Abstract

Shock metamorphic effects in samples from the Slate Islands, Lake Superior (48°40' N, 87°00' W) suggest that the islands are part of a meteorite impact structure. The islands form the central uplift of a complex crater and are ringed by a submerged trough and annular ridge with a diameter of 30 km. Precambrian bedrock units are locally brecciated and cut by allochthonous breccia dikes. These dikes contain clasts of identifiable country rock and also fragments of a sedimentary unit, possibly Upper Keweenawan in age, which is no longer present in outcrop. The orienta tions of shatter-cones present in the breccia host-rocks indicate the interior of the islands as the approximate shock centre. Microscopic planar features, equivalent to those described from other impact sites, occur in quartz and plagioclase and the level of shock deformation increases towards the interior of the islands. The shock event postdates Keweenawan igneous activity (about 1.1 b.y. old) and, on the basis of the erosion level, may be early Paleozoic in age.

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