  We investigate a quarter-filled two-band Hubbard model involving a
crystal-field splitting, which lifts the orbital degeneracy as well as an
inter-orbital hopping (inter-band hybridization). Both terms are relevant to
the realistic description of correlated materials such as transition-metal
oxides. The nature of the Mott metal-insulator transition is clarified and is
found to depend on the magnitude of the crystal-field splitting. At large
values of the splitting, a transition from a two-band to a one-band metal is
first found as the on-site repulsion is increased and is followed by a Mott
transition for the remaining band, which follows the single-band
(Brinkman-Rice) scenario well documented previously within dynamical mean-field
theory. At small values of the crystal-field splitting, a direct transition
from a two-band metal to a Mott insulator with partial orbital polarization is
found, which takes place simultaneously for both orbitals. This transition is
characterized by a vanishing of the quasiparticle weight for the majority
orbital but has a first-order character for the minority orbital. It is pointed
out that finite-temperature effects may easily turn the metallic regime into a
bad metal close to the orbital polarization transition in the metallic phase.
