Abstract

The late Cenozoic andesitic-dacitic volcanic rocks of southern Peru may be divided geographically and geochemically into the Arequipa volcanics and the Barroso volcanics. Although these rocks exhibit strong chemical affinities with calc-alkalic rocks of the island arcs, their Sr87/Sr86 ratios are significantly higher. The Rb-Sr isotope data for each volcanic association define two internally consistent and parallel pseudoisochrons (age ∼ 400 m.y.) that give “initial ratios” of 0.7061 and 0.7044 for the Arequipa and Barroso volcanics, respectively.

Comparison of trace-element and Sr-isotope data of the volcanic products with that of Precambrian sialic rocks indicates that the anomalously high Sr87/Sr86 is not due to upper crustal contamination nor is a lower crustal origin likely on the basis of geophysical arguments. We explain the observed variation of Sr87/Sr86 by means of a modified continental volcanic-arc subduction model in which the overlying continental plate (200 to 300 km thick) abuts the downgoing oceanic plate to form a lithosphere-lithosphere subduction boundary to depths in excess of those at which magma is generated. By this model, andesitic magma either is generated by subduction-zone melting of the ancient lithosphere of the South American plate or is derived from the downgoing oceanic plate but then is isotopically re-equilibrated with the overlying continental lithosphere. Thus, the Arequipa and Barroso volcanics are the products of magmas derived from, or isotopically equilibrated with, different parts of the thick lithosphere of the South American plate. Different Rb/Sr ratios frozen into different parts of the continental lithosphere during the course of its formation are the cause of both the anomalously high Sr87/Sr86 ratios and the regional variations observed.

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