Moser D.E., Cupelli C.L., Barker I.R., Flowers R.M., Bowman J.R., Wooden J., Hart J.R.
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
Department of Earth Sciences, The University of Western Ontario, London, ON N6A 5B7, Canada; Department of Geological Sciences, University of Colorado, Boulder, CO, United States; Department of Geology and Geophysics, University of Utah, Salt Lake City, UT, United States; Department of Geology and Environmental Science, Stanford University, Stanford, CA, United States; Themba Labs, Somerset West 7129, Johannesburg, South Africa
Moser, D.E., Department of Earth Sciences, The University of Western Ontario, London, ON N6A 5B7, Canada; Cupelli, C.L., Department of Earth Sciences, The University of Western Ontario, London, ON N6A 5B7, Canada; Barker, I.R., Department of Earth Sciences, The University of Western Ontario, London, ON N6A 5B7, Canada; Flowers, R.M., Department of Geological Sciences, University of Colorado, Boulder, CO, United States; Bowman, J.R., Department of Geology and Geophysics, University of Utah, Salt Lake City, UT, United States; Wooden, J., Department of Geology and Environmental Science, Stanford University, Stanford, CA, United States; Hart, J.R., Themba Labs, Somerset West 7129, Johannesburg, South Africa
Integrated electron nanobeam (EBSD, CL, EDS) and isotopic measurements (U-Pb, (U-Th)/He) of zircon from the collar and centre of the 80 km wide central uplift of the 2020 ± 3 Ma Vredefort impact structure reveal new shock features in a microstructural progression related to impact basin formation and degree of U-Pb age resetting: (1) planar fractures in {1K0} and {1K2} orientation during initial shock wave compression; (2) curviplanar fractures in {1K1} orientation, now annealed, which host glassy inclusions of partial melt of the host rock; (3) microtwin lamellae in an orientation of 65° about [110], attributed to shock wave rarefaction; (4) nucleation of impact-age crystallites, possibly on microtwins, during post-shock heating by impact melt; and (5) crystal-plastic deformation linked to crater modification of the core of the central uplift. Planar fracturing and microtwinning ≥20 GPa in "cold shock" zircon in granitoid at a radial distance of 25 km failed to reset zircon age. Single-grain ID-TIMS data extend between pre-impact age of 2077 ± 11 Ma and a secondary Pb-loss event at ca. 1.0 Ga - the latter reflecting Kibaran igneous activity between 1.110 and 1.021 Ga. Age resetting by the impact event operated in an ~ 15 km wide "hot shock" zone of impact-elevated temperatures ≥700°C at the core of the central uplift. Mechanisms include internal recrystallization, defect-accelerated Pb diffusion via shock microstructures and melt films, and late crystal-plastic deformation. Igneous zircons from a 2019 ± 2 Ma foliated norite impact melt yield a mean (U-Th)/He date of 923 ± 61 Ma, indicating exposure of the present surface after this time.