Petrology, geochemistry and regional relationship of the Neoproterozoic-Cambrian units of the Sierra de Guasayán, Sierras Pampeanas Orientales, Northwest Argentina

Abstract

The Sierra de Guasayán is part of de Eastern Sierras Pampeanas, in the province of Santiago del Estero, NW Argentina, being mainly formed by several granite bodies emplaced in the regional low-grade metamorphic basement. The latter is represented by slates and phylites in the northern part of the range, whereas in the southern part, banded schists with quartz-feldspar veins are predominant. On the basis of their sedimentary, petrological and general stratigraphic relations, the metasedimentary units of the northern sector are related here to the Puncoviscana basin. Cordieritic hornfelds developed by thermal metamorphism have been recognized in the granite-metamorphites contact zone.  The composition of the granitoids is mainly tonalitic-granodioritic to the north, and granitic-granodioritic to the south. Porphyritic textures and medium grained are predominant, while there is common occurrence of small rounded to elongated xenoliths. Geochemically, the granitoids are calc-alkaline, weakly peraluminous with ASI values between 1-1.09. Normal trends of evolution are show in the Harker diagrams. The petrographic and geochemical uniformity of the granitic rocks suggests a common genetic source. Comparatively, the granitoids of Sierra de Guasayán are similar to the Cambrian granitoids present to the north, in the Batolito de Tastil, Cordillera Oriental, and partly to those of the Batolito Sierra Norte-Ambargasta, to the south. All of them constitute the Pampean Magmatic Arc, developed on the western margin of Gondwana during the Late Neoproterozoic - Lower Cambrian.