Combine first 5 ingredients in a small saucepan. Bring to a boil; cook 1 minute. Remove from heat. Gently stir in tangelo and orange sections. Cool completely.
Brush both sides of cake slices with margarine. Place a large nonstick skillet over medium-high heat until hot. Add cake slices; cook 2 minutes on each side or until lightly browned. Cut each cake slice in half diagonally. Serve cake with fruit mixture. Garnish with mint sprigs, if desired.