I don't offhand, but you can find out yourself by placing blocks side-by-side, starting from each outer edge. Block on the left, block on the right, repeat until they meet and you have your center on one axis (X or Z). Mark it, delete everything else, and repeat the trick with the other two sides to get the center on the other axis.
Or you could use coliseum walls and be done it it in seconds, i really think being a couple tenths of a world-unit of wont matter. In fact it may be easier to eyeball it as well since the exact center of a non-defined shape is ridiculously hard to find accurately, even more so because its 3D. The math equation would have like seventy different co-ordinates alone if you wanted any semblance of accuracy, and i dont want to think about how long it would take to factor all those together. Really, just doing what looks right is your safest bet here......
Basically eyeball what you think is exact center, then line up some 5x5 until you hit the edges and see what needs adjusting. What you can do to mark the center is to place down a golf hole, with the flag in the center point.
quarry midpoint I was wondering this myself, so I put together a quick measurement tool, if someone wants to improve on it. The middle coordinates, as best I can figure, are: x - 62.8 & y - 213.3