  We present a detailed chemical analysis of 22 stars along the
colour-magnitude sequence of the intermediate-age (1.7 Gyr) open cluster IC
4651. We find for the cluster a well-defined Fe abundance of [Fe/H]= 0.10 +/-
0.03 (internal errors), with a reddening E(b-y)=0.091. We also derive
abundances for the alpha elements Mg, Si, Ti, and Ca and find a moderate
enhancement of the three former elements. Among the Fe group elements, Cr and
Ni are slightly overabundant ([X/Fe] ~ 0.05). The Na abundance among the giants
is more than 0.2 dex higher than in the dwarfs. We interpret this as due to
dredge-up of 23Na in the giants. Li abundances show a well-defined pattern: the
Solar-type stars have an almost constant Li abundance, just below that of the
Hyades, and the Li-dip is pronounced and well determined. Turnoff stars just
above the dip have a `cosmic' Li abundance, but within a very small range of
magnitudes (0.25 mag) higher on the turnoff, Li drops by more than a factor 10
implying that some extra mixing is required. Among the giants, two probable
clump stars show detectable Li, while all the other (likely) RGB stars do not.
We show that rotating stellar models including the most recent developments for
meridional circulation and turbulence by shear instabilities explain very well
the observed Li pattern. The possibility remains open that the giants
exhibiting the highest Li abundances are actual RGB bump stars which have just
been through the so-called "lithium flash".
