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Late Paleozoic-Early Mesozoic Tectonics in South China Around Yangtze Massif: Closing Process of the Paleo-Tethys
RODINIA, GONDWANA AND ASIA
into many terranes (Fig. l),based on their pre-Cenozoic geologic characteristics. Based on formation ages, these can be grouped into three: Pre-Jurassic terranes (Pa, Pb), Jurassic terranes (Ja, Jb, Jc) and Post-Jurassic terranes (Ka, Kb, Kc). The Jurassic terranes, which is a main subject of this paper, is distributed two-fold, the Tanba-Mino-Ashio Terrane in the Inner Zone and the Chichibu Terrane in the Outer zone. These terranes are composed mainly of Jurassic accretionary complexes including the oceanic plate stratigraphic components of Late Paleozoic to Jurassic age. The Tanba-Mino-Ashio Terrane is distributed tectonically below the pre-Jurassic terrane group in the north and changes gradually into the Cretaceous metamorphic belt (Ryoke Belt) to the south. The Chichibu Terrane develops as a large-scale nappe tectonically above the post-Jurassic terrane group (Sanbagawa Belt and Shimanto Terrane). The Kurosegawa Terrane, which belongs to the Pre-Jurassic terrane group, is intermittently distributed as a klippe tectonically above the central zone of the Chichibu Terrane. Based on plate tectonic concept, several models have been proposed for the pre-Neogene terrane arrangement of Southwest Japan (Fig. 2). An essential point of difference among these models is the interpretation of geologic relationship between the Kurosegawa Terrane and the Chichibu Terrane. On the basis of their interpretation, the models are grouped into [A] and [B]. According to model [A], the Kurosegawa Terrane has been tectonically introduced between the Northern and Southern Chichibu terranes (e.g., Maruyama et al., 1984; Hada et al., 1996; Yamakita, 1998). On the other hand, the Kurosegawa Terrane is regarded as a nappe on the Chichibu Terrane in model [B]. From field observation and tectonic interpretation, the author asserts model [B]. This model can be subdivided into
833
[B-11, [B-21 and [B-31 based on interpretation of the root of the Kurosegawa Terrane. The root is interpreted as the PaleoRyoke Continent in the [B-l] model (Miyamoto and Hara, 1996), as the preJurassic terranes of the Inner Zone of Southwest Japan in the [B-21 model (Isozaki and Itaya, 1991), and as the preJurassic terranes distributed to the southwest of Southwest Japan in the [B-31 model. From paleo-biogeographical data and tectonic features of the Chichibu and Tanba terranes, the author asserts the [B-31 model.
References Hada, S., Ishii, S., Landis, C.A. and Aitchison, J. (1996) Mid-Permian fusulinacean territories and the identity of the Kurosegawa terrane in Southwest Japan. Japan Contribution to the IGCP, 1996, IGCP National Committee of Japan, pp. 27-34. Isozaki, Y.and Itaya, T. (1991) Pre-Jurassic klippe in northern Chichibu belt in west-central Shikoku, Southwest Japan, Kurosegawa terrane as a tectonic outlier of the pre-Jurassic rocks of the inner zone. J. Geol. SOC.Japan, v. 97, pp. 431-450. Maruyama, S., Banno, S., Matsuda, T. and Nakajima, T. (1984) Kurosegawa zone and its bearing on the development of the Japanese Islands. Tectonophys., v. 110, pp. 47-60. Miyamoto, T. and Hara, I. (1996) Structural geology of Southwest Japan during Cretaceous age, with special reference to the formation and collapse of the ryoke magma arc and tectonics of its joining with the high-pressure type Sambagawa belt. Tectonics and Metamorphism (The Hara Volume), SOUBUN Co., Ltd., pp. 87-99. Yamakita, S. (1998) What belongs to the Northern Chichibu belt? tectonic division between the Northern Chichibu belt and the Kurosegawa belt-. J. Geol. SOC.Japan, v. 104, pp. 623-633. Yao, A. (2000) Terrane arrangement of Southwest Japan in view of the Paleozoic - Mesozoic tectonics of East Asia. The association for the Geological collaboration in Japan, Monograph, v. 49, pp. 145-155.
