Modeling the geochemical evolution of impact melts in terrestrial impact basins: Vredefort granophyre dikes and Sudbury offset dikes

Abstract

The Vredefort impact structure, South Africa, is comparable to the Sudbury impact structure, Canada, in size, age, and target rock composition. Both impact structures feature impact melt dikes. The melt sheet of the Sudbury impact (Sudbury Igneous Complex; SIC) is genetically linked to the Sudbury offset dikes in the underlying target rock. At Vredefort, the melt sheet was eroded so that only the granophyre dikes retain compositional melt sheet characteristics. XRF analyses of 43 samples from four granophyre dikes are similar to previous studies, but identify an anomalous mafic phase within one of the dikes. The results from the Vredefort granophyre dikes are compared to the Sudbury offset dikes and shown to follow similar geochemical trends, controlled by crystallization of feldspar and pyroxene. The mafic granophyre phase is compositionally remarkably similar to the offset dike compositions. The program Rhyolite-MELTS was used to test possible melt sheet compositions. Modeling results are broadly consistent with the overall chemical and mineral composition of the dikes. Modeling is consistent with offset dikes being derived from the basal mafic layer of the SIC, and the granophyre dikes being derived from alkalidepleted bulk continental crust. For all modeled compositions, crystallization primarily occurred at temperatures between 1150°C and 1000°C. The emplacement of the felsic granophyre dikes from a homogenized crustal melt suggests emplacement within tens of years after the impact event. The presence of the mafic phase in one of the granophyre dikes is explained by its emplacement following some differentiation of the Vredefort melt sheet.

Post-Impact Faulting of the Holfontein Granophyre Dike of the Vredefort Impact Structure, South Africa, Inferred from Remote Sensing, Geophysics, and Geochemistry

“…The dikes generally crop out as continuous lines of disaggregated outcrops, or, less frequently, as discontinuous pavements with sharp contacts. The dikes occur both as clast-rich and clastpoor varieties, where the former have an inhomogeneous distribution of clasts both across and along the strike [2,3,13]. Clast-rich zones are characterized by up to 70 vol.% clasts with an average clast content of 10-20 vol.% [13,24].…”

“…The granophyre dikes have long been known to have been emplaced at the latest stages of development of the Vredefort Impact Structure based on the fact that the dikes cross-cut the uplifted collar rocks (e.g., [24]). The core dikes were likely derived from a homogenized melt sheet [2], and thus must have formed after the initial cratering process. The emplacement mechanism of the dikes is not fully understood [25], but is generally thought to have been associated with the opening of fractures deep in the crust, either by hydrostatic pressure-opening faults [46] or by faults opening through a process ongoing during post-impact crustal adjustment (e.g., [15]).…”

“…These various lines of evidence can be interpreted as the emplacement of a single dike at a late stage during the development of the Vredefort Impact Structure, with the melt being homogeneous (likely as a result of a homogeneous source for the melt; [2]). The emplacement of the dike likely required a pre-existing fracture to be present for the dike to exploit such that the host granites may have been offset from one another prior to or coeval with the emplacement of the dike.…”

“…The Vredefort Impact Structure in South Africa is one of the oldest and largest known impact structures on Earth and forms an impact basin (Figure 1A) [1]. Such impact basins form voluminous melt sheets on the surface that feed dikes, which descend into underlying country rocks at the late stages of the impact cratering process [2]. At Vredefort, these intrusions are known as Vredefort Granophyre Dikes (e.g., [3]).…”

Ocean resurge-induced impact melt dynamics on the peak-ring of the Chicxulub impact structure, Mexico

“…In the terrestrial environment, the proportion of pre-impact sedimentary cover rock with respect to crystalline basement rock decreases with impact magnitude in the terrestrial environment. Thus, impact melt rock in large impact structures, such as Chicxulub, formed chiefly from basement rock (≥ 90%: Kring and Boynton 1992;Barton et al 2010;Huber et al 2020).…”