Potassium-argon determinations on two amphiboles from syenite give an average age of 642 m.y. Structural and petrological features had previously suggested that the intrusive was related to the Monteregian Hills of Cretaceous age, but this correlation is now shown to be erroneous. The Chatham-Grenville intrusive is the youngest Precambrian intrusive yet recorded from this part of the Canadian Shield.

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