Continental Rifting in Central Ethiopia

The Pennsylvania State University The Graduate School College of Earth and Mineral Sciences CONTINENTAL RIFTING IN CENTRAL ETHIOPIA: GEOCHEMICAL AND ISOTOPIC CONSTRAINTS FROM LAVAS AND XENOLITHS A Thesis in Geosciences by Tyrone O. Rooney © 2006 Tyrone O. Rooney Submitted in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy August 2006 The thesis of Tyrone O. Rooney was reviewed and approved* by the following: Tanya Furman Professor of Geosciences Thesis Adviser Chair of Committee Richard R. Parizek Professor of Geology and Geo-Enviormental Engineering Andrew A. Nyblade Associate Professor of Geosciences David H. Eggler Emeritus Professor of Petrology Derek Elsworth Professor of Energy & Geo-Enviornmental Engineering Barry B. Hanan Professor of Geosciences, Dept. of Geological Sciences, San Diego State University. Special Member Katherine H. Freeman Professor of Geosciences Associate Head of Graduate Programs and Research *Signatures are on file in the Graduate School. ii ABSTRACT This dissertation will integrate geochemical and geophysical constraints from central-northern Ethiopia, moving closer to an integrated tectonic, structural and magmatic model of rifting process, from its initiation through the final transition to seafloor spreading. I focus on Quaternary magmatism within the Main Ethiopian Rift (MER), which is concentrated in extensional faults belts on the rift floor, specifically - the Wonjii Fault Belt and the Silti-Debre Zeyit Fault Zone. The location of these Quaternary eruptives within presently active extensional fault belts of the MER, presents the opportunity to deduce the primitive source(s) of rift magmatism, probe crustal structure and assess the role of magmatism in continental rifting. Mass balance of bulk rock & phenocryst composition and thermodynamic modeling of mafic lavas from both zones of extension indicates heterogeneity between the two extensional fault belts in the MER. Fractionation occurs at a shallower level (~1 kb) beneath the Wonjii Fault Belt in comparison to the Silti-Debre Zeyit Fault Zone, where fractionation occurs at various levels in the crust (<1–35 km). Shallow fractionation of the Wonjii Fault Belt lavas is consistent with calderas observed along the belt (e.g. Kone, Gedemsa) that are largely absent from the Silti-Debre Zeyit Fault Zone, where explosive maars associated with basaltic eruptions are more common. Given the strong association between these extensional fault belts and magma facilitated crustal modification (e.g. dyking, calderas), the sub-aerial distribution of Quaternary volcanism outlines the spatial distribution of lithospheric modification attendant to rifting and extension in central Ethiopia. We present lithospheric xenolith data that clearly show magmatic intrusions in the lithosphere beneath these active zones of extension, supporting geodynamic models of magma-assisted rifting. The broadly basaltic composition of these lithospheric xenoliths confirms the important role of mafic volcanism in modifying crustal structure. The Debre Zeyit and Butajira volcanic zones exhibit non-overlapping 87Sr/86Sr and 207Pb/204Pb ratios (0.7038-0.7045, 15.57-15.60 in Debre Zeyit; and 0.7036-.7038, 15.54-15.56 in Butajira). While other isotopic systems do not exhibit such variation (143Nd/144Nd 0.51261-0.51283, Debre Zeyit and 0.51278 to 0.51282, Butajira; 206Pb/204Pb 18.15-18.58, Debre Zeyit and 18.17-18.52, Butajira; 208Pb/204Pb 38.38-38.60, Debre Zeyit and 38.17 to 38.45, Butajira). The ubiquitous signature of the Afar Plume in Quaternary volcanism throughout the region supports magma assisted rifting models. However, a depleted mantle signature is observed in southern samples (Butajira), indicative of intrusion into the SCLM prior to the current episode of volcanism. This intrusion of depleted mantle derived melt into the lithosphere may reflect decompression melting due to the northward propagation of the MER, suggesting that the widespread influence of the Afar plume may be a more recent phenomenon. Combining these observations with existing geophysical data, I present a new geodynamic model for rift evolution in Ethiopia. This model associates processes active to the northeast of the Boru- Toru Transfer Zone with the Afar Depression and incipient seafloor spreading. To the southwest, continental rifting dominates and extension occurs in tectono-magmatic belts close to the rift margins with an overall orientation similar to the rift border faults. Within the framework the Wonjii Fault Belt is considered the southward propagation of the Red Sea Rift while the Silti-Debre Zeyit Fault Zone represents the northward propagation of the Main Ethiopian Rift. iii TABLE OF CONTENTS List of Tables vi List of Figures vii Acknowledgements ix Chapter One. Overview and Introduction 1 Chapter Two. Structure of the Ethiopian Lithosphere: Evidence from 21 the Main Ethiopian Rift 