For all the lamentations it has spawned this sorry season, Broadway is still capable of astonishing acts. Marsha Norman's " 'night, Mother," an unflinching two-character play about the 90 minutes preceding a young woman's suicide, is perhaps the most astonishing of all. But in a handsomely slaphappy fashion, "My One and Only," the new musical built around the indestructible tunes of George and Ira Gershwin, is an unexpected coup as well.

Opening, as they have, within hailing distance of one another, these two shows are a salutary reminder that Broadway is still capable of intoxicating diversity. The spectrum is not as full as it could be. (Discounting revivals, " 'night, Mother" is one of the few straight plays on Broadway this season with any apparent staying power.) On the other hand, there is a certain daring in both enterprises, even if the risks "My One and Only" is running are primarily those of style, not substance.

In Norman's play, the winner of this year's Pulitzer Prize, the challenge is as blunt as it is courageous. Suicide may be a form of escapism, but it is hardly the sort of escapism Broadway has traditionally promulgated. The curtain has barely risen on " 'night, Mother," however, before Jessie Cates (Kathy Bates) announces in a tone bereft of self-pity--a tone so matter-of-fact that she could be announcing her decision to try a new shade of lipstick--that she has resolved to kill herself in a couple of hours.

Being a methodical person, she has certain preparations to make. The gun that belonged to her deceased father must be fetched from the attic and cleaned. And since her mother (Anne Pitoniak) will soon find herself on her own, the sundry details of running a house must be gone over--where the fuses are kept and what number to call when the washer-dryer goes on the fritz and which days the grocery store delivers. People make similar preparations for a long vacation. Here they are being made for death.

Unlike "K-2," which relegates its life-and-death meditations to an ice-bound ledge on the world's second highest mountain and further exoticizes the subject with its breathcatching special effects, " 'night, Mother" is resolutely domestic. It is a play about a one-time-only routine in a slipcovered suburban home like every other slipcovered suburban home. The mother is not too bright and the daughter has no surprising rationale up her sleeve. Having surveyed the flat landscape of her life, she has simply concluded that it isn't worth further exploration.

The daughter's biography, as it seeps through, is largely unexceptional. Her husband walked out on her some time ago; her teen-age son, like many his age, is a troublemaker. She herself has a history of epilepsy, but as she points out, her memory's back, her gums are no longer swollen and she's feeling as good as she ever has. But somewhere along the line, she lost herself. The person she was supposed to become, or might have become, just didn't show up. She tried life. Suicide is "the other thing I'm trying."

Norman resolutely refuses to tame her subject with easy outs--the fatal illness or humiliating paralysis that is the usual motive in plays of this ilk. "Mama," says Jesse, "I'm just not havin' a very good time," and that's about as philosophical as matters get. At first the mother is incredulous. Then she tries every ploy in her limited arsenal to dissuade her daughter--tears, scorn, pleading, anger. To no avail. With the ritual cup of nightly cocoa, the preparations continue. The daughter even advises her mother how to behave once the shot has been fired. Calls will have to be made, naturally, but then the mother should probably go to the sink and scrub the cocoa pan. Over and over, if need be, until the police arrive. Over and over. It will give her something to do.

The contrast between the enormity of the deed in the making and the mundane preoccupations preceding it generates a colossal theatrical charge. " 'night, Mother" is a play of huge significance, but Norman makes no claims for its importance beyond those inherent in the situation. Her dialogue could be described as representing the leaner end of realism, I suppose, although that does not account for its extraordinary evocative power. Merely by going about its business--as if the business of living were not all that different from the business of dying--it miraculously puts an audience in touch with its own mortality.

The acting is unimpeachably accurate. Pitoniak, long one of the pillars of the Actors' Theater of Louisville, gives us a fussy, silly woman with a frumpy wardrobe and an insatiable sweet tooth. Without sacrificing her flibberty-gibbet ways, she suffers, and through her suffering, acquires a helpless dignity.

Bates is wan, overweight, puffy with fatigue. Yet a primal law of theater prevails: honesty is beauty. By the play's end, Bates is heartrendingly beautiful. Under director Tom Moore's lucid eye--an eye that does not overlook the flashes of humor--both actresses chip away at the layers of humdrum until the humdrum falls away, revealing humanity's elemental emotions.

This is the agony of leave-taking, the pain of bewilderment, the abiding ache of emptiness in an empty world. When, as a parting gesture, Jessie brings out a shoe box of gift-wrapped trinkets--"little presents for whenever you need one"--and offers them to her mother, the only sound in the hushed auditorium is that of tears falling.

