Looks like a Black Locust tree to me (Robinia pseudoacacia). Like a real Acacia its really a 'weed' tree and has nitrogen fixing bacteria on its shallow root system, (Also like a Birch). This enables it to grow...fast!!!...even in stoney unfertile ground. When its starts getting larger the roots especially become a real problem, especially since it suckers so freely from the rootstock.

Nice tree though I reckon but watch for those roots!!! Albizia julibrissin is an alternative to try if the Robinia gets too big - its definitely a lot hardier than its made out to be! (Survived -16 here for two weeks) and has delicate Acacia like foliage, although deciduous.