A TEM INVESTIGATION OF SHOCK METAMORPHISM IN QUARTZ FROM THE VREDEFORT DOME, SOUTH-AFRICA

The origin of the Vredefort structure in South Africa is still debated . Several causes have been discussed, namely asteroid impact, internal gas explosion or tectonic processes. Evidence of dynamic rock deforma tion is pervasive in the form of planar features in quartz grains, sha tter cones, veins of pseudotachylite and occurrence of coesite and sti shovite (high-pressure quartz polymorphs). A number of these character istics is widely believed to support an impact origin. However, the pl anar features in quartz, which are generally considered as one of the strongest indicators of impact, are in the Vredefort case considered a s anomalous when compared with those from accepted impact structures. We have investigated by optical and transmission electron microscopy ( TEM) the defect microstructures in quartz grains from different lithol ogies sampled at various places at the Vredefort structure. Whatever t he locality, only thin mechanical Brazil twin lamellae in the basal pl ane are observed by TEM. So far, such defects have only been found in quartz from impact sites, but always associated with sets of thin glas s lamellae in rhombohedral planes {101nBAR} with n = 1, 2, 3, and 4. A t the scale of the optical microscope, Brazil twins in (0001) are easi ly detected in Vredefort quartz grains because of the numerous tiny fl uid inclusions which decorate them. Similar alignments of tiny fluid i nclusions parallel to other planes are also detected optically, but at the TEM scale no specific shock defects are detected along their trac es. If these inclusion alignments initially were shock features, they are now so severely weathered that they can no longer be recognized as unambiguous shock lamellae. Fine-grained coesite was detected in the vicinity of narrow pseudotachylite veinlets in a quartzite specimen, b ut stishovite was not found, even in areas where its occurrence was pr eviously reported. Finally, definite evidence of high-temperature anne aling was observed in all the samples. These observations lead us to t he conclusion that our findings regarding microdeformation in quartz a re consistent with an impact origin for the Vredefort structure. Most of the original shock defects are now overprinted by an intense post-s hock annealing episode. Only the thin mechanical twin lamellae in the basal plane have survived.