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The metamorphic rocks regarded as Precambrian are widely exposed in the Tarma area of the central cordillera, central Peru. Discordant Rb-Sr ages of 230 and 147m. y. on biotite were obtained from a sillimanite-biotite migmatitic gneiss and a garnet porphyrobast biotite schist, respectively, in the metamorphic mass of the area. The presence of the retrogressive imprint observed in the gneiss suggests that the older age is a minimum one of the regional metamorphism in the area. It also suggests that a main phase of its metamorphism belongs to an upper Paleozoic orogeny revealed by Rb-Sr whole-isochron ages from a Chilean metamorphic basement. The younger age might indicate the effect of a late Jurassic-early Cretaceous platonic mass or a tectonic event of this time.

Rb-Sr ages of metamorphic rocks from the Tarma area of the central Andean cordillera,central Peru.