Twenty-five U/Pb zircon ages, mostly for the Lima segment, are reported, giving concordant ages in the range 101-36 m.y. with a typical accuracy of + or - 0.5%. The results confirm the episodic nature of magmatism. Comparison with K/Ar ages suggest rapid cooling of the plutons following emplacement. Whereas the early basic intrusions are diachronous, the major dyke swarm is closely bracketed at 73-71 m.y. The centred complexes are short-lived and essentially synchronous at 68-65 m.y. The Paccho and Santa Rosa superunits may contain a few previously unrecognized Tertiary plutons. The Linga superunit near Arequipa is comparable with the 67 m.y. Yarabamba superunit. The main phase of porphyry-Cu mineralisation took place 61 m.y. ago.-R.J.P.