Ultramafic rocks at the center of the Vredefort structure: Further evidence for the crust on edge model

Ultramafic rocks from the center of the Vredefort structure previously have been interpreted either as Early Proterozoic igneous rocks genetically rel ated to the Bushveld complex, or as Archean upper mantle rocks. Re-Os isoto pic data from these rocks display very low Os-187/Os-188 and Re-187/Os-188 suggesting that they are not magma chamber cumulates, but rather the residu es of melt extraction, similar in Re-Os systematics to subcontinental litho spheric mantle. Whole-rock Re depletion model ages for two samples are 3.3 and 3.5 Ga, indicating that the postulated melt extraction event must have occurred in the Archean, On the basis of these data we conclude that the ul tramafic rocks are not related to the Bushveld event, but rather represent upper mantle that rebounded to crustal levels following an impact event. Th is conclusion strengthens evidence for a model whereby the rocks exposed in the core of the Vredefort structure represent the lower parts of a crust o n edge profile that possibly contains a paleo-Moho very near the Earth's su rface.