The Tectonostratigraphy, Granitoid Geochronology and Geological Evolution of the Precambrian of Southern Ethiopia B

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Journal of African Earth Sciences 34 (2002) 57–84 www.elsevier.com/locate/jafrearsci

The tectonostratigraphy, granitoid geochronology and geological evolution of the Precambrian of southern Ethiopia B. Yibas a, W.U. Reimold a,*, R. Armstrong b, C. Koeberl c, C.R. Anhaeusser a,d, D. Phillips b a Impact Cratering Research Group, School of Geosciences, University of the Witwatersrand, Private Bag 3, Wits 2050, Johannesburg 2001, South Africa b Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia c Institute of Geochemistry, University of Vienna, Althan Str. 14, A-1090 Vienna, Austria d Economic Geology Research Institute, School of Geosciences, University of the Witwatersrand, Private Bag 3, Wits 2050, Johannesburg 2001, South Africa Received 7 July 2000; accepted 1 June 2001

Abstract Two distinct tectonostratigraphic terranes, separated by repeatedly reactivated deformation zones, are recognised in the Pre- cambrian of southern Ethiopia: (1) granite-gneiss terrane, which is classified into sub-terranes and complexes, and (2) ophiolitic fold and thrust belts. The granite-gneiss terrane consists of para- and orthoquartzofeldspathic gneisses and granitoids, intercalated with amphibolites and sillimanite–kyanite-bearing schists. The paragneisses resemble gneisses from northern Kenya that were derived from sediments that filled the Kenyan sector of the ‘‘Mozambique Belt basin’’ between 1200 and 820 Ma. The volume of sediments formed during this period is comparatively small in southern Ethiopia, implying that the ‘‘Mozambique Belt basin’’ became progressively narrower northwards. The granitoid rocks in the study area vary from granitic gneisses to undeformed granites and range compositionally from diorites to granites. The granitoid gneisses form an integral part of the granite-gneiss terrane, but are rare in the ophiolitic fold and thrust belts. The ophiolitic fold and thrust belts are composed of mafic, ultramafic and metasedimentary rocks in various proportions. Undeformed granitoids are also developed in these belts. Eight granitoids from southern Ethiopia have been dated by U–Pb single zircon SHRIMP and laser probe 40Ar–39Ar dating. The SHRIMP ages range from 880 to 526 Ma, and are interpreted as close approximations of the respective magmatic emplacement ages. The 40Ar–39Ar data range from 550 to 500 Ma. The available geochronological data and field studies allowed classification of the granitoids of the Precambrian of southern Ethiopia into seven generations: Gt1 (>880 Ma); Gt2 (800–770 Ma); Gt3 (770–720 Ma); Gt4 (720–700 Ma); Gt5 (700–600 Ma); Gt6 (580–550 Ma); and Gt7 (550–500 Ma). The period 550–500 Ma (Gt7) is marked by emplacement of late- to post-tectonic and post-orogenic granitoids and presumably represents the latest tectonothermal event marking the end of the East African Orogen. Five tectonothermal events belonging to the East African Orogen are recognised in the Precambrian of southern Ethiopia: (1) Adola (1157 2 to 1030 40 Ma); (2) Bulbul–Awata (876 5 Ma); (3) Megado (800–750 Ma); (4) Moyale (700–550 Ma); and (5) Berguda (550–500 Ma). Ó 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Southern Ethiopia; Geological evolution

1. Introduction relationship between the roughly contemporaneous high-grade metamorphic Mozambique Belt and the low- The Precambrian of southern Ethiopia occupies an grade Arabian–Nubian Shield in northeast Africa is still important position within the Pan-African Mozambique a subject of debate (e.g., Key et al., 1989). Belt and the Arabian–Nubian Shield, which, together, In the Kenyan part of the East African Orogen, form the East African Orogen (Stern, 1994) (Fig. 1). The useful contributions have resulted from mapping in western (Vearncombe, 1983) and north-central (Key * Corresponding author. Tel.: +27-11-716-2946; fax: +27-11-339- et al., 1989) Kenya. Investigations aimed at understand- 1697. ing the geology of selected areas in southern Ethiopia E-mail address: [email protected] (W.U. Reimold). have also been reported (e.g., Lebling, 1940; Jelenc, 1966;

Fig. 1. Geological map of northeastern Africa, modiﬁed after Worku and Schandelmeier (1996) and Shackleton (1997), showing the Precambrian of southern Ethiopia within the conﬁnes of the East African Orogen.

Gilboy, 1970; Chater, 1971; Kazmin, 1971, 1972; Kaz- pian Precambrian terranes, in general, and for southern min et al., 1978; Kozyrev et al., 1985; Map of Adola Ethiopia, in particular, are very limited (Table 1). In the Belt, 1990; Gichile, 1991; Woldehaimanot, 1995; Worku 1990s, however, geochronological data have slowly and Schandelmeier, 1996). Asystematic study for the emerged (Ayalew et al., 1990; Alemu, 1997; Tadesse Precambrian geology of the southern Ethiopian region et al., 1997, 2000; Worku, 1996; Mechessa, 1996; Teklay has been provided recently by Yibas (2000). et al., 1998) (Table 1). Although these geochronologi- Although the Precambrian of southern Ethiopia oc- cal data for the study area are, to some extent, useful cupies an important position within the East African in shedding light on the timing of magmatic activity, Orogen (Fig. 1), geochronological data for the Ethio- they still fall short of providing a comprehensive Table 1 Available geochronological data for Precambrian rocks of southern Ethiopia Lithotectonic terrane Complex Locality Rock type Mi.–WR Age (Ma) Reference Gneiss–granitoid Moyale–Sololo Complex El Der Hornblende–biotite gneiss Bi (K–Ar) 749 15 Rogers et al. (1965) terrane Genale–Dolo Complex Negele Biotite gneiss Bi (K–Ar) 516 5 Rogers et al. (1965) Zembaba village Porphyritic granite (ET9) Zr (Pb–Pb) 752 6 Teklay et al. (1998) 57–84 (2002) 34 Sciences Earth African of Journal / al. et Yibas B. Zembaba village Metarhyolite (ET10) Zr (Pb–Pb) 605 7 Teklay et al. (1998) Alge,a west of the Bulbul Belt Mylonitic tonalite (Alge Gneiss of Zr (Pb–Pb) 557 9 Teklay et al. (1998) Kazmin, 1972) 35 km SW Negele (Bulbul) Porphyritic granodiorite gneiss Zr (Pb–Pb) 884 7 Teklay et al. (1998) Adola Complex Mormora River Granite Bi (K–Ar) 504 20 Jelenc (1966) Burjiji Granite Zr (U–Pb) 612 6 Worku (1996) Gariboro Pegmatite Ms (Rb–Sr) 530 10 Gilboy (1970) Granite WR (Rb–Sr) 680 Gilboy (1970) Granite WR (Rb–Sr) 515 10 Gilboy (1970) Granite Zr (U–Pb) 646 30 Worku (1996) Lega Dima Granite Zr (U–Pb) 550 18 Worku (1996) Robele Granite Zr (U–Pb) 554 13 Abraham et al. (1992) Alghea (west of Digati) Alghe granite gneiss Zr (U–Pb) 722 2 Worku (1996) Sebeto Tonalite gneiss Zr (U–Pb) 765 3 Abraham et al. (1992) Burji–Finchaa Complex Yabello town Yabello granite gneiss Zr (Pb–Pb) 716 8 Teklay et al. (1998) Agere Mariam Foliated granite Zr (U–Pb) 708 3 Abraham et al. (1992) Near Agere Mariam Berguda granite Zr (U–Pb) 529 11 Abraham et al. (1992) Konso, west of Burji– Konso granulite Zr (Pb–Pb) 720 7 Teklay et al. (1998) Finchaa Complex

Ophiolitic fold and Megado Belt Megado Megado metabasic rocks WR (Sm–Nd) 789 36 Worku (1996) thrust belts Moyale–El Kur Belt Moyale Quartz diorite Bi (K–Ar) 526 5 Rogers et al. (1965) Moyale Meta-trondhjemite Zr (Pb–Pb) 658 8 Teklay et al. (1998) El Der Amphibolite WR (K–Ar) 647 20 Rogers et al. (1965) Moyale Amphibolite Zr (U–Pb) 700 10 Teklay et al. (1998) WR ¼ whole rock, Bi ¼ biotite, Zr ¼ zircon, Ms ¼ muscovite. a Note that Alghe and Alge are two different localities, the former located in the western margin of the Megado Belt near Digati village, and the latter in the westernmost part of the Genale–Dolo complex, west of the Bulbul Belt. 59 60 B. Yibas et al. / Journal of African Earth Sciences 34 (2002) 57–84 understanding of the evolution of the Precambrian ter- complexes are represented in the study area. Although rane of southern Ethiopia. This is primarily due to the this classification is still in use, its validity is diminishing lack of systematic geological and structural work in this in the light of newly emerging geological and, especially, region, which has severely constrained the use of geo- geochronological data (e.g., Ayalew et al., 1990; Teklay chronological data in interpreting the geological evolu- et al., 1998; Worku, 1996; Yibas, 2000; Yibas et al., tion. 2000a,b). In this paper, a new map depicting the Precambrian The geological map of the study area, which occupies geology of southern Ethiopia is presented together with an area of over 88 000 km2, is shown in Fig. 2. Approxi- SHRIMP and laser probe 40Ar–39Ar geochronological mately 60% of the area shown was mapped recently by data for selected granitoids of the study area. These new Yibas (2000). The previous work of Genzebu et al. results complement the existing chronological data, and, (1994), Gobena et al. (1997), Kozyrev et al. (1985), the together with the systematic geological mapping of the geological map of the Bulbul area (Geological map of area (Yibas, 2000; Yibas et al., 2000a), make a signifi- Bulbul area, 1988), the geological maps of the Moyale cant contribution to deciphering the tectonic evolution (EIGS, 1997) and Sololo (EIGS, 1997) areas and TM of the Precambrian of southern Ethiopia. The implica- Landsat images were also used to compile the geological tions with regard to the relationship of this region to the and structural database for southern Ethiopia (Yibas, Mozambique Belt, the Arabian–Nubian Shield and the 2000). East African Orogen are discussed. A new tectonostrati- The Precambrian geology of Ethiopia consists of two graphic classification for the study area is provided. distinct lithotectonic terranes which show contrasting lithological association, internal structures and grade of metamorphism (Yibas, 2000; Yibas et al., 2000a): (1) the 2. Geology and tectonostratigraphy granite-gneiss terrane, consisting of high-grade para- and orthogneisses and deformed and metamorphosed The Precambrian terrane of southern Ethiopia is granitoids; and (2) the ophiolitic fold and thrust belts, overlain to the west by Cainozoic volcanics and sedi- consisting of low-grade, mafic–ultramafic and sedimen- ments associated with the Main Ethiopian Rift System tary assemblages. Major fault and/or shear zones dis- (Figs. 2 and 3). The Precambrian rocks comprise high- playing multiple deformation features (Yibas, 2000) grade ortho- and paragneisses and migmatites, low- separate these two terrane types. grade volcanosedimentary–ultramafic assemblages and The bulk of the granite-gneiss terrane is underlain by granitoids of variable composition. Lebling (1940) was para- and orthoquartzofeldspathic gneisses, intercalated among the earliest workers, who broadly classified the with amphibolites, sillimanite–kyanite-bearing schist geology of the Precambrian of southern Ethiopia into and marbles, and granitoids, and extends into northern gneissic, granitoid and ‘‘green’’ rock terranes and pro- Kenya. Lithologically, the paragneisses resemble the vided the first lithological descriptions. This was fol- gneisses of northern Kenya (Yibas, 2000; Yibas et al., lowed by the classification of Jelenc (1966) for the Adola 2000a). rocks into the Gariboro Series (granitic and psammitic gneisses) and the Adola Series (basic and pelitic rocks). 2.1. Granite-gneiss terrane Kazmin (1972) and Kazmin et al. (1978) (Table 2) classified the Precambrian rocks of Ethiopia into Upper, The granite-gneiss lithotectonic terrane has been Middle and Lower Complexes, following the threefold subdivided into two sub-terranes (Burji–Moyale, Adola– classification of Gilboy (1970) and Chater (1971) for the Genale), and four complexes (Burji–Finchaa, Moy- Adola rocks. According to this classification, the Lower ale–Sololo, Adola and Genale), based on the spatial Complex, consisting of relatively high-grade gneisses, associations of the rocks, internal structures, interrela- represents the older (presumably Archaean) cratonic tionships of the various rock types and lithostructural basement, upon which the Middle Complex (Lower to similarity (Figs. 2 and 3; Table 3). Middle Proterozoic, platform cover of clastic sediments) The NW–SE trending Geleba–Chelanko Shear–Fault was deposited. The Upper (Neoproterozoic) Complex Zone (Figs. 2 and 3) separates the Burji–Moyale and consists of low-grade volcanosedimentary assemblages the Adola–Genale sub-terranes and has also acted as the of ophiolitic affinity (Kazmin et al., 1978). All three locus of subsequent granitic magmatic activity (e.g., the

Fig. 2. Geological map of southern Ethiopia (scale 1:1 000 000), refer to maps attached with this issue. B. Yibas et al. / Journal of African Earth Sciences 34 (2002) 57–84 61

Fig. 3. Tectonostratigraphic map of the Precambrian geology of southern Ethiopia with sample location for dated rocks (modiﬁed from Yibas, 2000): 1 ¼ Metoarbasebat granite; 2 ¼ Meleka granodiorite; 3 ¼ Digati diorite gneiss; 4 ¼ Wadera megacrystic diorite gneiss; 5 ¼ Wadera deformed granite dyke; 6 ¼ Moyale granodiorite; 7 ¼ Melka Guba megacrystic diorite gneiss; 8 ¼ Bulbul mylonitic diorite gneiss.

