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The Cenozoic Magmatism of East Africa: Part II – Rifting of the Mobile Belt

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The Cenozoic Magmatism of East Africa: Part II – Rifting of the Mobile Belt The Cenozoic Magmatism of East Africa: Part II – Rifting of the Mobile Belt Tyrone O. Rooney Dept. of Earth and Environmental Sciences, Michigan State University, East Lansing, MI 48823, USA Abstract Despite the foundational role East Africa has played in the advancement of our understanding of continental rifting, there remains substantial ambiguity as to how magmatism has evolved during progressive rift development. This contribution is a comparative study that explores the temporal development of magmatism in both rifts, and within the interposing Turkana Depression. Notwithstanding the independent evolution of the Main Ethiopian Rift and Kenya Rift prior to their quaternary linkage, magmatic events within them show remarkable parallelism. Following an initial pulse of basaltic magmatism ca. 20 Ma, more evolved compositions (flood phonolites in Kenya, rhyolites in Ethiopia) dominated the landscape until ca. 12 Ma. From ca. 12 Ma to 9 Ma, a renewed phase of widespread basaltic volcanism (Mid-Miocene Resurgence Phase) impacted the entire region from the Afar margin to Kenya, though activity in the south slightly predates equivalent basaltic events to the north. Following this widespread basaltic event, silicic magmatism again dominated the now nascent rifts until a renewed phase of basaltic activity commenced ca. 4 to 1.6 Ma termed the ‘Stratoid Phase’. Following the termination of the Stratoid Phase, the modern expression of volcanism in both rifts has been dominated by central silicic volcanoes. Magmatic activity within the Turkana Depression broadly parallels that in both rifts – basaltic events are evident at ca. 20 Ma, 12 Ma, and 4 Ma, however 1 Figure 1 Afar Main Ethiopian Rift Turkana Toro Kenya Ankole Rift Eastern Kivu & Branch Virunga NTD Western Modern East African Rift Branch Cretaceous Rifting Cenozoic Lavas of East Africa Neoproterozoic Rungwe Mesoproterozoic and older Extent of Craton Figure 2 DM Afar 0.5130 Plume 0.5128 HIMU Nd EM2 144 0.5126 Nd/ 143 0.5124 Yemen Afar MER Turkana 0.5122 North Kenya Rift South Kenya Rift/Tanzania Plateaus Rungwe Kivu-Virunga Carbonatites PA 0.5120 0.7030 0.7040 0.7050 0.7060 0.7070 0.7080 0.7090 0.7100 87Sr/86Sr Figure 3 Afar Main Ethiopian Rift Turkana Nyanza Kenya Volcanics Rift Early Miocene Resurgence Samburu Event Kenya Ethiopia Volcanics Rift Early Miocene Resurgence Samburu Event Rift Basins (22-17 Ma) Tendaho-Manda Hararo Erta 'Ale Range Tadjoura Dabbahu Alid Asal Alayta Transverse Volcanics Manda Inakir Figure 44 A. D. 0.5129 3.5 3 143 0.5128 Nd/ 144 (wt. %) (wt. 2.5 2 Nd 0.5127 