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The Cenozoic Magmatism of East Africa: Part IV – the Terminal Stages of Rifting Preserved in the Northern East African Rift System

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The Cenozoic Magmatism of East Africa: Part IV – the Terminal Stages of Rifting Preserved in the Northern East African Rift System The Cenozoic Magmatism of East Africa: Part IV – The Terminal Stages of Rifting Preserved in the Northern East African Rift System Tyrone O. Rooney Dept. of Earth and Environmental Sciences, Michigan State University, East Lansing, MI 48823, USA Abstract The Afar Depression is a broad region of subdued topography wherein the Red Sea, Gulf of Aden, and East African Rift System converge at a triple junction. This mature sector of the East African Rift System is characterized by attenuated continental lithosphere, with the implication that lavas erupted here can provide insights into the composition of sub-lithospheric magma reservoirs. Magmas in Afar and Yemen largely have geochemical characteristics consistent with a mixture of melts derived from the Afar plume, and from a hybrid endmember comprised of the depleted upper mantle and Pan-African lithosphere. Lavas erupted in the Erta ‘Ale range do not comport with this model and instead appear to have been derived from melting of a lithospheric mantle metasome that is likely related to prior Afar plume-lithosphere interaction. The extant data show no significant change in the reservoirs contributing to magmatism during the development of the rift in this region, however the surface distribution of magmatism reveals a relationship between rift evolution and magmatism. In Yemen, magmatic activity is limited to the regional Mid Miocene Resurgence event at ca. 10 Ma, and small-volume alkaline volcanism along the evolving Gulf of Aden margin from 6 Ma to present. In contrast, a much more extensive magmatic record is preserved in Afar. Coincident with the development of the rift margin, initial magmatism in Afar during this period (20 to 10 Ma) was dominantly in the form of 1 Figure 1 Jalua - Alid Red Sea Red Sea Rift Arabian plate Erta 'Ale Danakil Block Yemen Nubian plate Dabbahu Manda Hararo Asal Rift Gulf of Aden L. Abhe Tendaho-Goba'ad Discontinuity Wonjii Fault Belt Main Ethiopian Rift Somalian plate Figure 2 Yemen (6 Ma - Recent) Yemen (11 - 9.5 Ma) Yemen Gulf of Aden Mablas Dahloid Gulf Series Dahla Afar Stratoid Axial Series Figure 3 4 A. D. 0.5130 3 0.5129 143 Nd/ 0.5128 144 (wt. %) (wt. 2 2 Nd TiO 0.5127 1 0.5126 0 0.5125 0.8 B. E. 0.7070 0.6 3 0.7060 87 O Sr/ 2 86 0.4 Sr 0.7050 CaO/Al Al Harf Dabab 0.2 Jabal An Nar