The Rb-Sr composition of eight melt rock and three basement samples from the East Clearwater impact structure, Quebec, and two basement samples from the West Clearwater structure has been determined. The whole rock 87Sr/86Sr ratios of the melt samples, 0.7167–0.7253, are within the range of the basement samples, 0.7054–0.7322, and provide further evidence that the melt rocks represent shock-melted basement. A mineral isochron obtained from a relatively coarse grained melt rock gives an age of 287±26 Ma for the crystallization age of the melt. This is equivalent to K-Ar whole-rock ages of 285±30 Ma and 300±30 Ma and a Rb-Sr age of 266±15 Ma obtained on melt rocks from West Clearwater and confirms the previously generally held assumption that the East and West Clearwater structures resulted from the simultaneous impact of two bodies at 285–300 Ma ago.
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