arm chair lawyer time: I seriously doubt he had any legal right to pursue Trayvon to detain/question him, even if he caught Trayvon in the act of a crime.
You can follow someone, and I do not know when in America it became illegal to approach someone and ask them what they are up to. Neighborhood watch usually are to be vigilant and if they see something call it in to the police, they are indeed not police themselves.
Would Martin have to answer to Zimmerman when approached / asked what he was doing? no- he could have told the "creepy ass cracker" to leave him alone. I seriously doubt that is how it happened however- and this is all speculation and personal thoughts- but I am fairly certain that Martin started the aggression in this situation.
This witness that was on the phone with him "in the end" is terribad- the news spin on what she has said is so bad- listen / watch (if you can bear more than a few moments of it) her actual testimony- oh my goodness.
As for detain Martin, no Zimmerman I do not believe had any right to detain him... question him? I think anyone can question anyone at any time- hi how are you? hey are you going to steal my bread? and what is pursue, I can follow whomever I want around all day as long as they do not go into a restricted area I am not allowed to go.