Petrology and geochemistry of syn- to post-collisional metaluminous A-type granites—a major and trace element and Nd–Sr–Pb–O-isotope study from the Proterozoic Damara Belt, Namibia

Abstract

Leucocratic A-type granites of the Proterozoic Damara orogen, Namibia are a distinctive groups of rocks based on their mineralogical, chemical and isotopic characteristics. A-type granites can be distinguished from the leucocratic S-type granites on the basis of Fe-rich amphibole and biotite and higher abundances of Zr, Nb, Y and LREE but lower values of 87 Sr/ 86 Sr (500 Ma) and δ 18 O in the A-type granites. In comparison with A-type granites described in the literature, the Damaran A-type granites are enriched in CaO, Al 2O 3, MgO, FeO, Ba, Sr and Eu but depleted in Ga and Zn. Low K/Rb, Rb/Sr, Rb/Ba and Sr/Ba ratios indicate that these melts are not fractionation products of calc-alkaline source rocks and therefore, these chemical criteria are interpreted as a feature of the source rocks, i.e., undepleted metatonalites. The A-type granites have high calculated zircon and monazite saturation temperatures mostly in excess of 800°C and it is concluded that these granites represent high-temperature partial melts. The relationship between major- and trace element data and isotopic composition demonstrates that the A-type granites originate from several sources and have undergone different processes. The lack of evidence for shallow crustal contamination (e.g., lack of country rock xenoliths), the low viscosities of the melts (as inferred from their high calculated temperature) and their high Nd concentrations relative to the country rocks suggest that the isotopic characteristics were acquired from an enriched mantle. Initial Sr isotopic compositions vary over a narrow range (0.70486–0.70928) but the initial Nd isotopic compositions vary considerably with initial ε Nd values between −0.03 and −6.35. This wide range in the Nd isotope values is probably the result of interaction of mantle-derived magmas with crustal rocks. A-type granites at Baukwab have negatively correlated initial ε Nd values and 207 Pb/ 204 Pb and 206 Pb/ 204 Pb ratios indicating interaction of mantle-derived magmas (low initial ε Nd, low 207 Pb/ 204 Pb and 206 Pb/ 204 Pb ) with continental crust (high initial ε Nd, high 207 Pb/ 204 Pb and 206 Pb/ 204 Pb ). A-type granites from Albrechtstal and Oetmoed have positively correlated Pb isotope ratios and show also positive correlations between Nd isotopes and Pb isotopes, suggesting interaction of at least two different source materials. The relationship between Nd isotopes and Nd concentrations precludes simple assimilation, instead mixing of different sources seems to be more realistic. For the A-type granites from Oetmoed, oxygen isotopes are negatively correlated with Sr isotopes indicating that the crustal end-member has high δ 18 O values and rather low initial 87 Sr/ 86 Sr ratios whereas a hypothetical mantle component has lower δ 18 O values but higher initial 87 Sr/ 86 Sr ratios. The chemical and isotope data suggest that the Damaran A-type granites were generated by a variety of processes including partial melting of mantle-derived tonalitic sources, limited crystal fractionation and interaction of these magmas with crustal material. These data indicate that the Damaran A-type suites mark episodes of continental growth during which mantle material, although limited in extent, is added to the crust.