What species of tree is this (found in Navarre, Spain, many tiny flowers)?
I believe you are looking at Box Elder, Acer negundo.  It's a North American species but according to Wikipedia it is naturalized in Europe.  If you search online for images, you will see the leaves and flower structures in your own pictures.  Specimens I have seen in my area (Indiana, USA) are very similar to your pictures, even identical, and the trunk is gnarly, as you observed.