I'd just say it was time for you to move on, you're happy to have been part of the team, but you know that the new person will do a great job. I don't think you owe anything long.
I agree with domerjen. If your goal is no-drama and informational, then that's the email to send. It's also your best chance for not getting a lot of "OMG are you okay?" Calls/emails. Although nothing can eliminate it completely.