Abstract

Otish Gabbro sills intrude sedimentary rocks in the Otish Basin of the southeastern Superior Province. Here, deposition of Otish Supergroup sediments had previously been thought to be older than K–Ar and Sm–Nd ages of ca. 1750–1710 Ma for Otish Gabbro sills, and younger than ca. 2515–2500 Ma U–Pb ages of underlying Mistassini dykes. However, a much older U–Pb baddeleyite age of 2169.0 ± 1.4 Ma is presented here for an Otish sill, indicating that they are coeval with, and likely genetically related to, the giant 2172–2167 Ma Biscotasing dyke swarm to the southwest and (or) the Cramolet sills and Payne River dykes to the north. The new date also indicates that the age of the Otish Supergroup falls between ca. 2515 Ma and ca. 2169 Ma, only a little different from the ca. 2450–2217 Ma bracket for the Huronian Supergroup of the Southern Province, and is consistent with both supergroups spanning the oxy-atmo inversion. The Otish Supergroup could also be coeval with the Sakami Formation to the north, but is likely older than the Richmond Gulf Group on the east coast of Hudson Bay. Early paleomagnetic study of Otish sills yielded a remanence ∼20° from that expected for Biscotasing-aged intrusions. This may indicate that too few distinct sills were studied to average out paleosecular variation, that demagnetization techniques failed to fully remove unstable magnetization components, or that the remanence is a stable secondary overprint, perhaps acquired during a fluid event related to uranium mineralization at ca. 1720 Ma.

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