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首页> 外文期刊>Contributions to Mineralogy and Petrology >Zircon ages in granulite facies rocks: decoupling from geochemistry above 850 degrees C?
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Zircon ages in granulite facies rocks: decoupling from geochemistry above 850 degrees C?

机译：锆石年龄在颗粒体相岩石中：从850摄氏度以上的地球化学去耦？

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Granulite facies rocks frequently show a large spread in their zircon ages, the interpretation of which raises questions: Has the isotopic system been disturbed? By what process(es) and conditions did the alteration occur? Can the dates be regarded as real ages, reflecting several growth episodes? Furthermore, under some circumstances of (ultra-)high-temperature metamorphism, decoupling of zircon U-Pb dates from their trace element geochemistry has been reported. Understanding these processes is crucial to help interpret such dates in the context of the P-T history. Our study presents evidence for decoupling in zircon from the highest grade metapelites ( 850 degrees C) taken along a continuous high-temperature metamorphic field gradient in the Ivrea Zone (NW Italy). These rocks represent a well-characterised segment of Permian lower continental crust with a protracted high-temperature history. Cathodoluminescence images reveal that zircons in the mid-amphibolite facies preserve mainly detrital cores with narrow overgrowths. In the upper amphibolite and granulite facies, preserved detrital cores decrease and metamorphic zircon increases in quantity. Across all samples we document a sequence of four rim generations based on textures. U-Pb dates, Th/U ratios and Ti-in-zircon concentrations show an essentially continuous evolution with increasing metamorphic grade, except in the samples from the granulite facies, which display significant scatter in age and chemistry. We associate the observed decoupling of zircon systematics in high-grade non-metamict zircon with disturbance processes related to differences in behaviour of non-formula elements (i.e. Pb, Th, U, Ti) at high-temperature conditions, notably differences in compatibility within the crystal structure.
机译：肉芽饼相岩石经常在锆石年龄展现出大幅蔓延，其解释提出了问题：同位素系统是否受到干扰？改变发生了哪些过程和条件？日期可以被视为真正的年龄，反映了几集吗？此外，在某些情况下（超级）高温变质形状，报道了锆石U-Pb枣的去耦，据报道了其微量元素地球化学。了解这些过程至关重要，以帮助在P-T历史的上下文中解释这些日期。我们的研究提出了沿Ivrea区（NW Italy）中连续的高温变质场梯度沿着最高级的从最高级的元素（＆ 850℃）在锆石中解耦的证据。这些岩石代表了具有长期高温历史的二叠系较低的大陆地壳的良好特征。阴极发光图像显示中倍虫岩面部中的锆石主要以狭窄的过度生长保护脱滴核心。在上部倒角和颗粒状面部，保存的滴乳芯减少，变质锆石数量增加。在所有样本中，我们根据纹理记录了一系列四个轮辋世代。除了从来自肉芽石相的样品外，u-pb级和Ti-in-in-in-in-in-in-in-in-in-in-in-in-in-in-in-in-in-in-in-in-in-in-in-in-in-in-in-in-in-in-in-in-in-in-in-in-in-in-in-in-in-in-in-in-in-in-in-in-in-in-in-in-in-in-in-in-in-in-in--in-in-in-in-in-in-in-in-in-in-in-in-in-in-in-up，除了肉芽酸盐相的样本，其在年龄和化学中显示出显着的散射。我们将观察到的Zircon Systematics在高档非计量锆石中与高温条件（即Pb，Th，U，Ti）的差异相关的扰动过程，特别是在内部的兼容性差异晶体结构。

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