GARYVILLE, La.—A huge Louisiana refinery was rocked by an explosion and fire Monday. Marathon Petroleum Corp. said six workers suffered minor injuries. Officials later gave the all-clear sign, but it was not immediately clear what caused the morning blast at the Marathon Petroleum Corp. plant in Garyville, between New Orleans and Baton Rouge. Six contract workers were injured, Marathon spokesperson Joe Gannon said in an emailed statement. He said four were treated at the plant and three were treated and released at a health care facility where they were taken as a precaution. “The safety of responders, employees, contractors, and the community are our top priority, and we will conduct a full investigation to determine the cause of the fire,” he wrote. According to Marathon’s website, the facility along the Mississippi River has a refining capacity of 578,000 barrels per calendar day, making it one of the largest in the …