There's something unsettling about Colagens, the first album from Sao Paolo-based saxophonist Marcelo Coelho's MC4+ group. On the surface, it's a simple enough affair: a quartet with guest trombonists running through a series of original tunes that have wide-open harmonies and shifting grooves. There's only one track on the album that could be described as up-tempo, and even that is hardly a sprint from start to finish. If you merely skimmed over the record, you might have the impression of a laid-back session.

Yet far from being content to lounge, Colagens is a record of agitation. As the music begins to feel comfortable, melodic lines glance off one another, the rhythm lags then leaps, the harmony refuses to resolve itself in a typical way. Much of this restless energy sprouts from guitarist Lupa Santiago, whose playing has a diffuse texture that creates a kind of harmonic hovering. The music moves forward, but part of it hangs back for a second, content to move at its own pace.

On "Tormenta," a track that owes a debt to the Dave Holland Quintet, the sharpness of Vincent Gardner's trombone and Coelho's sax plays off of the ethereality of Santiago's guitar. For much of the tune, Gardiner and Coelho engage in a staccato spar as drummer Carlos Ezequiel taps out sparse support. If it weren't for the differences in style between Coelho and Chris Potter—Coelho's sax is warmer; Potter's more dexterous—parts of the track would sound a lot like the title track on the Dave Holland Quintet's Prime Directive (ECM, 2000). When Santiago and bassist Guto Brambilla enter, the tone of the music transforms—the lines lengthen, and even though Ezequiel continues to tap hard and fast, everything feels a little slower, less jittery.

The textural contrast between the horns and the rhythm section—especially Santiago's guitar—defines many of the pieces on Colagens. Even as Santiago plays single-note melodic lines on the weaving improvisations of the title track, his presence marks a contrast with Coelho and trombonist Paulinho Malheiros (Gardner and Malheiros share trombone duty throughout the album).

"S.U.Y.E," the most ballad-like number on the album, showcases Coelho's preference for polyrhythmic restlessness over easy swoons. During his solo, Coelho hits upon a melodic phrase that echoes Gershwin's "Embraceable You," but instead of digging in, he drops back, leaving us with only a bare-bones sketch of a lush melody.

Coelho's decision to end Colagens with a restrained duet between him and Santiago, "Madrugada," is a masterstroke. By the end of this busy album of clashes and inventions, we're ready for something quieter, and Coelho gives us a delicate dialogue that is romantic without ever dipping into sap.

Colagens is a work of creativity and vigor. At times, the openness of the harmonies, and the music's refusal to stay in one place for very long, prove frustrating—demanding serious listening before they can be appreciated. Yet a record of surprises and tremendous musicianship should be celebrated, and Colagens has plenty of both.