Without a picture it is hard to say, but it could be winter damage or what we call Southwest Injury. This can cause the trunk to split or crack open and then the bark will peel away from that area. If you can attach photos here, try it and I will take a look. If not, you could take digital photos of the tree and take them into your county extension office or email them to the office and they can have the horticulture specialist that serves your county look at them.The big, black ants are usually of no concern. They typically do not do any damage to a plant. Some plant's flower produce a sweet nectar that the ants like, so that's why you often see ants on flowering plants.