Evan Turner, director of the 100-year-old Philadelphia Museum of Art, yesterday astonished his staff by submitting his resignation, effective June 30.

"He stunned us," says one curator, "and his timing was exquisite."

Turner, 49, issued his announcement at a meeting called to thank the staff for the successful completion of the museum's $9 million Centennial program. The last phase of the project --new American art wing -- took place yesterday.

Turner, who told his staff he was leaving to complete a book on Thomas Eakins, the Philadelphia painter, came to the museum 13 years ago. "As you can imagine, this place is splintering with rumors, but there is no trace of scandal," the curator observed.

Under Turner's directorship, the museum has opened new departments of American and 20th-century art, and established the Alfred Stieglitz Center for the study of photography. In the past two years, the building has been air-conditioned and all of its exhibits have been reinstalled.

Turner was trained at Harvard where he wrote his doctoral thesis on his namesake, the English painter William Joseph Mallord Turner. He came to Philadelphia from Montreal, where he was one of the youngest museum directors on the continent. In his letter of resignation he noted the museum "is at a major turning point in its history. . . This is an appropriate time for me to resign." No successor has been named.