson, D.R., Crawford, A.J., McCulloch, M.T., 1984.
Nd–Srisotope and geochemical systematics in Cambrian boninites
andtholeiites from Victoria, Australia. Contrib. Mineral Petrol.
88,164–172.
Pearce, J.A., 1975. Basalt geochemistry to investigate past
tectonicenvironments on Cyprus. Tectonophysics 25, 41–67.
Pearce, J.A., 1982. Trace element characteristics of lavas
fromdestructive plate boundaries. In: Thorpe, R.S. (Ed.),
Andesites.Unwin Hyman, London, pp. 525–548.
Pearce, J.A., 1983. Role of the sub-continental lithosphere
inmagma genesis at active continental margins. In:
Hawkesworth,C.J., Norry, M.J. (Eds.), Continental Basalts and
MantleXenoliths. Shiva, Nantwich, pp. 230–249.
Pearce, J.A., Cann, J.R., 1973. Tectonic setting of basic
volcanicrocks determined using trace element analyses. Earth
Planet.Sci. Lett. 19, 290–300.
Pearce, J.A., Gale, G.H., 1977. Identification of ore
depositenvironment from trace element geochemistry of
associatedigneous host rocks. Geol. Soc. Lond. Spec. Publ. 7,
14–24.
Pearce, J.A., Lippard, S.J., Roberts, S., 1984. Characteristics
andtectonic significance of supra-subduction zone ophiolites.
In:Kokiaar, B.P., Howells, M.F. (Eds.), Marginal Basin
Geology.Geol. Soc. Lond. Spec. Publ. 16, 77–94.
Pearce, J.A., Norry, M.J., 1979. Petrogenetic implications of
Ti,Zr, Y, and Nb variations in volcanic rocks. Contrib.
MineralPetrol. 69, 33–47.
Pharaoh, T.C., Pearce, J.A., 1984. Geochemical evidence for
thegeotectonic setting of early Proterozoic metavolcanic sequencein
Lapland. Precambrian Res. 25, 283–308.
Rieschmann, T., Kröner, A., Basahel, A., 1984.
Petrography,geochemistry, and tectonic setting of metavolcanic
sequencesfrom the Al Lith area, southwestern Arabian Shield.
In:Proceedings of the IGCP Project-164, vol. 6. Pan-AfricanCrustal
Evolution in the Arabian-Nubian Shield. Faculty ofScience, King
Abdulaziz University, Jeddah, pp. 365–379.
Rollinson, H.R., 1993. Using Geochemical Data:
Evaluation,Presentation and Interpretation. Wiley, New York, 351
pp.
Shackleton, R.M., 1986. Precambrian collision tectonics in
Africa.In: Coward, M.P., Ries, A.C. (Eds.), Collision Tectonics.
Geol.Soc. Lond., Spec. Publ. 19, 329–349.
Shackleton, R.M., 1996. The final collision zone between East
andWest Gondwana: where is it? J. Afr. Earth Sci. 23, 289–310.
Shervais, J.W., 1982. Ti–V plots and the petrogenesis of
modernand ophiolitic lavas. Earth Planet. Sci. Lett. 59,
101–118.
Stern, R.J., 1993. Tectonic evolution of the late
ProterozoicEast African Orogen: constraints from crustal evolution
of theArabo-Nubian Shield and the Mozambique Belt. In:
Thorweihe,
-
B. Yibas et al. / Precambrian Research 121 (2003) 157–183
183
U., Schandelmeier, H. (Eds.), Proceedings of the
InternationalConference Geoscientific Research in Northeast Africa,
Berlin,Germany, pp. 73–74.
Stern, R.J., 1994. Arc assembly and continental collision inthe
Neoproterozoic East African Orogen: implication for
theconsolidation of Gondwana. Ann. Rev. Earth Planet. Sci.
22,319–333.
Sun, S.S., Nesbitt, R.W., 1977. Chemical heterogeneity of
theArchean mantle, composition of the Earth and mantle
evolution.Earth Planet. Sci. Lett. 35, 429–448.
Teklay, M., Kröner, A., Mezger, K., Oberhänsli, R., 1998.
Geo-chemistry, Pb–Pb single zircon ages and Nd–Sr
isotopecomposition of Precambrian rocks from southern and
easternEthiopia: implications for crustal evolution in East Africa.
J.Afr. Earth Sci. 26, 207–227.