TiO 2 1.5 0.5126 1 1.4 Tendaho-Manda Hararo Samburu Basalts 0.7065 B. Dabbahu E. Getra Basalts Alayta Early Miocene 0.7060 1.2 Erta 'Ale Range Flood Phonolites Alid 0.7055 3 Simien-Axum Transverse Volcanics 87 O 1 Early Rift Dev. Sr/ 2 Tadjoura 0.7050 Asal 86 Sr 0.8 Manda Inakir 0.7045 CaO/Al 0.6 0.7040 0.7035 0.4 0.7030 C. F. 100 Nb (ppm) Nb 200 Zr (ppm)Zr 100 10 4 6 8 10 12 14 4 6 8 10 12 14 MgO (wt. %) MgO (wt. %) 20 G. H. HIMU EM2 40 PAL 10 Afar Plume 208 39 Pb/ 5 204 Pb Zr/Nb 38 2 >5% MgO 37 DMM 5 10 15 20 25 30 35 18 19 20 21 22 La/Yb 206Pb/204Pb Figure 5 Samburu Basalts 100 Getra Basalts Early Miocene Resur. 10 Early Rift Dev 100 Flood Phonolite 10 Sample/Chondrite Simien-Axum 100 10 La Ce Pr Nd(Pm)Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Figure 6 100 Samburu Basalts Getra Basalts Early Miocene Resur. 10 100 Flood Phonolite Early Rift Dev 10 Sample/Primitive Mantle Sample/Primitive 100 Simien-Axum 10 RbBaTh U NbTa K LaCePb Pr Sr P Nd Zr HfSmEu Ti GdTbDyHo Y ErTmYbLu Figure 7 Afar Main Ethiopian Rift Turkana Nyanza Ethiopia Volcanics Rift Kenya Flood Phonolites Rift Kenya Volcanics Flood Phonolites Rift Basins (22-17 Ma) Figure 8 15 phonolite trachyte trachydacite 12.5 tephri- rhyolite phonolite 10 phono- tephrite O (wt%) 2 trachyandesite 7.5 foidite tephrite basaltic O+K (ol<10%) trachy- 2 trachy- andesite Na basanite basalt 5 (ol>10%) dacite Balchi alkaline Wechacha sub-alkaline basaltic 2.5 picro- basalt andesite andesite Shebele basalt Axum Flood Phonolites 0 40 50 60 70 SiO2 (wt%) Tendaho-Manda Hararo Erta 'Ale Range Tadjoura Dabbahu Alid Asal Alayta Transverse Volcanics Manda Inakir Figure 9 104 A. D. 1000 Ba(ppm) 100 1000 Ti (ppm) Ti 10 100 1 1000 B. E. 200 100 (ppm) Sr 100 Rb (ppm)Rb 10 50 C. F. 500 1000 Nb(ppm) 200 Zr (ppm) 500 100 50 55 60 65 70 75 55 60 65 70 75 SiO2 (wt. %) SiO2 (wt. %) 0.5134 DMM G. 0.5132 Balchi Wechacha Shebele 0.5130 HIMU Axum Flood Phonolites Nd 0.5128 144 Afar EM2 Nd/ 0.5126 143 Plume 0.5124 0.5122 PAL 0.702 0.704 0.706 0.708 87Sr/86Sr Figure 10 1000 Shebele 100 10 1000 Simien-Axum 100 Sample/C1 Chondrite Sample/C1 10 1000 Wechacha 100 10 La Ce Pr Nd(Pm)Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Figure 11 1000 >55% SiO2 100 10 1 Axum-Adwa (other) Sample/Primitive Mantle Sample/Primitive Axum-Adwa (high Ti) 0.1 RbBaTh U NbTa K LaCePb Pr Sr P Nd Zr HfSmEu Ti GdTbDyHo Y ErTmYbLu Figure 12 Afar Main Ethiopian Rift Turkana Ethiopia Volcanics Mid-Miocene Resurgence Kenya Volcanics Kenya Mid-Miocene Resurgence Rift Rift Basins (13-10 Ma) Nyanza Rift Tendaho-Manda Hararo Erta 'Ale Range Tadjoura Dabbahu Alid Asal Alayta Transverse Volcanics Manda Inakir Figure 13 A. 4 D. 0.5129 3.5 0.5128 3 143 Nd/ 2.5 0.5127 144 (wt. %) (wt. 2 2 0.5126 Nd TiO 1.5 0.5125 1 0.5124 0.5 1 Tendaho-Manda Hararo 0.7065 B. Dabbahu E. Alayta 0.7060 0.8 Erta 'Ale