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

摘要

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 petro-genesis. 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 percent MgO suggests derivation from a moderately enriched lithospheric upper mantle (~(87) Sr/~(86) Sr: 0.705. epsilon Nd: -2, delta ~(18)O: per thousand, ~(206)Pb/~(204)Pb: 19.40, ~(207)Pb/~(204)Pb: 15.82). More evolved quartz syenites show increasing ~(87) Sr/~(86)Sr ratios, increasing delta 5~(18)O values but less radiogenic epsilon Nd values and Pb isotopes with decreasing MgO. indicating assimilation of ca. 10 percent Archaean to Proterozoic local lower crust with unradiogenic epsilon Nd, high ~(87)Sr/~(86)Sr and low U/Pb. For samples from outside the shear zone a hypothetical parental melt with 10 wt percent MgO has dis-tinctly more radiogenic Sr but less radiogenic Nd iso-topic composition (~(87)Sr/~(86)Sr: 0.712, c Nd: -13), with strongly unradiogenic Pb isotope ratios (~(206)Pb/~(204)Pb: 17.40, ~(207)Pb/~(204)Pb: 15.50), suggesting another strongly enriched lithospheric mantle source for these rocks. Differentiated syenites from outside the shear zone show decreasing ~(87) Sr/~(86)86 Sr, increasing delta ~(18)O values, more radiogenic epsilon 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.