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首页> 外文期刊>Contributions to Mineralogy and Petrology >Geochemistry of ultrahigh-pressure anatexis: fractionation of elements in the Kokchetav gneisses during melting at diamond-facies conditions
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Geochemistry of ultrahigh-pressure anatexis: fractionation of elements in the Kokchetav gneisses during melting at diamond-facies conditions

机译：超高压麻醉剂的地球化学：钻石相条件下熔化过程中Kokchetav片麻岩中元素的分馏

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摘要

The Kokchetav complex in Kazakhstan contains garnet-bearing gneisses that formed by partial melting of metasedimentary rocks at ultrahigh-pressure (UHP) conditions. Partial melting and melt extraction from these rocks is documented by a decrease in K_2O and an increase in FeO + MgO in the restites. The most characteristic trace element feature of the Kokchetav UHP restites is a strong depletion in light rare earth elements (LREE), Th and U. This is attributed to complete dissolution of monazite/ allanite in the melt and variable degree of melt extraction. In contrast, Zr concentrations remain approximately constant in all gneisses. Using experimentally determined solubilities of LREE and Zr in high-pressure melts, these data constrain the temperature of melting to ~ 1,000 °C. Large ion lithophile elements (LILE) are only moderately depleted in the samples that have the lowest U, Th and LREE contents, indicating that phengite retains some LILE in the residue. Some restites display an increase in Nb/Ta with respect to the protolith. This further suggests the presence of phengite, which, in contrast to rutile, preferentially incorporates Nb over Ta. The trace element fractionation observed during UHP anatexis in the Kokchetav gneisses is significantly different from depletions reported in low-pressure restites, where generally no LREE and Th depletion occurs. Melting at UHP conditions resulted in an increase in the Sm/Nd ratio and a decoupling of the Sm-Nd and Lu-Hf systems in the restite. Further subduction of such restites and mixing with mantle rocks might thus lead to a distinct isotopic reservoir different from the bulk continental crust.
机译：哈萨克斯坦的科克切塔夫（Kokchetav）复合物含有石榴石片麻岩，这些片麻岩是在超高压（UHP）条件下部分沉积岩的部分熔融形成的。从这些岩石中的部分熔融和熔体提取可以通过在钾钙钛矿中K_2O的减少和FeO + MgO的增加来证明。 Kokchetav UHP重晶石最典型的痕量元素特征是轻稀土元素（LREE），Th和U的大量消耗。这归因于独居石/尿囊石在熔体中的完全溶解和不同程度的熔体萃取。相反，在所有片麻岩中，Zr浓度保持大致恒定。使用实验确定的LREE和Zr在高压熔体中的溶解度，这些数据将熔体温度限制在〜1,000°C。 U，Th和LREE含量最低的样品中仅适度消耗了大型离子亲石元素（LILE），这表明锂铁矿在残留物中保留了一些LILE。某些辉石相对于原石显示出Nb / Ta的增加。这进一步表明存在方铁矿，与金红石相反，它比Ta优先掺入Nb。在Kokchetav片麻岩中，UHP麻醉过程中观察到的微量元素分馏与低压重晶石中所报告的损耗显着不同，低压重晶石中通常没有LREE和Th损耗。在UHP条件下熔化会导致Sm / Nd比增加，并且在重晶石中Sm-Nd和Lu-Hf系统解耦。因此，进一步减少这些辉石并与地幔岩混合可能会导致形成一个不同于大块大陆壳的独特同位素储集层。

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