Fluid action on zircon growth and recrystallization during quartz veining within UHP eclogite: Insights from U–Pb ages, O–Hf isotopes and trace elements
Fluid action on zircon growth and recrystallization during quartz veining within UHP eclogite: Insights from U–Pb ages, O–Hf isotopes and trace elements
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DOI：
10.1016/j.lithos.2011.06.012
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发表时间：
2012-04
期刊：
影响因子：
3.5
通讯作者：
Yingming Sheng;Yong‐Fei Zheng;Ren‐Xu Chen;Qiu-li Li;M. Dai
中科院分区：
文献类型：
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作者：
Yingming Sheng;Yong‐Fei Zheng;Ren‐Xu Chen;Qiu-li Li;M. Dai
对大别造山带某石英脉及其超高压榴辉岩中的锆石进行了原位西姆斯和LA-（MC）-ICPMS的O、U-Pb同位素及微量元素、U-Pb、Lu-Hf同位素分析。综合研究结果，揭示了在大陆俯冲带变质作用中，水流体对变质锆石新生长和原岩锆石重结晶的影响。大多数锆石在CL图像上显示出明显的核（幔）环结构。微量元素含量低，Th/U比值低（0.003 ~ 0.06），MREE-HREE配分模式浅，Eu负异常不明显，表明它们是由变质流体形成的。矿脉边缘的U-Pb年龄为222±6 ~ 212± 4 Ma，反映了深俯冲陆壳折返过程中的长期生长。而榴辉岩的边缘则给出了两组一致的U-Pb年龄，分别为221±3 Ma和208± 3 Ma，对应于两次流体作用。所有边缘的负δ 18 O值相对一致，脉为−2.59至−1.23‰，榴辉岩为−3.37至−0.33‰，表明它们是由相同来源的负δ 18 O变质流体形成的。岩芯呈明显的振荡或斑块状分带，微量元素含量高，Th/U比值高（0.13 ~ 2.12），MREE-HREE模式陡峭，Eu负异常明显。给出了脉岩和榴辉岩的新元古代上交点U-Pb年龄。大多数岩心的U-Pb年龄不一致，δ 18 O值为正，两者之间没有相关性。这表明，在水流体的存在下，变质重结晶的原岩锆石的非线性同位素蚀变。与新元古代U-Pb年龄一致的最高δ 18 O值（4.76‰）可作为原岩锆石原始δ 18 O值的代用指标。因此，这些核可能是新元古代低δ 18 O岩浆结晶的产物，并在三叠纪与变质成因的负δ 18 O流体发生了相互作用。特别是，脉中的核是被榴辉岩的变质流体捕获的捕虏晶体。因此，岩浆核和变质生长体的同时出现表明，变质流体下沉到榴辉岩裂隙中导致了原岩锆石的物理迁移和Zr元素的化学迁移。
In-situ SIMS analyses of O and U–Pb isotopes as well as LA-(MC)-ICPMS analyses of trace elements, U–Pb, and Lu–Hf isotopes were carried out for zircon from a quartz vein and its host UHP eclogite in the Dabie orogen. The results are integrated to decipher the effect of aqueous fluid on the new growth of metamorphic zircon and on the recrystallization of protolith zircon during continental subduction-zone metamorphism. Most zircons show distinct core–(mantle)–rim structures in CL images. The rims are unzoned and have low trace element contents, low Th/U ratios of 0.003 to 0.06 and shallow MREE–HREE patterns, and insignificant negative Eu anomalies, indicating their growth from metamorphic fluid. The rims from the vein have concordant U–Pb ages of 222±6 to 212±4Ma, suggesting protracted growth during exhumation of the deeply subducted continental crust. In contrast, the rims from the eclogite gave two groups of concordant U–Pb ages at 221±3 and 208±3Ma, respectively, corresponding to two episodes of fluid action. The all rims have relatively uniformly negative δ18O values of −2.59 to −1.23‰ for the vein and −3.37 to −0.33‰ for the eclogite, suggesting their growth from negative δ18O metamorphic fluids of the same origin. The cores exhibit clearly oscillatory or patchy zonation, high trace element contents, high Th/U ratios of 0.13 to 2.12, steep MREE–HREE patterns, and significant negative Eu anomalies. They gave discordia Neoproterozoic upper-intercept U–Pb ages for the both vein and eclogite. Most cores have discordant U–Pb ages and variably positive δ18O values, with no correlation between the two variables. This suggests non-linear isotope alteration of the protolith zircon by metamorphic recrystallization in the presence of aqueous fluid. The highest δ18O value of 4.76‰ with the concordant Neoproterozoic U–Pb age is considered as a proxy for the primary δ18O value for the protolith zircon. In this regard, the cores would crystallize from slightly low δ18O magmas in the Neoproterozoic, and interacted with the negative δ18O fluids of metamorphic origin in the Triassic. In particular, the cores in the vein are the xenocrystal that was trapped by the metamorphic fluids from the eclogite. Therefore, the occurrence of the both magmatic cores and metamorphic growth in the vein suggests that sinking of the metamorphic fluids into eclogite factures brought about both the physical transport of protolith zircon and the chemical transport of element Zr during the continental collision.