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

拟议的项目涉及南比亚西部梅苏姆碱性环杂岩碱性深成岩的主量和微量元素地球化学及同位素组成。基于与来自Etendeka高原的拉斑玄武岩不同的地球化学成分，可以认为碱性岩石的来源与纳米比亚西部的其他白垩纪火成岩（沃尔维斯岭玄武岩和Etendeka溢流玄武岩序列）不同。原始碱性辉长岩（辉长岩）可能代表了地壳下岩石圈地幔的熔融。NE-正长岩要么是辉长岩的分馏产物，要么也是上地幔源熔体演化的产物。Qtz规范正长岩可能是ne规范正长岩的AFC产物。将使用最先进的ICP-MS技术研究岩石的Nd、Sr、Pb、Hf全岩同位素和高精度HFSE（高场强元素; Zr-Nb-Ta-Hf）丰度。结果将限制可能的不均匀性的上地幔在裂谷相关的地区，也可以用来区分第二级过程（地壳污染或没有分离结晶，岩浆混合）从一级过程（富集地幔源的部分熔融）。该项目旨在了解复杂的软流圈-岩石圈相互作用，因此非常适合增加有关裂谷相关大陆背景下岩石圈动力学的知识。

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.