U–Pb geochronological analyses of five zircon fractions from a lineated and foliated monzonite sill on the west side of Kootenay Lake are discordant and yield a lower intercept age of 173 ± 5 Ma, interpreted as the minimum crystallization age. An upper intercept of 1710 ± 180 Ma is interpreted as the average age of inherited components, and is consistent with contamination by Middle Proterozoic detritus in Upper Proterozoic to lower Paleozoic strata. The sills are interpreted as pre- to syn-kinematic with respect to regional second-phase or possibly third-phase deformation, thus further constraining the timing of Mesozoic orogeny in the Kootenay Arc, and may represent an early, foliated phase of the Nelson Batholith.

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