Arero granitic complex and the Altuntu megacrystic have ages between 725 and 708 Ma (Gichile, 1991; granite were intruded along its extent). Teklay et al., 1998). The most important lithologic units in the Burji– The granitoids in the Burji–Finchaa complex are Finchaa granite-gneiss complex are the Burji gneiss, the predominantly high-K granites, with minor shoshonitic Bari Dome gneisses, the Finchaa quartzofeldspathic peralkaline granites, tonalites, trondhjemites and gran- gneisses and the Yabello, Altuntu, Finchaa and Soyoma odiorites (Yibas, 2000; Yibas et al., 2000d). The Moyale– granitoid gneisses (Fig. 2). With the exception of the Sololo granite-gneiss complex is composed mainly of Berguda granitoid complex, which has an age of circa granitic and granodioritic gneisses and subordinate par- 530 Ma, the other granitoids so far dated in this complex agneisses (biotite gneiss, mylonitic quartzofeldspathic 62

Table 2 Stratigraphy of the Precambrian geology of Ethiopia (after Kazmin, 1972; Kazmin et al., 1978) Era Age (Ma) Southern Ethiopia Western Ethiopia Northern Ethiopia Eastern Ethiopia Complex .Ybse l ora fArcnErhSine 4(02 57–84 (2002) 34 Sciences Earth African of Journal / al. et Yibas B. Group Units Group Formation Eocambrian 500 Matheos Formation Upper Shiraro Formation Dadikama Forma- tion Neoproterozoic 700 Tambien Group Amota Formation Mai Kenetal Forma- tion Arequa Formation 1000 Mormora Group Birbir Group Tsaliet Group Soka and Boje Series and Kajimiti Beds 1600 Adola Group Greenstones and amphibolites Palaeo- to Mes- Wadera Group Middle oproterozoic Archaean 2500 Burji gneiss Gneisses undiffer- Gneisses undifferen- Mica schists Lower entiated tiated Arero Group Yavello gneiss Gneisses undifferentiated Awata gneiss Alghe gneiss ? ? Beles granulites Konso gneiss Table 3 Tectonostratigraphy of the granite-gneiss complexes of the Precambrian of southern Ethiopia (compare Yibas, 2000) Era/epoch Major geologic activity Age (Ma) Burji–Moyale sub-terrane Adola–Genale sub-terrane Burji–Finchaa granite- Moyale–Sololo granite- Adola gneiss–granitoid Genale–Dolo granite- gneiss complex gneiss complex complex gneiss complex Early Cambrian Post-tectonic to post- 550–500 Berguda charnockitic Metoarbasebat granite e.g., Robele granite orogenic magmatism granite (528 8:4to (526 5Ma) (554 23 Ma); Lega 538 3Ma) Dima granite (550 8 Ma) Late-Neoproterozoic Arc- and collisional 650–570 Meleka foliated granodi- Wadera megacrystic di- granitic magmatism orite (610 9); Gari- orite gneiss (579 5 57–84 (2002) Ma); 34 Sciences Earth African of Journal / al. et Yibas B. boro–Burjiji granitic Wadera foliated granitic massif (646–602 Ma) dyke (576 5Ma) Early–Middle Neoprote- Granitic magmatism as- 700–900 Soyoma charnockitic Mount Garara mylonitic Wadera mylonites and Mylonitic quartzofelds- rozoic sociated with the opening granite gneiss granite gneiss gneisses pathic gneisses and closure of marginal Finchaa biotite granite Raro–Kakissa–El Gof Alghe granite gneiss Zembaba granite gneiss basins (708 5 Ma); Yabello magnetite-bearing quar- (722 2Ma) (756 6 Ma); Melka charnockitic granite gne- tzofeldspathic gneiss and Guba megacrystic diorite iss (716 5 Ma); Sagan granite gneisses, diop- gneiss (778 23 Ma) basic charnockites side-bearing gneiss; Sod- (725 3 Ma); Altuntu igga granite gneisses, megacrystic granite gne- mylonitic quartzofelds- iss; Sebeto tonalite gneiss pathic gneisses, migmat- (765 3Ma) itic gneisses, and charnockitic granite Awata granitoid and Bulbul–Alge granitoid migmatitic assemblage and migmatitic assemblage

Late Mesoproterozoic Sedimentation in the 1050 Paragneisses with subor- Roukka quartzofelds- Kenticha Formation: Hornblende–biotite gne- ‘‘Mozambique Belt dinate foliated granites pathic gneisses, biotite– biotite–quartz–feldspar iss, with interbeds of basin’’ hornblende gneisses, gneiss, garnet staurolite amphibole schist minor quartzofeldspathic gneiss and amphibolite schists Finchaa gneiss; Bari Mega–Hidilola biotite– Aflata Formation Genale Formation Dome gneisses amphibole gneisses, intercalated with amphibolites Burji gneisses Sololo quartzofelds- Arero gneisses; Zembaba Quartzofeldspathic pathic gneisses Formation gneisses and associated schists Palaeo- to Mesoprotero- Pre-Pan-African Crust 1250–2050 Inferred from ages of zoic zircon xenocrysts (Yibas, 2000; Yibas et al., 2000a,b, 2000c, 2000d) 63 64 B. Yibas et al. / Journal of African Earth Sciences 34 (2002) 57–84 gneiss, and muscovite–quartz–feldspar gneiss; Fig. 2). Aflata Formation (e.g., the 765 3 Ma Sebeto tonalite Geochemical studies of the complex confirm the domi- and the 722 2 Ma Alghe granite gneiss) are subduc- nance of orthogneisses ranging from monzonitic to tion-related granitoids (Gichile, 1991; Worku, 1996; monzodioritic granitoids of both volcanic-arc and Yibas, 2000; Yibas et al., 2000d). The major rock units within-plate tectonic affinity (Yibas, 2000; Yibas et al., in the Kenticha Formation are pelitic gneisses and 2000d). Lithologic similarity between the biotite–horn- mica schists, with minor occurrences of amphibolites, blende gneisses in the western part of the Moyale–Sololo marbles and graphite schists. granite-gneiss complex and the Burji gneisses in the The Genale–Dolo granite-gneiss complex forms the north suggests that these gneisses might belong to the largest segment of what was previously known as the same facies. However, continuity cannot be proven, as Alghe gneiss. Kazmin (1972, 1975) described these rocks the area between these two gneisses is covered by Cai- as uniform biotite- and biotite–amphibole gneisses nozoic volcanic rocks (Figs. 2 and 3). The Moyale– grading into banded migmatites and grey granodioritic Sololo paragneisses also extend southwards into Kenya rocks (Kazmin also regarded these gneisses as part of (Key et al., 1989). the oldest, Archaean rock sequences in Ethiopia). The The southwestern part of the Adola–Genale granite- paragneisses in the Genale–Dolo granite-gneiss com- gneiss sub-terrane is dominated by the Bulbul–Awata plex, in the northeastern as well as in the central and granitoid–migmatitic assemblage, which is subdivided western regions of the study area, can be classified into into the Awata and Bulbul–Alge migmatitic–granitoid three major assemblages, which are intruded by different gneisses (Fig. 2). In the Alge area and to the west of the phases of orthogneisses. However, the mylonitic gneisses Bulbul Belt these rocks are represented by strongly de- that are widely developed throughout the complex are formed granitoids. interpreted as the result of the Wadera shearing event at Although significant amounts of paragneisses and about 580 Ma (cf. below; Yibas et al., 2000a,b). The schists occur in the Adola granite-gneiss complex, or- Bulbul–Alge granitoid–migmatitic gneisses form part of thogneisses are dominant (Woldehaimanot, 1995; the Bulbul–Awata granitoid–migmatite assemblage. Worku, 1996; Yibas, 2000; Yibas et al., 2000b). Kozyrev Minor basic intrusives result in positive topographic et al. (1985) classified the Adola gneisses into five for- features in the northeastern part of the study area (the mations––the Zembaba, Aflata, Kenticha, Buluka and largest occurrence is the circular dome of Mount Ka- Bore Formations. Yibas (2000) and Yibas et al. (2000a), lido; Gobena et al., 1997). however, argued that both the Buluka and Bore for- The equilibrium paragenesis commonly observed in mations are dominated by granitoid gneisses and the gneissic rocks of southern Ethiopia is representative migmatitic gneisses, which are similar to the Awata of the medium-temperature part of the amphibolite fa- granitoids and migmatitic gneisses, and, hence, mapped cies, although evidence of prograde metamorphism to- these units together as the Awata–Bulbul granitoid– wards the higher-temperature part of the amphibolite migmatitic assemblage. The Zembaba Formation, which facies can be observed locally (e.g., Chater, 1971; forms the eastern part of the Adola Complex, is the Worku, 1996; Yibas, 2000). lowest stratigraphic unit in this complex. It consists mainly of quartzofeldspathic gneisses, with a minor 2.2. Ophiolitic fold and thrust belts amount of leucocratic biotite gneiss, that are intercalated with biotite–muscovite schists. Four mafic–ultramafic–sedimentary assemblages (each Kazmin (1972), Kazmin et al. (1978) and Kozyrev composed of varying proportions of mafic, ultramafic et al. (1985) regarded most of these rocks as parag- and sedimentary rocks) are recognised in the Precam- neisses, but Worku (1996) emphasised the presence brian of southern Ethiopia (Yibas, 2000; Yibas et al., of a significant proportion of gneisses derived from 2000c, and references therein). These are the (1) Bulbul; anorogenic granites in addition to paragneisses––an (2) Kenticha; (3) Megado; and (4) Moyale–El Kur belts observation that was supported by Yibas (2000). In the (Figs. 2 and 3; Table 4). The mafic rocks that form the south-central part of the Adola Complex, in the vicinity bulk of these assemblages are dominantly sub-alkaline, of Arero village, the Zembaba gneisses appear to be low-Ti tholeiites and boninites (Yibas, 2000; Yibas et al., paragneisses, as they display prominent compositional 2000c). The mafic–ultramafic–sedimentary assemblages and grain size variation. The Aflata Formation crops can be referred to as fold and thrust belts in view of their out in the western and central parts of the Adola deformational styles and as ophiolites in as far as their Complex. The textural and compositional variations origin and lithological associations are concerned (Yi- may suggest heterogeneity of the protoliths of the rocks bas, 2000; Yibas et al., 2000c). of this formation. The rocks mapped as Aflata Forma- The contacts of the ophiolitic fold and thrust belts tion by Kozyrev et al. (1985) were earlier regarded by with the adjacent granite-gneiss complexes are re- Kazmin (1972, 1975) as Archaean in age (Table 2). activated deformation zones, where evidence of both However, most of the granitoids mapped as part of the thrusting and strike-slip faulting could be found (Figs. 2 B. Yibas et al. / Journal of African Earth Sciences 34 (2002) 57–84 65

