New Type of Evidence for Ancient Impacts?

Discovery of mafic impact melt in the center of the Vredefort dome: Archetype for continental residua of early Earth cratering?

Authors:

Cupelli et al

Abstract:

Melting by impact heating is thought to have been a significant process in the modification of early planetary crusts; however, crustally derived melt bodies in ancient terrestrial crust are frequently presumed to be absent due to erosion. Here we demonstrate that in the central basement uplift of the 2.020 Ga Vredefort impact basin (South Africa), components of mafic impact melt have survived amid Archean gneiss as decimeter-scale dikes and lenses of variably foliated gabbronorite. Zircon microstructural, trace element, and isotopic analyses (U-Pb, Lu-Hf) of the gabbronorite reveal a dominant population of 2.02 Ga unshocked igneous zircon with apparent Ti-in-zircon temperatures of 800–900 °C, similar to those from the mafic sublayer of the Sudbury impact melt sheet. Highly negative subchondritic εHf values of –1.4 ± 1.1 to –7.9 ± 1.4 are consistent with a depleted mantle model age of ca. 3 Ga and gabbronorite derivation from the once superjacent Witwatersrand basin lithologies. The recrystallized igneous mineral textures and Archean felsic gneiss inclusions in the gabbronorite are attributable to the effects of emplacement and crater modification following ∼20 km elevation of the central uplift. Long mistaken as preimpact basement, the setting and characteristics of the Vredefort gabbronorite may provide new benchmarks in the search for remnants of large cratering events and melt residua on Earth's cratons.