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首页> 外文期刊>Contributions to Mineralogy and Petrology >Anomalous alkaline rocks of Soustov, Kola: evidence of mantle-derived metasomatic fluids affecting crustal materials
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Anomalous alkaline rocks of Soustov, Kola: evidence of mantle-derived metasomatic fluids affecting crustal materials

机译：可乐Soustov的异常碱性岩石：地幔衍生的交代流体影响地壳物质的证据

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摘要

The intrusive complexes of Grerniakha-Vyrmes and Soustov represent the two extremes of the Early Proterozoic alkaline plutons of Kola, predominantly composed of feldespathoidal syenites. Gremiakha-Vyrmes rocks (zircon age: 1,884+-6 Ma) have trace-element and isotope signatures (~ (87)Sr/~ (86)Sr_t≈0.704, implied by Nd_t≈-3-1.3) compatible with an ultimate mantle origin. Soustov syenites (zircon age: 1,872+-8 Ma) are totally different and show an acute crustal imprint. They have sodaline and analcite instead of nepheline, contain a plethora of REE-HFSE-rich accessories, and are characterised by elevated contents of F, Cl, REE, Y, Th, U, Zr, Hf, Nb, Ta, Sn, Be, Li, Rb, Tl, Pb and Cs, negative Eu anomalies, K/Rb≈190-160, Nd/Th ≈3, and Nb/Ta≈12, with extremely high ~(87)Sr/~(86)Sr_t (>0.720) and, at the same time, relatively high implied by Nd_t (≈1.6-1.7). In this paper, we explore the idea that the anomalous features of Soustov syenites can be explained if we assume they are derived from a metasomatic agent, initially an H_2O-CO_2 supercritical fluid released by alkaline mafic magmas, that was profoundly contaminated during percolation through crustal materials. As percolation advanced, the bulk composition of the fluid solute changed from alkali halides and carbonates to a silica-undersaturated alkaline melt. When the fluid cooled to a temperature of approx 550-600 deg C, it reached the point at which vapor and melt were no longer miscible and split into two components, a vapour phase and a Cl- and F-rich silica-undersaturated silicate melt that crystallised to produce Soustov syenites. To study this process, we have developed a numerical method for modelling the solute composition of the fluid during the infiltration metasomausm. Our results, using the LRFE abundances and the Sr and Nd isotope composition of a Gremiakha-Vyrmes pegmatite as the starting solute composition of the fluid, and the mode and mineral trace-element and isotope composition of a common Kola gneiss as representative of percolated materials, indicate that the fluid would have acquired a signature closely matching Soustov's, even in the case of Nd isotopes, if the gneiss age is 2.9 Ga, near its real age. This model is still a mere working hypothesis that needs further refinements, but may represent a reasonable explanation of the genesis of anomalous alkaline rocks with high ~(87)Sr/~ (86)Sr_t and implied by Nd_t >= 0, either saturated or undersaturated, which are difficult to understand in terms of magmatic fractionation/contamination.
机译：Grerniakha-Vyrmes和Soustov的侵入复合体代表了Kola早期元古代的碱性Pluton的两个极端，主要由长石块状正长岩组成。 Gremiakha-Vyrmes岩石（锆石年龄：1,884 + -6 Ma）具有痕量元素和同位素特征（〜（87）Sr /〜（86）Sr_t≈0.704，由Nd_t≈-3-1.3暗示）与最终地幔相容起源。苏斯托夫正长岩（锆石年龄：1,872 + -8 Ma）完全不同，并表现出强烈的地壳烙印。它们具有苏打水和方钠石而不是霞石，含有大量富含REE-HFSE的附件，并且其特征在于F，Cl，REE，Y，Th，U，Zr，Hf，Nb，Ta，Sn，Be的含量升高，Li，Rb，Tl，Pb和Cs，负Eu异常，K /Rb≈190-160，Nd / Th≈3和Nb /Ta≈12，具有极高的〜（87）Sr /〜（86）Sr_t （> 0.720），同时Nd_t（≈1.6-1.7）暗示相对较高。在本文中，我们探索了这样一个想法，即如果我们假设苏斯托夫正弦岩的异常特征是源于碱性镁铁质岩浆释放的交代剂（最初是H_2O-CO_2超临界流体）而产生的，那么这种异常特征在地壳的渗流过程中受到了严重污染。材料。随着渗滤的进行，流体溶质的整体组成从卤化碱和碳酸盐转变为二氧化硅不足的碱性熔体。当流体冷却到大约550-600摄氏度的温度时，它达到了蒸汽和熔体不再可混溶的状态，并分裂成两部分：气相和富含Cl和F的二氧化硅不饱和硅酸盐熔体结晶产生苏斯托夫正长岩。为了研究这一过程，我们开发了一种数值方法，用于对渗入渗流过程中流体的溶质组成进行建模。我们的研究结果以Gremiakha-Vyrmes伟晶岩的LRFE丰度和Sr和Nd同位素组成作为流体的初始溶质组成，而普通可乐片麻岩的模式，矿物微量元素和同位素组成作为渗滤材料的代表，表明如果片麻岩年龄为2.9 Ga（接近其实际年龄），则即使在Nd同位素的情况下，流体也将获得与Soustov十分相近的特征。该模型仍只是一个需要进一步完善的工作假设，但可能代表对〜（87）Sr /〜（86）Sr_t高且由Nd_t> = 0暗示的饱和或高碱度异常岩石的成因的合理解释。饱和度不足，就岩浆分离/污染而言很难理解。

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