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Geology, Petrology, and Geochemistry of the Basaltic Rocks of the Axum Area, Northern Ethiopia

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Topics in Igneous Petrology

Abstract

The Axum volcanic rocks constitute an important outcrop of trap basalts at the northern end of the great Ethiopian flood basalt province. They constitute about 5–480-m-thick sequences exposed around Axum and Shire regions covering a total area of about 900 km2. Even though the Axum volcanic rocks were previously assigned to the low-Ti (LT) tholeiitic basalts of the northwestern Ethiopian volcanic province, our geochemical studies on rocks from this region reveal that these rocks are alkaline to transitional in composition with distinct geochemical characteristics. Their petrologic diversity is very significant, the lavas ranging from basanites to tephrites and trachy-andesites. The variation in the major element (TiO2 and Fe2O3) concentrations, incompatible trace elements, and the rare earth element (REE) concentrations and ratios (e.g., (La/Yb)n = 5.0–14.0) indicate a complex petrogenetic evolution of the flood basalts. The Axum volcanic rocks are entirely alkaline to transitional in nature and are classified here into the Lower Sequence (Low-Ti) basalts and the Upper Sequence (High-Ti) basalts. Both these sub-divisions are also reflected in their unique chondrite-normalized REE patterns and multi-element diagrams: the Lower Sequence basalts show enhanced REE fractionation and higher concentrations of the highly incompatible trace elements; whereas the Upper Sequence basalts exhibit less REE fractionation and relatively high concentrations of the moderately incompatible trace elements. The mineralogical and geochemical variations among the basalts of Axum provide clues about the complexity of the northern and northwestern Ethiopian flood basalts and their petrogenetic evolution.

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Acknowledgments

The authors are very grateful to Drs. M. Koch and M. DiBlasi from the Centre for Remote Sensing, Boston University, USA, for their collaboration during the first field session and the local people of Axum for their invaluable help during the field work. We also thank the Tigray Water, Mines and Energy Bureau for their professional and material support during the second fieldwork. We would like to appreciate Axum University staff, especially Drs. K.W-Aregay and W. Fitehanegest, for their invaluable support and discussions during the field session. We thank the reviewers, H.C. Sheth and an anonymous colleague, for their useful suggestions and comments. M.H. is supported by an ÖAD (Austrian Academic Exchange Program) North-South doctoral scholarship at the University of Vienna, Austria.

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Author notes

  1. Christian Koeberl

    Present address: Natural History Museum, Burgring 7, A-1010, Vienna, Austria

Authors and Affiliations

  1. Department of Lithospheric Research, University of Vienna, Althanstrasse 14, A-1090, Vienna, Austria

    Miruts Hagos, Christian Koeberl & Friedrich Koller

  2. Department of Earth Sciences, Mekelle University, 231, Tigray, Ethiopia

    Kurkura Kabeto

Authors
  1. Miruts Hagos
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  2. Christian Koeberl
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  3. Kurkura Kabeto
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  4. Friedrich Koller
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Corresponding author

Correspondence to Miruts Hagos .

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Editors and Affiliations

  1. Dept. Geology, University of Calcutta, Ballygunge Circular Rd. 35, Kolkata, 700019, India

    Jyotisankar Ray

  2. Dept. Earth Sciences, Florida Center for Analytical, Florida International University, Miami, 33199, Florida, USA

    Gautam Sen

  3. , Department of Geology, University of Calcutta, Ballygunge Circular Road, Kolkata, 700019, India

    Biswajit Ghosh

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Hagos, M., Koeberl, C., Kabeto, K., Koller, F. (2011). Geology, Petrology, and Geochemistry of the Basaltic Rocks of the Axum Area, Northern Ethiopia. In: Ray, J., Sen, G., Ghosh, B. (eds) Topics in Igneous Petrology. Springer, Dordrecht. https://doi.org/10.1007/978-90-481-9600-5_4

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