  We investigate geometric phase of fermion states under relative vibrations of
two sublattices in graphene by solving time-dependent Sch\"{o}dinger equation
using Floquet scheme. In a period of vibration the fermions acquire different
geometric phases depending on their momenta. There are two regions in the
momentum space: the adiabatic region where the geometric phase can be
approximated by the Berry phase and the chaotic region where the geometric
phase drastically fluctuates in changing parameters. The energy of fermions due
to vibrations shows spikes in the chaotic region. The results suggest a
possible dephasing mechanism which may cause classical-like transport
properties in graphene.
