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Shear zone-related syenites in the Damara belt (Namibia): the role of crustal contamination and source composition

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Abstract

Geochemical and Nd-Sr-Pb-O isotope data for a suite of syn-collisional (ca. 520 Ma) syenites associated with a major shear zone in the Proterozoic Damara orogen (Namibia) constrain their sources and petrogenesis. Major rock types from within and outside the shear zone range from highly potassic nepheline syenites to quartz syenites and were primarily generated by fractional crystallization from a mantle-derived alkaline magma. Even the most primitive samples show pronounced depletion in Nb, Ti, Sr and P on a primitive mantle-normalized diagram, indicating the involvement of a recycled crustal component in the source. Extrapolation of the Sr-Nd-Pb-O isotope composition of the syenites from within the shear zone back to a hypothetical parental melt with 10 wt% MgO suggests derivation from a moderately enriched lithospheric upper mantle (87Sr/86Sr: 0.705, ε Nd: −2, δ18O: 6‰, 206Pb/204Pb: 19.40, 207Pb/204Pb: 15.82). More evolved quartz syenites show increasing 87Sr/86Sr ratios, increasing δ18O values but less radiogenic ε Nd values and Pb isotopes with decreasing MgO, indicating assimilation of ca. 10% Archaean to Proterozoic local lower crust with unradiogenic ε Nd, high 87Sr/86Sr and low U/Pb. For samples from outside the shear zone a hypothetical parental melt with 10 wt% MgO has distinctly more radiogenic Sr but less radiogenic Nd isotopic composition (87Sr/86Sr: 0.712, ε Nd: −13), with strongly unradiogenic Pb isotope ratios (206Pb/204Pb: 17.40, 207Pb/204Pb: 15.50), suggesting another strongly enriched lithospheric mantle source for these rocks. Differentiated syenites from outside the shear zone show decreasing 87Sr/86Sr, increasing δ18O values, more radiogenic ε Nd values and Pb isotope ratios with decreasing MgO indicating interaction with a lithospheric component with low Rb/Sr but high Sm/Nd and U/Pb.

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Acknowledgements

The analytical work was supported by grants from the Max-Planck-Society and A.W. Hofmann is thanked for hospitality and free access to mass spectrometry facilities while S. J. held a post-doc position in Mainz. Iris Bambach (Max-Planck-Institut, Mainz) did a superb job in managing the line drawings. Special thanks go to D. Dohle from the Bonn fluorine-lab crew for determination of the oxygen isotope ratios. S. Göbeler (Marburg) helped with Fe2+ and REE determinations. E. Hoffer (Marburg) is thanked for logistic help in the field in 1998 and for free access to samples originally collected by members of the SFB 48 at the University of Göttingen. We would like to thank T. Green for making his unpublished phlogopite and amphibole trace element analyses available in 1998. CMP reviewers, two of them anonymous, as well as R. Trumbull and M. Okrusch made numerous comments which helped to clarify various aspects of the original manuscript. The patient editorial handling by J. Hoefs is gratefully acknowledged.

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  1. Fachbereich Geowissenschaften, Philipps-Universität Marburg, Lahnberge/Hans-Meerwein-Straße, 35032, Marburg, Germany

    S. Jung

  2. Abt. Geochemie, Max-Planck-Institut für Chemie, Postfach 3060, 55020, Mainz, Germany

    S. Jung

  3. Institut für Mineralogie, Universität Münster, Corrensstrasse 24, 48149, Münster, Germany

    K. Mezger

  4. Mineralogisch-Petrologisches Institut der Universität Bonn, Poppelsdorfer Schloß, 53115, Bonn, Germany

    S. Hoernes

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Jung, S., Mezger, K. & Hoernes, S. Shear zone-related syenites in the Damara belt (Namibia): the role of crustal contamination and source composition. Contrib Mineral Petrol 148, 104–121 (2004). https://doi.org/10.1007/s00410-004-0589-8

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