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首页> 外文期刊>Earth and Planetary Science Letters: A Letter Journal Devoted to the Development in Time of the Earth and Planetary System >Dating of eclogite-facies zircons: the age of Alpine metamorphism in the Sesia-Lanzo Zone (Western Alps)
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Dating of eclogite-facies zircons: the age of Alpine metamorphism in the Sesia-Lanzo Zone (Western Alps)

机译：榴辉岩相锆石的年代：Sesia-Lanzo地区（西阿尔卑斯山）的高山变质时代

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Zircons from eclogite-facies rocks of the Sesia-Lanzo Zone have been investigated by cathodoluminescence and dated by ion microprobe (SHRIMP). Most of the zircons show a oscillatory or sector zoned core, which is surrounded by a weakly zoned or unzoned rim, irregular in shape. The U-Th-Pb analyses revealed that, opposite to the cores, the zircon rims have low U- and Th-contents, very low Th/U ratios and that their U-Pb systems were reset or they lost lead at the time of metamorphism. This study provides evidence that the zircon rims recrystallized and changed their internal zoning as well as their chemical and isotopic composition during high-pressure (15-18 kbar) moderate temperature (550-600 deg C) metamorphism. In the Sesia-Lanzo Zone, eclogite-facies zircons from three different localities and four different rock types were analysed. The age of metamorphic zircon rims was obtained from an eclogite from Monte Mucrone (65 +- 5 Ma), an eclogitic micaschist from the Aosta Valley (65 +- 3 Ma) and two mafic rocks from Cima di Bonze (68 +- 7 Ma). The approx 65 Ma age is further supported by the ages of the youngest zircon rims in three other samples. These results constrain the age of eclogite-facies metamorphism in the Sesia-Lanzo Zone. An age of 76 +- 1 Ma determined on zircons from a hydrothermal vein is interpreted as dating the prograde formation of the vein at temperatures well below 550 deg C. The age results suggest a maximum rate of 0.7 cm/y for the subduction of the Sesia-Lanzo Zonte to eclogite-facies conditions. The Cretaceous-Tertiary age for the eclogite-facies metamorphism in the Sesia-Lanzo Zone implies that this continental unit was not subducted together with either portions of the Adriatic margin or the neighbouring Mesozoic Tethys. We suggest that the Sesia-Lanzo Zone behaved as a separate continental block at the time of Alpine convergence.
机译：已经通过阴极发光研究了Sesia-Lanzo地区的榴辉岩相岩中的锆石，并通过离子微探针（SHRIMP）确定了年代。大多数锆石显示出一个振荡的或扇形的带状核，周围被形状不规则的弱带或无带轮辋包围。 U-Th-Pb分析显示，与核相反，锆石轮辋的U-和Th含量低，Th / U比非常低，并且它们的U-Pb系统被重置，或者在铸造时失去了铅。变质。这项研究提供了证据，表明在高压（15-18 kbar）中等温度（550-600摄氏度）的变质过程中，锆石边缘会重结晶并改变其内部区域以及化学和同位素组成。在Sesia-Lanzo地区，分析了来自三个不同地区和四种不同岩石类型的榴辉岩相锆石。变质锆石轮缘的年龄是从蒙克·莫克龙（Monte Mucrone）（65 +-5 Ma）的榴辉岩，奥斯塔山谷（65 +-3 Ma）的火山岩云母和从Cima di Bonze（68 +-7 Ma）的两个镁铁质岩石获得的。 ）。其他三个样本中最年轻的锆石轮辋的年龄进一步支持了约65 Ma的年龄。这些结果限制了Sesia-Lanzo地区的榴辉岩相变质时代。在热液脉的锆石上确定的年龄为76±1 Ma的年龄被解释为是在远低于550℃的温度下形成脉的渐进形成的。年龄结果表明，俯冲作用的最大速率为0.7 cm / y。 Sesia-Lanzo Zonte到榴辉岩相状态。 Sesia-Lanzo地区的榴辉岩相变质的白垩纪-第三纪表明该大陆单元没有与亚得里亚海边缘部分或邻近的中生代特提斯俯冲。我们建议，Sesia-Lanzo区在高山汇合时表现为一个单独的大陆块。

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