A few days later I photographed another one that turned out (I think) to be an Atlantis Fritillary (Speyeria atlantis).
While cruising through my photos, I notice that the first one looked an awful lot like the second one. HM. Now I think they are both Atlantis.
The differences are difficult to see, if you don't know what you are looking for. When you know the answer, of course, it's easy!
So, for your elucidation, it seems that for the Atlantis, the borders of all four wings have solid black margins.
The Artic has solid black triangles sitting along a black diamond margin (and we really are a bit south for this species).
The Aphrodite has a solid black margin along the outer edge of the forewing only.
The Spangled has no solid black margins at all.