How do I keep and gain muscle on a low calorie diet?

I am an 18 years old,male,5'8,and 325 pounds I've been on a diet for the last 5 weeks and have lost 15 pounds.I take in 1200-1500 calories a day and lift weights 3 days a week unfortunately I've lost muscle.Does anyone have suggestions on what I can do to keep and gain muscle on a low calorie diet?