Impact melt is generated following hypervelocity impact events. Emplacement of impact melt dikes, such as the Vredefort Granophyre Dikes, allow for this high temperature melt to come into contact with deeply-buried target rocks after the cratering process is completed. Our study analyzes the effects of this interaction by examining the direct contact between the Vredefort Granophyre and the granitic host at the Kopjeskraal and Lesutoskraal Granophyre Dikes using scanning electron microscopy (SEM), energy-dispersive spectrometry (EDS), electron backscatter diffraction (EBSD), and X-ray micro-computed tomography (μCT). A several-mm-thick transition zone between the host rock and the impact melt is enriched in SiO and indicates preferential melting of feldspar and mica in the host rock by interaction with the impact melt. Immiscible droplets of newly-formed silicate melt migrated from the transition zone into the impact melt. We observe inundations of the impact melt along narrow fractures into the host rocks, which, in some cases, surround and incorporate fragments of the host rock into the melt body. We suggest three possible mechanisms by which components of the host rock can enter the impact melt: 1) fragmentation of the host rock prior to melt emplacement and subsequent entrainment into the melt; 2) inundations of melt around fragments of host rock at the contact, followed by incorporation of the host rock into the melt; 3) melting of the host rock and immiscible migration of melt fragments within the impact melt. The lack of observed assimilation of the granitic fragments into the impact melt, either because of silica saturation or viscosity contrast between the melts, suggests that the bulk composition of the Granophyre Dike matrix approximately represents the composition of the impact melt sheet.

中文翻译：

---

撞击熔化接触处目标岩石的陨石坑后熔化：来自南非弗里德堡撞击结构的观测

超高速撞击事件后会产生撞击熔化。冲击熔岩堤的安置，例如 Vredefort Granophyre 岩堤，允许高温熔体在陨石坑过程完成后与深埋的目标岩石接触。我们的研究通过使用扫描电子显微镜 (SEM)、能量色散光谱 (EDS)、电子背散射衍射 (EBSD) 检查 Vredefort 花斑岩与 Kopjeskraal 和 Lesutoskraal 花斑岩岩脉中花岗岩主体之间的直接接触来分析这种相互作用的影响和 X 射线微计算机断层扫描 (μCT)。母岩和冲击熔体之间的几毫米厚的过渡区富含 SiO，表明母岩中的长石和云母通过与冲击熔体的相互作用而优先熔融。新形成的硅酸盐熔体的不混溶液滴从过渡区迁移到冲击熔体中。我们观察到冲击熔体沿着狭窄的裂缝涌入主岩，在某些情况下，主岩的碎片包围并合并到熔体中。我们建议母岩成分进入冲击熔体的三种可能机制：1）在熔体侵位之前母岩破碎并随后夹带进入熔体； 2) 熔体淹没接触处母岩碎片周围，随后母岩并入熔体中； 3）母岩的熔融和冲击熔体内熔体碎片的不混溶迁移。由于二氧化硅饱和度或熔体之间的粘度差异，没有观察到花岗岩碎片同化到冲击熔体中，这表明花斑岩岩脉基质的整体成分大致代表了冲击熔体片的成分。