  An analysis of globular cluster systems associated with galaxies included in
the Virgo and Fornax HST Advanced Camera Surveys reveals distinct (g-z) colour
modulation patterns. These features appear on composite samples of globular
clusters and, most evidently, in galaxies with absolute magnitudes Mg in the
range from -20.2 to -19.2. These colour modulations are also detectable on some
samples of globular clusters in the central galaxies NGC 1399 and NGC 4486 (and
confirmed on data sets obtained with different instruments and photometric
systems), as well as in other bright galaxies in these clusters. After
discarding field contamination, photometric errors and statistical effects, we
conclude that these supra-galactic colour patterns are real and reflect some
previously unknown characteristic. These features suggest that the globular
cluster formation process was not entirely stochastic but included a fraction
of clusters that formed in a rather synchronized fashion over large spatial
scales, and in a tentative time lapse of about 1.5 Gy at redshifts z between 2
and 4. We speculate that the putative mechanism leading to that synchronism may
be associated with large scale feedback effects connected with violent star
forming events and/or with super massive black holes.
