Where does a '60s singer-songwriter turn for inspiration after exhausting her own muse over the course of three decades? For Joni Mitchell, the answer is to explore the rich catalogue of classic popular songs that dominated American music from the late '30s to the early '50s. They are the core of her new album, "Both Sides Now," and they were the heart of Mitchell's concert Thursday night at Merriweather Post Pavilion.

At the start, Mitchell promised "a musical adventure through romance." She delivered a program of cinematic proportions, underscored by a 70-piece orchestra whose lengthy overture--full of melancholy strains and subtle shadings--defined the emotional parameters of the material.

Mitchell, dressed in a blue satin gown, opened with the infatuated exuberance of "You're My Thrill" and ecstatic release of "At Last" before delivering the triumphant concession of "Comes Love." The latter song slyly explored the notion that when it comes to love, "nothing can be done," and Mitchell's bluesy bent notes suggested she wouldn't have it any other way.

She followed with a catalogue of the heart's inevitable aches: the sullen recognition that "You've Changed" and the dismissed entreaties, "Answer Me, My Love" and "Don't Go to Strangers," the bittersweet intoxication of Mitchell's own "A Case of You" and the false post-breakup bravado of "Don't Worry 'Bout Me." The turbulence of romance was suggested in a brilliantly sketched "Stormy Weather," while the heart's resilience powered a bouncy, irony-tinged "I Wish I Was in Love Again."

Mitchell ended the night's romantic journey with a luminous reading of her 30-year-old classic, "Both Sides Now." And as she sang the familiar lyrics--"Something's lost and something's gained from living every day"--it was clear Mitchell's hard-won perspective enriches both her own ruminations and the melancholy masterpieces she's now mastered.

After decades of unrepentant smoking, Mitchell's voice is deeper, rougher, ravaged, but she uses it to wonderful effect on the more confessional songs. There were moments when Mitchell was tentative, or rhythmically unanchored. And sometimes the orchestra overwhelmed her. But conductor Vince Mendoza's arrangements were brilliant, particularly in a closing segment of originals that ranged from Gil Evans-style brassiness ("Be Cool" and "Hejira") and art song ("Ludwig's Tune") to the reflective "For the Roses" and a sassily funkified cover of Marvin Gaye's "Trouble Man." Mitchell got a standing ovation at both ends of the concert, suggesting fans' acceptance of these new avenues of exploration.