Radiometric dating of igneous rocks in the Gulquac Lake area of west-central New Brunswick confirms that rhyolitic volcanism and shallow granitic intrusive activity were closely associated in time. Rb–Sr whole rock isochrons yield an age of 409 ± 35 Ma for the volcanic rocks and 409 ± 25 Ma from the intrusive rocks (λ = 1.42 × 10−11 year−1). Structural relationships in the area show that emplacement of the granite coincided with an episode of intense faulting and uplift of sillimanite-grade Cambro–Ordovician terrain.The combined radiometric data indicate a minimum age for the Silurian–Devonian boundary of 409 ± 20 Ma.

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