Table 4 Correlation of the main lithologies in the ophiolitic fold and thrust belts of the Precambrian of southern Ethiopia Megado Belt Kenticha Belt Bulbul Belt Moyale–El Kur Belt Moyale sub-belt Jimma–El Kur sub-belt Kajimiti Beds: Conglomer- ates and greywackes Megado metasediments: Kenticha metasediments: Bulbul metasediments: Metasediments interlayered Metasediments interlayered Greywackes, lithic are- Staurolite–sillimanite- Metasandstones, graphites with metabasic rocks and with minor metabasic rocks nites, quartzites and phyl- bearing biotite schists, Fe– and biotite phyllites ultramafic schists: Gra- and meta-ultramafic rocks: lites Mn quartzites, marbles (Kazmin et al., 1978) phitic schists, amphibolite Quartz–feldspar schists, and siliceous metapelites and amphibole gneiss conglomeratic gneiss, kyanite–sillimanite schist, quartzites and marbles, intercalated with minor me- tabasite and ultramafic schists Megado metabasic rocks: Kenticha metabasic rocks: Bulbul metabasic rocks: Metabasic rocks: Amphi- Metabasic rocks: Amphi- Amphibole–chlorite Amphibole–chlorite schists Amphibole–chlorite bole–chlorite schist, am- bole–chlorite schists, am- schists, amphibolites and and amphibolites schists, amphibolites and phibolites and phibolites and subordinate metagabbros metagabbros metagabbros metagabbros Meta-ultramafic rocks: Meta-ultramafic rocks: Meta-ultramafic rocks:N– Meta-ultramafic rocks: Meta-ultramafic rocks: Serpentinites, talc schists Talc–tremolite schists, S elongated lenticular Serpentinites, talc–tremo- Serpentinites, talc–tremo- and talc–chlorite–tremolite talc–anthophyllite schists bodies of ultramafic schists lite schist, talc–serpentine lite schist and birbirites schists and talc serpentinite, mi- (mainly composed of talc, rocks with antigorite at the forming the base of the nor chlorite–talc schists, chlorite, tremolite and ac- core, and birbirites (talc– sub-belt chlorite–anthophyllite–ser- tinolite, in various propor- magnetite serpentinite pentine schists; occur in tions) tectonically rocks) two sub-parallel zones: the interlayered with meta- eastern sheared zone and gabbros and with basic the western zone forming schists the base and the upper part of the Kenticha Belt, respectively and 3; Yibas, 2000). N–S trending curvilinear shear Metabasic and metasedimentary rocks are dominant zones separate the Kenticha and Megado belts from over ultramafic rocks in the Megado Belt. In contrast, the adjacent granite-gneiss complexes (Woldehaimanot, ultramafic rocks (serpentinite, talc–tremolite and talc– 1995; Worku, 1996; Yibas, 2000). These zones are the anthophyllite schists) are dominant over the mafic rocks Digati–Meleka, Lega Dembi–Aflata, Dermi Dama and in the Kenticha Belt. Staurolite- and sillimanite-bearing Kenticha Shear Zones and represent major thrust faults biotite schists and minor occurrences of Fe–Mn quartz- reactivated by N–S trending, sinistral strike-slip faults. ites, marbles and siliceous metapelites are also recogn- The contacts of the Bulbul rocks with the adjacent high- ised in the Kenticha Belt (Kazmin, 1976; Kazmin et al., grade gneisses to the west are characterised by easterly 1978; Woldehaimanot, 1995; Worku, 1996; Yibas, 2000; dipping, curvilinear, reverse faults, which have been re- Yibas et al., 2000a). The meta-ultramafic rocks in the activated by a strong shearing event (Yibas, 2000). Kenticha Belt comprise two sub-parallel zones of talc Along these faults, sectors of the Bulbul Belt are inter- and talc–serpentine rocks, which are 2–5 km wide and layered with thin zones of mylonitic diorites. The west- extend N–S for at least 60 km (Fig. 2). Sandwiched ern contact of the Moyale–El Kur Belt is characterised between these two ultramafic zones are biotite schists, by curvilinear fault zones (Roukka Fault) that trend epidote–amphibole gneisses and amphibolites. The east- from N–S to NNW–SSE. In most cases, the faults dip ern ultramafic zone is in contact with the metasediments moderately to steeply in easterly direction. The northern of the Kenticha Formation. The westerly dipping con- boundary of this ophiolitic belt with the Moyale–Sololo tact, where exposed, is marked by altered and sheared granite-gneiss is the major, NW–SE trending, dextral El ultrabasic bodies. These rocks may represent the lower Gof–Ketema Fault (Figs. 2 and 3). part of the ultramafic sequence in this zone, consisting The Kenticha and Megado mafic–ultramafic–sedi- of lenticular bodies of serpentinised peridotite enveloped mentary assemblages form two N–S trending lin- by talc–serpentine schist (Kazmin et al., 1978; Wold- ear belts, separated by quartzofeldspathic gneisses and ehaimanot, 1995). Ashear zone dipping steeply to the granitoids of the Adola granite-gneiss complex (Fig. 2). west marks the contact between the eastern ultramafic 66 B. Yibas et al. / Journal of African Earth Sciences 34 (2002) 57–84 zone and the basic rocks. The western ultramafic zone, Talc and talc–tremolite schists are the dominant ultra- which may form the upper part of the Kenticha Belt, mafic rocks in the Moyale area, but these rocks are lacks lenticular serpentinised peridotite and is domi- highly altered, and birbirites (talc–magnetite–serpenti- nated by tremolite–talc and chlorite schists. To the west, nite rocks) are common in the area. Good exposures of the ultramafic–mafic zones are in contact with metase- birbirite occur on Toukka Hill of western Moyale and diments (graphitic phyllite and biotite schists, calcareous are characterised by boxworks of silica and magnetite metasandstone; marbles and calc–silicate rocks; Kazmin veinlets. The less altered ultramafic rocks are composed et al., 1978). of serpentine, talc, calcite, dolomite and iron oxides. The The Bulbul Belt occurs in the most easterly part of the most prominent ultramafic body in the area is Mount study area, where the Dawa River (Fig. 2) exposes the Agal Guda, in the eastern part of the Moyale sub-belt eastern contact of the belt with the adjacent biotite- and (Fig. 2). amphibole-bearing gneisses of the Genale–Dolo Com- The ultramafic rocks in the Jimma–El Kur sub-belt plex. Mesozoic sediments overlie most of the eastern are composed of serpentinites, talc–tremolite schists and parts of the Bulbul Belt. The main rock types consti- birbirites, and form positive topography at the west- tuting the Bulbul Belt are mafic rocks with minor ul- ern margin of the sub-belt. These ultramafic rocks are tramafic rocks. In the west, slices of quartzofeldspathic strongly sheared at their base and are in tectonic contact gneisses and dioritic mylonites are tectonically inter- with the mylonitic megacrystic granodiorite to the west. layered with basic rocks. Diorites and plagiogranites The main exposure of this unit occurs around Mount separate the mafic–ultramafic rocks into two zones. The Jeldesa (Fig. 2), where the rocks are brownish to buff in Bulbul Belt might represent a part of a regional, U- colour and form a cryptocrystalline mass with criss- shaped, folded ophiolitic belt, which is believed to ex- crossing magnetite and silica veinlets. Large magnetite tend eastwards into Somalia where it forms the Inda Ad crystals are common throughout this unit. Formation, and possibly separates the Bur and Harar The mafic rocks in these belts comprise amphibole granite-gneiss massifs (Kazmin, 1975). Its existence to schist, amphibole–chlorite schist, massive amphibolite the east has been confirmed by deep drilling in eastern and metagabbro. The amphibole schists and amphibo- Ogaden (Eastern Ethiopia), where some boreholes pen- lites are the dominant lithologies and generally surround, etrated low-grade basement rocks underlying the Me- or are, in places, gradational into, the metagabbros. sozoic sediments (Kazmin, 1975, and references therein). Significant proportions of metasedimentary rocks Yibas (2000) showed that the Moyale Belt is more occur intercalated with the metabasic rocks, particularly extensive than previously reported and extends to the in the Megado Belt where they are mainly represented Jimma–El Kur area in the east––hence the name Moy- by greywackes with subordinate lithic arenites and ale–El Kur fold and thrust belt (Fig. 2). The belt is rare quartz-arenites. Conglomerates and greywackes subdivided into the Moyale and the Jimma–El Kur sub- (Kajimiti Beds) form the upper part of the sedimentary belts, which are separated by a narrow belt of quartzo- sequence. The mineral assemblages in the meta-arkoses, feldspathic gneisses and schist that were deformed by a metapelites and metagreywackes typically comprise shearing event that gave rise to the N–S trending quartz, plagioclase, biotite, muscovite and accessory Roukka Shear Zone (Fig. 2). The Moyale sub-belt rutile, chlorite and epidote (Gilboy, 1970; Chater, 1971; covers the area around the town of Moyale and the Yibas, 1993, 1999). This indicates that the volcanosedi- region close to the border of Ethiopia and Kenya, and mentary rocks did not exceed greenschist facies meta- continues southwards into Kenya (Walsh, 1972; Shagi morphism, in contrast to the dominantly amphibolite et al., 1991). Exposures in the Jimma–El Kur sub-belt and, in places, granulite facies metamorphic grade of (Fig. 2) are scarce, as recent soils largely cover the flat the gneissic rocks and some of the metasediments of the landscape. However, some lenticular outcrops do occur, Kenticha Belt (Yibas, 1999, and references therein). The especially in the central and northern parts of the sub- metasediments in the Moyale sub-belt include graphite– belt, where variable rock types, including conglomer- quartz–mica schists/gneisses, quartz–feldspar schists and atic gneisses, micaceous quartzofeldspathic schists and quartzofeldspathic schists. gneisses, calc–silicate rocks, ultramafic–mafic schists, serpentinites, marbles and quartzites have been mapped (EIGS, 1997; Yibas, 2000; Yibas et al., 2000a). In 3. Geochronology the Moyale–El Kur Belt, ultramafic rocks (serpentinites, talc–tremolite schist and talc–serpentine rocks) are Several generations of felsic to intermediate intru- found intimately associated with amphibolites. Lenses sives occur in the Precambrian of southern Ethiopia of serpentinite bodies occur within the amphibolite (Fig. 2; Table 3). These granitoids range from gneisses gneiss and the graphitic gneiss/schist in western Moyale. to undeformed granites and vary compositionally from In the core of these lenses, antigorite-bearing rocks give diorites to granites, with the dioritic rocks being less way progressively outwards to talc-dominated schists. abundant. Amore detailed discussion of the composi- B. Yibas et al. / Journal of African Earth Sciences 34 (2002) 57–84 67 tions, textures, structures and contact relationships of successive metamorphic event(s) and partial or complete the granitoids with the adjacent lithologies can be found resetting of geochronometers, such as K–Ar, 40Ar–39Ar in Yibas (2000). and Rb–Sr, by younger tectonothermal events. The po- Although granitoid gneisses form an integral part of tential of within-grain analysis for determining the the granite-gneiss terrane, they are absent or rare in the crystallisation history of zircons has been appreciated ophiolitic fold and thrust belts. Afew deformed grani- since the early 1970s. This followed the discovery, from toid rocks do, however, occur in these belts. Notable conventional dating techniques, that old zircon grains among these are the Gayo mylonitic granite, the Digati had preserved their ages through Alpine-age granulite- diorite gneiss and the Geleba diorite in the Megado Belt, facies metamorphism, and that xenocrysts in some the Moyale granodiorite and mylonitic megacrystic granites had survived fusion temperatures (Compston granodiorite in the Moyale–El Kur Belt, and the Bulbul et al., 1992). mylonitic diorite gneiss in the Bulbul Belt. Hybrid The present study aims at establishing a geochrono- granitoids, represented by irregularly intercalated gabb- logical framework for the complex Precambrian terrane roic, dioritic and granodioritic intrusives, form con- of southern Ethiopia, using the SHRIMP dating tech- spicuous hills in the northern part of the Jimma–El Kur nique to determine precise U–Pb ages on single zircons sub-belt (Fig. 2). In the Moyale–El Kur Belt, granitic from selected lithologies. In total, eight samples were and aplitic dykes intrude the mafic–ultramafic rocks. analysed. Towards the west, close to the contact with the Moyale Zircons from the selected samples were separated granodiorite, NE–SW trending granitic dykes, probably using a Wilfley table, a magnetic separator and heavy offshoots of the Moyale granodiorite intrusion, are liquids at the Hugh Allsopp Laboratory, University common. Some of these dykes are strongly brecciated of the Witwatersrand, Johannesburg. Zircon separates and appear to have intruded syn-tectonically along were submitted to the Precise Radiogenic Isotopic Ser- contact zones between the mafic and ultramafic layers. vices (PRISE), at the Research School of Earth Sciences The compositions of some of the granitic dykes show (RSES), Australian National University (ANU), in similarity to the main Moyale granodiorite body to the Canberra. Zircon grains were handpicked under a bin- west (Yibas, 2000; Yibas et al., 2000d). ocular microscope and mounted in epoxy, together with Most of these relatively undeformed granites in the RSES SHRIMP zircon standards AS3 (Duluth southern Ethiopia occur within the granite-gneiss ter- Complex gabbroic anorthosite; Paces and Miller, 1989) rane. Their contact relationships with the country rocks, and SL13. The grains were then sectioned, polished and where exposed, are sharp, steep and truncate the re- photographed. All zircons were further examined by gional, mainly N–S trending, foliation. The size of these SEM cathodoluminescence imaging, a procedure that granitic bodies is variable, but they generally are smaller greatly enhances the quality of data produced in the when compared to the other suites of granites. subsequent ion microprobe sessions. Through cathodo- Four samples from the granitoid gneisses [viz., the luminescence imaging of the zircons, hidden and Bulbul mylonitic diorite gneiss (BY96-78); the Melka complex internal structures can be more accurately de- Guba megacrystic diorite gneiss (BY97-183); the Digati termined than in normal reflected or transmitted light. diorite gneiss (BY96-18); and the Wadera megacry- Consequently, the target area can be more reliably se- stic diorite gneiss (BY97-178)], three samples from the lected for analysis. Using this approach, it was also deformed granitoids [the Moyale granodiorite (BY97- possible to get more information on inheritance, meta- 79); the Meleka metagranodiorite (BY96-13A); and morphic overgrowths and protolith ages. The samples the Wadera foliated granite dyke (BY97-178A)], and were analysed at the RSES using both the SHRIMP I one from an undeformed granitoid [the Metoarbasebat and SHRIMP II instruments. The SHRIMP data have granite (BY96-13A), (Figs. 2 and 3)] were dated using been reduced in a manner similar to that described by SHRIMP and 40Ar–39Ar laser probe dating techniques. Compston et al. (1992) and Williams and Claesson These samples were selected on the basis of relative (1987). U–Pb isotopic ratios in the unknown samples deformation intensities and crosscutting relationships, were normalised to a 206Pb/238U value of 0.1859 (equiv- in order to analyse the different magmatic events that alent to an age of 1099.1 Ma) for AS3. The U and Th occurred in the Precambrian of southern Ethiopia. concentrations were determined relative to those measured in the SL13 standard. In most cases, ages were 3.1. Methodology calculated using the 206Pb/238U ratios, with the correction for common Pb being made using the measured 3.1.1. Ion microprobe (SHRIMP) U–Pb zircon dating 207Pb/206Pb and 206Pb/238U values following Tera and Despite the advances made in geochronological Wasserburg (1972) and as described in Compston et al. techniques, deciphering the geochronology of complex (1992). In those cases where it was more appropriate to metamorphic terranes is still challenging as problems use the radiogenic 207Pb/206Pb compositions to calcu- arise from overprinting of pre-existing assemblages by late ages, the ages were determined using the directly 68 B. Yibas et al. / Journal of African Earth Sciences 34 (2002) 57–84 measured 204Pb/206Pb ratios and the relevant model atmospheric contamination. Unless otherwise stated, common Pb composition after Cumming and Richards errors are 1r uncertainties and include errors in the J- (1975). Uncertainties in the isotopic ratios and ages in value estimates. the data tables (and in the error bars in the plotted data) are reported at the one sigma (1r) level, but final 3.2. Chronological results weighted mean ages are reported with 95% confidence limits (2r). 3.2.1. Metoarbasebat granite This granite occurs in the northern part of the 3.1.2. 40Ar–39Ar dating Moyale–Sololo granite-gneiss complex close to the 40Ar–39Ar data from biotite, amphibole and musco- Moyale ophiolitic fold and thrust belt (Figs. 2 and 3) vite were obtained at the Anglo American Research around Metoarbasebat village on both sides of the Laboratories (AARL), Johannesburg. Samples were main Addis Ababa–Moyale road. It occurs as circular to crushed and sieved. Some 20 biotite–muscovite–amphi- semi-elliptical domes. The granite is coarse-grained, bole grains were extracted for analysis. The grains were pink to grey, and composed of K-feldspar, plagio- ultrasonically cleaned in de-ionised water and acetone clase, hornblende and quartz. ANE–SW to ENE–WSW and visually inspected for signs of alteration. The grains trending fabric, defined by mineral (mainly plagioclase, were then packed into the center of aluminium foil hornblende and quartz grains) alignment, is discernible. pockets and placed in an evacuated quartz glass vial, The granite shows porphyritic texture with large phe- together with the irradiation standard Hb3gr (Turner, nocrysts of amphiboles and plagioclase. Coarse-grained 1971). Flux monitors were placed within 2 mm of the quartz veins cut across the granite. Mafic xenoliths with sample packet. Neutron irradiation was carried out in diameters of 10–20 cm have been observed. As this position B2W of the Safari-1 reactor, Pelindaba (South granite is spatially close to the Moyale ophiolitic fold Africa), for a period of 40 hours at 10 MW. After ir- and thrust sub-belt (Fig. 2), it is assumed that these radiation and cooling, grains were individually loaded xenoliths are derived from this sub-belt. into 1 mm diameter wells in a 22 mm diameter alu- The zircons from sample BY96-10B of this granite minium disk. The disk was then covered with a sapphire are small (80–120 lm in length), colourless to light glass cover slip to ensure containment and to protect the pink, and are mainly subhedral, with occasional acicular coated quartz glass window of the high-vacuum sample forms. Cathodoluminescence imaging revealed strong port. Prior to analysis the samples were baked at 200 compositional zoning in most of the zircons, which also °C for 12 hours to remove adsorbed volatiles. Fifty in- shows that some grains have cores of inherited zircon. dividual Hb3gr grains from five irradiation positions These cores are surrounded by zoned overgrowths (Fig. were analysed in order to calculate an average J-value. 4a). Core/rim analyses were carried out using single pulses The U–Th–Pb results of the SHRIMP analyses for from a focused infrared laser beam. Gas purification this granite are reported in Table 5 and are plotted on a was achieved by means of two AP10 getter pumps, op- Tera–Wasserburg U–Pb concordia diagram in Fig. 5a. erated at 400 and 20 °C, respectively. Argon analyses The data are plotted uncorrected for common Pb, with were carried out on a MAP 215-50 static mass spec- the measured analyses plotting on a mixing line between trometer, equipped with a Nier-type source and Johnson a radiogenic end-member (on concordia) and a common multiplier detector, and operated at a mass resolution of Pb composition. The lower the common Pb content of 600. Procedural blanks were 7 Â 10À17 moles for 40Ar, the zircon, the closer the data points plot to the con- 2 Â 10À16 moles for 39Ar and 38Ar, 5 Â 10À17 moles for cordia. The majority of the data cluster near the con- 37Ar, and 4 Â 10À18 moles for 36Ar. Mass discrimination cordia and, thus, have very low common Pb content was monitored by analyses of standard air volumes (Fig. 5a). One analysis (5.1 in Table 5) plots away from from a Doerflinger pipette system. Analyses of co- this group and has a significant common Pb component. irradiated calcium and potassium salts yielded the Fifteen of the 16 analyses can be combined as a sta- 39 37 following isotope production ratios: ð Ar– ArÞCa ¼ tistically coherent group, for which a weighted mean 36 37 206 238 2 0:0007540:000027; ð Ar– ArÞCa¼0:0003190:000010; Pb/ U age of 526 5 Ma has been calculated (v ¼ 40 39 ð Ar– ArÞK ¼ 0:031 0:005. These values compare 0:72). The single analysis excluded from this calcula- favourably with analyses of a previous batch of salts, tion (6.1 in Table 5) appears to have suffered Pb loss. which were analysed by R. Burgess at Manchester Combining the field and petrographic observations, the University. The latter analysis yielded ratios of U–Pb SHRIMP age of 526 5 Ma is interpreted as the 0:000708 0:000024; 0:000312 0:00002; and 0:036 magmatic emplacement age. 0:008, respectively. The mean 40Ar–39Ar age obtained from laser probe All reported data have been corrected for system dating on biotite grains (Table 6) from the Metoar- blanks, mass discrimination, radioactive decay of 39Ar basebat granite is 506 4 Ma and from hornblende it is and 37Ar, reactor interference, fluence gradients and 511 4 Ma, and there is no age difference between the B. Yibas et al. / Journal of African Earth Sciences 34 (2002) 57–84 69

