Pb- and Sr-isotopic data for subduction-related Late Cretaceous and Cenozoic granitoids of the coastal region and lowermost Pacific slope of the Western Cordillera of central Peru suggest that these granitoids are derived from an undepleted mantle source (OIB–type or enriched sub-continental). A depleted MORB–type mantle did not have a major role in the genesis of the magmas.

The mantle source appears to have changed slightly with time. The data suggest that the asthenospheric wedge was isotopically heterogeneous during the early stages of Coastal batholith emplacement. During this time, the subduction slab geometry changed from steeply to moderately dipping, and the stress regime in the overriding plate changed from extensional to compressional. Subduction-related mantle convection caused a progressive homogenization of the asthenospheric wedge between 100 and 80 Ma. Since then, the isotopic composition of the mantle component has not varied significantly. Those parts of the metasomatized asthenosphere that did not melt (or where trapped magmas did not escape upward) did not participate significantly in the convective cells and were dragged along the slab to greater depth.

Evidence for participation of a radiogenic crustal component in the evolution of these Peruvian granitoids is always present. Significant crustal contamination or assimilation cannot be ruled out, since the isotopic contrast between lower- and upper-crustal rocks and mantle-derived magmas is generally low, but contamination by highly radiogenic, in situ, upper-crustal rocks appears to be sporadic in time and space. The radiogenic crustal component is shown to be mainly a component recycled by subduction. We assume that it corresponds to fluids derived from the subducting slab and associated subducted sediments. This crustal component appears to have changed with time. The variations of Pb- and Sr-isotopic compositions are consistent with a two-stage model:

• (1) During the early stages of Coastal batholith emplacement (100 to 90 Ma) the crustal component was mainly derived from tectonic erosion of the Lower Cretaceous accretionary prism. The sediments in the accretionary prism were derived from the Brazilian Shield and had highly radiogenic Pb- and Sr-isotopic compositions similar to modern sediments of the Barbados ridge.

• (2) After 84 Ma, the crustal component was Pacific oceanic sediments (and a slab component) with Pb- and Sr-isotopic compositions like those of modern Pacific sediments.

These results suggest that participation of recycled oceanic sediments in the genesis of Andean calc-alkaline magmas is the rule, although in many cases the evidence is obscured by interaction of mantle-derived magmas with the continental crust.