This seems to fit the requirements for a Hickory Hairstreak as described in the ROM Guide to Ontario Butterflies. It did look different than the dozens of Banded Hairstreaks elsewhere in the park. The behaviour was also different: the others were perched on shrubs and very active chasing each other around. This one was on goldenrod and in a different part of the park.

I'm looking for confirmation if possible before I add it to a life list for the park.