"MY ONE and Only" is merely out to raise a few sagging spirits with some Gershwin songs, a tumble of tap dance and the ditsy romance between an English aquacade star with a past and a barn-storming pilot from Texas, who's looking to be the first man to fly the Atlantic nonstop. Although its elements are not what you'd exactly call radical, they have been given enough of a twist to beat the nostalgia charge. Compared to the elephantine "42nd Street," "My One and Only" is familiar in a very fresh way.

To begin with, the show manifests a certain willingness to buck conventional romantic interest by casting 6-foot-6 Tommy Tune as Billy Buck Chandler, the intrepid but bashful pilot; and Twiggy, the 1960s' favorite fashion stick, as Edith Herbert, whom her dastardly Russian impresario describes as "third woman to swim the English Channel, first attractive one." The casting is scarcely the triumph of androgyny you might expect. Twiggy proves to have a most engaging singing voice (it sounds, appropriately, as if it is coming out of a Victrola), a sweetly glamorous presence and the awkward charm that teen-agers often display on the dance floor, but which, in this case, probably stems as much from the startling discrepancy between her height and that of her dancing costar.

You won't quite think Astaire and Rodgers when she and Tune swirl about the stage, their arms extended like glider wings, to "He Loves and She Loves," but you will think appropriately lighthearted, airborne thoughts. Later in the first act, the two find themselves marooned on a desert isle, alone with their budding love and the wreckage of Billy's plane. Set designer Adrienne Lobel has provided them with a shallow pool that extends the length of the stage, and in no time, they're tapping away in the H2O, kicking up a rainbow shower to the strains of "S'Wonderful." It is.

The book by Peter Stone and Timothy S. Mayer is perfectly inconsequential, but it acquits itself of its primary duty, which is to pave the way without too great a lurch for such Gershwin standards as "High Hat," "Strike Up the Band," "Nice Work If You Can Get It," "Funny Face," "How Long Has This Been Goin' On?" and "My One and Only." The title number occasions yet another tap dance--this one sotto voce, if the term may be applied--between Tune and that veteran hoofer Charles "Honi" Coles, who runs a tonsorial parlor and dispenses elegant advice to the lovesick pilot. With sure grace, Cole lays out the steps, and Tune deftly echoes them.

In fact, Tune, who also staged the show with Thommie Walsh, has surrounded himself with top-notch dancers. (When the tale wends its way to Morocco, even the legionnaires tap on in formation, while the harem girls provide syncopation with finger cymbals.) Tune, never fear, can hold his own. What adds that extra cubit to his performance is not just the top hat, although that brings him dangerously close to the 7-foot mark. It's the clear delight he takes in his partners. "My One and Only" went through a hellish pre-Broadway tryout, but following Tune's lead, it exudes only generosity now.

Still, the musical would look just a little archaic were it not for Lobel's high-tech sets--1920s moderne reinvented for the computer age. Mostly in black, white, red and blue--and mostly flat panels that rise from the floor and descend from the rafters--they provide a clean, streamlined environment for the nonsense. The exotic island on which Billy and Edith are momentarily stranded (it turns out to be Staten Island, but no matter) consists of a bright yellow table top, tilted on one of its corners, and a single green palm frond against the deep blue sky.

There's not much for the intellect here, granted. But there's a lot for the eye, including Rita Ryack's spiffy costumes--domino chic by way of early Hollywood. The uncharitable will cry hokum. Let them. "My One and Only" breaks old ground zestfully.

Broadway, of course, wouldn't be Broadway if it were not also capable of astonishments of an entirely different order. Consider "Dance a Little Closer," the Alan Jay Lerner/Charles Strouse musical, which demonstrated the bravery of the reckless by opening--and promptly closing--last Wednesday. An update of Robert Sherwood's "Idiot's Delight," it involved a group of tourists stranded in a posh hotel in the Bavarian Alps on what may have been the outbreak of World War III. One of them (Len Cariou in bad voice) was a cheesy nightclub performer; another (Liz Robertson, whose most distinctive qualification seemed to be the fact that she is Lerner's eighth wife) was the mysterious woman out of his past; a third was a Kissinger knock-off (George Rose, surely kicking himself by now for abandoning "You Can't Take It With You").

In a show that kept hitting one depth after another, perhaps the most mortifying was a maudlin love duet between two young homosexuals, "Why Can't The World Go and Leave Us Alone?" The number, which took place on an ice rink, was sung by two actors who were patently uncomfortable with their skates (or the sentiments), but nonetheless wobbled their way gallantly to the end.

The Kennedy Center was one of the producers of "Dance a Little Closer," which was to have tried out in Washington, until costs became prohibitive. The Center may have dropped a bundle on this one, but it has at least spared itself a hot flush of embarrassment on the home turf. Broadway can take the rap this time.