Petrogenesis of Cogenetic Nepheline and Quartz Syenites and Granites (Northern Damara Orogen, Namibia): Enriched Mantle versus Crustal Contamination

Geochemical and Nd-Sr-O whole rock isotope data and Pb isotope data from leached feldspars for a suite of synorogenic (ca. 540 Ma) syenites and granites from the northern part of the Proterozoic Damara orogen ( Namibia) constrain their sources and petrogenesis. Major rock types are mildly potassic nepheline syenites and quartz syenites that 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 back to a hypothetical parental melt with 8 wt% - 10 wt% MgO suggests derivation from a moderately enriched lithospheric upper mantle (Sr-87/Sr-86 = 0.7055, epsilon(Nd) = -4, delta(18)O = 5 parts per thousand, Pb-206/Pb-204 = 17.60, Pb-207/Pb-204 = 15.58, Pb-208/Pb-204 = 37.50). More evolved quartz syenites show virtually unchanged Sr-87/Sr-86, Pb-206/Pb-204, Pb-207/Pb-204, and Pb-208/Pb-204 and Pb-208/Pb-204 ratios and epsilon(Nd) values. The most distinctive feature of the quartz syenites is their higher delta(18)O values up to 10.5 parts per thousand. These features seem to indicate that, unlike most other Damaran igneous rocks, the syenites described here were entirely formed within the lithospheric mantle and were not modified by assimilation processes of basement and/or Damaran metasedimentary rocks. In order to account for the substantial variation in O isotopes, the most likely source for these rocks is a heterogeneous association of mantle rocks and subducted upper crustal material, most likely altered basaltic rocks from subducted ocean floor. Additionally, low temperature alteration may have affected the quartz syenites. In contrast, although coeval granites with A-type affinities are interpreted as fractionation products of the syenites, their distinctly more negative epsilon(Nd epsilon) values ranging from - 9 to - 10 indicate that deep crustal assimilation processes of local Proterozoic basement rocks play an important role in the evolution of these granites.