  The halo and disc globular cluster population can be used as a tracer of the
primordial epochs of the Milky Way formation. In this work, literature data of
globular clusters ages, chemical abundances, and structural parameters are
studied, explicitly focussing on the origin of the known split in the
age-metallicity relation of globular clusters. When the alpha-element
abundances, which are less strongly affected by the internal light-element
spread of globular clusters (Si, Ca), are considered, a very low observational
scatter among metal-poor clusters is observed. A plateau at [SiCa/Fe]~0.35 dex,
with a dispersion of only 0.05 dex is observed up to a metallicity of about
-0.75 dex. Only a few metal-poor clusters in this metallicity interval present
low [SiCa/Fe] abundances. Moreover, metal-rich globular clusters show a knee in
the [alpha/Fe] versus [Fe/H] plane around [Fe/H] -0.75 dex. As a consequence,
if a substantial fraction of galactic globular clusters has an external origin,
they have to be mainly formed either in galaxies that are massive enough to
ensure high levels of [alpha/Fe] element abundances even at intermediate
metallicity, or in lower mass dwarf galaxies accreted by the Milky Way in their
early phases of formation. Finally, clusters in the metal-poor branch of the
AMR present an anti-correlation of [SiCa/Fe] with the total cluster magnitude,
while this is not the case for metal-rich branch clusters. In addition, this
lack of faint high-alpha clusters in the young metal-poor population is in
contrast with what is observed for old and more metal-poor clusters, possibly
reflecting a higher heterogeneity of formation environments at lower
metallicity. Accretion of high-mass satellites, as a major contribution to the
current Milky Way globular cluster system both in the metal-poor and the
metal-intermediate regime is compatible with the observations.
