EDADES ISOTOPICAS DEL BASAMENTO DE LA PROVINCIA DE BUENOS AIRES

Horacio Cazaneuve

Abstract


The object of this work was to ascertain absolute ages for the Basement of the Province of Buenos Aires. The dated samples belong to metamorphic rocks, consi- dered to belong to one of the first episodes of the metamorphísm. They are gneiss, anphí- bolíte and granite. Frorn these rocks, concentrates of biotite, muscovíte, feldspar and pla- giocIase have been seaparated and dated by both Rb-Sr and K-Ar methods. The isotope ages range from 600 to 2260 míllíon years, Highly díscordant ages were found for' coexist- ing biotite and feldspar from a gneiss sample of Balcarce. The feldspar isotopic age bk Rb-Sr is 2260 m.y., and by the K-Ar method is 815. The different degree of sensitivity of these minerals to thermal events. seems the most likely explanatíon of the difference. The ages found in Tandil and Sierra de la Ventana range from 600 to 650 m.y.

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