The Gabriel Alegria Sextet enlivened and intrigued the audience at The Seasons Saturday night. The group of five young Peruvians and a South African meld strains from American, Peruvian and African music into a sophisticated hybrid with which they are writing a new chapter in the history of Latin jazz. Since earning an advanced degree in music from the University of Southern California, Alegria has spent several years refining his concept of Afro-Peruvian music. His fluency as a trumpet player is matched by his skills as a composer, arranger and leader. He has assembled a band of kindred spirits whose joy in performing seduces his listeners to receive with enthusiasm music that is often as challenging as any in the most adventurous modern jazz.