Granulite-facies gneiss of the Arequipa massif extensively developed along the Coastal Cordillera of southern Peru gives a whole-rock isochron age of 1,811 ± 39 m.y. (λ 87Rb = 1.47 × 10−11yr−1) and an initial 87Sr/86Sr ratio of 0.7086 ± 0.0009. Migmatitic granite within the gneiss is probably of late Precambrian or early Paleozoic age. Mineral ages, both K-Ar and Rb-Sr, from the gneisses and granites are disturbed. Potassium feldspar from the gneiss appears to have been reset by the migmatitization event, whereas mica and potassium feldspar from the migmatitic granite appear to reflect established tectonic-thermal events of Late Devonian and Late Triassic time.

In both metamorphic grade and age pattern, these rocks are similar to those of the Trans-Amazonian nucleus of the Guianas, Brazil, Uruguay, and the Buenos Aires province of Argentina. We suggest that the Arequipa massif has always been an integral part of the Trans-Amazonian nucleus and that the Andean belt is ensialic, at least in Peru. The manner in which the structural trends in the Arequipa massif strike into the Pacific Ocean leads to speculation concerning the evolution of the Pacific. Simple rifting, tectonic erosion, and major transcurrent faulting are considered as possible mechanisms to explain the truncation.

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