Nathan Morton, who directed CompUSA's growth into the nation's largest chain of computer superstores, quit as chairman and chief executive. The news surprised investors. CompUSA shares tumbled 9%, closing down $2.125 at $20.375 on the NYSE. The company did not explain Morton's departure, and he did not return phone calls seeking comment. Its board named President James Halpin as chief executive and outside director Giles Bateman as chairman. Morton joined the Dallas-based firm in May, 1989.