The singer died Monday at his home in Detroit as a result of heart failure ... according to his family.

Duke's family tells his hometown newspaper, the Detroit Free Press ... "Our hearts are heavy as we mourn the loss of a trailblazer, icon and music legend who, through his 70-year music career, touched the lives of so many."

Duke enjoyed a 70-year career in music before retiring earlier this year, and the family adds ... "We find solace in Duke’s legacy living on through his music for generations to come."

Duke sang first tenor for the Four Tops with a high, smooth voice ... a complement to baritone lead vocalist Levi Stubbs.

The Four Tops was inducted into the Rock & Roll Hall of Fame way back in 1990 ... on the strength of being one of the defining groups of Motown.

With Duke on the mic, the Four Tops cranked out a ton of hits ... including "I Can't Help Myself (Sugar Pie Honey Bunch)" and "Reach Out I'll Be There" ... plus "Bernadette," "It's The Same Old Song," "Standing In The Shadows of Love" and "When She Was My Girl."

The Four Tops saw three of their founding members die between 1997 and 2005 ... leaving Duke as the last OG standing.

He was preceded in death by Lawrence Payton, Obie Benson and Levi ... all of whom he had known since their teenage years growing up in Detroit.

Despite the loss of his founding group mates, Duke kept the Four Tops name alive as long as he could ... bringing in new members and continuing to perform live.

Duke was 88.