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The Cenozoic Magmatism of East Africa: Part V – Magma Sources and Processes in the East African Rift

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1 The Cenozoic Magmatism of East Africa: Part V – Magma Sources and Processes in the East African Rift Tyrone O. Rooney Dept. of Earth and Environmental Sciences, Michigan State University, East Lansing, MI 48823, USA Abstract The generation of magmas in the East African Rift System (EARS) is largely the result of either: (A) melting of easily fusible compositions located within the lithospheric mantle due to thermobaric perturbations of the lithosphere, or (B) melting of the convecting upper mantle due to decompression caused by thinning of the plate during extension. Melt generated from amphibole- or phlogopite-bearing metasomes within the lithospheric mantle yields alkaline, silica-undersaturated lavas, while more silica-saturated lavas are primarily a function of melting material within the convecting upper mantle. Sourcing of silica-undersaturated melts within the lithospheric mantle is consistent with the observed tendency for initial melts within any given region to exhibit trace element characteristics consistent with melting of lithospheric metasomes, likely reflecting the initial destabilization and thinning of the lithospheric mantle. With continued lithospheric thinning, the trend towards more silica-saturated compositions coincides with a shift towards compositions interpreted as melting of the convecting upper mantle. Contributions from these two sources may oscillate where extension is pulsed – melts of the convecting upper mantle are favored during periods of plate thinning; melting of either existing or recently formed metasomes may be favored during periods of relative extensional quiescence. The isotopic systematics of East African magmatism reveals significant complexity 1 Figure 1 Crust lithosp. Disseminated mantle phases amphibole,amphibole Fe-Ti Oxides & other phases pyroxenites asthensp. metasomatic agent Figure 2 crust lithospheric mantle amphibole metasome pyroxenite metasome asthenosphere Figure 3 100 Type IIa Type IIb 10 Type Ib Type Ia 50 Type IV 20 Type III 10 5 Sample/Primitive Mantle Sample/Primitive 100 Type V 10 Type VI 1 RbBaTh U NbTa K LaCePr Sr P NdZr HfSmEu Ti GdTbDyHo Y ErYbLu Figure 4 Afar Main Ethiopian Rift Turkana Eastern Toro Branch Ankole Kenya Kivu & Rift Virunga NTD Modern East African Rift Cretaceous Rifting Cenozoic Lavas of East Africa Western Neoproterozoic Branch Mesoproterozoic and older Extent of Craton Rungwe Figure 5 Figure 6 Figure 7 MORB OIB Upper Continental Crust 10 Mobile Belt Zr/Nb Carbonatites 1 Yemen Afar MER Cratonic Turkana North Kenya Rift South Kenya Rift/Tanzania Rungwe Kivu-Virunga Carbonatites 1 10 (La/Sm)CN Figure 8 DM Afar Plume 0.5130 ("C" reservoir) 0.5128 HIMU Nd CLM EM2 144 0.5126 Nd/ 143 0.5124 Yemen Afar MER 0.5122 Turkana North Kenya Rift South Kenya Rift/Tanzania 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 9 0.5131 DM Yemen Afar MER Turkana North Kenya Rift 0.5130 Afar Plume ("C" reservoir) 0.5129 Nd HIMU 144 0.5128 Nd/ 143 0.5127 CLM 0.5126 PA 0.5125 0.7030 0.7040 0.7050 0.7060 0.7070 87Sr/86Sr Figure 10 silicic cinder Silti-Debre Zeyit Wonji center cone Fault Zone Fault Belt silicic 7 km fractionation 10 km system mafic stacked 0.2 - mush fractionation dikes system 0.4 km dikes 35 km Crustal Thickness Figure 12 15.9 HIMU Crustal 15.8 Contamination PA EM2 Pb Vector 204 Pb/ 15.7 207 15.6 Afar Plume DM ("C" reservoir) 15.5 0.710 0.709 Olkaria Olkaria EM2 0.708 PA Crustal 0.707 Contamination Sr 86 Vector 0.706 Sr/ 87 0.705 CLM 0.704 0.703 Afar Plume HIMU DM ("C" reservoir) DM 0.5132 Olkaria Afar Plume 0.5130 ("C" reservoir) HIMU Nd 0.5128 144 EM2 Crustal Nd/ 0.5126 CLM Contamination 143 Vector 0.5124 0.5122 PA 18 19 20 21 206Pb/204Pb Figure 13 41.5 15.9 HIMU 41.0 HIMU 40.5 15.8 PA EM2 EM2 208 Pb/ Pb PA 40.0 204 204 Pb Pb/ 15.7 39.5 207 Afar Plume ("C" reservoir) 39.0 15.6 Afar Plume 38.5 DM ("C" reservoir) DM 15.5 38.0 0.710 DM SKR/NTD 0.709 South Kenya Rift/Tanzania Kivu-Virunga Carbonatites Rungwe 0.5132 Rungwe Kivu-Virunga PA EM2 Carbonatites 0.708 Afar Plume ("C" reservoir) 0.5130 HIMU 143 0.707 Nd/ Sr 0.5128 86 0.706 144 Sr/ EM2 Nd 87 CLM 0.5126 0.705 CLM 