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

与超镁铁质岩石、黄长岩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，但在另一些情况下则是贫瘠的。在这个意义上，将在本项目中研究和比较的Kovdor(俄罗斯)和Gardiner(E Greenland)杂岩是这种相反情况的关键例子：Kovdor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矿化。相反，来自Gardiner的黄长石显示出钙钛矿的矿石级富集物，而伴生的碳酸盐岩则是贫瘠的。我们之前的研究(穹顶1)表明，来自Gardiner的超镁铁质和黄长石是地幔来源的原始富钛硅酸盐熔体的堆积体。相比之下，来自科夫多的超镁铁质岩石富含方解石和黑云母，缺乏堆积结构，严重亏损相容元素(如镍和铬)，这与堆积相反，但支持交代成因。我们推测，这些岩石是由地幔碳酸盐岩熔体和硅质寄主岩石反应形成的。该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岩(Gardiner)和晚期碳酸盐岩(Kovdor)中HFSE富集的过程。如果这种杂岩中的硅酸盐岩石可以通过地幔碳酸盐岩与硅质围岩的反应而形成，那么在这种情况下，HFSE的富集将受到碳酸岩熔体的控制，而不是由演化的硅酸盐岩浆中的分馏过程控制。这可能导致了完全不同的富集度和分馏模式(例如，Zr/Hf、Nb/Ta、La/Lu)。为了验证这一假设，我们将遵循两个互补的分析策略：(1)将通过最先进的分析方法(再均一化、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)研究所有主要岩石单元中的熔体和流体包裹体，以及(2)将使用原位方法(LA-MC-ICP-MS)和高精度ID-TIMS分析获得主要矿物的放射性成因同位素组成(Sr-ND-Pb)。预期的结果将允许在不同的演化阶段和相关的元素富集化过程中限制宿主熔体/流体的性质和组成。结合解决从复杂尺度到单个颗粒的同位素差异的放射性同位素示踪，这将澄清与碳酸岩共生的硅酸盐岩石是否确实代表碳酸岩和地壳寄主岩石之间不同阶段交代的产物。这一发现将对碳酸岩-碱性硅酸盐岩杂岩中高频地磁作用的成矿过程具有重大意义，这些杂岩的风格和时间根据岩浆作用或交代作用的主导而有很大不同。

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.