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

大型复杂撞击坑中中央隆起结构的形成：假无蹄石在南非弗里德堡圆顶发展中的作用

• 批准号: 41469035
• 负责人: Professor Dr. Wolf Uwe Reimold
• 金额: --
• 依托单位: Museum für Naturkunde - Leibniz-Institut für Evolutions- und Biodiversitätsforschung (MfN)
• 依托单位国家: 德国
• 项目类别: Research Grants
• 财政年份: 2007
• 资助国家: 德国
• 起止时间: 2006-12-31 至 2012-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/41469035?language=en
• 关键词: Formation; central; uplift; structures; large

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.

该提案是先前申请（RE 528/5-1）的延续，涉及拟议的目标和工作时间表。伪角砾岩角砾岩（PTB）是南非Vredefort Dome中最突出的撞击变形现象，Vredefort Dome是2.02 Ga侵蚀的中央隆起，原宽250 km。这种熔融角砾岩出现在微脉中，长达千米，宽达10米的角砾岩带中。这个项目的中心主题是关于这些熔融角砾岩的起源的争议，要么(1)剪切（摩擦熔融）；(2)激波压缩熔化；(3)冲击波传播后立即减压熔化；(4)这些过程的结合；(5)外来冲击熔体侵入。解决这一问题需要详细的多学科分析（野外和实验室、构造地质、微变形、岩石学和地球化学），以表征神秘的角砾岩及其发生模式。了解PTB的成因有助于评估PTB在隆升构造形成和崩塌过程中的作用。本研究采用了与之前不同的方法，主要是通过微观结构和（微观）化学分析对弗里德堡圆顶核心的一个尺寸采石场的抛光3 x 1.5 m花岗岩板进行宏观露头分析，并比较宏观和细观采石场分析。对兰德花岗岩采石场和花岗岩板的伪长柄角砾岩地质体和各自的主岩样品进行了微裂缝和熔融角砾岩的取向、密度和三维几何形状的研究，以及化学、矿物学和结构的观察。微化学分析表明，非常薄的PTB细脉中的可变和局部化学成分支持这些熔体的局部（粒度）起源。剩下的就是解开这个融化最初是如何开始的问题。首先，对微裂缝和ptb的微化学研究表明，非常小的静脉的形成涉及局部融化，可能是在早期冲击压缩阶段。摩擦熔化的证据非常有限，我们的结果排除了由冲击熔化侵入产生的PTB。需要更多的构造和微化学证据来确定角砾岩形成的确切机制，特别是关于中基性寄主岩中熔融角砾岩的形成。计划将这项工作扩展到弗里德堡圆顶北部和西部其他采石场的样本。样品已经存在并准备好进行分析。此外，减压熔融过程作为熔融角砾岩形成的可行机制迄今尚未得到研究，我们建议通过数值模拟来验证这一假设。我们申请将该项目延长1年，目的是：(1)完成岩相微化学数据采集，进一步研究岩性对熔融角砾岩化学的控制作用，以及寄主岩石中冲击诱发的冲击特征与角砾岩之间的可能联系，从而确定PTB脉体的产状；(2)建立了在离圆顶中心不同径向距离处减压熔化形成熔体的数值模型。