It is most definitely a crossguard from a sword. Judging by it's size in relation to that quarter I'd say it's ceremonial sword. Two fraternal societies immediately spring to mind, both are keenly into ceremonial swords: Freemasons and Knights of Pythias. Both also have a triangle as a prominent symbol.
I am guessing it's Masonic. What convinces me are the two obvious pillars on the sides with the arch above. If the arch is presumed to be "up" (towards the blade) then the triangle is upright. This kind of rules out KoP because their triangle is inverted. As for the symbol on the back I'm not too sure. It could be a crude /worn version of the all seeing eye typically illustrated inside of the triangle or it could also be a temple entrance which is also commonly illustrated with the pillars. This would make sense considering what appears to be steps below it.
Still a cool find.
P.S. One can only guess how just this part of the sword got where it did in order for you to find it and i assume you did not find the blade with it.If the whole thing is there in the ground (in pieces) It might be worth returning to that spot and trying to detect with no discrimination. The blade would have been steel. One more piece worth looking for would be the pommel cap. It would be a heavy and ornate brass piece roughly the size of that quarter that went on the butt end of the handle.
