It is most likely open in another quest. There are many areas that are blocked in specific quests to keep you on target. But on Patrol you can see they are all open (as far as I have seen).
Not 100% certain the one you mention is used in another mission, but that is what my money is on.
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No, you can't get there in any other way but on the Black Garden mission. I've seen it and also wounded what was up with it? It pushes you back and its a huge, light barrier.