0.704 0.5124 0.703 Afar Plume HIMU PA 0.5122 DM ("C" reservoir) 18 19 20 21 18 19 20 21 206Pb/204Pb 206Pb/204Pb Figure 14 Toro Ankole Virunga Kivu Kivu and Virunga Volcanics A. DMM 0.51325 Afar Plume ("C" reservoir) 0.51300 Nd 0.51275 HIMU 144 CLM EM2 Nd/ 0.51250 143 EACL 0.51225 PA 0.51200 0.702 0.703 0.704 0.705 0.706 0.707 0.708 87Sr/86Sr B. 40.00 HIMU EM2 PA Afar Plume Pb 39.00 ("C" reservoir) 204 Pb/ 208 38.00 Eastern Uganda Homa Bay Shombole/Kerimasi 37.00 Oldoinyo Lengai DMM Toro Ankole 18 19 20 21 22 206Pb/204Pb Figure 16 15.9 HIMU 15.8 PA EM2 15.7 Pb 204 Pb/ 207 15.6 Afar Plume ("C" reservoir) 15.5 Mobile Belt Mixing DM Craton Mixing 15.4 DM 0.5130 Afar Plume ("C" reservoir) 0.5128 HIMU CLM Nd 0.5126 EM2 144 Nd/ 143 0.5124 0.5122 PA 0.5120 0.710 Yemen Afar 0.709 MER Turkana NKR EM2 SKR/NTD 0.708 Rungwe PA Kivu-Virunga Carbonatites 0.707 Sr 86 Sr/ 0.706 87 Afar Plume CLM 0.705 ("C" reservoir) 0.704 DM HIMU 0.703 17 18 19 20 21 22 206Pb/204Pb Figure 17 PA 0.5130 DM Afar Plume 0.707 ("C" reservoir) 0.706 0.5128 HIMU 87 Nd Sr/ 144 0.705 86 Sr Nd/ 0.5126 143 Afar Plume ("C" reservoir) 0.704 0.5124 HIMU PA DM 0.703 0.5122 15.8 HIMU HIMU PA 40 15.7 207 Pb/ Pb PA 204 204 Pb Pb/ 208 39 Afar Plume 15.6 ("C" reservoir) Afar Plume ("C" reservoir) DM DM 38 15.5 20 10 HIMU 8 19 Afar Plume Zr/Nb Pb ("C" reservoir) 6 204 Pb/ 18 PA 4 206 DM 2 17 Yemen MER Afar Rungwe 0 2.5 5 7.5 10 12.5 15 17.5 2.5 5 7.5 10 12.5 15 17.5 3 4 3 4 He/ He (R/RA) He/ He (R/RA) Figure 18 40.0 PA EM2 EM2 15.7 PA 39.5 39.0 15.6 208 Pb/ Pb 38.5 204 Afar Plume Afar Plume 204 ("C" reservoir) Pb Pb/ 15.5 ("C" reservoir) 38.0 207 Yemen 37.5 Afar 15.4 MER Turkana North Kenya Rift 37.0 DM DM 15.3 36.5 PA DM Afar Plume 0.7070 0.5132 Yemen MER North Kenya Rift ("C" reservoir) Afar Turkana 0.7060 0.5130 143 CLM Nd/ Sr 0.5128 86 0.7050 144 Sr/ CLM Nd 87 0.5126 0.7040 EM2 0.5124 0.7030 Afar Plume 0.5122 DM ("C" reservoir) PA 20 DM Afar Plume Yemen MER North Kenya Rift ("C" reservoir) 17.5 Afar Turkana 15 Afar Plume 3 15 He/ ("C" reservoir) 12.5 4 He(R/R Hf ε 10 10 A DM ) 7.5 PA 5 PA 5 EM2 2.5 17.5 18 18.5 19 19.5 2017.5 18 18.5 19 19.5 20 206Pb/204Pb 206Pb/204Pb Figure 19 22% 207Pb Yemen DMM 24.25% Pb Afar 208 206 MER Pb Turkana 53.75% NKR 26.25% Pb 208 PA 206 52.5% Pb 21.25% 207Pb 207 Pb Afar Plume EM2 ("C" reservoir) Pb 206 208Pb 2 as to the specific reservoirs that may participate in the melting processes noted above. The lithospheric mantle beneath East Africa has undergone enrichment through the percolation of sub-lithospheric derived melts and fluids over an extended interval, which close to the Tanzania craton, has resulted in a layered lithospheric mantle exhibiting extreme isotopic ratios. Elsewhere, the lithospheric mantle has also undergone enrichment but given the more juvenile age of this lithosphere, less extreme isotopic values have developed. Material rising from the African Large Low Shear Velocity Province (LLSVP) has also metasomatized the lithospheric mantle, and thus lavas exhibiting a trace element signature linked to melting within the lithospheric mantle may exist as any number of reservoirs or mixtures of the same. Material derived from the convecting upper mantle incorporates the Afar Plume endmember, a depleted mantle endmember, and some form of lithospheric endmember. The isotopic characteristics of magma suites from throughout the region form arrays that broadly converge on the composition of the Afar Plume, despite some complexity where the plume material has formed a hybrid plume-lithosphere component. The convergence of these arrays strongly supports a model whereby the prevalent composition of material rising from the African LLSVP beneath the EARS is broadly equivalent to the composition of the Afar Plume. 1. Introduction The spatial and temporal variability in alkalinity and silica saturation of lavas within the East African Rift System (EARS) is a first order observation in any synthesis of regional magmatism (Baker, 1987; Foley et al., 2012). There is a general trend within the rift for a progression of decreasing alkalinity over time – this is particularly apparent in the Kenya Rift where transitional basalts do not manifest until ca. 7 Ma; the earlier magmatic periods are dominated by alkaline lava series.
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