Tonalitic–trondhjemitic–granodiorite (TTG)-like leucosomes, which are formed from anatectic melts, record crustal growth and evolution and thus provide constraints on the process of anatexis. However, potential Mg isotope disequilibrium during crustal anatexis makes interpreting the genesis of anatectic melts challenging, thereby potentially hindering the application of the Mg isotope system in investigations of crustal evolution and crustal anatexis. A key factor in Mg isotope fractionation during anatexis is the behaviour of garnet, which becomes preferentially enriched in light Mg isotopes compared with other Mg-hosting minerals during partial melting processes. In this study, a combination of petrography, whole-rock geochemistry, and mineral microanalysis reveals that garnets in leucosomes from the Dulan area of the North Qaidam orogen, were formed by dehydration melting of hornblende. Garnet-bearing leucosomes yield low δMg values (−0.70‰ to −0.33‰) compared with garnet-free leucosomes (−0.19‰ to −0.16‰) and global arc rocks (−0.35‰ to 0.06‰). The different Mg isotopic compositions are attributed to the effect of peritectic garnets on the Mg isotope system in anatectic melts. Quantitative modelling further indicates that the lower Mg isotopic values of garnet-bearing leucosomes require the incorporation of ∼5% peritectic garnets into anatectic melts during magma ascent. Our results demonstrate the influence of peritectic garnets on the Mg stable isotope system and highlight that this influence on the Mg isotopic compositions of peritectic minerals during crustal anatexis must be considered in interpretations of crustal evolution using the Mg isotope system.

中文翻译：

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地壳深熔过程中包晶石榴石对镁同位素组成的影响：柴北缘类TTG隐色体的制约

英闪长岩-长角闪长岩-花岗闪长岩（TTG）类隐色体是由深熔熔体形成的，记录了地壳的生长和演化，从而对深熔过程提供了限制。然而，地壳深熔过程中潜在的镁同位素不平衡使得解释深熔熔体的成因具有挑战性，从而可能阻碍镁同位素系统在地壳演化和地壳深熔研究中的应用。深熔过程中镁同位素分馏的一个关键因素是石榴石的行为，与其他含镁矿物相比，石榴石在部分熔融过程中优先富集轻镁同位素。在这项研究中，综合岩相学、全岩地球化学和矿物微量分析表明，柴北缘造山带都兰地区隐色体中的石榴石是由角闪石脱水熔融形成的。与不含石榴石的隐色体（-0.19% 至 -0.16%）和全球弧岩（-0.35% 至 -0.06%）相比，含石榴石的隐色体产生较低的 δMg 值（-0.70% 至 -0.33%）。不同的镁同位素组成归因于包晶石榴石对深熔熔体中镁同位素系统的影响。定量模型进一步表明，含石榴石隐色体的镁同位素值较低，需要在岩浆上升过程中将~5%的包晶石榴石掺入深熔熔体中。我们的结果证明了包晶石榴石对镁稳定同位素系统的影响，并强调在使用镁同位素系统解释地壳演化时必须考虑这种对地壳深熔过程中包晶矿物镁同位素组成的影响。