Post-cratering melting of target rocks at the impact melt contact: Observations from the Vredefort impact structure, South Africa

Abstract

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 SiO2 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.