Diatremes and shock features in Precambrian rocks of the Slate Islands, northeastern Lake Superior: Reply
The Precambrian rocks of the Slate Islands exhibit shock features that have been attributed to meteorite impact. The islands occur at the intersection of two major regional faults, one of which controls the location of late Precambrian alkalic magmatism. Alkalic intrusive events north of Lake Superior occurred on the Slate Islands. These alkalic intrusions are represented by a carbonatite dike in the southeast corner of the island and a set of alkalic diabase dikes exposed at several locations on the island. Diatreme breccias cut these alkalic rocks and are interpreted to be late-stage phases of this volatile-rich alkalic magmatism. Shatter cones appear spatially related to the diatremes, and quartz grains displaying deformation lamellae are present in the matrix of the breccias. Shatter-cone structures and deformation lamellae are considered to be indicative of shock events. On the basis of field observations and data, the shock and diatreme events can be correlated and related to volcanism or alkalic magmatic processes associated with major regional fractures and not to meteorite impact.