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.

在某些情况下，与超镁铁质岩石，粉状岩石和小石相关的碳岩含有经济的HFSE矿化，但在其他情况下却贫瘠。从这个意义上讲，将在该项目中进行研究和比较的科夫多（俄罗斯）和加德纳（E Gradiner）综合体，代表了此类相反情况的关键例子：科夫多尔的碳酸盐是hfse矿物质的，仅在相关的硅酸盐岩石中只有较小的HFSE浓度。相比之下，加德纳（Gardiner）的Melilitolites显示出钙钛矿的矿石级富集，而相关的碳矿物质则贫瘠。我们先前的研究（圆顶1）表明，来自Gardiner的Ultramafics和Melilitolites是地幔衍生，原始和富含Ti的硅酸盐熔体的累积。相比之下，来自科夫多尔的超镁铁质岩石是方解石和生物岩富含的，缺乏累积的纹理，并且在兼容的元素（例如Ni和Cr）中大量耗尽，这与累计相矛盾，而是支持跨质量的起源。我们假设这些岩石是由地幔衍生的碳质熔体和硅质宿主岩石之间的反应形成的。该项目旨在详细比较这两种复合物，以解读导致早期累积（Gardiner）和晚期碳酸盐（Kovdor）（Kovdor）中HFSE增强的过程。如果这种配合物中的硅酸盐岩石可以通过地幔衍生的碳岩与硅质壁岩石的反应形成，那么在这种情况下，HFSE的富集将由碳酸盐熔体控制，而不是通过硅酸盐岩浆生成硅酸盐的分离过程。这可能导致完全不同的富集和分级模式（例如Zr/HF，NB/TA，LA/LU）。为了检验这一假设，我们将遵循两种互补的分析策略：（1）所有主要岩石单位中的熔体和流体包含将通过最先进的分析方法（重新启动，微温度测量，微隔离，综合性拉曼光谱法，高分辨率X射线计算机，X射线X射线学，EPMA，LA-ISPM）和PICP（2）和（2）radi（2）radi（2）radi（2）radi（2）radi（2）将使用ID-TIMS使用原位方法（LA-MC-ICP-MS）和高精度分析来获得主要矿物。预期的结果将允许在可变进化阶段和相关元素富集过程下约束宿主熔体/流体的性质和组成。结合放射性同位素示踪剂，可以解决从复合尺度到单个晶粒的同位素差异，这将阐明是否确实代表了碳酸盐和地壳宿主岩石之间可变阶段的硅酸盐岩石。这一发现将对HFSE的矿石 - 碱金属盐硅酸盐岩体复合物中的矿石形成过程产生重大影响，该过程在样式和时机上的差异很大，具体取决于岩浆或变质过程是占主导地位的。