Rb–Sr Study of Shock-metamorphosed Inclusions from the Onaping Formation, Sudbury, Ontario

Abstract

Sixteen inclusions from the Onaping Formation, a possible meteorite impact breccia at Sudbury, Ontario, were analyzed for Rb, Sr, and Sr isotopic composition. Inclusions ranged from weakly shocked granitic rocks to heterogeneous glassy inclusions (impact melt?). Strongly shocked and glassy inclusions yield separate isochrons of 1430 ± 65 m.y. and 1480 ± 55 m.y., respectively (λ 87 Rb = 1.39 × 10 −11 y −1 ). Twenty-eight samples, including data for Onaping Formation samples from the literature, have an isochron age of 1515 ± 65 m.y. Isochron initial 87 Sr/ 86 Sr ratios range from 0.714 to 0.717, The Onaping samples thus appear anomalously younger than the 1700 m.y. ages reported for the geologically younger Nickel Irruptive and Whitewater sediments. If the highly shocked granitic inclusions and glasses in the Onaping Formation are relatively susceptible to radiogenic Sr loss during subsequent thermal events, the data are compatible with an impact origin for the Sudbury structure. The whole-rock ages, combined with reported mineral ages, suggest a metamorphic event in the Sudbury area 1400–1500 m.y. ago.