2.1 Introduction 21 2.2 Study Location 24 2.3 Methods 25 2.4 Results 26 2.4.1 Petrography 26 2.4.2 Geochemical Characterization of Mafic 43 Host Lavas 2.5 Discussion 47 2.5.1 Conditions of Melt and Melt Segregation 50 2.5.2 Genesis of Mafic Host Lavas 50 2.5.3 Genesis of Debre Zeyit-Butajira Xenoliths 52 2.5.4 Crustal and Lithospheric Structure 59 2.6 Summary 61 Chapter Three. Lithospheric Structure in the Main Ethiopian Rift: The 64 transition from continental rifting to seafloor spreading 3.1 Introduction 64 3.1.1 Background 64 3.1.2 Tectonic Setting 67 3.2 Methods 68 3.3 Results 72 3.3.1 Petrography 72 3.3.2 Major and Minor Elements 73 3.3.3 Trace Elements 86 3.4 Discussion 91 3.4.1 Geodynamic models of rifting and crustal 91 structure 3.4.2 Geophysical evidence of crustal structure in 92 the MER 3.4.3 Geochemical Indicators of Crustal Structure 95 3.4.4. Synthesis Model of Crustal Structure and 113 Implications for Rifting 3.5 Conclusions 123 Chapter Four. Source reservoirs of East African Rift magmatism: A story of continental rifting and mantle plumes. 125 4.1 Introduction 125 4.2 Location and geologic background 127 iv 4.3 Results 129 4.4 Discussion 135 4.4.1 Geochemical Source Reservoirs 135 4.4.2 Pseudo-Binary Arrays 140 4.4.3 Geodynamic Implications 141 4.5 Conclusions 143 References 145 v LIST OF TABLES Chapter One Chapter Two Table 2.1 Representative microprobe analysis of olivine 27 from Butajira and Debre Zeyit. Table 2.2 Representative microprobe analysis of pyroxene 29 from Butajira and Debre Zeyit. Table 2.3 Representative feldspar microprobe analysis. 31 Table 2.4 Representative microprobe analysis of spinels 33 and Fe-Ti oxides. Table 2.5 Xenolith and host lava components with 38 temperature and pressure estimates Table 2.6 Whole rock major, minor, and trace element 44 analysis results for the DZBJ host lavas. Chapter Three Table 3.1 Major, Minor and trace element analysis of the 70 MER basalts. Table 3.2 Representative microprobe analysis of olivine 75 from the Ethiopian rift. Table 3.3 Representative microprobe analysis of pyroxene 78 from the Ethiopian rift. Table 3.4 Representative feldspar microprobe analysis from 81 the Ethiopian rift Table 3.5 Representative microprobe analysis of spinels 84 and Fe-Ti oxides in the Ethiopian rift. Table 3.6 Mass balance fractional crystallisation modelling 96 of the Wonjii Fault Belt Basalts. Table 3.7 Xenolith and host lava components with 101 temperature and pressure estimates Table 3.8 Thermodynamic modelling of Wonjii Fault Belt 103 Basalts using MELTS Chapter Four Table 4.1 Isotopic ratios for the Quaternary Basalts erupted 130 at Debre Zeyit and Butajira. vi LIST OF FIGURES Chapter One Figure 1.1 Figure 1.1. Colour shaded reflief image of Africa. 3 Figure 1.2 30 Millions years of Ethiopian magmatism 5 Figure 1.3 Topographic image of the Northern EARS. 7 Figure 1.4 Location Diagram 12 Figure 1.5 Three stage block model for progressive continental 14 rifting Figure 1.6 Four-component source reservoir model 19 Chapter Two Figure 2.1 Map of the study area 23 Figure 2.2 Core compositions of pyroxene and olivine crystals in 35 mafic Butajira and Debre Zeyit lavas. Figure 2.3 Phenocryst and xenocryst feldspar zoning and 35 composition in the Debre Zeyit and Butajira areas. Figure 2.4 Photomicrograph of sample BJ-1045 showing kink 41 banding in an olivine crystal. Figure 2.5 IUGS classification of the Debre Zeyit and Butajira 41 series. Figure 2.6 Selected major element oxide abundances and 47 incompatible trace element abundances plotted against MgO for Debre Zeyit and Butajira mafic lavas Figure 2.7 Primitive mantle normalized trace element profiles. 47 Figure 2.8 Abundances of elements expected to behave 48 incompatibly during partial melting of anhydrous spinel-lherzolite Figure 2.9 FeO* vs. SiO2 for selected EARS mafic suites and 49 experimental melts in equilibrium with mantle peridotite Figure 2.10 Whole-rock major and trace element ratios from the 51 DZBJ volcanics compared with other regional mafic basalts Figure 2.11 Cartoon of pressure-temperature variations for DZBJ 54 xenoliths and host lavas Figure 2.12 Microprobe clinopyroxene and olivine chemical 57 compositions plotted as a function of Mg# and MgO in host lava and xenoliths throughout the Debre Zeyit and Butajira region Figure 2.13 Debre Zeyit Quaternary basalt region and sample 60 locations in comparison to shallow seismic tomography Chapter Three Figure 3.1 Digital Elevation model of the central MER with 65 sample locations Figure 3.2 Total alkali-silica diagram showing all Quaternary 69 MER basalts Figure 3.3 Phenocryst assemblage of the MER lavas. 69 Figure 3.4 Major element X-MgO variation diagrams. 74 Figure 3.5 Variation of CaO/Al2O3 with MgO 87 Figure 3.6 Incompatible and compatible trace element X-MgO 88 variation diagrams. Figure 3.7 Geophysical results from EAGLE project.

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