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Late Cenozoic calc-alkaline lavas of the central Andean continental volcanic arc are characterized by high 87Sr/86Sr ratios (0.705 to 0.708) and high K, Rb, Sr, Ba, LREE, and Ni concentrations relative to island-arc volcanics of comparable SiO2 content. Conflicting published interpretations of these “anomalous” compositions include (1) sialic contamination of a mantle-derived magma of low 87Sr/86Sr and (2) melting of enriched continental lithospheric mantle. The δ18O values for these volcanics, however, lie almost entirely in the range +7.0 to +7.50/00, slightly higher than accepted mantle values of +5.5 to +6.50/00, but too low to be compatible with the large degree of crustal contamination required to alter the 87Sr/86Sr ratios of a mantle-derived magma. Moreover, δ18O values tend to decrease slightly with increasing 87Sr/86Sr. Fractional crystallization as a major factor is precluded by the strong covariance of trace-element concentrations with Sr-isotope ratios.

Melting of sialic material in the subduction zone can account for observed 87Sr/86Sr ratios, δ18O values, and trace-element abundances in the lavas of southern Peru. The data fit a petrogenetic model similar to that proposed by Nicholls and Ringwood in which magma derived from a subducted slab of oceanic crust and continental material rises into and reacts with overlying mantle to yield calc-alkaline magma. Sr-isotope ratios and trace-element abundances of magmas generated by this process are dominated by the slab-derived material, whereas O-isotope ratios and major-element abundances are dominated by the volumetrically more important mantle material.

O-isotope data on the Peruvian volcanics suggest that about one part slab melt equilibrates with four parts mantle material. Sr-isotope ratios and Sr abundances indicate that approximately 60% to 90% of the slab melt is derived from sialic material and the remainder from quartz eclogite. Predicted magmatic compositions match observed compositions when partial melting of the slab is assumed to be 10% to 20% and partial melting of modified mantle is assumed to be 7% to 15%.

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