Tropidurus torquatus — Amazon Lava Lizard
Some other names for this species:
Eastern Collared Spiny Lizard
These lizards were common around Iguaçu National Park. The first one I saw was the handsome tree-hugging adult pictured above. Most of the others I saw were on rocks, which befits their "Lava Lizard" name. The second photo here is a juvenile on a mossy forest rock. The third photo shows one of several lizards that were hanging out on partly-submerged boulders with crashing waterfalls only a few yards away.
I'm pretty sure that these three are all the same species. I'm quite confident that they are Tropidurus, and I think the species is T. torquatus, since that seems to be the only species mentioned from this location on the web.