Formation and emplacement of two contrasting late-Mesoproterozoic magma types in the central Namaqua Metamorphic Complex (South Africa, Namibia): Evidence from geochemistry and geochronology

•We investigated granitoid rocks occurring in the Namaqua Metamorphic Complex.•One type of magmas is interpreted as secondary products of primary mantle magmas.•The other type formed by fluid-present partial melting of sedimentary crustal sources.•Both magmas were formed and emplaced between 1220 and 1180 Ma.•The most likely geotectonic environment is a continental back-arc mobile belt.

The Namaqua Metamorphic Complex is a Mesoproterozoic low-pressure, granulite facies belt along the southern and western margin of the Kaapvaal Craton. The NMC has formed between ~ 1.3 and 1.0 Ga and its central part consists essentially of different types of granitoids intercalated with metapelites and calc-silicate rocks. The granitoids can be subdivided into three major groups: (i) mesocratic granitoids, (ii) leucocratic granitoids and (iii) leucogranites. The high-K, ferroan mesocratic granitoids (54–75 wt% SiO2) have a variable composition ranging from granitic to tonalitic, and contain biotite and/or hornblende or orthopyroxene. They are strongly enriched in REE and LILE, indicating A-type chemical characteristics, and are depleted in Ba, Sr, Eu, Nb, Ta and Ti. The leucocratic granitoids and leucogranites (68–76 wt% SiO2) differ from the other group in having a granitic or slightly syenitic composition containing biotite and/or garnet/sillimanite. They have lower REE and MgO, FeOt, CaO, TiO2, MnO concentrations, but higher Na2O and K2O contents. Compositional variations in mesocratic granitoids indicate their formation by fractional crystallization of a mafic parental magma. Leucocratic granitoids and leucogranites lack such trends, which suggests melting of a felsic crustal source without subsequent further evolution of the generated magmas. The mineralogical and geochemical characteristics of the mesocratic granitoids are consistent magmatic differentiation of a mantle derived, hot (> 900 °C) parental magma. The leucocratic granitoids and leucogranites granites were formed from low-temperature magmas (< 730 °C), generated during fluid-present melting from metasedimentary sources.New U-Pb zircon ages reveal that both magma types were emplaced into the lower crust within a 30–40 million years interval between 1220–1180 Ma. In this time period the crust reached its thermal peak, which led to the formation of the leucocratic granitoids and leucogranites. A prolonged period of relatively high crustal temperatures is followed by a second heat pulse at ~ 1100 Ma, that was intense enough to facilitate zircon growth in the older plutons and it produced a younger granite suite. The crust cools down below amphibolite facies conditions after a further 100 million years. The prolonged high-temperature history is best compatible with steady and long-lasting heat transfer from mantle sources, suggesting a continental back-arc situation as the most likely setting of the NMC in the late Mesoproterozoic.