The Age of the Sudbury Nickel Irruptive and the Murray Granite

Abstract

Whole-rock samples of norite from the North and South Ranges of the Sudbury Irruptive yield a rubidium strontium isochron of 1956 ± 98 Ma age and (87Sr/86Sr)o = 0.7063 (λ87Rb = 1.39 × 10−11 a−1). Five samples of ore bearing 'sub-layer' from the South Range are compatible with this age. Four new determinations of South Range and six published analyses of North Range micropegmatite define an isochron of age = 1680 ± 30 Ma with (87Sr/86Sr)o = 0.708.The norite age is believed to represent the 'Sudbury event', a local event marked by the development of shock metamorphism and intrusion of the nickel irruptive. The 1680 Ma micropegmatite age is believed to be secondary and related to greenschist-facies metamorphism in the center of the Sudbury Basin during the Penokean Orogeny. Evidence supporting a later separate intrusion of granophyre-rich micropegmatite suggests that the time of this intrusion was relatively soon after the norite rather than 1680 Ma ago.Analyses of Murray granite samples indicate a pre-norite age of 2260 Ma. Most of the Murray granite shows evidence of later metamorphic events including the Sudbury event. Late (post-norite and post-Murray granite) granitic dikes (i.e., the Murray offshoots) intrude the norite near the Murray granite. These and other smaller granitic dikes appear to have formed 1700 to 1800 Ma ago. The origin of these dikes remains problematical.