Fig. 4. Cathodoluminescence images of dated zircon grains from the granitoid rocks of the Precambrian of southern Ethiopia: (a) Metoarbasebat granite; (b) Meleka granodiorite; (c) Digati diorite gneiss; (d) Wadera megacrystic diorite gneiss; (e) Wadera deformed granite dyke; (f) Moyale granodiorite; (g) Melka Guba megacrystic diorite gneiss; (h) Bulbul mylonitic diorite gneiss. rim and core analyses. The age obtained from horn- NE–SW direction on slip planes and indicate that the blende is preferred, as hornblende has better Ar reten- rock is deformed. tion properties than biotite (Guo and Dickin, 1996). The heterogeneity of the zircon population of the This age is younger than the SHRIMP U–Pb zircon age Meleka granodiorite sample is shown in Fig. 4b. (526 5 Ma) and might indicate the cooling age of the Most grains exhibit lamellar (in fragments) or regular pluton. (in complete grains) and multiple zonation, resembling zonation typically observed in magmatic zircons. In 3.2.2. Meleka foliated granodiorite addition, a significant number of grains contain cores of The Meleka foliated granodiorite occurs in the possible xenocrystic nature (not analysed) with the ex- northernmost part of the Adola area (Figs. 2 and 3). ception of grain 7.1 (Table 5). Afew others have narrow This granodiorite intrudes the granitic gneisses of the overgrowths or exhibit embayments (analysis 9.2). This western part of the Adola granite-gneiss complex, close complexity is also reflected in the geochronological to its contact with the Megado Belt. The Meleka gran- results and the widely divergent U and Th contents and odiorite (sample BY96-13A) is composed of K-feldspar, U/Th ratios derived from the SHRIMP analyses (Table plagioclase, quartz and minor amounts of muscovite 5). Seventeen analyses on 15 grains were carried out and and biotite. It is weakly foliated and shows subtle NNE– are plotted, uncorrected for common Pb, on a Tera– SSW compositional layering defined by grey and pink Wasserburg U–Pb concordia diagram (Fig. 5b). Two layers. Aggregates of sulphide minerals are stretched in main observations can be made from this plot: (1) high 70 B. Yibas et al. / Journal of African Earth Sciences 34 (2002) 57–84

Fig. 4 (continued)

common Pb contents of some zircons; and (2) consid- gives a weighted mean 206Pb/238U age of 610 9 Ma, erable spread of ages for the analysed zircons. Some of and a second group of data gives a weighted mean 206Pb/ this complexity undoubtedly results from radiogenic Pb 238U age of 560 8 Ma (Age II). Three of the analyses loss (e.g., analysis 1.2 from a rim, Table 5), where ex- for this sample represent a much younger age group tremely high U concentrations (up to 4140 ppm) have (Age III) with ages between 489 and 515 Ma (compare resulted in extensive radioactive damage. This produces Table 5, not given in Fig. 5b). too young apparent ages. Anumber of analyses also The 40Ar–39Ar ages for biotite grains gave two groups plot to the left of the main cluster, which suggests the of ages: an older mean age of 534:5 8 and a younger presence of older, inherited components. Overall de- age of 512 4 Ma. The muscovite grains, however, gave tailed comparison of the data in Table 5 with the spots a mean age of 515 3 Ma, similar to the youngest age analysed at various places (cores rims, embayments, obtained from the biotite. various bands in complexly zoned grains) does not yield Age III (489–515 Ma) represents the youngest obvious correlation. For example, it is not always pos- SHRIMP data from zircons of the Meleka foliated sible to link data obtained from spots in outer zones or granodiorite and is similar to the ages obtained for the at rims of certain grains with the youngest ages deter- Metoarbasebat granite. The 40Ar–39Ar ages obtained mined. Thus, it is very difficult to anticipate a defi- from both granites are also similar. This suggests that nite magmatic emplacement age. It is possible, however, this age represents the youngest tectonothermal and based on a statistical assessment of the data, to define magmatic event to have occurred in the region. Age II of three groups of ages. The older group of data (Age I) 560 8 Ma is correlative with the emplacement age of B. Yibas et al. / Journal of African Earth Sciences 34 (2002) 57–84 71

Table 5 Summary of SHRIMP U–Pb zircon results for granitoids of the Precambrian of southern Ethiopia