Jabal Kharaz 0.7040 Perim Pliocene-Recent 0 0.7030 C. 1000 F. 100 Nb(ppm) 50 Zr (ppm) 20 100 0 2 4 6 8 10 120 2 4 6 8 10 12 MgO (wt. %) MgO (wt. %) Figure 4 0.5134 A. DMM 0.5132 0.5130 HIMU Nd 0.5128 144 Afar Plume EM2 Nd/ 0.5126 143 0.5124 0.5122 PAL 0.702 0.703 0.704 0.705 0.706 0.707 0.708 87Sr/86Sr B. HIMU 40 EM2 PAL Afar Plume Pb 39 204 Pb/ 208 38 Jabal An Nar Jabal Kharaz 37 Perim DMM Pliocene-Recent 18 19 20 21 22 206Pb/204Pb C. DMM 20.0 15.0 Afar Plume Hf 10.0 ε EM2 5.0 HIMU 0.0 PAL −2.5 0 2.5 5 7.5 10 12.5 εNd Tendaho-Manda Hararo Erta 'Ale Range Tadjoura Dabbahu Alid Asal Alayta Transverse Volcanics Manda Inakir Figure 5 4 A. D. 0.5130 3 0.5129 143 Nd/ 0.5128 144 (wt. %) (wt. 2 2 Nd TiO 0.5127 1 0.5126 0 0.5125 0.8 Tendaho-Manda Hararo B. Dabbahu Aden E. Alayta Bir Ali-Balhaf 0.7070 Erta 'Ale RangeDhamar Rada 0.6 Alid Little Aden 3 Transverse MaribVolcanics 0.7060 87 O Sr/ 2 Tadjoura Ras Imran Asal Sana'a 86 0.4 Manda InakirShuqra Sr Um Birka 0.7050 CaO/Al 0.2 0.7040 0 0.7030 100 1000 C. F. 50 Nb(ppm) Zr (ppm) 20 100 0 2 4 6 8 10 120 2 4 6 8 10 12 MgO (wt. %) MgO (wt. %) Figure 6 Bir Ali-Balhaf 100 U anomalies 10 Sana'a 100 Sample/Primitive Mantle Sample/Primitive 10 Rb Ba Th U Nb Ta K La Ce Pb Pr Sr P Nd Zr Hf Sm Eu Ti Gd Tb Dy Ho Y Er Tm Yb Lu Figure 7 0.5134 A. DMM 0.5132 0.5130 HIMU Nd 0.5128 144 Afar Plume EM2 Nd/ 0.5126 143 0.5124 0.5122 PAL 0.702 0.703 0.704 0.705 0.706 0.707 0.708 87Sr/86Sr B. HIMU 40.00 EM2 PAL Afar Plume Pb 39.00 204 Pb/ Aden 208 38.00 Bir Ali-Balhaf LIttle Aden Marib Ras Imran Sana'a 37.00 Shuqra DMM Um Birka 18 19 20 21 22 206Pb/204Pb C. DMM 20.0 15.0 Afar Plume Hf 10.0 ε EM2 5.0 HIMU 0.0 PAL −2.5 0 2.5 5 7.5 10 12.5 εNd Figure 8 Axial 0.16 0.3 Series 0.5 1 Gulf Series Busidima Formation 1.6 2 Upper Stratoid Series 2.7 2.8 2.8 3 2.9 Lower Hadar Stratoid Formation Series 3.8 3.9 4 3.9 Sagantole Dalhoid Formation 5 Series 5.6 5.6 5.6 Age (Ma) Age 6 Adu-Asa Formation >6.4 Magmatic Stratigraphy 7 Sedimentary Formations 7 7.03 Chorora Formation Chorora Dalha 8 Series 9 9.41 Chifra & 10 10 10 Finto Mablas Series Series 20 23.5 Figure 9 Red Sea Danakil Yemen Block Yemen Gulf of Aden L. Abhe Afar Dahla Miocene 23 to 7 Ma Mablas Afar Faults Main Ethiopian Rift Mablas Dahloid Gulf Series Dahla Afar Stratoid Axial Series Figure 510 A. D. 0.5130 4 0.5129 143 3 0.5128 Nd/ 144 (wt. %) (wt. 2 2 0.5127 Nd TiO 0.5126 1 0.5125 0 1 B. E. 0.7070 0.8 3 0.7060 87 O Sr/ 2 0.6 86 0.7050 Sr CaO/Al 0.4 Mablas Series Dalha Series 0.7040 0.2 Dalhoid Series Afar Strt. Series Gulf Series 0 0.7030 C. F. 1000 100 Nb(ppm) Zr (ppm) 10 100 1 0 2 4 6 8 100 2 4 6 8 10 MgO (wt. %) MgO (wt. %) 2.2 50 G. H. 2 1.8 20 Zr/Nb CN 1.6 (Tb/Yb) 