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

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The Tectonostratigraphy, Granitoid Geochronology and Geological Evolution of the Precambrian of Southern Ethiopia B 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 pro- gressively 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 Æ 2 to 1030 Æ 40 Ma); (2) Bulbul–Awata (876 Æ 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; 0899-5362/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S0899-5362(01)00099-9 58 B. Yibas et al. / Journal of African Earth Sciences 34 (2002) 57–84 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 Æ 15 Rogers et al. (1965) terrane Genale–Dolo Complex Negele Biotite gneiss Bi (K–Ar) 516 Æ 5 Rogers et al. (1965) Zembaba village Porphyritic granite (ET9) Zr (Pb–Pb) 752 Æ 6 Teklay et al. (1998) B. Yibas et al. / Journal of African Earth Sciences 34 (2002) 57–84 Zembaba village Metarhyolite (ET10) Zr (Pb–Pb) 605 Æ 7 Teklay et al. (1998) Alge,a west of the Bulbul Belt Mylonitic tonalite (Alge Gneiss of Zr (Pb–Pb) 557 Æ 9 Teklay et al. (1998) Kazmin, 1972) 35 km SW Negele (Bulbul) Porphyritic granodiorite gneiss Zr (Pb–Pb) 884 Æ 7 Teklay et al. (1998) Adola Complex Mormora River Granite Bi (K–Ar) 504 Æ 20 Jelenc (1966) Burjiji Granite Zr (U–Pb) 612 Æ 6 Worku (1996) Gariboro Pegmatite Ms (Rb–Sr) 530 Æ 10 Gilboy (1970) Granite WR (Rb–Sr) 680 Gilboy (1970) Granite WR (Rb–Sr) 515 Æ 10 Gilboy (1970) Granite Zr (U–Pb) 646 Æ 30 Worku (1996) Lega Dima Granite Zr (U–Pb) 550 Æ 18 Worku (1996) Robele Granite Zr (U–Pb) 554 Æ 13 Abraham et al. (1992) Alghea (west of Digati) Alghe granite gneiss Zr (U–Pb) 722 Æ 2 Worku (1996) Sebeto Tonalite gneiss Zr (U–Pb) 765 Æ 3 Abraham et al. (1992) Burji–Finchaa Complex Yabello town Yabello granite gneiss Zr (Pb–Pb) 716 Æ 8 Teklay et al. (1998) Agere Mariam Foliated granite Zr (U–Pb) 708 Æ 3 Abraham et al. (1992) Near Agere Mariam Berguda granite Zr (U–Pb) 529 Æ 11 Abraham et al. (1992) Konso, west of Burji– Konso granulite Zr (Pb–Pb) 720 Æ 7 Teklay et al. (1998) Finchaa Complex Ophiolitic fold and Megado Belt Megado Megado metabasic rocks WR (Sm–Nd) 789 Æ 36 Worku (1996) thrust belts Moyale–El Kur Belt Moyale Quartz diorite Bi (K–Ar) 526 Æ 5 Rogers et al. (1965) Moyale Meta-trondhjemite Zr (Pb–Pb) 658 Æ 8 Teklay et al. (1998) El Der Amphibolite WR (K–Ar) 647 Æ 20 Rogers et al. (1965) Moyale Amphibolite Zr (U–Pb) 700 Æ 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.
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