Late Paleozoic-Early Mesozoic Tectonics in South China Around Yangtze Massif: Closing Process of the Paleo-Tethys Akira Yaol, Yoichi Ezakil, Kiyoko Kuwaharal, Weicheng Hao2and Jianbo Liu2
' Department of
Geosciences, Graduate School of Science, Osaka City University, Osaka 558-8585, Japan Department of Geology, Peking University, Beijing 100 871, P.R. China
The East Asian Continent is composed of various allochthonous continental terranes and various accretionary terranes (Fig. 1; Yao, 2000). The Yangtze Massif is one of the typical continental terranes that had its origin on the northeast margin of Gondwanaland (Metcalfe, 1999). The accretionary terranes around the Yangtze Massif were formed through the pre-Jurassic subduction and collision processes that are divided in to the Early Paleozoic and the Late Paleozoic - Triassic stages. We have investigated the Changning-Menglian Terrane of the western Yunnan area and the Yunkai Terrane of the southern Guangxi area, China. The pre-Jurassic geologic bodies of the western Yunnan area are divided into the Yangtze, Ailaoshan, Simao, Lancangjian, Lincang, Changning-Menglian and Baoshan-Gengma terranes from east to west. The Changning-Menglian Terrane (Fig. 1) Gondwana Research, V. 4, No. 4,2001
represents the Late Paleozoic - Triassic accretionary terrane that consists of Late Paleozoic - Triassic sediments formed in the Paleo-Tethys.The sediments of the Changning-Menglian Terrane are grouped into three facies (Yao and Kuwahara, 1999), namely the accretionary sedimentary complexes, greenstone-limestone sequences and shallow sea cover sediments. The accretionary sedimentary complexes of the area indicate two modes of occurrence, namely melanges and thrust sheets. The case of thrust sheet is mainly represented by the repeated occurrence of chert-clastics sequence ranging from Devonian to Middle Triassic. These geologic components aid in the reconstruction of stratigraphy of the terrane in terms of the subduction of the Paleo-Tethys oceanic plate. The Yunkai Terrane (Fig. 1) of the southern Guangxi area is a narrow zone between the Yangtze - Jiangnan terranes and
834
I
RODINIA, GONDWANA AND ASIA
lOOE
120E
140E
Fig. 1.
the Cathaysia terrane. This terrane extends northeastward along the Tanlu Fault. Furthermore, it may correspond to the northern zone of the Jiangnan Terrane in northeast Jiangxi Province where the Late Paleozoic ophiolites are distributed (He et al., 2000). The Yunkai Terrane is subdivided into three zones, namely northern, central and southern zones based on their lithologic and structural features. The northern zone is characterized by a repeated occurrence of Devonian - Permian chert sheets that dip to north. The central zone is represented by the Devonian clastics. The southern zone is mainly composed of the Permian clastics accompanied by cherts. On the basis of radiolarian biostratigraphical research, there is a younging age polarity among the chert sheets of the northern zone where the upper sheets (mainly Permian) are younger than the lower sheets (mainly Devonian). Although the Changning-Menglian Terrane and the Yunkai Terrane possess almost same age of formation through the Indosinian movement and have similar relationship situation with the Yangtze Terrane, there are some lithologic and structural differences between both terranes. The lithological difference is primary based on the broadness of sedimentary basins and the feature of sea floor in the Paleo-Tethys. The ChangningMenglian Terrane originated from t h e oceanic basin accompanied by basic volcanic seamounts, which constitute the basement comprising greenstone-limestone sequences. The
Distribution of continental and accretion a ry terranes of East Asia (modified from Yao, 2000).
accretionary sedimentary complexes of this terrane were formed by the subduction of the Paleo-Tethys oceanic plate. The tectonic features are similar to those of Jurassic terranes in Southwest Japan that were formed by the subduction of the Panthalassa oceanic plate. The original basin for the Yunkai Terrane, distributed between the Yangtze Massif and the Cathaysia Massif during Devonian to Permian times, was not so broad and its floor does not possess typical oceanic feature. It is considered that the structural features of this terrane represent the collision of the Yangtze Jiangnan terranes and the Cathaysia Terrane. The acidic igneous activity of Late Permian - Triassic times around the Yunkai Terrane (Fang, 1989) is consistent with the collision of both continental terranes.
References Fang, Q. (1989) The plate-tectonic environment and genetic mechanism of the hypersthene granite at Darongshan, Guangxi. J. Southeast Asian Earth Sci., v. 3 , pp. 271-279. He, K., Nic, Z., Zhao, C., Ye, N., Zhou, Z., Lc, C. and Tai, D. (2000) Discovering fossils on the late Paleozoic radiolarian in northeast Jiangxi Province. J. Graduate School, China University of Geosci., V. 14, pp. 1-8. Metcalfe, I. (1999) Gondwana dispersion and Asian accretion: an overview. In: Metcalfe, I. (Ed.), Gondwana Dispersion and Asian Accretion. Balkema, pp. 9-28.
