It was a surprise, looking up on stage and seeing the familiar angular face, watching the wide mouth open and hearing jazz-scatting come out. But it was indisputably Joni Mitchell up there and it was jazz she was singing.

Mitchell first performed the new songs she had written with Charles Mingus about a year ago, at a big, open-air concert in Berkeley to benefit Bread and Roses, a charitable group. It was night time and the fog swirled above the theatre, floodlights snatching the vapors near the stage where Mitchell sang and Herbie Hancock played piano.

As Mitchell went into a dreamy, forties-style lyric about Manhattan, a prop-driven plane droned through the fog. It was such a perfect touch, I suspected the promoters of hiring the plane. I looked around, half-expecting to see the casually dressed audience transformed, the men in evening dress, the women with gardenias in their hair.

Joni Mitchell's newest album, Mingus (Asylum 5R-505), is a collection of songs written with the late jazz bassist and composer and inspired by him. Mingus died in Mexico early this year at 56, before the collaboration he initiated could be completed; but the six songs, snippets of taped interviews with Mingus, and the bold, colorful paintings that wrap this package are evidence that the unorthodox pairing worked.

Mingus is the first Joni Mitchell album I've liked since the first one, back in 1968, that introduced her own versions of the songs already made popular by Judy Collins and other performers. As the lank-haired archetype for art students everywhere, the rock groupie whose relentlessly autobiographical songs told more than I ever wanted to know about life and love among the cocaine and Jack Daniels set and, finally the jaded international artiste, Mitchell's subsequent ventures onto vinyl made me cringe. She was, I thought, a female Woody Allen - not in her performing style or appearance, but in her calculating solipsism and her knack for making surface descriptions sound profound.

So when I heard Mitchell was working with Mingus on an album, I expected it to be her Interiors - bleak, ponderous and ultimately flat-footed. But it's not, it's her Annie Hall - occasionally self-conscious, but otherwise warm and intelligent, with a wit and elasticity all the more affecting for being unexpected.

In Mingus, Mitchell has done a very difficult thing. She's taken the differences in age, race, gender, nationality and class (Mitchell, as her songs on earlier albums remind us, is rich; Mingus never was) between Mingus and herself and used them in counterpoint, to establish herself as both a unique observer and participant in his life.

In perhaps the album's finest moment, Mitchell takes "Goodbye Pork Pie Hat," one of Mingus' best known compositions, and turns it into a commentary on the life and times of Mingus, the late saxophonist Lester Young (for whom Mingus wrote the music) and herself. She does this by contrasting the racism that drove Young and Mingus underground as young black musicians and the heady openness with which she and Mingus embrace in a racing New York night.

And she does it in jazz time, her voice supple and expressive, her specially written lyrics evoking the big city energy that inspired the older men: "We came up from the subway / On the music midnight makes / To Charlie's bass and Lester's saxophone / In taxi horns and brakes."

Mitchell expands the song into a tribute to both Young, the original subject, and Mingus, the original composer, making it her own without doing violence to the generous spirit of the original work. It's a moving performance she can be proud of.

There are other bright moments on Mingus - Mitchell's hilarious lyrics to "The Dry Cleaner from Des Moines," her chilling rendition of "The Wolf that Lives in Lindsey." Her lyrics are vivid and uncharacteristically unsentimental, most of the time. Her singing is fluid, with an occasional affecting break in her voice reminiscent of Billie Holliday. An all-star band of jazz-fusion players, including Hancock and Weather Report's Wayner [sic] Shorter and Jaco Pastorius, provides added texture and nuance.

Mitchell plans to tour the country late this summer with some of these same musicians, giving fans who cherish her folk and rock hits a chance to hear her speak in the rich vocabulary of jazz. It's an adventuresome move, and I hope it succeeds. Joni Mitchell could easily go on playing "Both Sides Now" and "Woodstock" for the rest of her life, never changing a note, and be hugely popular.

Instead, she's chosen to grow. For encouraging that decision, she, and we, can thank an extraordinary musician who left us a vibrant musical legacy. His name, of course, is Mingus.