K-AR AND RB-SR GEOCHRONOLOGY OF IGNEOUS ROCKS FROM THE SIERRA DE PAIMAN, NORTHWESTERN ARGENTINA

Epizonal igneous and metamorphic rocks in northwestern Argentina are exposed in the Sierra de Paiman. The metamorphic rocks are quartzites, phyllites, and slates with soft-body impressions and fossil traces that suggest a late Precambrian-Cambrian age. The igneous rocks were intruded during two major magmatic events according to K-Ar and Rb-Sr data. The older event is represented by different kinds of granitoids and gabbroids, intruded 437-459 Ma. These rocks were emplaced syntectonically in a shear zone that remained active after emplacement, causing extensive mylonitization on the east side of the range. The granitoids show Sr isotopic disturbances possibly related to magma mixing events. Petrologic, geochemical and siotopic data for these rocks suggest a volcanic-arc setting, probably related to the back arc of the eastward-dipping continental arc of the Famatina Belt. During the younger event (ca. 379 Ma) stocks and dikes of leucogranites were emplaced post-tectonically with respect to the last episode of mylonitization. The leucogranites have syn-collisional signatures and may thus represent the culmination of the volcanic arc of the Famatina Belt.