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.

与超镁铁岩、镁铁质岩和碎屑岩共生的碳酸岩在某些情况下含有经济的HFSE矿化，但在其他情况下是贫瘠的。从这个意义上说，科夫多尔（俄罗斯）和加德纳（格陵兰东部）的复杂，这将在本项目中进行研究和比较，代表了这种相反的情况下的关键例子：在科夫多尔碳酸岩HFSE矿化，只有少量的HFSE相在相关的硅酸盐岩石。相比之下，加德纳的黄长石显示钙钛矿的矿石级富集，而相关的碳酸岩是贫瘠的。我们以前的研究（DOME 1）表明，超镁铁岩和镁铁岩从加德纳是一个幔源的，原始的和富钛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富含方解石和黑云母，缺乏堆积结构，并且强烈缺乏相容元素（例如，Ni和Cr），这与堆积相矛盾，而是支持交代成因。我们推测，这些岩石是由幔源碳酸岩熔体和地幔岩之间的反应形成的。该项目旨在详细比较这两个复合体，以分别解释导致早期堆晶岩（加德纳）和晚期碳酸岩（科夫多尔）中HFSE富集的过程。如果硅酸盐岩石在这样的复杂可以形成的反应幔源碳酸盐岩与围岩，富集HFSE在这种情况下将控制碳酸盐熔体，而不是在不断发展的硅酸盐岩浆分馏过程。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，我们将遵循两个互补的分析策略：（1）将采用最先进的分析方法研究所有主要岩石单元中的熔体和流体包裹体（再均质化、显微测温、共焦拉曼光谱、高分辨率X射线计算机断层扫描、EPMA、LA-ICP-MS），（2）利用LA-MC-ICP-MS原位分析和ID-TIMS高精度分析，获得主要矿物的放射性同位素组成（Sr-Nd-Pb）。预期的结果将允许约束的性质和组成的主机熔体/流体在可变的演变阶段和相关的元素富集过程。结合放射性同位素示踪剂，解决同位素的差异，从复杂的规模，以个人的颗粒，这将澄清，如果硅酸盐岩石与碳酸盐岩确实代表的产品之间的交代作用的变化阶段的碳酸盐岩和地壳寄主岩石。这一发现将有重大影响的成矿过程HFSE在碳酸盐岩碱性硅酸盐岩复合体，强烈不同的风格和时间取决于是否岩浆或交代过程占主导地位。