We describe a scheme for the formation of globular cluster (GC) systems employing a semi-analytic model of galaxy formation. We assume that metal-poor GCs are formed in gaseous proto-Galactic fragments at high redshift (z>5) and that the merging of these fragments leads the formation of metal-rich GCs. We find that such continued gaseous merging leads to significant age-structure amongst the metal-rich GCs. We suggest that the formation of the metal-poor GCs is closely related to the star formation rate in the proto-Galactic fragments, which may have been significantly higher at early times.