* France launched Mali military offensive in January
* U.N. peacekeeping force to be made up of African troops
* Peacekeeping force will be U.N.'s third largest (Adds detail on U.N. special envoy for Mali final paragraph)
By Michelle Nichols
UNITED NATIONS, April 25 (Reuters) - The U.N. Security Council unanimously approved on Thursday the creation of a 12,600-strong peacekeeping ...