204 206 Grain spot U (ppm) Th (ppm) Th–U Pb (ppm) Pb– Pb f206 (%) Radiogenic Age (Ma) 206Pb–238U 206Pb–238U Sample BY96-10B from the Metoarbasebat granite, northwest of Moyale town 1.1 315 131 0.42 26 0.000465 0.375 0.082 0.0039 506 23 2.1 265 100 0.38 23 0.000378 0.483 0.084 0.0014 523 8 3.1 224 124 0.55 20 0.000196 0.281 0.086 0.0011 531 7 4.1 311 110 0.35 27 0.000476 0.306 0.087 0.0011 536 7 5.1 168 123 0.73 14 0.000527 1.081 0.076 0.0016 474 10 6.1 34 24 0.70 3 0.002475 4.094 0.084 0.0019 523 11 7.1 312 55 0.18 26 0.000148 – 0.086 0.0014 534 8 7.2 34 18 0.54 3 0.000831 0.919 0.084 0.0023 520 14 8.1 99 43 0.43 9 0.000148 0.228 0.085 0.0017 528 10 9.1 261 95 0.37 23 0.000349 0.335 0.086 0.0012 535 7 10.1 253 102 0.40 22 0.000344 0.384 0.084 0.0011 520 7 11.1 256 95 0.37 22 0.000394 0.269 0.084 0.0011 520 7 12.1 73 44 0.59 7 0.000648 1.004 0.083 0.0018 513 10 13.1 299 101 0.34 26 0.000205 0.090 0.085 0.0011 526 6 14.1 253 46 0.18 21 0.000128 – 0.084 0.0013 521 7 15.1 145 70 0.48 13 0.000472 0.492 0.085 0.0012 525 7 Sample BY96-13A from the Meleka granodiorite, northwest of the Adola granite-gneiss complex 1.1 259 251 0.97 25 0.002471 5.29 0.0832 0.0023 515 14 1.2 4140 900 0.22 55 0.028255 50.3 0.0128 0.0006 82 4 2.1 400 404 1.01 47 0.000220 0.32 0.1004 0.0015 617 9 3.1 204 154 0.75 23 0.000381 0.6 0.1003 0.0013 616 8 4.1 777 310 0.40 71 0.003047 5.05 0.0899 0.0011 555 7 5.1 77 25 0.33 7 0.001717 2.6 0.0916 0.0014 565 8 6.1 588 168 0.29 48 0.002619 4.07 0.0820 0.0014 508 8 7.1 123 75 0.61 15 0.002633 4.81 0.1151 0.0022 702 13 8.1 259 254 0.98 17 0.010832 19 0.0573 0.0010 359 6 9.1 123 69 0.57 10 0.007849 13.9 0.0788 0.0018 489 11 9.2 580 80 0.14 49 0.000351 0.62 0.0897 0.0015 553 9 10.1 268 116 0.43 29 0.006025 13.8 0.1043 0.0055 639 32 11.1 244 159 0.65 26 0.020009 32.8 0.0982 0.0018 604 11 12.1 123 48 0.39 12 0.001634 2.72 0.0967 0.0020 595 12 13.1 613 1394 2.27 86 0.000197 0.2 0.0927 0.0016 572 9 14.1 190 146 0.77 21 0.000834 1.42 0.0979 0.0019 602 11 15.1 159 89 0.56 15 0.001332 2.49 0.0904 0.0016 558 9 Sample BY96-18B from the Digati diorite gneiss, southern part of the Megado Belt 1.1 114 110 0.96 13 0.000343 0.47 0.0938 0.0019 578 11 2.1 65 102 1.56 8 0.001312 1.17 0.0925 0.0019 570 11 3.1 279 32 0.11 27 0.000238 0.55 0.1035 0.0018 635 11 4.1 144 135 0.93 16 0.000386 0.059 0.0939 0.0017 578 10 5.1 127 203 1.59 16 0.000858 1.06 0.0917 0.0017 566 10 6.1 54 75 1.38 6 0.001765 4.20 0.0905 0.0020 558 12 7.1 101 152 1.51 12 0.000312 1.02 0.0926 0.0018 571 11 7.2 547 40 0.07 44 0.000133 0.20 0.0856 0.0053 529 31 8.1 59 77 1.30 7 0.001872 2.82 0.0965 0.0022 594 13 9.1 185 179 0.97 19 0.000340 0.38 0.8884 0.0016 566 9 10.1 135 115 0.85 14 0.000703 0.96 0.0918 0.0016 566 10 11.1 122 113 0.92 13 0.001036 1.05 0.0908 0.0017 560 10 12.1 107 101 0.95 11 0.001203 1.47 0.0920 0.0017 567 10 13.1 175 169 0.97 16 0.000484 0.93 0.0779 0.0029 483 17 14.1 85 79 0.93 8 0.000386 2.81 0.0745 0.0048 463 29 Sample BY96-178 from the Wadera megacrystic diorite gneiss, Wadera Shear Zone 1.1 481 78 0.16 41 0.000370 0.10 0.0897 0.0011 554 6 1.2 102 78 0.77 11 0.001623 0.45 0.0926 0.0016 571 9 2.1 123 133 1.08 13 0.000073 0.25 0.0923 0.0018 569 10 3.1 83 134 1.62 10 0.000634 0.32 0.0926 0.0015 571 9 3.2 179 130 0.73 17 0.000202 – 0.0848 0.0012 524 7 4.1 291 6 0.02 23 0.000063 – 0.0866 0.0011 535 7 5.1 551 444 0.81 59 0.000035 – 0.0957 0.0011 589 6 (continued on next page) 72 B. Yibas et al. / Journal of African Earth Sciences 34 (2002) 57–84

Table 5 (continued)

204 206 Grain spot U (ppm) Th (ppm) Th–U Pb (ppm) Pb– Pb f206 (%) Radiogenic Age (Ma) 206Pb–238U 206Pb–238U 5.2 61 43 0.71 6 0.000155 0.37 0.0960 0.0018 591 11 6.1 136 119 0.87 15 0.000441 0.09 0.0936 0.0015 577 9 7.1 218 169 0.77 24 0.000010 0.11 0.0967 0.0012 595 7 8.1 138 122 0.88 15 0.000297 0.25 0.0938 0.0018 578 10 9.1 224 175 0.78 24 0.000202 0.04 0.0943 0.0013 581 7 10.1 124 112 0.90 13 0.000476 0.89 0.0917 0.0013 566 8 11.1 237 15 0.06 21 0.000106 0.11 0.0934 0.0012 575 7 11.2 92 89 0.96 10 0.000010 0.42 0.0949 0.0017 584 10 11.3 40 37 0.92 4 0.001384 1.02 0.0910 0.0020 561 12 12.1 142 63 0.44 13 0.000354 0.43 0.0884 0.0015 546 9 12.2 38 56 1.46 5 0.000136 1.13 0.0928 0.0019 572 11 Sample BY96-178A from a granitic dyke crosscutting the Wadera megacrystic diorite gneiss 1.1 357 287 0.81 38 0.000235 0.20 0.0940 0.0016 579 10 2.1 182 142 0.78 19 0.000179 0.59 0.0922 0.0016 568 10 3.1 203 162 0.80 21 0.000642 0.22 0.0929 0.0017 572 10 4.1 224 175 0.78 24 0.000322 0.40 0.0935 0.0019 576 11 5.1 163 140 0.85 18 0.000367 – 0.0985 0.0020 606 12 6.1 171 144 0.84 18 0.000153 0.45 0.0936 0.0018 577 11 7.1 156 97 0.62 16 0.000736 0.86 0.0960 0.0020 591 12 8.1 151 157 1.04 17 0.000010 0.45 0.0962 0.0018 592 11 9.1 186 149 0.80 19 0.000282 0.63 0.0898 0.0018 554 11 10.1 44 41 0.92 5 0.000269 3.89 0.0914 0.0028 564 17 10.2 286 238 0.83 30 0.000212 – 0.0924 0.0017 570 10 11.1 198 82 0.42 19 0.000247 0.41 0.0938 0.0020 578 12 12.1 71 62 0.88 7 0.001083 2.01 0.0833 0.0250 516 151 13.1 168 127 0.76 16 0.000635 0.61 0.0858 0.0018 531 10 Sample BY96-79 from the Moyale granodiorite, central part of the Moyale Belt, southern Ethiopia 1.1 283 68 0.24 29 0.000268 0.39 0.1064 0.0013 652 8 1.2 756 115 0.15 79 0.000049 0.21 0.1093 0.0012 669 7 2.1 487 97 0.20 52 0.000026 0.09 0.1099 0.0012 672 7 3.1 442 133 0.30 47 0.000049 0.07 0.1084 0.0012 663 7 4.1 856 200 0.23 81 0.000776 1.36 0.0969 0.0011 596 6 4.2 463 98 0.21 50 0.000035 0.09 0.1106 0.0013 676 7 5.1 496 122 0.25 51 0.000201 0.34 0.1048 0.0012 643 7 6.1 1035 186 0.18 101 0.000643 1.19 0.1015 0.0011 623 6 7.1 1375 207 0.15 143 0.000251 0.47 0.1089 0.0012 667 7 7.2 380 172 0.45 43 0.000016 0.14 0.1098 0.0013 672 8 8.1 283 131 0.46 32 0.000169 0.13 0.1098 0.0015 672 9 9.1 846 359 0.42 92 0.000156 0.30 0.1057 0.0012 648 7 10.1 1062 186 0.17 111 0.000134 0.31 0.1088 0.0011 666 7 11.1 932 179 0.19 0.92 0.000224 0.51 0.1024 0.0011 628 7 12.1 909 92 0.10 94 0.000108 0.31 0.1098 0.0012 672 7 Sample BY97-81 from the Bulbul mylonitic diorite gneiss, Bulbul Belt 1.1 349 27 0.08 49 0.000016 0.45 0.1515 0.0018 910 10 1.2 405 50 0.12 55 0.000060 0.38 0.1452 0.0016 874 9 1.3 89 34 0.38 13 0.000156 0.64 0.1388 0.0019 838 11 2.1 89 75 0.84 15 0.000215 0.78 0.1448 0.0020 872 11 3.1 577 405 0.70 56 0.003061 5.31 0.0903 0.0011 557 6 4.1 255 77 0.30 37 0.000096 0.32 0.1454 0.0017 875 10 5.1 246 116 0.47 37 0.000069 0.42 0.1465 0.0018 881 10 6.1 468 231 0.49 67 0.000079 0.50 0.1388 0.0016 838 9 7.1 139 50 0.36 20 0.000081 0.30 0.1410 0.0083 850 47 8.1 556 229 0.41 83 0.000045 0.30 0.1459 0.0016 878 9 9.1 57 44 0.77 9 0.000370 0.44 0.1481 0.0027 890 15 10.1 723 322 0.44 107 0.000018 0.30 0.1439 0.0015 866 9 11.1 182 103 0.57 28 0.000214 0.52 0.1451 0.0017 874 10 12.1 134 81 0.61 21 0.000185 0.37 0.1469 0.0019 883 11

206 Note: Uncertainties given at the 1r level; f206 (%) denotes the percentage of Pb that is common Pb; – denotes no common Pb detected; correction for common Pb made using the measured 238U–206Pb and 207Pb–206Pb ratios following Tera and Wasserburg (1972), as outlined in Compston et al. (1992). B. Yibas et al. / Journal of African Earth Sciences 34 (2002) 57–84 73

Fig. 5. Tera–Wasserburg U–Pb concordia plots of SHRIMP data (uncorrected for common Pb) for samples of granitoid rocks from southern Ethiopia: (a)–(g) and (j) are Tera–Wasserburg U–Pb concordia plots of SHRIMP data (uncorrected for common Pb) for samples from granitoids of southern Ethiopia: (a) Metoarbasebat granite; (b) Meleka granodiorite; (c) Digati diorite gneiss; (d) Wadera megacrystic diorite gneiss; (e) Wadera deformed granite dyke; (f) Moyale granodiorite; (g) Melka Guba megacrystic diorite gneiss (X ¼ age data which give a mean 207Pb/206Pb age of 778 23 Ma, and filled circles represent age data which give a mean 207Pb/206Pb age of 525 25 Ma––compare text); (j) Bulbul mylonitic diorite gneiss; (h) is a conventional (Wetherill) U–Pb plot; and (i) is a Th–U versus age plot for the two main groups of SHRIMP age data for the Melka Guba megacrystic diorite gneiss (filled circles represent younger age data, which give a mean age of 525 12 Ma, and filled squares represent older age data, which give a mean age of 778 23 Ma––compare text). the Digati diorite gneiss (discussed below). Although for a probable xenocrystic core provides an upper limit Ages II and I are statistically possible, the zircons from for the formation of this pluton. This 700 Ma age cor- which these data were obtained are indistinguishable, relates well with the age of emplacement of the Moyale and, thus, do not allow interpretation of any one of ophiolitic amphibolite, as constrained by U–Pb zircon these two ages as the magmatic age for the Meleka fo- ages by Teklay et al. (1998). liated granodiorite. The bulk of the SHRIMP age data for the Meleka zircons (between 550 and 639 Ma) is 3.2.3. Digati diorite gneiss interpreted to bracket the emplacement age for the The Digati diorite gneiss (Sample BY96-18B) occurs Meleka foliated granodiorite, whereas the 700 Ma age along the western tectonic boundary between the 74 B. Yibas et al. / Journal of African Earth Sciences 34 (2002) 57–84

Fig. 5 (continued)

