PELITE MELTING IN A MEGA-IMPACT - ANATEXIS OR SHOCK MELTING?

Although it has been known that passage of a shock wave through a mineral generates a residual heating proportional to the magnitude of the shock wave, it is only recently that studies of meteorite impact structures have been expanded to include an analysis of metamorphic effects generated by this heating. The 2.02 Ga Vredefort impact structure in South Africa is the oldest, potentially largest (original diameter 200-300 km), and most deeply eroded impact structure currently known on Earth. Investigation of the rocks exposed within 10 km of the center of the Vredefort impact structure has revealed several features that are consistent with recent 2-D hydrocode modeling which suggests that, within a radius of up to 10-20 km from the impact center, common crustal rocks could have experienced post-shock temperatures in excess of their fluid-absent solidii. The Vredefort rocks comprise predominantly granitic to granodioritic amphibolite- to granulite-facies Archean gneisses with scattered mafic and sedimentary greenstone remnants that show evidence of having experienced mid-crustal (ca. 0.5 GPa) fluid-absent partial melting at ~3.1 Ga. At a distance of ~7 km from the center of the impact structure, pelitic granulites largely preserve their Archean gneissic migmatite textures but display post-impact symplectitic crd+opx+spl±pl±ksp coronas around garnet and biotite. Within 4 km of the center, however, pelitic rocks comprise highly aluminous alkali feldspar+crd+spl±sill±crn±rut granofelses, with no trace of the Archean parageneses preserved. These granofelses are closely associated with small 2017 ± 5 Ma leucogranite pods. Thermobarometric constraints suggest temperatures in excess of 900-1000 *C at pressures of 0.2-0.3 GPa. The granofelses and leucogranites are interpreted as the products of high-proportion partial melting of biotite-bearing precursors. In contrast, clastic melt breccias in the enclosing granitoid gneisses are attributed to shock melting at temperatures exceeding 1200-1300 *C. Shock melting appears to have been highly heterogeneous, suggesting either highly variable shock pressure distribution or a significant frictional heating component along shear surfaces.