DISSERTATION Titel der Dissertation ˝Geochemical and Petrographic Studies of the Volcano- Tectonic Evolution of Northern Afar: Implications for the Structural Setup of the Actively Expanding Erta’Ale Depression˝ Verfasser Mag. Miruts Hagos angestrebter akademischer Grad Doktor der Naturwissenschaften (Dr. rer. nat.) Wien, im November 2010 Studienkennzahl lt. Studienblatt: A 091 426 Dissertationsgebiet lt. Studienblatt: Erdwissenschaften Betreuer: Prof. Dr. Christian Koeberl PREFACE: This thesis presents the results of three full years of work conducted at the Department of Lithospheric Research, University of Vienna, on the rock samples and structural data collected from the Ethio-Afar volcanic province, Ethiopia. The main objective of this study was to establish the Oligocene–Present volcano-tectonic evolution of the northern Afar Depression and set the appropriate nature of the Afar Depression lithosphere. This work was done at the megascopic scale for the regional geological map and structural setup of the northern Ethiopian plateau and the northern Afar Depression volcanic provinces. The petrogenetic link between the Ethiopian flood and the Danakil Depression basalts was evaluated, using petrographic analyses of the samples together with their major and trace element composition, as well as the Sr and Nd isotopic composition of selected samples. Geochemical and isotopic data were obtained using instrumental neutron activation analysis (INAA), X-ray fluorescence (XRF) spectrometry, electron microprobe (EMP), and thermal ionization mass spectrometry (TIMS) at the Department of Lithospheric Research, University of Vienna. This thesis consists a total of eight chapters with a much summarized general introduction of the East African Rift System (Chapter 1) and review of the regional tectonics and magmatism throughout East Africa (Chapter 2) followed by short description of the methodologies used (Chapter 3). The other five chapters are dealing with the specific tectonic and volcanics details of the Afar Depression. Chapter 4 (a conference proceedings) presents the structural and kinematic aspects of the young ocean basin (Afar Depression), followed by characterization of the pre- and post-rift volcanism of the rift bounding plateau (Chapters 5 and 6). The compositional differences of basaltic rocks in a highly evolved rift segments is presented in Chapter 7, and plume-head to tail magmatic evolution of the Afar volcanic province is summarized in Chapter 8. Chapter 4 (a conference paper, presented at the 7th Congress of the Ethiopian Geosciences and Mineral Engineering Association, August 27–28, 2010, Addis Ababa, Ethiopia) presents an overview of the structural and kinematic analysis of the Afar Depression starting from continental breakup to incipient sea-floor spreading. Measured structural data together with the existing data shows that the Afar Depression geometric make-up is under progressive transformation. Incipient breakup of the Afar Depression was mainly due of the slab pull at the i Zagros Orogenic Front leading to an initial oblique extension similar to that of the Red Sea and Gulf of Aden. The increasing influence of the Afar mantle plume and the westwards propagation of the Gulf of Aden enabled the depression to change its geometric and kinematic characteristics and rotate the eastern flank (Danakil Block) in a counter clockwise direction. After 30–25 My of deformation, extension in the Central and Northern Afar Depression is orthogonal to its axial zone. Chapter 5, a manuscript currently in press in a book called ‘Topics on Petrology’ at Springer publishers, presents and discusses major and trace element data of the northern Ethiopian flood basalts, Axum area. Basically, two distinct lava groups have been discovered in the region: the first one (Axum flood basalt sequence) is part of the Ethiopian flood basalts; whereas the second group (Axum post-trap basalt sequence) is totally distinct from the flood basalts. The chemical difference is observed in both the major (TiO2, Fe2O3, MgO, CaO) and trace (Rb, Ba, Nb, Th, Ta, La) element contents, reflecting variation in degree and depth of melting among the two groups. Chapter 6, a paper accepted for publication in the Austrian Journal of Earth Sciences, discusses the geochemical characteristics of the Axum basaltic series and the phonolite/trachyte plugs. Unlike any other region in the northern Ethiopian volcanics, the Axum–Adwa area is shattered by a series of E–W aligned trachyte and phonolite plugs. This chapter presents the first detailed study of the Axum volcanics (Alkali basalts and phonolites/trachytes). Two major episodes of volcanic activities occurred in the region. The Axum flood basalt sequence erupted contemporaneously with the major