HAWTHORNE — A mortar shell explosion killed at least seven Marines and injured several more during mountain warfare training in Nevada’s high desert, prompting the Pentagon to immediately halt the use of the weapons until an investigation can determine their safety, officials said Tuesday. The explosion occurred Monday night at the Hawthorne Army Depot, a sprawling facility used by troops heading overseas, during an exercise involving the 2nd Marine Expeditionary Force from Camp Lejeune, N.C. Several Marines from the unit were injured in the blast, authorities said. The mortar round exploded in its firing tube during the exercise, Brig. Gen. …