I believe this is among the tongue and wreath buckles that were commercially available, popular with gold miners in the 1850's. The vast majority are found in California or Australia (again, in mining regions). While its possible that a militia member might wear one they haven't been attributed to any in particular.
-but I can always be wrong.
nhbenz and DCMatt are exactly right. Great find by the way. Also I noticed that DCMatt is from Herndon, Virginia my oldest daughter lives in Oakton, Virginia....small world.