Fresh off their Grammy win for 2013 Album of the Year, Marcus Mumford and Winston Marshall of British folk outfit Mumford & Sons begin a two night stand at the Patriot Center in Fairfax, VA. (Kyle Gustafson/FOR THE WASHINGTON POST)

Pop music’s annals teem with odd and short-lived trends; yes, kids, there really was a time when disco and wimp rock were unironically cool to lots of folks. Yet the recent epic-folk binge may one day rank among the oddest sonic booms ever.

Combos that rely on the sorts of instrumentation and arrangements that had been relegated to mountain hootenannies and European pubs for the past couple of centuries, or at least since Dylan went electric, are now filling basketball arenas.

On Wednesday, Mumford & Sons, a British quartet that is leader of this unplugged brigade and the main reason you’re more likely to hear a banjo on rock radio than a country station these days, played the first of two sold-out shows at the Patriot Center.

The band came to town fresh from winning album of the year at Sunday’s Grammy Awards for its second and most-recent record, “Babel.”

The industry doesn’t quite know what to make of this act just yet, however: The Mumfords were also nominated in two rock categories, although the group’s most popular tunes are a mishmash of Celtic and hillbilly styles, played almost entirely on acoustic instruments, and have more to do with “Puff the Magic Dragon” than “Smoke on the Water.”

Not that there’s an absence of electricity at a Mumford & Sons show. Early in the 100-minute set, on the radio smash “Little Lion Man,” Marcus Mumford, the frontman (and only member actually surnamed “Mumford”), wowed the crowd by simultaneously strumming a flattop guitar, working kick drum and tambourine pedals with separate feet, and bellowing the tune’s mostly romantic lyrics in an aching voice.

Although he appeared completely sober, Mumford intentionally slurred the song’s profane chorus, turning the f-word into the fffff-word, as an angry drunk at last call might. “Below My Feet” exuded the energy of the best gospel tunes and would have fit nicely on another Grammy album of the year winner: the “O Brother, Where Art Thou” collection, which won in 2001.

Speaking of movie music: The chart-busting “I Will Wait” and deep-album cut “White Blank Page,” a breakup song with its super-sappy lyrics (“Tell me now where was my fault / In loving you with my whole heart?”) and a Springsteenish “aaaah-ah-ah-ah-ah” coda that made the audience swoon, were among many Mumfords tunes played on this night that, no matter how long the epic-folk genre remains trendy, should eventually find homes on chick-flick soundtracks.

For the encore, the Mumfords set up shop around a single microphone, as an old-timey bluegrass band might, and tried an a cappella version of an early melancholy original, “Sister.”

Fans all around the venue, who had screamed with joy for every finger-picked note to that point, started clapping rhythmically during the first verse.

The unsolicited percussion brought Marcus Mumford to the brink of a Joni Mitchellesque meltdown, and he waved his hands frantically until the worshipers stopped disturbing his art. O brother.

McKenna is a freelance writer.