Gondwana Research, V. 4, No. 4,2001
RODINIA, GONDWANA A D ASIA Yao, A. (2000) Terrane arrangement of Southwest Japan in view of the Paleozoic - Mesozoic tectonics of East Asia. The Association for the Geological Collaboration in Japan, Monograph, No. 49, pp. 145.155.
835
Yao, A. and Kuwahara, K. (1999) Paleozoic and Mesozoic radiolarians from the Changning - Menglien Terranes, Western Yunnan, China. In: Yao, A. et al. (Eds.), Biotic and Geological Development of the Paleo-Tethys in China, Peking University Press, pp. 14-42. ~~
~~~
~
The Geodynamic Evolution of the Neoproterozoic of Southern Ethiopia B. Yibasl, W.U. Reimold’ and C.R. Anhaeusser1f2 Geology Department, University of the Witwatersrand, Private Bag 3, Wits 2050, Johannesburg,South Africa Economic Geology Research Institute, Geology Department, University of the Witwatersrand, Private Bag 3, Wits 2050, Johannesburg,South Africa Two distinct tectonostratigraphic terranes, separated by repeatedly reactivated deformation zones, are recognised in the Precambrian 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 ortho-quartzofeldspathic gneisses and granitoids, intercalated with amphibolites and sillimanitekyanite-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 t o 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 trust 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 dating. The U-Pb single zircon SHRIMP and laser probe 40Ar-39Ar 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: Gtl (>850 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 to 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 (1157f 2 to 1030f 40 Ma); (2) Bulbul-Awata (-876+ 5 Ma); (3) Megado (800-750 Ma); (4) Moyale (700-550 Ma); and (5) Berguda (550-500 Ma). It can, therefore, be argued that the geodynamic evolution of the Precambrian of southern Ethiopia spans the period from the Palaeoproterozoic to the Neoproterozoic ( < ~ 2 0 5 0to 500 Ma). It began with the formation of a passive continental margin Gondwana Research, V: 4,No. 4,2002
and the development of an ocean basin in southern Ethiopia a n d northeastern Kenya. This ocean developed o n a Palaeoproterozoic ‘pre-Mozambique Belt’ crust, inferred from xenocryst ages (2051+ 82 to 1362k 43 Ma). Plate collision and metamorphism between 1250 and 1050 Ma was responsible for the closure and gneissification of the marginal basin sediments. The opening of the first Pan-Africanocean in southern Ethiopia must have occurred > >880 Ma ago, which is consistent with the argument that the oldest Pan-African igneous rocks erupted at about 880 Ma in an intra-oceanic arc setting and with the age of an initial ocean magmatism of 842f 1 7 Ma in the northern part of the East African orogen associated with the break-up of a larger continental block into West and East Gondwana at about this time. Initiation of rifting for the formation of the second oceanic basin (the Megado basin) in southern Ethiopia occurred simultaneously with the subduction of the Bulbul-(Kenticha?) ocean basin in the east, above a westerly dipping subduction zone where the western spreading arm of the oceanic crust was subducted and led to the generation of boninites and low-Ti tholeiite beneath a fore-arc at Megado at about 789+ 36 Ma. Closure of the Megado basin and obduction of the ophiolite was followed by arc and collisional magmatism and charnockitization at about 760 to 722 Ma. The opening of the youngest marginal basin (Moyale ocean basin) at about 700 Ma, in a fore-arc supra-subduction setting, began in a similar manner described to that for the Megado basin formation. However, the Moyale oceanic basin probably closed before a proper back-arc-type basin could form and, hence, no clastic sedimentation could be recorded. This was followed by subduction of the Moyale basin, which took place at about 660 Ma (age of the Moyale arc-granodiorite, SHRIMP U-Pb age, this study). This was followed by arc and collisional magmatism and granulite facies metamorphism due to collisional tectonism, which continued until 550 Ma ago and was responsible for the present structural and lithological set-up of the Precambrian of southern Ethiopia. The final stage in the geodynamic evolution of the Precambrian of southern Ethiopia occurred between 550 and 500 Ma, which was a period of emplacement of post-tectonic and post-orogenic granitoids accompanied by thrusting, transcurrent faulting, and shearing associated with uplifting and final cooling - marking the end of the East African Orogen. The presence of strongly deformed and metamorphosed charnockitic
