  Globular cluster systems in most large galaxies display bimodal color and
metallicity distributions, which are frequently interpreted as indicating two
distinct modes of cluster formation. The metal-rich (red) and metal-poor (blue)
clusters have systematically different locations and kinematics in their host
galaxies. However, the red and blue clusters have similar internal properties,
such as the masses, sizes, and ages. It is therefore interesting to explore
whether both metal-rich and metal-poor clusters could form by a common
mechanism and still be consistent with the bimodal distribution. We show that
if all globular clusters form only during mergers of massive gas-rich
protogalactic disks, their metallicity distribution could be statistically
consistent with that of the Galactic globulars. We take galaxy assembly history
from cosmological dark matter simulations and couple it with the observed
scaling relations for the amount of cold gas available for star formation. In
the best-fit model, early mergers of smaller hosts create exclusively blue
clusters, whereas subsequent mergers of progenitor galaxies with a range of
masses create both red and blue clusters. Thus bimodality arises naturally as
the result of a small number of late massive merger events. We calculate the
cluster mass loss, including the effects of two-body scattering and stellar
evolution, and find that more blue clusters than red clusters are disrupted by
the present time, because of their smaller initial masses and larger ages. The
present-day mass function in the best-fit model is consistent with the Galactic
distribution. However, the spatial distribution of model clusters is much more
extended than observed and is independent of the parameters of our model.
