I'm more sure than about most things that this is Magnolia tripetala, in the rhododendron garden in Stanley Park at Ceperley Meadow. Except, I've just seen two old postings in which @Ron B mentioned how malodorous the flowers are of this species. These flowers were right down at nose level, and I didn't notice any smell at all. You can see that there were new flowers, and senescent ones. No smell. Still, I can't imagine this being anything else. I didn't measure the leaves, but they were probably around two hand-spans in length.