Petrogenesis of alkaline rift-related ring complexes constrained from combined Sr, Nd, Pb, and Hf isotopes and HFSE systematics

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The proposed project deals with the major and trace element geochemistry and isotope composition of alkaline plutonic rocks from the Messum alkaline ring complex in western Nambia. Based on the distinct geochemical compositions relative to the tholeiitic flood basalts from the Etendeka plateau, it can be suggested that the alkaline rocks originated from sources distinct to those envisaged for other Cretaceous igneous rocks from western Namibia (Walvis Ridge basalts and Etendeka flood basalt sequences). Primitive alkaline gabbros (theralites) probably represent melts of the subcrustal lithospheric mantle. Ne-normative syenites are either fractionation products of the theralites or represent also evolved melts from upper mantle sources. Qtz-normative syenites are likely AFC products of the ne-normative syenites. The rocks will be investigated for Nd, Sr, Pb, Hf whole rock isotopes and high-precision HFSE (High-Field-Strength Elements; Zr-Nb-Ta-Hf) abundances using state-of-the-art ICP-MS techniques. The results will place constraints on possible heterogeneities of the upper mantle in rift-related areas and can also be used to distinguish second-order processes (crustal contamination with or without fractional crystallization, magma mixing) from first-order processes (partial melting of enriched mantle sources). The project aims at understanding complex asthenosphere-lithosphere interactions and is therefore ideally suited to add knowledge on lithosphere dynamics in rift-related continental settings.