FOR A WHILE now most of the real creative energy in American popular misic has been pulsing out of Nashville. And all of a sudden people are starting to notice. Not only have Willie Nelson and Dolly Parton exploded into national stardom on their own (that is, without special benefit of television), but they have also influenced a whole school of rock artists from Linda Ronstadt to groups like the Eagles and Kansas, so that the general drift of popular music today seems to be more and more in the direction of country.

And at just this moment country music itself is also moving through a major metamorphosis, one that began way back in the '60s with Mickey Newbury, John D. Loudermilk, and Kris Kristofferson and continues today with young talents such as Lee Clayton, Guy Clark, and Sonny Throckmorton. Country lyrics by these younger writers are dealing with a wider range of emotions, exploring a greater number of situations than ever before. They are writing songs about stuff that Kitty Wells never even heard about in school.

There is a wacky, outrageous quality to much of the new material. Younger writers, such as Paul Craft, are satirizing all that the traditional country audience holds dear. You would know him, if at all, as the author of that off-the-wall bit of nonsense titled "Dropkick Me, Jesus, Through the Goalposts of Life." Well, he also wrote another one, nearly as improbable as country material, called "It's Me Again, Margaret," about an obscene phone caller who, when at last apprehended by the police, directs his single, alloted call to his favorite target for sexual harrassment, Margaret. Or, finally, what about the song that Craft addressed to Linda Lovelace, star of Deep Throat ? (I'm sure I couldn't get its title past the editors of this family newspasper, so I won't even try.) But does that sound to you like something Roy Acuff and the Smoky Mountain Boys might record? No? Well, that's just the point.

You can, admittedly, listen to country music's top-10 singles week after week and not notice much of a change. The earlier-mentioned Sonny Throckmorton, for example, is about the hottest songwriter in Nashville right now. He wrote a song called "Knee Deep in Loving You" that made it to number-one country a while back on a recording by Dave and Sugar. "Knee Deep" is nice enough, bright and spritely with a straight-ahead beat, but it could have been written by any one of a dozen established song writers in Nashville a decade ago. You can, however, get an idea of what it is that makes Sonny Throckmorton a lot different from that same dozen of 10 years back with his new album, The Last Cheater's Waltz (Mercury SRM-1-3736). On it, you'll find not just his chart-contenders, but also the more personal, venturesome numbers which have won him his reputation for real quality.

This latter material is what Sonny Throckmorton is all about. Listen to him wail out the wistful lyrics of the album's title track in his Neil Young-like voice, and you may well suspect that he has written country music's definitive "cheatin' song," one a little more bitter and a lot closer to the bone than the thousand or so that preceded it. His "Middle Age Crazy" is a touching portrait of an ordinary man in mid-life crisis, "40 years old, going on 20." There is in this none of the jeering, baiting tone that was so often heard in rock lyrics treating the same material. Throckmorton simply sings with subtle sympathy of one who is "middle age crazy, trying to prove he still can."

And speaking of that, there is a good deal of subtlety of a different sort in a country-chart song that Throckmorton wrote in collaboration with Glenn Martin called, "If We're Not in Love by Monday." The chord progression is a lot more complex and interesting on this one than is usual in country, and it has sophisticated lyrics to match. It is the kind of song that would fit very well into the repertoire of a club singer such as Nancy Wilson or Marlena Shaw.

Good as this album is, you shouldn't get the idea that The Last Cheater's Waltz is the only one of its kind coming out of Nashville today, nor that Sonny Throckmorton is somehow singlehandedly changing things there. No, he is, whether he knows it or not, part of a movement. You could call him one of country's avant-garde if music as commercial as it is were not inimicable to the very notion of an avant-garde. So maybe it's more a matter of a new generation taking over. In this way maybe country is growing up. Maybe this younger generation of country writers is showing greater maturity than the older one over did. There is something in their work even for those who may have turned a deaf ear to the music before.