Ethiopian flood basalt and the second (the Axum post-trap volcanics) erupted late, most probably at the end of Miocene and beginning of Pliocene. Chapter 7 presents various geochemical studies of the Erta’Ale axial range (northern Afar Depression). Using the XRF, INAA EMP and TIMS data, together with the petrographic study, different series of basalts were identified. The TIMS analysis also assisted in deciphering the possible source of the basaltic rocks of the region. Chapter 8 deals mainly on the comparison of the Ethiopian Plateau basalts (plume-head) the Danakil Depression basalts (Plume-tail). A comparison was made based on the chemical analyses used in this thesis and other analyses from the Ethiopian flood basalts. ii ACKNOWLEDGEMENTS: There are many people and organizations that deserve special thanks for their valuable contributions to the accomplishment of this study. I am most grateful to my supervisor Christian Koeberl for his tremendous professional support, encouragement, persistent help in writing scientific papers, and for showing me his limitless hard work. Without his contribution this work would never have been feasible. I will always remain grateful for his invaluable professional and personal support during the last three solid years. I greatly appreciate the help of Professor Friedrich Koller for valuable discussions, critical review of my work, and encouragement. I really appreciate the sharing of knowledge on petrographic analyses of volcanic rocks. Thanks are due to Dieter Mader for his unreserved all rounded assistance during my three year stay in the department. He put tremendous support, especially with the Instrumental Neutron Activation Analysis (INAA) and interpretation of the chemical data. His frequent encouragements, discussions, and translations from and to German language (including the thesis abstract) are very much appreciated. He was really everything for me and what he did is all memorable. I would like to pass my sincere thanks to the following persons for their help with the various analytical techniques: Peter Nagl (Univ. Vienna) for his continuous help, explanation and measurements during the X- ray fluorescence (XRF) spectrometry analyses. Martin Thöni and Monika Horschinegg for the TIMS analysis. Franz Kiraly (Univ. Vienna) for introducing me with the electron MicroProbe (EMP) and data analyses. W. Füzi, L. Slawek, and S. Hrabe (Univ. Vienna) for thin section preparations. I thank Tamara Goldin, Katerina Bartosova and Tsolmon Amgaa for their continuous support and reviewing part of my work. Special thanks goes to all colleagues and students from our Impact Cratering Group, namely Ludovic Ferriere, Lidia Pittarello, Liang Chen, Bettina Schenk, Ildiko Gyollai, Beatrica Cecchin, iii Anna Losiak, Mara Bellavita, and Matthew Huber for their day to day support, encouragement, help, and friendly discussions. I thank my close friend, Tenday G/Medhin and his family, who gave me tremendous support encouragement and above all made me feel like at home. My heartfelt thank goes to the Austrian Academic Exchange Program (ÖAD) for fully supporting my scholarship. With out their support this work would never have been in this state and shape. At last but not list, I want to convey my deep gratitude to all friends and colleagues who expressed their good wishes, supported me morally and material during the past three or more years. iv ABSTRACT: The Ethio-Afar mantle plume is one of the youngest plume structures on earth that continually pours significant amounts of magmatic products to the surface of the earth for the last 30 Ma or more and reshaping the sublithospheric crust of east Africa. As a result of the complex geodynamic nature of the region, the petrologic diversity is very significant, the lavas ranging from basanites to tholeiitic basalts and phonolites to trachytes. This thesis presents geochemical and petrographic investigations of 90 volcanic rocks, mainly basaltic, from both the northern Ethiopian flood basalts and northern Afar Depression, the Erta’Ale Range. It also incorporates the kinematic analysis of the northern and central Afar Depression from incipient breakup to seafloor spreading. Rock sampling was performed in two field seasons: the first was in 2007 from the Ethiopian highland volcanic rocks and the second in 2008/09 from the northern Afar Depression axial basalts. Many petrologically distinct volcanic sequences in the northern Ethiopian plateau as well as Afar Depression have been identified. The oldest volcanic rocks of the Afar mantle plume are represented by the high-Ti (HT) and low-Ti (LT) groups of the Oligocene flood basalts. However, compositionally distinct magmatic series have been sporadically erupted in the Ethiopian highlands. One of these volcanic