Linking mineral growth and time is required to unravel the evolution of metamorphic rocks. However, dating early metamorphic stages is a challenge, due to subsequent retrograde overprinting. A perfectly fresh eclogite and a former eclogite retrogressed under amphibolite facies from the southern French Massif Central (Levezou massif, Variscan belt) were investigated with a large panel of geochronometers (U-Pb on zircon, rutile and apatite, Lu-Hf and Sm-Nd on garnet) in a petrological context tightly constrained by petrographic observation, trace element analyses and phase equilibrium modelling. Both samples recorded similar HP conditions at 18-23 kbar and 680-800°C, while the retrogressed eclogite later equilibrated at 8-9.5 kbar and ca. 600°C. In the retrogressed sample, most of the zircon grains are characterized by negative Eu anomalies and HREE enrichment, and yield an Ordovician U-Pb date of 472.3 ± 1.7 Ma, interpreted as the emplacement age of the mafic protolith. In agreement with other data available for the Variscan belt, and based on zircon trace elements record and whole rock geochemistry, this age is considered to represent the magmatism associated with the extreme thinning of the continental margins during the Ordovician. In the same sample, few zircon rims show a weaker HREE enrichment and yield a date of 378 ± 5.7 Ma, interpreted as a prograde pre-eclogitic age. Lu-Hf garnet dating from both samples yields identical dates of 357 ± 13 Ma and 358.0 ± 1.5 Ma inferred to approximate the age of the high-pressure metamorphic peak. Fresh and retrogressed samples yield respectively 350.4 ± 7.7 Ma and 352 ± 20 Ma dates for Sm-Nd garnet dating, and 367.8 ± 9.1 Ma and 354.9 ± 9.5 Ma for U-Pb rutile dating. Apatite grains from the retrogressed sample give a mean age of 351.8 ± 2.8 Ma. The similarity between all recorded ages from distinct chronometers and radiometric methods (U-Pb – rutile, apatite; Lu-Hf – garnet; Sm-Nd – garnet) combined with P–T estimations from high-pressure metamorphic rocks equilibrated under different conditions testifies to very fast processes that occurred during the Variscan orogeny, highlighting a major decompression of 15-8.5 kbar in less than 7 Myr, and suggesting mean exhumation rates in excess of 6.3 mm/yr.

中文翻译：

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从埋藏到挖掘：通过多方法岩石年代学约束的镁铁质榴辉岩地体的就位和变质，来自 Lévézou 地块（法国地块中部，瓦里斯干带）的案例研究

需要将矿物生长和时间联系起来才能解开变质岩的演化。然而，由于随后的逆行套印，确定早期变质阶段是一个挑战。使用大量地球计时仪（锆石、金红石和磷灰石上的 U-Pb、Lu-Hf 和 Sm-石榴石上的 Nd）在岩石学环境中受到岩石学观察、微量元素分析和相平衡建模的严格限制。两个样品在 18-23 kbar 和 680-800°C 下都记录了类似的 HP 条件，而退化榴辉岩后来在 8-9.5 kbar 和约 600°C。在回归样本中，大部分锆石颗粒以Eu负异常和HREE富集为特征，奥陶纪U-Pb日期为472.3±1.7 Ma，解释为镁铁质原岩的侵位年龄。与 Variscan 带的其他可用数据一致，并根据锆石微量元素记录和全岩石地球化学，该时代被认为代表了与奥陶纪大陆边缘极度变薄相关的岩浆作用。在同一样品中，很少有锆石边缘显示出较弱的 HREE 富集，并产生 378 ± 5.7 Ma 的日期，解释为前榴辉岩时代。两个样品的 Lu-Hf 石榴石测年结果相同，分别为 357 ± 13 Ma 和 358.0 ± 1.5 Ma，推测近似高压变质峰的年龄。新鲜和退化样品产量分别为 350.4 ± 7。Sm-Nd 石榴石测年为 7 Ma 和 352 ± 20 Ma，U-Pb 金红石测年为 367.8 ± 9.1 Ma 和 354.9 ± 9.5 Ma。来自退化样品的磷灰石颗粒的平均年龄为 351.8 ± 2.8 Ma。来自不同天文钟和辐射测量方法（U-Pb – 金红石、磷灰石；Lu-Hf – 石榴石；Sm-Nd – 石榴石）的所有记录年龄之间的相似性，结合来自不同条件下平衡的高压变质岩的 P-T 估计，证明了到 Variscan 造山运动期间发生的非常快速的过程，突出显示在不到 7 Myr 的情况下出现了 15-8.5 kbar 的主要减压，并表明平均折返率超过 6.3 毫米/年。