Ten K–Ar dates, including four coexisting mineral pairs, have been obtained for metamorphic rocks of the Horsethief Creek Formation (Proterozoic) of the Esplanade Range, British Columbia.The oldest date obtained is 181 m.y. on a single hornblende, but it is possible that this sample contains excess Ar. The oldest concordant biotite–hornblende date is 113 m.y.Muscovite that coexists with post-tectonic (F2 folding) biotite gave an age of 73 m.y. This age suggests that the temperature of the rocks had dropped below that for argon diffusion 73 m.y. ago.

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