On the coexistence of rift-related silica-undersaturated and silica-saturated alkaline rocks – The cretaceous Messum alkaline complex (Namibia) revisited

Abstract

The ∼132 Ma-old Messum igneous complex, a circular structure of ∼18 km diameter, belongs to the Damaraland intrusive complexes that intruded into the Pan-African Damara basement of Namibia. The plutonic rocks of the core of the complex comprise olivine-gabbro, biotite-gabbro, nepheline-syenite and quartz-syenite as well as minor alkali gabbro and coexist with the Etendeka volcanic sequences of the Goboboseb Mountains, comprising basalts (Tafelkop and Tafelberg types) that are overlain by deposits of several quartz latite eruptions. New major and trace element and Sr, Nd and Pb isotope data are reported for nepheline-syenite and quartz-syenite. Major and trace element data are consistent with the development of nepheline-syenites through fractional crystallization processes. The nepheline-syenite samples have initial ⁸⁷Sr/⁸⁶Sr of 0.7046–0.7049 and initial εNd ranging from +0.53 to +1.47, indicating a mantle source with time-integrated lithophile element depletion. Initial Pb isotopes are rather uniform (²⁰⁶Pb/²⁰⁴Pb:17.89–18.14, ²⁰⁷Pb/²⁰⁴Pb: 15.56–15.58, ²⁰⁸Pb/²⁰⁴Pb: 37.78–38.00). Uniform initial Sr and Pb isotope ratios for nepheline-syenites suggest that the syenite magmas evolved through fractional crystallization from an inferred mafic alkaline magma without crustal contamination. However, the range in Nd isotope composition is too large to be related to crystal fractionation alone and imply a heterogeneous source with respect to the Nd isotope composition. The quartz-syenites show a large range in initial εNd (+1.0 to −4.4) and initial ⁸⁷Sr/⁸⁶Sr (0.7047–0.7115). Initial Pb isotope ratios are also heterogeneous (²⁰⁶Pb/²⁰⁴Pb: 18.29–18.74, ²⁰⁷Pb/²⁰⁴Pb: 15.60–15.65, ²⁰⁸Pb/²⁰⁴Pb: 37.43–38.49). These data, together with major and trace element constraints and results from AFC modeling substantiate the production of the quartz-syenites by fractional crystallization of clinopyroxene, amphibole and K-feldspar accompanied by assimilation of up to about 30 % pre-Pan African lower crust. Isotope and chemical data do not support derivation from a single batch of magma undergoing contamination, but suggest that a large magma body at depth evolved largely by fractionation with batches of melt extracted from this chamber being variably contaminated at lower crustal levels.