The Vredefort impact structure in South Africa is deeply eroded to its lowermost levels. However, granophyre (impact melt) dykes in such structures preserve clasts of supracrustal rocks, transported down from the uppermost levels of the initial structure. Studying these clasts is the only way to understand the properties of already eroded impactites. One such lithic clast from the Vredefort impact structure contains a thin pseudotachylite vein and is shown to be derived from the near-surface environment of the impact crater. Traditionally, impact pseudotachylites are referred to as in situ melt rocks with the same chemical and isotopic composition as their host rocks. The composition of the sampled pseudotachylite vein is not identical to its host rock, as shown by the micro-X-ray fluorescence (µXRF) and energy-dispersive X-ray (EDX) spectrometry mapping. Mapping shows that the melt transfer and material mixing within pseudotachylites may have commonly occurred at the upper levels of the structure. The vein is spatially related to shocked zircon and monazite crystals in the sample. Granular zircons with small granules are concentrated within and around the vein (not farther than 6–7 mm from the vein). Zircons with planar fractures and shock microtwins occur farther from the vein (6–12 mm). Zircons with microtwins (65°/{112}) are also found inside the vein, and twinned monazite (180°/[¹⁰¹]) is found very close to the vein. These spatial relationships point to elevated shock pressure and shear stress, concentrated along the vein’s plane during impact.

中文翻译：

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大型冲击盆地上层与冲击有关的假速滑石及相关的冲击锆石和独居石的性质：以弗雷福德冲击结构为例

南非的Vredefort冲击结构已被深深侵蚀到最低水平。但是，这种结构中的花岗石（冲击熔岩）堤坝保留了上壳岩的碎屑，这些碎屑从初始结构的最高层向下输送。研究这些碎屑是了解已经腐蚀的冲积岩特性的唯一方法。Vredefort冲击结构中的一种这样的石屑包含一条细速滑石脉，并显示出它来自冲击坑的近地表环境。传统上，撞击假速闪石被称为原位熔岩，其化学和同位素组成与其宿主岩相同。所采样的假速滑石脉的成分与其宿主岩不同，如微X射线荧光（µXRF）和能量色散X射线（EDX）光谱图所示。作图表明，假速晶石中的熔体转移和材料混合可能通常发生在结构的较高层。静脉在空间上与样品中的锆石和独居石晶体有关。带有小颗粒的颗粒状锆石集中在静脉内和周围（距静脉不超过6-7毫米）。带有平面裂缝和微晶双绞线的锆石出现在距静脉较远的地方（6-12毫米）。在静脉内还发现了具有微孪晶（65°/ {112}）的锆石和孪晶独居石（180°/ [ 静脉在空间上与样品中的锆石和独居石晶体有关。带有小颗粒的颗粒状锆石集中在静脉内和周围（距静脉不超过6-7毫米）。带有平面裂缝和微晶双绞线的锆石出现在距静脉较远的地方（6-12毫米）。在静脉内还发现了具有微孪晶（65°/ {112}）的锆石和孪晶独居石（180°/ [ 静脉在空间上与样品中的锆石和独居石晶体有关。带有小颗粒的颗粒状锆石集中在静脉内和周围（距静脉不超过6-7毫米）。带有平面裂缝和微晶双绞线的锆石出现在距静脉较远的地方（6-12毫米）。在静脉内还发现了具有微孪晶（65°/ {112}）的锆石和孪晶独居石（180°/ [¹⁰¹ ]）非常靠近静脉。这些空间关系表明冲击力和剪切应力升高，并且在冲击过程中沿静脉平面集中。