Rare earth element distribution in Plio-Quaternary volcanic rocks from southern Peru

Rare-earth element abundances of calc-alkaline andesitic rocks from southern Peru show that these rocks cannot be produceed by a single stage process. The high content of LILE, particularly LREE requires their derivation from a source already enriched in these elements and having a distinctly fractionated REE pattern. It is suggested that ascending hydrous fluids, released from the subducted oceanic lithosphere, enriched the upper mantle in LILE by zone refining. The partial melting of such an enriched upper mantle, followed by fractional crystallization, could produce andesitic rocks. REE data indicate that shoshonitic rocks from southern Peru can be derived from an unfractionated garnet-bearing peridotite by a low degree of partial melting.