Discussion of “New zircon shock phenomena and their use for dating and reconstruction of large impact structures revealed by electron nanobeam (EBSD, CL, EDS) and isotopic U–Pb and (U–Th)/He analysis of the Vredefort dome”1Appears in Canadian Journal of Earth Sciences, 2011, 48(2): 117–139 [doi: dx.doi.org/10.1139/e11-011

Abstract

Spodumene pegmatites occur in the southeastern sector of the 2.02 Ga Vredefort impact structure in South Africa. Moser et al. (published in 2011) presented an isotope dilution – thermal ionization mass spectrometry (ID–TIMS) age of 1136 ± 47 Ma for these spodumene pegmatites. This discussion questions the reliability of this age. Field relations indicate that the spodumene pegmatites are not affected by the structural fabric that affected the host amphibolites. The crosscutting impact-related veins of pseudotachylitic breccia clearly argues a preimpact age for the spodumene pegmatites. The high-uranium content of one of the two zircons used by Moser et al. to obtain the age is correlated to the ∼1.1 Ga Pb-loss event, widely observed in different rocks within the Vredefort impact structure.