It might not be possible to find out the species, but I put it up anyway..
There are 38 sps. in the region,
Mimosa bimucronata is the closest I found, but the flowers of mine are not so pristine white, more yellowish white, and the flower head in bud looks more elongated..
https://www.google.be/search?q...
No match here either for Mimosa..
http://www.ufrgs.br/fitoecolog...
I googled all 38 sps, phewww.. most of them have no pics on the web..
I think this one could be the same species, it was a tall tree I met in a large community property..
