Less than 30% of the surnames in the Buenos Aires phone book (looking back to when they last had them) are "Spanish" (from Spain or the various regions in Spain). Over 40% are of Italian origin, with large groups of German, English, Irish, French, Russian and Polish names, as well as a huge Lebanese heritage community.
And that doesn't include the Germans from the migration of 1945-46 who assumed Spanish (or other ethnic) names for obvious reasons!