geochemistry of the maﬁc rocks of the ophiolitic fold and ......precambrian research 121 (2003)...

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).


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


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


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,

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Table 1Representative major, minor and trace elements of the mafic rocks of the Precambrian of southern Ethiopia: major element data (in wt.%) by XRF

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Table 1 (Continued)

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Table 1 (Continued)

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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.


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


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.


Fig. 4. Zr vs. rare earth elements (data in ppm) in the Precambrian metabasic rocks of southern Ethiopia. Symbols as inFig. 3a.


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.


(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


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.


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).



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,


Fig. 8. Geodynamic evolution of the Precambrian of southern Ethiopia during the East African Orogeny (900–500 Ma).


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


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.


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.


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,


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

geochemistry of the maﬁc rocks of the ophiolitic fold and ......precambrian research 121 (2003)...

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