Abstract

A prominent magnetic anomaly in the northern Powder River Basin of Wyoming defines a postulated mafic / ultramafic body in the basement that is 4 miles wide and 40 miles long (6.5 × 65 kilometers) in its present configuration, although originally it was longer, as both north and south ends have been truncated by faulting. It is thus similar to the Great Dyke of Rhodesia which averages 4 miles wide, but is 325 miles long. Both bodies were intruded into a 3.5-2.6 billion year old (Ga) granite / gneiss terrane containing 2.7-2.6 Ga greenstone belts. Age of the Great Dyke of Rhodesia is 2.6 Ga; the age of the Great Dike of Wyoming is unknown, but measured ages of other mafic / ultramafic bodies in the Wyoming Craton range from 2.7 to 2.0 Ga, with a peak at 2.7 Ga. Given the great mobility of the earth’s crust, apparently increasing backwards in time, and the many fragmentation and accretionary events that have affected the earth’s continents, it is possible that these two bodies formed in close spatial proximity to one another and that fragments of continental crust containing them or adjacent bodies have been subsequently dispersed and accreted onto other continents.