Zircon U–Pb Geochronology, Whole-Rock Geochemistry and Petrogenesis of Biotite Granites in the Gaudeanmus Area, Namibia

The Gaudeanmus area is located at the southern Central Zone of the Damara orogenic belt in south-western Africa. In this paper, we investigate the whole rock major and trace element compositions and Sr–Nd–Pb isotopic compositions of the biotite granite, and determine the age of the samples utilising U–Pb zircon dating methods. Our results provide an LA–collector inductively plasma mass spectrometer (ICP–MS) zircon U–Pb age for the biotite granite of 540 ± 4 Ma (i.e., earliest Cambrian). The biotite granites show the characteristics of metaluminous compositions belonging to high-K calc–alkaline to shoshonite series. The granites contain high alkali and rare earth elements (REE), are enriched in large-ion lithophile elements (Rb, K, Pb), and depleted in high field-strength elements (Nb, Ta, Ti). The REE patterns are characterised by enrichment of LREEs relative to HREEs and medium negative Eu anomalies in the chondrite-normalised REE diagram. These rocks have high initial 87Sr/86Sr ratios (0.71400–0.71768); low εNd(t) value (−12.0 to −7.1); Sm–Nd isotope crust model ages ranging from 1711 to 2235 Ma; and large variations in 206Pb/204Pb (18.0851–19.2757), 207Pb/204Pb (15.6258–15.7269), and 208Pb/204Pb ratios (38.7437–40.5607). Such geochemical signatures indicate that the biotite granite rocks derive mainly from partial melting of ancient crustal rocks resembling the local basement meta-sedimentary rocks. However, minor mantle-derived materials may have also been involved in the formation of these rocks. Combining with regional tectonic evolution, we consider that the biotite granite intrusions in the Gaudeanmus area formed in a transitional tectonic regime that went from compressional to extensional tectonics.