PUSHING THE LIMITS OF ULTRAMETAMORPHISM – IMPACT-INDUCED UHT-LP METAMORPHISM IN EXHUMED LOWER CRUST IN THE VREDEFORT DOME (SOUTH AFRICA)

The 90-km-wide Vredefort Dome exposes a 40-km-wide core of polymetamorphosed mid- to-lower-crustal Archean gneisses that were exhumed by up to 15 km as a result of a giant 2.02 Ga impact event. Mafic, ultramafic and metasedimentary xenoliths within the predominantly granitoid gneisses record dynamothermal mid-crustal upper amphibolite to granulite facies metamorphism at 3.1 Ga; however, in the innermost core, rocks display evidence of a subsequent, highly localized, ultra-high-T, low-P thermal metamorphism. THERMOCALC modelling of aluminous granofels assemblages (Crd-Ksp-Spl±Crn±Rut±Sill±Pl and Crn-Pl-Spl-Bt±Rut) suggests peak metamorphic conditions of 1150-1200 °C, 0.25-0.3 GPa, with the assemblages representing peritectic crystallization products after high-percentage (>90%) partial melting of Archean Grt-Bt-Crd/Opx granulites similar to those found beyond the granofels terrane. An age of 2.017 ± 0.005 Ga (U-Pb single-zircon) for a small biotite granite body found in close proximity to these granofelses confirms that this event is linked to the impact. Mafic, ultramafic and ironstone gneisses show localized reaction coronae, with mm- to cm-scale granitoid (Cpx/Opx-Bt-Pl-Ksp-Qtz) melt lenses typically localised between mafic and felsic bands. The exceptionally fine grain size of both the reaction products and melts (usually <50 μm), points to a short-lived metamorphic event. In general, the rocks show little evidence of melt escape, providing an unusual opportunity to study natural melting phenomena in an UHT environment. The evidence for UHT-LP metamorphism is consistent with numerical modelling of the Vredefort impact, which suggests that anadiabatic heating during shock compression (shock P > 40 GPa) would have raised temperatures in already pre-heated mid- to lower crustal rocks directly below the point of impact to > 1100 °C, and that granulite conditions would only have persisted for a few hundred thousand years after the impact.