Rb-Sr AGES OF METAMORPHIC ROCKS FROM THE TARMA AREA OF THE CENTRAL ANDEAN CORDILLERA, CENTRAL PERU

Abstract

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.