U2's "Joshua Tree" was an unlikely candidate to become a No. 1 album, the centerpiece of this year's most eagerly awaited tour.

The Irish quartet approached their latest project with the kind of restraint that rarely wins mass adulation. Instead of the crashing climaxes and soaring melodies of their past hits, the band built their new songs around rumbling rhythms and circular guitar figures that always seem on the brink of breaking loose but just barely don't. Bono Hewson's lyrics are elliptical references to a quest that is never quite satisfied.

Fortunately, the band had already won a large, loyal following that was willing to follow them into new territory. "Joshua Tree" rewards such faith with a brilliant song suite that captures the exquisite tension between idealism and realism. The quest may be spiritual (as on "I Still Haven't Found What I'm Looking For"), romantic ("With or Without You") or political ("Red Hill Mining Town"), but the irony is the same in every instance.

The many references to America, a country where ideals and reality collide with special force, only help the songs. Hewson, perhaps the best rock singer of his generation, does his best work ever here, but the record's mood owes just as much to the inspired instrumental trio that backs him. Well worth searching out are the album's singles, which each contain two non-LP songs that fit into the same suite.

Also of interest to U2 fans: the soundtrack album to the film "Captive." U2 guitarist Dave "The Edge" Evans co-wrote and co-performed all the tracks with British keyboardist Michael Brook. The result is moody instrumental music marked by Evans' circular guitar figures. The only song that really sounds like U2, though, is the one vocal number, "Heroine," which features an Annie Lennox-like vocal by Sinead O'Connor and powerful drumming by U2's Larry Mullen. U2 --

"Joshua Tree" (Island 90581-1).

THE EDGE & MICHAEL BROOK --

"Captive: Original Motion Picture Soundtrack" (Virgin 7 90609-1).

U2 appears Sunday with Los Lobos at RFK Stadium.