A TEM investigation of shock metamorphism in quartz from the Sudbury impact structure (Canada)

Abstract

The Sudbury basin in Canada is an elliptical feature (approx. 60 × 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 Ga ago and was subsequently deformed and metamorphosed. Despite prolonged debate over the origin and original size of the Sudbury structure, strong evidence for meteorite impact has been provided by the discovery in its rocks of a wide range of shock-metamorphic features, especially the optically observable traces (fluid inclusion arrays) of former Planar Deformation Features (PDFs) in quartz parallel to {10 1 3} planes. In this new examination of Sudbury samples, no preserved original PDFs (glassy lamellae) were observed optically in quartz grains from basement rock fragments in the Onaping Formation, a unit interpreted as a ‘fallback breccia’ deposited within the original crater. However, TEM revealed other thin lamellar features preserved in the quartz; these are identified as Brazil twin lamellae parallel to the basal plane (0001). These lamellae are 15–200 nm thick, show a typical spacing of 30 nm to several microns apart, and are occasionally decorated with fluid inclusions (≤ 0.5 μm in size), which probably formed during post-shock alteration and annealing. Numerous subgrain boundaries (SGBs) were also detected, many of them oriented roughly parallel to the basal plane (0001). The SGBs apparently formed during post-shock recrystallization, leading to the local disappearance 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 accepted as an ancient impact structure about 2.0 Ga old. The recognition of basal Brazil twins in quartz at Sudbury provides additional evidence for meteorite impact origin and also emphasizes the high value of these durable shock features for identifying old and tectonically metamorphosed impact structures.