A 51-year-old Peabody man died after being injured in an explosion at a Dow Chemical Co. plant in North Andover on Wednesday, the Essex district attorney’s office said. According to state Fire Marshal Stephen Coan, the unidentified man was flown to a hospital, where he died overnight. The explosion caused a fire, which was contained by the plant’s sprinkler system, Coan said. Other workers escaped unharmed, according to the district attorney’s office. Coan said the preliminary determination was that the blast was an accident.