Spilitic basalts from the Proterozoic Seton Formation at the type locality of Seton Island in the East Arm of Great Slave Lake, Northwest Territories, have a Rb–Sr isotopic age of 1872 ± 10 m.y. The age, representing either the time of eruption or a subsequent metasomatic (spilitization) event, provides a minimum age for the Great Slave Supergroup of the district. An initial 87Sr/86Sr ratio of 0.7018 ± 0.0005 for the lavas is, together with their age, in accordance with previously proposed theories on continental separation and mantle derived magmatism during Aphebian times.

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