By the time Gregory Allen finished the E'tudes Tableaux of Rachmaninoff that ended his Terrace Theater concert on Saturday afternoon it was quite clear just how and why he had won first prize in the 1980 Arthur Rubinstein Competition.

The tall young man from Cheyenne, Wyo., who plays with his boots on, presented a program that was as welcome for its novelties as for the elegance and mounting excitement of the playing. He began with a set of variations by Leo Smit. That these were written over 30 years ago but were heard for the first time by many of the pianists in Saturday's audience says much about the routine content of too many programs.

Allen played Smit's fascinating variations with probing intellectual and emotional power. Though the following Chopin nocturne and Polonaise-Fantasie held more of the former power than the latter, they were always beautiful.

Scriabin's "White Mass" sonata was supercharged with nuance and tonal splendor, while a Faure' Nocturne and Rachmaninoff group were totally satisfying. As if to show that perfect precision and charming style fit easily together, Allen tossed off the Rachmaninoff version of Mendelssohn's "Midsummer Night's Dream" scherzo in a way that left his listeners breathless.