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首页> 外文期刊>Contributions to Mineralogy and Petrology >Lead isotopic evidence for interaction between plume and lower crust during emplacement of peralkaline Lovozero rocks and related rare-metal deposits, East Fennoscandia, Kola Peninsula, Russia
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Lead isotopic evidence for interaction between plume and lower crust during emplacement of peralkaline Lovozero rocks and related rare-metal deposits, East Fennoscandia, Kola Peninsula, Russia

机译：俄罗斯科拉半岛东芬诺斯坎迪亚（East Fennoscandia）在过碱性Lovozero岩石和相关稀有金属矿床放置过程中羽流与下地壳相互作用的铅同位素证据

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摘要

The Lovozero alkaline massif-an agpaitic nepheline syenite layered intrusion-is located in the central part of the Kola Peninsula, Russia, and belongs to the Kola ultramafic alkaline and carbonatitic province (KACP) of Devonian age. Associated loparite and eudialyte deposits, which contain immense resources of REE, Nb, Ta, and Zr, constitute a world class mineral district. Previous Sr, Nd, and Hf isotope investigations demonstrated that these rocks and mineral deposits were derived from a depleted mantle source. However, because the Sr, Nd, and Hf abundances in the Kola alkaline rocks are significantly elevated, their isotopic compositions were relatively insensitive to contamination by the underlying crustal rocks through which the intruding magmas passed. Pb occurring in relatively lower abundance in the KACP rocks, by contrast, would have been a more sensitive indicator of an acquired crustal component. Here, we investigate the lead isotopic signature of representative types of Lovozero rocks in order to further characterize their sources. The measured Pb isotopic composition was corrected using the determined U and Th concentrations to the age of the crystallization of the intrusion (376 +/- 28 Ma, based on a Pb-206/Pb-204 versus U-238/Pb-204 isochron and 373 +/- 9 Ma, from a Pb-208/Pb-204 versus Th-232/Pb-204 isochron). Unlike the previously investigated Sr, Nd, and Hf isotopes, the lead isotopic composition plot was well outside the FOZO field. The Pb-206/Pb-204 values fall within the depleted MORB field, with some rocks having lower Pb-207/Pb-204 but higher Pb-208/Pb-204 values. Together with other related carbonatites having both lower and higher Pb-206/Pb-204 values, the combined KACP rocks form an extended linear array defining either a similar to 2.5-Ga secondary isochron or a mixing line. The projection of this isotopic array toward the very unradiogenic composition of underlying 2.4-2.5-Ga basaltic rocks of the Matachewan superplume and associated Archean granulite facies country rock provides strong evidence that this old lower crust was the contaminant responsible for the deviation of the Lovozero rocks from a presumed original FOZO lead isotopic composition. Evaluating the presence of such a lower crustal component in the Lovozero rock samples suggests a 5-10 contamination by such rocks. Contamination by upper crustal rock is limited to only a negligible amount.
机译：Lovozero碱性地块是一株霞石正长岩层状侵入体，位于俄罗斯科拉半岛中部，属于泥盆纪科拉超基性碱性碳酸盐省（KACP）。伴生的薄石和钠长石矿床含有大量的稀土元素、铌、钽和锆资源，构成了一个世界级的矿区。先前的锶、钕和高f同位素研究表明，这些岩石和矿床来自贫瘠的地幔来源。然而，由于科拉碱性岩石中的Sr、Nd和Hf丰度显著升高，它们的同位素组成对侵入岩浆通过的下伏地壳岩石的污染相对不敏感。相比之下，在KACP岩石中以相对较低的丰度出现的Pb将是获得地壳成分的更敏感的指标。在这里，我们研究了具有代表性的Lovozero岩石类型的铅同位素特征，以进一步表征其来源。使用确定的U和Th浓度将测量的Pb同位素组成校正到侵入体结晶的年龄（376 +/- 28 马，基于Pb-206 / Pb-204与U-238 / Pb-204等时线和373 +/- 9 马，来自Pb-208 / Pb-204与Th-232 / Pb-204等时线）。与之前研究的Sr、Nd和Hf同位素不同，铅同位素组成图远远超出了FOZO场。Pb-206/Pb-204值属于枯竭的MORB场，一些岩石的Pb-207/Pb-204值较低，但Pb-208/Pb-204值较高。与其他具有较低和较高 Pb-206/Pb-204 值的相关碳酸盐岩一起，组合的 KACP 岩石形成了一个扩展的线性阵列，定义了类似于 2.5-Ga 的次级等时线或混合线。该同位素阵列向Matachewan超羽流的下伏2.4-2.5-Ga玄武岩和相关太古宙花岗岩相乡村岩石的非常非放射性成分的投影提供了强有力的证据，证明这种古老的下地壳是导致Lovozero岩石偏离假定的原始FOZO铅同位素组成的污染物。评估Lovozero岩石样品中是否存在这种下地壳成分表明，这种岩石污染了5-10%。上地壳岩石的污染仅限于可以忽略不计的量。

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