Abstract Age determination of impact structures via the zircon U-Pb system remains challenging and often ambiguous due to highly variable effects of shock metamorphism on U-Pb geochronology. It is, therefore, crucial to link the observed zircon microtextures, including their temperature and pressure conditions associated with their formation, directly to the U-Pb ages preserved. Here, we analyzed three recrystallized zircon grains and one plastically deformed zircon crystal from the medium-sized Rochechouart impact structure in the northwestern Massif Central of France. For the Rochechouart impact structure the impact age (206.92 ± 0.32 Ma [40Ar/39Ar]) as well as the tectono-themal history is well established making this study site ideal to test concepts about U-Pb systematics in shocked zircon and to differentiate between shock-driven age resetting and pre-impact crystallization and metamorphic overprinting. Zircon microstructures were studied using scanning electron imaging, cathodoluminescence imaging, and electron backscatter diffraction (EBSD) mapping. Further, we conducted U-Pb Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) depth-profiling analysis, allowing us to interpret the resultant age data in discrete steps with increasing ablation time/depth. The U-Pb depth profiling data demonstrate that plastically strained grains are incompletely reset and preserve rim and interior age domains reflecting typical pre-impact (pre-Variscan and Variscan) regional tectonic ages. Our results also reveal that the granular crystals encountered contain microstructural evidence for “former reidite in granular neoblastic” (FRIGN) zircon, reflecting both high-pressure (⩾30 GPa) and high-temperature (⩾1200 °C) conditions. This signifies that FRIGN zircon is now known from an additional, medium sized, impact structure further supporting the hypothesis that this impact induced microstructure is commonly preserved. In addition, FRIGN zircon has a high potential to preserve completely impact-reset crystal domains (∼204 to 207 Ma) that can be identified by our combined analytical approach of U-Pb depth profiling and EBSD mapping, and thus are suitable for determining reliable impact ages.

中文翻译：

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冲击重结晶锆石的空间 U-Pb 年龄分布——来自法国 Rochechouart 撞击结构的案例研究

摘要 由于冲击变质作用对 U-Pb 地质年代学的高度可变影响，通过锆石 U-Pb 系统确定撞击结构的年龄仍然具有挑战性且常常是模棱两可的。因此，将观察到的锆石微观结构（包括与其形成相关的温度和压力条件）与保存的 U-Pb 年龄直接联系起来至关重要。在这里，我们分析了来自法国西北部地块中部的中型 Rochechouart 撞击结构的三颗再结晶锆石颗粒和一颗塑性变形锆石晶体。对于 Rochechouart 撞击结构，撞击年龄 (206.92 ± 0. 32 Ma [40Ar/39Ar]) 以及构造热历史已经确立，这使得该研究地点非常适合测试有关冲击锆石中 U-Pb 系统学的概念，并区分冲击驱动的年龄重置和冲击前结晶和变形叠印。使用扫描电子成像、阴极发光成像和电子背散射衍射 (EBSD) 映射研究了锆石微结构。此外，我们进行了 U-Pb 激光烧蚀电感耦合等离子体质谱 (LA-ICP-MS) 深度剖面分析，使我们能够随着烧蚀时间/深度的增加，以离散步骤解释所得的年龄数据。U-Pb 深度剖面数据表明，塑性应变颗粒未完全重置并保留了边缘和内部年龄域，反映了典型的撞击前（前瓦里斯坎和瓦里斯坎）区域构造年龄。我们的结果还表明，遇到的粒状晶体包含“粒状新母细胞中的前雷德石”（FRIGN）锆石的微观结构证据，反映了高压（⩾30 GPa）和高温（⩾1200°C）条件。这意味着 FRIGN 锆石现在是从一个额外的中等大小的撞击结构中得知的，进一步支持了这种撞击引起的微观结构通常被保留的假设。此外，