Context :

Aims : We investigate the colour-magnitude relation of metal-poor globular clusters , the so called blue tilt , in the Hydra and Centaurus galaxy clusters and constrain the primordial conditions for star cluster self-enrichment .

Methods : We analyse U , I photometry for about 2500 globular clusters in the central regions of Hydra and Centaurus , based on FORS1 @ VLT data .
We measure the relation between mean colour and luminosity for the blue and red subpopulation of the globular cluster samples .
We convert these relations into mass-metallicity space and compare the obtained GC mass-metallicity relation with predictions from the star cluster self-enrichment model by Bailin & Harris ( 2009 ) .
For this we include effects of dynamical and stellar evolution and a physically well motivated primordial mass-radius scaling .

Results : We obtain a mass-metallicity scaling of Z \propto M ^ { 0.27 \pm 0.05 } for Centaurus GCs and Z \propto M ^ { 0.40 \pm 0.06 } for Hydra GCs , consistent with the range of observed relations in other environments .
We find that the GC mass-metallicity relation already sets in at present-day masses of a few 10 ^ { 5 } M _ { \odot } and is well established in the luminosity range of massive MW clusters like omega Centauri .
The inclusion of a primordial mass-radius scaling of star clusters significantly improves the fit of the self-enrichment model to the data .
The self-enrichment model accurately reproduces the observed relations for average primordial half-light radii r _ { h } \sim 1 - 1.5 pc , star formation efficiencies f _ { \star } \sim 0.3 - 0.4 , and pre-enrichment levels of [ Fe / H ] ~ { } -1.7 dex .
The slightly steeper blue tilt for Hydra can be explained either by a \sim 30 % smaller average r _ { h } at fixed f _ { \star } \sim 0.3 , or analogously by a \sim 20 % smaller f _ { \star } at fixed r _ { h } \sim 1.5 pc Within the self-enrichment scenario , the observed blue tilt implies a correlation between GC mass and width of the stellar metallicity distribution .
We find that this implied correlation matches the trend of width with GC mass measured in Galactic GCs , including extreme cases like Omega Cen and M54 .

Conclusions : First , we found that a primordial star cluster mass-radius relation provides a significant improvement to the self-enrichment model fits .
Second we show that broadened metallicity distributions as found in some massive MW globular clusters may have arisen naturally from self-enrichment processes , without the need of a dwarf galaxy progenitor .