Petrogenesis of synorogenic diorite–granodiorite–granite complexes in the Damara Belt, Namibia: Constraints from U–Pb zircon ages and Sr–Nd–Pb isotopes

Abstract

The synorogenic Palmental complex (central Damara Belt, Nambia) consists of ca. 545Ma old quartz diorites and rare granodiorites and ca. 520Ma-old leucogranites, representing one of the earliest and most primitive phase of crustal plutonism predating the main high-T regional metamorphism. Most quartz diorites and one granodiorite evolved through multistage, polybaric evolutionary processes involving fractionation from a lithospheric mantle-derived melt, followed by fractional crystallization of mainly hornblende, plagioclase and apatite which is shown by decreasing MgO, FeO, CaO, TiO2 and P2O5 with increasing SiO2. Assimilation of felsic basement gneisses was also important during formation of these granitoids. Although their chemical characteristics (high LILE, low HFSE) resemble those of quartz diorites and granodiorites with calc-alkaline affinity, they differ in their enriched Sr (initial 87Sr/86Sr: 0.7061–0.7098) and Nd (initial εNd: −2.7 to –9.9) isotopic composition. Neodymium depleted mantle mean crustal residence ages range from 1.3 to 1.9Ga for the quartz diorites including the granodiorite. These model ages correlate with major and trace element abundances, further substantiating that AFC processes modified the initial isotopic systematics. Lead (206Pb/204Pb: 17.43–17.68, 207Pb/204Pb: 15.61–15.66, 208Pb/204Pb: 38.19–38.49) isotopic composition are unradiogenic relative to other Damaran intrusive rocks but plot above the Stacey and Kramers (1975) reference line, indicating that the source underwent an ancient (pre Pan-African) increase in U/Pb and Th/U, followed by more recent U-depletion. Some variation in 206Pb/204Pb at high 207Pb/204Pb further indicates involvement of ancient crustal material, most likely through AFC processes. A cross-cutting leucogranite dyke has also evolved isotopic compositions (initial 87Sr/86Sr: 0.7326; initial εNd: −15.6; 206Pb/204Pb: 17.42, 207Pb/204Pb: 15.62, 208Pb/204Pb: 38.16) but in view of the apparent younger age of the granite, this rock type cannot be considered as a fractionation product of the quartz diorites and therefore represents likely a melting product of the underlying felsic basement. We also report data from two quartz diorite samples collected in the north and east of the Palmental area. They plot off the trends defined by the other quartz diorites. Palaeoproterozoic Nd model ages of 2.1Ga and distinct Pb isotopic data indicate a Palaeoproterozoic mean crustal residence age for the source material. We conclude that these samples belong to the nearby Goas and Okongava quartz diorites. For these rocks, based on a previous model and a comparison with results from amphibolite-dehydration melting experiments, a high-K meta-tholeiite melted at 1000–1050°C is a likely source. Isotopic data obtained on samples of the Palmental complex make a Pan African subduction zone setting unlikely for these rocks. It is concluded that the most primitive Palmental quartz diorites recording moderately evolved Nd and Sr isotope data (εNd(t): −2.7, 87Sr/86Sr: 0.706) reflect an isotope composition typical of aged subcrustal lithospheric mantle.