Tolessa, S., Bonavia, F.F., Meshesha, S., Eshete, T.,
1991.Structural pattern of Pan-African rocks around Moyale,
southernEthiopia. Precambrian Res. 52, 179–186.
Turner, F.J., 1981. Metamorphic Petrology: Mineralogical and
FieldTectonic Aspects. New York, McGraw-Hill, 541 pp.
Walsh, J., 1972. Geology of the Moyale area. Degree Sheet
14.Ministry of Natural Resources, Geological Survey of Kenya.Report
No. 89, 32 pp.
Winchester, J.A., Floyd, P.A., 1977. Geochemical discrimination
ofdifferent magma series and their differentiation products
usingimmobile elements. Chem. Geol. 20, 325–343.
Wolde, B., Asres, Z., Desta, Z., Gonzalez, J.J., 1996.
Neoprotero-zoic zirconium-depleted boninite and tholeiite series
rocks fromAdola southern Ethiopia. Precambrian Res. 80,
261–279.
Woldehaimanot, B., Behrmann, J.H., 1995. A study of
metabasitesand metagranite chemistry in the Adola Region (south
Ethiopia):implication for the evolution of the East African Orogen.
J.Afr. Earth Sci. 21, 459–476.
Worku, H., 1996. Geodynamic development of the Adola
Belt(southern Ethiopia) in the Neoproterozoic and its control
ongold mineralisation. Ph.D. thesis. Berlin Tech. Univ.,
Germany,156 pp.
Worku, H., Schandelmeier, H., 1996. Tectonic evolution of
theNeoproterozoic Adola Belt of southern Ethiopia: evidencefor
Wilson cycle process and implications for oblique platecollision.
Precambrian Res. 77, 179–210.
Worku, H., Yifa, K., 1992. The tectonic evolution of
thePrecambrian metamorphic rocks of the Adola Belt
(southernEthiopia). J. Afr. Earth Sci. 14, 37–55.
Yibas, B., 1993. The geochemistry of the metabasic rocks of
theAdola volcano–sedimentary–ultramafic assemblage: evidencefrom
supra-subduction zone (SSZ) ophiolitic sequence, Adola,southern
Ethiopia. Report, Ethiopian Institute of GeologicalSurveys, Addis
Ababa, 18 pp.
Yibas, B., 2000. The Precambrian geology, tectonic evolutionand
controls of gold mineralisations in southern Ethiopia.Ph.D. thesis
(unpublished). University of the Witwatersrand,Johannesburg, 448
pp.
Yibas, B., Reimold, W.U., Anhaeusser, C.R., 2000a. The geologyof
the Precambrian of southern Ethiopia: I. The tectono-stratigraphic
record. Economic Geology Research InstituteInformation Circular No.
344, University of the Witwatersrand,Johannesburg, 30 pp.
Yibas, B., Reimold, W.U., Armstrong, R., Phillips, D., Koeberl,
C.,2000b. The geology of the Precambrian of southern Ethiopia:II.
U–Pb single zircon SHRIMP and laser argon dating ofgranitoids.
Information Circular, Economic Geology ResearchInstitute University
of the Witwatersrand, Johannesburg, No.345, 21 pp.
Yibas, B., Reimold, W.U., Koeberl, C., Anhaeusser, C.R.,2000c.
Geochemistry of the granitoids in the Precambrian ofsouthern
Ethiopia: constraints on the tectonic regime duringthe
Neoproterozoic-early Palaeozoic (900–500 Ma). InformationCircular,
Economic Geology Research Institute, University ofthe
Witwatersrand, Johannesburg, No. 350, 28 pp.
Yibas, B., Reimold, W.U., Armstrong, R., Koeberl, C.,
Anhaeusser,C.R., Phillips, D., 2002. The tectonostratigraphy,
granitoidgeochronology and geological evolution of the Precambrian
ofsouthern Ethiopia. J. Afr. Earth Sci. 34, 57–84.
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)IntroductionDescriptions of the mafic rocksMegado mafic
rocksKenticha mafic rocksBulbul mafic rocksMoyale-El Kur mafic
rocks
GeochemistrySample preparation and chemical analysesGeneral
geochemical characteristics and rock classificationREE geochemistry
and spider diagramsTholeiitic rocksMegado tholeiitesMoyale
tholeiites
Calc-alkaline rocksBoninites
Tectonic setting
Discussion and conclusionsGeochemistry and palaeotectonic
settingThe geodynamic evolution of the Precambrian of southern
Ethiopia
AcknowledgementsReferences