Megado ophiolitic fold and thrust belt and the western 5). Most of the analyses plot as a coherent group, but a part of the Adola granite-gneiss complex (Fig. 2). It is number of data scatter around it (Fig. 5c). The main well banded and composed of feldspars, quartz, am- group of data yields a weighted mean 206Pb/238U age of phibole, biotite and garnet. The garnet porphyroblasts 570 7 Ma and is interpreted to provide the age of contain inclusion trails, probably on earlier S-surfaces. emplacement to the diorite. Two generations of biotite are evident in thin section. Those analyses which fall outside of this group in- The earlier generation defines the foliation together with clude: (1) one analysis (3.1 in Table 5, Fig. 5c) that quartz and feldspar grains, whereas the younger biotites is significantly older (635 11 Ma) and was deter- clearly post-date deformation and are randomly dis- mined in an intermediate zone of a subhedral grain; it tributed and sometimes cluster in knots. is plausible to interpret this analysis to indicate inher- Basic dykes intruded the Digati diorite gneiss con- itance from an older crustal component; (2) several cordant to the gneissic foliation. These basic dykes are zircons (9.1, 13.1 and 14.1 in Table 5), which have fine-grained, dark grey and well foliated, and are com- obviously lost radiogenic Pb; (3) a single analysis of a posed mainly of amphibole and plagioclase. The basic complex zircon (7.2 in Table 5) from an area which had dykes are, in turn, intruded by felsic veinlets, which are been identified through cathodoluminescence imaging tightly folded parallel to the foliation. as an overgrowth. This new zircon growth post-dates Mineralogical studies of the zircon grains from the the main magmatic growth and also has a lower Th/U sample collected from the Digati diorite gneiss show the ratio than the older zircon. This feature is common presence of different populations of zircon in terms of among metamorphic zircons; hence, this analysis was both their size and morphology (Fig. 4c). The dominant excluded from the final age calculation and this age is zircon population is translucent to cloudy and long to interpreted as the metamorphic age for the Digati di- short prismatic, with rounded to sub-rounded to bipy- orite gneiss. ramidal bases. Some of the long prismatic grains with The 40Ar–39Ar laser age determination on biotite bipyramidal faces are fractured. grains gave a mean age of 502 4 Ma (Table 6). The Fifteen SHRIMP U–Pb zircon analyses were made core of the biotite grains gave younger ages than the on 14 zircon grains from the Digati diorite gneiss (Table rims. B. Yibas et al. / Journal of African Earth Sciences 34 (2002) 57–84 75

Table 6 Laser probe 40Ar–39Ar dating results for single mineral grains from Precambrian rocks in southern Ethiopia (N ¼ number of analysed points per sample) Belt/complex Sample no. Rock type Mineral Grain no. Analysis position Age Mean age (Ma) Moyale–Sololo BY96-10B Metoarbasebat Biotite BY2-1 Rim 493 7 506 4(N ¼ 5) granite-gneiss granite Core 504 7 complex BY2-2 Rim 510 7 Core 501 7 BY2-3 Rim 511 7 Core 502 10 Hornblende BY2-4 Rim 505 9 511 4(N ¼ 3) Core 515 16 BY2-5 Rim 513 8 Core 462 21 Adola granite- BY96-13AMeleka foliated Biotite BY3-1 Rim 540 7 512 4(N ¼ 4) gneiss complex granodiorite Core 529 9 BY3-2 Rim 515 6 Core 512 6 BY3-3 Rim 512 6 Core 505 9 Muscovite BY3-4 Rim 512 6 515 3(N ¼ 4) Core 491 12 BY3-5 Fusion 512 7 Megado ophio- BY96-18B Digati diorite Biotite BY4-1 Rim 506 5 502 4(N ¼ 4) litic fold and gneiss thrust belt Core 496 5 (Digati) BY4-2 Rim 503 5 Core 502 5 Bulbul Belt BY96-81 Bulbul diorite Biotite BY5-1 Rim 514 6 495 5(N ¼ 4) mylonite gneiss Core 511 6 BY5-2 Rim 484 5 Core 475 4 BY5-3 Rim 491 5 BY5-4 Core 421 4

