  We present the results of a Spitzer Infrared Spectrograph (IRS) survey of
24um-selected luminous infrared galaxies (LIRGs, L_IR > 10^11 L_sun) in the
rich cluster Cl0024+16 at z=0.4. Optically, these LIRGs resemble unremarkable
spiral galaxies with e(a)/e(c) spectral classifications and [Oii]-derived star
formation rates (SFRs) of <2 M_sun/yr, generally indistinguishable from the
'quiescent' star forming population in the cluster. Our IRS spectra show that
the majority of the 24um-detected galaxies exhibit polycyclic aromatic
hydrocarbon (PAH) emission with implied SFRs ~30-60 M_sun/yr, with only one
(<10%) of the sample displaying unambiguous evidence of an active galactic
nucleus in the mid-infrared. This confirms the presence of a large population
of obscured starburst galaxies in distant clusters, which comprise the bulk of
the star formation occurring in these environments at z~0.5. We suggest that,
although several mechanisms could be at play, these dusty starbursts could be
the signature of an important evolutionary transition converting gas-rich
spiral galaxies in distant clusters into the passive, bulge-dominated
lenticular galaxies that become increasingly abundant in the cores of rich
clusters in the ~4Gyr to the present day.