into many terranes (Fig. l),based on their pre-Cenozoic geologic characteristics. Based on formation ages, these can be grouped into three: Pre-Jurassic terranes (Pa, Pb), Jurassic terranes (Ja, Jb, Jc) and Post-Jurassic terranes (Ka, Kb, Kc). The Jurassic terranes, which is a main subject of this paper, is distributed two-fold, the Tanba-Mino-Ashio Terrane in the Inner Zone and the Chichibu Terrane in the Outer zone. These terranes are composed mainly of Jurassic accretionary complexes including the oceanic plate stratigraphic components of Late Paleozoic to Jurassic age. The Tanba-Mino-Ashio Terrane is distributed tectonically below the pre-Jurassic terrane group in the north and changes gradually into the Cretaceous metamorphic belt (Ryoke Belt) to the south. The Chichibu Terrane develops as a large-scale nappe tectonically above the post-Jurassic terrane group (Sanbagawa Belt and Shimanto Terrane). The Kurosegawa Terrane, which belongs to the Pre-Jurassic terrane group, is intermittently distributed as a klippe tectonically above the central zone of the Chichibu Terrane. Based on plate tectonic concept, several models have been proposed for the pre-Neogene terrane arrangement of Southwest Japan (Fig. 2). An essential point of difference among these models is the interpretation of geologic relationship between the Kurosegawa Terrane and the Chichibu Terrane. On the basis of their interpretation, the models are grouped into [A] and [B]. According to model [A], the Kurosegawa Terrane has been tectonically introduced between the Northern and Southern Chichibu terranes (e.g., Maruyama et al., 1984; Hada et al., 1996; Yamakita, 1998). On the other hand, the Kurosegawa Terrane is regarded as a nappe on the Chichibu Terrane in model [B]. From field observation and tectonic interpretation, the author asserts model [B]. This model can be subdivided into
833
[B-11, [B-21 and [B-31 based on interpretation of the root of the Kurosegawa Terrane. The root is interpreted as the PaleoRyoke Continent in the [B-l] model (Miyamoto and Hara, 1996), as the preJurassic terranes of the Inner Zone of Southwest Japan in the [B-21 model (Isozaki and Itaya, 1991), and as the preJurassic terranes distributed to the southwest of Southwest Japan in the [B-31 model. From paleo-biogeographical data and tectonic features of the Chichibu and Tanba terranes, the author asserts the [B-31 model.
References Hada, S., Ishii, S., Landis, C.A. and Aitchison, J. (1996) Mid-Permian fusulinacean territories and the identity of the Kurosegawa terrane in Southwest Japan. Japan Contribution to the IGCP, 1996, IGCP National Committee of Japan, pp. 27-34. Isozaki, Y.and Itaya, T. (1991) Pre-Jurassic klippe in northern Chichibu belt in west-central Shikoku, Southwest Japan, Kurosegawa terrane as a tectonic outlier of the pre-Jurassic rocks of the inner zone. J. Geol. SOC.Japan, v. 97, pp. 431-450. Maruyama, S., Banno, S., Matsuda, T. and Nakajima, T. (1984) Kurosegawa zone and its bearing on the development of the Japanese Islands. Tectonophys., v. 110, pp. 47-60. Miyamoto, T. and Hara, I. (1996) Structural geology of Southwest Japan during Cretaceous age, with special reference to the formation and collapse of the ryoke magma arc and tectonics of its joining with the high-pressure type Sambagawa belt. Tectonics and Metamorphism (The Hara Volume), SOUBUN Co., Ltd., pp. 87-99. Yamakita, S. (1998) What belongs to the Northern Chichibu belt? tectonic division between the Northern Chichibu belt and the Kurosegawa belt-. J. Geol. SOC.Japan, v. 104, pp. 623-633. Yao, A. (2000) Terrane arrangement of Southwest Japan in view of the Paleozoic - Mesozoic tectonics of East Asia. The association for the Geological collaboration in Japan, Monograph, v. 49, pp. 145-155.
