Abstract

Zircon U-Pb ages on 50 plutonic samples from super-units in the Lima, Arequipa, and Toquepala segments of the Peruvian Coastal batholith range from 188 to 37 m.y., revealing previously unrecognized Jurassic elements. The Jurassic plutons represent a distinct, time-defined, continental arc separated from the Cretaceous arc by a quiescent period of 50 m.y.

Only 4 of the 50 samples have discordant zircons, and these are all from the Arequipa and Toquepala segments. This regionally limited discordance, interpreted to be due to inherited radiogenic Pb from the Precam-brian basement, suggests that there are fundamental differences in the nature of the crust beneath the Lima and the Arequipa/Toquepala segments.

On a grand scale, magmatism in the Coastal batholith generally evolved through time from mafic to siliceous melts, but zircon U-Pb ages show that the occurrence of small bodies of old siliceous and young mafic rocks was also important. Close age association between all rock types raises the possibility that mantle-derived mafic magmas provided the heat that produced siliceous magmas in either the underplated wedge of the subduction complex or the lower crust throughout the period of batholith emplacement.

In general, age results confirm that super-units have short time spans. For the Santa Rosa, Paccho, and Linga “super-units,” however, mapping generalizations were made in the absence of thorough isotopic age and trace-element data. Remapping of these “super-units” with supporting geochemical and geochronological studies will be required before the emplacement history of the batholith is fully understood.

Dated plutons in the “Paccho super-unit” yield ages between 64.0 and 39.0 m.y. They are thus considerably younger than has been previously recognized and cannot belong to one super-unit. Their young ages also verify the eastward migration of the locus of magmatism through time. A migration rate of 1.3 mm/yr, computed for the rocks along latitude 11°08.1′S on the basis of the ∼105-m.y.-old Patap super-unit and a 61-m.y.-old pluton in the “Paccho super-unit,” suggests that the geometry of the Andean subduction zone in Peru was very stable during batholith emplacement.

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