Stable zirconium (Zr) isotopes have become the focus of attention as potential tracers of magma crystallization and differentiation processes. However, the effects of partial melting, especially disequilibrium melting, on Zr isotope fractionation remain unclear. Here we report in-situ zircon UPb ages, major and trace elements, and ZrHf isotope data for the migmatites in the eastern Gangdese arc, South Tibet. The leucosomes from the metatexite and diatexite migmatites are generally concordant with the foliation of the paleosome and display diffuse and gradational contacts with the melanosomes, indicating that they are in-source leucosomes. Zircon in the paleosome, the protolith of the migmatites, have UPb age of 361 ± 2 Ma and low Hf/Hf ratios of 0.282399–0.282482. They exhibit intragrain variation in Zr values, increasing from the core (−0.10 ‰ to +0.36 ‰) to the rim (+0.14 ‰ to +0.55 ‰) domains. The negative correlation between Zr values and Zr/Hf ratios suggests preferential incorporation of light Zr isotopes during the crystallization of the zircon in the protolith. Most zircon in the leucosomes from the metatexite and diatexite migmatites exhibit distinct overgrowth rims around the inherited cores. The inherited cores have consistent UPb ages (ca. 360 Ma), trace elements, and Hf isotopic compositions (0.282362–0.282449) with zircon in the paleosome, indicating in situ partial melting. The inherited cores display Zr values ranging from −0.12 ‰ to +0.31 ‰, consistent with the zircon core domains in the paleosome. The overgrowth rims have UPb ages of ca. 102 Ma and display higher Hf/Hf ratios (0.282592–0.282900) and Zr values (+0.01 ‰ to +0.63 ‰) than their inherited cores. The elevated Hf/Hf for the overgrowth rims resulted from the retention of unradiogenic Hf in the residue due to limited zircon dissolution and more release of radiogenic Hf to the melts because of the garnet breakdown, indicating disequilibrium melting. The elevation of Zr for the overgrowth rims relative to the inherited cores was not caused by the disequilibrium melting of other Zr-bearing minerals but was mainly controlled by the Zr isotopic composition and dissolution degree of the zircon in the protolith. The preferential melting of isotopically heavy zircon rims would elevate the Zr values in the melts relative to their zircon-bearing protoliths. Crustal anatexis may be another important mechanism contributing to the Zr isotope fractionation during crustal differentiation.

中文翻译：

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地壳深熔过程中锆石Zr[sbnd]Hf同位素不平衡：藏南冈底斯弧东部中生代混合岩的记录


稳定锆（Zr）同位素作为岩浆结晶和分异过程的潜在示踪剂已成为人们关注的焦点。然而，部分熔化，特别是不平衡熔化对Zr同位素分馏的影响仍不清楚。在这里，我们报告了藏南冈底斯弧东部混合岩的原位锆石 UPb 年龄、主量元素和微量元素以及 ZrHf 同位素数据。来自后质岩和杂质岩混合岩的隐色体通常与古体的叶理一致，并与黑素体表现出弥漫性和渐变性的接触，表明它们是源内隐色体。古体中的锆石（混合岩的原岩）的 UPb 年龄为 361 ± 2 Ma，Hf/Hf 比率较低，为 0.282399–0.282482。它们表现出晶粒内 Zr 值的变化，从核心（−0.10 ‰ 至 +0.36 ‰）到边缘（+0.14 ‰ 至 +0.55 ‰）域增加。 Zr 值与 Zr/Hf 比率之间的负相关表明，在原岩中锆石结晶过程中优先掺入轻 Zr 同位素。来自变质岩和辉长岩混合岩的无色体中的大多数锆石在继承的核心周围表现出明显的过度生长边缘。继承的岩心具有一致的 UPb 年龄（约 360 Ma）、微量元素和 Hf 同位素组成（0.282362–0.282449），与古体中的锆石一致，表明原位部分熔融。继承的岩心显示 Zr 值范围为 -0.12 ‰ 至 +0.31 ‰，与古体中的锆石岩心域一致。过度生长的边缘具有约 UPb 年龄。 102 Ma 并显示出比其继承核心更高的 Hf/Hf 比 (0.282592–0.282900) 和 Zr 值（+0.01 ‰ 至 +0.63 ‰）。 过度生长边缘的 Hf/Hf 升高是由于锆石溶解有限，导致非放射性 Hf 保留在残渣中，并且由于石榴石分解，更多放射性 Hf 释放到熔体中，表明熔化不平衡。生长环相对继承核的Zr升高并非由其他含Zr矿物的不平衡熔融引起，而主要受原岩中锆石的Zr同位素组成和溶解程度控制。重同位素锆石边缘的优先熔化将提高熔体中相对于其含锆石原岩的 Zr 值。地壳深熔可能是地壳分异过程中 Zr 同位素分馏的另一个重要机制。