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Petrology, geochronology and Sr–Nd isotopic geochemistry of the Konso pluton, south-western Ethiopia: implications for transition from convergence to extension in the Mozambique Belt

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Abstract

Granites were shown to be excellent geochronological, structural and geodynamic markers. Among several generations of granites described in the Neoproterozoic of Ethiopia, we studied the post-tectonic Konso pluton to characterise the post-Pan-African evolution of the Mozambique Belt (MB) of southern Ethiopia. The Konso pluton is a composite intrusion of slightly peraluminous and ferro-potassic, bt (biotite)–leucogranites, bt–hbl (hornblende)–granites and subordinate coeval metaluminous monzodiorites, intruded into high-grade gneiss–migmatite associations of the MB. The whole suite displays chemical features of A-type granites. It is LIL- and HFS-elements enriched with Y/Nb and Yb/Ta≥1.2. The granites and leucogranites show non-fractionated to fractionated REE patterns [(La/Yb)N=0.3–9.4] with strong negative Eu anomalies. The monzodiorites show fractionated REE patterns [(La/Yb)N=5.5–7.4] with negligible negative Eu anomaly. The low initial (87Sr/86Sr)450 ratios (0.70113–0.70441) and positive εNd(450) values (+1.8 to +3.3) suggest an isotopically primitive source. The Konso granites are likely to be derived from a basaltic parent, with minor contamination by crustal material with high Y/Nb and low Sr initial isotopic ratios. Age of pluton emplacement is constrained by a Rb–Sr isochron and zircon U–Pb data at 449±2 Ma. The Konso pluton is, therefore, the witness of an Ordovician A-type magmatic event, which marks a change from convergence, related to the Pan-African collision, to extension in the Mozambique Belt of southern Ethiopia.

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Acknowledgements

We are grateful to C. Spatz, D. Mangin and M. Champenois for technical assistance. Our sincere gratitude goes to T. Nardos and D. Hailu for their invaluable assistance during the field work. The two anonymous referees are thanked for their valuable comments that contributed to improve the manuscript. We are indebted to the Department of Geology and Geophysics, Addis Ababa University, for the logistical support during the field work. This work was supported by a research grant from the French Ministry of Foreign Affairs and financial support from INSU-Ethiopie-2000 Project.

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  1. Department of Geology and Geophysics, Addis Ababa University, P.O. Box 1176, Addis Ababa, Ethiopia

    Asfawossen Asrat

  2. CRPG-CNRS, 15 rue Notre-Dame des Pauvres, B.P. 20, 54501 Vandoeuvre-lès-Nancy Cedex, France

    Pierre Barbey

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  1. Asfawossen Asrat
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Correspondence to Asfawossen Asrat.

Appendix

Appendix

Analytical methods

Mineral compositions were analysed with a CAMECA SX-50 Electron Microprobe at the University Henri Poincaré (Nancy). Operating conditions were 20-nA sample current, 15-kV accelerating potential, counting times of 20 s and a beam diameter of 1 µm. Calibration was made on a combination of silicates and oxides. Data reductions were performed using the PAP correction procedure (Pouchou and Pichoir 1991).

Whole-rock major and trace elements were analysed by ICP-AES and ICP-MS at CRPG-CNRS (Nancy). Analytical uncertainties are given as 2% for major elements, and as 5 or 10% for trace element (except REE) concentrations higher or lower than 20 ppm, respectively. Precision for REE is estimated at 5% when chondrite-normalised concentrations are >10 ppm and at 10% when they are lower.

Separation of Rb–Sr and Sm–Nd was made according to the methods of Michard et al. (1985) and Boher et al. (1992). Rb, Sr, Sm and Nd concentrations were determined using the isotope dilution method. Rb was analysed by thermal ionisation on a Cameca TSN 206 SA mass spectrometer, whereas Sr and Nd isotopic ratios and Sm concentration were measured using a Finnigan MAT 262 mass spectrometer. Measured 87Sr/86Sr and 143Nd/144Nd ratios were normalised to 86Sr/88Sr=0.1194 and 146Nd/144Nd=0.7219, respectively. Repeated analyses of standards NBS 987 for Sr and JM for Nd gave average ratios of 87Sr/86Sr=0.710205±23 (2σ) and 143Nd/144Nd=0.511095±16 (2σ). The blanks for Sr and Nd were negligible (<2 ng for Sr and 0.4 ng for Nd) compared with the extracted quantities from samples. Analytical errors on isotopic ratios (error in the last two digits of the Sr and Nd isotopic ratios) are expressed as 2σ (=two standard errors of the mean).

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Asrat, A., Barbey, P. Petrology, geochronology and Sr–Nd isotopic geochemistry of the Konso pluton, south-western Ethiopia: implications for transition from convergence to extension in the Mozambique Belt. Int J Earth Sci (Geol Rundsch) 92, 873–890 (2003). https://doi.org/10.1007/s00531-003-0360-9

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