  Homogeneous abundances of light elements, alpha and Fe-group elements from
high-resolution FLAMES spectra are presented for 76 red giant stars in M54, a
massive globular cluster (GC) lying in the nucleus of the Sagittarius dwarf
galaxy. We also derived detailed abundances for 27 red giants belonging to the
Sgr nucleus. Our abundances assess the intrinsic metallicity dispersion (~0.19
dex, rms scatter) of M54, with the bulk of stars peaking at [Fe/H]~-1.6 and a
long tail extending to higher metallicities, similar to omega Cen. The spread
in these probable nuclear star clusters exceeds those of most GCs: these
massive clusters are located in a region intermediate between normal GCs and
dwarf galaxies. M54 shows the Na-O anticorrelation, typical signature of GCs,
which is instead absent in the Sgr nucleus. The light elements (Mg, Al, Si)
participating to the high temperature Mg-Al cycle show that the pattern of
(anti)correlations produced by proton-capture reactions in H-burning is clearly
different between the most metal-rich and most metal-poor components in the two
most massive GCs in the Galaxy, confirming early result based on the Na-O
anticorrelation. As in omega Cen, stars affected by most extreme processing,
i.e. showing the signature of more massive polluters, are those of the
metal-rich component. This can be understood if the burst of star formation
giving birth to the metal-rich component was delayed by as much as 10-30 Myr
with respect to the metal-poor one. The evolution of these massive GCs can be
reconciled in the general scenario for the formation of GCs sketched in
Carretta et al.(2010a) taking into account that omega Cen could have already
incorporated the surrounding nucleus of its progenitor and lost the rest of the
hosting galaxy while the two are still observable as distinct components in M54
and the surrounding field.
