Impact‐induced melting of Archean granulites in the Vredefort Dome, South Africa. I: anatexis of metapelitic granulites

Abstract

Mid-crustal Archean pelitic granulites in the Vredefort Dome experienced a static, low-P granulite facies overprint associated with the formation of the dome by meteorite impact at 2.02 Ga. Heating and exhumation were virtually instantaneous, with the main source of heat being provided by energy released from nonadiabatic decay of the impact shock wave. Maximum temperatures within a radius of a few kilometres of the centre of the structure exceeded 900 uC and locally even exceeded 1350 uC. This led to comprehensive melting of the precursor Archean granulite assemblages (Grt+Bt+Qtz+Pl+Ksp¡ Crd¡Opx¡Sil) followed by peritectic crystallization of aluminous alkali feldspar+Crd+Spl¡Crn¡Sil parageneses and the segregation of small, evolved, biotite leucogranite bodies. However, at a distance of c. 6 km from the centre pre-impact rock features are largely preserved, although partial replacement of garnet by symplectitic coronas of Crd+Opx¡Spl¡Pl and biotite by orthopyroxene indicate that peak temperatures approached 775¡50 uC. Thin interstitial moats of K-feldspar are closely associated with the orthopyroxene coronas; they are interpreted as the remnants of low-proportion partial melts generated by biotite breakdown. Both the textures and mineral compositional data support reduced equilibration volumes in these rocks, which re¯ect rapid isobaric cooling following shock heating and exhumation. The high temperatures and strong lateral thermal gradient are consistent with the modelled impact-induced isotherm pattern for a 200±300 km diameter impact crater.

New U–Pb SHRIMP zircon age for the Schurwedraai alkali granite: Implications for pre-impact development of the Vredefort Dome and extent of Bushveld magmatism, South Africa

“…8. Vredefort impact event at 2.023 Ma (Kamo et al, 1996) with resultant shock and transient thermal metamorphism (Gibson et al, 1997Gibson, 2002), and development of granophyric impact melt (French and Nielsen, 1990;Koeberl et al, 1996). 7.…”

“…The first predates the Vredefort impact and is ascribed to a period of high heat flow connected to the Bushveld magmatic event (Gibson and Wallmach, 1995;Gibson and Reimold, 2001). The second event, contemporaneous with and resulting from the impact episode, caused transient thermal effects that overprint the existing mineralogical and structural features (Gibson et al, 1997Gibson, 2002).…”

Arrested development – a comparative analysis of multilayer corona textures in high-grade metamorphic rocks

“…Typically, the latter scenario arises in contact aureoles characterized by rapid heating and cooling (Johnson et al, 2004 - Fig. 8g; Mcfarlane et al, 2003;Ings and Owen, 2002;Barboza and Bergantz, 2000;Wheeler et al, 2004;Daczko et al, 2002;Dasgupta et al, 1997;Joesten and Fisher, 1988), but it can also occur in shock-heated rocks within large impact structures (Gibson, 2002;Ogilvie, 2010).…”

“…Nonprogressive coronas develop in a separate P -T event to those that generate the peak assemblage (e.g. Johnson and Carlson, 1990;Gibson, 2002;McFarlane et al, 2003).…”

Observations and interpretations at Vredefort, Sudbury, and Chicxulub: Towards an empirical model of terrestrial impact basin formation

“…They further suggested that the pervasive network of veins of pseudotachylitic breccia may have provided the means for large-scale continuous rock flow during structural uplift. The originally deepest rocks of the crystalline core of the Vredefort Dome are from mid-crustal levels (Lana et al 2003), and there is a progressive increase in recorded postimpact temperatures from ~300 °C in the outer collar rocks to >700 °C, and locally as high as 1000 °C, in the central core due to the geothermal gradient prior to impact-induced uplift and shock heating (Gibson et al 1998;Gibson 2002;Gibson and Reimold 2005). These high temperatures may have also assisted in mechanical strength degradation during structural uplift and modification.…”