Geochemical and geochronological analyses of granulitic and calc-silicate rocks of northwestern Sierras Pampeanas

Abstract

The calcsilicate and granulites, show different tipology depending on they belong to the western or eastern belts of the northwestern Sierras Pampeanas. The mineral associations vary according to the metamorphic grade and the chemical compositions; they vary from rich in carbonates to rich in silica. They also look very different from the amphibolites. The calc-silicate in the Cumbres Calchaquíes, Aconquija and Ancasti ranges, show mineral associations with quartz, plagioclase, clinozoicite, garnet, hornblende, titanite, diopside, biotite and calcite; while in the sierra de Quilmes, in granulite facies conditions, hypersthene and scarce biotite are present, and both are different to calc-silicate from sierra de Fiambalá. The geochronologic data obtained, allows to correlate to the event of very-low to low metamorphic grade of the Puncoviscana Formation, with ages of 530-568 Ma, with their equivalent ones deeper in the sierra de Quilmes, represented by calc-silicates with ages of 571 Ma, and granulites with 480 Ma, followed by anatexis phenomena, like those to the Cafayate granite with 453 Ma, and the migmatites of Loma Colorada with 446 Ma. This event is reiterated in the El Infiernillo Tonalite of 419 Ma, in the syntectonic-granodiorites of El Portezuelo Formation, with 445 Ma; in the gneiss of La Puntilla with 483 Ma, and in the gneiss of Sierra de Fiambalá with 440 Ma. On the other hand, the deformation zone of the Sierra de Copacabana, with an age of 388 Ma, is related to the granitoids of Zapata with 395 Ma; La Punta with 405 Ma, and Señor de la Peña, of 376 Ma. These values demonstrate that metamorphism, deformation and magmatism of the Famatinian Cycle, are superimposed over the Pampean cycle.