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Although the original poster of the photo hasn't been on this forum for over five years, I'd still like to give an answer (a nice change from asking questions!).

Given the stated body length of about 10 cm and the lack of pronotal and cephalic armor, this can only be a pretty large female of a Megasoma species (Scarabaeidae: Dynastinae). Other South American dynastids, including Golofa, don't reach this size. Given the location, it's most likely a M. mars since the other large species of Megasoma don't reach this far south. For more info on the distribution and habitus of various species of Megasoma I refer to this website.

Best regards,

Scarabaeoid