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麦苏姆碱性环杂岩中碱性深成岩的常量元素和微量元素地球化学及同位素组成。根据与Etendeka高原拉斑玄武岩不同的地球化学组成，可以推测碱性岩石的来源与纳米比亚西部其他白垩纪火成岩的来源不同(Walvis Ridge玄武岩和Etendeka溢流玄武岩序列)。原始碱性辉长岩可能代表了地壳下岩石圈地幔的熔融。北东向正长岩要么是拉斑玄武岩的分馏产物，要么代表上地幔源区的演化熔体。QTZ规范正长岩可能是Ne规范正长岩的AFC产物。这些岩石将利用最先进的电感耦合等离子体质谱(ICPMS)技术，研究全岩的Nd、Sr、Pb、Hf同位素和高精度的HFSE(高场强元素；Zr-Nb-Ta-Hf)丰度。这一结果将限制裂谷相关地区上地幔可能存在的不均一性，并可用于区分二级过程(有或不有分离结晶的地壳污染、岩浆混合)和一级过程(富集地幔来源的部分熔融)。该项目旨在了解复杂的软流圈-岩石圈相互作用，因此非常适合在与裂谷有关的大陆环境中增加关于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.