A TEM INVESTIGATION OF SHOCK METAMORPHISM IN QUARTZ FROM THE SUDBURY IMPACT STRUCTURE (CANADA)

The Sudbury basin in Canada is an elliptical feature (approx. 60 X 27 km in size) that is now widely believed to be part of a large (approx. 200 km diameter) meteorite impact structure which formed about 1.85 G a ago and was subsequently deformed and metamorphosed. Despite prolong ed debate over the origin and original size of the Sudbury structure, strong evidence for meteorite impact has been provided by the discover y in its rocks of a wide range of shock-metamorphic features, especial ly the optically observable traces (fluid inclusion arrays) of former Planar Deformation Features (PDFs) in quartz parallel to {<10(1)over b ar 3>} planes. In this new examination of Sudbury samples, no preserve d original PDFs (glassy lamellae) were observed optically in quartz gr ains from basement rock fragments in the Onaping Formation, a unit int erpreted as a 'fallback breccia' deposited within the original crater. However, TEM revealed other thin lamellar features preserved in the q uartz; these are identified as Brazil twin lamellae parallel to the ba sal plane (0001). These lamellae are 15-200 nm thick, show a typical s pacing of 30 nm to several microns apart, and are occasionally decorat ed with fluid inclusions (less than or equal to 0.5 mu m in size), whi ch probably formed during post-shock alteration and annealing. Numerou s subgrain boundaries (SGBs) were also detected, many of them oriented roughly parallel to the basal plane (0001). The SGBs apparently forme d during post-shock recrystallization, leading to the local disappeara nce of Brazil twin lamellae. Experimental evidence suggests that such basal Brazil twins are the unique product of high-pressure shock waves . The features in the Sudbury samples are identical to those observed in the Vredefort structure, South Africa, which is also widely accepte d as an ancient impact structure about 2.0 Ga old. The recognition of basal Brazil twins in quartz at Sudbury provides additional evidence f or meteorite impact origin and also emphasizes the high value of these durable shock features for identifying old and tectonically metamorph osed impact structures.