HFSE enrichment in carbonatites and associated peridotite-melilitolite-foidolite complexes: the role of fractionation versus metasomatism in ore-forming processes

碳酸盐岩和相关橄榄岩-黄橄榄石-磷硅石复合体中的 HFSE 富集：成矿过程中分馏与交代作用的作用

• 批准号: 521502267
• 负责人: Privatdozent Dr. Michael Marks
• 金额: --
• 依托单位: Department of Earth and Atmospheric Sciences
• 依托单位国家: 德国
• 项目类别: Priority Programmes
• 资助国家: 德国
• 项目状态: 未结题
• 来源: https://gepris.dfg.de/gepris/projekt/521502267?language=en
• 关键词: HFSE; enrichment; carbonatites; associated; peridotite

Carbonatites associated with ultramafic rocks, melilitolites and foidolites contain economic HFSE mineralizations in some cases, but are barren in others. In that sense, the Kovdor (Russia) and Gardiner (E Greenland) complexes, which will be studied and compared in this project, represent key examples for such opposite cases: carbonatites at Kovdor are HFSE-mineralized, with only minor HFSE-phases in the associated silicate rocks. In contrast, melilitolites from Gardiner show ore-grade enrichment of perovskite, while the associated carbonatites are barren. Our previous study (DOME 1) suggests that ultramafics and melilitolites from Gardiner are cumulates of a mantle-derived, primitive and Ti-rich silicate melt. In contrast, ultramafic rocks from Kovdor are calcite- and biotite-rich, lack cumulate textures and are strongly depleted in compatible elements (e.g., Ni and Cr), which contradicts a cumulate, but rather supports a metasomatic origin. We hypothesize, that these rocks formed by a reaction between mantle-derived carbonatitic melts and silicic host rocks. This project aims at comparing these two complexes in detail to decipher the processes leading to the HFSE-enrichment in early cumulates (Gardiner) and late-stage carbonatites (Kovdor), respectively. If silicate rocks in such complexes can form by reaction of mantle-derived carbonatites with silicic wall rocks, enrichment of HFSE in such cases would be controlled by carbonatitic melts, and not by fractionation processes in evolving silicate magmas. This probably leads to completely different enrichment and fractionation patterns (e.g., Zr/Hf, Nb/Ta, La/Lu). To test this hypothesis, we will follow two complementary analytical strategies: (1) melt and fluid inclusions in all major rock units will be investigated by a state-of-the-art analytical approach (re-homogenization, microthermometry, confocal Raman spectroscopy, high-resolution X-ray computed tomography, EPMA, LA-ICP-MS), and (2) radiogenic isotope compositions (Sr-Nd-Pb) will be obtained for major minerals using both in-situ methods (LA-MC-ICP-MS) and high-precision analysis by ID-TIMS. The expected results will allow to constrain the nature and composition of the host melt/fluid at variable evolutionary stages and associated element enrichment processes. Combined with radiogenic isotope tracers which resolve isotopic differences from complex-scale to individual grains, this will clarify if silicate rocks associated with carbonatites indeed represent products of variable stages of metasomatism between carbonatite and crustal host rocks. This finding would have major implications for ore-forming processes of HFSE in carbonatite-alkaline silicate rock complexes, which differ strongly in style and timing depending on whether magmatic or metasomatic processes dominate.

与超镁铁质岩、千英石和橄榄岩伴生的碳酸盐岩在某些情况下含有经济的HFSE矿化作用，但在其他情况下则是贫瘠的。从这个意义上说，本项目将研究和比较的科夫多尔（俄罗斯）和加德纳（格陵兰东部）杂岩，代表了相反情况的关键例子：科夫多尔的碳酸盐岩是hfse矿化的，在伴生的硅酸盐岩石中只有少量的hfse相。相比之下，Gardiner的千英石显示钙钛矿的矿级富集，而伴生的碳酸盐岩则是贫瘠的。我们之前的研究（DOME 1）表明，Gardiner的超镁铁岩和千英石是地幔衍生的原始富钛硅酸盐熔体的堆积物。相比之下，科夫多尔的超基性岩石则富含方解石和黑云母，缺乏堆积结构，并且相容性元素（如Ni和Cr）强烈缺乏，这与堆积相矛盾，而是支持交代成因。我们假设，这些岩石是由地幔衍生的碳酸盐岩熔体和硅质寄主岩之间的反应形成的。本项目旨在对这两个复合体进行详细比较，以分别破译导致早期堆积（Gardiner）和晚期碳酸盐岩（Kovdor）中hfse富集的过程。如果幔源碳酸盐与硅质围岩反应形成该类杂岩，则该类杂岩的富集将受碳酸盐熔体控制，而不是受演化中的硅酸盐岩浆分馏作用控制。这可能导致完全不同的富集和分馏模式（例如，Zr/Hf, Nb/Ta, La/Lu）。为了验证这一假设，我们将采用两种互补的分析策略：(1)通过最先进的分析方法（再均质、显微测温、共聚焦拉曼光谱、高分辨率x射线计算机断层扫描、EPMA、LA-ICP-MS）来研究所有主要岩石单元中的熔体和流体包裹体；(2)通过原位方法（LA-MC-ICP-MS）和ID-TIMS高精度分析来获得主要矿物的放射性成因同位素组成（Sr-Nd-Pb）。预期的结果将允许约束宿主熔体/流体在不同演化阶段的性质和组成以及相关的元素富集过程。结合从复杂尺度到单个颗粒的同位素差异的放射性成因同位素示踪剂，这将澄清与碳酸盐岩相关的硅酸盐岩石是否确实代表了碳酸盐岩和地壳主岩之间不同阶段交代作用的产物。这一发现将对碳酸盐岩-碱性硅酸盐杂岩中HFSE的成矿过程产生重大影响，这些成矿过程在风格和时间上存在很大差异，这取决于岩浆作用还是交代作用占主导地位。