Formation of central uplift structures in large, complex impact craters: The role of pseudoachylitic in the development of the Vredefort Dome, South Africa

本建议书是先前申请（RE 528/5-1）的延续，涉及拟议目标和工作进度。假玄武质角砾岩（PTB）是南非Vredefort Dome（2.02 Ga，最初宽250 km）中最突出的撞击变形现象。这种熔融角砾岩出现在微观脉和长达公里和10米宽的角砾岩带中。该项目的中心主题是关于这些熔融角砾岩的起源的争议，通过（1）剪切（摩擦熔融）;（2）冲击压缩熔融;（3）冲击传播后立即减压熔融;（4）这些过程的组合;和（5）外来冲击熔融的侵入。解决这个问题需要详细的多学科（现场和实验室，结构地质，微变形，岩相学和地球化学）分析，以表征神秘的角砾岩和发生模式。了解PTB的成因，然后可以评估PTB在隆起结构的形成和坍塌过程中的作用。本研究采用了与之前不同的方法，主要是宏观露头分析的微观结构和（微观）化学分析的抛光3 x 1.5米花岗岩板从维雷德福圆顶核心的尺寸采石场和比较宏观到中观采石场分析。已对来自兰德花岗岩采石场和花岗岩板的假玄武质角砾岩和相应寄主岩石样品的基质进行了微裂缝和熔融角砾岩方向、密度和3D几何形状以及化学、矿物学和结构观察的调查。微观化学分析表明，变量和本地化的化学成分在非常薄的PTB细脉支持本地（晶粒尺度）这些熔体的起源。剩下的就是要解开这种融化最初是如何起源的问题。第一次微观化学研究的微裂缝和PTB表明，非常小的细脉的形成涉及局部熔融，可能在早期冲击压缩阶段。摩擦熔化的证据是非常有限的，我们的研究结果排除PTB一代的冲击熔体侵入。需要更多的结构和微化学证据来确定角砾岩成因的确切机制，特别是关于中基性寄主岩中熔融角砾岩的生成。计划将这项工作扩展到来自Vredefort Dome北方和西部其他采石场的样品。样品已经存在，可以进行分析。此外，减压熔融过程作为一个可行的机制熔融角砾岩形成尚未调查到目前为止，我们建议测试这一假设的数值模拟。我们申请将该项目延长一年，以便（1）完成岩石学-微化学数据采集，以进一步调查岩性对熔融角砾岩化学的控制作用，以及主岩中撞击诱发的冲击特征与角砾岩之间的可能联系，从而发现PTB脉;（2）建立了在距拱顶中心不同径向距离处减压熔化形成熔体的数值模型。

This proposal is for a continuation of the previous application (RE 528/5-1), with respect to both the proposed goals and the work schedule. Pseudotachylitic breccias (PTB) are the most prominent impact-induced deformation phenomenon in the Vredefort Dome, the eroded central uplift of the 2.02 Ga, originally 250 km wide Vredefort Impact Structure, South Africa. Such melt breccias occur in microscopic veins and up to kilometer long and 10s of meters wide breccia zones. The topic central to this project is the controversy about the origin of these melt breccias, by either (1) shearing (friction melting); (2) shock compression melting; (3) decompression melting immediately after shock propagation; (4) combination of these processes; and (5) intrusion of allochthonous impact melt. Resolving this problem requires detailed multidisciplinary (field and laboratory, structural geological, microdeformation, petrographic, and geochemical) analysis to characterize the enigmatic breccias and modes of occurrence. Understanding the genesis of PTB allows then to assess the role of PTB during formation and collapse of the uplift structure. This study uses a different approach from the previous, largely macroscopic outcrop analysis by microstructural and (micro)chemical analysis of a polished 3 x 1.5 m granite slab from a dimension stone quarry in the core of the Vredefort Dome and comparative macro- to mesoscopic quarry analysis. Investigation of microfracture and melt breccia orientation, density and 3D geometry, as well as chemical, mineralogical and textural observations from the groundmass of pseudotachylitic breccia and respective host rock samples from the Rand Granite Quarry and the granite slab has been carried out. Microchemical analysis has shown that variable and localised chemical compositions in very thin PTB veinlets support local (grain scale) origin of these melts. What remains is to unravel the problem of how this melt originated in the first place. First microchemical investigations of microfractures and PTBs have indicated that formation of very small veinlets involved local melting, likely during the early shock compression phase. Evidence for friction melting is very limited, and our results exclude PTB generation by intrusion of impact melt. Additional structural and microchemical evidence to assign the exact mechanism(s) for breccia genesis is needed, especially with respect to melt breccia generation in intermediate to mafic host rock. It is planned to extend the work to samples from other quarries in the northern and western part of the Vredefort Dome. Samples already exist and are ready for analysis. Furthermore, the process of decompression melting as a viable mechanism for melt breccia formation has not been investigated to date, and we propose to test this hypothesis by numerical modelling. We apply for a 1-year extension of this project in order to (1) complete the petrographic-microchemical data acquisition for further investigation of the role of lithological control on melt breccia chemistry and the possible link between impact-induced shock features in the host rocks to the breccias and, thus, the occurrence of PTB veins; and (2) to develop a numerical model of melt formation by decompression melting at different radial distances from the center of the dome.