Genesis of the mafic impact melt rock in the northwest sector of the Vredefort Dome, South Africa

Abstract

Genesis and emplacement of Vredefort Granophyre, the impact melt rock exposed on the Vredefort Dome, the erosional remnant of the central uplift of the Vredefort impact structure, South Africa, have long been debated. This debate was recently reinvigorated by the discovery that besides the previously known felsic variety of >66 wt% SiO 2 , a second, somewhat more mafic phase of <66 wt% SiO 2 occurs along a Granophyre dike on farms Kopjeskraal and Eldorado in the northwest sector of the dome. Two hypotheses have been put forward to explain the genesis and emplacement of this second phase: (1) successive injections of impact melt into extensional fractures opened in the course of central uplift formation/crater modification, with melts of distinct compositions derived from a differentiating impact melt body in the crater, and (2) generation of the more mafic phase as a product of admixture/ assimilation of a mafic country rock component, either the so-called epidiorite of possible Ventersdorp Supergroup affiliation or the Dominion Group meta-lava (DGL), to Felsic Granophyre. In the latter model, contamination with mafic country rock would have occurred during downward intrusion and stoping into and below the crater floor. The so-called Mafic Granophyre has previously only ever been sampled on a single site (Farm Kopjeskraal). In this study, samples of Granophyre occurring along the southerly extension of this dike on farm Rensburgdrif, and from a second dike on the Rietkuil property further southwest were investigated by field work, and petrographic, geochemical, and isotopic analysis. The mafic phase indeed occurs in the interior of the dike at Rensburgdrif, and also on Rietkuil. New geochemical and Sr-Nd isotope data support the hypothesis that the Mafic Granophyre composition represents a mixture between Felsic Granophyre and a mafic country rock. A 20% admixture of epidiorite or DGL to Felsic Granophyre provides an excellent match for the chemical composition of the Mafic Granophyre. The Sr-Nd isotope data indicate that this admixture likely involved the epidiorite component rather than DGL. Together with earlier Sr-Nd-Os-Se isotopic data, and other geochemical data, these results further support formation of the Mafic Granophyre by local assimilation/admixture of epidiorite to Felsic Granophyre.

Shallow depth extent of the Lesutoskraal granophyre dike in the Vredefort impact structure: Geophysical surveys and trenching data

“…Notably, the bulk rock composition of the granophyre has not been affected by interaction with the host dolerite. This is in contrast with the prediction of Reimold et al (2021Reimold et al ( , 2023, who proposed that interaction of the impact melt with mafic material would result in assimilation of the mafic FIGURE 10. Classification of the two rock samples taken from the trench by means of the total alkali-silica diagram.…”

Terrestrial impact sites as field analogs for planetary exploration

“…The second melt rock type of the Vredefort Dome is the Vredefort Granophyre, the true impact melt rock, which occurs in the form of nine dikes within the basement core and along the core/collar boundary. Recent discussions and reviews have been published by Reimold et al (2017Reimold et al ( , 2021Reimold et al ( , 2023. The granophyre dikes provide excellent exposures of impact melt rock.…”

“…The granophyre dikes provide excellent exposures of impact melt rock. Two chemically different variants of impact melt rock, known as felsic and mafic granophyre, respectively, occur in two dikes in the northwestern sector of the dome Reimold et al (2017Reimold et al ( , 2021Reimold et al ( , 2023. The reason for this duality is currently strongly debated, with either 1) consecutive intrusions of material from a differentiating impact melt sheet or 2) assimilation of a mafic country rock (epidiorite) by intruding felsic granophyre being favored (Reimold et al, 2021;2023).…”

Assessment of Radon and Naturally Occurring Radionuclides in the Vredefort Meteorite Crater in South Africa

“…Amongst the key areas that Bezuidenhout identified was the Vredefort Crater, the largest and oldest verified impact crater on the planet, which is located around 120 km southwest of Johannesburg, South Africa. Due to the meteoric impact, this area is rich in unique geological formations [9] and various studies have been performed in this area [10][11][12].…”