Unpublishing edited posts

Hi!
I am aware of the existence of the Contributor role, who can write posts but not publish them. And the Author, who can publish posts and edit them as well. I need something in-between.
Let’s say we have an author with a published post and that he edits it.
I need a system to render edited posts unpublished, so that the admins can review them after the modified content goes live.
Does anybody know whether there is there a plugin (or some code addition in the theme’s functions.php) that would make this happen?
The need for this arises because I want admins to give users ownership of posts created from the admins themselves, in order to let them edit, but I want to be able to moderate the edits.
Cheers!
Alex