Late Paleozoic-Early Mesozoic Tectonics in South China Around Yangtze Massif: Closing Process of the Paleo-Tethys Akira Yaol, Yoichi Ezakil, Kiyoko Kuwaharal, Weicheng Hao2and Jianbo Liu2
' Department of
Geosciences, Graduate School of Science, Osaka City University, Osaka 558-8585, Japan Department of Geology, Peking University, Beijing 100 871, P.R. China
The East Asian Continent is composed of various allochthonous continental terranes and various accretionary terranes (Fig. 1; Yao, 2000). The Yangtze Massif is one of the typical continental terranes that had its origin on the northeast margin of Gondwanaland (Metcalfe, 1999). The accretionary terranes around the Yangtze Massif were formed through the pre-Jurassic subduction and collision processes that are divided in to the Early Paleozoic and the Late Paleozoic - Triassic stages. We have investigated the Changning-Menglian Terrane of the western Yunnan area and the Yunkai Terrane of the southern Guangxi area, China. The pre-Jurassic geologic bodies of the western Yunnan area are divided into the Yangtze, Ailaoshan, Simao, Lancangjian, Lincang, Changning-Menglian and Baoshan-Gengma terranes from east to west. The Changning-Menglian Terrane (Fig. 1) Gondwana Research, V. 4, No. 4,2001
represents the Late Paleozoic - Triassic accretionary terrane that consists of Late Paleozoic - Triassic sediments formed in the Paleo-Tethys.The sediments of the Changning-Menglian Terrane are grouped into three facies (Yao and Kuwahara, 1999), namely the accretionary sedimentary complexes, greenstone-limestone sequences and shallow sea cover sediments. The accretionary sedimentary complexes of the area indicate two modes of occurrence, namely melanges and thrust sheets. The case of thrust sheet is mainly represented by the repeated occurrence of chert-clastics sequence ranging from Devonian to Middle Triassic. These geologic components aid in the reconstruction of stratigraphy of the terrane in terms of the subduction of the Paleo-Tethys oceanic plate. The Yunkai Terrane (Fig. 1) of the southern Guangxi area is a narrow zone between the Yangtze - Jiangnan terranes and
834
I
RODINIA, GONDWANA AND ASIA
lOOE
120E
140E
Fig. 1.
the Cathaysia terrane. This terrane extends northeastward along the Tanlu Fault. Furthermore, it may correspond to the northern zone of the Jiangnan Terrane in northeast Jiangxi Province where the Late Paleozoic ophiolites are distributed (He et al., 2000). The Yunkai Terrane is subdivided into three zones, namely northern, central and southern zones based on their lithologic and structural features. The northern zone is characterized by a repeated occurrence of Devonian - Permian chert sheets that dip to north. The central zone is represented by the Devonian clastics. The southern zone is mainly composed of the Permian clastics accompanied by cherts. On the basis of radiolarian biostratigraphical research, there is a younging age polarity among the chert sheets of the northern zone where the upper sheets (mainly Permian) are younger than the lower sheets (mainly Devonian). Although the Changning-Menglian Terrane and the Yunkai Terrane possess almost same age of formation through the Indosinian movement and have similar relationship situation with the Yangtze Terrane, there are some lithologic and structural differences between both terranes. The lithological difference is primary based on the broadness of sedimentary basins and the feature of sea floor in the Paleo-Tethys. The ChangningMenglian Terrane originated from t h e oceanic basin accompanied by basic volcanic seamounts, which constitute the basement comprising greenstone-limestone sequences. The
Distribution of continental and accretion a ry terranes of East Asia (modified from Yao, 2000).
accretionary sedimentary complexes of this terrane were formed by the subduction of the Paleo-Tethys oceanic plate. The tectonic features are similar to those of Jurassic terranes in Southwest Japan that were formed by the subduction of the Panthalassa oceanic plate. The original basin for the Yunkai Terrane, distributed between the Yangtze Massif and the Cathaysia Massif during Devonian to Permian times, was not so broad and its floor does not possess typical oceanic feature. It is considered that the structural features of this terrane represent the collision of the Yangtze Jiangnan terranes and the Cathaysia Terrane. The acidic igneous activity of Late Permian - Triassic times around the Yunkai Terrane (Fang, 1989) is consistent with the collision of both continental terranes.
References Fang, Q. (1989) The plate-tectonic environment and genetic mechanism of the hypersthene granite at Darongshan, Guangxi. J. Southeast Asian Earth Sci., v. 3 , pp. 271-279. He, K., Nic, Z., Zhao, C., Ye, N., Zhou, Z., Lc, C. and Tai, D. (2000) Discovering fossils on the late Paleozoic radiolarian in northeast Jiangxi Province. J. Graduate School, China University of Geosci., V. 14, pp. 1-8. Metcalfe, I. (1999) Gondwana dispersion and Asian accretion: an overview. In: Metcalfe, I. (Ed.), Gondwana Dispersion and Asian Accretion. Balkema, pp. 9-28.
