The Controversy about the Vredefort Granophyre (Vredefort Impact Structure, South Africa): New Insights from Multi-Isotope Analysis

The Controversy about the Vredefort Granophyre (Vredefort Impact Structure, South Africa): New Insights from Multi-Isotope Analysis

Yeimmy A. Gutierrez Pardo, Wolf Uwe Reimold, Natalia Hauser, Roger L. Gibson, Alisson Lopes Oliveira, Christian Koeberl, Toni Schulz

Impact melt rock complexes in large terrestrial impact structures constitute important quasi-igneous systems that may also host economic mineralization (e.g., Sudbury Complex, Canada). They provide essential information about the physico-chemical processes (e.g., melting of particular target rocks; mixing; assimilation of clastic material) in the transient crater cavity; the composition of the extraterrestrial impactor; the nature of the melted target rock volume as a function of impact magnitude; and evidence of shock metamorphism in rock clasts.

The 27 x 60 x 2.5 km Sudbury Complex, which hosts world-class Ni deposits, is the remnant of a 1.86 Ga impact melt sheet that underwent differentiation during cooling. Intrusions of the melt into the footwall of the Complex (the so-called Offset Dikes) show a bimodal composition thought to indicate two successive pulses of intrusion from the differentiating melt complex.

Since 2011, a similar process has also been invoked to explain chemical variation in dikes of impact melt rock in the 2.02 Ga Vredefort impact structure (South Africa). Deep erosion of the originally 250-300 km diameter crater has removed the original impact melt sheet, but has exposed the remnants of nine dikes of impact melt rock (Vredefort Granophyre) in the deeply eroded central uplift complex (Vredefort Dome). In general, the dikes show an unusual, but remarkably homogeneous, chemical composition. However, two ca. 20 m wide dikes in the NW sector of the Dome display a composite character involving a felsic (>66 wt% SiO₂) marginal facies and a comparatively more mafic (<66 wt% SiO₂) central facies. This duality has been interpreted by some researchers to indicate Offset Dike-like successive emplacements of these two types of melt. However, modeling of major and trace element chemistry of the two granophyre varieties, compared to the compositions of mafic wall-rocks exposed in the vicinity of the dikes, suggested that admixture of a mafic country rock to the felsic melt variety could also explain the mafic melt composition. Wannek (2015) proposed an "assimilation hypothesis", whereby rafts of mafic wall-rock would have been incorporated into the melt descending into fractures in the crater floor, becoming partially assimilated in the process. This mixing relationship has since been strongly supported with multi-isotope data (Sr, Nd, Os, Se, U-Pb; Reimold et al., 2023) data. However, it has not been possible yet to verify which country rock could qualify as the mafic endmember for this assimilation process.

This study presents geochemical and Sr-Nd isotope whole-rock data for the two principal mafic rock types found in close proximity to the composite dikes – basaltic meta-lava of the ~3.0 Ga Dominion Group and meta-dolerite sills (the so-called “Epidiorite”) related to the 2.8 Ga Klipriviersberg Group. This work also includes a provenance (U-Pb on zircon) study to investigate if the zircon populations in the felsic and mafic Granophyre facies were derived from different sources that might be linked to the meta-lava or Epidiorite. The geochemical and isotopic whole-rock data are compared to data from an impact-related pseudotachylitic breccia in the meta-lava to further investigate the generation of this second intriguing melt rock type in the Vredefort Dome.

References:

Reimold, W.U. et al., 2023. Genesis of the mafic impact melt rock in the northwest sector of the Vredefort Dome, South Africa. Meteorit. Planet. Sci., 58, 907-944.

Wannek, D.S., 2015. Geologische Untersuchungen zur Genese des Vredefort Granophyrs im Nordwesten des Vredefort Doms. M.Sc. Thesis (unpubl.). Freie Universität Berlin, Germany. 186 pp.