We　describe　a　scheme　for　the　generation　of　macroscopic　quantum-interference　states　for　a　collection　of　trapped　ions　by　a　single　geometric　phase　operation.　In　the　scheme　the　vibrational　mode　is　displaced　along　a　circle　with　the　radius　proportional　to　the　number　of　ions　in　a　certain　ground　electronic　state.　For　a　given　interaction　time,　the　vibrational　mode　returns　to　the　original　state,　and　the　ionic　system　acquires　a　geometric　phase　proportional　to　the　area　of　the　circle,　evolving　from　a　coherent　state　to　a　superposition　of　two　coherent　states.　The　ions　undergo　no　electronic　transitions　during　the　operation.　Taking　advantage　of　the　inherent　fault-tolerant　feature　of　the　geometric　operation,　our　scheme　is　robust　against　decoherence.　©　2010　Chinese　Physical　Society　and　IOP　Publishing　Ltd.