Alochthonous Suevitic Breccias at Manicouagan

This work presents results of the first detailed investigation of suevitic rocks associated with the 214 Ma, 80–100km diameter Manicouagan impact structure, since the work of Murtaugh (1976). Suevite refers to an allochthonous impact breccia comprising a predominantly clastic matrix with blebs of melt and/or glass. At Manicouagan, suevite has so far only been positively identified sporadically developed at the basal contact of impact melt (IM) breccia and basement target lithologies towards the periphery of Renee Levasseur Island. Breccias present at the base of thick (up to 1.1km) IM sections in the centre of the structure may represent thermally annealed suevites. Suevites range in colour from green to brown to red and typically comprise fine grained, friable matrices with mm–10's cm size, generally rounded, mineral and lithic fragments, which can exhibit shock, e.g., planar deformation features in quartz, and evidence of partial melting. Brown, fluidal melts, as well as rock clasts rimmed with melt are also present. Field evidence indicates movement of the suevite relative to the base of the impact melt sheet. Complex relationships are observed between the two, suggesting that neither was solid nor consolidated when juxtaposed. Such features include flow textures at the base of the impact melt sheet immediately adjacent to the suevite and interfingering of both lithologies. Analytical Scanning Electron Microscopy of the melt blebs and stringers within the suevite points to an IM source. Preliminary trace element data indicate that the suevites contain more Ni than both average impact melt rocks and those in the immediate vicinity, implying a possible projectile component. The suevite probably originated close to the point of impact and, critically, may reveal early-stage excavation processes.

J. G. Murtaugh. 1976. Quebec Ministry of Natural Resources Report 432 p. 93-98.