10 1.4 1.2 5 MgO > 5 wt. % MgO > 5 wt. % 1 1.5 2 2.5 3 3.5 2.5 5 7.5 10 12.5 15 (La/Sm)CN La/Yb Figure 11 Mablas Series 100 10 Dalha Series 100 10 Dalhoid Series 100 Sample/Chondrite 10 Afar Stratoid Series 100 10 Gulf Series 100 10 La Ce Pr Nd(Pm)Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Figure100 12 Mablas Series 10 1 100 Dalha Series 10 1 100 Afar Stratoid Series 10 Sample/Primitive Mantle Sample/Primitive 1 100 Dalhoid Series 10 1 100 Gulf Series 10 1 Rb Ba Th U Nb Ta K La Ce Pb Pr Sr P Nd Zr Hf Sm Eu Ti Gd Tb Dy Ho Y Er Tm Yb Lu Figure 13 0.5134 A. DMM 0.5132 0.5130 HIMU Nd 0.5128 144 EM2 Nd/ Afar Plume 0.5126 143 0.5124 0.5122 PAL 0.702 0.703 0.704 0.705 0.706 0.707 0.708 87Sr/86Sr B. HIMU 40 EM2 PAL Afar Plume Pb 39 204 Pb/ 208 38 Mablas Series Dalha Series Dalhoid Series 37 Afar Strt. Series DMM Gulf Series 18 19 20 21 22 206Pb/204Pb C. DMM 20.0 15.0 10.0 Hf Afar Plume ε EM2 5.0 HIMU 0.0 PAL −5.0 −2.5 0 2.5 5 7.5 10 12.5 εNd Figure 14 Gulf Series Afar Stratoid Series Afar Faults Red Sea Yemen Danakil Yemen Block Gulf of Aden L. Abhe Afar Tendaho-Goba'ad Discontinuity Main Ethiopian Rift Figure 15 Red Sea Danakil Yemen Block Yemen Gulf of Aden L. Abhe Afar Tendaho-Goba'ad Discontinuity Gulf of Aden Main Ethiopian Rift Main Red Sea Rift Ethiopian Rift Transverse Afar Faults Tendaho-Manda Hararo Erta 'Ale Range Tadjoura Dabbahu Alid Asal Alayta Transverse Volcanics Manda Inakir Figure 516 A. D. 4 0.5130 143 3 Nd/ 144 (wt. %) (wt. 2 Nd 2 0.5129 TiO 1 0 0.5128 1 Tendaho-MandaTendaho-Manda Hararo Hararo B. E. DabbahuDabbahu 0.7050 AlaytaAlayta 0.8 ErtaErta 'Ale 'Ale Range Range AlidAlid 3 Transverse Volcanics Transverse Volcanics 0.7045 87 O Tadjoura Sr/ 2 0.6 TadjouraAsal AsalManda Inakir 86 Manda Inakir Sr 0.4 CaO/Al 0.7040 0.2 0.7035 0 1000 C. F. 100 Nb(ppm) Zr (ppm) 10 100 0 2.5 5 7.5 10 12.5 15 0 2.5 5 7.5 10 12.5 15 MgO (wt. %) MgO (wt. %) 2.2 20 G. H. 2 1.8 10 Zr/Nb CN 1.6 (Tb/Yb) 1.4 5 1.2 MgO> 5 wt. % MgO> 5 wt. % 1 1 2 3 4 5 6 0 5 10 15 20 25 30 35 (La/Sm)CN La/Yb Figure 17 Tadjoura Manda Hararo 100 10 Asal Dabbahu 100 10 Manda Inakir Erta 'Ale 100 Sample/Chondrite 10 Transverse Volcanics Alid 100 10 La Ce Pr Nd(Pm)Sm Eu Gd Tb Dy Ho Er Tm Yb LuLa Ce Pr Nd(Pm)Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Figure 18 100 Tadjoura Manda Hararo 10 1 100 Asal Dabbahu 10 1 100 Manda Inakir Erta 'Ale Sample/Primitive Mantle Sample/Primitive 10 1 100 Transverse Volcanics Alid 10 1 Rb Ba Th U Nb Ta K La Ce Pb Pr Sr P Nd Zr Hf Sm Eu Ti Gd Tb Dy Ho Y Er Tm Yb Lu Rb Ba Th U Nb Ta K La Ce Pb Pr Sr P Nd Zr Hf Sm Eu Ti Gd Tb Dy Ho Y Er Tm Yb Lu Figure 19 0.5134 A.
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