Generation of magnesian, high-K alkali-calcic granites and granodiorites from amphibolitic continental crust in the Damara orogen, Namibia

Abstract

The Pan-African Damara orogen of Namibia is characterized by large-scale granitoid intrusions. Two plutons in the Northern Central Zone (NCZ) of the Damara orogen within the Okombahe district have U–Pb zircon ages of 576.2 ± 5.7 Ma and 570.9 ± 4.9 Ma that predate the time of high grade regional metamorphism which occurred between 540 and 480 Ma. The intrusive rocks are magnesian high-K alkali-calcic granodiorites to granites, are enriched in HFSE and REE, and have undergone only a limited degree of fractional crystallization, and do not contain xenoliths of local country rocks. Initial isotope compositions are unevolved with 87Sr/86Sr between 0.704 and 0.706 and initial εNd ranging from − 1.9 to − 3.9. Lead isotopes are radiogenic (206Pb/204Pb: 18.32 to 18.61, 207Pb/204Pb: 15.61 to 15.69 and 208Pb/204Pb: 37.87 to 39.29) with variable 207Pb/204Pb ratios at almost constant 206Pb/204Pb and 208Pb/204Pb ratios, suggesting a derivation from ancient sources with comparatively high U/Pb but low Th/Pb ratios. The limited variations in Sr, Nd and Pb isotopes were not caused by crustal contamination or magma mixing, but instead reflect source heterogeneities. Strontium and Nd isotope compositions suggest mafic lithologies similar to amphibolites from the Kalahari Craton basement as potential sources. A comparison with amphibolite melting experiments confirms the possible derivation of the granodiorites from an amphibolitic source. Calculated maximum zircon saturation temperatures at insignificant amounts of inherited zircon, indicate intrusion temperatures of up to 900 °C. Apatite saturation temperatures are higher, up to ca. 950 °C. Pressures of 5 to 10 kbar are determined through Qz-Ab-Or systematics and are interpreted as minimum pressures at the site of melting suggesting that the granodiorites/granites represent high temperature partial melts generated in the lower crust. Although there are some compositional similarities with granites generated in subduction zones, radiogenic Pb isotope ratios and high δ18O values suggest that reprocessed amphibolitic rocks are more likely sources.