That large cast aluminum device forms the upper half of the gear housing(as you can probably already see), and there are two steel alignment pins in it that I have to assume are used to help orient the lower gear housing. You can see the end of one of them right there adjacent to the blown out piece. It's possible that the pin began to swell with corrosion and that led to the failure you see there, but that's just speculation on my part. Since the broken off section is no longer serving any useful purpose, I'd try pulling it off to get a look at that pin.
There should be as much material behind the pin as there was in front of it, so unless it's blown out too you may not have an oil leak here. But if it was my machine I'd want to disassemble things enough to make sure there were no cracks that went all the way through. If they aren't available(though I suspect they are)gaskets can be made without too much trouble, and then you'd have a good opportunity to change the oil as well.