Contrasting Lu–Hf and U–Th–Pb isotope systematics between metamorphic growth and recrystallization of zircon from eclogite-facies metagranites in the Dabie orogen, China
Contrasting Lu–Hf and U–Th–Pb isotope systematics between metamorphic growth and recrystallization of zircon from eclogite-facies metagranites in the Dabie orogen, China
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DOI：
10.1016/j.lithos.2009.04.015
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发表时间：
2009-10
期刊：
影响因子：
3.5
通讯作者：
Qiong-Xia Xia-Qiong-Xia-Xia-104886927;Yong‐Fei Zheng;Honglin Yuan;Fu-Yuan Wu
中科院分区：
文献类型：
--
作者：
Qiong-Xia Xia-Qiong-Xia-Xia-104886927;Yong‐Fei Zheng;Honglin Yuan;Fu-Yuan Wu
通过岩相学、U-Th-Pb 和 Lu-Hf 元素以及大别造山带榴辉岩相变花岗岩锆石同位素的综合研究，识别出不同类型的变质锆石。结果提供了变质生长和重结晶之间在原岩继承、流体和熔融效应方面的岩石学区别。对变花岗岩进行锆石 U-Pb 测年，得出两组年龄分别为 778±13 Ma 和 223±4 Ma，对应于新元古代的原岩形成和三叠纪的变质变质。水相流体中变质生长的锆石具有三叠纪 U-Pb 年龄一致、U 含量相对较高但 Th 含量较低、Th/U 和 176 Lu/177 Hf 比值较低以及 Hf 同位素比值较高的特征。来自含水熔体的变质生长的锆石显示出一致的三叠纪U-Pb年龄、非常高的Th和U含量、升高的176Hf/177Hf比率，但几乎没有变化176Lu/177Hf比率。变质重结晶通常与新元古代和三叠纪之间不一致的 U-Pb 年龄有关，但流体/熔体的可用性决定了原岩锆石的内部结构、形态、U-Th-Pb 和 Lu-Hf 元素以及同位素系统通过变质脱水和部分熔融而改变的程度。虽然锆石 U-Th-Pb 同位素系统可以通过固态重结晶进行不同程度的重置，但其初始 Hf 同位素特征在相同条件下保持不变。另一方面，溶解重结晶导致 U-Th-Pb 计时系统几乎完全重置为变质时期的一致年龄，但它不会显着改变 Lu-Hf 同位素组成。置换重结晶可不同程度地重置锆石 U-Th-Pb 和 Lu-Hf 同位素系统，具体取决于变质流体/熔体的活动。因此，五种类型的变质锆石不仅区分水性流体和含水熔体的生长，而且区分通过固态、置换和溶解机制的重结晶。这提供了对俯冲带变质作用期间锆石行为的见解，特别是折返期间脱水熔融的行为。
Different types of metamorphic zircon are recognized by a combined study of petrography, U–Th–Pb and Lu–Hf elements and isotopes in zircons from eclogite-facies metagranites in the Dabie orogen. The results provide petrological distinction between metamorphic growth and recrystallization with respect to protolith inheritance, fluid and melt effects. Zircon U–Pb dating for the metagranites yields two groups of ages at 778±13 Ma and 223±4 Ma, respectively, corresponding to protolith formation in the Neoproterozoic and metamorphic modification in the Triassic. Metamorphically grown zircons from the aqueous fluid are characterized by concordant Triassic U–Pb ages, relatively high U contents but low Th contents, low Th/U and176Lu/177Hf ratios, and elevated Hf isotope ratios. Metamorphically grown zircons from the hydrous melt show concordant Triassic U–Pb ages, very high contents of both Th and U, elevated176Hf/177Hf ratios, but almost unchanged176Lu/177Hf ratios. Metamorphic recrystallization is commonly associated with discordant U–Pb ages between Neoproterozoic and Triassic, but availability of fluid/melt dictates the extent to which internal structure, morphology, U–Th–Pb and Lu–Hf element and isotope systems of protolith zircon were modified by metamorphic dehydration and partial melting. While the zircon U–Th–Pb isotope systems can be variably reset by solid-state recrystallization, its initial Hf isotope signature keeps unchanged at the same conditions. On the other hand, dissolution recrystallization causes almost complete resetting of the U–Th–Pb chronometric systems to concordant ages at the metamorphic time, but it does not significantly change the Lu–Hf isotope compositions. Replacement recrystallization resets the zircon U–Th–Pb and Lu–Hf isotope systems to variable degrees, depending on the activity of metamorphic fluid/melt. Consequently, the five types of metamorphic zircon are distinguished not only between growth from the aqueous fluid and the hydrous melt but also between the recrystallization via the solid-state, replacement and dissolution mechanisms. This provides insights into the behavior of zircon during subduction-zone metamorphism, particularly that concerning dehydration melting during exhumation.