Elevation of zircon Hf isotope ratios during crustal anatexis: Evidence from migmatites close to the eastern Himalayan syntaxis in southeastern Tibet
Elevation of zircon Hf isotope ratios during crustal anatexis: Evidence from migmatites close to the eastern Himalayan syntaxis in southeastern Tibet
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地壳深熔过程中锆石Hf同位素比的升高：来自西藏东南部喜马拉雅构造结东部附近混合岩的证据
DOI：
10.1016/j.lithos.2022.106592
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发表时间：
2022
期刊：
影响因子：
3.5
通讯作者：
Yong-Fei Zheng
中科院分区：
文献类型：
--
作者：
Qiong-Xia Xia;Yi-Xiang Chen;Ren-Xu Chen;Yong-Fei Zheng
高级变质岩中锆石的初始Hf同位素组成是原岩成因和陆壳生长演化的重要示踪剂。然而，陆壳部分熔融过程中Lu-Hf同位素在包晶反应中的行为较少受到关注。本文对藏东南东喜马拉雅构造带附近的混合岩进行了锆石U-Pb年龄、Lu-Hf同位素和微量元素以及岩石学和全岩地球化学的联合研究。结果表明，混合岩中包晶锆石的176Hf/177Hf明显高于原岩中的锆石。更有趣的是，包晶锆石的176Lu/177Hf比值和高场强元素(如Nb、Ta、Hf)和重稀土元素的含量显著高于原岩锆石。这表明高Lu/Hf比值的石榴石和其它Lu/Hf比值适中的矿物(如钛铁矿、钛铁矿、角闪石和黑云母)在地壳深熔过程中发生了分解，形成了包晶锆石的Hf同位素组成。岩相学观察证实，石榴石和黑云母以残留物或残留物的形式存在于中颗粒(黑云片麻岩)和一些白云母(长英脉)中，但在目标混合岩的黑素体中不存在。因此，包晶锆石是通过包晶反应与深熔熔体一起产生的，如果在地壳深熔过程中没有共生包晶石榴石，则获得了比原岩继承的锆石更高的176Lu/177Hf比值。在这方面，深熔花岗岩中包晶锆石的Hf同位素组成大幅升高，不能真实反映母岩的Hf同位素组成。因此，在使用Hf同位素比值追踪岩浆过程和地壳演化方面的母岩性质时，必须非常小心。
The initial Hf isotope composition of zircon in high-grade metamorphic rocks is an important tracer for protolith origin as well as for the growth and evolution of continental crust. However, less attention has been paid to the behavior of Lu-Hf isotopes in peritectic reactions during partial melting of the continental crust. In this paper, we present a combined study of zircon U-Pb ages, Lu-Hf isotopes and trace elements as well as petrology and whole-rock geochemistry for migmatites close to the eastern Himalayan syntaxis in southeastern Tibet. The results show that peritectic zircons in the migmatites have significantly higher176Hf/177Hf ratios than their protolith zircons. More interestingly, the176Lu/177Hf ratios and the contents of high field strength elements (such as Nb, Ta, and Hf) and heavy rare earth elements are significantly elevated in the peritectic zircons compared to the protolith zircons. This suggests that garnets with high Lu/Hf ratios and other minerals (such as titanite, ilmenite, amphibole, and biotite) with moderate Lu/Hf ratios were decomposed during crustal anatexis, contributing to the Hf isotope composition of peritectic zircons. This is confirmed by petrographic observations that garnet and biotite occur as residues or remnants in mesosomes (biotite gneiss) and some leucosomes (felsic veins), but are absent in the melanosomes of the target migmatites. Therefore, the peritectic zircons were produced together with anatectic melts through peritectic reactions and acquired the elevated176Lu/177Hf ratios compared to the inherited zircons from the protoliths if no peritectic garnet was coevally produced during the crustal anatexis. In this regard, the greatly elevated Hf isotope compositions of peritectic zircons in anatectic granites cannot faithfully reflect the Hf isotope composition of parental rocks. As such, great care must be taken when using Hf isotope ratios to trace the nature of parental rocks with respect to magmatic processes and crustal evolution.