Gondwana Research, V. 4, No. 4,2001
RODINIA, GONDWANA A D ASIA Yao, A. (2000) Terrane arrangement of Southwest Japan in view of the Paleozoic - Mesozoic tectonics of East Asia. The Association for the Geological Collaboration in Japan, Monograph, No. 49, pp. 145.155.
835
Yao, A. and Kuwahara, K. (1999) Paleozoic and Mesozoic radiolarians from the Changning - Menglien Terranes, Western Yunnan, China. In: Yao, A. et al. (Eds.), Biotic and Geological Development of the Paleo-Tethys in China, Peking University Press, pp. 14-42. ~~
~~~
~
The Geodynamic Evolution of the Neoproterozoic of Southern Ethiopia B. Yibasl, W.U. Reimold’ and C.R. Anhaeusser1f2 Geology Department, University of the Witwatersrand, Private Bag 3, Wits 2050, Johannesburg,South Africa Economic Geology Research Institute, Geology Department, University of the Witwatersrand, Private Bag 3, Wits 2050, Johannesburg,South Africa Two distinct tectonostratigraphic terranes, separated by repeatedly reactivated deformation zones, are recognised in the Precambrian 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 ortho-quartzofeldspathic gneisses and granitoids, intercalated with amphibolites and sillimanitekyanite-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 t o 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 trust 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 dating. The U-Pb single zircon SHRIMP and laser probe 40Ar-39Ar 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: Gtl (>850 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 to 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 (1157f 2 to 1030f 40 Ma); (2) Bulbul-Awata (-876+ 5 Ma); (3) Megado (800-750 Ma); (4) Moyale (700-550 Ma); and (5) Berguda (550-500 Ma). It can, therefore, be argued that the geodynamic evolution of the Precambrian of southern Ethiopia spans the period from the Palaeoproterozoic to the Neoproterozoic ( < ~ 2 0 5 0to 500 Ma). It began with the formation of a passive continental margin Gondwana Research, V: 4,No. 4,2002
and the development of an ocean basin in southern Ethiopia a n d northeastern Kenya. This ocean developed o n a Palaeoproterozoic ‘pre-Mozambique Belt’ crust, inferred from xenocryst ages (2051+ 82 to 1362k 43 Ma). Plate collision and metamorphism between 1250 and 1050 Ma was responsible for the closure and gneissification of the marginal basin sediments. The opening of the first Pan-Africanocean in southern Ethiopia must have occurred > >880 Ma ago, which is consistent with the argument that the oldest Pan-African igneous rocks erupted at about 880 Ma in an intra-oceanic arc setting and with the age of an initial ocean magmatism of 842f 1 7 Ma in the northern part of the East African orogen associated with the break-up of a larger continental block into West and East Gondwana at about this time. Initiation of rifting for the formation of the second oceanic basin (the Megado basin) in southern Ethiopia occurred simultaneously with the subduction of the Bulbul-(Kenticha?) ocean basin in the east, above a westerly dipping subduction zone where the western spreading arm of the oceanic crust was subducted and led to the generation of boninites and low-Ti tholeiite beneath a fore-arc at Megado at about 789+ 36 Ma. Closure of the Megado basin and obduction of the ophiolite was followed by arc and collisional magmatism and charnockitization at about 760 to 722 Ma. The opening of the youngest marginal basin (Moyale ocean basin) at about 700 Ma, in a fore-arc supra-subduction setting, began in a similar manner described to that for the Megado basin formation. However, the Moyale oceanic basin probably closed before a proper back-arc-type basin could form and, hence, no clastic sedimentation could be recorded. This was followed by subduction of the Moyale basin, which took place at about 660 Ma (age of the Moyale arc-granodiorite, SHRIMP U-Pb age, this study). This was followed by arc and collisional magmatism and granulite facies metamorphism due to collisional tectonism, which continued until 550 Ma ago and was responsible for the present structural and lithological set-up of the Precambrian of southern Ethiopia. The final stage in the geodynamic evolution of the Precambrian of southern Ethiopia occurred between 550 and 500 Ma, which was a period of emplacement of post-tectonic and post-orogenic granitoids accompanied by thrusting, transcurrent faulting, and shearing associated with uplifting and final cooling - marking the end of the East African Orogen. The presence of strongly deformed and metamorphosed charnockitic