3.2.4. Wadera megacrystic diorite gneiss row rims of possible metamorphic origin can be ob- The mylonitic Wadera diorite gneiss (sample BY97- served. 178) is coarse-grained with megacrystic K-feldspar The U–Pb SHRIMP data for this sample (Table 6) grains. It is composed of quartz, K-feldspar, plagioclase, are plotted on a Tera–Wasserburg diagram (Fig. 5d). amphibole and biotite. The megacrysts were modified by There is a well-constrained group of data for grains deformation and exhibit augen (flaser) texture. This body from both zircon populations, near the concordia curve, intrudes concordantly into the amphibole gneiss of the but also some evidence of Pb loss in four of the analyses, Genale–Dolo granite-gneiss complex. The megacrystic which are spread out to the right of the main group, diorite gneiss has been intruded later by a number of giving relatively younger apparent 206Pb/238U ages. The granitic dykes that are discordant to the pervasive foli- main group of data gives a weighted mean 206Pb/238U ation and both are deformed. age of 579 5Ma(v2 ¼ 1:38; N ¼ 14), which is inter- Two main zircon populations were separated from preted as the magmatic age of the body. The other four the sample collected from the Wadera megacrystic di- analyses gave younger ages of 554–524 Ma. These ages orite gneiss (Fig. 4d). The first one is dominated by long, are all from rim overgrowths of zoned grains and likely medium-grained, prismatic zircon grains, with rounded represent post-magmatic zircon growth. terminations and less prominent pyramidal faces, and that are smoky, yellow or transparent. Afew grains with 3.2.5. Wadera deformed granitic dyke relatively well-developed pyramidal faces were also This is a foliated granitic dyke (sample BY97-178A) observed in this group. The second population is char- which discordantly intrudes the Wadera megacrystic acterised by fine-grained zircon crystals that are domi- diorite gneiss. The dyke is composed of quartz, K-feld- nated by oval to round shapes and that are of probable spar and plagioclase, which is strongly altered to epidote metamorphic origin. Grains of both populations exhibit group minerals, and accessory biotite and sphene. The intricate multiple zonation of the type that could mainly zircon population of the sample comprises a significant be attributed to magmatic zircon growth. Rarely, nar- proportion of rounded to sub-rounded, short prismatic 76 B. Yibas et al. / Journal of African Earth Sciences 34 (2002) 57–84 crystals, as well as long-prismatic, intricately zoned and probably magmatic population. The U–Pb data for 15 variably terminated (pyramidal or stubby terminations) analyses on 12 zircon crystals are presented in Table 5 grains (Fig. 4e). The SHRIMP U–Pb analyses for the and are plotted on a Tera–Wasserburg concordia plot in zircons from the Wadera foliated granitic dyke sample Fig. 5f. Eleven of the analyses define a statistically ro- are listed in Table 5 and plotted in Fig. 5e. Apart from bust group with a weighted mean 206Pb/238U age of one analysis (13.1, Table 5), which suggests that this 666 5Ma(v2 ¼ 1:49). The weighted mean 207Pb/206Pb grain has suffered Pb loss, and analysis 12.1, which is age for all analyses gives a less precise date of 669 10 from an apparently late overgrowth, all data can be Ma (v2 ¼ 1:03; N ¼ 15), but this age is in excellent combined to produce a well-defined weighted mean agreement with the 206Pb/238U age and is regarded as 206Pb/238U age of 576 7Ma(v2 ¼ 1:31)––a slightly the age of magmatic emplacement of the granodiorite. younger age than the age obtained for the host, the Teklay et al. (1998) obtained a 657:9 0:8 Ma single megacrystic diorite gneiss, but within its error limits. zircon 207Pb/206Pb evaporation age for a sample from the Had it not been for the clearly discordant field rela- Moyale granodiorite, which is also in good agreement tionship between the host megacrystic diorite and this with the results reported here. dyke, one would interpret the results for the granite dyke and the megacrystic diorite as indicating syn-tec- 3.2.7. Melka Guba megacrystic dioritic gneiss tonic, near-coeval formation of both phases. While it The Melka Guba megacrystic diorite gneiss (sample cannot be excluded that the host diorite was thermally BY97-183) occurs in the southwesternmost part of the affected by the intrusion of the dyke, the detailed field, Genale–Dolo granite-gneiss complex (Fig. 2) close to the petrographic and isotopic evidence that could perhaps Melka Guba Bridge crossing the Dawa River. The di- support this view are not as strong as the bulk of the orite gneiss is mainly composed of amphibole, feldspar isotopic evidence. We prefer to interpret these data as and quartz, and is well banded with prominent K-feld- indicating that both the dyke and the host diorite were spar augen. It is strongly deformed with characteristic formed within a few Ma of each other and that the hook folds and transpositional gneissic layering. Two 579 5 Ma and 576 7 Ma ages represent likely mag- populations of zircon grains are present (Fig. 4g). The matic ages for the host diorite and the granitic dyke, first population is dominated by cloudy and long-pris- respectively. This indicates magmatism at 570–580 Ma matic zircon grains, which have bipyramidal termina- that was followed by partial resetting at 550–525 Ma. tions. Afew acicular, smoky brown grains with slightly However, the distinct differences in fabrics of the two to moderately rounded bipyramidal bases were also lithologies imply that a deformation has clearly occurred observed in this group. The second population is dom- prior to the intrusion of the dyke into the diorite, which inated by light-coloured, less cloudy to translucent was followed by deformation. The mylonitic fabric (a few smoky), and rounded to oval-shaped (possibly of the gneisses observed throughout the Genale–Dolo metamorphic) zircon grains. Afew of these grains show granite-gneiss complex are interpreted as the result of new growth of zircon on one of their tips. the Wadera shearing event at about 580 Ma, coeval with The cathodoluminescence images of the complex this magmatic event. zircon population (Fig. 4g) and the SHRIMP data (Table 5; Fig. 5g) manifest the structural complexity of 3.2.6. Moyale granodiorite these zircons. The oldest analysis (10.2) was obtained The Moyale granodiorite occurs as a NW–SE trend- from a core of a small zircon crystal. The apparent age ing body in and around Moyale town. This granodiorite of this obviously inherited component is approximately intruded the Moyale fold and thrust sub-belt, as wit- 2050 Ma (Fig. 5g). Two groups of analyses plot near the nessed by the nature of its contact with the amphibolites concordia (Figs. 5g and h), with the older group show- to the east and the migmatised amphibole gneiss to the ing some scattering that suggests either Pb loss or grains west (Fig. 2). Xenoliths of amphibolites are common of different ages. Aweighted mean 207Pb/206Pb age of along and near the intrusive contact. Plagioclase, quartz 778 23 Ma can be calculated from five of the six an- and K-feldspar are the dominant minerals in the gran- alyses within this group, but the spread in U–Pb ratios odiorite, with minor to trace amounts of biotite, clino- (Fig. 5h) precludes the calculation of any meaningful zoisite, epidote, sphene, apatite and chlorite. 206Pb/238U age. The other group also shows some spread The zircon population of the Moyale granodiorite in U–Pb ratios (Fig. 5h), but all six analyses give a (sample BY97-79) is relatively homogeneous, the dom- weighted mean 207Pb/206Pb age of 525 12 Ma (Fig. 5g). inant population characterised by long-prismatic forms. The different age groups determined cannot be consis- Some of the grains are slightly rounded, transparent to tently matched to zircon morphology. For example, the translucent crystals, with a subordinate amount of short data of the 778 Ma group are obtained dominantly from prismatic grains (Fig. 4f). Apart from variable degrees core analyses, but some are from outer zones on mag- of Pb loss in a number of zircon grains, the zircons from matic, euhedral zircons. There is also a definite differ- this sample appear to belong to a relatively simple, ence in Th/U ratios of the grains from the two age B. Yibas et al. / Journal of African Earth Sciences 34 (2002) 57–84 77 groups (Fig. 5i). The lower Th/U ratios of the younger than those from the rims. The 40Ar–39Ar age falls within phase of zircon growth might be indicative of meta- the age range obtained for the Metoarbasebat unde- morphic origin (Tera and Wasserburg, 1972). formed granite and is interpreted as signature of the The older group of data with a weighted mean age of youngest tectonothermal event in the basement complex 778 23 Ma is interpreted as the magmatic emplace- of southern Ethiopia. ment age for the pluton, which can be bracketed by the 807 20 and 703 17 Ma old data. The youngest age group can also be resolved into 552 13 and 529 16:5 Ma sub-groups. These data are all obtained from over- 4. Discussion growths. The results are consequently interpreted to represent post-magmatic events that are correlative with 4.1. Tectonostratigraphy the emplacement of the Digati diorite gneiss and Met- oarbasebat undeformed granite, respectively. Consider- The general tectonostratigraphic classification (Lower, ing the strong gneissic fabric developed on the Melka Middle and Upper Complexes––after Kazmin, 1972; Guba megacrystic diorite, it appears plausible to inter- Kazmin et al., 1978) for the Precambrian of Ethiopia pret the 552 13:25 Ma age as a metamorphic age for (Table 2) was based mainly on metamorphic grade and the pluton. deformational differences. This classification suggested a prevalence of Archaean gneisses in the Precambrian of 3.2.8. Bulbul mylonitic diorite gneiss southern Ethiopia, but was not based on absolute geo- The Bulbul mylonitic diorite gneiss occurs within the chronological data. The validity of this classification has Bulbul fold and thrust belt (Figs. 2 and 3). The diorite is been challenged recently following U–Pb zircon dating coarse-grained, grey, strongly sheared and well foliated. of various gneisses (Gichile, 1991; Ayalew et al., 1990; It is composed of plagioclase, quartz, amphibole, brown Teklay et al., 1998; Worku, 1996; Yibas, 2000; Yibas biotite and accessory sphene and zircon, and is charac- et al., 2000a,b). The rocks mapped as the Aflata Forma- terised by ophitic feldspars rotated during the develop- tion by Kozyrev et al. (1985) were earlier regarded by ment of the N–S trending mylonitic fabric. Kazmin (1972, 1975) as Archaean in age (Table 2). Two dominant zircon populations are identified in However, most of the granitoids mapped as part of the the Bulbul mylonitic diorite (sample BY96-81): the Aflata Formation are Neoproterozoic in age (e.g., the first population comprises long-prismatic crystals, with 765 3 Ma Sebeto tonalite and the 722 2MaAlghe rounded to semi-rounded bases, and the second popu- granite gneiss) and are subduction-related granitoids lation is dominated by short-prismatic grains with (Gichile, 1991; Worku, 1996; Yibas, 2000; Yibas et al., rounded to semi-rounded bases (Fig. 4h). 2000d). The Bulbul mylonitic diorite, which has a vol- As shown on a Tera–Wasserburg concordia diagram canic-arc geochemical affinity (Yibas, 2000; Yibas et al., (Fig. 5j), the SHRIMP analyses on zircons from the 2000d), yielded a SHRIMP age of 876 5 Ma, repre- Bulbul diorite show that there is significant complexity senting the oldest emplacement age so far obtained in and heterogeneity within individual separates. The ma- the Precambrian of southern Ethiopia. jority of the data points cluster near the concordia Recent geological mapping, supported by geochemi- curve. For this group, a 206Pb/238U age of 876 7Mais cal and U–Pb zircon geochronological studies, has dem- calculated (v2 ¼ 0:39; N ¼ 10), which appears to be onstrated that the Precambrian of southern Ethiopia is the magmatic age for this sample. Three analyses (1.3, dominated by granitoids and orthogneisses emplaced 3.1 and 6.1) fall to the right of the main group and between 900 and 550 Ma (Yibas, 2000; Yibas et al., give lower apparent ages. Cathodoluminescence imaging 2000a,b, 2000c, 2000d). Some paragneisses are also showed that these zircons are not part of a younger present and are dominantly quartzofeldspathic gneisses overgrowth phase, and, accordingly, it is concluded that intercalated with amphibolites or amphibole-bearing these grains have suffered Pb loss. Analysis 1.1 from gneisses, sillimanite–kyanite-bearing schist and marbles. the core of a zoned grain gave a significantly older age The paragneisses extend southwards into northern (910 10 Ma) compared to the overgrowth on this Kenya and are lithologically similar to the arenaceous grain, which is part of the 876 7 Ma age group. This clastic successions, basinal shales and shelf sediments age is regarded as the time of emplacement of this plu- (carbonaceous limestones, and aluminous shales) that ton, which makes the Bulbul diorite the oldest, so far filled the Mozambique Belt basin of Kenya between dated, pluton in the basement complex of southern 1200 and 820 Ma (Mosley, 1993). In southern Ethiopia, Ethiopia. this event must have occurred between about 1050 and Laser 40Ar–39Ar dates for biotite grains from this 880 Ma, as indicated by the oldest recognised meta- sample ranges from 514 6 to 421 4 Ma. The lower morphic ages (Rb/Sr; Chater, 1971) and the earliest arc ages are too young when compared with the other data magmatism (Bulbul diorite; Yibas, 2000; Yibas et al., set. The ages from the cores of biotite grains are younger 2000b,d). 78 B. Yibas et al. / Journal of African Earth Sciences 34 (2002) 57–84 Mosley (1993) argued that the Mozambique Belt granodiorite. This age correlates well with the age of basin of Kenya was formed across a thinned and down- emplacement of the Moyale ophiolitic amphibolite, as warped basin or rift sited along, and to the east of, the constrained by U–Pb zircon ages obtained by Teklay exposed craton margin of Pre-Pan-African crust. This et al. (1998). The main group of SHRIMP data from crust could be of Archaean to Mesoproterozoic age the Digati diorite gneiss yields a weighted mean 206Pb/ (Shackleton, 1994). The volume of paragneisses formed 238U age of 570 7 Ma, which is interpreted as the from sediments of this period is small in southern provide age of emplacement for this diorite. Asingle Ethiopia, in comparison to that of Kenya. Hence, it can analysis of a complex zircon, which also has a lower be argued that this ‘‘Mozambique Belt basin’’ became Th/U ratio than the older zircon, post-dates the main progressively narrower northwards and that the devel- magmatic growth and is interpreted as a metamorphic opment of Mozambique Belt gneisses in the Precam- age. The zircon ages obtained from both the Wadera brian of southern Ethiopia is much less than previously megacrystic diorite gneiss and the deformed granite thought (Yibas, 2000; Yibas et al., 2000a). dyke intruded into the diorite are interpreted as indi- Based on the new geological and geochronological cating magmatism at 570–580 Ma, during which pe- data presented here, coupled with the geochemistry and riod both the dyke and the host diorite were formed petrogenesis of the granitoids, mafic and sedimentary within a few million years of each other, followed by rocks (Yibas, 1999, 2000; Yibas et al., 2000c,d), a new partial resetting at 550–525 Ma. On account of the tectonostratigraphic classification of the Precambrian of strong gneissic fabric of the diorite and the magmatic southern Ethiopia is proposed. This classification is nature of its zircons it is suggested that the 579 5 and summarised in Table 8. the 575 6 Ma ages are the emplacement ages for the host diorite and the dyke, respectively. The 778 23 4.2. Granitic magmatism in southern Ethiopia Ma SHRIMP age obtained from cores and intermediate zones of magmatic zircons from the Melka Guba The geochronological data obtained from SHRIMP megacrystic diorite gneiss is most likely the mag- and 40Ar–39Ar dating helped considerably to decipher matic emplacement age for this pluton, whereas the age the sequence of major magmatic and tectonothermal groups of 552 13 and 525 12 Ma, which were all events that affected the Precambrian of southern Ethi- obtained from overgrowths, represent post-magmatic opia between 900 and 500 Ma. The presence of Ar- events that are coeval with the same emplacement of chaean rocks in southern Ethiopia has, thus far, not the Digati and Metoarbasebat granitic bodies, respec- been supported by geochronological data. The ever- tively. The 552 13 could possibly approximate the age increasing geochronological database (Teklay et al., of deformation that imparted the strong gneissic fabric 1998; Worku, 1996; Yibas, 2000; Yibas et al., 2000b; this on the diorite. The 876 7 Ma zircon age obtained for work) now suggests that the Precambrian terrane is the Bulbul mylonitic diorite is regarded as the mag- dominated by granitoids and ophiolites emplaced be- matic age for the pluton. This is the oldest magmatic tween 900 and 700 Ma, which was followed by colli- age obtained so far in the Precambrian basement of sional granitic magmatism and coalescence between 700 southern Ethiopia. and 550 Ma. The presence of Palaeoproterozoic ‘‘Pre- The SHRIMP zircon geochronological data, when Mozambique Belt’’ crust in southern Ethiopia can, coupled with the results of field studies (Yibas, 2000; however, be inferred from xenocryst ages ranging from Yibas et al., 2000b), permitted the classification of the 2050 82 to 1362 43 Ma (SHRIMP, zircon ages, granitoids into: Gt1 (> 880 Ma), Gt2 (800–770 Ma), Yibas, 2000; Yibas et al., 2000b; this work; for meta- Gt3 (770–720 Ma), Gt4 (720–700 Ma), Gt5 (700–600 rhyolite: 1125 2:5 and 1656:8 1:9 Ma, single zircon Ma), Gt6 (580–550 Ma) and Gt7 (550–500 Ma) (Table 7; evaporation technique, Teklay et al., 1998). In Kenya, Yibas et al., 2000a,b,c,d). evidence for Palaeoproterozoic Ubendian events (2000– The mean 40Ar–39Ar ages obtained from laser probe 1800 Ma), which are common in other parts of east and dating on biotite, muscovite and hornblende grains central Africa, is scarce and represented only by Nd ages range from 500 Ma for the Bulbul diorite to 515 3 of 1940 Ma (Harris et al., 1986). Ma for the Meleka granodiorite. There is no obvious The U–Pb SHRIMP age of 526 5 Ma obtained for age difference for rims and cores of most grains. With the Metoarbasebat granite is interpreted as the mag- the exception of the Metoarbasebat granite, for which matic emplacement age for the pluton, which is one the 40Ar–39Ar ages are similar to the SHRIMP U–Pb of the youngest for the granitoids of southern Ethio- zircon age, the 40Ar–39Ar laser ages for the other pia. The magmatic age for the Meleka granodiorite granitoids are distinct from the corresponding SHRIMP is bracketed by the 550–639 Ma SHRIMP data ob- zircon ages. This suggests that the 40Ar–39Ar ages for the tained from the non-xenocrystic zircons of this sample, deformed granitoids and granitoid gneisses represent whereas the 700 Ma age for a probable xenocrystic core resetting by relatively young tectonothermal and mag- provides an upper limit for the formation of this matic events. B. Yibas et al. / Journal of African Earth Sciences 34 (2002) 57–84 79

Table 7 Geochronological classiﬁcation of granitic magmatism in the Precambrian of southern Ethiopia during the East African Orogeny Granitic phase [age in Ma] Dated granites Age (Ma) Zircons (U–Pb) 40Ar–39Ar laser Gt7 [550–500] Metoarbasebat granite 526 5a 506 4 (Bt); 511 3 (Hb) Berguda charnockitic granite 528 8:4b (rim) 538 3b (core) Lega Dima granite 550 18c Robele granite 554 23d Gt6 [580–550] Wadera foliated granite 576 5a Wadera megacrystic diorite gneiss 579 5a Digati dioritic gneiss 570 5a 502 4 (Bt) Gt5 [700–600] Burjiji granitic massif 602c Meleka foliated granodiorite 610 9a 512 4 (Bt), 515 4 (Mu) Gariboro granite 646c Moyale granodiorite 666 5a Gt4 [720–700] Finchaa biotite-foliated granite 708 5d Yabello charnockitic granite gneiss 716 5e Gt3 [770–720] Alghe granite gneiss 722 2e Sagan basic charnockite 725e Zembaba granite gneiss 756 6e Sebeto tonalite gneiss 765 3d Gt2 [800–770] Melka Guba megacrystic granodiorite gneiss 778 23a Gt1 [> 880] Bulbul diorite mylonite 876 7a 495 5 (Bt) Hb ¼ hornblende, Bt ¼ biotite, Mu ¼ muscovite. a Data from this study; SHRIMP, U–Pb (all others ¼ U–Pb single zircon evaporation method). b Gichile (1991). c Worku (1996). d Genzebu et al. (1994). e Teklay et al. (1998).

