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

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, Nd: –2, ¹⁸O: 6, ²⁰⁶Pb/²⁰⁴Pb: 19.40, ²⁰⁷Pb/²⁰⁴Pb: 15.82). More evolved quartz syenites show increasing ⁸⁷Sr/⁸⁶Sr ratios, increasing ¹⁸O values but less radiogenic Nd values and Pb isotopes with decreasing MgO, indicating assimilation of ca. 10% Archaean to Proterozoic local lower crust with unradiogenic 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, 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 ¹⁸O values, more radiogenic 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.