Range Alid 3 Transverse Volcanics 0.7055 87 O Sr/ 2 0.6 Tadjoura Asal 0.7050 86 Manda Inakir Sr CaO/Al 0.4 Eth Mid Miocene 0.7045 Kele Basalts Kenya Mid Miocene 0.7040 0.2 Gombe Tirr Tirr YTVL Mid Miocene 0.7035 0 Bofa C. F. 1000 100 Nb(ppm) Zr (ppm) 100 10 0 2.5 5 7.5 10 12.5 15 17.5 0 2.5 5 7.5 10 12.5 15 17.5 MgO (wt. %) MgO (wt. %) 2.00 G. H. 1.80 10 Zr/Nb CN 1.60 5 (Tb/Yb) 1.40 1.20 2 >5% MgO >5% MgO 1.00 2 3 4 5 6 7 8 7.5 10 12.5 15 17.5 20 (La/Sm)CN La/Yb Figure 14 100 Kele Basalt Kenya Mid Miocene Eth. Mid Miocene YTVL Mid Miocene 10 100 Bofa Stratoid Series Gombe Stratoid Series Sample/Chondrite 10 La Ce Pr Nd(Pm)Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Figure 15 100 Kele Basalt Kenya Mid Miocene Eth. Mid Miocene YTVL Mid Miocene 10 Bofa Stratoid Series 100 Sample/Primitive Mantle Sample/Primitive Gombe Stratoid Series 10 RbBaTh U Nb Ta K LaCePb Pr Sr P Nd Zr HfSmEu Ti GdTb DyHo Y ErTmYb Lu Figure 16 0.5134 0.51290 A. DMM 0.5132 0.51285 HIMU 0.5130 Afar Plume 0.51280 Nd 0.5128 Nd 144 144 0.51275 EM2 Nd/ 0.5126 Nd/ 143 143 0.51270 0.5124 0.51265 0.5122 >5% MgO PAL 0.51260 0.702 0.703 0.704 0.705 0.706 0.707 0.708 0.7035 0.704 0.7045 0.705 87Sr/86Sr 87Sr/86Sr B. 39.75 HIMU 40 EM2 PAL 39.50 Afar Plume 39.25 Pb 39 Pb 204 204 39.00 Addado-Hertali Pb/ Pb/ Eth Mid Miocene Fantale-Dofan 208 38 208 38.75 Kele Basalts Bosetti-Kone Kenya Mid Miocene Debre Zeyit Gombe Stratoid 38.50 Aluto-Gedemsa Tirr Tirr Butajira 37 Mid Miocene YTVL Shalla Bofa Stratoid 38.25 Diguna >5% MgO DMM Arba Minch 18 19 20 21 22 18 18.5 19 19.5 206 204 Pb/ Pb 206Pb/204Pb Figure 17 Afar YTVL Main Ethiopian Rift Turkana Ethiopia Volcanics Early Rift Development Phase Kenya Volcanics Early Rift Development Phase Rift Basins (10-6 Ma) Nyanza Rift Kenya Rift Figure 18 Afar Main Ethiopian Rift Turkana Kenya Volcanics Stratoid Phase Ethiopia Volcanics Quaternary Plateau Kenya Rift Pyroclastics & Flows Stratoid Phase - Basalts & Trachytes Rift Basins (5-3 Ma) Tendaho-Manda Hararo Erta 'Ale Range Tadjoura Dabbahu Alid Asal Alayta Transverse Volcanics Manda Inakir Figure 19 A. D. 4 0.5129 3 143 Nd/ 144 (wt. %) (wt. 2 2 0.5128 Nd TiO 1 0.5127 0 1 0.7060 B. Tendaho-Manda Hararo E. Dabbahu 0.7055 0.8 Alayta Erta 'Ale Range Alid 0.7050 3 Transverse Volcanics 87 O 0.6 Sr/ 2 Tadjoura Asal 0.7045 86 Sr 0.4 Manda Inakir CaO/Al Pliocene Shields 0.7040 High Ti Shields 0.2 Plio-Quat E. Plateau Plio-Quat W. Plateau 0.7035 Bale - Plio Bale - Quat 0 0.7030 C. F. 1000 100 Nb(ppm) Zr (ppm) 100 10 0 2.5 5 7.5 10 12.5 15 17.5 0 2.5 5 7.5 10 12.5 15 17.5 MgO (wt. %) MgO (wt. %) 10 G. HIMU H. 40 EM2 PAL Afar Plume 5 Zr/Nb Pb 39 204 Pb/ 208 38 2 37 DMM >5% MgO 18 19 20 21 22 7.5 10 12.5 15 17.5 20 22.5 25 206Pb/204Pb La/Yb Figure 20 100 Pliocene Shields High Ti Shields 10 Plio-Quat W.
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