4.3. Correlation with neighbouring regions of the East granitoid gneisses in northern Somalia and southern African Orogen Ethiopia, in which these old zircon xenocrysts were found, have magmatic ages ranging from 780 to 840 Ma. In general, the ages of selected granitoids of the Pre- In the western part of the crystalline basement of north- cambrian of southern Ethiopia show similarity to the ern Somalia, zircon xenocrysts gave Palaeoproterozoic Neoproterozoic 830–540 Ma ages obtained from the U–Pb single zircon evaporation ages of 1400 to 1820 granitoids of the western Ethiopian Precambrian terrane Ma in 720–840 Ma old granitoids (Krooner€ and Sassi, (Ayalew et al., 1990). It is also evident that the dominant 1996). granitic magmatism in the area occurred during the period between 750 and 500 Ma; only 10 out of the 42 4.4. Tectonomagmatic events, metamorphism and post- ages now available are older than 750 Ma (Fig. 6). Pre- orogenic cooling Neoproterozoic ages are recognised from zircon xenocrysts in some of the granitoids of southern Ethiopia, The following tectonothermal sequence is proposed such as the Melka Guba diorite gneiss, from which xe- for the Precambrian of southern Ethiopia, based on field nocryst SHRIMP ages ranging from 2050 to 1362 Ma mapping and available geochronological data. Possible were obtained. Teklay et al. (1998) also obtained xeno- correlation of these tectonothermal events with those of crystic single zircon evaporation ages of 1125 2:5 and the adjacent Neoproterozoic terranes of western Ethio- 1656:8 1:9 Ma from a metarhyolite in southern Ethi- pia, northern Kenya, Somalia and the northern part of opia. These xenocryst ages strongly indicate the presence the Arabian–Nubian Shield is also suggested. of Palaeo- to Mesoproterozoic continental crust in the Precambrian of southern Ethiopia, which has been re- 4.4.1. Adola tectonothermal event (1157 2to 1030 40 juvenated during the Neoproterozoic. Palaeoproterozoic Ma) to Archaean ages for xenocryst zircons were also found This tectonothermal event is inferred from U–Pb ages in granitoid gneisses in northern Somalia (Krooner,€ 1993) (1125 2:5 and 1156:8 1:9 Ma, Teklay et al., 1998; and in eastern Ethiopia (Teklay et al., 1998). The 1362 43 Ma, this study) of zircon xenocrysts obtained 80 B. Yibas et al. / Journal of African Earth Sciences 34 (2002) 57–84 4.4.3. Megado tectonothermal event (770–720 Ma) ASm–Nd isochron age of 789 36 Ma for the Me- gado metavolcanics (Worku, 1996), a 778–760 Ma U– Pb zircon SHRIMP age (this study) and a U–Pb zircon age (Genzebu et al., 1994) for some granitoids in the study area imply the presence of a strong tectonothermal event during this period. These granitoids were geochemically classified into those emplaced either in attenuated continental crust or in a subduction-related setting associated with the closure of the Megado oceanic basin and ophiolite obduction (Yibas, 2000). The 749 15 Ma K–Ar whole-rock age for the El Der hornblende–biotite gneiss (Rogers et al., 1965) is now interpreted to approximate the peak metamorphism associated with the closure of the Megado ocean. In the western part of the study area, the granulite facies metamorphic pressure of 9 kbar indicates significant crustal thickening related to collision-induced thrusting (Gichile, 1991, 1992). This is recognised in western Ethiopia as a period of regional metamorphism (Ayalew et al., 1990) coincident with the age of many ‘‘ophiolites’’ in the Arabian–Nubian Shield (Mosley, 1993). Emplacement of ophiolites and volcanosedimentary Fig. 6. Histogram of available geochronological data for the Pre- sequences, coupled with thrusting and imbrication of cambrian rocks of southern Ethiopia (for details on the data see Tables paragneisses in Kenya, took place at about 760 Ma 1 and 8). (Cahen et al., 1984). The period between 710 and 730 Ma was dominated by emplacement of charnockitic rocks in southern Ethiopia (Teklay et al., 1998). This event could be correlative with the widespread presence from the eastern part of the granite-gneiss terrane of of charnockitic granites and granulite facies metamor- southern Ethiopia. The oldest whole-rock Rb-Sr meta- phism from Sudan to Tanzania (Stern and Dawoud, morphic age (1030 40 Ma) obtained for the metase- 1991; Maboko et al., 1995) and further south into Ma- dimentary rocks of Adola (Chater, 1971) is used to lawi (Krooner,€ 1993). constrain the upper limit of this tectonothermal event. In north-central Kenya, the existence of ‘‘cold migmat- 4.4.4. Moyale tectonothermal event (700–550 Ma) itic basement’’ of probable Kibaran age was inferred by The fourth tectonothermal event in southern Ethio- Key et al. (1989) based on a Rb/Sr isochron age of pia, which might have taken place between 700 and 550 1200 Ma, which was interpreted as the age of migmati- Ma, is related to the closure of the Moyale oceanic basin sation. and a protracted period of subduction-related granitic magmatism (Yibas, 2000). The major granitoids em- 4.4.2. Bulbul–Awata tectonothermal event (876 5 placed during this event include: the Moyale granodi- Ma) orite (666 5 Ma); Digati diorite gneiss (570 7 Ma); Geochemical data for the granitoids of southern Wadera megacrystic diorite gneiss (579 5 Ma); the Ethiopia (Yibas, 2000; Yibas et al., 2000d) suggest that Gariboro collisional granitoids (602–646 Ma); and the the 876 5 Ma old Bulbul mylonitic diorite was em- Meleka granodiorite (570–610 9 Ma). The whole rock placed in a subduction-related setting regarded to rep- Rb–Sr ages of 680 10 Ma (Gilboy, 1970) and 594– resent the first subduction-related tectonothermal event 605 Ma (Worku, 1996) obtained for the Gariboro and in the East African Orogen of southern Ethiopia. The Burjiji granitoids, respectively, might indicate a colli- earliest tectonothermal event associated with the Mo- sion-induced metamorphic event (Gilboy, 1970; Worku, zambique Belt in Kenya is the Samburian–Sabachian 1996; Yibas, 2000). This is possibly correlative with event at about 830 Ma (Key et al., 1989). This event is the 630–580 Ma, Baragoian–Barsaloian tectonothermal characterised by amphibolite–granulite facies meta- event, which resulted in amphibolite facies metamor- morphism and recumbent folding related to oblique phism in Kenya (Key et al., 1989). The event could collision, between the Archaean Tanzanian Craton in possibly be correlated with whole-rock Rb–Sr isochron the west and an eastern Kibaran Craton to the east (Key and U–Pb zircon ages ranging from 582 to 541 Ma, et al., 1989; Charsley, 1987). which are interpreted to bracket the last metamorphism B. Yibas et al. / Journal of African Earth Sciences 34 (2002) 57–84 81 and deformation in western Ethiopia (Ayalew et al., dicate a final tectonothermal event referred to here as 1990). the Berguda tectonothermal event. In Kenya, this event correlates with a 500–480 Ma event, which has been interpreted as a period of uplift, thrusting, emplacement 4.4.5. Berguda tectonothermal event (550–500 Ma) of pegmatites and melts, and final cooling (Key et al., This period, which marks the end of the Neoprote- 1989; Mosley, 1993). rozoic orogeny, is manifested by emplacement of late- to post-tectonic granitoids throughout southern Ethiopia. However, evidence of granulite facies metamorphism during this period is obvious in the presence of the 5. Conclusions charnockitic Berguda granitoid in the west (Gichile, 1992). Gichile (1992) recognised a granulite facies meta- 1. New geological and geochronological data call for morphism and intense deformation in the core of this a revised tectonostratigraphic classification of the Pre- layered granitoid and suggested that this pluton is a cambrian rocks of southern Ethiopia. Geological and tectonically uplifted block. U–Pb zircon ages of structural maps for the study area (Yibas, 2000), to- 538 2:8 and 528 8:4 Ma were obtained for this gether with geochemical (Yibas, 2000; Yibas et al., pluton (Genzebu et al., 1994), one for the granulitic 2000c,d) and geochronological data and interpretation core and another for an intermediate zone, respectively. and consideration of the major fault–shear zones that These ages are similar to those of granulites in other separate the lithologic associations (Yibas, 2000), allow parts of the East African Orogen, where granulite facies the establishment of a plausible tectonostratigraphy for metamorphism and associated deformation occurred the Precambrian of the study area (Table 8). between 553 and 521 Ma, as a result of initial Gondw- 2. Two distinct lithotectonic terranes, which differ in ana amalgamation (Jacobs et al., 1995). The presence of terms of lithological association, internal structures and these charnockitic plutons in proximity with unde- grade of metamorphism, are recognised. These terr- formed granitoid, with similar U–Pb zircon ages, sug- anes include: (1) the granite-gneiss terrane composed of gests emplacement of both granitoid types into different high-grade para- and orthogneisses and deformed and levels of the crust and subsequent juxtaposition by metamorphosed granitoids; and (2) ophiolitic fold and thrusting, and strike-slip faulting and shearing associ- thrust belts composed of low-grade, volcanosedimen- ated with uplifting and final cooling. tary–ultramafic assemblages. Major shear zones, which The youngest granite age thus far recorded in are characterised by repeatedly reactivated strike-slip southern Ethiopia is that of the 526 Ma Metoarbasebat and/or thrust faults, separate these distinct terranes granite. 40Ar–39Ar ages from biotite, muscovite and (Figs. 2 and 3). hornblende grains from selected granitoids range from 3. The granite-gneiss terrane is classified into 500 to 515 3 Ma. These ages are interpreted to in- the Burji–Moyale and Adola–Genale granite-gneiss

Table 8 Simplified tectonostratigraphy of the Precambrian of southern Ethiopia (modified from Yibas, 2000 and Yibas et al., 2000a) Era/epoch Age (Ma) Lithologic group/complex/belt Early Cambrian 550–500 Post-tectonic, post-orogenic granitoids, e.g., Berguda charnockitic granite (528 8:4 to 538 3 Ma), Metoarbasebat granite (526 5 Ma), Robele granite (554 23 Ma), Lega Dima granite (554 8 Ma) Late Neoproterozoic 700–570 Arc and collisional granitoids, e.g., Wadera megacrystic diorite gneiss (579 5 Ma), Digati diorite gneiss (570 7 Ma), Meleka foliated granodiorite (610 9 Ma), Gariboro–Burjiji granitic massif (646–602 Ma), Moyale granodiorite (680 Ma) 700 Moyale–El Kur Belt: essentially mafic–ultramafic rocks with minor metasediments; mafic rocks ¼ low-Ti tholeiites and boninites 790 Granitoids, e.g., Melka Guba diorite gneiss (750 Ma), Sebeto tonalite gneiss (780 Ma) Megado Belt: mafic–ultramafic and metasedimentary rocks; mafic rocks ¼ low-Ti tholeiites and boninites ?? Kenticha Belt: mafic–ultramafic and metasedimentary rocks; mafic rocks are mainly calc– alkaline with minor tholeiites 900 Granitoids, e.g., Bulbul diorite gneiss Bulbul Belt: mainly mafic rocks, minor ultramafic and metasedimentary rocks; mafic rocks are dominantly calc–alkaline with a minor tholeiite component Mesoproterozoic 1050 Burji–Finchaa paragneisses Moyale–Sololo Adola gneisses Genale–Dolo parag- paragneisses neisses Palaeo- to Mesoprotero- 2050–1250 Pre-Pan-African Crust zoic 82 B. Yibas et al. / Journal of African Earth Sciences 34 (2002) 57–84 sub-terranes, which are separated by the major Sebeto– 10. Five tectonothermal events in the evolution of the Chelanko Fault Zone. The Burji–Moyale sub-terrane is East African Orogen are recognised in the Precambrian further divided into the Burji–Finchaa and the Moyale– of southern Ethiopia: the Adola (1157 2 to 1030 40 Sololo complexes. The Adola–Genale sub-terrane is di- Ma), Bulbul–Awata (876 5 Ma), Megado (800–750 vided into the Adola and Genale–Dolo complexes. Ma), Moyale (700–550 Ma) and Berguda (550–500 Ma) 4. Four ophiolitic fold and thrust belts have been tectonothermal events. recognised. These are the Bulbul, Kenticha, Megado and Moyale–El Kur fold and thrust belts, which are composed of mafic, ultramafic and metasedimentary Acknowledgements rocks in various proportions. Felsic volcanic rocks are virtually absent in these belts. This paper resulted from the Ph.D. project of Bisrat 5. The granitoid rocks of southern Ethiopia vary Yibas, with generous financial assistance from Anglo from granitoid gneisses to undeformed granites and American Prospecting Services (AAPS) and the New compositionally from diorites to granites. The gneissose Mining Business Division (now Exploration and Ac- granitoids form an integral part of the granite-gneiss quisition Division) of the Anglo American Corporation terrane and are rare in the ophiolitic fold and thrust of South Africa Limited. The first author is particularly belts. grateful to Roy Corrans, Steve Marsh, Nick Franey and 6. The combined use of SHRIMP and laser probe Kirsty Reid of the New Mining Business Division for 40Ar–39Ar dating, coupled with previously available data their support and encouragement. The Geological Sur- and field studies, suggests that the granitoids of the vey of Ethiopia is also thanked for granting permission Precambrian of southern Ethiopia can be classified into: to conduct this study. Ronel Malan, Lyn Whitfield and Gt1 (> 880 Ma), Gt2 (800–770 Ma), Gt3 (770–720 Ma), Di Du Toit provided expert drafting support. The con- Gt4 (720–700 Ma), Gt5 (700–600 Ma), Gt6 (580–550 structive comments of Mohammed Abdelsalam, Tse- Ma) and Gt7 (550–500 Ma). All granitoid suites are haye Woldai, Taddese Yehunie and an anonymous deformed, with the exception of the Gt7 granitoids reviewer are greatly appreciated. This is University of (Yibas, 2000). the Witwatersrand Impact Cratering Research Group 7. The period between 710 and 730 Ma was domi- Contrib. No. 40. nated by the emplacement of charnockitic rocks (Ya- bello granite gneiss, Sagan basic charnockite and Konso granulite) in southern Ethiopia. This event may corre- References late with the widespread presence of charnockitic granites and granulite facies metamorphism from Sudan to Abraham, A., Hassen, N., Yemane, T., Genzebu, W., Seyid, G., Tanzania (Maboko et al., 1995) and further south into Mehari, K., Alemu, T., 1992. The geological evolution of the € Proterozoic of southern Ethiopia, Abstract. In: 29th International Malawi (Krooner, 1993). 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